EFFICIENT TREATMENT OF WASTEWATER USING ELECTROCHEMICAL CELL
Field of the invention
The present invention relates to methods and systems for the electrochemical treatment of waste water.
In particular, it relates to methods and systems for the removal of organic pollutants and oxidation of
inorganic compounds using solid polymer membrane electrolyte electrochemical cells.
Background
There is substantial growth in the demand for new wastewater treatment technologies that is being driven
by population growth and increasing volumes of wastewater produced, tighter wastewater quality
regulations, increasing cost of clean water and water shortages, awareness for the protection of clean
water sources and replacement of aging wastewater systems. Industries are specifically being forced
both by tougher discharge standards and cost pressures to eliminate their recalcitrant wastewater
pollutants prior to discharge, and to adopt on-site water reuse and recycling systems to avoid rising water
supply and effluent discharge costs. The requirement is for cost-effective, sustainable water treatment
technology that does not require the addition of chemicals and does not produce secondary pollution, is
compliant with stringent water quality standards, and has minimal operational and maintenance
requirements.
Industrial wastewater can contain organic compounds, many of which are toxic, persistent and resist
conventional biological and chemical wastewater treatment. The best approach to treat recalcitrant
wastewater is by non-chemical oxidation techniques that can mineralize the pollutants and reduce the
organic load and toxicity of the waste, such as electrochemical oxidation. Electrochemical oxidation is
sustainable, safe and has a high treatment efficacy eliminating a wide variety of pollutants such as
persistent organic pollutants, dioxins, nitrogen species (e.g. ammonia), pharmaceuticals, pathogens,
microorganisms, a majority of priority pollutants and pesticides. There are two main approaches to
electro-oxidation of pollutants in wastewater. The first is to oxidize pollutants by indirect electrolysis,
generating a redox reagent in situ as a chemical reactant. The mediator can be a metallic redox couple or
a chemical reagent (e.g. chlorine, ozone, peroxides). These processes require the addition of a large
amount of chemicals and/or feed oxygen, and produce secondary pollution leading to additional costs for
the disposal of the treated wastewater and operation and maintenance of the process. The second
approach is to use direct electrochemical oxidation, where the organic pollutants are oxidized on the
anode surface.
A variety of cell configurations that include flow-through parallel plates, divided chambers, packed bed
electrodes, stacked discs, concentric cylinders, moving bed electrodes and filter-press have been
developed for direct electrochemical wastewater treatment. However, common to all these
electrochemical cell configurations is poor operational efficiency leading to high energy consumption.
The wastewater is utilized as electrolyte, and in the case of divided cells, both anolyte and catholyte.
Due to very low ionic conductivity of wastewater though, the addition of a supporting electrolyte is
required to improve the cell efficiency and obtain reasonable cell voltages. This generally results in salt,
base and/or acid concentrations that exceed allowable pollutant discharge limits thereby adding the cost
for both the disposal of the treated wastewater and the balance of plant costs of liquid electrolyte
handling. Large electrode gaps and low surface area electrodes are also contributors to efficiency losses
and increased energy consumption. Slow mass transport in the pores of the porous beds, non-optimized
catalyst materials with poor reaction kinetics, high electrode overpotentials, and catalysts with low over
potentials for side reactions (e.g. oxygen evolution) also contribute to lower performance and efficiency
losses. The use of cell component materials which passivate quickly and increase cell resistivity and
instabilities, contribute to efficiency losses. Operating conditions also contribute to efficiency losses.
With high mass and ionic transfer losses, at nominal operating current densities, the voltages are too low
such that incomplete destruction of organic contaminants occurs and an organic film blocks catalyst sites
reducing performance and requiring the use of cell reversal techniques to clean the electrode surfaces.
For instance, published PCT application WO9901382 discloses an electrolytic cell method and apparatus
for the decontamination of fluids. The system advantageously comprises means for adding one or more
chemical substances into the fluid to be treated (e.g. an acid, carbon dioxide, an alkali, hydrogen
peroxide, or a salt.) In another example, Andrade et al. in J . Haz. Mats. 153, 252-260 (2008) disclose the
use of a divided electrolytic cell to treat model phenol wastewater. A supporting electrolyte of sulfuric
acid was required.
To eliminate the requirement for supporting electrolyte addition, various methods have been developed
that reduce the electrode gap in single compartment electrochemical cell configurations. For example,
US6328875 discloses the use of a porous anode allowing wastewater to penetrate through the anode to
flow through the capillary inter-electrode gaps. However, the energy consumption was still high when
run without a supporting electrolyte. As with all single chamber electrochemical systems, hydrogen is
simultaneously produced and wastewater constituents are reduced on the cathode, which consume much
energy. Fouling of the cathode commonly occurs from these reaction products, decreasing the cell
efficiency and leading to increased energy consumption. Another problem encountered in single
chamber systems during oxidation is the production of intermediate compounds. These compounds are
reduced at the cathode and are then reoxidized at the anode decreasing cell efficiency and increasing
energy consumption.
An approach to eliminate the requirement for addition of a supporting electrolyte addition is to use a
solid polymer electrolyte (SPE) in the electrolytic cell. SPE technology has been developed for other
purposes including the production of hydrogen by water electrolysis and of energy using polymer
electrolyte membrane fuel cells. For instance, in the system disclosed in WO03093535, dehalogenation
of halogenated organic compounds and destruction of nitrates is conducted on the cathode by
electrochemical reduction. In this configuration, the anode and cathode compartments are divided by an
ion exchange membrane and an anolyte and halogen-containing catholyte are passed through their
respective chambers. Although the system operated without supporting electrolytes, in order to operate
at low current density (high cell efficiency), a supporting electrolyte was required in the anolyte and/or
catholyte. Murphy et al. in Wat. Res. 26(4) 1992 443-45 1 used a SPE electrolytic cell to treat
wastewaters with low or negligible supporting electrolyte content. The wastewater was re-circulated
through both the anode and cathode. The energy consumption was very high however, and was
attributed to low rates of phenol oxidation and side reactions, primarily oxygen evolution from water.
J.H. Grimm et al. in J . Appl. Elect. 30, 293-302 (2000) used a SPE electrolytic cell to treat model phenol
containing wastewater. The wastewater was pumped through the anode and cathode chambers in series.
The energy consumption however was also high for phenol removal, which was attributed by the authors
to the loss in current efficiency due to side reactions such as oxygen evolution. Further, A. Heyl et al. in
J . Appl. Electrochem. (2006) 36: 1281-1290 investigated a range of SPE electrolytic cell configurations at
higher temperatures to de-chlorinate 2-chlorophenol model wastewater. In all cases, the wastewater was
pumped across the membrane from either the cathode or anode to the opposite chamber through
perforations in the membrane or by assisted electro -osmotic drag of treated membranes. The energy
consumption was found to be unpractically high for the untreated membrane, lower for the chemically
treated membrane, and lowest for the perforated membrane. However, the best mineralization was
obtained with anodic oxidation first followed by cathodic reduction with higher energy consumption.
Still further, another approach for treating low conductivity wastewater without the use of supporting
electrolytes was disclosed in WO2005095282. The system used a solid polymer electrolyte sandwiched
between anode and cathode electrodes place in a single chamber of low conductivity wastewater. The
energy consumption for pollutant mineralization of this setup was high due to the high voltages required.
Systems have also been developed in the art to reduce the cost of producing hydrogen by electrolysis by
integrating electrolytic treatment of wastewater therewith. The electrolytic cells involved can use
anolytes containing organic pollutants. For instance, Park et al. in J . Phys. Chem. C. 112(4) 885-889
(2008) used a single chamber cell to treat aqueous pollutants and produce hydrogen. As with all single
chamber systems, a supporting electrolyte was required. The hydrogen generated was contained in a
mixed product gas that required further treatment to recover usable hydrogen. Similar single chamber
configurations were disclosed by T. Butt & H. Park in WEFTEC 2008 Conference Proceedings and by J .
Jiang et al. in Environ. Sc. & Tech. 42(8), 3059 (2008). Divided cell configurations were disclosed for
instance in WO2009045567 and by Navarro-Solis et al. in I J Hydrogen Energy 35 (2010) 10833-10841.
The preceding systems all involved the use of additional supporting electrolytes. Systems without
supporting electrolytes have also been disclosed for instance by F. Kargi in I . J . Hydrogen Energy 36
(201 1) 3450-3456.
Systems using a solid polymer electrolyte based electrolytic cell have also been disclosed in the art to
generate hydrogen and to treat wastewater. For instance, US65333919 discloses a method for
electrolysis of an aqueous solution of an organic fuel. In this system, permeation of unreacted methanol
to the cathode (fuel crossover) takes place and causing high cathode overpotentials and requiring the
addition of a hydrogen gas cleaning operation. Further, E.O. Kilic et al. in Fuel Proc. Tech. 90 (2009)
158-163 disclose a system to treat formic and oxalic acid and generate hydrogen. However, the specific
energy consumption was high due to the higher current densities required.
Notwithstanding the substantial developments in the art, there remains a continuing need for more
efficient and cost effective methods for wastewater treatment. The present invention addresses this need
while additionally providing other benefits as disclosed herein.
Summary of the invention
Methods and systems have been discovered for the energy efficient treatment of polluted wastewater
using certain electrolytic cell designs and a combination of voltage and current density limitations. A
lower current density results in better efficiency, and a lower voltage results in less unwanted side
reaction (e.g. oxygen evolution). A higher flow rate results in lower energy consumption. Improved
energy efficiency can be achieved while essentially removing all the pollutant.
The electrolytic cells employed comprise a solid polymer electrolyte electrolytic cell comprising an
anode, a cathode, and a solid polymer membrane electrolyte separating the anode and the cathode. The
anode comprises an anode catalyst layer, and the anode catalyst layer comprises an anode catalyst. In a
like manner, the cathode comprises a cathode catalyst layer and the cathode catalyst layer comprises a
cathode catalyst. The cathode in the electrolytic cell is liquid-electrolyte free. That is, the cathode
comprises no liquid catholyte nor liquid supporting electrolyte.
The present method comprises providing at least a first and second solid polymer electrolyte electrolytic
cell stack, supplying a flow of wastewater comprising a pollutant to the anode of each of the first and
second electrolytic cell stacks at a flow rate and flow pressure, providing a voltage less than about 3 volts
across each of the cells in the first and second electrolytic cell stacks wherein the anode is positive with
respect to the cathode, operating each of the cells in the electrolytic cell stacks at an operating
temperature and a current density less than about 20 mA/cm2, and particularly less than about 10
mA/cm2. This results in the pollutant being degraded and hydrogen gas being generated at the cathode.
The generated hydrogen gas is exhausted from the cathode. The flow of wastewater can be supplied to
the anode without an added supporting electrolyte, and the electrolytic cell can be operated over a wide
range of wastewater temperatures. In particular here, either a stack component in each of the two stacks
is different or an operating condition of the two stacks is different. In other words, at least one of a stack
component in the first solid polymer electrolyte electrolytic cell stack and an operating condition of the
first solid polymer electrolyte electrolytic cell stack is different from the stack component in the second
solid polymer electrolyte electrolytic cell stack and the operating condition of the second solid polymer
electrolyte electrolytic cell stack.
In embodiments in which a stack component differs between the first and second solid polymer
electrolyte electrolytic cell stacks, the different stack component can be selected from the group
consisting of an anode fluid delivery layer, the anode catalyst, the anode catalyst layer, an anode flow
field plate, an anode filter layer, the solid polymer electrolyte membrane, and the number of cells in the
stack.
In embodiments in which the different stack component is the anode catalyst layer, it can be at least one
of the catalyst loading and catalyst active area in the anode catalyst layer of the first solid polymer
electrolyte electrolytic cell stack that differs by more than about 5% from that of the catalyst loading and
catalyst active area in the anode catalyst layer of the second solid polymer electrolyte electrolytic cell
stack. In certain embodiments, it can be that the catalyst loading or catalyst active area in the anode
catalyst layer of the first solid polymer electrolyte electrolytic cell stack differs by more than about 10%
from that of the catalyst loading and catalyst active area in the anode catalyst layer of the second solid
polymer electrolyte electrolytic cell stack.
In embodiments in which an operating condition differs between the first and second solid polymer
electrolyte electrolytic cell stacks, the different operating condition can be selected from the group
consisting of the flow rate of the wastewater, the flow pressure of the wastewater, the voltage, the
operating temperature, and the current density. The different operating condition of the first solid
polymer electrolyte electrolytic cell stack can differ by more than about 5% from that of the second solid
polymer electrolyte electrolytic cell stack, and in certain embodiments the operating conditions can differ
by more than about 10%.
In the methods and systems of the invention, the first and second solid polymer electrolyte electrolytic
cell stacks may each comprise a single electrolytic cell. Alternatively, either or both of the first and
second solid polymer electrolyte electrolytic cell stacks may comprise more than one electrolytic cell.
Further, additional solid polymer electrolyte electrolytic cell stacks may be employed.
In one configuration of the invention, the first solid polymer electrolyte electrolytic cell stack is
connected upstream and in series flow with the second solid polymer electrolyte electrolytic cell and the
anode outlet from the first solid polymer electrolyte electrolytic cell stack is connected to the anode inlet
of the second solid polymer electrolyte electrolytic cell stack second cell stack. In certain such
embodiments, the first and second solid polymer electrolyte electrolytic cell stacks can share common
end plates.
In another configuration of the invention, the first and second solid polymer electrolyte electrolytic cell
stacks are connected in parallel flow with the second solid polymer electrolyte electrolytic cell and the
supplied wastewater is divided between the anode inlets of the first and second solid polymer electrolyte
electrolytic cell stacks.
In the methods and systems of the invention, one or more treatment units may be incorporated in the flow
of wastewater. The treatment unit or units may be a filter, a degas unit, and a pH controller. Such
treatment units may be incorporated at various locations throughout a system, including upstream or
downstream of either the first or the second solid polymer electrolyte electrolytic cell stack.
The method is suitable for removing a variety of pollutants from wastewater, e.g. an organic or mixture
of organics, inorganics such as ammonia or hydrogen sulfide, or mixtures of organics and inorganics. As
demonstrated in the Examples, the method is suitable for removing an organic pollutant such as Acid
Blue dye, phenol, acetaminophen, formic acid, ibuprofen, or a mixture of organic pollutants from Kraft
pulp and paper mill effluent. Pollutants oxidized using the method include dissolved organics,
biological oxygen demand (BOD), chemical oxygen demand (COD), total organic carbon (TOC),
recalcitrant organics that remain after biological treatment processes, ammonia, dissolved gases (VOC
light hydrocarbons and H2S hydrogen sulfide), microorganisms, pathogens, and metal ions.
In a wastewater mixture of pollutants, the energy requirement and cell operating conditions for optimized
decomposition and/or oxidation is not equal for all constituents. Also, different catalysts are able to
accelerate oxidation and/or decomposition of specific constituents. Therefore, the decomposition and
oxidation of wastewater pollutants can be optimized to lower treatment cost by combining stacks in
series and parallel that operate at a combination of lower and higher voltage, current density,
temperature, pressure, and flow rate.
Additionally, or alternatively, each of the cell stacks may comprise different component designs and
materials, such as anode fluid delivery layers, anode filter layers, catalyst composition and loadings, flow
field plate type and design, polymer electrolyte membranes, electrode active areas, and cell count.
Again, each stack of electrolytic cells may be optimized for different types of contaminants that may be
in the wastewater stream. For instance, as shown in the following Examples, certain anode catalysts have
a higher affinity for oxidizing different contaminants. Therefore, in wastewater streams with several
different contaminants, one skilled in the art will be able to determine the best anode catalyst (or
membrane electrode assembly design) for a particular type of contaminant, and use them in separate
groups of segmented electrolytic cells or stacks of electrolytic cells for improved contaminant removal
from the wastewater stream.
Embodiments of the system may comprise multiple electrolytic cells in stacks, in either series and/or
parallel flow arrangements. For example, wastewater can be split and supplied to multiple stacks of cells
and the flows combined thereafter at the cell or stack outlets. Each of the stacks of electrolytic cells may
be different, for example, operating at different operating conditions and/or comprising different
components. This is particularly useful for wastewaters that comprise appreciable concentrations of
oxidizable constituents such as ammonia, hydrogen sulfide, metals and inorganics or constituents that
decompose at low electrolytic potentials, decompose with heat, and with different catalysts. By
optimizing the design of each stack for particular types of contaminants that may be in the wastewater
stream, improved contaminant removal, energy efficiency, operating costs and cell lifetime may be
achieved. As shown in the Examples below, cell (and membrane electrode assembly) designs and
operating conditions have a large effect on removal of different contaminants.
Pollutant specific decomposition and oxidation catalysts may be desirably incorporated into the anode
flow field plate, an anode filter layer, the anode fluid diffusion layer, or anode catalyst layer. These
provide for the decomposition and/or oxidation of the pollutants at lower voltage, higher flow rates and
lower energy consumption. Pollutant specific decomposition catalysts may be desirably incorporated
into the catalyst layer to provide for faster rate of pollutant removal. This may allow higher flow rates
and lower cell active area required to reduce energy consumption. The anode catalyst layer may
alternatively only contain catalyst particles that speed up decomposition of a specific pollutant, e.g.
Mn0 2 decomposes H20 2, but does not react with the other pollutants in the wastewater ft may be
beneficial to remove the resulting products quickly in this manner before oxidizing the remaining
organics and inorganics. For example, if the products are gases and/or solids, the wastewater can be
advantageously de-gassed or filtered in an intermediate step to prevent them from interfering with
downstream processes.
The total energy required to remove pollutants from a mixed wastewater can be reduced by configuring
the system to remove the easily decomposed or oxidized pollutants at lower energy first. The wastewater
may pass through a series of stacks, each with a catalyst layer that targets one or more pollutants to
decompose and/or oxidize them at low energy. For wastewater with rapid oxidation and decomposition
of pollutants, the flow rate may be increased and the total cell area or number of cells reduced to lower
cost and system footprint.
For pollutants that oxidize and/or decompose into gases, one or more degas units or methods may be
employed in the system to remove resulting product gases. Dissolved gases (e.g. C0 2, 0 2) may need to be
removed due to corrosion and/or undesirable reactions in downstream equipment and processes. For
example, in water with low concentrations of minerals, carbon dioxide forms carbonic acid which is
corrosive. Degasification methods include heating (e.g. deaerating heaters), reducing pressure (e.g.
vacuum deaerators), membrane processes (e.g. membrane contactors), air stripping, substitution with
inert gas (e.g. bubbling with argon), vigorous agitation, contact with catalytic resins, and freeze-thaw
cycling. For dissolved oxygen, chemical oxygen scavengers may also be added (e.g. ammonium
sulfite). For dissolved carbon dioxide additional methods of removal include contact with limestone
and/or magnesium oxide (to form carbonates and bicarbonates), chemical reaction with a solution of
sodium carbonate to form sodium bicarbonate, and carbonic acid neutralization by controlling the pH
between 7.5 and 8.5.
The following are operating conditions that may be adjusted individually or in combination from stack to
stack in a system to provide for lower energy consumption:
• the voltage may be reduced in one or more stacks for easily decomposed or oxidized species to
lower the energy required in one or more stacks,
· the pressure may be increased in one or more stacks to keep gases dissolved,
• the flow rate may be increased in one or more stacks,
• the temperature may be increased in one or more stacks to increase reaction kinetics,
• the current density may be reduced in one or more stacks.
For pollutants that oxidize and/or decompose into gases, one or more degas units or methods may be
employed in the system to remove resulting product gases. The method can comprise a degas step and/or
an intermediate step between stacks, and/or a post-degas step.
The method can comprise a pre-filtration step and/or intermediate step between stacks, and/or a postfiltration
step.
The method can comprise a combination of stacks in which certain cells comprise anode filter layers and
others do not. The anode filter layers may be either electrically conductive or non-conductive. The
anode and cathode flow field plates may also be electrically conductive or non conductive or a
combination of both. The anode catalyst layers may have different compositions of catalysts and different
concentrations of ionomer.
The method can comprise a pre- pH adjustment step and/or an intermediate step between stacks, and/or a
post-pH adjustment step. This is advantageous for removing pollutants e.g. increase pH to precipitate
metals or decrease pH to precipitate silica and for reducing corrosiveness of wastewaters with acids to
permit the use of less corrosion resistant cell components and thereby reducing stack cost.
The method can additionally comprise a post treatment step for removing free chlorine selected from the
group consisting of: reducing electrochemically, adsorbing, decomposing by contacting a transition
metal, reacting with a salt, reacting with a chemical reducing agent, reacting with organic matter,
decomposing by contacting a redox filter, decomposing by light exposure, and decomposing by heating.
Further, the method can comprise a step for preventing formation of chlorine selected from the group
consisting of: controlling the pH of the wastewater to be greater than about 2, increasing the ionomer
concentration at the anode fluid delivery layer, increasing the ionomer concentration at the anode catalyst
layer, and incorporating materials that are known to catalyze the decomposition of free chlorine into the
anode. The latter materials can include transition elements such as iron, copper, manganese, cobalt and
nickel, Raney metals of copper, nickel and cobalt, their oxides and spinels and can be mixed into the
anode catalyst layer. Alternatively, such materials can be applied as coatings to the anode fluid delivery
layers and/or anode plates to effect decomposition of free chlorine.
Further still, the method can additionally comprise a cleaning step selected from the group consisting of:
purging the anode and cathode with a cleaning solution, flowing solid particles through anode flow -field,
in-situ backwashing, oxygen scouring, chemical cleaning, ultrasonic cleaning, gas purging, liquid
purging, potentiostatic cleaning, flowing water, and generating chlorine and intermediates of oxygen at
higher anodic voltages.
Brief Description of the Drawings
Figure 1 shows a schematic of one embodiment of the inventive system and was used to perform the
laboratory scale wastewater treatment in the Examples.
Figure 2 shows a schematic of the solid polymer electrolyte cell used in the system of Figure 1.
Figure 3 shows a schematic of an alternative embodiment of an electrochemical cell suitable for use in
the inventive system.
Figure 4 shows a schematic of an embodiment of the inventive system having more than one
electrochemical stack.
Figure 5 shows a schematic of an embodiment of an electrochemical cell stack in cross section suitable
for use in the inventive system, wherein the stack comprises segmented cell groups.
Figure 6 shows a schematic of another embodiment of the inventive system having more than one
electrochemical stack.
Figure 7 shows a schematic of yet another embodiment of the inventive system having more than one
electrochemical stack.
Figure 8 is a qualitative prior art illustration showing how the change in original compound concentration
can differ from that of the COD over the course of oxidation for refractory organic compounds such as
phenol.
Figure 9 compares the average actual hydrogen generated from a number of tests performed at several
different currents on phenol contaminated wastewater to ideal or perfect hydrogen generation.
Detailed Description
Certain terminology is used in the present description and is intended to be interpreted according to the
definitions provided below. In addition, terms such as "a" and "comprises" are to be taken as openended.
Further, all US patent publications and other references cited herein are intended to be
incorporated by reference in their entirety.
Herein, SPE stands for solid polymer electrolyte and can be any suitable ion conducting ionomer, such as
Nafion®. A SPE electrolytic cell is thus a cell comprising a SPE as the electrolyte to which electrical
energy is supplied to effect a desired electrochemical reaction (with a positive voltage being applied to
the anode of the cell).
Herein, unless otherwise specified, when referring to a numerical value the term "about" is intended to be
construed as including a range of values within plus or minus 10% of the value being referred to.
An electrode in the cell is "liquid-electrolyte free" means that no significant ion containing liquid is
deliberately provided to the electrode, such as is done in certain systems in the prior art. However, it is
not intended at the cathode for instance to exclude minor amounts of wastewater which may cross over
through a solid polymer electrolyte.
An "electrolytic cell stack" refers to a series stack of electrolytic cells comprising one or more
electrolytic cells.
A "stack component" refers to any of the components making up a solid polymer electrolyte electrolytic
cell stack of the invention. It includes but is not limited to an anode fluid delivery layer, the anode
catalyst, an anode filter layer, the anode catalyst layer, an anode flow field plate, the solid polymer
electrolyte membrane, and the number of cells. Further, it includes any special sublayers employed, the
cathode catalyst, the cathode catalyst layer, a cathode gas diffusion layer, and a cathode flow field plate.
An "operating condition" refers to any of the variable operating conditions employed in the operation of
a solid polymer electrolyte electrolytic cell stack of the invention. It includes but is not limited to the
flow rate of the wastewater, the flow pressure of the wastewater, the voltage, the operating temperature,
and the current density.
The energy efficient system of the invention employs a simple, compact electrolytic cell architecture to
minimize ionic, ohmic and mass transport resistances, and is characterized by a reduced operating
voltage and current density, low-cost components, a chemically stable, non-liquid electrolyte membrane,
and low-cost, durable and high performance electrode and catalyst designs. Recovery of high purity, by
product hydrogen is possible for enhanced efficiency.
An exemplary system is shown in the schematic of Figure 1. System 100 comprises SPE electrolytic cell
101 for the direct electrochemical treatment of organic contaminated wastewater. A controlled flow of
wastewater 102 is supplied to anode inlet 11 of cell 101 by some suitable delivery means, e.g. peristaltic
pump 103. After sufficient treatment/transit time in cell 101, the treated wastewater exits at anode outlet
12 and as shown here is delivered to treated effluent tank 104 where entrained or by-product gases
generated during treatment (e.g. carbon dioxide, nitrogen, oxygen) are allowed to vent to atmosphere.
For monitoring and flow control purposes, pressure gauge 105, valve 106, and flowmeter 107 are
provided in the anode outlet line.
Electrical energy is provided to cell 101 by DC power supply 108 and the temperature of cell 101 is
monitored and controlled by temperature controller 109. Hydrogen is generated at the cathode of cell
101 as a result of the electrochemical treatment and is exhausted at cathode outlet 13. As shown in
Figure 1, the relatively pure hydrogen may be collected and stored in storage container 110 for later use
in the generation of electricity or as a fuel or chemical reactant.
Figure 2 shows a detailed schematic of solid polymer electrolyte electrolytic cell 101. Cell 101
comprises SPE membrane electrolyte 6. The cell anode comprises anode catalyst layer 8 and anode fluid
delivery layer 9. The cell cathode comprises cathode catalyst layer 5 and cathode gas diffusion layer 4.
Anode flow field plate 10 is provided adjacent anode fluid delivery layer 9. Anode flow field plate 10
comprises flow field channel/s 10a which are fluidly connected to anode inlet 11 and anode outlet 12.
Wastewater 101 is delivered uniformly to and from anode fluid delivery layer 9 by directing it through
flow field channel/s 10a. Cathode flow field plate 3 is provided adjacent cathode gas diffusion layer 4.
Cathode flow field plate 3 comprises flow field channel/s 3a which are fluidly connected to cathode
outlet 13. Since no catholyte nor other liquid nor fluid is supplied to the cathode, a cathode inlet is not
required. Hydrogen gas generated during the electrochemical treatment of wastewater 101 however is
collected from the cathode and directed out of the cell by way of flow field channel/s 3a. Leads 2 are
provided at the cell electrodes in order to make electrical connections to power supply 108. Mechanical
support is provided to the components in cell 101 by way of end plates 1 which clamp the cell together.
Sealing is provided to the cell by seals 7. A drain port (not shown in Figure 2) may be incorporated at
the cathode for purposes of cleaning and/or purging any water crossover accumulation. Finally, Figure 2
shows heating elements 14 which may be used to control the cell temperature during operation.
Cell 101 may optionally include an anode filter layer (not shown) which is incorporated between anode
flow field plate 10 and anode fluid delivery layer 9. During assembly, such a filter layer may be applied
to anode fluid delivery layer 9, or applied to anode flow field plate 10, or applied as a discrete component
to be clamped in place by end plates 1. Such filter layers may be provided to prevent particulate and
suspended solids contaminants from entering anode fluid delivery layer 9. The average pore size of such
a filter layer is chosen to remove the particles expected to be found in the wastewater. For example, to
filter particles of 50 micrometer size or greater, the average pore size would be less than 50um. Further,
the average pore size of a filter layer is preferably smaller than the average pore size of the adjacent
anode fluid delivery layer 9. In situations where the pore size throughout anode fluid delivery layer 9 is
not uniform, the average pore size of a filter layer is preferably less than that of the flow field side of
anode fluid delivery layer 9. This configuration provides for removal of contaminants while still allowing
filtered wastewater to access the anode catalyst layer for treatment. Cake formation on the surface of a
filter layer is promoted and this prevents clogging of both the filter layer and anode fluid delivery layer 9.
Also oxidation products can readily be removed from the catalyst layer.
Unexpectedly high energy efficiency can be obtained from system 100 and can result from appropriate
limitations to the voltage and current density applied to the cell and by adoption of some of the designs
and components used in advanced SPE fuel cells for the generation of electricity. In particular, the
voltage applied across electrolytic cell 101 (or across individual cells if more than one is employed in a
system) should be less than about 3 volts. The current density is limited to below about 20 mA per cm2
of electrode area. And as discussed further below, certain catalyst choices, catalyst layer constructions,
fluid delivery layer and gas diffusion layer constructions can benefit operating efficiency.
The reasons for the improved efficiency of the instant invention are not completely understood.
However, without being bound by theory, several mechanisms may be involved at the anode for the
mineralization of organic pollutants. Oxygen for the "electrochemical incineration" of organic pollutants
in the wastewater is obtained from water from an oxygen evolution reaction. Adsorbed hydroxyl radical
and oxygen radical species generated on the surface of the anodic catalyst can mineralize organic
pollutants present. In addition, for certain n-type semiconductor oxide catalyst, anionic (oxygen)
vacancies can preferentially react with water and generate OH* radicals. Oxidation via intermediates of
oxygen evolution/hydroxyl radicals at anodic potentials in the region of water discharge can mineralize
or partially oxidize organic pollutants. Direct oxidation of ammonia to nitrogen may occur. Further,
indirect electrochemical oxidation may take place by inorganic oxidants generated by anodic oxidation of
sulfate, carbonate, or phosphate ions in the wastewater. Decomposition of pollutants may occur. Direct
electrolysis of pollutants may occur. And further still, there may be indirect electrochemical oxidation by
redox reagents electrochemically generated from a mediator present in the wastewater.
The chemical reactions involved at the anode can include:
Direct electrolysis of organic compound R by electron transfer:
R - P + e
For the mineralization of organic compounds, R, through oxygen transfer from water and evolved
oxygen:
n
R — H 0 ®mineralization products [C0 +salts, etc.] + H -|- n e
2H 0 0 + 4H4 -I- 4 e
n
R - —0 ®mineralization products [C0 +salts, etc.] + H + e
For hydroxyl and oxygen radicals, and intermediates of 0 2 evolution on a catalyst surface:
H20 ® OH* + H+ + -
2 ® (OH ) + H+ + -
R + [OH*radicate/ 0 *species /intermediates]^
min lizati products [C0 + salts, etc.] + H + + e
For the oxidation of ammonia
4NH 3+30 ®2N + H 0
NH3/NH 4 + OH* N2 +H20 + H+ + e
and if the wastewater is alkaline, removal via free chlorine
HOC1 + 2/3NH 3 1/3N2 + H20 + H+ + CI
N H3/NH 4 + HOCl/OCl N2 + H20 + H+ + CI
For the formation of inorganic oxidants, e.g.:
2HS04- - S20 2-+2f +2
For the generation of oxidants in-situ, e.g. NaCl in wastewater:
2C1- ®Cl + 2e-
- C + H 0 H0C1+ H + +C1
2
HOCl ®H + +OCl
For H2S :
H S S° + 2H+ + 2e
And if the wastewater is alkaline, via electrochemical decomposition (see "A Modified
Electrochemical Process for the Decomposition of Hydrogen Sulfide in an Aqueous Alkaline Solution",
Z. Mao, A. Anani, Ralph E. White, S. Srinivasan & A. J . Appleby. Journal of the Electrochemical
Society, 1991, pages 1299-1303.) A pH control apparatus may be employed to facilitate alkaline
decomposition
And for metal ions [e.g. transition metal ions such as iron, manganese]:
oxidization via hydroxyl radicals and oxygen
oxidation via hydroxyl radicals, e.g. Mn + OH* Mn 1 + OH
or oxidation with oxygen, e.g.
2Fe2+ + l/20 2 + 5H20 2Fe(OH)3j + 4H+
Mn2+ + I/2O 2 +H20 Mn0 2| + 2H+
For such purposes, oxygen generating electrocatalysts may desirably be incorporated into a catalyst layer
deposited on a fluid diffusion layer. Further, the residence time of wastewater in contact with the catalyst
layer may be increased to complete oxidation, and a microfilter may be employed in the system to
remove resulting metal precipitates.
And catalytic decomposition:
H20 2 H20 + I/2O 2
Pollutant specific decomposition and oxidation catalysts may be desirably incorporated into the anode
flow field plate, an anode filter layer, the anode fluid diffusion layer, and anode catalyst layer. These can
provide for the decomposition and/or oxidation of the pollutants at lower voltage, higher flow rates and
lower energy consumption.
Pollutant specific decomposition catalysts may be desirably incorporated into the anode catalyst layer to
provide for faster rate of pollutant removal at higher voltages. This may allow higher flow rates and
lower required cell active areas so that smaller stacks at lower cost may be employed.
For pollutants that oxidize and/or decompose into gases, one or more degas units or methods may be
employed in the system to remove resulting product gases.
Meanwhile at the cathode, hydrogen evolution occurs as:
H + + T e " ®
Kinetic effects generally are believed to dominate at the low current densities involved in the present
method, and thus the catalysts used may have a great effect. A high active surface area may allow more
OH radicals to be available, the electron and proton transfer media present (e.g. conductive particles and
ionomer) enhance charge transfer, and additional particles may also contribute to generate local oxygen
(e.g. high surface area graphite particles). The use of advanced fuel cell components may also assist in
improved mass transfer and current collection and local mixing of fluids at the catalyst surfaces if there is
not excessive oxygen generation at the anode.
In the present invention, there may be a preferred amount of oxygen produced where too little means not
enough is present for the pollutant removal related reactions to take place at a reasonable rate and yet
where too much oxygen production is parasitic and the current density shoots up while the rate of
contaminant removal remains the same. In the list of preceding anode reactions, the mineralization of
organic compound reactions is frequently cited in the literature. However, the reactions for hydroxyl and
oxygen radicals, and intermediates of 0 2 evolution on a catalyst surface may be of importance. A small
amount of locally generated oxygen may occur on alternative particles without compromising catalytic
sites for OH radicals. In effect, this may result in increased reaction kinetics, and the same organic
pollutant removal rate might be achieved at lower applied voltage and current densities. For electrodes in
the prior art, in order to obtain a decent level of OH radicals, the applied voltage needs to be increased
thereby driving the cells into a substantial range for oxygen production that may then compete with
radical production sites. That is, higher voltages and current densities may be needed in the prior art to
get an equal amount of OH radicals.
Regardless, unexpected improved energy efficiency has been obtained when appropriately limiting the
applied voltage and current density as mentioned previously and also by using certain electrolytic cell
designs and components. Energy efficiency can be further improved by using a combination of
electrolytic stacks at limited applied voltage and current along with faster wastewater flow rates. SPE
membrane electrolyte 6 is a suitable proton conducting solid polymer electrolyte and is preferably a thin,
extended life material choice to increase efficiency (e.g. sulfonated tetrafluoroethylene based
fluoropolymer -copolymer such as Nafion ® in a thickness less than about 30 micrometers). However,
for durability and/or high temperature service, membrane electrolyte thickness may desirably be
increased to between 50 and 100 micrometers (e.g. by laminating thinner membranes together or using
thicker membranes).
With regards to the anode catalyst, platinum, tin oxide, antimony tin oxide, manganese oxide and
mixtures thereof have been used successfully in the Examples. In the case of antimony tin oxide, heat
treatment to improve its electrical conductivity or doping, for instance with Nb, may be considered to
improve durability. Manganese oxide can be considered for purposes of decomposing any hydrogen
peroxide which may be formed at the anode. Other n- and p-type semiconductor oxides, perovskite-like
oxide classes, and amorphous or nanocrystalline transition metal oxides (e.g. Mo0 2) may also be
considered as anode catalysts. Further, spinels of cobalt and nickel, and high surface area nickel oxides
may also be considered. As is known in the art, use of supported catalysts (e.g. Pt dispersed on carbon or
antimony tin oxide on high surface area graphite or Nb particles) can improve the dispersion of the
catalytic materials and thus utilization and also the interaction between certain catalysts and supports can
enhance catalytic activity and durability. Generally dopants can be employed to improve electrical
conductivity (e.g. antimony doped Sn0 2, chlorine and fluorine doped Sn0 2) or to improve durability and
stability at elevated voltages (e.g. cobalt, nickel, palladium, niobium, tantalum, platinum, palladium,
iridium, ruthenium, vanadium, rhenium), and mixtures of such dopants to improve both electrical
conductivity and stability/durability (e.g. Sn0 2 doped with Nb and a dopant selected from the group Sb,
Fe, F, Pt and Ni). Other possible dopants include Mo, Cr, Bi, and W.
Pollutant specific decomposition and oxidation catalysts may be desirably incorporated into the anode
flow field plate, an anode filter layer, the fluid diffusion layer and/or the anode catalyst layer. These
provide for the decomposition and/or oxidation of the pollutants at lower voltage, higher flow rates and
lower energy consumption. Pollutant specific decomposition catalysts may be desirably incorporated
into the catalyst layer to provide for faster rate of pollutant removal at higher voltages. This may allow
higher flow rates and lower cell active area required so smaller stacks and lower capital cost.
Depending on the composition of the wastewater, the decomposition and pollutant specific catalysts may
be Mn0 2, Pb0 2, Fe20 3, metal oxides, mixed metal oxides, doped metal oxides, metal oxides and mixed
metal oxides and doped metal oxides supported on zeolites, aluminum oxides, calcium oxides and
potassium oxides; metal sulfides, bimetallic catalysts such as Ni-Pt and Pt-Sn, cobalt compounds,
transition metals and alloys, Pt, Au, Ru, Ir, Pd, Au and their compounds, noble metals supported on metal
oxides such as alumina, silica, & ceria, transition metal-exchanged zeolites, perovskites,
metalloporphyrins, silver compounds, activated carbon, Cu, Ce0 2 promoted cobalt spinel catalysts,
carbides, nitrides and functionalized silica catalysts.
The composition of the catalyst layer may also include higher concentrations of ionomer as it acts as an
acid catalyst.
The selected catalyst materials are catalytic at lower voltages for the organic contaminants (i.e. have a
lower overpotential) so the applied voltage required is lower and consequently, the current density is
lower. Such catalyst materials have a high overpotential for water electrolysis, so that the generation of
oxygen can be controlled at the operating voltage thereby reducing the current density associated with
this reaction.
Other considerations in the selection of anode catalyst include use of nanoparticles, nanostructured and/or
mesoporous materials to obtain high surface areas. Supported catalysts may be employed using supports
of graphite. If stability of graphite at elevated anodic voltages is an issue, stable, conductive particles
including carbides, nitrides, borides, corrosion resistant metals, alloys, and metal oxides (e.g. Nb, Nb20 5,
ZnO, NbC and/or mixtures thereof) can be employed. Additives can include perovskite -based metal
oxides that exhibit mixed electronic and ionic conductivity.
Anode catalyst layer 8 generally comprises particles to improve electron conduction, ionomer (e.g.
similar to that used in the membrane electrolyte) for ion conduction and to serve as a binder, and material
to control the wetting characteristics (e.g. dispersed PTFE). Pore size and overall porosity can be
engineered to some extent by choice of particle size and agglomerate size (which can be modified for
instance by controlling the high shear mixing rate during preparation of a catalyst ink or slurry used to
make the catalyst layer). The pore characteristics of the anode catalyst layer, the surface chemistry and
surface area can be important with regards to the mass transport of wastewater to the catalyst and the
removal of product gas such as carbon dioxide. Preferably, the pore structure and hydrophobic surfaces
of the anode catalyst layer facilitate bubble detachment so that gas blanketing and/or pore blockage does
not occur. A graded particle size and pore size distribution can be employed in catalyst layer 8 to allow
deeper penetration of wastewater and greater catalyst surface area utilization.
Anode fluid delivery layer 9 is provided to readily deliver fluids to and from anode catalyst later 8 in a
uniform manner. In addition, it provides electrical contact and mechanical support thereto. Carbon fibre
paper, foams, and other materials commonly employed in SPE fuel cell embodiments may be
contemplated here as substrates. And materials for electrical conduction and wettability may be added
thereto. As with anode catalyst layer 8, the pore size distribution and bulk porosity of anode fluid
delivery layer 9 is carefully controlled as it can be important with regards to carbon dioxide bubbles
formed (effecting size and mixing) and their effect on mass transport. Sublayers (not shown in Figure 2)
commonly used in fuel cell embodiments may be incorporated in anode fluid delivery layer 9 and located
adjacent to anode catalyst layer 8 in order to improve contact to the latter and to provide an asymmetric
pore size distribution across layer 9 (e.g. to provide larger pores on the side adjacent anode flow field
plate 10 which may act as a pre-filter preventing suspended solids from blocking catalyst sites).
If elevated anode potentials are involved, dissolution of materials such as carbon fiber paper may occur.
In such cases, more stable media can be employed including metal coated (e.g. nickel coated) carbon
fiber paper or woven cloth, metal screen/gauze/cloth (particularly with 2 or more ply screens with
different mesh sizes and the smaller closest to membrane, with weave patterns to promote in-plane water
permeability, flattened and diffusion bonded or spot welded together), sintered metal screen/gauze/cloth
(again with 2 or more ply screens to improve current distribution and flattened), expanded metal
foil/film/membrane (with 1 or more plies and flattened), sintered metal fiber and powder media (again
with 1 or more plies and flattened, having asymmetric pore size and with the smaller pore diameter
located closest to membrane, and having high in-plane water permeability), flattened photo-etched
media, chemically etched media, micro-perforated plate, or combinations thereof. The preceding
materials are electrically conductive and can be corrosion resistant types [stainless steel, inconel, monel,
titanium, alloys, valve metals] or have corrosion resistant coatings applied thereto [e.g. carbides, nitrides,
borides, noble & valve metals & metal alloys, metal oxides]. Conductive coatings may be applied to the
surfaces contacting the catalyst coated membrane if the corrosion resistant materials employed form
passive layer. Sublayers can be applied incorporating corrosion resistant and electrically conductive
particles [e.g. carbides, nitrides, borides, noble & valve metals & metal alloys, metal oxides] . For
monopolar designs, high in-plane conductivity is desirable, suggesting use of corrosion resistant,
conductive, materials and coatings therefor.
The cathode catalyst can be selected from the group of conventional catalysts commonly used for
hydrogen evolution, including platinum or supported platinum (e.g. carbon supported platinum),
palladium, palladium alloys, supported Pd/C, nickel & oxides thereof, rhodium (e.g. metals where
significant coverage by H2 species is possible), molybdenum disulfide, perovskite -based metal oxides
that exhibit mixed electronic and ionic conductivity, amorphous or nanocrystalline transition metal
oxides, and high surface area Raney metals and metal blacks. In addition, manganese oxide, graphite,
and carbon may also be employed at the cathode. Again, manganese oxide may be beneficial to
decompose any hydrogen peroxide present. Along with cathode catalyst, cathode catalyst layer 5 also
generally can comprise particles to improve electron conduction, ionomer for ion conduction and to serve
as a binder, and material to control the wetting characteristics. Cathode catalyst layer 5 can be prepared
by coating onto cathode gas diffusion layer and sintering at an appropriate temperature (e.g. about 150°C
or 370°C respectively depending on whether ionomer or PTFE is employed). Conductive particles in
layer 5 can desirably be mixed to provide a size distribution that optimizes current distribution and
porosity for hydrogen recovery. If erosion is an issue, PTFE and/or other stable binders in catalyst layer
5 can be employed for improved erosion/wear resistance.
Cathode gas diffusion layer 4 is provided to readily deliver gases to and from cathode catalyst later 5 in a
uniform manner. Layer 4 is desirably designed to repel wastewater which may cross -over from the
anode side through the membrane electrolyte, while still permitting ready removal of generated hydrogen
gas. Thus, a hydrophobic construction may be employed, for instance a teflonated stainless steel mesh
substrate. Further, use of a hydrophobic sublayer with a small pore structure adjacent cathode catalyst
layer 5 may also serve to prevent wastewater cross-over from entering the rest of the cathode. In turn,
this can reduce or eliminate parasitic reactions and contamination at the cathode and thereby help keep
the current density low. In general, materials similar to those employed in anode fluid delivery layer 9
may be considered. For monopolar designs, high in-plane conductivity is desirable, suggesting use of
corrosion resistant and hydrogen resistant, conductive, materials and coatings therefor (e.g. nickel,
palladium alloys, titanium nitride, etc.).
The flow field channels 3a, 10a in the cathode and anode flow field plates 3, 10 can have numerous
configurations, including single serpentine, interdigitated, and/or multiple linear designs, and the crosssections
can have various shapes. Designs for gravity assist may be employed. Accommodating the
hydrogen generated at the cathode is relatively straightforward and one end of the cathode flow field may
be dead-ended. At the anode, channel design preferably maximizes residence and encourages uniform
mixing of the liquids and generated gases. It can be useful to provide for turbulence to promote the
mixing of gas and liquid and to prevent bubble coalescence and large plugs of gas from forming. This
may be accomplished by providing static means for in-line mixing in the channels, e.g. spiral tape,
twisted tape, or helical static mixing elements in various locations within flow field channels 10a. Such
mixing can serve various purposes including reducing a concentration overvoltage at anode, eliminating
radial gradients in temperature, velocity and material composition, and improving mass transport of the
wastewater allowing larger channels and higher wastewater flows to be used without any loss to
performance. Appropriate mixing components would continuously mix the wastewater and direct the
wastewater flow to the outer perimeter so that pollutants are efficiently delivered to the catalyst layer and
gas bubbles are contacted with the porous plate surfaces for removal.
Figures 1 and 2 depict one possible embodiment of the system and electrolytic cell and versions of this
were used in the Examples to follow. However, many other variations are possible and include a
monopolar cell design comprising non-conducting plastic plates with conductive film on landings for
current collection or with a metal substrate used in the anode fluid delivery layer for current collector.
Other monopolar and bipolar variations may be contemplated including bipolar pairs within a monopolar
stack. Plate materials in such cases can be varied. In monopolar designs, plates can be electrically
insulating and made of plastic, composite (e.g. glass fiber reinforced plastic), ceramic, or metals coated
with insulating, corrosion resistant coatings. In bipolar designs, plates are electrically conductive and can
be made of composites (carbon plastic, fiber reinforced where fibers are conductive metals, carbides,
nitrides, etc.), metals, alloys, and substrates comprising appropriate coatings (similar to those of anode
delivery layer 9 on the anode side and gas diffusion layer 4 on the cathode side). In a monopolar stack
comprising bipolar pairs, an electrically conductive cathode plate can be employed in between two
electrically insulating anode plates.
Dissolved gases (e.g. C0 2, 0 2) may need to be removed due to corrosion and/or undesirable reactions
in downstream equipment and processes. For example, in water with low concentrations of minerals,
carbon dioxide forms carbonic acid which is corrosive. Degasification methods include heating (e.g.
deaerating heaters), reducing pressure (e.g. vacuum deaerators), membrane processes (e.g. membrane
contactors), air stripping, substitution with inert gas (e.g. bubbling with argon), vigorous agitation,
contact with catalytic resins, and freeze-thaw cycling. For dissolved oxygen, chemical oxygen
scavengers may also be added (e.g. ammonium sulfite). For dissolved carbon dioxide additional methods
of removal include contact with limestone and/or magnesium oxide (to form carbonates and
bicarbonates), chemical reaction with a solution of sodium carbonate to form sodium bicarbonate, and
carbonic acid neutralization by controlling the pH between 7.5 and 8.5.
Also possible are designs employing a porous anode plate, e.g. porous graphite or porous metal plates
with small pores for degassing the wastewater. In such a design, the channel surfaces can be made
hydrophobic to prevent water ingress with the maximum pore size dependent on contact angle of plate
surface and operating pressure of the wastewater flow. Figure 3 shows a schematic of such an alternative
embodiment 111 based on a porous anode plate option. (In Figure 3, like numerals have been used to
indicate components similar to those shown in Figures 1 and 2.) Here, the electrolytic cell comprises
porous anode plate 15 and gas collection manifold 16. A vacuum assist at the anode outlet is also
provided by vacuum pump 17 to assist in the removal of gases. Other options include the use of a 2-
stage system, instead of a single stage, in which two electrolytic cells are employed in series with the
anode outlet from one being connected to the anode inlet of the other, and in which generated hydrogen
is collected from both cathodes.
The energy efficient benefits of the invention are obtained by limiting the current density and the voltage
applied per electrolytic cell in the system. Other operating conditions are fairly flexible. Any operating
temperature between the freezing point and boiling point of the wastewater may be considered (e.g. from
about 3 to 95 °C) although temperatures modestly elevated above ambient may be useful in increasing
reaction rates (e.g. from about 25 to 50 °C). Wastewater may typically be supplied at pressures from
about 0 to 30 psi. The transit time or residence time of the wastewater is selected in order to ensure
adequate removal of pollutants from the wastewater.
Depending on what is specifically in the wastewater, certain modifications can be considered. For
instance, if the wastewater contains acid, base, alkali and/or other ionic species that make it conductive,
ionomer may not be required in the catalyst layer and an alternative binder may be employed (e.g.
PTFE). If high chloride ion levels are present in the wastewater, it may react at anode electrocatalytic
sites to produce free chlorine (defined as dissolved Cl2 gas, hypochlorous acid HOC1 and/or hypochlorite
ion OC in equilibrium together and whose concentrations are a function of pH). Here, pH may be
controlled to prevent dissolved Cl2 gas (pH > 2). And divalent ions can be added to the wastewater to
increase the concentration therein (such as sulphate S0 4
2 and/or sulphate salts such as NaS0 4) . Such
divalent ions preferentially adsorb onto the electrode, catalyze oxygen formation, and inhibit the
oxidation of chloride ions. Further, transition elements such as iron, copper, manganese, cobalt and
nickel, Raney metals of copper, nickel and cobalt, their oxides and spinels can be mixed into the catalyst
layer that are known to catalyze the decomposition of free chlorine. Such materials can be applied as
coatings to the anode fluid delivery layers and/or anode plates to effect decomposition of free chlorine.
Further, a post treatment step may be employed to remove free chlorine, including: electrochemical
reduction, adsorption by granular activated carbon or kaolinite clay, decomposition by contacting
transition metals (especially copper, iron, nickel and cobalt and/or their oxides and spinels such as
substituted cobalt oxide spinels), reacting with salts such as ammonium acetate, ammonium carbonate,
ammonium nitrate, ammonium oxalate, and ammonium phosphate, reacting with chemical reducing
agents such as sodium metabisulfite, reacting with organic matter such as glycerol, decomposition by
contacting redox filters such as copper/zinc alloys, decomposition by light exposure (especially UV), and
decomposition by heating the solution. Further still, the ionomer concentration at the anode fluid
delivery layer or catalyst layer may be increased to block chloride ions from catalytic reaction sites.
In certain cases during operation, species can undesirably migrate into regions of the electrolytic cell. For
instance, if the wastewater contains high levels of metallic ions that are not all oxidized, a portion can
diffuse into the membrane. This problem may be addressed by performing an in-situ ion exchange
cleaning procedure, or alternatively a pre-treatment step may be employed to remove or reduce these via
chemical coagulation-flotation/filter/clarifier, electro-coagulation & flotation/filter/clarifier, lime
softening, chemical precipitation, and so on. Further, one or more of the following may be performed to
reduce fouling and cleaning requirements: removal of suspended solids, particulate matter, and colloidal
particles (e.g. filtering, gravity separation by coagulation, flocculation & clarification), removal or
reduction of scale-forming minerals (e.g. lime softening, deionization and ion exchange), and removal of
free fats, oil and grease (e.g. coagulation, flotation, and filtration). When metal ion leakage into the
cathode is undesirably encountered, the following procedures or modifications may be considered: a
purge or flush step of the cathode with deionized water, acid, base, chelating agent, or other cleaning
solution, a potentiostatic cleaning procedure, a modification to the ion-exchange membrane to make it
more selective for protons with respect to metallic cations, and/or a modification of the cathode catalyst
layer and gas diffusion layer to make them more hydrophobic to facilitate cleaning. When sodium ion
( a+) ion leakage into the membrane is undesirably encountered, an in-situ ion exchange cleaning
procedure may be performed. And, when sodium ion leakage into the cathode is undesirably
encountered, as above a purge or flush step of the cathode with deionized water, acid, base or other
cleaning solution may be used. In particular, a deionized water purge that results in formation of sodium
hydroxide can provide a valuable by-product which can be recovered. And when oxygen leakage into
the cathode is undesirably encountered, Mn0 2 or other catalyst can be incorporated into the cathode gas
diffusion layer and/or catalyst layer in order to decompose hydrogen peroxide. To provide for certain of
the preceding cleaning processes, the cell and/or system may, at the cathode side, comprise a drain for
cleaning solutions and a valve at the hydrogen gas outlet to prevent solution entering the gas line during
cleaning. Drains may be incorporated generally which drain into the wastewater outlet or other general
disposal. For clean in place capability, power would be turned off to the cell or cells, and a valve at the
wastewater inlet employed to bypass the wastewater and to hook up a cleaning solution line. A valve at
the exit may be employed in order to collect the cleaning solution. A similar process could be used on
the hydrogen line.
One of ordinary skill in the art can be expected to appreciate the factors involved and to be able to
determine what is adequate and how to adjust parameters such as flow rates, etc. accordingly. As shown
in the Examples, model wastewater can be treated without fouling the cell electrodes. Oxygen evolution
on the anode side due to water electrolysis as a side reaction can help keep the electrode free from any
organic film buildup. However, in other situations, occasional cleanup of the electrodes may be required
and accomplished by temporary cell reversals or other techniques known to those in the art.
The advantages of the present methods and systems are numerous. Primarily, they offer improved
energy efficiency in the treatment of polluted wastewater. No solid waste or sludge is produced, nor
toxic by-product gases which otherwise would need to be treated later. No catholyte is employed at the
cathode, no fresh water is needed to generate hydrogen, and no waste is produced there. Thus, no
additional chemicals need be added nor later removed to accomplish treatment. The system is versatile
and can effectively treat effluents from industrial and municipal wastewaters and can mineralize many
pollutants and microorganisms under the same operating conditions, thus combining organic pollutant
removal and disinfection in a single step. Fundamentally, a wide operating range of temperatures,
pressures, and variable effluent flow rates may be used. The system is scaleable and can be considered
for treatment of wastewater quantities ranging from milliliters to millions of liters. The electrolytic cell
components are suitable for low cost, high volume manufacturing processes and/or are already being
mass produced. Along with low cost construction, operating costs and energy consumption are low,
especially considering the possible capture of high purity by-product hydrogen for energy recovery, or
use in other industrial operations.
Embodiments of the system may comprise multiple electrolytic cells in stacks, in either series and/or
parallel flow arrangements, as shown in Figures 4 to 7. For example, wastewater can be split and
supplied to multiple stacks of cells and the flows combined thereafter at the cell or stack outlets, as
shown in Figures 6 and 7. Each of the stacks of electrolytic cells may be different, for example,
operating at different operating conditions and/or comprising different components. This is particularly
useful for wastewaters that comprise appreciable concentrations of oxidizable constituents such as
ammonia, hydrogen sulfide, metals and inorganics, and/or constituents that decompose at low electrolytic
potentials, decompose with heat, or decompose with different catalysts. By optimizing the design of each
stack for particular types of contaminants that may be in the wastewater stream, improved contaminant
removal, energy efficiency, operating costs and cell lifetime may be achieved. As shown in the
Examples below, cell (and membrane electrode assembly) designs and operating conditions have a large
effect on removal of different contaminants.
In Figure 4, first cell stack 201 is connected upstream and in series with second cell stack 202, wherein
the anode outlet from first cell stack 201 is connected to the anode inlet of second cell stack 202, with
hydrogen generated being collected from all cathodes via stream 203. At least one of first cell stack 201
and second cell stack 202, operates at different operating conditions than the other cell stack, such as
anolyte flow rate, current, voltage, temperature and/or pressure.
In Figure 5, three stacks sharing end plates are connected in flow series such that reactant flows from first
first stack 204, then to second stack 205, and then to third stack 206. Each of stacks 204, 205, 206 may be
a monopolar design or a bipolar design, as described in the foregoing, and may comprise of one or more
cells. Each of the stacks is sandwiched between shared end plates 207, 208, which may contain pockets
(not shown) for each of the stacks for keeping them in place. The end plates may be electrically
insulating or electrically conductive. If the end plates are electrically insulating, then current or voltage
control may take place at each of the anode and cathode electrodes to provide different operating
conditions to each of the cells. Alternatively, each stack may have conductive end plates (placed in the
pocket of the insulating end plate), and current or voltage control may take place at the end plates so that
operation of each cell in the stack will be the same, but may be different from other stacks. In yet
another alternative, conductive end plates may be used and, current or voltage control may take place at
the conductive end plates so that each of the stacks will operate at the same conditions. Furthermore,
there may be a collection means between stacks for collecting wastewater for sampling and determining
pollutant levels that can optionally be used for controlling current or voltage of the downstream segments
(not shown). One skilled in the art will appreciate the advantages of such a design, such as requiring less
hardware and balance of plant, while providing a smaller footprint. In addition, in a monopolar design,
each of the cells can be operated at different current or voltage and, thus, the performance of each cell
can be controlled to be the same without affecting other cells. (Electrical connections have been omitted
from Figure 5 for ease of understanding.) While that stacks are shown to be connected in flow series, it
will be understood that the stacks may also be connected in different ways, such as that described for
Figures 6 and 7.
In another embodiment, as shown in Figure 6, first cell stack 210 is connected upstream and in series
with second cell stack 2 11 and third cell stack 212, with hydrogen generated being collected from all
cathodes via stream 213. In this embodiment, at least one of first cell stack 210, second cell stack 2 11,
and third cell stack 212 operates at different operating conditions than the other cell stacks, such as
anolyte flow rate, current, voltage, temperature and/or pressure. The anolyte flow from first cell stack
210 is divided among second and third cell stacks 2 11, 212 via valve 214. One skilled in the art will
appreciate that the first stack may be in a reverse position than is shown in Figure 5, that is, the second
stack and the third stack are in a parallel configuration with the first stack in series downstream from the
second and third stacks such that the two anolyte streams exiting from second and third stacks combine
into a single anolyte stream that is then fed to the first stack.
In yet another embodiment, as shown in Figure 7, first cell stack 210, second cell stack 2 11, and third cell
stack 212 are connected in a parallel arrangement, such as the anolyte flows are divided among stacks
210, 2 11, and 212 via valve 215. In this embodiment, at least one of first cell stack 210, second cell stack
2 11, and third cell stack 212 operates at different operating conditions than the other cell stacks. Such an
arrangement may be useful in situations where a portion of the wastewater is reused for other
applications that can tolerate a higher contaminant level while another portion of the wastewater is
recycled and requires a lower contaminant level. For example, a portion of the wastewater can be treated
to reduce chemical oxygen demand (COD) or ammonia or hydrogen sulfide to about 99% removal level
so that the treated wastewater can be reused in a reverse osmosis unit or other industrial process requiring
a very low COD or other pollutants, while the remaining portion of wastewater only needs to be treated
to a 50% COD level for discharge and/or for disinfection.
In further embodiments, treatment unit 218, which may be a filter, a degas unit, a pH controller, or the
like, may be placed upstream of stacks, between stacks and/or downstream at the end of the stacks, as
shown in Figures 5 and 6, or between stacks in Figure 4 (treatment units 218 not shown). As a filter,
treatment unit 218 captures oxidation and decomposition products between stacks to prevent such
products from entering downstream stacks. As a filter, treatment unit 218 may be cleaned by any method
known in the art, such as backwashing, air or gas scouring, and filter cartridge replacement. As a degas
unit, treatment unit 218 removes product gases to prevent such products from entering downstream
stacks. As a pH controller, treatment unit 218 adjusts wastewater pH to prevent corrosion of downstream
components, precipitates pollutants and/or delivers effluent with requested pH.
For example, for wastewater with organic contaminants, ammonia, and hydrogen sulfide, two electrolytic
cell stacks may be connected in series. The first cell stack may be set at a lower voltage and current and
higher anolyte flow rate to oxidize the ammonia, hydrogen sulfide and other easily oxidized organics.
The second cell stack may be set at a higher voltage and current, and at the same or lower anolyte flow
rate to oxidize the remaining organics. A filter between the first and second cell stack may be employed
to remove oxidation products, such as sulfur-containing oxidation product. Also, a degas unit may be
placed in front of the first stack, between the first and second stack and/or at the end of the second stack.
Also pH may be adjusted after the first stack and/or after the second stack.
In another example, for wastewater containing various molecular weight organic contaminants, such as
volatile and petroleum hydrocarbons, a first cell stack may be operated at a lower voltage and/or current
to oxidize the bulk of the easily oxidized organic constituents and partially oxidize the large molecular
weight organics, and a second cell stack connected in series and operated at a higher voltage and/or
current to oxidize the remaining species.
Additionally, or alternatively, each of the cell stacks may comprise different components, such as anode
fluid delivery layers, anode filter layers, catalyst composition and loadings, flow field plate type and
design, polymer electrolyte membranes, electrode active areas, and cell count. Again, each stack of
electrolytic cells may be optimized for different types of contaminants that may be in the wastewater
stream. For instance, as shown in the Examples (see Tables 1 through 9), certain anode catalysts have a
higher affinity for oxidizing different contaminants. Therefore, in wastewater streams with several
different contaminants, one skilled in the art will be able to determine the best anode catalyst (or
membrane electrode assembly design) for a particular type of contaminant, and use them in separate
groups of segmented electrolytic cells or stacks of electrolytic cells for improved contaminant removal
from the wastewater stream.
The method can comprise a combination of stacks with filter layers and stacks without. The filter layers
may be either electrically conductive or non-conductive. The anode and cathode flow field plates may
also be electrically conductive or non conductive or a combination of both.
For influent wastewaters comprising solid particles, the method can comprise stacks with an anode filter
layer adjacent the anode fluid diffusion layer and stacks without a filter layer. For instance, if particles
are in the incoming wastewater, a first set of stacks may desirably have such filter layers. An
intermediate filtration step may then be performed and the next stacks may not require such filter layers.
For influent wastewaters without any particles, the first set of stacks may not have such filter layers but if
solid particles are produced in the oxidation and/or decomposition process, the subsequent stacks amy
desirably then have such filter layers. Also intermediate and post-filtration steps and/or pH adjustment
steps and/or degas steps may also be employed.
The anode flow field plate may be electrically conductive with a conductive anode filter layer and fluid
diffusion layer. Alternatively, for instance if the wastewater is very corrosive, the flow field plate may be
made of plastic, the anode filter layer may be a non-conductive filter layer, while the fluid diffusion layer
will be electrically conductive.
n one example, for landfill leachates, which typically contain ammonia nitrogen, organics, and chloride
ions the wastewater treatment system may comprise three separate electrolytic cell stacks connected in
series. The first cell stack is designed to oxidize ammonia at lower voltages, such as about 1.4 to about
2.0 V, while running the anolyte at a relatively faster flow rate than the second cell stack because of the
faster reaction rate and/or lower contaminant concentration. The stack may also comprise fewer cells
than the second cell stack because of the faster reaction rate. The reactions in the first electrolytic cell
stack may include the following:
2NH3 + 60Ff N2 +6H20 +6e (1)
2C1 C 12 + 2e (2)
Cl2 + H20 HOC1 + FT + CI (3)
2/3NH3 + HOC1 1/3N2 + H20 + H+ + CI (4)
The second electrolytic cell stack is designed to oxidize organics, and may operate at a higher voltage
than the first electrolytic cell stack such as greater than about 2.5 V, while running the anolyte at a
relatively slower flow rate than the other cell stack because of the slower reaction rates and/or higher
concentration of contaminants. Additionally, or alternatively, the stack may also comprise more cells
than the other cell stacks because of the slower reaction rates and/or higher concentration of
contaminants.
The third electrolytic cell stack is designed to remove the remaining microorganism contaminants from
wastewater stream by disinfection. Therefore, the third cell stack may comprise means for generating
free chlorine from chloride ions in chloride-containing wastewaters. The third cell stack may operate at a
lower voltage than the second cell stack because the voltage for generating free chlorine is generally
lower than the voltage required to generate hydroxyl radicals, and may comprise fewer cells than the
second cell stack because less residence time is required for the wastewater since once free chlorine is
generated, free chlorine will disinfect the wastewater and has residual disinfection.
One skilled in the art will recognize that the embodiments described above can be further combined in a
number of different ways to optimize contaminant removal from the wastewater stream as well as
improving energy efficiency and lifetime of the electrolytic cells. The embodiments described above
merely illustrate several aspects of the invention and should not be construed to be limiting in any way.
In the construction of multiple cell systems, conductive layers may be employed between the fluid
diffusion layers and plates or between the gas diffusion layers and plates. Alternatively, conductive foils
or membranes may be welded to the fluid diffusion layers or gas diffusion layers.
The following examples are provided to illustrate certain aspects of the invention and particularly the
various results obtained when employing electrolytic cells of the invention with different cell
components and/or which are operated under different conditions. These examples however should not
be construed as limiting in any way.
EXAMPLES
Numerous laboratory scale solid polymer electrolyte electrolytic cells were constructed as shown
generally in Figure 2 and were used to remove contaminants from wastewater samples via the method of
the invention. The contaminants removed were either Acid Blue 29, phenol, acetaminophen, ibuprofen,
Kraft mill effluent, or formic acid and these were present in different concentrations as indicated below.
The test electrolytic cells all employed a single membrane electrode assembly (MEA) comprising fluid
and gas distribution layers adjacent to each of the anode and cathode electrodes. The fluid distribution
layers were made of various porous carbon papers on which various microporous sublayers had been
applied (as indicated below) and niobium mesh with a tungsten gauze sublayer. In some cases,
commercially obtained MEAs were used and in other cases, catalyst layers comprising special catalyst
compositions were prepared and applied to the fluid distribution layers (again as indicated below). The
MEAs with fluid diffusion layers were clamped between graphite resin composite plates in which
serpentine flow field channels had been machined. The size of the MEA varied somewhat from cell to
cell as indicated below, but was of order of 50 cm2 in size.
In these laboratory scale tests, several thicknesses of porous graphite paper from Toray were used as
substrates for the fluid diffusion layers (i.e. Toray™ TGP-H-030= 110 m h, TGP-H-60 = 190 mih, TGPH-
90=280 mih, TGP-H-120=370 mih) . The papers were impregnated with PTFE using multiple
successive conventional dip or flow techniques to build up the thickness of the PTFE coating slowly
without forming cracks. Each coating layer was dried to remove water at 80°C. The PTFE impregnated
substrate was either sintered at 400°C for 10 minutes to increase the hydrophobicity of the surface before
applying the microporous sublayer coatings, or was left unsintered to allow for controlled penetration of
microporous coating solution.
Microporous sublayer coatings were then applied to the fluid diffusion layer substrates. Suspensions of
electrically conductive particles and hydrophobic PTFE were prepared in solutions comprising water,
wetting agent, and pore formers as indicated in Table 1 below. First, the electrically conducting particles
were suspended in water and wetting agent by dispersing/mixing at 1500 rpm for 5 minutes. Then, the
PTFE and pore former in water were added and mixed at 2500 rpm using a high shear mixer for 30
minutes or longer until no agglomeration is present (determined by fineness of grind gage). The sublayer
suspension was then applied to the substrates either by rod or blade coating. The coated substrates were
heated to remove water and then were calendared. Finally, both the wetting agent and pore former were
removed and the applied PTFE was sintered by heating the coated substrates for 10 minutes at 400°C.
Table 1 below summarizes the various sublayer compositions of the 8 different sublayers appearing in
these Examples. Sublayer #s 4, 5, and 6 had the same composition and were made in the same manner
but were applied in different amounts to the substrates involved.
Table 1.
4, 5, 6 5 wt.% Timrex KS25™ 1 wt.% 1 wt.% HPMC + 0.4 wt.% Tergitol™
graphite 92.6 wt.% H20
7 5.5 wt.% Timrex KS25™ 2 wt.% 1 wt.% HPMC + 9 1 0.5 wt.% Tergitol™
graphite wt.% H20
8 3.5 wt.% Timrex KS25™ 2 wt.% 1 wt.% HPMC + 0.5 wt.% Tergitol™
graphite + 1.5 wt.% Mn0 2 91.5 wt.% H20
9 5 wt.% Niobium 1 wt.% 1 wt.% HPMC + 0.4 wt.% Tergitol™
92.6 wt.% H20
Notes:
Timrex HSAG300™ graphite has a particle size distribution with 90% < 32 mih, and a surface area = 280
m2/g
Super P-Li™ conductive carbon black has 40 nm particle size and a surface area of 62 m2/g
Timrex KS150 synthetic graphite has a particle size distribution with 95% <180 mih
Timrex KS25 synthetic graphite has a particle size distribution with 90% < 27.2 mih and a surface area of
12 m2/g
Mn0 2 powder has < 5m h particle size distribution
HPMC stands for hydroxypropyl methylcellulose
>95% of niobium was -325 mesh powder
Nine different anode catalyst layers (denoted Al to A9) and five different cathode catalyst layers
(denoted CI to C5) appear in these Examples. The various catalyst layer and preparation suspension
compositions are summarized in Table 2 below. Al and CI were commercially obtained platinum
catalyst layers coated on a membrane electrolyte which were provided as a complete catalyst coated
membrane (CCM) product from Ion Power, Inc. and thus do not appear in Table 2. The catalyst layers
appearing in Table 2 were applied in the form of a suspension to the sublayer coated fluid diffusion
layers or membrane electrolytes as indicated in Tables 4-7 below. The suspensions were prepared by
adding the indicated catalyst and electrical conductor powder to a liquid carrier. The suspension was
mixed at 2500-3500 rpm for about 30 minutes after which the proton conductor (electrolyte) was added
and mixed further at 2500 rpm for 15 minutes. The catalyst coating suspension was then sparingly
sprayed using multiple passes onto each surface of the membrane (CCM) or onto the fluid distribution
layer and cathode gas diffusion layer (electrodes) using an air-powered, gravity-fed spray gun. The
coating was dried between passes until the desired coating weight was reached.
Table 2.
Notes:
ATO(l) stands for antimony tin oxide nanoparticles; ratio of Sb20 5:Sn0 2 is 10:90 wt%; 22-44 nm
particle size; and surface area of 20-40 m2/g
ATO(2) was ATO(l) which had been heat treated for 4 hours at 550°C in air
ATO(3) was antimony tin oxide decorated Timrex HSAG300™ graphite
ATO(4) was Nb and Sb doped tin oxide particles; Nb20 5:Sb20 5:Sn0 2, nominal ratio 5:10:85 wt.%
The platinum used was HiSPEC 4100™; nominally 40% by weight on carbon support
Timrex HSAG300™ graphite is a conductive, high surface area graphite having a particle size
distribution in which 90% < 32 mih; and a surface area of 280 m2/g
Super P-Li™ was a conductive carbon black; with 40 nm particle size; and a surface area of 62 m2/g
The silver used was a spherical powder, 99.9%(metals basis), having a particle size distribution of 1.3-3.2
mih; and a surface area of 0.3-0.7 m2/g
Mn0 2 powder had <5m h particle size distribution
Sn-Ag was an alloy nanopowder, with <150 nm particle size, 3.5% Ag
Sn0 2 was -325 mesh powder
Nafion™ EW1100 was a dispersion comprising colloidal particles in a 10 wt.% solution
Ta was -325 mesh powder
Nb was -325 mesh powder
TiC powder had < 4 mih particle size distribution
Further, in the above, the ATO(3) was prepared by dissolving 9.5 gm SnCl2-2H20 and 0.5 gm SbCl3 in
10 ml concentrated HC1 acid. The mixture was stirred until the solution was clear. 10 gm of pre -treated
Timrex HSAG300™ graphite was then dispersed in 100 ml ethanol. This graphite suspension was
heated to 80-90°C and the acid solution was added slowly while continuing to stir. Heating and stirring
continued until the ethanol evaporated. The powder product was filtered and washed with de-ionized
water and then dried in an oven at 100°C. In this procedure the Timrex HSAG300™ had been pretreated
by first combining 0.25 gm PdCl2, 12.5 gm SnCl2-2H20 , 150 ml de-ionized water, and 75 ml
concentrated HC1 acid, stirring at room temperature until green in colour (> 1 hr), then adding 20 gm of
the graphite powder to this suspension, stirring for 1-3 minutes, and finally filtering, rinsing and drying
the powder.
The test MEAs comprising the fluid diffusion layers were bonded together into unitary assemblies before
testing. When employing commercially obtained and in-house manufactured catalyst coated membrane
electrolytes, these were placed between an appropriate anode fluid distribution layer and cathode fluid
distribution layer (henceforth referred to as cathode gas diffusion layer because the fluid at the cathode
side was always gaseous) and were either hot pressed at 140°C for 5 minutes or left un-bonded for
testing. When employing the catalyst coated fluid diffusion layers described herein, these electrodes were
placed on either side of a commercially obtained membrane electrolyte and hot pressed at 140°C for 5
minutes to bond them together. PTFE tape was used to mask the edges of un-bonded CCMs to provide a
dimensionally stable perimeter for the cell assembly.
The compositions and loadings of the various catalyst layers and fluid distribution layers used in the
MEAs in these Examples are summarized in Table 3 below.

Table 3 coiit.
' es otherwise i di ted the substrate with PTFE ot wintered before sublayer coating
The electrochemical cell assembly was completed by sandwiching the test MEAs between anode and
cathode flow field plates made of polymer-graphite composite. A 4 pass serpentine channel had been
machined in the cathode flow field plate with a 1mm channel width, 1mm channel height, 1mm landing
width and a geometric area of 50 cm2. Two different anode flow field plates were used; the first having a
4 pass serpentine channel machined in the flow field plate with a 1mm channel width, 1 mm channel
height, 1 mm landing width and a geometric area of 50 cm2, and the second having a single channel
machined therein with a 5 mm channel width, 8mm channel height, 2 mm landing width, and a geometric
area of 50 cm2. A spiral in-line mixing component, manufactured from twisted PTFE tape, 2mm in width,
was used with the single channel anode flow field plate and the channel interior was coated with PTFE.
The sealing gaskets used were made of Viton® and Gore®, the current collectors were gold coated
copper, and the end compression plates were made of steel and contained interior electrical resistance
heating elements. In all the experimental tests below, the 4 pass channel design was used except for the
test involving MEA K2 in Table 5 which used the single channel and in-line mixing component.
Testing then involved preparing model contaminated wastewaters (>1 L of solution) with the specified
pollutant in de-ionized water. The electrochemical cell temperature was kept constant using the internal
resistive heating elements, a temperature controller, and thermocouple. Several test temperatures were
used as indicated below. Wastewater comprising the indicated contaminant was then flowed through the
anode of the test cell using a peristaltic pump at a rate of 270 mL/hour while a constant DC voltage was
applied to the current collectors. The valve downstream from the anode exhaust was used in selected
trials to provide pressurized flow. The cathode inlet of the test cell was sealed and the cathode exhaust
was also provided with a valve downstream to provide slightly pressurized hydrogen gas exhaust. The
majority of tests were run at atmospheric pressure at the anode exhaust and slight pressure (< lpsi) at the
cathode exhaust as a result of filling the hydrogen storage container. No water or purge gases were used
or required on the cathode. No supporting electrolyte of any kind was used at the cathode in any test. The
wastewater effluent was collected in a plastic jug and the product gases were released to the atmosphere.
Tables 4, 5, 6, 7, 8 and 9 below summarize the results obtained for the tests involving Acid Blue 29 dye,
phenol, acetaminophen, formic acid, ibuprofen, and Kraft effluent respectively.
In the case of the Acid Blue 29 dye pollutant, colour measurements were used to quantify the efficacy of
treatment. The % of colour removal was determined with a U VIS Spectrophotometer by comparing
absorbance against samples of known concentrations.
In the case of the other pollutants tested, the chemical oxygen demand (COD) was used to quantify the
efficacy of treatment. COD is used as a measurement of pollutants in wastewaters and natural waters.
Both organic and inorganic components of a sample are subject to oxidation, but in most cases the
organic component predominates and is of the greatest interest (ref. Standard Methods for the
Examination of Water and Wastewater, 2 1st Edition, APHA, AWWA, WEF, ©2005). In general, the
oxidation of specific compounds is characterized by the extent of degradation of the final oxidation
products (ref: Industrial Water Quality, 4th edition, W. Wesley Eckenfelder, Jr., Davis L. Ford and
Andrew J . Englande, Jr. McGraw-Hill Companies, Inc. © 2009). The reason for this is that the
degradation of the pollutant can be referred to in several ways. There is: (1) Primary degradation which
involves a structural change in the parent compound; (2) Acceptable degradation (defusing) which
involves a structural change in the parent compound to the extent that toxicity is reduced; (3) Ultimate
degradation (mineralization) which involves conversion of organic carbon to inorganic C0 2; and (4)
Unacceptable degradation (fusing) which involves a structural change in the parent compound resulting
in an increase in toxicity. Any degradation process that does not lead to total mineralization of the
organic constituents may potentially form end products that can be more toxic than the original
compounds. Figure 4 is a prior art illustration of how the change in original compound concentration can
differ from that of the COD over the course of oxidation for refractory organic compounds such as
phenol. Although at point A, the amount of original/parent compound has decreased to zero, the COD of
the wastewater does not meet discharge limit for COD concentration.
Therefore, to quantify the pollutant removal efficacy of the system/process, ultimate degradation
(mineralization) of the organic compounds is preferably measured by the chemical oxygen demand
(COD). COD will report virtually all organic compounds, and is used for monitoring and control of
discharges in industrial applications, discharge permits, and for assessing treatment plant performance.
COD is a measure of the total quantity of oxidizable components in a sample (e.g. carbon, hydrogen from
hydrocarbons, nitrogen, sulfur, and phosphorus) and was measured here by Method 5220 C (EPA
approved - Standard Methods for the Examination of Water and Wastewater, 2 1st edition).
Samples of the treated wastewater were taken throughout the test periods and average values for colour
and COD were determined in accordance with the pollutant present. The current across the test cells was
generally stable and the average current density was also determined as reported below.
Tables 4 to 9 also list the energy consumption (the product of voltage, average current, and time over all
the passes through the cell) per unit volume of wastewater. Where appropriate, the specific energy
consumption per unit mass of COD mineralized is also listed.
Further, the hydrogen gas volume produced was measured in each case at the storage device. And from
this, the efficiency of H2 electrolysis was determined and listed in the Tables. Under ideal circumstances
it requires 39.4 kWh of electricity at normal conditions (25°C and 1 atm) to make 1 kg of hydrogen. This
represents the higher heating value (HHV) of hydrogen, which includes the total amount of energy
(thermal and electrical) to disassociate water at normal conditions. System efficiency is calculated by
dividing the heating value (HHV) by the real energy input in units of kWh/kg. Industrial electrolyzer
efficiencies generally are in the range of 52% to 82% (HHV).
Table 4 . Colour Removal
Note: In Table; 4 to 9 . * indicate? that the catalyst layer coated onto fluid and gas distribution layers: all the other MEA comprise catalyst layers coated
onto the membrane. All CCM based M As wer e tested unbounded while the others were tested bonded.
Table 5. Phenol Removal
Tabl . Acetaminophen Removal
Table . . Foniiic Acid Removal
Table 6 . I p fen Removal
Table 9 . Kraft effluent removal
The results using these laboratory test cells show that electrochemical cells with non-liquid, polymer
electrolytes, that contain no other added chemicals, and comprising low cost catalysts and other electrode
components can provide equal or better removal efficiency as comparative prior art systems for
recalcitrant Acid Blue 29 dye, phenol, acetaminophen, formic acid, ibuprofen, and Kraft pulp and paper
mill effluent. In particular, these results can be obtained with substantially lower energy inputs (i.e. at
current densities less than about 10 mA/cm2 and applied voltages less than about 3 V), in some instances
with greater than 60% energy reduction at 80% COD removal, with greater than 80% energy reduction at
95% COD removal and this is without including recoverable energy contributions from the hydrogen
produced. A 20% increase in current efficiency was observed for Acid Blue dye 29, and over 60%
increase for phenol and acetaminophen. Certain specific in-house prepared catalyst choices and
electrode designs can lead to > 40% improvement in performance.
Further still however, the inventive method efficiently produces hydrogen at a purity equivalent to
commercial electrolyzers and in sufficient amounts such that an estimated additional 15-35% reduction in
net energy consumption may be achieved depending on wastewater composition (assuming conversion of
hydrogen back to electricity using a fuel cell stack operating at 50% efficiency and assuming 95% of the
hydrogen was recovered). For illustrative purposes, Figure 5 shows the average actual hydrogen
generated from a number of tests performed at several different currents on phenol contaminated
wastewater compared to ideal or perfect hydrogen generation. As can be seen, there is a high conversion
of phenol contaminant to hydrogen.
In addition, the recoverable energy in a realistic scaled industrial system can be estimated based on the
above. Assuming state-of the art fuel cells are used to convert the generated hydrogen back into
electricity at 50% efficiency, Table 10 shows the expected recoverable energy in an industrial system
operating as per the three data points shown in Figure 5 above. In this Table, the system has been scaled
up to treat lm /hr 500mg/l phenol wastewater, and it is assumed that the hydrogen generated is converted
back to electricity with 95% utilization using 5kW fuel cells operating at 50% efficiency.
Table 10.
All of the above mentioned U.S. patents and applications, foreign patents and applications and non-patent
publications referred to in this specification, are incorporated herein by reference in their entirety.
While particular embodiments, aspects, and applications of the present invention have been shown and
described, it is understood by those skilled in the art, that the invention is not limited thereto. Many
modifications or alterations may be made by those skilled in the art without departing from the spirit and
scope of the present disclosure. The invention should therefore be construed in accordance with the
following claims.

What is claimed is:
1. A method for the energy efficient treatment of polluted wastewater comprising:
providing at least a first and second solid polymer electrolyte electrolytic cell stack, the cells in
the first and second stacks each comprising an anode comprising an anode catalyst layer and the
anode catalyst layer comprising an anode catalyst, a cathode comprising a cathode catalyst layer
and the cathode catalyst layer comprising a cathode catalyst wherein the cathode is liquidelectrolyte
free, and a solid polymer membrane electrolyte separating the anode and the cathode;
supplying a flow of wastewater comprising a pollutant to the anode of each of the first and
second electrolytic cell stacks at a flow rate and flow pressure;
providing a voltage less than about 3 volts across each of the cells in the first and second
electrolytic cell stacks wherein the anode is positive with respect to the cathode;
operating each of the cells in the electrolytic cell stacks at an operating temperature and a current
density less than about 20 mA/cm2, thereby degrading the pollutant and generating hydrogen gas
at the cathode; and
exhausting the generated hydrogen gas from the cathode;
wherein at least one of a stack component in the first solid polymer electrolyte electrolytic cell
stack and an operating condition of the first solid polymer electrolyte electrolytic cell stack is
different from the stack component in the second solid polymer electrolyte electrolytic cell stack
and the operating condition of the second solid polymer electrolyte electrolytic cell stack.
2. The method of claim 1 wherein the different stack component is selected from the group
consisting of an anode fluid delivery layer, the anode catalyst, the anode catalyst layer, an anode flow
field plate, an anode filter layer, the solid polymer electrolyte membrane, and the number of cells in the
stack.
3. The method of claim 2 wherein the different stack component is the anode catalyst layer and at
least one of the catalyst loading and catalyst active area in the anode catalyst layer of the first solid
polymer electrolyte electrolytic cell stack differs by more than about 5% from that of the catalyst loading
and catalyst active area in the anode catalyst layer of the second solid polymer electrolyte electrolytic cell
stack.
4. The method of claim 3 wherein at least one of the catalyst loading and catalyst active area in the
anode catalyst layer of the first solid polymer electrolyte electrolytic cell stack differs by more than about
10% from that of the catalyst loading and catalyst active area in the anode catalyst layer of the second
solid polymer electrolyte electrolytic cell stack.
5. The method of claim 1 wherein the different operating condition is selected from the group
consisting of the flow rate of the wastewater, the flow pressure of the wastewater, the voltage, the
operating temperature, and the current density.
6. The method of claim 5 wherein the different operating condition of the first solid polymer
electrolyte electrolytic cell stack differs by more than about 5% from that of the second solid polymer
electrolyte electrolytic cell stack.
7. The method of claim 6 wherein the different operating condition of the first solid polymer
electrolyte electrolytic cell stack differs by more than about 10% from that of the second solid polymer
electrolyte electrolytic cell stack.
8. The method of claim 1 comprising operating each of the cells in the electrolytic cell stacks at a
current density less than about 10 mA/cm2
9. The method of claim 1 comprising supplying the flow of wastewater to the anode of each of the
cells in the electrolytic cell stacks without an added supporting electrolyte.
10. The method of claim 1 wherein the cathode of each of the cells in the electrolytic cell stacks
comprises no liquid catholyte nor liquid supporting electrolyte.
11. The method of claim 1 wherein the first and second solid polymer electrolyte electrolytic cell
stacks each comprise a single electrolytic cell.
12. The method of claim 1 wherein the first and second solid polymer electrolyte electrolytic cell
stacks comprise more than one electrolytic cell.
13. The method of claim 1 wherein the first solid polymer electrolyte electrolytic cell stack is
connected upstream and in series flow with the second solid polymer electrolyte electrolytic cell and the
anode outlet from the first solid polymer electrolyte electrolytic cell stack is connected to the anode inlet
of the second solid polymer electrolyte electrolytic cell stack second cell stack.
14. The method of claim 13 wherein the first and second solid polymer electrolyte electrolytic cell
stacks comprise common end plates.
15. The method of claim 1 wherein the first and second solid polymer electrolyte electrolytic cell
stacks are connected in parallel flow with the second solid polymer electrolyte electrolytic cell and
wherein the supplied wastewater is divided between the anode inlets of the first and second solid polymer
electrolyte electrolytic cell stacks.
16. The method of claim 1 comprising incorporating a treatment unit in the flow of wastewater.
17. The method of claim 16 wherein the treatment unit is selected from the group consisting of a
filter, a degas unit, and a pH controller.
18. The method of claim 16 comprising incorporating the treatment unit upstream or downstream of
either the first or the second solid polymer electrolyte electrolytic cell stack.
19. A system for the treatment of wastewater comprising at least a first and second solid polymer
electrolyte electrolytic cell stack, the cells in the first and second stacks each comprising an anode
comprising an anode catalyst layer and the anode catalyst layer comprising an anode catalyst, a cathode
comprising a cathode catalyst layer and the cathode catalyst layer comprising a cathode catalyst wherein
the cathode is liquid-electrolyte free, and a solid polymer membrane electrolyte separating the anode and
the cathode; wherein the system and the first and second solid polymer electrolyte electrolytic cell stacks
are configured to operate according to the method of claim 1 and wherein a stack component in the first
solid polymer electrolyte electrolytic cell stack is different from the stack component in the second solid
polymer electrolyte electrolytic cell stack.
20. A system for the treatment of wastewater comprising at least a first and second solid polymer
electrolyte electrolytic cell stack, the cells in the first and second stacks each comprising an anode
comprising an anode catalyst layer and the anode catalyst layer comprising an anode catalyst, a cathode
comprising a cathode catalyst layer and the cathode catalyst layer comprising a cathode catalyst wherein
the cathode is liquid-electrolyte free, and a solid polymer membrane electrolyte separating the anode and
the cathode; wherein the system and the first and second solid polymer electrolyte electrolytic cell stacks
are configured to operate according to the method of claim 1 and wherein a configured operating
condition of the first solid polymer electrolyte electrolytic cell stack is different from the configured
operating condition of the second solid polymer electrolyte electrolytic cell stack.