Digestion of organic sludge
[0001] The invention pertains to a process of treating organic substances such as waste
water treatment sludge, manure etc. by anaerobic digestion.
Background
[0002] Organic waste streams or sludge, such as waste water treatment sludge, biomass
waste streams, waste streams from food industry and the like, are commonly treated by
anaerobic digestion so as to reduce the organic matter content and produce biogas. As
used herein "sludge" is understood to mean any slurry or suspension of organic material,
generally regarded as waste, in an aqueous medium, having a dry solids content between
2 and 12 wt.%, preferably between 3 and 9 wt.%. Such rather thick sludges are difficult
to digest in conventional reactors for anaerobic digestion. The methane (in the biogas)
can be used as a fuel for the energy input in the (digestion) processes at the wastewater
treatment plant (WWTP), or sold externally.
[0003] The anaerobic digestion process has four main enzyme-catalysed stages: (1)
hydrolysis by hydrolytic enzymes excreted by the microorganisms, resulting in
breakdown of polymeric and glyceridic structures, in particular of proteins and
polysaccharides, and fats, respectively, to small peptides, and amino acids, monomeric
sugars such as glucose, and fatty acids; (2) acidogenesis, resulting in conversion of the
products of hydrolysis to smaller fatty acids, lactic acid, alcohols (ethanol, methanol)
and other ones; (3) acetogenesis, resulting in conversion of the smaller fatty acids and
alcohols to carbon dioxide, hydrogen and acetic acid; and (4) methanogenesis, resulting
in conversion of hydrogen, propionic acid and acetic acid to methane and carbon dioxide
The latter three stages take place inside the cells of the corresponding bacteria. The first
stage, hydrolysis, is usually the rate-limiting step of the breakdown process.
[0004] Anaerobic digestion is conventionally carried out in complete stirred tank
reactors (CSTR). The shape and arrangement of the reactors, as well as the further
process features, such as flow management, mixing, residence times, can vary widely.
The process can be mesophilic, i.e. at temperatures between 25 and 45°C, typically
around 36°C, or thermophilic, at temperatures between 45 and 60°C or even higher and
typically around 55°C.
[0005] A drawback of performing anaerobic digestion in CSTR is that a part of the
biomass which is to be digested is not involved in the entire process and leaves the
reactor untreated. Also, the four stages of anaerobic digestion, from hydrolysis to
methanogenesis, are performed simultaneously, resulting in sub-optimum process
conditions for most if not all of the process stages. In addition, sludge retention times
(SRT) and hydraulic retention times (HRT) are necessarily the same in CSTR, and as a
result, optimum treatment of all sludge is usually not achievable.
[0006] Attempts to overcome the drawbacks of CSTR treatment for anaerobic digestion
mainly focused on plug-flow treatment, or combinations of plug-flow and CSTR
treatment. While such arrangements can result in higher efficiencies or conversion rates,
they require relatively expensive equipment cost, because of the required high
height/depth (H/D) ratios.
[0007] WO 03/006387 (= US 2004/0172878) discloses an installation for anaerobic
conversion of 'dry' biomass into biogas wherein hydrolysis and methane fermentation
are separated and fermentation is performed in two fermentation tanks, the first one
operated at mesophilic and the second one at thermophilic temperatures. The recycle
stream of this document is related to leak water from the conveyors and press water from
the expeller; no sludge is recycled. WO 201 1/1 12736 discloses an installation for
producing biogas from biomass in anaerobic CSTR reactor(s). The oxidation-reduction
potential (ORP), pH, temperature and gas production in the reactor are monitored and
used for controlling the amount of biomass fed to the reactor, and the biomass retention
time within the reactor is increased by a biomass recycle after separation of the liquid
phase. No means, other than storage, for accommodating fluctuations in the incoming
stream are suggested. Both documents do not provide for optimization of the different
microbiological steps in the breakdown of (complex) organic substrates to mainly
methane and carbon dioxide. EP2345090 describes a combination of pH and redox
meters for controlling aerobic treatment of wastewater to optimize the removal of
nitrogen and does not aim at controlling different stages in methane formation.
Summary of the invention
[0008] It was found that the overall efficiency of anaerobic digestion of sludge as
defined above can be significantly improved by combining the advantages of a CSTR
treatment and a plug-flow process, by carrying out the anaerobic digestion in a series of
at least three reactors, allowing the partial processes of the anaerobic digestion
(hydrolysis, acidogenesis, acetogenesis and methanogenesis) to be performed under
more optimum conditions. The conditions are further optimised by combining the
improved reactor arrangement with a control of the flows between the reactors by means
of reaction (pH, redox) and flow (sludge load, gas production) data input.
Description of the invention
[0009] The invention thus pertains to a process and an installation for treating sludge by
anaerobic digestion. In the process of the invention, the sludge is treated serially in at
least three anaerobic reactors comprising a first, a second and a last reactor, wherein:
a major part of the influent sludge stream is fed to said first reactor,
a major part of the liquid effluent of said first reactor is fed to said second reactor,
a major part of the liquid effluent of said second reactor is fed to said last reactor,
directly or indirectly, i.e. optionally through one or more further reactors which may be
located between the second and last reactors, and
a major part of the liquid effluent of said last reactor is disposed and/or optionally
further treated, and
wherein a controlled variable part of the effluent of said last reactor is fed to said first
reactor, and
a controlled variable part of the influent sludge stream is fed to said second reactor;
the level of said variable parts being controlled by means of the pH and/or redox values
in all reactors.
[0010] The process and the installation of the invention preferably comprise a third
reactor between said second reactor and said last reactor, wherein a major part of the
liquid effluent of said second reactor is fed to said last reactor via the third reactor, i.e. a
major part of the liquid effluent - in particular the entire liquid effluent - of the second
reactor is fed to the third reactor, and a major part of the liquid effluent of the third
reactor is fed to the last reactor. If desired, a fifth and even a sixth reactor can be
provided between the third and last reactors; however, such further reactors only
marginally further improve the efficiency and/or the anaerobic conversion rate of the
array of reactors.
[0011] While all stages of the anaerobic digestion can and will take place in each of the
reactors, hydrolysis (stage 1) predominantly takes place in the first reactor and, to some
extent, also in the second and even in the third reactor. Acidogenesis and acetogenesis
take place in all reactors, and depend strongly on the production of intermediates formed
by the hydrolysis process. Acidogenesis and acetogenesis are also strongly inter
dependent. Methanogenesis occurs in all reactors because acetic acid is formed in all
stages, and it is the major process of the last reactor.
[0012] The organic material or sludge, as defined in the introductory part above, which
is treated in the process of the invention, may e.g. be sludge issuing from an (aerobic or
anaerobic) waste water treatment plant (WWTP), manure, biomass waste streams, waste
streams from food industry and slaughter houses, and the like. The sludge may comprise
various polysaccharides such as cellulose, hemicelluloses, starches, gums another fibres,
as well as lower sugars, proteins, and fats, as well as lower levels of other biological
materials such as alcohols, organic acids, hydrocarbons, lignins, etc., all of which are
digested to at least some extent, depending of the degree of ready biodegradability.
[0013] Herein, the "liquid effluent" of a reactor is understood to mean the liquid and any
solid or semi-solid material suspended or floating in the liquid. Thus the liquid includes
most of the (organic/biological, i.e. comprising the bacteria and enzymes of the
anaerobic flora) sludge present in the liquid. The transfer of such suspended or (sem
solid material from one reactor to another is important in the present invention. Such
suspended or solid material (sludge) is distinguished from the solid, non-digestible
material which will settle in the reactor despite some mixing.
[0014] Usually, the solid material introduced in the anaerobic digestion system, also of
the invention, will comprise such non-digestible, largely inorganic material, such as
sand, clay-like material and precipitates, which will not be converted in the reactor
system. This part may vary from about 10% to about 40% of the total dry solids of the
influent. The heavier part of the inorganic material can be discharged from any of the
reactors, preferably at least from the first reactor, by appropriate settling using the
generally higher density of this inorganic material. The non-settling part of the inorganic
material and the newly formed precipitates will leave the last reactor together with the
effluent, and can be separated off downstream if desired.
[0015] In the process and the installation of the invention, solid material which is formed
during the anaerobic digestion, in particular struvite (ammonium magnesium phosphate),
can also be collected at the bottom of one or more of the reactors, preferably the secondlast
(e.g. the third of four) reactor. If necessary, magnesium, at levels corresponding to
the stoichiometry of the phosphorus load of the treated sludge, can be introduced in the
reactor(s), for optimum struvite yield and hence for optimum phosphate recovery.
Magnesium can suitably be added as its salt, e.g. magnesium chloride, hydroxide, oxide,
acetate or sulphate. By lowering the phosphate concentration during the digestion
process by forming struvite in the digester itself, unplanned struvite formation after the
digesters can be prevented.
[0016] The reactors are advantageously provided with controllable mixing devices,
allowing the mixing of the sludge but also the ratio of SRT to HRT to be controlled.
Preferably, at least the first and second reactors are provided with such mixing devices.
[0017] In an advantageous embodiment of the process and the installation of the
invention, the at least three reactors are vertically separated compartments of a single
tank. The single tank can effectively be a cylindrical container tank with three or four or
even more radially separated compartments constituting the various reactors, but other
shapes or even separated reactors for the different compartments are perfectly possible.
The height of the reactors and of the tank is not critical for the operation of the anaerobic
digestion, although a minimum height of 4 m is generally preferred. Typically heights
are between 5 and 15 m for practical reasons, preferably between 6 and 11 m. The
reduced height without any reduction of performance, compared to existing plug-flow
digesters of the prior art, constitutes an important advantage of the invention. The tank
diameter - for a cylindrical radially divided tank - may advantageously be between 10
and 25 m, preferably between 15 and 22 m, allowing an effective total reactor content of
between 750 and 4000 m , preferably between 1500 and 3000 m . However, smaller and
larger equipments will essentially be equally effective.
[0018] The "major part" (or "a" "major part", which is the same) of an effluent which is
fed from one reactor to another reactor, as defined above, is preferably at least 85% (by
volume of the total effluent from the relevant reactor per time unit), preferably at least
90%, most preferably at least 96%. The "major part" of effluent which is disposed from
the last reactor (and/or optionally further treated) is preferably at least 50% (by volume
of its total effluent, including any recycle), more preferably at least 70%, in particular at
least 85% , even more in particular at least 90%. Under undisturbed steady operation
with constant influent characteristics, the major parts may be 100% for some or all of the
effluents, i.e. no recycling. The controlled variable ("minor") part which is fed from a
downstream reactor to a more upstream reactor is therefore preferably less than 15 % (by
volume), preferably less than 10%. The controlled variable parts which are recycled, are
preferably mixed, i.e. contain essentially the same levels of sludge as the reactor content
from which the part is discharged; if necessary, the reactor content is mixed before or
while the variable part is being fed to an upstream reactor; on the other hand, recycling
of part of the effluent from one reactor to a more upstream reactor can be done without
upfront mixing of the reactor content, depending on the feed material, as long as at least
a part of the semi-solid (suspended) material including the anaerobic bacteria, is carried
along with the recycled effluent. Where reference is made herein to volumes per time
unit or per day, this is to be understood as average volumes per time unit over a longer
period of e.g. one or several hours or days, thus averaging possible short term
differences resulting from recycle events being less frequent than loading events.
[0019] The controlled variable part of the effluent of said last reactor being fed to said
first reactor is preferably somewhat higher than other recycle streams, in particular, it is
at least 2 % in volume of said major part of the liquid effluent being disposed and/or
further treated, per time unit, e.g. up to 10%. More preferably, e.g. in order to allow
accommodation of more demanding and/or more fluctuating feed streams, the controlled
variable part which is fed from the last reactor to the first (and optionally partly to the
second) reactor is between 2 and 50% (by volume) of the total effluent issuing from the
last reactor, even more preferably between 5 and 30%, most preferably between 10 and
20 % of the total effluent from the last reactor. In terms of total reactor content, the
recycle rate is preferably between 0 and 15 vol.% of the total reactor content per day,
more preferably 0.5-12.5 vol.% per day, most preferably between 1.25 and 7.5 vol.% or
even between 2.5 and 5 vol.% per day.
[0020] Also a controlled variable part of the effluent of said last reactor can be fed to
said second reactor. That part will generally be smaller than the part recycled from the
last to the first reactor, unless acidity is too high, not only in the first reactor (pH too
low, e.g. below 6.0, especially below 5.0), but also in the second reactor (pH e.g. below
6.5, especially below 6.0). The part recycled from the last to the second reactor is
preferably between 0 and 50% of the total volume recycled from the last reactor (to the
first and second reactors together), more preferably between 0 and 25% thereof, most
preferably between 0 and 10% thereof, per time unit, e.g. per day. In terms of the total
effluent issuing from the last reactor, the part being fed to the second reactor is
preferably between 0 and 30% (by volume), more preferably between 0 and 10%, while
it may be between 5 and 30%, preferably between 10 and 25% in case of too high acidity
of the second reactor. In an alternative definition, the part fed from last to second reactor
is preferably between 0 and 25% in volume, more preferably between 0 and 10 %, in
volume of said major part of the liquid effluent being disposed and/or further treated,
more preferably between 0 and 4 %. Most preferably essentially 0 vol.% per time unit
(whether on the basis of total effluent, or total recycle, or non-recycled part, or on
reactor content) is returned to the second reactor, if all the influent is being fed to the
first reactor and if the pH in the second reactor is not decreasing. But if the pH of the
second reactor decreases, the effluent of said last reactor can be fed to said second
reactor to a maximum of 25 % in volume of the total recycle.
[0021] Alternatively or additionally, a controlled variable part of the effluent of said
second last reactor (i.e. the reactor preceding the last reactor, which may be the second
reactor, or, preferably, the third reactor) can be fed to the first reactor. This controlled
part may for example be between 0 and 25 vol.%, preferably between 0 and 10 vol.%,
preferably between 0 and 4 vol.% of the effluent of the second-last reactor.
[0022] As an exception, the "major part" of the influent sludge stream which is fed to
the first reactor may be as low as 50%. The controlled variable part of the influent
sludge stream being fed to said second reactor varies largely with the volume properties
of the influent sludge stream. For example, if the influent sludge has already undergone
hydrolysis to some extent or has a high amount of easily degradable material (peak
load), the controlled part being directly fed ("by-passed") to the second reactor can be
relatively high, as high as 50%. Preferably, under more steady conditions, the major part
being fed to the first reactor and the controlled variable part being fed to the second
reactor are more than 75% and less than 25%, respectively. Preferably, the controlled
variable part ("minor part") of the influent being fed to the second reactor is as low as
possible, depending on the amount that can be put through the first reactor, which will
be controlled by pH and/or redox measurement.
[0023] An important feature of the process and the installation of the invention is the
flow control from one reactor to another, including the bypasses and recycles, which can
be achieved by measuring the pH and/or redox potential (oxidation-reduction potential,
ORP) in at least the first two reactors, but preferably in all reactors. Also, influent and
effluent flow rates, in particular the flow rate of the incoming sludge (by volume) and
exiting (bio)gas (by volume), can be used for controlling the flow of the various recycles
and bypasses. Thus, the level of said variable parts being fed from one reactor to another
one, in particular from the last reactor to the first and/or second reactor, as well as from
the influent to the second reactor, thereby bypassing the first reactor, are controlled by
means of the pH and/or redox values measured in the first and second reactor in
combination with the measured influent load to be treated and, preferably also on the
basis of the measured gas production of the combined reactors, or of the individual
reactors. The input data of pH, redox and flow rates is introduced into a data processor,
which, once adjusted to the characteristics of the reactor array and the types and amounts
of sludge to be treated, will translate the input to increases or decreases of one or more
of the bypass or recycle flow rates by opening (widening) or closing (narrowing) the
respective valves or by controlling the different pump flows.
[0024] As to the pH and the redox potential, it is commonly known that the Eh and pH of
a solution are related. For a half cell equation, conventionally written as reduction
(electrons on the left side):
iA =bB +n[e]=h[lt] =cC +dD
[0025] The half cell standard potential E0 is given by:
¾ volts) = —— —t
wherein G the standard Gibbs free energy change, is the number of electrons
involved, and F is Faraday's constant. The Nernst equation relates pH and Eh.
Eh =Eo - 0.05916/« * log ( {A}a{B}b)l ({C} {D}d) ) - 0.05916/ /« *pH
where curly brackets indicate activities and exponents are shown in the conventional
manner. This equation is the equation of a straight line for Eh as a function of pH with a
slope of -0.05916/z/h volt (pH has no units.) This equation predicts lower Eh at higher pH
values. This is observed for reduction of 0 2 to OFT and for reduction of H+ to H2.
[0026] A normal range for pH in anaerobic (CSTR) reactors is in the range of 6.0 - 8.0,
where a pH of 6.0 is low, increasing the risk of acidification of the reactor. To prevent
loss of activity by too low pH values, the recycle flow is controlled to keep the
installation in optimum condition. A problem of only using the pH for monitoring
performance and controlling flows is that pH values are influenced by the carbonate
equilibrium in one direction (alkalinity of the reaction mixture) and in the opposite
direction by the fatty acid concentrations, which makes the pH measurement alone not a
very reliable value to control the digestion process. The redox measurement gives an
indication of the total amount of oxidising and reducing ions in the medium and is less
dependent on flattening effects of a few components. Depending of the influent flow and
the speed of degradation, a lower pH (lower than 6.5 or even lower than 6.0) can be
maintained for some time in the first and/or the second reactor.
[0027] The pH control of the recycle and bypass flows can for example be performed as
follows: if the pH in the first reactor does not sufficiently drop compared to the influent
pH, i.e. if it is around 7.3 or above and the recycle from the last reactor is low, this may
indicate that the digestion capacity of the first reactor is insufficient, because of too
complex influent components which need more hydrolysis time. In such case, the part of
the influent being fed to the second reactor ("bypass") can be increased. When the pH
decreases well below around 7.0, in particular below 6.8, the bypass flow can be
decreased. If, on the other hand, the pH drop in the first reactor is too large, e.g. to 6.5 or
lower, the recycle flow from the last reactor (and optionally to a minor extent from the
second-last, e.g. third, reactor) can be increased, e.g. to around 25 %, or to 43% or even
up to 100 % (by volume influent per day), which values correspond to up to 20%, or up
to 30%, or even up to 50%, respectively, of the total effluent volume of the last reactor.
Levels of 3-6%> or even 6-10 % > (by volume of the total effluent of the last reactor) are
also feasible. When the pH rises above 6.8, the flow of the recycle part from the last
reactor may be decreased.
[0028] Typically, the pH of the first reactor will be kept between 4.5 and 7.5, preferably
between 5.0 and 7.3, in particular between 6.0 and 7.3. The pH of the second reactor will
typically be between 6.0 and 8.0, preferably between 6.5 and 7.5. The pH of the third
reactor (if there is one) and of any further reactors before the last one will typically be
between 6.8 and 8.0, while the pH of the last reactor will preferably be between 7.0 and
8.0, ideally between 7.0 and 7.5. However, deviations outside these typical ranges can
still be acceptable, at least temporarily, especially in the first and second reactors.
[0029] If the gas production increases above the maximum amount that can be treated
per time unit, the flow of the incoming sludge can be reduced or the total incoming flow
can be fed to only the first reactor instead of to the first two reactors. The degradability
and thus the gas production will drop if the load of the first reactor increases above the
optimum load. If the incoming flow is too high to be treated in only the first reactor, part
of the flow can be put into the second reactor. Reduction of the flow of incoming sludge
can be effected by diverting a part of the incoming sludge to a buffer tank and returning
it e.g. when the gas production has decreased below the maximum level. In this way, the
gas production data are used to adapt the various flows, bypasses and recycles. Also the
gas composition can be used for this purpose. Thus a relatively high methane content of
the total gas production, or especially of the gas issuing from the first and second
reactor, is indicative of a relatively swift degradation process, allowing the recycle from
the last to the first reactor(s) to be decreased, and/or the bypass of the first reactor to be
adapted. Vice versa, a relatively low methane content compared to e.g. carbon dioxide
content, of the of the total gas production, or especially of the gas issuing from the last
(and possibly second-last) reactor, is indicative of a non-optimum degradation process,
which can be improved by increasing the recycle from the last to the first reactor(s).
[0030] The redox control of the recycle and bypass flows can be performed similarly.
For example, if the redox potential in the first reactor is relatively high (= less negative,
e.g. > -330 mV, in particular > -300 mV), this is an indication of insufficient anaerobic
capacity of the first reactor, and the recycle proportion of the effluent of the last reactor
being fed to the first reactor is increased, up to around 20% or even up to 50% (based on
total effluent of last reactor). If, on the other hand, the redox potential becomes too low,
e.g. well below -420 mV, in particular < -450 mV, the recycle from the last reactor may
be decreased and/or the bypass rate to the second reactor may be increased. The precise
levels of the redox potential which should be used as an indicator and thus a controlling
criterion for adjusting the various flows, will depend on the types of sludge, and can be
routinely determined by the skilled person. As indicated above, an advantage of using
the redox potential is that it is more directly indicative of the anaerobic digestion
process, while the pH effect (acidification) may be offset by a higher alkalinity of the
influent mixture. Therefore, both the pH and the redox potential are advantageously used
for controlling the recycle and bypass flows.
[0031] Thus, in a preferred embodiment of the process of the invention, the controlled
variable part of the effluent of the last reactor which is fed to the first reactor can be
increased when the pH in the first reactor is 6.0 or below and/or the redox value in the
first reactor is above -330 mV, especially when it is above -300 mV. On the other hand
the controlled variable part of the effluent of said last reactor being fed to the first
reactor can be decreased when the pH in the first reactor is above 6.8 and/or the redox
value in the first reactor is below -420 mV especially when it is below -450 mV.
[0032] Similarly, in a preferred embodiment, the controlled variable part of the influent
sludge stream which is fed to the second reactor (i.e. bypassing the first reactor) can be
increased when the pH in the first reactor is 6.0 or below and the recycle flow to the first
reactor is between 60 and 100%, especially between 80 and 100 % of the throughput (i.e.
of the same volume as the effluent which is disposed or further treated) and the pH in the
second reactor is 6.8 or above and/or the redox value in the first reactor is below -330
mV or below -300 mV. The controlled variable part of the influent sludge stream which
is fed to the second reactor is decreased when the pH in the second reactor is below 6.0
and/or the redox value in the second reactor is above -300 mV or above -330 mV.
[0033] It is possible, but generally unnecessary or even undesired, to provide pH control
in the first reactor by addition of caustic or acid. It is strongly preferred, however, to
control the pH in the various reactors, especially the first and second, by adjusting the
bypass and/or recycle flows as described above.
[0034] The influent flows and effluent flows, and - where applicable - bypass and/or
recycle flows are adjusted in such a way that the total hydraulic retention time (HRT) in
the series of reactors (three, preferably four, or more) is between 3 and 10 days,
preferably between 4 and 8 days. The HRT in each reactor (or compartment) is between
1 and 3 days, preferably between 1.25 days (30 h) and 2 days. Therefore, an increase of
the recycle, e.g. from the last to the first reactor, is effected by discharging an additional
flow from the last reactor for recycle, and keeping the flow rate of the non-recycled part
of the effluent (to be discharged or further treated downstream) essentially constant.
[0035] The process of the invention can be performed in a continuous, semi-continuous
or batch-wise mode. Preferably the process is operated in a semi-continuous mode,
comprising a loading stage and a reaction stage. In the loading stage, a pre-determined
volume of influent is loaded to the first reactor, and optionally also to the second reactor,
and as a result, a similar volume is transferred from one reactor to the next reactor and a
similar volume is discharged from the last reactor. In a first part of the reaction stage, the
content of the reactors may be mixed, especially in the first and second reactor. In a
second part of the reaction stage, mixing can be interrupted and solids are allowed to
settle; however, at least part of the bacterial sludge will remain in suspension despite the
interruption of the mixing, and will be transferred during the subsequent loading stage.
The length of the various stages may vary; for instance, the loading stage may take
between one minute or less up to e.g. 10 minutes, the mixing stage may be as short as
e.g. 5 minutes or as long as e.g. an hour, and the settling stage may range from e.g. 10
minutes to one hour. During the settling stage, heavy solids - as distinguished from
lighter (bacterial) sludge - will be collected at the bottom part of the reactor, and can be
continuously or periodically discharged, e.g. before or during the loading stage. This
applies particularly to struvite (having a specific density in the order of 1.5 to 1.7 g/cm ),
which is preferably precipitated - with the aid of added magnesium salts - in the last, or
more preferably the second last (third) reactor. The recycling operations may be
continuous, or they may coincide with the loading stages or they may be more frequent
or less frequent than the loading stages.
[0036] The effluent of the last reactor can be treated in various ways, either
anaerobically in a stirred tank reactor (CSTR) for polishing or directly to the final
treatment of the sludge, like the dewatering installation. It is also feasible to feed the
effluent of the last reactor to the first reactor of a second set of at least three anaerobic
digesting reactors as described herein, i.e. using two sets of reactors of the invention in
series. However, the use of a CSTR type reactor for polishing and accommodating
surplus levels of influent sludge is preferred. In many cases it is possible to upgrade
existing digestion installations by changing one of the digester tanks to a configuration
of the new digester concept, especially in the embodiment where the four (or three or
five) reactors are compartments of a single tank.
[0037] The installation for carrying out the anaerobic digestion of organic sludge of the
invention can comprise an array of at least a first reactor, a second reactor and a last
reactor. The first reactor is provided with a sludge inlet, a controllable mixer, a pH meter
and/or a redox meter, a gas outlet, a liquid (overflow) outlet to said second reactor, a
liquid overflow inlet from said last reactor and a solid outlet at the bottom. The second
reactor is provided with a sludge inlet, a liquid overflow inlet from said first reactor, a
controllable mixer, a pH meter and/or a redox, a gas outlet, a liquid overflow outlet to a
third reactor or to said last reactor and preferably a solid outlet at the bottom. The last
reactor is provided with a liquid (overflow) inlet from said second reactor or from a third
or further intermediate reactor, a controllable mixer, a gas outlet, a liquid overflow outlet
to said first reactor, a liquid overflow outlet for disposing liquid and a solid outlet at the
bottom. Regulating devices are provided for regulating the flow of at least one of said
liquid overflow outlets using pH and/or redox information input from said pH and/or
redox meters, and preferably also input of incoming sludge flow data and discharge
(bio)gas flow data. The gas collecting system of different compartments can be
combined, and be provided with a gas flow measuring device.
[0038] All reactors can be separate reactors or compartments in one or more tanks. For
example, the array of reactors is advantageously a cylindrical container and said first
reactor, second reactor, optional third reactor further intermediate reactors, and a last
reactor are vertically separated compartments of said cylindrical container. The
connecting lines (outlet from first reactor to inlet to second reactor; outlet from second
reactor to inlet to next (third) reactor; etc.) can be vertical subsectors or pipes
neighbouring the respective two reactors, wherein the outlet from the preceding reactor
is an overflow at the top and the inlet into the next reactor is at the bottom, as further
shown in Figure 2 .
[0039] The installation as defined preferably further comprises, in said array, a third
reactor provided with a liquid overflow inlet from said second first reactor, a
controllable mixer, a gas outlet, a liquid overflow outlet to said last reactor, and a solid
outlet at the bottom. The solid outlets at the bottom of the various reactors, especially the
first and the last reactor, can simply be openings at the lowest point of the bottom of the
reactor, which may be flat or, preferably inclined, and may be provided with devices to
move the solids to the outlet opening, such as pumps or jet streams, if desired.
[0040] The installation can also advantageously be provided with a buffer tank which
can be connected to the sludge supply line by controllable valves for diverting part of the
incoming sludge to be stored in the buffer tank and, later on, returning the stored sludge
to the supply line for feeding to the array of reactors.
Description of thefigures
[0041] Figure 1 shows a set of reactors according to the invention. The figure shows an
installation having four anaerobic reactors, 1, 2, 3, 4 . Each reactor is equipped with a
mixing unit ( 11, 21, 31, 41), a liquid/sludge inlet (12, 22, 32, 42), a liquid/sludge outlet
(13, 23, 33, 43), a solid outlet at the bottom (14, 24, 34, 44), which may be provided
with means for directing solids towards the outlet, such as a jet stream (not shown), a gas
outlet (15, 25, 35, 45), connected to gas lines (16, 26, 36, 46), a pH and/or redox meter
(17, 27, only shown for reactors 1 and 2), and liquid/sludge lines (10, 19-20, 29-30, 39-
40, 49) feeding and discharging the major parts of the sludge to and from the reactors.
The liquid/sludge outlets 13, 23, 33, 43 are arranged as overflow pipes in the Figure,
which may be located inside or outside the reactor.
[0042] Incoming sludge 7 is fed to the first reactor through line 7 1 after passing flow
meter 75. Bypass lines 72, 73, and 74 may be provided to pass a minor part of the
incoming sludge to the second, and optionally the third and fourth reactor, the various
flows being controlled by valves 99. Outlet lines 29, 39 and 49 are provided with side
lines 91, 92, 95, for recycling a minor part of the effluent of the second, third and fourth
reactor, respectively, to the first and optionally second reactor, wherein lines 92 and 95
may be divided into lines 93, 94 and 96, 97, respectively.
[0043] Solids from the first, and optionally the second reactor (and exceptionally the
third, not shown), discharged from outlets 14 and 24 respectively, largely consist of sand
and other inorganic solids present in the starting sludge, and are carried off through lines
8 1 and 82, to a inorganic solids disposal 85 or to the digested sludge buffer. Solids from
the fourth and/or third (and exceptionally the second reactor, not shown) discharged
from outlets 34 and 44 respectively, largely consist of struvite formed in the respective
reactor(s), typically the third reactor, and are carried off through lines 83 and 84, to a
struvite disposal 86. A magnesium salt (e.g. magnesium chloride) supply 87 is fed to the
third reactor through line 88. Instead of being combined with feed line 40 of reactor 3, it
may enter reactor 3 elsewhere, or it may be fed to reactor 4, e.g. combined with feed line
40.
[0044] Gas issuing from the various reactors is carried off through lines 16, 26, 36 and
46, and after being combined, passes through a flow meter (64), which may also contain
a unit for assaying the gas composition, and is fed to a biogas collecting unit 67. Both
the gas flow rates and the gas compositions can also be measured for the gas streams
from the individual reactors, i.e. by flow meters and gas composition assaying units in
the individual lines 16, 26, 36 and 46 (not shown).
[0045] Data from the pH and redox meters 17, 27, the incoming sludge flow indicator
75, the gas effluent flow (and optionally composition) indicators 64 (which may also be
indicators in the individual lines 16, 26, 36 and 46) and optionally 66 and optionally the
liquid effluent flow indicator 62, are fed through connections 18, 28, 76, 63, 65,
respectively (which may be wired or wireless) to data processing and control unit 6 .
Controlling signals for the various valves 99 and mixing devices 11, 21, 31, 4 1 are
transmitted from the processing and control unit 66 through exits 6 1 (lines to the valves
not shown).
[0046] Downstream of the set of anaerobic reactors, a finishing reactor, such as a CSTR
reactor 5 may be provided. It has feed line 50, mixing device 51, liquid inlet 52, liquid
outlet 53, gas outlet 55 connected to the biogas collecting unit 67 through line 56 and
indicator 66. The treated liquid is discharged through line 59, and either post-treated or
discharged.
[0047] Figure 2 shows a cross-sectional view of an alternative arrangement of the
installation of the invention, wherein reactors 1, 2, 3 and 4 are compartments of a
cylindrical tank 9 . The same numbers as in Figure 1 represent the same parts. Thus, each
compartment is equipped with a mixing device 11, 21, 31, 41, and further with similar
inlets, outlets, lines, valves, gauges and devices as in Figure 1 (not separately shown in
Figure 2). Incoming sludge through supply line 10 is introduced into column 12, which
serves as a sludge inlet for reactor 1 though an overflow, or through an external line
including a pump. Overflow outlets 13, 23 and 23 provide the connection between the
reactors, thus avoiding separate (external lines). Reactor 4 is provided with a sludge
outlet in the form of shaft 43, from which the sludge is carried off to a sludge buffer
preceding a dewatering installation/unit through line 49.
[0048] Figure 3 shows the comparative results in terms of breakdown of sludge (%
Organic Dry Substance: ODS) during 9 weeks of operation of the process of the
invention (■ ), a reference process using a CSTR only (¨), and a digester with a Thermal
Pressure Hydrolysis unit (A). The curves are drawn using moving averages over 4 days.
Example
[0049] Since 201 1 the applicant has been testing the new digester concept of the
invention with many different sludges in comparison of a reference reactor without any
additional treatment. The comparative tests were conducted under fully controllable
circumstances. Each test consists of a period of stabilization in which 20 litre CSTR Lab
reactors are compared with the full scale installation on the WWTP. To achieve an HRT
of e.g. 20 days in the CSTR Lab reactors, 1 liter of feed sludge originating from a
digester of a WWTP is daily pumped into the reactors in a semi-continuously way. If the
CSTR Lab reactor results are comparable with the full scale CSTR results, the new
digester concept is placed in front of one of the two Lab CSTR reactors. This new
digester concept with a HRT of 6 days is filled with digested sludge from the previous
stabilization period. The reference reactor is kept under the same conditions during the
whole test period. After placing the new digester concept of 8.5 1in front of the CSTR,
the feed flow is increased to 1.43 1/d to keep the overall HRT at 20 days.
[0050] The results are presented in Figure 3 . During this test period also the effect of
Thermal Pressure Hydrolysis (TPH) was tested in a third parallel reactor, where sludge
was pre-treated at 160 °C at 6-8 bar during 2 hours. The TPH technology for reductive
treatment of organic waste is described e.g. WO96/09882 (Cambi). As can be seen in the
graph, the new digester concept results are close to that of the TPH treatment. The
advantage of the new digester concept in comparison with TPH is that no high pressures
or high temperatures are necessary which is highly desirable for a positive energy
balance.
[0051] During the test period the average breakdown in the reference reactor was 42 %
and in the combination of the new digester concept and a CSTR 50%. In the TPH
reactor, the organic matter reduction amounted to 53 %. If these results are translated to
a full scale installation, only the new digester concept gives a positive contribution to
significantly lower the operational costs of the WWTP, since no additional energy is
needed like for TPH.

Amended claims [clean copy]
1. A process for treating organic sludge by anaerobic digestion, comprising serially
treating the sludge in at least three anaerobic reactors comprising a first, a second
and a last reactor, wherein at least 50 vol% of the influent sludge stream is fed to
5 said first reactor,
at least 85 vol% of the liquid effluent of said first reactor is fed to said second
reactor,
at least 85 vol% of the liquid effluent of said second reactor is directly or indirectly
fed to said last reactor, and
10 at least 85 vol% of the liquid effluent of said last reactor is disposed and/or
optionally further treated, and
wherein a controlled variable part of the effluent of said last reactor is fed to said
first reactor, and/or
a controlled variable part of the influent sludge stream is fed to said second reactor;
15 the level (volume per time unit) of said variable parts being controlled by means of
the pH and/or redox values in at least said first reactor,
wherein the controlled variable part of the effluent which is fed to said first reactor
and/or wherein the controlled variable part of the influent sludge stream which is
fed to said second reactor is increased when the pH in the first reactor is 6.0 or
20 below and/or the redox value in the first reactor is above 330 my, and decreased
when the pH in the first reactor is above 6,8 and/or the redox value in the first
reactor is below 420 mV.
2. The process according to claim 1, wherein said at least three reactors are four or
more reactors and comprise a third reactor between said second reactor and said fast
25 reactor, the major part of the liquid effluent of said second reactor being fed to said
third reactor, and a major part of the liquid effluent of said third reactor being
directly or indirectly fed to said last reactor.
3. The process according to claim 1 or 2, wherein said at least three reactors are
vertically separated compartments of a single tank.
30 • 4. The*process according to claim 3, wherein said single tank has a height of between
5 and 15 in, preferably between 6 and.11m.
AMENDED SHEET
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2
5. The process according to any one of the preceding claims, wherein the levels of said
variable parts are further controlled by means of the pH and/or redox values in said
second reactor.
6. The process according to any one of the preceding claims, wherein the level of said
5 variable parts are further controlled by means of the influent sludge stream load
and/or the flow rate of the gas collected from the combined or individual reactors.
7. The process according .to any one of the preceding claims, wherein at least one of
said reactors is provided with controllable mixing devices.
8. The process according to any one of the preceding claims, wherein struvite is
10 collected at the bottom of said last and/or said third reactor.
9. The process according to any one of the preceding claims, wherein said controlled
variable part of the effluent of said last reactor being fed to said first reactor is
between 2 and 10 % in volume of said major part of the liquid effluent being
disposed and/or further treated, per time unit.
15 10. The process according to any one of the preceding claims, wherein the effluent of
said last reactor is treated in a stirred tank reactor.
11. The process according to any one of the preceding claims, wherein the total
hydraulic retention time (HRT) in the series of reactors is between 3 and 10 days,
preferably between 4 and 8 days.
20 12. An installation for anaerobic digestion of organic sludge, comprising an array of at
least a first reactor, a second reactor and a last-reactor,
said first reactor being provided with inlet from the sludge supply, a controllable
mixer, a pH meter and/or a redox meter, a gas outlet, a liquid overflow outlet to said
second reactor, a liquid inlet from said last reactor and a solid outlet at the bottom;
25 said second reactor being provided with a sludge inlet,a liquid overflow inlet from
said first reactor, a controllable mixer, a pH meter and/or a redox meter, a gas
outlet, a liquid overflow outlet to a third reactor or to said last reactor and a solid
outlet at the bottom;
said last reactor being provided with a sludge inlet, a liquid overflow inlet from said
30 second reactor or from a third or further reactor, a controllable mixer, a gas outlet, a
AMENDED SHEET
1
PCT/NL 2014/050 621 — 03.12.2015
3
liquid outlet to said first reactor, a liquid overflow outlet for disposing liquid and a
solid outlet at the bottom; regulating devices being provided for regulating the flow
of at least said liquid inlet from said last reactor in said first reactor and said liquid
inlet from the sludge supply in said second reactor, us0i ng pH and/or redox
5 information input from said pH and/or redox meters.
13. The installation according to claim 12, further comprising in said array a third
reactor provided with a liquid overflow inlet from said second reactor, a
controllable mixer, a gas outlet, a liquid overflow outlet to said last reactor and a
solid outlet at the bottom.
10 14. The installation according to claim 12, wherein said array is a cylindrical tank and
said first reactor, second reactor, optional third reactor and a last reactor are
vertically separated compartments of said cylindrical tank.