The invention relates to treatment of industrial effluents. In particular, the invention
provides an apparatus and method for treatment of industrial effluents that contain a plurality of
soluble or insoluble contaminants including sodium sulphate such as may be discharged from a
textile dyeing process.
Background
[002] Industrial effluents contain a variety of soluble and I or insoluble impurities. The presence
of these impurities in waste water discharged from an industrial plant is understood to be toxic or
environmentally harmful. Industrial plants are therefore typically required to treat waste water and
effluents prior to release of such waste water or effluents to ensure that harmful impurities are
removed and safely disposed of. Given the high risk of ground water and river water contamination
arising from release of industrial waste water or effluents, regulatory requirements go a step further,
and mandate that certain types of industrial plants implement "zero-discharge" or "zero-liquid
discharge" effluent treatment systems - i.e. the treatment systems eliminate release of waste water or
liquid effluent entirely, while enabling safe disposal of solid impurities or industrial waste in landfills
or through other solid waste disposal arrangements.
[003] Industrial plants have a further incentive to implement effluent treatment systems - in that if
properly recovered (i.e. in sufficient quantity and I or to a sufficient level of purity), certain
contaminants within the effluent stream can be reused within the manufacturing process. In addition
to meeting regulatory requirements, implementation of efficient recovery systems can therefore also
contribute to improved resource utilization and profitability within the plant.
[004] An instance of the above considerations is particularly observed in industries that use
sodium sulphate as a part of the manufacturing process, such as textile industries. Sodium sulphate
has significant applications within the textile and dyeing industry - since it improves the affinity of
dyes towards fabric, and contributes towards migration, adsorption and fixation of dye to a textile
substrate. Commercially, the textile industry uses anhydrous or hydrated sodium sulphate (such as
for example, sodium sulphatedecahydrate (Na2S04. !OH20))- which is also known as Glauber's salt
or Glaber's salt. Glauber's salt has a high affinity for water and is added to dyeing vats during the
dyeing process. Once the dyeing process is completed, sodium sulphate comprises one of several
soluble or insoluble contaminants within an effluent stream resulting from the dyeing or textile
manufacturing process.
[005] Previously known solutions for removing or recovering Glauber's salt from industrial
effluent streams have not been commercially successful, owing to high levels of impurities found in
2
the recovered salt. Yet further, the treated effluent stream retains significant levels of sodium
sulphate, which is irretrievably lost when the treated effluent stream is discharged. Moreover, the
earlier known solutions for removing Glauber's salt have not been implemented within zero liquid
discharge systems.
[006] The present invention seeks to address the above drawbacks in the state of art, and to
provide an effluent treatment system that improves commercial and environmental sustainability.
Summary
[007] The invention comprises a method for reducing discharge of an industrial effluent
containing a plurality of soluble or insoluble contaminants including sodium sulphate. The method
comprises a first treatment stage and a second treatment stage. The first treatment stage may
comprise (i) cooling a received effluent to a temperature that enables precipitation of sodium
sulphate crystals (ii) separating a first yield of sodium sulphate crystals from a corresponding first
mother liquor and (iii) evaporating the first mother liquor to form a residue of contaminants. The
second treatment stage may comprise (i) forming an aqueous solution comprising at least a part of
the first yield of sodium sulphate crystals dissolved in water (ii) cooling the aqueous solution to a
temperature that enables precipitation of sodium sulphate crystals and (iii) separating a second yield
of sodium sulphate crystals from a corresponding second mother liquor. The invention may
additionally include a recirculation stage which comprises combining the second mother liquor with
an effluent that is received at the first treatment stage.
[008] In an embodiment of the method, the temperature that enables precipitation of sodium
sulphate crystals at either or both of the first treatment stage and the second treatment stage may be
between ooc and 32.4°C. In a more specific embodiment, the temperature that enables precipitation
of sodium sulphate crystals at either or both of the first treatment stage and the second treatment
stage may be between soc and l5°C. In a preferred embodiment, the temperature that enables
precipitation of sodium sulphate crystals at either or both of the first treatment stage and the second
treatment stage is l2°C.
[009] Separating either the fttst yield of sodium sulphate crystals from the corresponding first
mother liquor or the second yield of sodium sulphate crystals from the corresponding second
mother liquor may include centrifugation.
[0010] In certain method embodiments, the first mother liquor may be evaporated in a thin film
dryer or an agitated thin film dryer.
[0011] Sulphuric acid may be added to the aqueous solution prior to prec1p1tation of sodium
sulphate crystals. In an embodiment, the aqueous solution may be pH neutralized prior to
precipitation of sodium sulphate crystals.
3
[0012] In a method embodiment, the temperature of the second mother liquor may be between 0°C
and 32.4oC.
[0013] The second mother liquor may be combined with a received effluent at the first treattnent
stage after the received effluent is cooled to a temperature that enables precipitation of sodium
sulphate crystals.
[0014] In a particular method embodiment, the received effluent may comprise a concentrated
effluent stream received from an evaporator. The concentrated effluent stream may have been
subjected to one or more of filtering, homogenizing, cooling, pH neutralization, microbial action,
and reverse osmosis.
[0015] In an embodiment, the sodium sulphate crystals may include crystallized sodium
sulphatedecahydrate.
[0016] The invention additionally includes an apparatus for reducing discharge of an industrial
effluent containing a plurality of soluble or insoluble contaminants including sodium sulphate. The
apparatus comprises (i) a first stage cooling unit for cooling a received effluent to a temperature that
enables precipitation of sodium sulphate crystals (ii) a first stage separator for separating a first yield
of sodium sulphate crystals from a corresponding first mother liquor (iii) an evaporator for
evaporating the first mother liquor to form a residue of contaminants (iv) a reservoir for forming an
aqueous solution comprising at least a part of the first yield of sodium sulphate crystals dissolved in
water (v) a second stage cooling unit for cooling the aqueous solution to a temperature that enables
precipitation of sodium sulphate crystals (vi) a second stage separator for separating a second yield
of sodium sulphate crystals from a corresponding second mother liquor and (vii) a recirculator for
combining the second mother liquor with an effluent that is received at the first stage cooling unit.
[0017] In various apparatus embodiments, either or both of the first stage cooling unit and the
second stage cooling unit may be configured to cool to a temperature between ooc and 32.4°C, or
between soc and 15°C, or preferably to a temperature of 12°C.
[0018] In an embodiment, either or both of the first stage separator and the second stage separator
may comprise a centrifuge. In another embodiment, a single cooling unit may operate as the first
stage cooling unit and the second stage cooling unit. Likewise a single separator may operate as the
flrst stage separator and the second stage separator.
[0019] In an apparatus embodiment, the evaporator may comprise a thin film dryer or an agitated
thin film dryer.
4
[0020] The aqueous solution formed in the reservoir may include sulphuric acid. The aqueous
solution formed in the reservoir may be substantially pH neutral.
[0021] The second stage separator may output the second mother liquor at a temperature between
0°C and 32.4°C. The recirculator may combine the second mother liquor with a received effluent
after the received effluent is cooled by the first stage cooling unit.
[0022] In an apparatus embodiment, the received effluent may comprise a concentrated effluent
stream received from an evaporator, which may in a more specific embodiment be received from a
multi-effect evaporator.
[0023] The received concentrated effluent stream may have been subjected to one or more of
filtering, homogenizing, cooling, pH neutralization, microbial action, and reverse osmosis.
[0024] In a particular apparatus embodiment, the sodium sulphate crystals may include crystallized
sodium sulphate decahydrate.
[0025] The invention additionally provides a method for reducing discharge of an industrial effluent
containing a plurality of soluble or insoluble contaminants including sodium sulphate discharged
from a textile dyeing process. The method comprises (i) a first treatment stage comprising (a)
cooling an effluent discharged from a textile dyeing process, to a temperature that enables
precipitation of hydrated sodium sulphate crystals (b) separating a first yield of hydrated sodium
sulphate crystals from a corresponding first mother liquor and (c) evaporating the first mother liquor
to form a residue of contaminants (ii) a second treatment stage comprising (a) forming an aqueous
solution comprising at least a part of the first yield of hydrated sodium sulphate crystals dissolved in
water (b) cooling the aqueous solution to a temperature that enables precipitation of hydrated
sodium sulphate crystals and (c) separating a second yield of hydrated sodium sulphate crystals from
a corresponding second mother liquor and (iii) a recirculation stage comprising combining the
second mother liquor with an effluent discharged from the textile dyeing process that is received at
the first treatment stage.
[0026) The invention additionally provides an apparatus for reducing discharge of an industrial
effluent containing a plurality of soluble or insoluble contaminants including sodium sulphate
discharged from a textile dyeing process. The apparatus comprises (i) a first stage cooling unit for
cooling an effluent discharged from a textile dyeing process, to a temperature that enables
precipitation of hydrated sodium sulphate crystals (ii) a first stage separator for separating a first
yield of hydrated sodium sulphate crystals from a corresponding first mother liquor (iii) an
evaporator for evaporating the first mother liquor to form a residue of contaminants (iv) a reservoir
for forming an aqueous solution comprising at least a part of the first yield of hydrated sodium
sulphate crystals dissolved in water (v) a second stage cooling unit for cooling the aqueous solution
to a temperature that enables precipitation of hydrated sodium sulphate crystals (vi) a second stage
5
separator for separating a second yield of hydrated sodium sulphate crystals from a corresponding
second mother liquor and (vii) a recirculator for combining the second mother liquor with an
effluent discharged from the textile dyeing process, that is received at the first stage cooling unit.
Brief description of the accompanying drawings
[0027] Figure 1 is a flowchart of steps associated with recovery of sodium sulphate from an effluent
stream.
[0028] Figure 2 is a block diagram illustrating a primary treatment system for treatment of an
effluent stream.
[0029] Figure 3 is a block diagram illustrating an apparatus for reducing liquid discharge of an
industrial effluent.
[0030] Figures 4A and 4B are mass balance diagrams illustrating recovery of sodium sulphate from
a treated effluent stream.
Detailed Description
[0031] The present invention is directed towards an apparatus and method for treatment of
industrial effluents that contain a plurality of soluble or insoluble contaminants including sodium
sulphate. The apparatus and method of the present invention achieve recovery of sodium sulphate
crystals of sufficiendy high purity, so as to enable reuse of the recovered crystals in an industrial
process. Implementation of the apparatus and method within an effluent treatment system
additionally ensures that the treatment system is a "zero-discharge" system.
(0032] Figure 1 is a flowchart briefly illustrating steps associated with recovery of sodium sulphate
from an effluent stream.
(0033] Step 102 of Figure 1 represents an industrial process that results in an effluent stream
containing a plurality of contaminants including sodium sulphate. It would be understood that this
industrial process can represent any industrial process resulting in effluent discharge. In an
embodiment, the industrial process of step 102 may represent a textile manufacturing process or a
textile dyeing process which uses Glauber's salt to improve the affinity of dye towards fabric, and
which results in a wastewater stream containing dissolved sodium sulphate.
[0034] Step 104 comprises subjecting the effluent stream to a one or more primary treatment
processes for recovering clean water from the effluent stream. Exemplary primary treatment
processes of the type that may be implemented at step 104 are discussed in detail in connection with
Figure 2. Implementation of the one or more primary treatment processes results in (i) recovery of
6
upto 92% clean (or pure) water from the effluent stream and (ii) a concentrated effluent stream
containing a significandy higher concentration of soluble impurities than concentration of impurities
in the original effluent stream arising at step 102.
[0035] Step 106 comprises cooling of the concentrated effluent stream, which cooling may be
achieved by a variety of methods, including for example by means of one or more heat exchangers.
The objective of cooling step 106 is to lower the temperature of the concentrated effluent stream to
a crystallization temperature of sodium sulphate (i.e. a temperature or temperature range at or within
which precipitation of anhydrous or hydrated sodium sulphate crystals occurs).
[0036] As a consequence of cooling the concentrated effluent stream to a crystallization
temperature, at step 108 sodium sulphate crystallizes out of the concentrated effluent stream. Step
110 comprises separating the sodium sulphate crystals from the corresponding (or remaining)
mother liquor. In an invention embodiment, the sodium sulphate crystals resulting from
crystallization step 108 are crystallized as sodium sulphate decahydrate (Na2S04. 10H20).
[0037] Figure 2 comprises a block diagram illustrating an exemplary primary treatment system 200
configured for implementing one or more processes or steps associated with primary treatment of
an effluent stream as discussed at step 104 of Figure 1.
[0038] As illustrated in Figure 2, an industrial effluent enters screening tank 202. Screening tank 202
comprises one or more flltration or straining devices which remove suspended and floating solid
matter and particulate matter from the effluent stream. Exemplary embodiments of the flltration or
straining devices may include one or more of mesh fllters, screen fllters, rotating screen filters or
disk ftlters.
[0039] The flltered effluent stream thereafter enters homogenizing tank 204 and is permitted to rest
in the tank for a predefined period. Homogenizing tank 204 may include mixers and / or aerators
which, over the predefined period, homogenize the resting effluent. The homogenizing processes
within homogenizing tank 204 ensure that the effluent achieves uniform effluent characteristics in
terms of suspension, impurity content, pH, temperature etc. In an embodiment, the flltered effluent
stream may rest in the homogenizing tank for upto 24 hours or even beyond.
[0040] The homogenized effluent stream thereafter passes through cooling tower 206 for the
purpose of reducing temperature of the effluent stream. In an embodiment, the temperature of the
homogenized effluent stream is reduced from between 45° to 4 7°C to less than 36° C.
[0041] The cooled effluent stream thereafter passes into neutralization tank 208 to neutralize pH of
the effluent stream. Neutralization tank 208 may include one or more automated systems including
dosing pump 208a, acid tank 208b and pH sensor 208c. On detection of a pH imbalance by pH
7
sensor 208c, dosing pump 208a can pump a specified quantity of acid from acid tank 208b into
neutralization tank 208 to neutralize pH of the effluent contained within neutralization tank 208.
[0042] The neutralized effluent is subsequently passed to biological treatment tank 210, where
organic matter within the effluent stream is removed by micro-organisms or microbes that are
within or introduced into biological treatment tank 210. The microbial action converts insoluble
organic matter within the effluent stream into an organic sludge, which organic sludge is separated
from the effluent stream using membranes provided within membrane bioreactor tank 212. The
separated organic sludge may optionally be dewatered (such as for example, using a belt press
system) and the resulting residual solid waste may be disposed of through any appropriate solid
waste disposal system (including landfills or by using such solid waste as manure or fertilizer).
[0043] The remaining effluent stream passes from membrane bioreactor tank 212 to reverse
osmosis system 214, which may contain one or more than one reverse osmosis stages. Reverse
osmosis system 214 typically results in upto 92% of the effluent stream being converted to a reverse
osmosis permeate stream (i.e. clean water I pure water) while the remainder of the effluent stream
(8% or more) comprises the reverse osmosis reject stream (i.e. an effluent stream with a high
concentration of impurities).
[0044] The reverse osmosis reject stream is passed to evaporator 216, which further concentrates
the effluent stream by the process of evaporation. In an exemplary embodiment, evaporator 216 is a
multi-effect evaporator wherein evaporated liquid (as steam) from one effect or stage serves as a
heating medium in a successive effect or stage. Passing of the reverse osmosis reject stream through
evaporator 216 results in a yet more concentrated effluent stream.
[0045] The concentrated effluent stream output from evaporator 216 passes to thickener tank 218
which includes one or more devices (such as mixers and I or aerators) for homogenizing the
concentrated effluent stream.
[0046] In the exemplary embodiment of primary treatment system 200, output from thickener tank
218 (a concentrated effluent stream) may be used as a feedstream for the subsequent steps of
cooling (step 106) , crystallization (step 108) and separation (step 110) associated with recovery of
sodium sulphate crystals in accordance with the method illustrated in Figure 1.
[0047] Figure 3 is a block diagram illustrating an apparatus embodiment of the present invention,
which achieves reduction of liquid discharge of an industrial effluent which contains a plurality of
soluble or insoluble contaminants including sodium sulphate.
[0048] Apparatus 300 comprises first stage cooling unit 302, which receives an effluent stream from
the industrial plant. In the illustrated embodiment, first stage cooling unit 302 comprises chiller tank
302a, plate heat exchanger 302b and cooling tower 302c - which in operation reduce the
8
temperature of the effluent stream received from the industrial plant. It will however be understood
that any other combination of components can equally be used to function as a first stage cooling
unit for the purpose of reducing temperature of the effluent stream.
[0049] The effluent received at first stage cooling unit 302 comprises one or more soluble or
insoluble contaminants, including sodium sulphate. In an embodiment of the invention, first stage
cooling unit 302 receives a concentrated effluent stream of the kind that is output from evaporator
216 of primary treatment system 200 described in connection with Figure 2.
[0050] In an embodiment of the invention, first stage cooling unit 302 reduces the temperature of
the received effluent stream to a crystallization temperature for sodium sulphate (or for hydrated
sodium sulphate) i.e. to a temperature at or below 32.4°C, and preferably to a temperature between
0°C and 32.4°C. In a more preferred embodiment, the first stage cooling unit reduces the
temperature of the received effluent stream to a temperature between soc and l5°C, and even more
preferably to 12°C .
[0051] The cooled effluent is passed on to first stage crystallization tank 304, in which as a
consequence of the reduction in temperature of the effluent to a crystallization temperature,
formation of sodium sulphate crystals is stimulated. The cooled effluent may be allowed to rest in
first stage crystallization tank 304 for a duration sufficient to enable crystal formation. In an
embodiment of the invention, the crystals formed in first stage crystallization tank 304 are sodium
sulphatedecahydrate crystals. In an embodiment of the invention, cooling by first stage cooling unit
302 and crystallization may occur within a single reservoir.
[0052] The contents of first stage crystallization tank 304 are subsequendy transferred to first stage
separator 306, which first stage separator 306 separates a first yield of sodium sulphate crystals from
a corresponding first stage mother liquor. First stage separator 306 may comprise any one or more
separating devices that may be appropriate to achieve the desired result - including without
limitation, one or more filters or centrifuges. In an embodiment of the invention, first stage
separator 306 may comprise a pusher centrifuge.
[0053] The first mother liquor arising from the crystallization and separation steps discussed above
contains high levels of impurities. This mother liquor cannot be discharged without potential
environmental consequences or without violating regulatory norms in this regard. The first mother
liquor is accordingly transferred to an evaporator 308- which evaporates liquid content of the first
mother liquor, and forms a dry or caked residue of solid contaminants. The residue of solid
contaminants may be safely disposed of in a landfill or by way of any other appropriate form of solid
waste management. In an invention embodiment, evaporator 308 may comprise a thin f11m dryer or
an agitated thin film dryer.
9
(0054] While the first mother liquor is likely to include dissolved sodium sulphate as a contaminant,
the first mother liquor is not recirculated within the effluent treatment system. This is for the reason
that the first mother liquor contains an exceeding high level of contaminants or impurities in
concentrated form, and recirculating the concentrated impurities within the effluent treattnent
system would result in a systematic increase of contaminants within the system -leading to a steady
reduction in effectiveness of the effluent treatment system, and eventual choking or shutdown of the
system.
[0055] The first yield of sodium sulphate crystals separated at first stage separator 306 are likely to
have unacceptable levels of impurities or contaminants, as well as discolorations which make the
first yield of crystals unacceptable for industrial applications (such as within the textile industry).
(0056] First stage cooling unit 302, first stage crystallization tank 304, first stage separator 306 and
evaporator 308 may together comprise a first treatment stage within a process for effluent treatment.
(0057] The flrst yield of sodium sulphate crystals is thereafter transferred to aqueous solution
reservoir 310, in which an aqueous solution is formed by dissolving the first yield of sodium
sulphate crystals in water. In an embodiment of the invention, an appropriate concentration of acid
may be added to the aqueous solution reservoir to neutralize pH of the aqueous solution. In a
preferred embodiment, the acid added for pH neutralization is sulphuric acid (H2S04), which has
been found to improve subsequent yields of sodium sulphate crystals. It would be understood that
neutralization of pH may be manually achieved, or may be achieved by way of an automated pH
sensing and neutralization system of the kind discussed in connection with neutralization tank 208
of Figure 2. The concentration of dissolved sodium sulphate within the resulting aqueous solution
may be between 20% to 35% w /w. In an embodiment of the invention, preparation of the aqueous
solution requires combining of hydrated sodium sulphate crystals with water in not less than a 40:60
w /w proportion upto a 70:30 w /w proportion. Owing to the high moisture content within hydrated
sodium sulphate crystals (over 50%), the concentration of dissolved sodium sulphate within the
resulting aqueous solution is significantly less. In preferred embodiment of the invention,
preparation of the aqueous solution involves addition of 96 grams of hydrated sodium sulphate
crystals per litre of water, which may result in a concentration of dissolved sodium sulphate within
the aqueous solution of approximately 40 gm/1. In an invention embodiment, the step of combining
sodium sulphate crystals with water is carried out at room temperature.
(0058] The aqueous solution is thereafter transferred from aqueous solution reservoir 310 to a
second stage cooling unit 312. In the illustrated embodiment, second stage cooling unit 312
comprises chiller tank 312a, plate heat exchanger 312b and cooling tower 312c- which in operation
reduces the temperature of the aqueous solution received from aqueous solution reservoir 310. It
will however be understood that any other combination of components can equally be used to
function as a second stage cooling unit for the purpose of reducing temperature of the aqueous
solution.
10
[0059] In an embodiment of the invention, the second stage cooling unit 312 reduces the
temperature of the received aqueous solution to a crystallization temperature for sodium sulphate
(or for hydrated sodium sulphate) i.e. to a temperature at or below 32.4°C, and preferably to a
temperature between 0°Cand 32.4°C. In a more preferred embodiment, second stage cooling unit
312 reduces the temperature of the received aqueous solution to a temperature between soc and
15°C, and even more preferably to 12oc.
[0060] The cooled aqueous solution is passed on to second stage crystallization tank 314, in which
as a consequence of the reduction in temperature of the aqueous solution to a crystallization
temperature, formation of sodium sulphate crystals is stimulated. The cooled effluent may be
allowed to rest in second stage crystallization tank 314 for a duration sufficient to enable crystal
formation. In an embodiment of the invention, the crystals formed in second stage crystallization
tank 314 are sodium sulphatedecahydrate crystals.
[0061] In an embodiment of the invention, cooling by second stage cooling unit 312 and
crystallization may occur within a single reservoir.
[0062] The contents of second stage crystallization tank 314 are transferred to second stage
separator 316, which separates a second yield of sodium sulphate crystals from a corresponding
second stage mother liquor. Second stage separator 316 may comprise any one or more separating
devices that may be appropriate to achieve the desired result- including without limitation, one or
more filters or centrifuges. In an embodiment of the invention, second stage separator 316 may
comprise a pusher centrifuge.
[0063] In an embodiment of the invention, a single cooling unit may operate as the first stage
cooling unit as well as the second stage cooling unit. In another embodiment of the invention, a
single separator may operate as the first stage separator as well as the second stage separator.
[0064] Second stage cooling unit 312, second stage crystallization tank 314 and second stage
separator 316 may together comprise a second treatment stage within a process for effluent
treatment.
[0065] In comparison to the first yield of sodium sulphate crystals, the second yield of sodium
sulphate crystals is found to have a low level of impurity or contaminants, which makes the second
yield of crystals acceptable for industrial applications, including within the textile industry. The
second yield of sodium sulphate crystals accordingly may be reused within the industrial plant.
[0066] The second mother liquor is likely to include dissolved sodium sulphate but otherwise
insignificant or low levels of other contaminants. The second mother liquor can accordingly be
recirculated within the effluent treatment system to ensure that the dissolved sodium sulphate can be
11
extracted, while simultaneously avoiding discharge of liquid effluent. In an embodiment of the
invention, the second mother liquor is recirculated by combining said second mother liquor with an
effluent stream that is received for crystallization at first stage crystallization tank 304. In a tnore
specific embodiment of the invention, the second mother liquor is recirculated by combining said
second mother liquor with a cooled effluent stream that is output from first stage cooling unit 302.
In an exemplary embodiment, the second mother liquor may be combined with the cooled effluent
stream either within first stage crystallization tank 304 or prior to the cooled effluent stream entering
the first stage crystallization tank 304.
[0067] In a specific embodiment, apparatus 300 may include a recirculator (not illustrated) for
recirculating and combining the second mother liquor with a cooled effluent stream within (or as it
enters) first stage crystallization tank 304. Specific embodiments of the recirculator may include one
or more of a recirculation pump and/ or a pipeline or fluid connection operatively coupling an
output of second stage separator 316 with an input to first stage crystallization tank 304.
[0068] Tables 1 to 3 summarize the results of 3 sets of data arising from comparative analysis of a
first yield of hydrated sodium sulphate crystals with a second yield of hydrated sodium sulphate
crystals, that are formed in accordance with the description provided in connection with Figure 3
hereinabove. Each of the 3 comparative data sets represent a separate round of testing on first and
second yields of hydrated sodium sulphate derived in accordance with the present invention.
[0069] Table 1:
Comparative Data - Set 1
Parameters Units of Measurement First Yield of Hydrated Second Yield of
Sodium Sulphate Hydrated Sodium
Crystals (crystallized Sulphate Crystals
out of a solution (crystallized out of a
having a sodium solution having a
sulphate concentration sodium sulphate
of 40 gm/1) concentration of 40
gm/1)
pH 10.9 6.68
Total dissolved solids mg/1 24500 38940
Electrical conductivity uS/em 37700 59510
Total hardness mg/1 52 Below detection limit
Color PtCo 1020 220
Total alkalinity mg/1 310 40
12
[0070] Table 2:
Comparative Data - Set 2
Parameters Units of Measurement First Yield of Hydrated Second Yield of
Sodium Sulphate Hydrated Sodium
Crystals (crystallized Sulphate Crystals
out of a solution (crystallized out of a
having a sodium solution having a
sulphate concentration sodium sulphate
of 40 gm/1) concentration of 40
gm/1)
pH 10.81 6.81
Total dissolved solids mg/1 23880 38440
Electrical conductivity uS/em 36513 58710
Total hardness mg/1 56 Below detection limit
Color PtCo 1060 220
Total alkalinity mg/1 330 44
[0071] Table 3:
Comparative Data - Set 3
Parameters Units ofMeasurement First Yield of Hydrated Second Yield of
Sodium Sulphate Hydrated Sodium
Crystals (crystallized Sulphate Crystals
out of a solution (crystallized out of a
having a sodium solution having a
sulphate concentration sodium sulphate
of 40 gm/1) concentration of 40
_gm/1)
pH 10.73 6.62
Total dissolved solids mg/1 24990 38650
Electrical conductivity uS/em 38211 59250
Total hardness mg/1 58 Below detection limit
Color PtCo 1040 245
Total alkalinity mg/1 320 44
[0072] The data in Tables 1 to 3 establishes that the second yield of sodium sulphate crystals
consistently demonstrates higher purity and significantly lower levels of contaminants, impurities
and color arising from impurities in comparison with the first yield of sodium sulphate crystals.
Obtaining a second yield of sodium sulphate crystals therefore not only presents advantages in terms
of reducing contaminating discharge from an industrial plant, but also increases the yield of reusable
sodium sulphate, which presents significant economic advantages for manufacturing processes.
[0073] Figure 4A is a mass balance diagram corresponding to a single stage process for treating a
liquid effluent containing sodium sulphate. It will be observed that the illustrated apparatus 400A for
13
treating liquid effluent comprises cooling unit 402 (which in tum comprises chiller tank 402a, plate
heat exchanger 402b and cooling tower 402c), crystallization tank 404 and separator 406.
[0074] As illustrated in Figure 4A, from the total mass of dissolved sodium sulphate (100% sodium
sulphate) within the concentrated effluent received at cooling unit 402, recovered sodium sulphate
within the eventual crystal yield is between 85% to 90%, while unrecoverable sodium sulphate in the
mother liquor is between 10% to 15%. The crystal yield recoverable from the apparatus 400A is
likely to have high levels of impurity and discoloration, making the yield unsuitable for reuse or
further industrial application.
[0075] Figure 4B is a mass balance diagram corresponding to a two stage process for treating a
liquid effluent containing sodium sulphate. It will be observed that the illustrated apparatus 400B for
treating the liquid effluent is substantially comparable to the apparatus illustrated and described in
connection with Figure 3. The apparatus 400B comprises first stage cooling unit 402 (which in turn
comprises chiller tank 402a, plate heat exchanger 402b and cooling tower 402c), first stage
crystallization tank 404,first stage separator 406, evaporator 408, aqueous solution reservoir 410,
second stage cooling unit 412 (which in tum comprises chiller tank 412a, plate heat exchanger 412b
and cooling tower 412c), second stage crystallization tank 414 and second stage separator 416.
[0076] As illustrated in Figure 4B, out of a total mass of dissolved sodium sulphate (1 00% sodium
sulphate) within the concentrated effluent that ·is received at first stage cooling unit 402, recovered
sodium sulphate in the first yield of sodium sulphatecrystals is between 85% to 90% of the total
mass of dissolved sodium sulphate. Said first crystal yield is dissolved within aqueous reservoir 410,
resulting in an aqueous solution having 85% to 90% of the total mass of dissolved sodium sulphate,
which is co.oled at second stage cooling unit 412 and passes on to second stage crystallization tank
414 and second stage separator 416. The second yield of sodium sulphate crystals recovered from
second stage separator 416 is between 75% to 80% of the total mass of dissolved sodium sulphate,
while the second mother liquor has between 10% to 15% of the total mass of dissolved sodium
sulphate - which 10% to 15% of total mass of dissolved sodium sulphate is recirculated to first
crystallization tank 404. The mass of unrecoverable sodium sulphate within a solid residue output
from evaporator 408 is between 20% to 25% of the total mass of dissolved sodium sulphate.
[0077] The total mass of recovered sodium sulphate from the two stage treatment apparatus of
Figure 4B is less than the total mass of recovered sodium sulphate from the single stage treatment
apparatus of Figure 4A. However, the second yield of crystals recovered from the two stage
treatment apparatus of Figure 4B has high purity and significandy lower levels of discoloration,
which makes such crystals suitable for reuse and further industrial application, particularly within the
textile or dyeing industries.
[0078] The method and apparatus of the present invention may be used to treat any liquid effluent
discharge from an industrial plant. In a preferred embodiment, the method and apparatus may be
14
used to treat a sodium sulphate containing liquid effluent discharged from a dyeing step or process
within a textile manufacturing or dyeing plant.
[0079] While the exemplary embodiments of the present invention are described and illustrated
herein, it will be appreciated that they are merely illustrative. It will be understood by those skilled in
the art that various modifications in form and detail may be made therein without departing from or
offending the spirit and scope of the invention as defined by the appended claims. Additionally,
while exemplary method and apparatus embodiments of the present invention are described in
terms of applicability to the textile industry, it would be understood that the invention is equally
implementable within any industrial plant which generates an effluent stream containing one or
more contaminants including sodium sulphate.

We claim:
1. A method for reducing discharge of an industrial effluent containing a plurality of soluble or
insoluble contaminants including sodium sulphate, the method comprising:
a first treatment stage comprising:
cooling a received effluent to a temperature that enables precipitation of sodium sulphate
crystals;
separating a first yield of sodium sulphate crystals from a corresponding first mother liquor;
evaporating the first mother liquor to form a residue of contaminants;
a second treatment stage comprising:
forming an aqueous solution comprising at least a part of the first yield of sodium sulphate
crystals dissolved in water;
cooling the aqueous solution to a temperature that enables precipitation of sodium sulphate
crystals;
separating a second yield of sodium sulphate crystals from a corresponding second mother
liquor;
a recirculation stage comprising combining the second mother liquor with an effluent that is
received at the first treatment stage.
2. The method as claimed in claim 1, wherein the temperature that enables precipitation of
sodium sulphate crystals at either or both of the first treatment stage and the second treatment stage
is between ooc and 32.4"C.
3. The method as claimed in claim 1, wherein the temperature that enables precipitation of
sodium sulphate crystals at either or both of the first treatment stage and the second treatment stage
is between soc and 15°C.
4. The method as claimed in claim 1, wherein the temperature that enables precipitation of
sodium sulphate crystals at either or both of the first treatment stage and the second treatment stage
is 12oC.
16
5. The method as claimed in claim 1, wherein separating either the first yield of sodium
sulphate crystals from the corresponding first mother liquor or the second yield of sodium sulphate
crystals from the corresponding second mother liquor includes centrifugation.
6. The method as claimed in claim 1, wherein the first mother liquor is evaporated in a thin
film dryer or an agitated thin film dryer.
7. The method as claimed in claim 1, wherein sulphuric acid is added to the aqueous solution
prior to precipitation of sodium sulphate crystals.
8. The method as claimed in claim 7, wherein the aqueous solution is pH neutralized prior to
precipitation of sodium sulphate crystals.
9. The method as claimed in claim 1, wherein the temperature of the second mother liquor is
between oac and 32.4°C. -
10. The method as claimed in claim 9, wherein the second mother liquor is combined with a
received effluent at the first treatment stage after the received effluent is cooled to a temperature
that enables precipitation of sodium sulphate crystals.
11. The method as claimed in claim 1, wherein the received effluent comprises a concentrated
effluent stream received from an evaporator.
12. The method as claimed in claim 11, wherein the concentrated effluent stream has been
subjected to one or more of filtering, homogenizing, cooling, pH neutralization, microbial action,
and reverse osmosis.
13. The method as claimed in claim 1, wherein the sodium sulphate crystals include crystallized
sodium sulphatedecahydrate.
14. An apparatus for reducing discharge of an industrial effluent containing a plurality of soluble
or insoluble contaminants including sodium sulphate, the apparatus comprising:
a first stage cooling unit for cooling a received effluent to a temperature that enables precipitation of
sodium sulphate crystals;
a first stage separator for separating a first yield of sodium sulphate crystals from a corresponding
first mother liquor;
an evaporator for evaporating the first mother liquor to form a residue of contaminants;
17
a reservoir for forming an aqueous solution comprising at least a part of the first yield of sodium
sulphate crystals dissolved in water;
a second stage cooling unit for cooling the aqueous solution to a temperature that enables
precipitation of sodium sulphate crystals;
a second stage separator for separating a second yield of sodium sulphate crystals from a
corresponding second mother liquor; and
a recirculator for combining the second mother liquor with an effluent that is received at the first
stage cooling unit.
15. The apparatus as claimed in claim 14, wherein either or both of the first stage cooling unit
and the second stage cooling unit are configured to cool to a temperature between oac and 32.4°C.
16. The apparatus as claimed in claim 14, wherein either or both of the first stage cooling unit
and the second stage cooling unit are configured to cool to a temperature between sac and 15°C.
17. The apparatus as claimed in claim 14, wherein either or both of the first stage cooling unit
and the second stage cooling unit are configured to cool to a temperature of 12°C.
18. The apparatus as claimed in claim 14, wherein either or both of the first stage separator and
the second stage separator comprise a centrifuge.
19. The apparatus as claimed in claim 14, wherein a single cooling unit operates as the first stage
cooling unit and the second stage cooling unit.
20. The apparatus as claimed in claim 14 wherein a single separator operates as the first stage
separator and the second stage separator.
21. The apparatus as claimed in claim 14, wherein the evaporator is a thin film dryer or an
agitated thin film dryer.
22. The apparatus as claimed in claim 14, wherein aqueous solution formed in the reservoir
includes sulphuric acid.
23. The apparatus as claimed in claim 22, wherein the aqueous solution formed in the reservoir
is substantially pH neutral.
18
24. The apparatus as claimed in claim 14 wherein the second stage separator outputs the second
mother liquor at a temperature between ooc and 32.4°C.
25. The apparatus as claimed in claim 14, wherein the recirculator combines the second mother
liquor with a received effluent after the received effluent is cooled by the first stage cooling unit.
26. The apparatus as claimed in claim 14, wherein the received effluent comprises a
concentrated effluent stream received from an evaporator.
27. The apparatus as claimed in claim 26, wherein the concentrated effluent stream is received
from a multi-effect evaporator.
28. The apparatus as claimed in claim 27, wherein the concentrated effluent stream has been
subjected to one or more of filtering, homogenizing, cooling, pH neutralization, microbial action,
and reverse osmosis.
29. The apparatus as claimed in claim 14, wherein the sodium sulphate crystals include
crystallized sodium sulphatedecahydrate.
30. A method for reducing discharge of an industrial effluent containing a plurality of soluble or
insoluble contaminants including sodium sulphate discharged from a textile dyeing process, the
method comprising:
a first treatment stage comprising:
cooling an effluent discharged from a textile dyeing process, to a temperature that enables
precipitation of sodium sulphate crystals;
separating a first yield of sodium sulphate crystals from a corresponding first mother liquor;
evaporating the first mother liquor to form a residue of contaminants;
a second treatment stage comprising:
forming an aqueous solution comprising at least a part of the first yield of sodium sulphate
crystals dissolved in water;
cooling the aqueous solution to a temperature that enables precipitation of sodium sulphate
crystals;
19
separating a second yield of sodium sulphate crystals from a corresponding second mother
liquor;
a recirculation stage comprising combining the second mother liquor with an effluent discharged
from the textile dyeing process that is received at the first treatment stage.
31. An apparatus for reducing discharge of an industrial effluent containing a plurality of soluble
or insoluble contaminants including sodium sulphate discharged from a textile dyeing process, the
apparatus comprising:
a first stage cooling unit for cooling an effluent discharged from a textile dyeing process, to a
temperature that enables precipitation of sodium sulphate crystals;
a first stage separator for separating a first yield of sodium sulphate crystals from a corresponding
first mother liquor;
an evaporator for evaporating the first mother liquor to form a residue of contaminants;
a reservoir for forming an aqueous solution comprising at least a part of the first yield of sodium
sulphate crystals dissolved in water;
a second stage cooling unit for cooling the aqueous solution to a temperature that enables
precipitation of sodium sulphate crystals;
a second stage separator for separating a second yield of sodium sulphate crystals from a
corresponding second mother liquor;
a recirculator for combining the second mother liquor with an effluent discharged from the textile
dyeing process, that is received at the first stage cooling unit.