Effluent Treatment in Textile, Leather and Alcohol Industries

C H Y S f C A b 1. IMPURtTlES

5 “ C U E M I C A L ~ M P U A ~ 11 E 5

-c

O R G A N I C

9 M p U R l T i E S

U N A E S T U E T I C C € I N B i t l O N S ..

c. A. Sistry

Centre for BioSciences and Biotechhology, bepat-tmeht of Chemical Engineering

Itldidn Institute of Technology, Madras - 600 036

I 4 - . -

L A N D U S E . TREATMENT PROCESS 1 ‘’ FOR \

# v DISCUAR G E INTO f N L A N D S U R F A C E

W A T E R S A N 0

MARINE W A T E R S

R E M O V A L / RfDUCTlON

OF THE IMPURITIES , -. .r) A N D

D I S C I I A R C E INTO

P U B L I C S E W E R S

REACH S P E C I F I E D r

SC AS T O . B € F I T FOR L.ll 1 I M l T S FOR THESE

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INTRODUCTION

Waste is a misplaced resource and the challenge in wastewater management is to identify a scheme of modifying inputs, modifying production processes and recovering byproducts to result in profits for the industry while satisfying efBuen t standards stipulated by the Pollution Control Boards. Link between occurance and disposal/reuse of wastewaters is given in figure 1.

The possibility of developing fundamentally new pro- cesses for wastewater treatment is remote. Only an innovative use of existing technology to arrive at functional and cost-effective management scheme best suited to local conditions would afford the possibility of a productive proposition. There are situations where non-availability of water forces curtailment of indust-

production in summer. A waste management scheme resulting in the reuse of treated wastewater would encourage initial investment in treatment that could be recovered in due course of time. Then there

#

are industries where land is scarce and only a recoursc of phisico-chemical treatmeut would produce an effluent acceptable to the Polluti satisfying land constraint. On the ample land is available simple biological prdcesses exploiting the natural advantage of bright sunshine and high temperature with a minimum reqai equipment, energy and skilled manpower would result in an appropriate and reliable freatment .system regional wastewater treatment municipal and industrial efflu ment of effluents from seve result not only in a less ex economy of scale and concomitant neutralisation, equalisation and proportioning but also in a cost sharing financial system based on ‘polluter-pays' principle. Wastewater treatment is essentially a management problem. More often than not this management problem could result in profits for the industry whilst fulfilling its commitment to environ- ment.

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WA.STE WATER TR EATME N 7’ DISPOSAL 1 REUSE

Figure 1. Link between occurance and disposal 1 reuse of wastewaters

IND3AN 3. EMVIRONMENTAL P E O m O N , VOL. 5, NO. 2, APRIL 198

ideal situation is to have more reusable water than needed and excess back to the drain; And (B) Reuse of clean water with treatment-( i ) treatment required, ( ii ) treated water quality needed and produced, and ( iii ) storage and handling systems. In general, if reuse does not require treatment, a separate water system will be necessary in most cases. The separate systems would be a drinking and high quality use system plus a lower quality service water system. In some plants that already have a separate water and service water system, a third water system may be necessary. It can be noted that this will increase the total water cost as the raw water use is reduced.

DISTILLERY WASTESWATER TREATMENT

There are at present 128 distilleries in India producing 800 million lit of alcohol and 12,000 million lit of spent wash per year. The wastewater from these industries is highly polluting. In contrast to many industries 80% of the raw material used ends up as a wastewater in alcohol distilleries. The polluting effect of these waste- waters are, therefore, mainly due to the high concentra- tion of organic matter contained in them. This organic matter is readily decomposed by biological action as a consequence of which discharge of these waters into surface waters usually cause severe deoxygenation conditions. Those wastes are usually acidic with a pH of 4.0-4.5 and they contain high amounts of sulpha- tes and potash. They are coloured and have a high BOD and COD. Characteristics of wastes are given in table 1.

Table 1. Characteristics of spent wash of the distillery, in mg/Iit

Characteristics Range values

PH Total acidity ( CaCos ) Total solids

Suspended solids ~

BOD COD Sulphate ( SO4 )

Total nitrogen ( N ) Potash ( K )

3.6 8,200

69,330

2,000

35,000

68,000 1,000

950

4,000

- 4.4 - 16,150

- 87,360

- 8,000

- 54,000 - 1,90,000

- 7,500

- 1,200

- 10,Ooo

96

Water reuse and b ypmduct recovery from distillery wastes

There is not much scope for water reuse in molasses distilleries*as spent wash is very strong and there is no economical method for colour removal from spent wash. However, one of the distilleries in Karnataka is collecting steam condensate and reusing after coafing. There is lot of scope for byproduct recovery. Complete recovery of grains in the form of distiller's dried p i n s and 'distiller's dried soluble as cattle feed has bocn tin accepted practice abroad in distilleries using @in. Mixing feeds are manufactured abroad from spat wasb of molasses distilbries by a process of cbnmntratiola: One such semi-solid product is marketed in with 45% solids content. Recovery of potash centration of molasses durjng the I World War and upto late thirties. .

process consists of washing th molasses and other soluble and partial autolysis by press tion and concentration and drying under va preparation thus obtained

India on methane production using distillery work. Sen and Bhaskaran in 1962 have reported results of labo- ratory scale digestion studies on distillery spent wash with a BOD of 18,000-37,OOO mg/lit at 37 O

loading of 3.008 kgl,day/cu m and a digest 10 days. BOD reduction of over 90% and gas ( 60% methane) of 25 volumes/volume of waste accrued during this investighion. Digestion peri provided better stability to the process even BOD loadings (7.36 kg/day/cu m ) days digestion period at loading of 3.76 caused sour digester. A method for preparatio core binder using spent wash was patented.

Treatment of distillery wastewaters

The methods suggested in India include : (a) lagoon followed by aerated lagoon, (b) anaero followed by dilutian and agricultural ut methane recovery by anaerobic digestion activated sludge process, (d) potash rec centration to 60% solids and disposal, (f) acid segregation process followed by aerobic t

INDIAN J. ENVIRONMEN NCT..'ZyL,' A

filter or aqaerobic activated

with methane recovery is indeed pproach in present times of energy

resting to note that the Central rd document on distillery wastes n for treatment with anaerobic ons and potash recovery as

respectively of the annual turn A cost effective process design m the pTovision of optimal

like temperature, pH, mixing, design alternatives for

lory spent wash may be nvention anaerobic digestion process,

aerobic activated sludge e segregation process. These

ed based on the criteria , functionality, stability and

parameters for design under

filter process has been developed to treat s-g wastes. The reactor contains a solid support,

piastic matrix or stones. The active micro- sms grow attached to the support material or as

blockr-h. the interstices. Mean cell residence time of the order of 100 day could be obtained

rt hydraulic retention time waste admitted at the bottom of the reactor.

Adcq-tc data for design of the process for distillery ash is not yet available in India.

The anaerobic activated sludge process was developed in 1966 with the objective of conserving active methane formers and achieving high mean cell residence time with low hydraulic retention time for maximising substrate utilisation. The sludge formed during anae- robic digestion is light in weight due to mineralisation and bulky due to entrained biogas, and accordingly tends to float and escape with the supernatent causing substantial loss of methanogens. Recirculation of this sludge maintains a sound population dynamics of methanogens and achieves active digestion.

Acid methane segregation is a microbial segregation process where acid formation is restricted in an open digester with pH reaching low values of 4.5-5.0 enhanc- ing the efficacy of substrate transformation. Surges in substrate are effectively attenuated in this phase where- as the effluent is adjusted for optimal environmental conditions in methane digester for improved kinetics of substrate utilisation. The kinetic coefficients for methane and acid formation stages have been estimated from extensive laboratory scale experimentation on distillery spent wash in 1981 for use in process design relationships derived from mass balance concept.

The maximum yield coefflcient, Y ( ratio of mass of cells formed to mass of substrate utilised ), maximum rate of substrate utilisation per unit mass of micro- organisms, K ( per unit time ); half-velocity constant, that is substrate concentration at one half the maximum growth rate, K, ( mass/unit voulme) and decay coefficient Kd ( per unit time ) for methane formation stage have been estimated as 0.163 mg microorglmg

Figore 2. Layout plan of efluent treatment plant for distillery spent wash 1. Mineral acid neutralidation tank, 2. Lime slurry tank with pH control, 3. Pump settler, 4. Hopper buttom settler, 5. Pump to filter press, 6. Filter press, 7. 'step down heat exchanger, 8. Acid formation reactors, 9. Holding tank, 10. Pump to methane reactors, 11. Methane reactors, 12. Gas storage tank, 13. Degassification chamber, 14. Secondary clarifier, 15. Sludge pump, 16. Sludge drying beds, and 17. Control room-cum-chem_ica.l house.

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COD utilised; 1.4 per day; 7970 mg/lit and 0.028/day respectively resulting in mean cell residence time of 5.57 day at an operating temperature of 34°C. Similarly estimated values of Y, K, Kd ( 0.1477 mg/ mg, 18.05 /day and 0.0125/day respectively ) result in residence time of 9.08 hr in acid formation stage. The overall treatment efficiency of 95% produces effluent with a BOD of 2500 mg/lit. Loadings and gas yields have been observed to be comparable with those reported by Sen and Bhaskaran in 1962.

TEXTILE MILL WASTEWATER TREATMENT

Cotton textile industry with an annual production of about 5,000 million m of cloth is one of the biggest in the country employing nearly 1 million workers in about 700 cotton textile mills discharge 20 to 25 lit of wastewater per m of cloth produced. BOD load varies from 6 to 10 gm / m of cloth processed. Pollution load originates from two sources-natural fibre impuri- ties (about 30% of load) and process chemicals (rest). Very little can be done to improve the waste contri- bution from natural impurities, but the range of BOD contribution from the process chemicals indicates that chemical substitution improve the situation. If instead of using starch sizing, polyvinyl alcohol is used, then BOD output is reduced by 95%.

Some of the wet processes used in textile industry include sizing, desizing, kiering, mercerising, bleaching, dyeing ( fancy, khakki and yarn ), screening and printing, and finishing. The' wastewaters resulting from these operations are laden with different types of chemicals in varying quantities due to which it may not be possible to discharge the untreated wastes either into a water course or into a municipal sewer without causing damage. Characteristics of textile mill wastes are given in table 2. The break-up of the water requirement for different purposes in a textile mill is as follows : Steam production-5.3% oofing waterd6.4%, D. M. water for selective process-7.8%, for process 72.3%, sanitary use-7.6%, and fire fighting, etc,,-0.6%. Characteristics of individual wastewaters are given in table 3.

Table 2. Characteristics of combined 'textile mill wastes, in mgllit

Characteristics Ahmedabad Mills B and C, Madras <

PH 7.0- 9.0 9.8- 11.8

Suspended solids 200- 550 500-1,850 BOD 100- 600 720- 840 COD 250- 800 1,160-1,790

Total dios. solids 2,400-3,(00 5,120-6,180

% Na 88- 92 -

Sizing Des i z i n g Kiering Scouring Bleaching Mercerising Dyeing Aniline black Basic Developed colours Direct Naphthol Sulphur Vats Printing

Table 3.

Process PH BOD kg BOD/I,OOO kg product

Characteristic of individual wastes from a textile mill

620-2,500 0.5- 5.0 7.0-9.5 -

10-13 -

8.5-9.6 5.5-9.6

- 6.0-7.5

5- 10

6.5-7.6 5- 10

8- 10

5- 10

6.7- 8.1

1,700-5,200 680-2,900 50- 110

90-1,700 45- 65

40- 55 100- 200 75- 200

220- 600 15- 675 11-1,800

125-1,50 0 135-1,380

14.8-16.1 1.5-1 7.5

1.36-3.02 5.0-14.8

10.5-13.5

t,

5- 10

15- 50

15- 20 1.3-1 1.7 2.5

2- 2.5 12- 30 2- 4

-311-L3111-

98 INDIAN J. ENVIRONMENTAL PROTECTION, VOL. 5, NO. 2, APRIL 1985

@ 1985 - Kalpana Corporation

ling water that dots not come in Contact with the cloth or have excess added chemicals can be

( i ) Cooling water in hydrosulphite operation RS indigo wash water.

( ii) In printing, cooling water reused to ‘ wash blankets or back greiges.

(iii) Cooling water as final rinse in dyeing. Later wash waters can be used for first or second

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(B) Treatment for reuse.

( i ) Ultrafiltration : The reports to date indicate that this can be a practical 1 approach in certain situations. Operation of this type of equip- ment,prodoces a water in quality only a little below reverse osmosis. This could be specially appealing, however, in a single dye-shade dye- house, then dyes and heat, as well as water could be reused. The cost is high.

( ii ) Chemical-biological treatment : The effluent from a fibre manufacturing biological treat- ment plant was treated chemically, then

reuse in air conditioning and other operatiOns for use in fibre manufactu-

from an integrated textile waste treatment ically after granular carbon was added

system. This produced water that Otle of the things that

e us6 of chemical coagulation with is .the high consumption required. In

ly certain chemicals seem to work, and the iron salts alone, or in combina-

olecular weight polymer; alum in mg/lit and FeCls from 500-800 mg/

lit without a polymer. The use of polymer will reduce chemical consumption approximately one-third, but requirement will be high in the 3-10 mg/lit range. The cost of storage and pumping facilities will be in addition to these costs. This reused water can be blended into and would replace a portion of raw water @r processing, keeping in mind that there will be an upper limit on dissolved solids that can be tolerated.

a d d be reused in processing.

Reuse of water in processing cafi be carried out tiuccessfully from an engineering and production point of view; however, the main objection to date is cost, which will be 2-3 times the present one. Nearly 70% of the total water consumption in 1 textile mill goes into its processing department. A batch of cloth, like fine poplins, may be subjected to 12 to 15 independent washes before it emerges as sparkling white cloth. Figure 3 shows the sequence of operations in a typical case. It also shows how the used wash waters from some of the later sequences can be collected in a sump and pumped back for direct use without any treatment. An average saving of 15 to 25% can be obtained in this manner.

Waste segregation

Waste segregation is practiced in the textile industry for many years. Heat recovery is an item that has been in use at Cannon Mills, U. S. A. for over 40 years. A dual water system was developed for the bleachery and provided a means of reuse of hot water that was dis- charged from steam plant. In Cannon Mills, one pipeline Collected the desizing wastes and transported them to the plant for immediate treatment. The second pipeline collected the highly alkaline wastes and transported them to a storage lagoon at the treatment plant. The third pipeline collected all of the remaining wastes from the bleachery. Operating efficiencies at the waste treatment plant were greatly improved, since it was possible to control the pH continuously.

A dye plant was constructed for continbous dyeing of towels and sheets, a system to segregate the dye wastes was provided at that time. All of the hot waste waters were collected and screened prior to heat recovery units. The heat recovered by these units saved the company approximately $600 / day. They also delayed cons- truction of an additional steam plant for 5 year. Three additional raw-water pipelines were made available to the waste programme in 1965. These pipelines provided ameans for planning further segregation at the two dye plants. Additional facilities were provided during 1967 to 1972 to improve treatment efficiency to 95%. Included in this construction were four concrete holding tanks used for storage of wastes to improve weekend operations.

Two tanks are used for die waste storage and are equipped with floating aerators to mix the waste and also to provide oxygen to waste. This arrangement pro- vided 3 million gallon of dye waste for the treatment

c

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KlER BOILING

is. RIVER

EQUALISATION CUM BOD 670ngl1, NEUTRALISATION - TANK

HOLDING TANK -

Figure 3. Treatmentflow sheet for textile industry

WINCH OVEING WASH -+ WATER

- plant when the mill is not in operation. Another tank of 2.6 million gallon capacity was used for caustic waste storage. This provides continuous pH control that is required for high efficiency operation. The final tank of 2.6 million gallon capacity was used for the storage of spills and overflows that occur in the bleachery and dye houses. There are now seven pipelines that are available to the waste treatment system. These types of wastes to be blended at the waste treatment plant. The blender is capable of supplying 5 biological treat- ment units with any combination of wastes, that is desired.

Process changes

(A) Washing focus for water economy. The greatest potential for improved water economy in the textile industry stems from the use of better washing methods. It is suggested that washing machines have not been conventionally designed with water economy as a high priority.

Rodney-Hunt in U. S. A., manufacturers of tensitrol washer, compared their washer with two tight strand washers and suggested that upto 85% less water use may be achievable.

EQUALSATION CUM NEUTRALISATION TANU

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MACHINE CLEANING 8

(B) Why use large quantities of water ?

Two major reasons were identified in a British survey. First, it was considered necessary to some for technical reasons. Other felt quality suffered if copious amounts of water were not used. Natural reluctance to change might also be added. Perhaps the best way to overcome such opposition is to run carefully supervised trials.

“k (C) Use more lots of less water.

A British survey of the processes involved in SCOW- ing wool revealed that considerable savings can be made-in one case a reduction f r m 24 gsrlldnjlb of wool to 3.3 gallonflb was made dsid five lots of water at a short liquorfcloth fd

(D) Water consumption rule.

As a rule of thumb, ‘ the shorter the wet process sequence the lower will be the wa Valid parameters in optimizing a minimal water consumption inch flow rates, fabric speed, water leve

Excess

INDIAN J. ENVIROESMENTAL $R

FLOOR WASHIMG ETC *

wash boxes save wastes.

the counter current system of y adoptable in J-box bleaching, to do away with storage tanks.

of pumps with filters are required.

ge considerations.

been suggested that if a cotton mill with ow bleaching, using a caustic saturator J-

washer peroxide saturator-J-Box washer ace, reused water from the peroxide

washer in the caustic washer, upto 33% saving in water might be accomplished. System using two

e peroxide and caustic steps might

BCOW rinse to desize.

suggested reusing cahstic scouring rinses Us for desizing, Storing these wastes equalise them and allow the high BOD

released evenly to treatment. Such a bined waste could contain 60-90% of BOD in of the water volume.

'.Heat reclamation recommended.

i suggested that heat exchangers be used, particularly since reduction in

sually means an increase in effluent . By simply installing copper piping in

lines, some mills have preheated water he dye house. Meyer suggested that a plant with a waste water flow of 1,000 a temperature of 140 OF may expect

ngs in recovered heat from this water that uld be equal to 30,OOO 1 b steam an hour.

(I) Reuse cooling water. Wherever possible cooling water should be reused.

(J) f Memerising caustic recovery. as been suggested that if there is as much as tonne of NaOH available annually, at a mini-

strength of 2% its recovery by evaporation for reuse may be economically feasible. Dialysis another way to recover caustic, provides a purer solution for reuse but requires more sophisticated control and attention.

(K) Riggs-Lombard solvent system. John Stewart ( Northern Textile Association )

described the potential application of a new Riggs- Lombard solvent system for reducing pollution load, total water consumption, chemicals used, labour costs and time spent.

(L) Pressure methods reduce carrier use.

In view of the toxic nature and high BOD potential of dye carriers used in dyeing, it is relevant to point out the pollution advantage gained by the use of pressure-dyeing techniques.

(M) Ammonia may help.

Ammonia for use in desizing, scouring, bleaching, mercerising and dyeing of cotton is receiving consi- derable attention. There appears to be some justification on the surface for the use of liquid ammonia, but lack of information concerning product quality, corrosion, toxicity, etc., puts the justification for use of ammonia technique in the early developmental stage.

(N) Zero discharge in dye operations.

Low volume processes, such as microfoam, pad-dry and solvent dyeing are still in relatively develop- mental stages but should be noted for their pollu- tion advantages.

Textlle mill wastewater treatment

Textile mill wastes are readily amenable for treatment in combination with domestic sewage in municipal waste treatment plants. The wastes, however, require pre- treatment by chemical coagulation with chemicals, like alum or ferrous sulphate. But municipal sewage pro- vides not only dilution of the waste but also the requi- site nutrient substances, like N and P, which are essential for biological treatment. Treatment consists of equalisation, primary treatment, chemical coagulation and biological treatment.

Equalisation : Equalisation of the waste over a period of time is an essential step towards regulation of waste characteristics. For instance, batch dumping of strong wastes from Kiers, often results in sudden peaks in waste characteristics. The Kier wastes should segregated and contained in a holding tank of 8-12 hr capacity and released gradually at a uniform rate so as to preven't shock loading. Besides, equalisation of wastes also serves to self neutralise the acidic and alka- line waste streams.

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Primary treatment : Primary treatment consisting of screening, grit and oil and grease removal.

Chemical coagulation : This is essential for removal of the colloidal impurities, colour and settleable solids. The common coagulants used are alum, ferrous sulphate, ferric sulphate, ferric chloride and chlorinated cooperas. Coagulant aids are also employed waste streams from dyeing and bleaching process or their combined streams are easily amenable for treatment with chemicals.

Biological treatment : Combind wastes from textile mill are treated in combination with sewage in a single stage by the trickling filter or activated sludge or by a two stage treatment consisting of high rate trickling filter followed by activated sludge. Kier wastes from a holding tank, bleach liquor, winch dyeing wastes may be treated with alum in a clarifloc- culator and then mixed with neutralised and equalised rest of the wastes including sanitary wastes and treated in an oxidation ditch. Textile mill wastes are deficient in N and P. After nutrient addition can be treated in an oxidation ditch or aerated lagoons. Reactive dye waste should be segregated and treated separately.

TANNERY WASTE TREATMENT

There are about 1,000 tanneries in India processing about 12 million pieces of skins and hides ( 60 million kg ). About 30 to 40 lit of wastewater is discharged per kg of hide or skin processed. BOD varies from 40 to 100 gm/kg of skin or hide processed. As judged by BOD load about 73-75% from pretanning opera- tions, 24-26% froin tanning and 1% from finishing operations. From a chrome tannery the quantity of trivalent chromium wasted to drain is about 0.35 to 0.45 kg/100 kg of skins or hides processed. Objection-

Table 4. Characteristics of dirernt tannery wastes

able constituents from tannery wastes include salt, lime, sulphide, fleshings, hair, shavings, am- monium salts, tannin, chromium, fat liquors, dyes, etc.

Some of the important unit operations generating wastewaters are : soaking, liming and hair removal, deliming. bating, pickling, tanning ( vegetable or chrome tanning ), and finishing ( fat liquoring, dyeings, finishing ). The characteristics of wastes are given in table 4.

Water use reduction

In addition to good housekeeping water usage in tanneries can be reduced by : ( a ) adoption of batch washing as an alternaitve to continuous washing using the lattice door, ( b ) alteration of process and use of low float systems to use less water, ( c ) use of drums in place of pits, ( d ) adoption of steps liking liming in second soak waters, chrome tanning in pickling liquor, etc., ( e ) separation of cleaner fractions of the waste and reuse without treatment, and ( f ) recycling after complete or partial treatment. Avoiding spilla- ges, leaks, carrying out the processes in a regulated manner can bring in considerable reduction in waste volume.

Direct reuse of water is possible in a tannery since some wash waters are relatively clean as from the washing after bating, pickling, neutralisation and dyeing can be reused for dirty tasks, such as washing after soaking, liming and floor washings. Paddles and drums instead of pits reduce water consumption considerably. Batch washing reduce water usage by 50%. ?

Operation PH Chlorides Suspended solid BOD COD

Soaking 7.8 11,600 3,860 2,800 6,200 9,200 20,130 Liming 10.2 1,100 5,200

Deliming 7.5 300 200 5,300 1 1,480

Vegetable tanning 5.0 500 30,( 00 10,500 26,900

Dyeing 6.2 ’ 300 1,200 820 1,700

Composite wastes 8 .O 3,200 1,400 2,000 4,200

Chrome tanning 6.8 1,100 2,400 3,500 10,200

- j 102 INDIAN J. ENVIRONMENTAL PROTECTION, VOL. 5, NO, 2, APRIL 1985

@ 1985 - Kdpana Corporation

SaIt dust Purification and reuse

Green fleshings For glue and animal feed

Hair , Lime sludge

For carpet, drugget industry For building construction, soil conditioning

x, t imed fleshings and trimmings For glue and gelatin production Vegetable tan bark As fuel

Vegetable tannery sludge Vegetable and ohrome tanned shavings and splits EtlBuent sludges Dewatering and incineration

Fertilizer, soil conditioner For leather boards, for reducing chrome liquor

L

A m n t development in South Africa has been the recycling of water from effluent ponds for use in the beam house thereby achieving a 30% economy in water with an equivalent reduction in evaporation pond area required. By adopting this method the water require- ment is brought down to 11.25 lit/kg of hide or skin processed.

Recovery and use of byprodacts

Large qnantities of solid wastes are produced in a tannery. AS far as possible attempts should be made to separate, reclaim and use all solid wastes, such as hair, fteshings, trimmings, shavings, lime sludge, tan bark, tan liquor sludge, and tanned trimmings and shavings ( Table 5 ).

I

Proass modilatious to redace waste concentration

Pollutants let out from a tannery include : (a) such constituents of raw hide or skin which must be removed during processing, like salt, fat, hair, non-collagenous protein, etc., (b) pollutants arising out of washing of floor and machines and sanitary use, and (c) the chemicals used in the process. It has been normal practice for tanners to use surplus amount of chemicals in various processes. It is possible to reduce these amounts of chemicals without affecting the quality of leather.

Thorough dusting and collection of dusted salt and preventing it from gaining access into the effluent can reduce the salt content of wastewater. Salt pollution

can be minimised by rousing the used salt for curing or using alternate curing agents. Dehairing process using mixed enzymes was developed recently. Jn this process the BOD of the effluent obtained is reduced by 50-70%. The use of drums for vegetable tanning is found to allow smaller floats and give higher fixation of vegetable tannins compared to pit vegetable tanning systems. LIRITAN process developed by Leather Industries Research Institute of South Africa involves very low effluent discharge from tanning.

Tn the case of chrome tanning, a reduced concentration of chromium in the effluent can be achieved by : (a) improving the fixation of chromium,(b) reuse of chrome liquor, and (c) precipitation of chromium from chrome liquor with alkali and redissolving and rinsing the chrome precipitate. Improving the fixation of chrome can be achieved by : (a) decreasing the acidity of the pickle and increase of basification after the tanning, (b) decrease of the amount of neutral salt present during the chrome tanning, (c) decrease of the volume of tanning liquor and chrome dosage, (d) increase of tanning temperature at the end of tanning and tanning time, and (e) presence of certain chemicals, like oxozolidine during the tanning. Judicious pH adjust- ment allows complete fixation of reagents during dyeing and fat liquoring.

Tannery wastewater treatment

Primary treatment : Studies carried out on full scale fill and draw settling tanks and sludge drying beds at Kanpur showed that a reduction of about 79-80% of

INDIAN J. ENVIRONMENTAL PROTECTION, VOL. 5, NO. 2, APRIL 1985

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suspended solids and 20-30% of BOD is effected in settling tanks. Tannery effluent sludge can be dried to 45% moisture in 4 to 6 day in summer and in 6 to 8 day in winter. Equalisation-cum-settling tank of 12-24 hr duration is recommended.

Chemical treatment : Tannery effluents can be treated by chemicals, like alum, COS from flue gas, HISO,, ferric chloride and lime. Good reduction in suspended solids and BOD reported but cost is high and presents problem of sludge disposal.

Hydrogen sulphide removal : The sulphide content of liming effluent could be greatly reduced by aeration with diffused aeration in presence of catalysts, like manganous salts.

Biological treatment : Tannery effluents can be treated satisfactorily either alone or in admisture with sewage by using biological methods, like anaerobic lagoon followed by aerated lagoon or oxidation ditch or activated sludge process.

INDIAN J. ENVIRONMENTAL PROTECTION, VOL. 5, NQ. 2, APRIZ, Ip@2 Q 1985 - Kalpam Corgozrrtioa