The following article will guide you about how to control pollution in tannery industry.
Control of Gaseous Pollution in Tannery Industry:
Source control is the most effective means of abating odour. Good sanitation practices are usually cheaper than control measures.
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Source control of odours can be carried out by following methods given below:
(i) Drawing the odorous air from the working atmosphere by exhaust fans and diluting with relatively clean air.
(ii) Removal of causative impurities from the tannery.
(iii) Masking the odour with objectionable additives.
(iv) Removal of odour bearing dusts by cyclone separators.
(v) Sorption of odorous gases through a granular sorbents like active carbon.
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Chemical air scrubber can be used to eliminate the odour nuisance in the tannery wastewater treatment plant. Odour masking chemicals can be added. Masking agents can also be used to control odours outdoors in such places as waste lagoons.
The smell caused by putrefaction of solid and liquid wastes generated during tannery operations can be reduced by quickly disposing them off without allowing for putrefaction.
Aerobic biological methods of effluent treatment and dewatering of sludge produced by methods like vacuum filtration, drying on sand drying bed will considerably reduce the smell caused by them.
Control of Water Pollution in Tannery Industry:
Tannery effluents if disposed off without any treatment either on land or in inland surface waters may create severe problems leading to damage of the environment. Tannin, trivalent chromium, proteinous matter, sulphides and high BOD/COD of the wastewater call for proper treatment.
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Tannery wastes in general, can be treated to get the desired end results in the following main stages:
(i) In-plant measures.
(ii) Primary treatment.
(iii) Chemical treatment.
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(iv) Secondary treatment
(i) In-Plant Control Measures:
In-plant measures include reduction of water consumption in the tannery, process modifications to reduce pollutional load of the waste being generated, segregation of soak liquor and lastly, recovery of the by-products and reuse of various process liquors.
Reduction of Water Usage in a Tannery:
To reduce the volume of the effluent, the water usage in tanneries can be considerably reduced by:
(i) Better housekeeping
(ii) Alteration of processes and low float systems to use less water
(iii) Separation of cleaner fractions of the waste for direct reuse without treatment
(iv) Recycle after complete or partial treatment
Process Modifications to Reduce Pollutional Load:
The quantities of pollutants which are inevitable in tannery wastewater are as follows:
Thus the total quantity of inorganic pollutants that are inevitable is 150 g/kg of hide or skin processed and the quantity of organic solids is 90 g/kg. In a normal tannery in India the pollutants are more than the theoretical minimum.
Therefore, there is a lot of scope for reducing the quantity of pollutants discharged from a tannery by adopting some of the measures viz.-
(i) Waste reduction during hide preservation
(ii) Waste reduction during beam house operations
(iii) Waste reduction during vegetable tanning
(iv) Waste reduction during pickling and chrome tanning
(v) Waste reduction from post chrome wet process
(vi) Reuse of process liquors.
a. Segregation of Salt Laden Effluents:
The high concentration of cationic portion of salts or minerals is reported to exert salt toxicity, thereby seriously interfering with the biodegradation of waste.
b. Recovery and Utilisation of By-Products:
The various wastes generated by the tannery have great potential for their reuse.
Tannery unhairing effluent and acid whey are the two waste liquids individually difficult to dispose off or to utilise, but if combined together, useful products can be recovered from this combination. These precipitated products have high contents of essential amino acids such as cysteine, lysine, valine, leucine etc.
(ii) Primary Treatment:
Most of the tanneries in India do not have any treatment facilities. In tanneries where treatment of effluents is carried out, it is limited to the mixing of the various effluents, followed by sedimentation.
Even those operations are not carried out satisfactorily. Screening for removal of coarser impurities, hair and fleshing followed by settling for atleast 4 hours in a continuous flow settling tank form the essential primary treatment of tannery wastewaters.
A simple system of three earth lagoons, each capable of holding an entire day’s flow has also been studied. They are operated in rotation on the fill and draw principle, filling during one day and discharging over the next 24 hours period, thus leaving the third day available for sludge removal. In a system of this type, removal of 80-90% of the suspended solids and 40-50% of the BOD has been reported.
Experiments on sedimentation with controlled pH indicated that pH of 8 is the optimum for sedimentation of the suspended solids, the removal of which in the form of COD is of the order of 45%. Tannin removal was also maximum at this pH although it was only 18%.
Primary sedimentation is highly desirable where subsequent biological treatment of the wastes is proposed in order to reduce the size of the biological stage where space is restricted. Settling is most conveniently effected in specially designed tanks.
These tanks may be of horizontal or vertical flow type. Although more difficult to construct, vertical flow tanks have the advantage of being self desludging when provided with side sloping at an angle of 60 degrees, sludge being removed via a valve fitted at the base of the tank.
(iii) Chemical Treatment:
Tannery effluent can be treated by chemical coagulants like alum, carbon dioxide from flue gas, sulphuric acid, ferric chloride and lime. In spite of the excellent results shown by some of these chemicals, chemical treatments are not widely adopted because of heavy cost and sludge handling problems involved in these methods.
A comparison of capital costs and annual operation costs for pre-treatment systems in class B tanneries reveals that the annual operation cost of system employing ferrous sulphate is substantially lower (0.33 times) despite higher capital investment (1.4 times) as compared to the system employing alum for comparable COD and tannin reduction (55 – 65% and 52 – 68% respectively).
Ferrous sulphate is not suitable for coagulation of vegetable tanning wastes, since it forms an ink with the gallic acid in the wastes giving them an intense black colour. If necessary, these wastes are treated with aluminium sulphate, which is much more effective if the wastes are previously acidified.
Experiments with potassium permanganate, alum, carbon dioxide and polymers showed that addition of alum was a cost effective means of reducing suspended solids. Alum doses of 250 mg/L can be utilised to coagulate combined beam and tan wastes and achieve 85% TSS and 62% BOD reduction.
The large quantity of lime present in tannery wastewaters provides sufficient alkalinity for floc formation and particle entrainment. The utilisation of passive tube settlers and rapid sand filtration further enhanced solids removal.
Most discrete leather particles and hair are easily removed by simple sedimentation, but proteins, metals and lime can only be precipitated by chemical flocculation. Some of the various coagulants referenced in the literature and the removal efficiencies attained with tannery wastewater are summarised in Table 21.7.
The removal of chromium from chrome liquor could be obtained by adjusting the pH of the wastewater using lime liquor. The optimum pH for effective removal of chromium from chrome liquor is 10.5. The percentage removal of chromium at pH 10.0 is about 96%.
Out of various alkalies such as lime, NaOH, Na2CO3 and NH4OH, lime was appeared to be the cheapest alkali for the economical removal and recovery of chromium. Using lime, about 98% chromium removal was possible at a pH value of 6.6.
Alum and ferric chloride for the chemical treatment of vegetable tan liquor and also of composite tannery waste gives fairly good removals of COD and tannin.
(iv) Secondary Biological Treatment:
Low cost technology and conventional treatment systems (both aerobic and anaerobic or in combination) can be employed for the secondary biological treatment of equalised and settled tannery wastes.
Tannery effluent can also be satisfactorily treated in admixture with sewage in a sewage treatment plant provided the proportion of tannery effluents is not high. Comparative statement for alternative type of treatment systems is shown in Table 21.8.
1. Aerobic Systems:
Attempts have been made in treating tannery effluents in an oxidation pond, but its application on full-scale operation is still awaited.
High rate biological filters provide a convenient means of removing a large proportion of the biologically oxidisable matter in a relatively small volume of plant for tannery wastewater.
Biological filters filled with stones and provided with recirculation can achieve removals of 70-90% BOD when loaded with rates as high as 2 kg BOD/m3/day. Effective under drainage is essential to collect percolated liquor and just as important the free access of air up the tower, as the process is aerobic.
Treatment of settled and 100% diluted vegetable and chrome tannery effluents on trickling filter has been studied in India. The average 5-day BOD value of pre-treated vegetable tannery effluent applied to the filter was 900 mg/l and the BOD of the filter effluent was in region of 158 mg/l at a hydraulic loading of 68 × 106 l/ha/day or 6 MGAD and without recirculation.
It was interesting to note that settlement of the filter effluent for 24 hours further brought down BOD to 56 mg/l. With the chrome tanning effluents, BOD was reduced from 821 mg/l when the filter effluent was settled for 24 hours. It was reported that practically complete removal of sulphides and chromium was effected by this treatment.
Tannery effluents can be treated successfully by activated sludge process. But until recently the principal drawbacks with this ‘process were the long aeration period required for any appreciable reduction in BOD.
Activated sludge process at an aeration solids concentration between 4000 to 6000 mg/l the BOD of the pre-settled and diluted vegetable tannery waste could be reduced from 988 mg/l to 684 mg/l with 6 hour aeration to 370 mg/l with 12 hour aeration, to 162 mg/l with 18 hour aeration and to 34 mg/l with 24 hour aeration. With chrome tannery waste equivalent reduction in BOD was obtained.
Case I – Constant feed of tannery wastewater alone. Tannery wastewater to be pretreated during initial period of operation. Equalisation basin to be provided. Influent BOD – 800 mg/l.
Case II – Varied feed of tannery wastewater alone. Tannery wastewater to be pretreated during initial period of operation. Influent BOD – 800 mg/l.
Case III – Varied feed of tannery wastewater plus constant feed of municipal wastewater. Influent BOD – 460 mg/l.
Oxidation ditch gives good performance in the secondary treatment of tannery wastes. The tannery waste could be successfully treated in oxidation ditches to reduce the BOD from 1000 to about 20 mg/l.
Oxidation ditch has an added advantage that it is capable of giving higher BOD removals (upto 98%) than the other similar treatment methods. It is estimated that the initial cost of an oxidation ditch including mechanical equipment and civil works could be about 45 to 50% of that for a conventional aerator using air blowers and diffusers.
2. Anaerobic Systems:
Some of the tanneries are using anaerobic lagoons with success. Experiments on anaerobic lagoon with the settled wastewater gave percentage reduction varied from 42.4% at a BOD loading of 0.41 kg/m3/day to 85.5% at a BOD loading of 0.14 kg/m3/day.
It was found that ten days detention time would be sufficient to bring BOD reduction of about 85%. If the final effluent has to be discharged on land, it would be sufficient to treat the settled waste in anaerobic lagoons for 10 days. Anaerobic lagoon treatment could reduce BOD by 88.5% but colour removal could not be obtained.
Anaerobic treatment of vegetable tannery wastewater is practically feasible at shorter time using fixed film reactors. The success in treatment is strongly dependent on the high concentration of active biomass retained within the reactor and on provisions of sufficient contact time between active biomass and wastewater.
The increased sludge retention enhances the rate of conversion of organic matter to methane and carbon dioxide. The reactors, having different methods of sludge retention investigated for the treatment of vegetable tannery wastewater corroborate the above facts.
3. Combined Process:
For complete treatment to meet the standards of discharge into river, anaerobic- treatment followed by aerobic process is necessary. The treatment of raw tannery wastewater having BOD concentrations of 2000 – 4000 mg/l is not economically feasible to treat by aerobic method due to oxygen transfer limitations.
The effluent produced after anaerobic treatment of tannery wastewater has COD and BOD concentrations permissible for discharge into public sewer. The anaerobic pretreatment with faster conversion rate at shorter time with added advantage of biogas generation will be a better approach to opt before aerobic polishing treatment.
In view of the promising results obtained in the above mentioned experiments more systematic and detailed evaluation of anaerobic pretreatment of tannery wastewater followed by aerobic treatment is necessitated to develop necessary design criteria for full scale treatment facility.
To overcome the difficulties of fixed-film reactors, the feasibility of anaerobic-aerobic concept of tannery waste treatment was further studied by the use of an anaerobic contact reactor in series with an aerobic activated sludge.
Anaerobic-aerobic reactor system exhibited the ability to provide 98% removal of suspended solids, 86% removal of COD and 70% removal of sulphides from raw combined tannery beam-house wastewater without pretreatment.
The conclusions drawn from anaerobic-aerobic treatment of tannery waste includes:
(i) Cost required for primary treatment (chemical cost) can be eliminated by directly taking raw waste after equalisation for anaerobic treatment.
(ii) Production of methane gas is high enough to be of value as non-conventional energy could potentially benefit tanners to offset some of the energy costs in the tannery.
(iii) In aerobic reactor, aeration time is much reduced as compared to the time required for direct aerobic treatment of tannery wastewater, thereby reducing electrical energy required for aeration.
Treatment through Non-Conventional Methods:
Use of non-conventional methods for tannery waste treatment includes:
(i) Use of water hyacinth.
(ii) Use of fungi.
(iii) Sand filters.
(iv) Agricultural utilisation.
(v) Disposal by spray irrigation.
Solid Waste Disposal:
The solid wastes problem arises in tanneries both in the tanning process and also during effluent treatment. Solid wastes from the tanning process are hair, fleshings, various leather trimmings, shavings etc. and lime sludge from liming pits.
The hair, trimmings and fleshings have various uses and are reclaimed as detailed in the following Table 21.9. The lime sludge is usually dumped but it can be used for reclaiming alkaline soils. Environmentally sound utilisation practices of solid waste disposal are presented in Table 21.9.
Environmentally sound processes of utilisation or disposal of solid wastes from tanneries are:
(i) Salt dust can be purified and reused
(ii) Green fleshings can be dried and immediately used for glue and animal feed manufacture etc.
(iii) Hair after washing, drying can be used in carpet industry etc.
(iv) Lime sludge – for building construction, soil conditioning.
(v) Limed fleshing and trimmings for glue and gelatine manufacture, animal feed, dissolving with the help of live steam for disposal.
(vi) Vegetable tan bark as fuel
(vii) Vegetable tan sludge as fertiliser soil conditioner
(viii) Vegetable and chrome tanned shavings and splits. Manufacture of leather boards, reducing chrome liquors etc.
(ix) Effluent sludges can be dewatered and incinerated.
Destruction by incineration is an effective method for solid waste disposal that yields minimal final residues and is attractive on account of the potential recovery of heat from the process.
During treatment of tannery wastes by conventional methods, biological sludges are produced in large quantities reaching 15% of the green weight of hides in terms of dry matter.
Sludge separated needs further processing without which the disposal process would not be complete. Sludge processing involves thickening, chemical conditioning, filtering through vacuum filtration and eventual disposal.
As far as the disposal of sludge is concerned, utilisation of the latter in agriculture has proved most useful. Dewatered sludge can be used for fertilising in doses of 300 – 400 g/ha. The amount of chromium contained in sludge should not exceed 1000 kg/ha.