Industrial sectors are divided into 3 categories according to the extent of pollution they can cause and the polluting nature of the wastes and effluents generated from them. The industries are categorized as Low polluting, Moderately polluting or Most polluting.
The tannery industry belongs to one of the most polluting industrial sectors. Almost every tannery industry uses significant amounts of chemicals in the process of transforming animal hides into leather.
Stages of Tanning
Liming and unhairing
De-liming and bating
Chrome-tanning (Chrome splitting)
Re-tanning and dying
Post-tanning and finishing process
Wastes from Tannery industries (Generated stagewise)
In the soaking stage, the most polluting stage of the tanning process contributes around 50–55% of the total pollution load of the tanning industry.
In the liming stage, protein, hair, skin and emulsified fats are removed from the hides, they are released in the effluent and increase its total solids contents.
The effluents from the tan-yard processes, de-liming and bating contain sulfides, ammonium salts, and calcium salts, and the effluent is slightly alkaline.
The pickling and chrome tanning effluents contain sulphuric acid, chrome, chlorides, sodium bicarbonate and sulfates.
The major pollutants of the post-tanning processare chrome salts, dyestuff residues, fatliquoring agents, syntans and other organic matter.
Nature of wastes
The tanning process is almost completely a wet process that consumes significant amounts of water and generates about 90% of the used water as effluent. Effluents from tannery industries carry heavy pollution loads due to a massive presence of highly colored compounds, sodium chloride and sulfate, various organic and inorganic substances, toxic metallic compounds, different types of tanning materials which are biologically oxidizable, and large quantities of putrefying suspended matter.
Auditing reports have found out that only about 20% of the large numbers of chemicals used in the tanning process is absorbed by leather and the rest is released as wastes. The tannery effluent damages the normal life of the receiving water bodies and land surface.
Characteristics of wastes
All effluent samples from the tannery industry were measured for parameters like pH, Electrical Conductivity (EC), Total Suspended Solids (TSS), Total Dissolved Solids (TDS), Biological Oxygen Demand (BOD5) in 5 days at 20 °Cand Chemical Oxygen Demand (COD). The final composite tannery effluents (wastewater) contain a high load of organic matter, dissolved and suspended solids, organic nitrogen, and ammonia, and possess high pH.
The pH values of the effluents at various leather manufacturing stages ranged from 3.8 to 12.5 depending on the strength and amounts of the reagents used. The low pH of the chrome-tanning effluents may be due to sulphuric acid added during the pickling stage for preparing the pickled pelt.
The highest mean pH was found in liming effluents because of the excessive use of lime and sodium sulfide in the production of the lime pelt.
TDS and TSS values
The Total Suspended Solids (TSS) of the effluents at different leather manufacturing stages were in the range approximately 830 to 11000 mg/L. These values are evidently higher than the permissible standards limits.
The mean Total Dissolved Solids (TDS) concentrations of effluents at different manufacturing units in the selected industries ranged from 4100 to 156700 mg/L. The high value of 156700 mg/L was due to pickling and chrome tanning and the low value of 4100 mg/L to fatliquoring. These levels of TDS values obtained in all samples were higher than the prescribed limits of 2100 mg/Lsuitable for discharging effluents into surface water.
BOD and COD values
The mean BOD5 and COD of the effluents at different manufacturing units of the selected industries were in the range 700 to 2400 mg/L and 7200 to 71040 mg/L, respectively.
The higher values (2400 mg/L and 71040 mg/L), belonging to re-tanning and dying effluents (stage 5) was due to the addition of many organic vegetable tanning materials and inorganic chemicals. The lowest values (700 mg/L and 4200 mg/L) belong to de-liming and bating effluents (stage 3).
Pollution and health concerns
Human health can also be affected by toxic hazards generated through the unskilled and unprotected handling of pesticides, tanning chemicals, and treated hides and skins. The lack of effective implementation of legislative control, poor processing practices and use of unrefined conventional leather processing methods have further aggravated the pollution problems.
The pH of this basic effluent was far above the standard limits. A large fluctuation in pH value exerts stress on the aquatic environment and may adversely affect some sensitive species of aquatic flora and fauna. The high levels of TDS in the effluent render it unsuitable for irrigation.
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