VPE 511: Environment and Environmental Hygiene (2+1)

TREATMENT OF TANNERY EFFLUENT

• Treatment of tannery waste water is required prior to discharge. The major prevention measures of tanning are flashing of hides after washing, recycling of spent chrome tanning solution, recycling and decontamination of toxic chemicals removing toxic sulphides and chromium. In all the processes dealing with solid and liquids, it is very important to define the optimum conditions to carry out the solid-liquid separation.
• The studies involve the definition of the type pollutant and treatment methods, including physical, chemical and biological methods. Sulphides are deadly toxic materials and must be destroyed chemically. The normal treatment system in the industry is to collect the entire sulphide containing wastes, and then oxidise the sulphides with air with a manganese suphate catalyst.
• Lime solution, free of sulphide, can be used to neutralize the acid wastes to adjust the pH to the acceptable range. Chromium present in very high concentration together with pentachlorophenol needs special attention for removal from the effluent.

Primary treatment of the effluent

• The mixing of the acid and alkaline wastes at a control pH results in the coagulation of the suspended solids.
• The removal of the coagulated materials by primary treatment results in a decrease of the suspended solids by about 80 percent and BOD by 50-70 percent.

Chemical treatment of the effluent

• Several physicochemical processes have been developed for the removal of pollutants from industrial effluents. Various chemical techniques such as coagulation, precipitation and reverse osmosis etc. are used to remove PCP from effluents.
• A chemical method for oxidative degradation of PCP in soil, under unsaturated condition and neutral pH was developed. Reagents used were heme (Fe++) as a catalyst and hydrogen peroxide as on oxidant. Heme and peroxide (Fenton reagent) could degrade PCP efficiently in a short period of time, either in liquid or unsaturated soil systems. However, the problems underlying the industrial non-acceptability of the physicochemical treatment technology are those associated with cost and reliability.
• Sulphides are deadly toxic materials and must be destroyed chemically. The normal treatment system in the industry is to collect the entire sulphide containing wastes, and then oxidize the sulphides with air and manganese sulphate catalyst. The lime solution, free of sulphide, can be used to neutrlise the acid wastes to adjust the pH to the acceptable range.
• Traditionally, chromium removal was made by chemical precipitation, oxidation / reduction, ion exchange, filtration, membranes and evaporation of all which are extremely expensive.
• The toxicity of heavy metal chromium is associated with a particular form of that metal, called Gr (IV). The toxicity may be removed by converting Cr (IV) to Cr (III), a non-toxic form of the metal, by adding more electrons to the metal through a chemical process called ‘reduction”.