WASTE MANAGEMENT IN LEATHER INDUSTRY

36 49wastewater streams, in particular for sulfide- and chrome- containing liquors. Andalthough a reduction of water consumption dose not reduce the load of manypollutants, concentrated effluents are often easier and more efficient to treat.Where segregation of flows is possible, through mixing of chrome- bearingeffluents and other effluent streams improves the efficiency of the effluenttreatment plant because the chromium tends to precipitate out with the proteinduring pretreatment.The sulfides in the deliming and pickling liquors can easily be oxides in thedrum adding hydrogen peroxide, sodium metabisulphite or sodium bisulphite.The associated emission level after treatment of sulfide is 2 mg/ l in a randomsample in the separate effluent. Where segregation of sulfide- bearing liquors isnot possible, the sulfides are generally removed by means of precipitation withiron (II) salts and aeration.[Treatment of Tannery Wastewater, (April 2002), from Protrade/GTZ “Ecologyand Environment in the Leather Industry- Technical Handbook”, Eschborn,1995,]6.2.1 Activated SludgeDuring a biological treatment by activated sludge, the wastewater to be treatedis introduced into a bank aerated by mechanical stirring or by compressed air.Here it mixes with the mass of bacteria flock maintained constantly insuspension. After sufficiently contact time, the mixture is clarified in a settlingpond and sludge is recycled in an aeration tank. The excess sludge from thesystem is treated with primary sludge.6.2.2 Aerated LagoonsAn aerated lagoon is an earthen basin in which the oxygen required by theprocess is supplied by surface aerators. In an aerobic lagoon, all the solids aremaintained as suspension.