The secondary treatment comprises two methods viz. activated sludge and bio composting method. Activated sludge process of destroying the COD/BOD comprises of treating the bio methanated effluent in aerobic lagoons where aeration is done mechanically and the activated sludge provides the bacterial action. This, however, does not result in zero effluent discharge or colour removal although COD/BOD may be brought down to the acceptable limits prescribed by CPCB. Besides it is highly energy intensive.

Bio composting makes use of the heat generated by the exothermic reactions of destroying the bio degradable constituents of the bio mass by aerobic reaction using thermophylic bacterial inocculum, to evaporate water from the bio methanated spent wash. Generally press mud from sugar mill with 70% moisture is used as bio mass and 2.5 T of water can be evaporated per tonne of press mud under most well modulated operation of the bio composting process. In view of the limited quantity of press mud availability vis-a-vis the production of spent wash, it is necessary to reduce the quantity of spent wash by physical evaporation in a multiple effect evaporator or in a reverse osmosis system. These once again involve heavy energy requirement although a valuable bio fertilizer, if marketed, judiciously, can generate enough remuneration to compensate for the energy consumption. This however, is very cumbersome and messy operation. Besides it involves a vast area (about 20 acres of land for a 100 KLD distillery) for composting. Also it involves a huge storage for the spent wash when the biocomposting operations would have to be stopped during rainy seasons. The only attractive aspect of the biocomposting technology is that it does result in zero liquid pollutant discharge.

The other technology for treatment of spent wash for pollution free and energy efficient situation is to incinerate the spent wash in a high pressure boiler and generate steam as well as power. In order to enable the spent wash to be burnt as a liquid fuel in the burner it is to be concentrated to a minimum of 60% TDS strength. It is observed that the spent wash at 60% TDS, has a gross cal. value of 3000 to 3300 K Cals per litre of concentrated spent wash. A high pressure boiler is envisaged in the distillery with a few modifications to burn the spent wash in as much as the ash constituents of spent wash have a low melting point (as low as 500C) and its vapors tend to deposit on the heating surface of the boiler. This technology has been in practice in western countries. Recently two plants based on incineration technology are commissioned in India and are in the process of stabilization. This technology is complete both for energy recovery and environment protection. As far as investment is concerned it is at par with other technologies.