Studies on co digestion of tannery fleshings primary sludge and secondary sludge

Abstract

Today tanneries in India face the twin problem of managing process solid wastes and effluent treatment plant sludge. Every tannery faces this enormous problem of managing the disposal of 75 tonnes of fleshings and 100 tonnes of sludge annually. With the inputs from the optimization studies, a detailed codigestion studies were carried out using volatile solids input of 38, 45, 53, 60, 68 and 145 grams in order to study the effect of semi-continuous feeding of the organic matter on the co-digestion process. It was observed that at the volatile solids load of 68 grams per cycle the biogas generation per gram of VSadded was 470 mL. At this volatile solids load, the maximum percentage volatile fatty acid reduction was observed and hence the volatile solids load of 68 grams per cycle was found to be optimum. It was found from the performance evaluation data on volatile fatty acid to alkalinity ratio that the stable conditions prevailed during co-digestion of tannery wastes. Enzyme application is an option to hasten the digestion process and in the present study one such enzyme namely steapsin, a commercial grade lipase, was used. Hence the present study also covers the effect of application of lipase on digestion process. Hence, lipase addition helps in reducing the residence time thereby reducing the reactor volume. In overall terms, the feasibility of co-digestion of fleshings along with the primary sludge and secondary sludge has been demonstrated satisfactorily for the first time. In the event of implementation of the findings of the study in the field, the entire fleshings generated in the tannery on the daily basis can be utilized as the main substrate, thus the solving the problem of fleshings management. However, only a part of the primary sludge and secondary sludge generated in a tannery could be utilized as part of co-substrates. Hence, the remaining amount of the primary and secondary sludges requires appropriate management.