Solubilisation of sludge by combined chemical and enzymatic treatment

In this study, the effects of cation-binding agents used alone and/or in combination with enzymes on solubilisation of municipal sludge and structure changes were investigated. Formic acid, citric acid, tartaric acid, EDTA, sodium tripolyphosphate (STPP), Zeolite A, sodium fluoride, sodium thiosulphate or sodium silicate were added to both biological and digested sludges. Citric acid ( 50 mmol/l) released the highest COD, amounting to 8 g/l from bio-sludge and 3 g/l from digested sludge. The highest specific dissolution rate was 0.5 g COD per mmol citric acid. COD released by STPP (50 mmol/l) was 3.3 g/l from bio-sludge and 2 g/l from digested sludge. STPP acted most efficiently to reduce suspended solids, 20% for digested and 40% for bio-sludge. The pre-treatment by the sequestering agents was followed by addition of three glycosidic enzymes. The used enzymes were more effective in hydrolysis of bio-sludge than in hydrolysis of the digested sludge. Additionally, after 4 h of incubation the remained enzymes activities in enzyme treated sludges were improved by up to 20%, indicating high stability of added enzymes.