Michael Recktenwald

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.

Enhanced efficiency of industrial-scale anaerobic digestion by the addition of glycosidic enzymes

The results of the action of two glycosidic enzymes added to an anaerobic digester processing mixed wastewater sludge are reported in this paper. The experiment was performed at a continuously operating full-scale wastewater treatment plant during a 6-month period. The addition of the enzyme mixture resulted in improved gas production and dewatering properties. Dewatering experiments, both on enzyme-treated and reference sludge on the local centrifuges, indicated that the carbonaceous matter was decreased by 7% in enzyme-treated sludge, and the average dry solids increased from 27% (reference) to 31% in enzyme-treated sludge. The polymer dosage was decreased from 8 to 5 kg/t dry solids. Additionally, practical experiences were gathered concerning the transformation from static laboratory-scale batch experiments to a dynamic continuous industrial scale setup. Preliminary cost calculations showed beneficial aspects and economical feasibility of enzyme addition to an anaerobic sludge digestion process.

Chemical sludge conditioning in combination with different conventional and alternative dewatering devices : Chamber filter press, decanter and Bucher press

The Kemicond process for sludge conditioning consists of chemical treatment with sulphuric acid and hydrogen peroxide at a pH-value of approximately 4 followed by a dewatering unit. It is shown that chemical treatment can improve the dewaterability of ferruginous digested sludge. It is concluded that the Fenton process as well as the oxidation of organics and the formation of iron hydroxo complexes are important reaction mechanisms. Furthermore, the organic matter changes through the acidic oxidative process. With the improvement in dewaterability, it is possible to achieve an increase in TS concentration, which affects a reduction of the sludge volume. Cost savings for sludge disposal can amortize the additional investment and operational costs for chemical treatment.