Anaerobic mesophilic digestion of sludge with extra-thermophilic and high ph pre-treatment

Abstract

In this study different methods of sludge pre-treatment with elevated temperatures andpH have been selected for investigation. Five sets of sludge samples were pretreated asfollows: heating at 70°C for 30 min, at 80°C for 20 min, at 90°C for 10 min, at 100°C forWWTP5 min and NaOH-treatment (pH 12 for 4 hours). For comparison a sample from Tallinn(3:1 mixture of primary (dry solids 5.2%) and activated sludge (dry solids 0.5%))was used. Extra-thermophilic pre-treatment increased the degree of hydrolyses of sludge,enabling the following anaerobic digestion process to proceed faster than that of rawsludge. However, extra-thermophilic pre-treatment was insufficient for removal ofnitrogen and phosphorous. Nitrogen can be easily solubilized during extra-thermophilicpre-treatment but solubilization of phosphorous occurs only through digestion. Theproduction of biogas during anaerobic digestion was also dependant on the quality of theinput of pre-treated sludge. The highest cumulative volume of biogas (124 mUg CODadded) was achieved by pre-treatment at 80°C. The percentage of mineralization of pretreated sludges on mesophilic digestion was the highest with 90°C sample - 65.9%.