Effects of enzymatic pre-treatment on aerobic and anaerobic digestion of wastewater sludges

Abstract

Aerobic and anaerobic digestion processes aim to degrade the organic matter content of the sludge, remove pathogenic microorganisms from sludge, and produce a stabilized sludge. In the last years, pre-treatment of sludge prior to the digestion process gained more importance due to the potential of increasing sludge solubilization and minimization. This study investigates the effect of enzymatic pre-treatment application prior to the aerobic and anaerobic digestion processes of wastewater sludges. For this purpose, sludge samples were obtained from four different urban and domestic wastewater treatment plants. Digestion tests were carried by operating one control reactor and one enzymatically pre-treated reactor for each sludge sample obtained. An enzyme mixture consisted of alpha-amylase, beta glucanase, lipase, protease, and celllulase was added to the sludge samples 0,5% by volume. The results of the study showed that the enzymatic pre-treatment plays an important role on the performances of the digestion processes. In aerobic digesters, minimization of sludge production was enhanced by the application of enzymatic pre-treatment. Enzyme added reactors showed better TS, VS, MLSS, and MLVSS removals by 43%, 51%, 44%, and 46% reduction in average, compared to the removals of control reactors by 24%, 31%, 28%, and 32% respectively, in average. Enzymes had great impact on the solubilization of large molecular compounds into smaller molecules by the destruction of cell structure. Compared to the control reactors, enzymatically pre-treated reactors showed better performances on the solubilization, which is determined by increased sCOD and DOC concentrations. Enzymatic pre-treatment also affected the release of extracellular polymeric substances from sludge flocs and enzyme addition led to an increase in the protein and carbohydrate contents of sludge. During the digestion process, COD, sCOD, and EPS contents of the sludge samples decreased, where pre-treated reactors showed better removal efficiencies. The dewaterability of sludge samples was deteriorated when they were enzymatically pre-treated at the initial stage. Then, by the aerobic digestion and the degradation of EPS, dewaterability was improved. Similar results were obtained for the anaerobic digesters in terms of sludge minimization and solubilization. In addition, enzymatic pre-treatment prior to the anaerobic digestion resulted in improved biogas production and increased methane potential. In both digestion processes, removal of pathogens was successfully achieved. At the end of the aerobic digestion, the reductions in TC, FC, and FS were about 99,78%, 99,84%, and 99,82%, respectively. At the end of the anaerobic digestion, the reductions in TC, FC, FS, and salmonella were found to be about 99,82%, 99,96%, <99,99%, and 99,98%, respectively. During the study, both urban and domestic wastewater sludge samples were tested but no relationship was found between the source of the sludge and the digestion process.