Study of concentrations of volatile fatty acids and potential methane during wastewater sludge digestion

Abstract

Anaerobic degradation of organic matter in wastewater is one of the means of generating biogas (methane) which is a source of fuel. Studies have established that volatile fatty acids (VFAs) can be adopted as indicators of process stability during anaerobic digestion. This study therefore aims at investigating changes in the concentrations of volatile fatty acids at 25^oC and at 37^oC during the anaerobic reduction of domestic wastewater sludge (DWS). Five litre composite samples of two substrates, primary and secondary sludge, were obtained from various domestic wastewater treatment plants in Scotland. Digested sludge was used as source of anaerobic microbial biomass in some of the tests, after degassing for 48 hours by incubating at 37^oC. Total solids, volatile solids, volatile fatty acids concentrations and pH were determined before and after the biochemical methane potential (BMP) tests. The results revealed higher reduction at 37°C than at 25°C for the volatile solids of both the primary and secondary sludge. Although the concentrations of VFAs in the tests with anaerobic biomass were substantially reduced compared to tests without anaerobic biomass, however, the tests without anaerobic biomass produced more methane per volatile solids reduced. The results indicate a more efficient conversion of the substrates to methane in anaerobic digesters with low anaerobic biomass to substrate ratio, provided the process is optimized to ensure efficient conversion of the available VFAs to methane.