Biogas recovery from microwave heated sludge by anaerobic digestion

Abstract

Biogas generated from sewage sludge, livestock waste, and food waste by anaerobic digestion is a valuable renewable energy resource. However, conventional anaerobic digestion is not an efficient process. A long hydraulic retention time and low biogas recovery rate hinder the applications of those resources. An effective pretreatment method to destroy sludge microbial cells has been one of the major concerns regarding improvement of the biogas production. This article focuses on the effects of microwave heating on sludge anaerobic digestion. Volatile suspended solid (VSS) and chemical organic demand solubilization of heated sludge were investigated. Microwave heating was found to be a rapid and efficient process for releasing organic substrates from sludge. The increase of organic dissolution ratio was not obvious when holding time was over 5 min with microwave heating. The effect of the VSS solubilization was primarily dependent on heating temperature. The highest value of VSS dissolving ratio, 36.4%, was obtained at 170°C for 30 min. The COD dissolving ratio was about 25% at 170°C. Total organic carbon of treated sludge liquor was 1.98 and 2.73 g/L at 150°C and 170°C for 5 min, respectively. A biochemical methane potential (BMP) test of excess sludge and a mixture of primary and excess sludge demonstrated an increase in biogas production. The total biogas from microwave treated mixture sludge increased by 12.9% to 20.2% over control after 30 days of digestion. Biogas production was 11.1% to 25.9% higher for excess sludge than for untreated sludge. The VS removal ratios of mixture sludge and excess sludge were 12% and 11% higher, respectively, compared to the untreated sludge.