Low temperature microwave and conventional heating pre-treatments to improve sludge anaerobic biodegradability

This paper presents the results of lab-scale experiments on low-temperature thermal pre-treatment (less than 100°C) prior to anaerobic digestion of sewage sludge. Two heating ways, microwave heating (MH) and conventional heating (CH), and two types of sludge, primary and waste activated sludge, were compared under the same experimental conditions. The degree of solubilisation produced by MH and CH up to 72°C, 82°C and 93°C was firstly estimated. For both types of heating, increase in soluble COD caused by the pre-treatment was about 14% on waste activated sludge and only 3% on primary sludge. The final temperature of 72ºC resulted as the most cost-effective in terms of additional soluble COD per unit of energy required. Subsequently, five series of Biochemical Methane Potential mesophilic assays were run in 120mL serum bottles on sludge samples pre-treated at 72ºC. When compared with control reaction vessels, no significant differences were noticed in net methane production of pre-treated primary sludge, whereas a relevant increase occurred regarding the pre-treated waste activated sludge.. It was also observed that the trend of methane content in biogas during the batch tests can be described by a second order polynomial.