The municipal WWTPs create two major sludge streams; primary sludge (PS) and waste activated sludge (WAS). These sludges have different biochemistries, leading to dissimilar anaerobic digestion (AD) results. Although the recovery of biogas from sewage sludge is an extensively applied process, there is no comprehensive literature review for the comparative quantification of methane production from these two streams. This paper attempted to fill this research gap by conducting a systematic review and meta-analysis based on 22 scientific papers reporting specific methane yields (SMY) from PS and WAS under controlled AD processes.

The results from the meta-analysis show that PS provides an average SMY of 0.298 L CH4/g VS (95% CI: 0.271-0.324), corresponding to a 52% greater production rate than WAS (average SMY: 0.196 L CH4/g VS; 95% CI: 0.174-0.218). The difference in yields is mainly caused by the increased hydrolysability of PS organic material in comparison to microbial biomass and Extracellular Polymeric Substances (EPS) in WAS. The subgroup analysis based on temperature shows that upper mesophilic digestion (40-45°C) produces the most reliable results (I2 = 52.1%), whereas thermophilic digestion (50-60°C) has considerable variability (I2 = 81.7%) and benefits depending on the setting. HRT has a positive and monotonically increasing impact on the methane yield, with HRTs greater than 20 days being essential for achieving optimal results in mixed sludges. Pretreatment of WAS using thermal hydrolysis, mechanical treatment, or both increases SMY by 19-37%; however, energy balance considerations are necessary during the procedure.

The results have implications for WWTPs in Sweden and the Nordic region in that energy requirements for thermophilic treatment together with the conservative HRT guidelines necessitate a scientific reassessment. The paper ends with some suggestions for operational optimization and future primary research directions.