Comparative Assessment of Sludge Pre-Treatment Techniques to Enhance Sludge Dewaterability and Biogas Production

Effectiveness of ultrasonication, microwave technologies, and enzyme addition prior to anaerobic digestion is investigated using sludge samples taken from the secondary settling tank of a domestic wastewater treatment plant to improve methane production, enhance dewaterability characteristics of the sludge, and to reduce excess sludge. Microwave pre-treatment (1500 W, 10 min at 175 °C) results in better extra digester performance (compared to the control reactor) in terms of methane production (25 m³ ton⁻¹ suspended solids (SS)) than ultrasound (no improvement) and enzyme pre-treatment (11 m³ ton⁻¹ SS). While methane production is not improved as a result of ultrasonication pre-treatment (15 000 kJ kg⁻¹ SS), a noticeable increase (19%) is observed in the case of microwave pre-treatment. Higher compactibility values are obtained after ultrasonication and microwave application compared to the control (i.e., from 7.1 to 8.7 and 9.2%, respectively) before anaerobic digestion. Although ultrasonication and microwave application decrease the dewaterability of the raw sludge (capillary suction time (CST) from 827 to 1364 and 2290 s, respectively), similar dewaterability results are obtained at the end of the anaerobic digestion process for all pre-treated sludge samples. An economic assessment of this study shows that pre-treatment with microwave results in more than 10-fold less net cost compared to the enzyme application.