Reviewing Improved Anaerobic Digestion by Combined Pre-Treatment of Waste-Activated Sludge (WAS)

Abstract

The anaerobic digestion of wastewater treatment sludge (WAS) produces a “green” biogas while reducing the amount of residual sludge. To increase the yield of biogas, several individual or combined pre-treatment methods of WAS can be used. These pre-treatment methods substantially reduce the amount of volatile suspended solids (VSSs) and their associated total chemical oxygen demand (TCOD). Pre-treating the sludge will increase the methane yield by 15 to 30%. Although the individual methods have been dealt with in research and large-scale operations, the combined (hybrid) methods have not previously been reviewed. Here, different hybrid treatment methods are reviewed, including (1) thermochemical hydrolysis pre-treatment, using an alkaline or acid addition to enhance solubilization of the sludge cells and increase biogas production; (2) alkaline and high-pressure homogenizer pre-treatment, combining a chemical and mechanical treatment; (3) alkaline and ultrasound pre-treatment, capable of solubilizing organic sludge compounds by different mechanisms, such as the fast and effective ultrasound disruption of cells and the increasing effect of the alkaline (NaOH) treatment; (4) combined alkaline and microwave pre-treatment, which enhances sludge solubilization by at least 20% in comparison with the performance of each separate process; (5) microwave (MW) and peroxidation pre-treatment of WAS suspended solids (SSs), which are quickly (<5 min) disintegrated by MW irradiation at 80 °C; (6) ultrasound and peroxidation pre-treatment, with ozone and peroxides as powerful oxidizing agents; and (7) pulsed electric field (PEF) pretreatment. All literature findings are assessed, discussing relevant operation conditions and the results achieved.