Mesophilic, thermophilic and thermal hydrolysis process coupled anaerobic digestion of sewage sludge: Biomethane potential, pathogen removal and energy feasibility

Abstract

Conventional Anaerobic digestion is one of the most preferred sludge management techniques however, it has limitations such as low hydrolysis rate, longer retention time, larger digester volume, and reduced methane generation etc., To overcome these problems, Thermal hydrolysis process (THP) has been used as an effective pretreatment method for enhancing methane generation. However, the effect of the thermal hydrolysis process in heavy metal solubilization and methane production from high SRT (solids retention time) sludge is still not clear. Anaerobic digestion of sewage sludge sourced from sequencing batch reactor (SBR) and conventional activated sludge process (CASP) systems were evaluated under mesophilic (MAD), thermophilic (TAD), and thermal hydrolysis process (THP) mediated anaerobic digestion (AD) in terms of methane generation and, VS and pathogen removal. THP resulted in almost 38% COD solubilization for SBR and CASP sludge. Anaerobic digestion of SBR sludge resulted in methane generation of 101, 166, and 390 mL/gVS under mesophilic, thermophilic, and THP pretreatment conditions, respectively. For CASP sludge, the highest methane generation was observed for THP pretreated sludge (587 mL/gVS), followed by thermophilic (298 mL/gVS) and mesophilic digestion (241 mL/gVS). The fecal coliforms in the mesophilic and thermophilic digested SBR sludge were 3000 and 620 MPN/g, respectively, while 6100 and 740 MPN/g were found in digested CASP sludge, respectively. The THP-MAD and TAD processed SBR, and CASP sludges met the Class A sludge fecal coliforms and Salmonella density criteria of <1000 MPN/g TS (dry basis) and 3 MPN/4g TS (dry basis), respectively, over MAD processed sludges. Even though THP pretreatment resulted in the release of heavy metals, the concentration of heavy metals in the THP effluent was lesser than the limits prescribed by USEPA. The energy demand of the THP unit (160 °C, 6 bar) was estimated to be 162 MJ/m3. The energy balance analysis revealed that THP pretreatment would lead to energy self-sufficiency upon integration in a wastewater treatment plant (WWTP). This study can provide an experimental basis to apply THP pretreatment to enhance hydrolysis rate in anaerobic digesters while maximizing methane production and achieving energy benefits over MAD and TAD.