THERMAL HYDROLYSIS PRETREATMENT OF SEWAGE SLUDGE

Abstract

Sewage sludge management is a serious issue faced by wastewater treatment plant (WWTP) operators due to the difficulties in sludge handling and the costs associated with it. Anaerobic digestion is an established technology and considered an essential part of modern WWTPs owing to its ability to transform organic matter into energy-rich biogas thereby reducing the amount of sludge and producing nutrient-rich biosolids as the end product. However, sludge hydrolysis is the rate-limiting step in anaerobic digestion resulting in poor process performance. It further demands longer Hydraulic retention time (HRT) resulting in large-size digesters thus increasing the capital cost. To accelerate rate-limiting hydrolysis and to enhance biogas production, various thermal-based pretreatment methods have been studied at lab- and full-scale worldwide. In thermal sludge pretreatment, the soluble intracellular material is released into the liquid phase, and the refractory organic material is transformed into biodegradable form. It enhances the sludge hydrolysis and downstream methanogensis. The thermal hydrolysis process (THP) is applied to improve the sludge solubilization and subsequent methane yield in the downstream anaerobic digestion (AD) process. The use of THP as a pretreatment technique to enhance the methane generation potential of sequencing batch reactor (SBR) sludge and the potential of THP in mitigating the risk of organic micropollutants remains unclear. In the first phase, sludge from an SBR system operating at a high solids retention time (SRT) of 40 days was subjected to THP using a 5L pilot plant at the temperature ranges of 120-180 ℃ for 30-120 min. The effect of THP on organics solubilization, methane yield, organic micropollutant removal, and microbial community dynamics was studied. The highest methane yield of 507 mL CH4/g VSadded and volatile solids (VS) removal of 54 % were observed at 160 ℃- 30 min THP condition, i.e., 4.1 and 2.6 times higher than the control (123 mLCH4/g VSadded, 20.7% VS removal), respectively. The experimental values of hydrolysis coefficient and methane yield have been predicted using Modified Gompertz, First order, and Logistics models. The observed values fitted well with all three models showing an R2 value between 0.96 to 1.0. THP pretreated sludges showed >80% removal of Trimethoprim, Enrofloxacin, Ciprofloxacin, and Bezafibrate. However, Carbamazepine, 17α-ethinylestradiol, and Progesterone showed recalcitrant behavior, resulting in less than 50% removal. Microbial diversity analysis showed the dominance of Proteobacteria, Firmicutes, Chloroflexi, and Bacteroidetes, collectively accounting for > 70-80% of bacterial reads. The THP-mediated anaerobic digestion of sludge shows better performance than the control digestion, improved methane yield, higher VS and micropollutants removal, and a diverse microbiome in the digester. In the second phase, 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 batch conditions. Anaerobic digestion of SBR sludge resulted in methane generation of 101, 166, and 390 mL CH4/g VSadded under mesophilic, thermophilic, and THP pretreatment conditions. For CASP sludge, the highest methane generation was observed for THP pre-treated sludge (587 mL CH4/g VSadded) followed by thermophilic (298 mL/gVSadded) and mesophilic digestion (241 mL CH4/g VSadded). The fecal coliforms in the mesophilic and thermophilic digested SBR sludges were 3000 and 620 MPN/g, while 6100 and 740 MPN/g were found in digested CASP, 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 3MPN/4g TS (dry basis), respectively, over MAD processed sludge. The energy demand of the THP unit (160 ℃, 6 bar) was estimated to be 162 MJ/m3. The energy balance analysis revealed that TH pretreatment would lead to energy self-sufficiency upon integration in a WWTP.