High pressure homogenization and temperature phased anaerobic digestion for enhanced biogas conversion from municipal waste sludge

This study, for the first time, compared advanced anaerobic digestion combining temperature-phased anaerobic digestion (TPAD) with high pressure homogenization (HPH) pretreatment to conventional anaerobic digestion of municipal sludge at laboratory scale.The study began with examination of thickened waste activated sludge (TWAS) solubilization due to HPH pretreatment at different pressure (0 to 12,000 psi) and chemical dose (0.009 to 0.036 g NaOH/g total solids). Homogenizing pressure was found as the most significant factor (p-value < 0.05) for increasing solubilization of particulate chemical oxygen demand (COD) and biopolymers in TWAS. Based on the preliminary results, a pretreatment with chemical dose of 0.009 g NaOH/g total solids and pressure of 12,000 psi was selected for digester studies.Upon acclimation of anaerobic inocula to pretreatments, a total number of twelve lab-scale digesters were operated under scenarios including single-stage (control), TPAD, and HPH coupled with TPAD (HPH+TPAD) at sludge retention times (SRTs) of 20, 14 and 7 days. Between mesophilic and thermophilic temperatures, mesophilic digestion was found to benefmore from pretreatments. Relative (to control) improvements in methane yield and volatile solids (VS) removals increased noticeably as SRT was shortened from 20 to 14 and 7 days. HPH+TPAD system was found to achieve maximum methane production and VS removals. Methane production from HPH+TPAD system ranged between 0.20 to 0.25 m3 CH4/kg CODadded at different SRTs whereas, VS removals ranged between 43.3 to 64%. Both TPAD and HPH+TPAD systems produced nearly Class A biosolids according to Organic Matter Recycling Regulations (OMRR) of British Columbia (BC), while control digestates were qualified as Class B biosolids only. Energy analysis indicated that all the digestion scenarios attained positive energy balance with HPH+TPAD system operated at 20 d SRT producing maximum net energy of 72.4 to 79.2 GJ/d.