Assessing the Optimum SRT for Anaerobic Digester with Sludge Pretreatment for Sulfide Control

Thermo-chemical pretreatment of municipal waste activated sludge (WAS) was conducted at 60±2°C with 0.6 mg H2O2+1.5 mg FeCl2/mg dissolved S2− prior to continuous anaerobic digestion at 10 days solids residence time (SRT). The experimental results showed that pretreatment significantly increased the daily methane production rate (L CH4/day) by 20%, and H2S and DMS concentrations in biogas decreased by 71%, and 41%, respectively. However, no enhancement was observed for the digestibility of particulates solids during the digestion of pretreated WAS, and overall (digestion + pretreatment) volatile suspended solids (VSS) removal was ∼10% higher than the control. Based on the experimental results, a process model using BioWin® 3.0 was developed, calibrated, and tested at different SRTs ranging from 3-30 days to assess the optimum SRT for pretreatment. Simulation results suggested that shorter SRTs (i.e., 5 days) are more advantageous for demonstrating the impact of pretreatment prior to the digestion to achieve higher methane production rate and VSS removal, whereas increasing the operating SRT to ≥20 days may diminish the impact (i.e., solids reduction and methane production) of using pretreatment.