What Is the Right Rate? Determining Digestibility Kinetics of Pretreated Waste Activated Sludge During Anaerobic Digestion

Pretreatment of municipal sludge has the potential to improve anaerobic digestion by increasing the ultimate biodegradability of particulate chemical oxygen demand (PCOD) to methane (CH4) gas. However, the effect of pretreatment, that is, increased soluble COD from solubilization of PCOD, makes it difficult to correctly estimate hydrolysis kinetics in biochemical methane potential tests. In this study, batch digestion tests were used to evaluate the general impact of alkaline and thermal pretreatment of waste activated sludge (WAS) and to determine a different method to estimate hydrolysis rates from pretreated streams. Throughout the 60-day batch experiments, liquid and gas samples were collected and analyzed for total (TCOD) and semi-soluble (SSCOD) COD, total (TSS) and volatile (VSS) suspended solids, pH, volatile fatty acids (VFAs), and ammonium concentration. Pretreatments significantly decreased VSS concentrations at the start of digestion, but all samples showed similar VSS destruction to the control at the end of the experiments. Ultimate CH4 production was significantly increased by alkaline and thermal pretreatments, which produced 203 and 219 mL CH4 gVSS(-1), respectively, versus 175 mL CH4 gVSS(-1) in the control. Although CH4 generation was a good proxy for solids hydrolysis of untreated WAS, CH4 generation significantly overestimated the rate of hydrolysis of pretreated solids. The overestimation was due to the abundance of SSCOD available at the start of the experiment being converting to CH4. Incorporating SSCOD or VFAs into the hydrolysis kinetics resulted in much slower hydrolysis rates that were similar to those measured by VSS destruction directly (e.g., 0.08 d(-1) with SSCOD + CH4 vs. 0.19 d(-1) from CH4 alone for thermal pretreatment vs. 0.05 d(-1) with VSS alone). The slower kinetics indicate that the necessity to understand the trade-offs between the performance advantages of pretreatment (i.e., solids destruction and energy conversion) versus the operational and financial impacts on large-scale digestion.

Automated summary

Pretreatment of municipal sludge has the potential to improve anaerobic digestion and increase methane production. However, the increased soluble COD from pretreatment makes it difficult to estimate hydrolysis kinetics accurately. In this study, batch digestion tests were conducted to evaluate the impact of alkaline and thermal pretreatment and to determine a different method to estimate hydrolysis rates. The inclusion of semi-soluble COD and volatile fatty acids in the hydrolysis kinetics model improved the accuracy of estimating the hydrolysis rates of pretreated solids.