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

Steven G. Hart, Michelle N. Young, Prathap Parameswaran, Bruce E. Rittmann, Cesar Torres

Research output: Contribution to journalArticlepeer-review


Pretreatment of municipal sludge has the potential to improve anaerobic digestion by increasing the ultimate biodegradability of particulate chemical oxygen demand (PCOD) to 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 CH4 potential tests. To address, batch digestion tests were conducted to investigate the general effect of alkaline and thermal pretreatment of waste activated sludge (WAS), as well as 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 (VFA), 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)/g(VSS), respectively, versus 175 mL(CH4)/g(VSS) 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 VFA into the hydrolysis kinetics led to much slower hydrolysis rates that were similar to those measured by VSS destruction directly. The slower kinetics indicated 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.

Original languageEnglish (US)
Pages (from-to)877-885
Number of pages9
JournalEnvironmental Engineering Science
Issue number11
StatePublished - Nov 2022


  • anaerobic digestion
  • hydrolysis
  • hydrolysis constant
  • pretreatment

ASJC Scopus subject areas

  • Environmental Chemistry
  • Waste Management and Disposal
  • Pollution


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