The study investigated the unbiodegradable fraction of particulate chemical oxygen demand (PCOD) in thickened waste activated sludge (TWAS) using semicontinuous-flow completely-mixed anaerobic digesters. A laboratory-scale semicontinuous stirred tank reactor was used to investigate TWAS anaerobic biodegradability at hydraulic retention time of 16.7 d to 33.3 d and organic loading rate (OLR) of 1.21 kg COD/m3•d to 3.47 kg COD/m3•d. The COD and volatile suspended solids (VSS) removal for TWAS were 37% to 44% and 39% to 42% at an OLR of 1.27 and 3.47 kg COD/m3•d, respectively. Using a biomass yield (<inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="00819-ilm01.gif"/>) of 0.29 g COD biomass/g COD substrate, decay rate (<inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="00819-ilm02.gif"/>of 0.015 d-1; and a solids retention time (SRT) of 16.7 d; two linear fits correlating the difference between effluent biomass; and effluent particulate COD with influent total COD, the unbiodegradable fractions of PCOD and VSS were estimated from the slopes of the linear fits to be in the range of 0.26 to 0.28.