Viability of Anaerobic Digester Sludge

Abstract

The viability of biological sludges is commonly expressed as the active bacterial concentration per unit mass volatile suspended solids (VSS). Due to the high concentration of paniculate matter in anaerobic sludge digester feed, digester volatile solids contain a large concentration of nonbacterial mass. Viability of anaerobic sludge based on sludge digester total VSS will therefore underestimate the active mass. Based on adenosine triphosphate (ATP) measurements, only 5 to 10% of the total VSS represents active bacterial biomass. Bacterial viability based on the biomass VSS fraction in the anaerobic sludge digester is much higher than viabilities based on total VSS. Using a kinetic model of anaerobic sludge digestion, the biomass VSS can be estimated. For sludge ages exceeding 10 days, less than 20% of the total volatile solids represents biomass—the remainder consists of biodegradable VSS not yet hydrolyzed for bacterial uptake and inert VSS. This means that 40 to 50% of the biomass VSS in anaerobic sludge represents active biomass. Based on ATP, anaerobic sludge viability ranges from 0.76 to 0.99 mg ATP/g biomass VSS for sludge ages between 10 and 40 days, which is comparable to reported aerobic sludge activity measurements.