Characterization of the heterotrophic biomass and the endogenous residue of activated sludge

The activated sludge process generates an endogenous residue (XE) as a result of heterotrophic biomass decay (XH). A literature review yielded limited information on the differences between XE and XH in terms of chemical composition and content of extracellular polymeric substances (EPS). The objective of this project was to characterize the chemical composition (x, y, z, a, b and c in CₓHyOzNₐPbSc) of the endogenous and the active fractions and EPS of activated sludge from well designed experiments. To isolate XH and XE in this study, activated sludge was generated in a 200L pilot-scale aerobic membrane bioreactor (MBR) fed with a soluble and completely biodegradable synthetic influent of sodium acetate as the sole carbon source. This influent, which contained no influent unbiodegradable organic or inorganic particulate matter, allowed the generation of a sludge composed essentially of two fractions: heterotrophic biomass XH and an endogenous residue XE, the nitrifying biomass being negligible. The endogenous decay rate and the active biomass fraction of the MBR sludge were determined in 21-day aerobic digestion batch tests by monitoring the VSS and OUR responses. Fractions of XH and XE were respectively 68% and 32% in run 1 (MBR at 5.2 day SRT) and 59% and 41% in run 2 (MBR at 10.4 day SRT). The endogenous residue was isolated by subjecting the MBR sludge to prolonged aerobic batch digestion for 3 weeks, and was characterized in terms of (a) elemental analysis for carbon, nitrogen, phosphorus and sulphur; and (b) content of EPS. The MBR sludge was characterized using the same procedures (a and b). Knowing the proportions of XH and XE in this sludge, it was possible to characterize XH by back calculation. Results from this investigation showed that the endogenous residue had a chemical composition different from that of the active biomass with a lower content of inorganic matter (1:4.2), of nitrogen (1:2.9), of phosphorus (1:5.3) and of sulphur (1:3.2) but a similar content of carbon (1:0.98). Based on these elemental analyses, chemical composition formulae for XH and XE were determined as CH₁.₂₄₀O₀.₃₇₅N₀.₂₀₀P₀.₀₁₇₂S₀.₀₀₇₀ and CH₁.₂₄₈O₀.₄₉₂N₀.₀₆₈P₀.₀₀₃₂S₀.₀₀₁₆, respectively. Data from EPS analyses also confirmed this difference in structure between XE and XH with an EPS content of 11–17% in XEversus 26–40% in XH.