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

The activated sludge process generates an endogenous residue (X_E) as a result of heterotrophic biomass decay (X_H). A literature review yielded limited information on the differences between X_E and X_H 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 C_xH_yO_zN_aP_bS_c) of the endogenous and the active fractions and EPS of activated sludge from well designed experiments. To isolate X_H and X_E in this study, activated sludge was generated in a 200 L 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 X_H and an endogenous residue X_E, 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 X_H and X_E 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 X_H and X_E in this sludge, it was possible to characterize X_H 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 X_H and X_E were determined as CH_1.240O_0.375N_0.200P_0.0172S_0.0070 and CH_1.248O_0.492N_0.068P_0.0032S_0.0016, respectively. Data from EPS analyses also confirmed this difference in structure between X_E and X_H with an EPS content of 11-17% in X_Eversus 26-40% in X_H.