Adaptive response of microbial communities to soluble microbial products.

We carried out two experiments to study the influence of soluble microbial products (SMP) on biomass concentration [defined as mixed liquor suspended solids (MLSS)] and removal of soluble biological and chemical oxygen demands (sBOD5 and sCOD): (1) SMP were allowed to accumulate, and (2) SMP content was artificially reduced by washing the biomass. The daily initial sCOD in both experiments was kept constant at 859+/-6 mg/l for 16 days. In experiment 1, the highest sCOD removal (80%) occurred during the first day. Thereafter, it decreased successively to 40% [sludge retention time (SRT), 12 days], after which it increased steadily to 50+/-4%. Variations in residual sCOD were accompanied by variations in sBOD5, showing that the biodegradability of the accumulated SMP components was changing. MLSS fluctuated within the range 1,200+/-25-1,993+/-58 mg/l. We attributed the irregular accumulation of the biomass to variations in the biodegradability of SMP components. The initial sBOD5/MLSS ratio varied according to variations in initial sBOD5 and MLSS, whereas the residual ratio was constant at 0.025+/-0.008. This indicated a direct relationship between the concentrations of biomass and SMP produced. In experiment 2, MLSS increased from 1,200+/-25 to a constant value (2,810+/-16 mg/l; SRT, 12 days). After this time, no decrease or increase in MLSS was observed. Correspondingly, sCOD and sBOD5 removal increased from 80-97 to 84-99%. A stable microbial community that could consume organic matter efficiently was developed under these conditions.