Uncoupling of metabolism to reduce biomass production in the activated sludge process

Abstract

Production of excess biomass during biological treatment of wastewaters requires costly disposal. Also with environmental and legislative constraints limiting disposal options, considerable impetus exists for reducing the amount of biomass produced. Uncoupling metabolism in activated sludge may reduce biomass production and this approach is investigated in conjunction with consequences upon substrate removal and population dynamics. To induce uncoupled metabolism, para-nitrophenol ( pNP) a known protonphoric uncoupler of oxidative phosphorylation, was introduced to a bench-scale activated sludge process. Microbial populations were monitored by both microscopic and by three methods of molecular analyses. Presence of the protonphore caused a shift in the microbial population with protozoa being washed out of the system and filamentous bacteria proliferating. The molecular composition of the microbial community was determined by PCR amplification of 16SrRNA genes and subsequent denaturing gradient gel electrophoresis (DGGE). Band Patterns obtained by both a direct and nested approach were similar. However, profiles derived from nested PCR contained more bands, indicative of the increased sensitivity of this approach. Analysis of the active biomass by Polyacrylamide Gel Electrophoresis (PAGE) of small molecular weight RNA (5mwRNA) showed that a sustained shift in the diversity of the predominant, metabolically active species present occurred within two days of the introduction of the protonphore. Biomass production was reduced by 49%, but the total substrate removal rate was also reduced by 25%. The combined effect was a 30% decrease in the biomass yield. Introduction of the protonphore caused substrate removal efficiency to decrease from a consistent value of 96% to 68.5% with considerable variance. This decline in overall process performance was attributed to a surmountable effect arising from the design of apparatus that resulted in a decrease in the reactor biomass concentration. Although the specific biomass volume was consistent throughout, decreased sedimentation resulted in solids being removed in the final effluent which decreased the amount of biomass which could be recycled. The catalytic efficiency of the biomass increased as reflected by a 3.3 fold increase in the specific substrate uptake rate.