The emphasis for operation of the activated sludge process has switched from the volume of wastewater turnover and effluent quality to minimisation of biomass produced. Attempts including extended aeration, promotion of bacterial predation by higher organisms and increasing process temperature have been successful in lowering yield. These all involve increased operating costs and capital works, whereas the process of chemical uncoupling may be cheaper. After initial chemical screening 2,4 dinitrophenol (2,4 DNP) was continuously dosed in an activated sludge simulation (31 working volume, 5.55h HRT). Effect on process efficiency was monitored by measuring BOD removal and biomass production by MLSS. After 7 weeks of chemical treatment the mean yield coefficient of the treated simulation (Y=0.30) was significantly lower than that of the control (Y=0.42). BOD removal from treated and control samples was comparable and not significantly different.

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