MLVSSoperation = MLVSSmeasured * (OURset-point/OURmeasured)
Figure A below shows oxygen uptake rates for three mixed liquor concentrations in which acetate was used as a standard substrate - as measured using a Challenge AER-200 respirometer system. The three tests show a consistent OUR fingerprint but with changes in OURmax that reflect changes in active biomass. Figure B shows a repeat test after adjusting the biomass concentration to give the desired Set-Point OUR. The relationship allows plant operators to maintain a constant loading rate capacity under all operating conditions. In this case, adjustment of the OUR for the 830 mg/L MLVSS sample and the 2,290 mg/L MLVSS sample to 1,685 mg/L brought all mixed liquors to the same OUR. Once a Set-Point OUR has been identified, the changes in MLVSS concentrations are relative so that the substrate type should not be a major concern as long as it is constant.