The methane production activity of anaerobic sludges is related to the history of the sludge, the biomass yield from the wastewater the sludge has been treating, the fraction of methanogens, and the presence of toxic substances. Best performance can be achieved from an anaerobic treatment plant if the operator measures the Specific Methanogenic Production (SMP) of the biomass contained within the sludge. The SMP is measured by placing a known amount of biomass in a respirometer vessel, adding a known amount of substrate – usually acetic acid – to the bottle at concentrations sufficient to cause maximum biomass activity, and measuring the resulting methane. The SMP is expressed as the COD equivalent of the methane production rate per gram of volatile solids, or gm COD/gm VSS/d. An example of an SMP measurement is shown in the figure below. Figure A shows methane production measured over a three-day period using a Challenge AER-200 aerobic/anaerobic respirometer. Figure B shows the calculated specific methane production rate. The maximum specific methane production rate represents the SMA – or the activity – of this sludge sample. SMAs between 1.0 and 2.0 indicate a highly active biomass. An SMA of 1.5 g COD/g VSS-d indicates that the biomass is 25% methanogens by weight.