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Biological sludge production is BOD removed times the observed yield, which falls with sludge age from about 0.6 at short SRT to 0.3 in extended aeration. Inert solids from the influent add to it. Wet volume depends entirely on solids content — sludge at 0.8% dry solids is over 100 times the volume of the dry mass.
Sludge production
Dry solids = BOD removed x observed yield + inert solids load
Wet volume
Volume (m3) = dry solids (kg) / (dry solids fraction x 1000)
Sewage sludge handling, storage and land application are regulated for pathogens, metals and nutrients. Confirm the applicable biosolids rules.
Longer retention allows more endogenous respiration, so a larger fraction of the biomass produced is oxidised again. Extended aeration therefore produces less sludge but consumes more air.
Volume scales inversely with solids content. Thickening from 0.8% to 4% cuts volume by 80%, which transforms the cost of digestion, transport and dewatering.