Biological treatment of municipal wastewater

Activated sludge is a selection device. Sewage enters, a mixed community is held in suspension with oxygen, and biomass is separated and partly returned. What decides the plant’s behaviour is which organisms are retained, and that is governed by one variable: sludge age, the mean time a cell stays in the system before being wasted.

Sludge age selects the community

An organism persists only if its growth rate exceeds its removal rate, so the sludge age must exceed the reciprocal of the specific growth rate of the slowest organism the plant needs. Heterotrophs oxidising organic carbon are fast and abundant, and a few days suffice. Ammonia-oxidising and nitrite-oxidising bacteria are autotrophs: they draw energy from oxidising nitrogen and carbon from carbon dioxide, which is a poor energy budget, so their yield is low and their maximum growth rate is roughly an order of magnitude below the heterotrophs'.

Temperature then sets the plant’s size. Nitrifier growth rate falls with temperature far more steeply than heterotrophic activity, with an Arrhenius coefficient near 1.10 per degree — a rate change of roughly a factor of two across ten degrees. A plant whose mixed liquor drops to 8–12 °C in winter must therefore hold two to three times the sludge age of a warm-climate plant to keep nitrification, and at a given mixed-liquor solids concentration that is directly a larger aeration tank. Cold-weather nitrification, not carbon removal, is the design case for most northern plants, and washout of nitrifiers after a cold snap or a toxic slug takes weeks to recover precisely because the organisms grow slowly.

Nitrification also consumes alkalinity — about 7.1 mg as calcium carbonate per milligram of ammonia nitrogen oxidised. In a soft-water catchment that drives pH down into the range where nitrification itself slows, and alkalinity must be dosed or recovered.

Nitrogen and phosphorus need conditions carbon removal does not

Nitrate is removed by denitrification, which requires an anoxic zone and an electron donor. Placing the anoxic zone ahead of aeration, with nitrate recycled into it, lets the incoming sewage’s own organic carbon serve as the donor and returns roughly half the consumed alkalinity — which is why pre-anoxic denitrification is the standard configuration rather than dosing methanol.

Enhanced biological phosphorus removal is stranger. Polyphosphate-accumulating organisms are given an anaerobic contact zone with no oxygen and no nitrate, where they take up volatile fatty acids and store them as polyhydroxyalkanoate, paying for it by hydrolysing internal polyphosphate and releasing phosphate. In the following aerobic zone they oxidise the stored polymer and take up more phosphate than they released. The phosphorus leaves the plant only in the wasted sludge, so the process is a sludge-production process; nitrate leaking into the anaerobic zone destroys the selective advantage, and competition from glycogen-accumulating organisms makes the process notoriously less reliable than chemical dosing.

What is not removed

Aeration is typically the largest single electricity consumer in a municipality. Sidestream deammonification treats warm, concentrated digester liquor with anammox bacteria at much lower oxygen and no organic carbon; applying it to the main stream is blocked by the same cold temperatures and by the difficulty of suppressing nitrite oxidisers there. Pharmaceuticals, contrast media and many micropollutants pass through or sorb to sludge at conventional sludge ages, which is why advanced polishing is treated as a separate stage.