Activated sludge

The activated sludge process is the most widely used biological method for treating municipal sewage. In an aeration tank, a dense population of bacteria and other microorganisms, the mixed liquor, is kept in suspension with air. The microbes consume dissolved and colloidal organic matter as food, converting it to carbon dioxide, water and new cells. The mixture then flows to a secondary clarifier, where the microbial flocs settle. Most of the settled sludge is returned to the aeration tank to keep the microbe population high; the excess is wasted to sludge treatment.

Control revolves around a few parameters. Mixed liquor suspended solids (MLSS), often 2 000 to 4 000 mg/L in conventional plants, measure the amount of biomass. The food-to-microorganism ratio (F/M) relates the daily BOD load to the biomass. The mean cell residence time, or sludge age, is the average time microbes stay in the system, controlled by how much sludge is wasted. Longer sludge ages favour nitrifying bacteria and give more stable sludge. Dissolved oxygen is kept around 1.5 to 2 mg/L. Sludge settleability is checked with the sludge volume index.

Variants include extended aeration with long sludge age and little excess sludge; sequencing batch reactors, which aerate, settle and decant in one tank in cycles; moving bed biofilm reactors with plastic carriers; and membrane bioreactors that replace the clarifier with membranes for very clear effluent. Aeration energy is the largest running cost, so fine-bubble diffusers and dissolved oxygen control pay back quickly. Bulking sludge, caused by filamentous organisms, is a common operational problem.