In aerobic biological waste water treatment, the biologically degradable load is reduced in the
presence of oxygen by digestion by microorganisms, generating biomass, carbon dioxide and
water.
After primary treatment, usually done to protect the biological treatment plant from excessive
loads and pollutant peaks, the biodegradable dissolved and colloidal material in the waste water
is transformed by microorganisms in the presence of air into solid cell substance (biomass) and
into carbon dioxide and water. After biodegradation, biomass and water are separated normally
using sedimentation tanks (clarification may be enhanced by flocculation etc.). Different
process designs are available, which can be classified in two main categories: activated sludge
systems (such as one- or two-stage activated sludge plants or aerated lagoons) and biofilm
systems (such as moving bed biofilm reactors (MBBR) or membrane bioreactors (MBR)). A
combination of biofilm and activated sludge systems is also used.