Biological processes enable dissolved biodegradable substances (organic matter) to be removed from wastewater. The organic matter provides a nutrient for microorganisms, and is converted under aerobic conditions into biomass, carbon dioxide and water.

Biological, aerobic water treatment by the biofilm process: trickling filter.

Wastewater treatment plant in laboratory scale: aerobic biological degradation of organic substances.

In contrast to the aerobic processes, the anaerobic degradation of organic substances takes place in the absence of oxygen. The anaerobic microorganisms use the organic substances as a source of nutrition, and so degrade them. This produces biogas, mainly comprised of methane (60%) and carbon dioxide (35%).

Anaerobic degradation of organic substances in the stirred tank and UASB reactor for biogas production (UASB: Upflow Anaerobic Sludge Blanket).

Two-stage continuous degradation of organic substances. First stage: hydrolysis and acidification, second stage: anaerobic degradation.