Biological treatment: removal of organic substances



Purifying micro-organisms in a biological wastewater treatment plant biodegrade the organic pollutants present in wastewater. During this process the micro-organisms consume oxygen which is provided by means of aerators. While metabolizing the organic pollution the microorganisms grow as sludge flocks. Afterwards the purified water is separated from the flocks by settling or membrane filtration.


Organic compounds + O2 --> CO2 + H2O + biological sludge


Biological wastewater treatment is fit for the purification of wastewaters which contain biodegradable organic pollutants (BOD, COD). While consuming organics, the biomass grows and also incorporates nitrogen and phosphorous from the wastewater. If nitrogen is present in excess, the wastewater treatment plant can be upgraded with Biological nitrogen removal. An excess of phosphorus is easily removed by dosing certain metal salts.


Biological wastewater treatment plants exist in several variations. The most important differences are found in the sludge/water separation process. In a continuous activated sludge system the flocks settle in a clarifier. In a sequencing batch reactor (SBR) the aeration and settling processes take place in the same reactor. In a membrane bioreactor (MBR) the separation is done by means of membrane filtration.

Example of a realisation

Biological WWTP: aeration basin

Operational costs

Besides the water treatment infrastructure there are also operational costs associated with water treatment. The most important among these are the energy cost for aeration and the handling of the produced biological waste sludge.

Approach Trevi

In the design of its biological wastewater treatment plants, Trevi uses a very energy-efficient fine bubble aeration system. Besides savings in the energy cost, this systems also allows for compact wastewater treatment plants with a low sludge production. To ensure the desired treatment results, the dimensioning of a new treatment plant is always supported by pilot tests.