Wastewater treatment plants

Processes in a wastewater treatment plant [WWTP] are designed to achieve considerable improvements in the quality of the wastewater discharged of into receiving surface waters especially in densely populated areas. Wastewater treatment is a multi-step process starting with the removal of suspended solids and other material present in wastewater via primary (mechanical) treatment. Primary treatment in a WWTP can reduce the Biological Oxygen Demand [BOD], a measure for the organic load, of the incoming waste water by 20-30% and the total suspended solids by some 50-60%.

In the second step of wastewater treatment (biodegradable) organic matter is removed by microorganisms that convert this matter in the presence of oxygen to yield carbon dioxide (CO2) and water (H2O) while producing the energy they need to thrive and multiply. This process is referred to as secondary (biological) treatment. The biological process is then followed by additional settling tanks (secondary sedimentation) to remove more of the suspended solids. About 85% of the suspended solids and BOD can be removed from the wastewater by a well run plant with secondary treatment. To reduce the excess sludge volume as well as the amount of organic matter adhered onto the sludge, most biological WWTPs treat excess sludge in an anaerobic digester tank.

Tertiary treatment will remove additional organic matter as well as nutrients, including nitrates and phosphates, which may cause the growth of unwanted plants and algae in receiving surface waters. However, tertiary treatment is not yet implemented as a standard wastewater treatment technique in Europe.

Most surfactants used in European household and homecare products are readily biodegradable and thus have a high BOD value. Hence, in a well operated plant with tertiary treatment more than 99 % of all the surfactants can be removed from sewage.