Wastewater treatment or sewage treatment is the process that removes the majority of the contaminants from waste-water or sewage and produces both a liquid effluent suitable for disposal to the natural Environment and a sludge. To be effective, sewage must be conveyed to a treatment plant by appropriate pipes and infrastructure and the process itself must be subject to regulation and controls. There are many and various forms of treatment processes. The site where the processes are conducted is called a wastewater treatment plant (WWTP). The flow scheme (see figure 1) of a conventional WWTP is generally the same in all countries and exists our of following physical-chemical elements:

• Mechanical treatment;

• Influx (Influent)

• Removal of large objects

• Removal of sand

• Pre-precipitation

• Biological treatment;

• Oxidation bed (oxidizing bed) or Aerated systems

• Post precipitation

• Effluent

• Chemical treatment (this step is usually combined with settling and other processes to remove solids, such as filtration.

Besides the physical-chemical classification the technical classification is based on the steps, which are performed one by one other:

• Primary treatment (see figure 1): to reduce oils, grease, fats, sand, grit, and coarse (settle able) solids. This step is done entirely with machinery.

• Secondary treatment (see figure 1) is designed to substantially degrade the solved content of the sewage within a biological degradation system, such as activated sludge systems. These systems use the capability of microorganism to degrade solved components in water. The final step in the secondary treatment stage is to separate the used biological media from the cleared sewage water with a very low levels of organic material and suspended matter.

• Tertiary treatment or advanced treatment (not in figure 1) is yet not applied widely. It provides a final stage to raise the effluent quality to the standard required before it is discharged to the receiving Environment. More than one tertiary treatment process may be used at any treatment plant. In most cases it is a further nitrogen or phosphate elimination and/or a disinfection. Additional steps like lagooning or Constructed wetlands are also counted as tertiary step if they are used after secondary treatment.

Figure 1: Wastewater treatment plant (Queens University 2004)