At wastewater treatment plants the grease and sand is removed by means of a grease and sand trap to protect the equipment in the following processes from damage. There are many different technologies for cleaning wastewater but the far most common is a combination of mechanical treatment and biological treatment. The wastewater is cleaned in steps ending with a biological process or aeration of the wastewater in order to start the nitrification and denitrification processes.
Mechanical wastewater treatment is used to remove solid matter in the water. It involves two processes: filtration (or separation) and sedimentation. Filtration is carried out in various stages using various processes. These can remove coarse impurities such as paper or leaves using a rake or rotary screen. In the primary sedimentation tank the purification (reduction of organic material) takes place.
In the biological stage of the wastewater treatment process the main impurities, which are suspended in the water and not picked up by the mechanical process, are treated. The biological wastewater treatment process takes place when the wastewater is being treated in the sludge activation tank. Other processes (e.g. trickling filter, immersion filter) are used in isolated cases.
Sludge is produced as a waste product in the wastewater treatment process. Treatment and preparation of sludge give rise to its own equipment requirements, which may vary significantly depending on the plant. The most common way of disposing of wastewater sludge is still using it in agriculture. This is why some countries utilize wastewater sludge thermally. In some cases, the wastewater sludge is delivered to an external biogas plant.