The residue known as purification sludge is generated by water treatment facilities. The content of this sludge is classified according to conducted studies. Wastewater purification sludge is made useable by various processes. This way, some agricultural uses are also derived. Before the ultimate disposal process, wastewater purification sludge is subjected to certain processes. One of these is biological purification of sludge. The pollution factor of purification sludge is determined by its biological properties. Biological Purification Methods aim to eliminate or minimize pathogens in the purification sludge during stabilization process. This process is applied in order to eliminate undesired odors. When raw purification sludge is stabilized the resulting material is called the bio-solid. The term bio-solid is also valid for defining processed purification sludge. Chemical methods, biological methods or thermal processes can be utilized for stabilization of purification sludge. A two stage process is performed in the anaerobic composting phase. After the first stage wherein organic materials are converted into organic acids, the second stage involves conversion of organic acids into carbon dioxide and methane. Thus, biogas is produced. The generated biogas is used for facility operation purposes. The facilities generate power and heat energy from this biogas.

In addition to the maturation periods, factors like mixing and temperature are also kept under control in regard of the sludge under aerobic composting process. Composting covers aerobic processes. Being an environmentally-friendly process, composting helps decrease carbon emissions and aims to create more useable lands. The goal of drying process is to decrease the moisture in the purification sludge. The lime stabilization process which comes thereafter is another part of the treatment. Slaked limes as well as non-slaked limes can be used in this process.

One of the methods used in lime stabilization process is dehydration, wherein lime is added to the purification sludge before dehydration process. Lime is also added to the purification sludge after dehydration. Afterwards, advanced lime stabilization technologies are applied. It is seen that processes applied at wastewater treatment centers require very large amounts of energy. In addition to transfer of wastewater, treatment and discharge of the water as well as processing of the generated purification sludge necessitate certain processes.

The stages requiring the largest amount of energy during these processes are the aeration and pumping phases. Also certain processes are required for the active sludge generated at the wastewater treatment facilities, and therefore the active sludge treatment system takes part in the process. Advanced purification methods are used in addition to biological purification. This method aims to provide higher quality purification in comparison to secondary purification processes.