Dewatering is used by large wastewater treatment plants to separate sludge into a liquid and solid. The principle methods in wastewater are belt filter presses and centrifuges.These systems are high maintenance and require a high degree of supervision and operator training. They are usually only implemented at larger facilities and are not cost efficient to be used on a small scale. This is only one part of the process of wastewater becoming treated water and biosolids. Primary treatment is essential prior to the dewatering. The filtrate or centrate liquid which is separated during the dewatering process must also be treated. This typically involves circulation to the headworks of the wastewater treatment plant.