An anaerobic lagoon is a deep impoundment, essentially free of dissolved oxygen that promotes anaerobic conditions. The process typically takes place in deep earthen basins, and such ponds are used as anaerobic pretreatment systems. Anaerobic lagoons are not aerated, heated or mixed. The typical depth of an aerated lagoon is greater than 2.44 m, with greater depths preferred. Such depths minimize the effects of oxygen diffusion from the surface, allowing anaerobic conditions to prevail. As such, anaerobic lagoons are different from shallower aerobic or facultative lagoons, making the process similar to that experienced with a single stage unheated anaerobic digester; except that anaerobic lagoons are in an open earthen basin. Conventional digesters are typically used for sludge stabilization in a treatment process, whereas lagoons are often used to pretreat raw wastewater. Pretreatment includes separation of settable solids, digestion of solids, and treatment of the liquid portion.