Anaerobic biogas settlers are air-tight reactors used for the pre-settlement of municipal wastewaters and the conversion of the settled sludge into biogas via anaerobic digestion. Biogas is recovered and can be transformed into heat, light or any other energy. The reactors are round or square - much like a septic tank, but with biogas collection. As the wastewater flows into the tank, organic solids start to settle to the ground. The liquid phase continuous to further treatment steps after a relatively short hydraulic retention time, while the sludge is kept in the tank for several months to years (long sludge retention time). The remaining slurry is almost pathogen free and can be used as soil amendment (optionally after an aerobic composting post-treatment or drying/humification in a sludge drying bed). Biogas settlers are most often used at community or institutional level as first treatment step of a decentralized wastewater treatment systems (DEWATS) for the pre-treatment of biodegradable domestic or industrial wastewaters.