An **anaerobic lagoon** is a wastewater treatment system that uses oxygen-free biological processes to degrade organic matter in sewage or industrial water. in these lagoons; anaerobic bacteria decompose organic compounds; generating biogas; mainly methane and carbon dioxide. they are common in low-cost treatment plants and require long retention times to allow for sedimentation and stabilization of solids. although they do not completely eliminate pathogens and require additional steps for complete treatment; they are efficient in reducing organic load and can be combined with aerobic systems to improve effluent quality.