An anaerobic lagoon is a deep earthen basin with sufficient volume to permit sedimentation of settlable solids, to digest retained sludge, and to anaerobically reduce some of the soluble organic substrate. Raw wastewater enters near the bottom of the pond and mixes with the active microbial mass in the sludge blanket. Anaerobic conditions prevail except for a shallow surface layer in which excess undigested grease and scum are concentrated. Sometimes aeration is provided at the surface to control odors. An impervious crust that retains heat and odors will
develop if surface aeration is not provided. The discharge is located near the side opposite of the influent. The effluent is not suitable for discharge to receiving waters. Anaerobic lagoons are followed by aerobic or facultative lagoons to provide required
treatment