Anaerobic lagoons have steep sides and can be as deep as 20 feet. They develop a thick crust on the surface which inhibits oxidization, but serves to trap in heat. This makes anaerobic lagoons more suited to colder climates. They typically use less land, but require a longer retention period than the others. They can cause odors, especially when being cleaned.
Facultative lagoons have an aerobic top layer and an anaerobic bottom layer.
The algae helps the treatment process by using nutrients in the wastewater. Facultative lagoons cause less odor, but may have problems functioning during cold periods when ice forms on the surface. Aerated lagoons and stabilization ponds are both aerobic systems. Stabilization ponds are the shallowest of all the systems, usually just 2 feet deep. They rely on surface diffusion of oxygen and algae growth to oxygenate the wastewater.