What type of pond has an aerobic top layer and an anaerobic bottom layer?

A facultative pond is designed to have distinct aerobic and anaerobic layers due to its biological processes. In these ponds, the upper layer receives sunlight, promoting the growth of algae and aerobic bacteria that utilize oxygen during the decomposition of organic matter. This aerobic layer is essential for breaking down waste, while the lower layer, being deeper and lacking sufficient light and oxygen, becomes anaerobic. Anaerobic bacteria thrive in this environment, helping to further decompose organic materials.

This dual-layer system effectively treats wastewater by balancing aerobic and anaerobic processes, which is not a characteristic of retention ponds, settling ponds, or infiltration ponds. Retention ponds primarily serve to collect and manage stormwater runoff with little focus on biological treatment. Settling ponds are designed to allow solids to settle out of the water, and infiltration ponds are used to let water seep into the ground, focusing on groundwater recharge rather than biological processes. Thus, the facultative pond's ability to facilitate both aerobic and anaerobic decomposition makes it particularly effective for wastewater treatment.