Recirculation is used to remove or lower levels of nitrates, BOD5, TSS and other contaminants in wastewater. A recirculation tank collects the primary treated effluent from a pretreatment component and mixes it with highly treated wastewater from an advanced pretreatment component to dilute the wastewater and end up with an overall cleaner effluent. A recirculation tank is similar to a septic tank, and is a watertight tank where solids can settle and additional treatment can occur.
The recirculation ratio is the amount of wastewater that flows through the advanced pretreatment component divided by the amount of wastewater that is sent to the final treatment and dispersal component. Recirculation ratios are usually between 3:1 and 5:1, and can be changed depending on the desired level of treatment. In a typical treatment train that involves recirculation, wastewater flows from a septic tank to the recirculation tank. It is then dosed to an advanced pretreatment component, for example a media filter, where it goes through secondary treatment and is collected and returned to either the recirculation tank or sent to the final treatment and dispersal component.