The solids retention time (SRT) is the time the solid fraction of the wastewater spends in a treatment unit. It is the quantity of solids maintained in the reactor divided by the quantity of solids coming out of the reactor each day: SRT = V *Cd / Qout* Cout. Where V is the digester volume; Cd is the solids concentration; Qout is the volume wasted each day and Cout is the solids concentration of the effluent. In a conventional, completely mixed, or plug flow reactor, the HRT equals the SRT. In retaining biomass reactors (e.g. anaerobic biogas settlers, septic tanks) the SRT exceeds the HRT.