Model-based comparison of sequencing batch reactors and continuous-flow activated sludge processes for biological wastewater treatment

This study compares, based on mathematical modelling, continuous-flow activated sludge processes (CAS) and sequencing batch reactors (SBR). The steady-state or periodic steady-state of CAS and SBR processes is compared for a feed composed of readily or slowly biodegradable substrates, for carbon and for carbon and nitrogen removal. The simulations are carried out for different values of the SRT (solids residence time), HRT (hydraulic residence time), number of cycles and internal recycle. If the SRT is large enough, the SBR can remove the influent biodegradable COD completely, while the CAS has a residual effluent COD concentration. For carbon and nitrogen removal, the SBR can remove the ammonia completely while the CAS has a residual effluent ammonia concentration. For typical values of the operating parameters, the CAS gives higher total nitrogen removal than the SBR, which becomes comparable to the CAS only for a large number of cycles per day.