An Anaerobic Baffled Reactor (ABR) is an improved septic tank because of the series of baffles under which the waste water is forced to flow. The increased contact time with the active biomass (sludge) results in improved treatment.
The majority of settleable solids are removed in the sedimentation chamber at the beginning of the ABR, which typically represents 50 % of the total volume. The upflow chambers provide additional removal and digestion of organic matter: BOD may be reduced by up to 90 %, which is far superior to that of a conventional septic tank. As sludge is accumulating, desludging is required every 2 to 3 years. Critical design parameters include a hydraulic retention time (HRT) between 48 to 72 hours, up-flow velocity of the wastewater less than 0.6 m/h and the number of up-flow chambers (2 to 3).