Nitrogen removal in a low-loaded single tank sequencing batch reactor

Biological processes for nitrogen removal from wastewaters have been extensively studied. System operation to successfully achieve nitrification and denitrification has led to many inno vative designs. While twoor three-sludge systems can offer conceptual appeal, single-sludge systems have been shown to produce sufficient removals and also reduce capital costs. In addition, systems designed to denitrify without supplemental carbon addition markedly cut operating costs. Within the past decade, several process configurations that stressed the alter nation of aerobic and anoxic conditions have been reported. These included: alternating the operation of two tanks in series;1 alternating aeration in a semi-plug flow, 2-cell system;2 an alternately aerated unit followed by a continuously aerated cell;3 a carousel arrangement with alternating aerobic/anoxic zones and stepfeeding;4 a plug flow compartmentalized reactor with aerobic and anoxic cells,5 and stepfeeding;4-5 intermittent operation of an extended aeration system;6 and operation of a concentric oxidation ditch system with stepfeeding.7 All these systems are continuous flow, and used organics in raw waste water as the carbon source for denitrification. Sequencing batch reactors (SBRs) provide a viable treatment alternative for nitrogen removal without supplemental carbon addition.8"10 Flexible operation of individual SBR modes (for example, fill, react, settle, draw, and idle) allow manipulation of operating strategy to suit various treatment requirements.11-12 Miller813 showed that nitrification easily occurred in low loaded SBRs, but that denitrification was difficult to maintain. Irvine et al.10 achieved approximately 90% nitrogen removal in a high loaded full-scale SBR. This was conducted to demonstrate how denitrification can be achieved in a low loaded SBR. In particular, the results demonstrated that an operating strategy which encouraged some denitrification to occur during fill, that is, when an exogenous electron donor was present, would remove a large percentage of the incoming nitrogen.