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Simple strategies to control the oxygen budget of an OLAND rotating biological contactor.

The oxygen-limited autotrophic nitrification/denitrification (OLAND) process is a one-stage process to remove ammonium from wastewaters with a low C/N ratio, combining partial nitritation and anammox. In OLAND, oxygen plays a crucial role in balancing the microbial functions, not only to provide sufficient oxygen for nitritation, but also to suppress nitratation and allow for sufficient anammox. A rotating biological contactor (RBC) is a simple and robust OLAND technology, yet it is not straightforward to control the oxygen load to the biofilm and the dissolved oxygen (DO) levels experienced by the biofilm when the biofilm is exposed to the atmosphere. Oxygen loads and bulk levels DO can be generically influenced by the rotation speed and the immersion levels of the discs, but the exact influences on the OLAND performance are not known. These aspects were studied in a stably performing OLAND 50-L RBC, which was fed with synthetic influent at a loading rate of 535 mg N/L.d and a hydraulic residence time of 46 h. At the references conditions with a disc rotation speed of 1.8 rpm and 50% immersion, a bulk DO of 0.2-0.9 mg O2/L and a stable nitrogen removal rate of 500 mg N/L.d were obtained, with no nitrite accumulation and low nitratation. These conditions indicate that the functions of the microbial community were ideally balanced. Currently, data are gathered from the effect of an increase of the immersion level to 75%, which will be followed by a disc rotation increase to 3.6 rpm, a decrease to 50% immersion and a decrease to 1.8 rpm. The direct effect of each of these actions is examined over a week. The results will lead to easy methods to control and restore the desired microbial OLAND balance and hence optimize nitrogen removal efficiencies.