Start-up of Autotrophic Nitrogen Removal Reactors via Sequential Biocatalyst Addition

A procedure for start-up of oxygen-limited autotrophic nitrification-denitrification (OLAND) in a lab-scale rotating biological contactor (RBC) is presented.In this one-step process, NH 4 + is directly converted to N 2 without the need for an organic carbon source.The approach is based on a sequential addition of two types of easily available biocatalyst to the reactor during start-up: aerobic nitrifying and anaerobic, granular methanogenic sludge.The first is added as a source of aerobic ammonia-oxidizing bacteria (AAOB), the second as a possible source of planctomycetes including anaerobic ammonia-oxidizing bacteria (AnAOB).The initial nitrifying biofilm serves as a matrix for anaerobic cell incorporation.By subsequently imposing oxygen limitation, one can create an optimal environment for autotrophic N removal.In this way, N removal of about 250 mg of N L -1 d -1 was achieved after 100 d treating a synthetic NH 4 + -rich wastewater.By gradually imposing higher loads on the reactor, the N elimination could be increased to about 1.8 g of N L -1 d -1 at 250 d.The resulting microbial community was compared with that of the inocula using general bacterial and AAOB-and planctomycete-specific PCR primers.Subsequently, the RBC reactor was shown to treat a sludge digestor effluent under suboptimal and strongly varying conditions.The RBC biocatalyst was also submitted to complete absence of oxygen in a fixed-film bioreactor (FFBR) and proved able to remove NH 4 + with NO 2 -as electron acceptor (maximal 434 mg of NH 4 + -N (g of VSS) -1 d -1 on day 136).DGGE and real-time PCR analysis demonstrated that the RBC biofilm was dominated by members of the genus Nitrosomonas and close relatives of Kuenenia stuttgartiensis, a known AnAOB.The latter was enriched during FFBR operation, but AAOB were still present and the ratio planctomycetes/ AAOB rRNA gene copies was about 4.3 after 136 d of reactor operation.Whether this relates to an active role of AAOB in the anoxic N removal process remains to be solved.