Start-up of moving bed biofilm reactors for deammonification: the role of hydraulic retention time, alkalinity and oxygen supply

Aerobic and anaerobic ammonium oxidation can be combined in a completely mixed moving bed biofilm reactor, allowing for single-stage ammonium removal from wastewater with low COD/N ratio unsuitable for conventional nitrification/denitrification processes (‘deammonification’). Mandatory preconditions are: (a) a low hydraulic retention time to wash out suspended cells competing with mass transfer limited biofilm cells for alkalinity as limiting substrate; and (b) an oxygen flux adapted to the surface loading rate to prevent complete nitrification to nitrate. pH control or ‘NH₃ inhibition’ of nitrite oxidation are neither useful nor necessary. By this strategy, oxygen limited biofilms with simultaneous presence of NH₄-N and NO₂-N were enriched, which allowed for growth of anaerobic ammonium oxidizers. It could be demonstrated that a deammonifying reactor can be purposefully started up within a reasonable span of time and without prior inoculation, if this explicitly described strategy is applied. Depending on surface loading and air flow rate, N removal rates of 4–5 g N/m² d could be achieved at DO concentrations between 1.0 and 4.0 mg/l.