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ANITA™Mox is a Veolia process using moving-bed biofilm reactor (MBBR) technology tested and validated in full-scale for energy- and cost-effective autotrophic N-removal from sidestream effluent using anammox (ANaerobic AMMonium OXidation) bacteria. In order to increase the ANITA™Mox process performances under different operating conditions (e.g. mainstream and sidestream application), substrate transport and accessibility inside the biofilm must be enhanced. In this work, (i) two laboratory scale biofilm ANITA™Mox reactors were operated using different configurations (IFAS - integrated fixed-film activated sludge - and MBBR) and (ii) the distribution of the anammox (AnAOB) and ammonia-oxidizing bacteria (AOB) in the suspended sludge and the biofilm was characterized using molecular tools (qPCR). This study showed that in IFAS configuration, the ANITA™Mox process achieved very high N-removal rate (up to 8 gN/m².d), which was three to four times higher than that achieved in the pure MBBR mode. The high concentration of suspended solids (mixed liquor suspended solids (MLSS)) in the bulk obtained within the IFAS mode induces a very efficient bacterial distribution between the AOB and AnAOB population. AnAOB activity mainly occurs in the biofilm (96% of total AnAOB in the reactor), whereas nitritation by AOB mostly takes place in the suspended phase (93% of total AOB). This spatial distribution observed in the IFAS reactor results from a natural selection due to more easily substrate accessibility for AOB in the bulk (NH4(+), O2) creating higher nitrite concentration in the bulk liquid compare to pure MBBR mode. The efficient control of MLSS level in the IFAS reactor is a key parameter to enhance the nitrite production by AOB and increase the substrate availability in the AnAOB-enriched biofilm leading to higher N-removal rate. These promising results obtained at laboratory scale have been further confirmed in on-going full-scale IFAS ANITA™Mox trials opening new roads for the widespread application of a very compact and robust ANITA™Mox process for sidestream but also mainstream cost-effective N-removal.

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