Elevated loading rates as a low operational intensity and small land footprint design strategy to achieve partial nitritation

An urgent need exists for cost-effective, low operational intensity and small land footprint ammonia treatment systems to meet stringent wastewater discharge regulations. A moving bed biofilm reactor system operating under a novel design strategy using elevated total ammonia nitrogen (TAN) loading rates ranging from 3 to 6.5 g TAN/m2·d has shown promise to achieve robust partial nitritation (PN) and the oxidation of TAN with limited oxidation of nitrite without the need for intense operational measures. An investigation of the biofilm, the viability of the embedded cells and the microbial ecology is necessary to fully understand this low operational intensity design. The investigation revealed that, at a loading rate of 6.5 g TAN/m2·d, the abundance of nitrite oxidizing bacteria (NOB) was 2 ± 0.46%; however, the NOB activity was negligible with 99 ± 0.2% nitrite accumulation; thus, the mechanism to attain robust PN is shown to be the suppression of NOB activity as opposed to NOB population suppression. This decrease in NOB activity is associated with an increase in thickness and mass of the biofilm, as well as the relative increase in ammonia oxidizing bacteria (AOB) that preferentially uptake oxygen compared to NOB. As such, this design strategy results in the suppression of the NOB activity due to limited access to oxygen via mass transfer limitations through thick biofilm and competition with an augmented AOB population in the biofilm. This research presents a low operational intensity and small footprint PN strategy that can be an effective alternative to more intensive strategies. [Display omitted] •Need for low operation intensive strategy to achieve stable partial nitritation.•MBBR operated at elevated loading rates of 3 to 6.5 g TAN/m2·d.•Stable partial nitritation achieved at loading rate of 6.5 g TAN/m2·d.•NOB activity suppression observed despite presence of NOB.•Proposed mechanism of NOB suppression is preferential oxygen uptake by AOB.