Effects of eukaryotic predation on nitrifying MABR biofilms

This research explored the effects of eukaryotic predation on nitrifying membrane-aerated biofilm reactor (MABR) biofilms. Past research on heterotrophic MABR biofilms showed that predation could create internal voids that promoted sloughing. However, the no past research addressed the effects of predation on nitrifying MABRs, even though nitrification is the most common MABR application. Nitrifying biofilms are typically denser, and ammonia oxidizing bacteria (AOB) form large, dense clusters within the biofilm. This could affect their susceptibility to predation. Nitrifying biofilms were grown in flat-sheet MABRs. Images of the biofilm were captured using optical coherence tomography (OCT). For detachment tests, an increased shear flow (Re≅140) was used, and a shear rheometer was used to measure the biofilm mechanical properties. The nitrifying community was analyzed with fluorescence in situ hybridization (FISH) and quantitative PCR (qPCR). Predation increased internal void ratios from 54 ± 5% to 69 ± 6%. Biofilms were weakened by predation, with a storage modulus (G') and loss modulus (G'') of 242 ± 135 and 1,649 ± 853 Pa with predation and 3,644 ± 1,857 and 23,334 ± 11,481 Pa for the control with suppressed predation. Predation increased the relative biofilm detachment from 4 ± 5 to 18 ± 12%, decreased the amount of biomass, i.e., the average biofilm thickness, from 502 ± 150 to 266 ± 54 µm, and decreased the nitrification flux from 1.00 to 0.61 g NH₄⁺-N/m²day. Also, predation decreased the abundance of nitrite oxidizing bacteria (NOB) relative to AOB, consistent with the observed nitritation. These results show that predation can significantly impact the structural stability, bacterial community and removal rates of nitrifying MABR biofilms. Lumping the effects of predation into the detachment or decay coefficients of biofilm models may not accurately reflect the behavior of nitrifying MABR biofilms.