Inhibition-level-based carrier selection and kinetic insights for optimised denitrification of saline wastewaters in MBBR

The moving bed biofilm reactor (MBBR) provides a high-rate treatment process for the denitrification of saline industrial wastewaters. The carrier structure regulates the biofilm-liquid contact area and can limit the biofilm exposure not only to substrate but also to inhibitors and thus, affects the process performance. However, the potential for biofilms on different carriers to perform differentially under varying salinities remains unexplored as MBBR optimisation strategy. In this study, the nitrogen removal kinetics and biofilm dynamics of methanol-driven denitrification in two MBBRs with a hollow-body and a porous-body carrier were systematically analysed to assess their best performance at salinities 1-9.5 %. Between 1 and 7.5 % salinity, both MBBRs removed over 90 % nitrogen, with the more exposed biofilm in the hollows resulting in higher removal rates. At 9.5 % salinity, the protected biofilm within the pores resulted in a more stable removal (89.5 ± 1.6 %) than in the more exposed hollows (80.9 ± 6.5 %). In both MBBRs, stable and similar denitrifying communities developed, dominated by Paracoccus, Methylovorus and Hyphomicrobium. Moreover, hypersaline (>3.5 %) wastewater from pulp mill NO_x-SO₂ scrubber was successfully denitrified in MBBRs, showing the potential of biofilm processes for its treatment. Eventually, denitrification kinetics under salinity inhibition in the biofilm was modelled for both carriers by introducing an inhibition factor to the existing Sumo©22 biofilm model. In summary, this study provides the basis for optimising saline industrial wastewater treatment in MBBR through selecting the typology of biofilm carrier based on the inhibition level and for predicting the biofilm process performance with a simplified modelling approach.