Denitrification Under Salinity Stress in Biofilm Reactors

Denitrification represents a sustainable solution to remove nitrate from saline wastewaters generated by many industrial activities such as food processing, aquaculture, flue gas washing, etc. To improve the process’ performance under salinity inhibition, biofilm reactors could represent an elegant strategy, but a systematic analysis focusing on the reactor system under salinity stress is needed for their optimisation. Moreover, since the electron donors available for the process may vary, the comparison between autotrophic and heterotrophic denitrification is relevant. The aim of this study was to investigate the potential and inhibition tolerance of methanol-driven and thiosulphate-driven denitrifying bacteria from non-saline sources for the treatment of saline wastewaters. Also, their denitrification performance in fluidised bed biofilm reactors (FBBR) and in moving bed biofilm reactors (MBBR) were studied, optimizing the process via reactor and biofilm carrier selection, and a salinity inhibition modelling strategy was proposed for the biofilm. Methanol-driven denitrification showed higher tolerance to inorganic ions than thiosulphate-driven, and for both, chloride had the most inhibitory role. This was also observed in the FBBRs, where the former showed higher removal rates (2.4 vs. 1.2 gN/L/d) at higher salinities (6.8 vs 5.6%), with stable denitrifying communities at increasing salinities, than the latter, dependent on Thiobacillus denitrificans activity. In the MBBR, where hollow- and porous- body carriers were compared for methanol-driven denitrification, a more exposed biofilm in the hollows showed higher removal rates at salinities <7.5%, while the less exposed biofilm in the pores had more stable nitrogen removal up to 9.5% salinity. With both carriers, an inhibition factor was implemented in an existing biofilm model and successfully reproduced denitrification kinetics under salinity stress. Eventually, the potential of denitrification for the pre-treatment of NOx-SO2 scrubber saline wastewater was confirmed, and pulp mill foul condensate as electron donor promoted nitrogen removal under salinity stress. Overall, this study showed that methanol- and thiosulphate- driven denitrification have potential for the treatment of industrial saline wastewaters, whose salinity tolerance and nitrogen removal performance can be improved by selecting biofilm reactors and optimised with suitable biofilm carriers.