Methanol and thiosulphate from pulp mill waste streams support denitrification under salinity stress

[Display omitted] •CH3OH and S2O32- driven denitrification under salinity stress was studied in FBBRs.•Heterotrophic denitrification was more salt resistant than autotrophic.•Heterotrophic denitrification had higher denitrification rates than autotrophic.•Foul condensate from pulp mill evaporators supported and promoted denitrification.•Denitrification of wet scrubber wastewater possible with in-mill electron donors. Methanol and thiosulphate are two common compounds found in pulp mill environment. In this study, for the first time, methanol driven, and thiosulphate driven denitrification under salinity stress were studied in fluidized bed bioreactors (FBBR) in a comparative manner. Concentration of salinity, due to Cl-, Na+, SO42- and K+ that are present in saline industrial wastewaters, and nitrate loading rate were gradually increased up to 8.1 % and 4.67 ± 0.30 gNO3--N/L/d to determine the reactors’ resistance and denitrification rate capacity. Moreover, the suitability of real pulp mill methanol-rich foul condensate as alternative electron donor was revealed in the heterotrophic reactor. With foul condensate, heterotrophic denitrification maintained > 97 % nitrate removal at a rate of 2.40 ± 0.10 gNO3--N/L/d under 6.8 % salinity, also decreasing nitrite accumulation as compared to pure methanol. Thiosulphate driven autotrophic denitrification showed lower salinity and nitrate loading tolerance than the methanol driven, removing nitrate > 97 % at salinity up to 5.6 % and rate of 1.27 ± 0.10 gNO3--N/L/d. Microbial community composition in the heterotrophic reactor remained stable at different salinities, with denitrifying genera Methylovorus, Paracoccus and Pseudomonas playing the major roles. On the other hand, in the thiosulphate driven reactor, inoculated with a pure culture of Thiobacillus denitrificans, many other microorganisms, including Moheibacter, became enriched. However, nitrate removal was mainly linked with T. denitrificans abundance as indicated by decreasing share of the community co-occurring with the decrease of the reactors’ performance. In summary, this study showed that thiosulphate and foul condensate supported denitrification under salinity stress, indicating potential for high-rate fluidized bed pre-treatment for nitrate removal from pulp mill NOx-SO2 scrubber wastewater prior to direction to subsequent activated sludge process.