Nitrogen removal from multi-electrolyte saline wastewater via mainstream anammox in warm climate conditions

High salts concentrations in wastewater hinder its biological treatment. Recent research has investigated the inhibitory effect of salinity on the anammox process, mainly focusing on NaCl. Thus, the inhibition caused by multi-electrolytes salinity on freshwater anammox bacteria remains unclear. In this study, the anammox process was evaluated for the treatment of multi-electrolyte saline wastewater (NaCl, MgCl₂, and CaCl₂) during 684 days in three operational phases. In Phase 1, the anammox inoculum was successfully adapted from sidestream (232 mgN.L^-1) to mainstream (60 mgN.L^-1) conditions, with no damage to the reactor performance, at an hydraulic retention time of 1.4 h. In Phase 2, salinity was gradually increased in the synthetic medium to adapt the freshwater anammox bacteria. The anammox bacteria tolerated a total salinity of 0.72 wt% (in g.L^-1: 4.7 NaCl, 2.0 MgCl₂, and 0.6 CaCl₂), achieving an 84.3 ± 0.8% nitrogen removal efficiency. The presence of salts favored the Ca. Jettenia genus over Ca. Brocadia after long-term exposure to salinity. Finally, in Phase 3, anaerobically pre-treated saline wastewater (0.72 wt%) was applied to the anammox reactor. The presence of residual organic matter (53 mgCOD.L^-1; COD/N of 0.86) resulted in partial deviation of the metabolic pathway from anammox to, especially, nitrite heterotrophic denitrification, resulting in the accumulation of ammonia-N in the effluent. Even so, the anammox process was predominant, being responsible for 83% of the nitrogen removal. The presence of both organic matter and salinity led to a shift in dominance from the Ca. Jettenia genus to Ca. Brocadia.