Single-stage versus two-stage partial nitritation - anammox reactor systems for deammoniafication under hypersaline conditions.

The production of increasing amounts of high salinity wastewaters in our industrialized society has prioritized their treatment to prevent environmental pollution. The partial nitritation - anammox (PN/A) process for nitrogen removal has been little investigated for hypersaline wastewaters (salinity greater than 3%). In the investigation presented here, single-stage versus two-stage partial nitritation - anammox (PN/A) reactor systems for deammonification at 4% (40 g/kg) saline conditions were investigated and compared in completely mixed fixed bed reactors. In the two-stage system, the first stage reactor achieved a nitritation rate of 1.9 gN/L-reactor/d. Effluent from the partial nitritation reactor was then fed to the second two-stage anammox reactor and the maximal nitrogen removal of 0.8 g/L-reactor/d was achieved. The dominant microbial species for the ammonia oxidizing and anammox reactions in the nitritation (first) reactor and the second reactor were identified as Nitrosococcus oceani and Candidatus Scalindua wagneri, respectively, both obligate halophiles. In the single-stage reactor, deammonification rates reached 0.6 gN/L-reactor/d. Nitrosomonas marina and Candidatus Scalindua wagneri were the dominant AOB and anammox bacteria, respectively. Maintaining free ammonia (FA) concentrations above 1 mg/L was found to selectively inhibit nitrite oxidizing bacteria (NOB) and resulted in long term stable nitritation. At FA concentrations lower than 1 mg/L, nitrate began to appear after 20 days of reactor operation. Nitritation was recovered after increasing FA in the reactor to inhibitory concentrations. Overall N₂O emissions were shown to be significantly lower in the single-stage PN/A reactor than the two stage PN/A reactor system.