pH role in partial nitritation of anaerobic digester reject water. Reaction rates, inhibition, and biomass properties

The nitritation process (ammonia oxidation to nitrite) is successfully applied in different processes used for reject water treatment. The impact of pH was investigated in experiments performed at pH 6.0, 6.5, and 7.0 using real dewatering liquor. The overall process performance in terms of the effluent quality and stability was compared. Efficient nitratation suppression was achieved at pH 6.5 and below due to the strong free nitrous acid (FNA) inhibition of nitrite oxidizers. The nxr gene activity showed the highest NOB suppression at pH 6.5. The process rate depended on the pH and nitrogen loading, and was in the range of 13.2–16.5, 8.7–23.3, and 34.0–37.9 g N/(kg·VSS·h), at pH 6.0, 6.5, and 7.0, respectively. The highest inhibition constant was estimated to be 0.905 g HNO2-N/m3 at pH 6.0. Decrease in pH also improved the AOB tolerance to free ammonia up to Ki, NH3 = 97.2 mgNH3-N/dm3. The affinity constants were 0.40, 0.34, and 0.12 mg NH3-N/dm3 at pH 6.0, 6.5, and 7.0, respectively. Microbial analysis revealed that the flocculation could be attributed to the Accumulibacter and Competibacter extracellular polymeric substance (EPS) production, which can also serve as a protector agent for nitrifiers.