Heterotrophic Nitrifiers Dominate Reactors Treating Incineration Leachate with High Free Ammonia Concentrations

Anaerobically treated leachate from municipal solid waste incineration plants contains extremely high free ammonia (FA) concentrations, which can hinder short-cut nitrification. pH adjustments made during the nitrification process kept FA concentrations below 110 mg/L, enabling reactors to operate when total ammonia nitrogen (TAN) concentrations were 1400 mg/L. Multiple lines of evidence showed that nitrite accumulation in these systems could be attributed to ammonia oxidation by heterotrophic nitrifiers from the genus Paracoccus. For example, (1) all of the amoA (ammonia monooxygenase subunit A) transcripts were clustered with Para coccus; (2) nitrite accumulated when membrane-bound nitrate reductase (Nar) was inhibited with sodium azide; (3) periplasmic nitrate reductase (nap) genes were not being actively transcribed; and (4) trace concentrations of nitrate in reactors were not sufficient to support nitrate reduction. Paracoccus species were actively transcribing genes from nitrification (amoA) and denitrification (narG, norB, nirS, nosZ) pathways in these systems even when FA concentrations reached 184 mg/L. Little is known about the role that heterotrophic ammonia oxidizing bacteria (AOB) play in nitrogen cycling in reactor systems. Therefore, this study is significant because this is the first investigation into the metabolism of heterotrophic nitrifiers in nitrogen removal systems with elevated FA concentrations.