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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 Paracoccus; (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 …