The inhibitory effects of reject water on nitrifying populations grown at different biofilm thickness

- Nitrifying MBBR biofilms were exposed to reject water to achieve NOB suppression.
- Two types of biofilm carriers, with different biofilm thickness, were compared.
- The biofilms differed considerably in microbial composition, structure and density.
- Reject exposure caused an increased nitrite production in the thinner biofilm.
- Free ammonia correlated with the reject exposure and may be the cause of inhibition.

Suppression of nitrite oxidizing bacteria (NOB) is of vital importance to achieve successful, energy efficient, mainstream anammox processes for wastewater treatment. In this study, biofilm carriers from a fully nitrifying MBBR system, fed with mainstream wastewater, were temporarily exposed to reject water from sludge dewatering, to evaluate this as a possible strategy to inhibit NOB and achieve nitrite production under realistic conditions. Two different carrier types were compared, in which biofilm thickness was maintained at approximately 400 and 50 μm, respectively, and reject treatment was tested at different exposure time and loading rates. Reject exposure almost always resulted in an increased nitrite production in the thinner biofilm, and overall, nitrifiers growing in the thin biofilm were more sensitive than those grown in the thicker biofilm. The effect from reject exposure remained in the systems for four days after returning to mainstream operation, with nitrite production gradually increasing for three days. Increased concentrations of free ammonia correlated with reject exposure and may be the cause of inhibition, although other factors cannot be excluded.

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