Factors affecting nitrite build-up in nitrifying biofilm reactor

Nitrite build-up in a nitrifying biofilm reactor for direct denitrification from the nitrite stage was investigated. At least three factors were found to influence the nitrite build-up: (i) the relative specific growth rates of Nitrosomonas to Nitrobacter, microNs/microNb in the biofilm; (ii) the relative initial ratio between Nitrosomonas and Nitrobacter on the support surface, (Mao)Ns/(Mao)Nb; (iii) the level of free ammonia, particularly at greater than 0.1 mg N/l that can be inhibitory to Nitrobacter. Results showed that without free ammonia inhibition, the first two factors determined the degree of nitrite build-up. The effect of (Mao)Ns and (Mao)Nb on nitrification kinetics were more significant than the specific growth rates of Nitrosomonas and Nitrobacter. By regulating effects of these three factors, extremely high concentrations of nitrite build-up could occur in a biofilm reactor. This offers a potential for achieving direct denitrification through a nitrate shunt.