Effect of solid hold-up on nitrite accumulation in a biofilm reactor -: molecular characterization of nitrifying communities

Biological ammonium oxidation was carried out in two inverse turbulent bed reactors fed with synthetic mineral wastewater containing a high ammonium concentration (100 mg N-NH4+/L). Both reactors were started-up and operated in the same conditions except for the solid carrier concentration: the solid hold-up ratios applied, defined as the ratios of static to expanded bed height, were 0.1 and 0.3 in reactors R-10 and R-30 respectively. These two solid hold-up ratios generate different particle-to-particle collision frequencies and, therefore, detachment forces. The influence of solid hold-up on biofilm growth and nitrifying performance was studied from a macroscopic (i.e. nitrate and/or nitrite production) and microbiological point of view. After 60 days of operation, both reactors contained the same amount of biomass. However, R-10 produced only nitrate while nitrite accumulated in R-30. A comparison of microbial populations in the reactors showed that R-10 contained both ammonium and nitrite oxidizing populations such as Nitrosomonas and Nitrospira, whereas in R-30, ammonium oxidizing populations were much greater than those of nitrite oxidizers. The major ammonium-oxidizing organism was not the same in both' reactors.