Effects of light and mass ratio of microalgae and nitrifiers on the rates of ammonia oxidation and nitrate production

[Display omitted] •The activity of enriched nitrifiers was significantly inhibited by light.•High concentrations of microalgae slowed oxygen production due to self shading effect.•The inhibitory effect of light on nitrifiers was alleviated in the co-culture.•Reactive oxygen species affected nitrification negatively.•Electricity consumption of coculture was lower than that of aerated bacteria culture. This study investigated the effects of microalgae and light on nitrifying bacteria. The oxygen production rates of microalgae were measured at various cell concentrations, and it was observed that 500–2000 mg/L of microalgal concentration supplied sufficient oxygen for bacterial nitrification without forced aeration. The effects of light on nitrification were investigated by determining the ammonia oxidation rate (AOR) and nitrate production rate (NPR) of enriched nitrifiers in the absence of microalgae with forced aeration, and it was observed that those rates were significantly lowered in the presence of light. However, the inhibitory effects of light on AOR and NPR were partially reduced in the co-culture owing to the self-shading effect. AOR and NPR of co-cultures with Chlorella vulgaris and Scenedesmus quadricauda were high at a low mass ratio of microalgae to enriched nitrifiers, while those with Desmedesmus communis increased as the mass ratio increased. These species-specific phenomena could be attributed to the different production of reactive oxygen species. Despite the negative effects of light and reactive oxygen species, it was estimated that the energy consumption of the co-culture was lower than that of the mechanically-aerated nitrifiers.