Inorganic carbon limitation decreases ammonium removal and N2O production in the algae-nitrifying bacteria symbiosis system

Abstract

Ammonium removal by a symbiosis system of algae (Chlorella vulgaris) and nitrifying bacteria was evaluated in a long-term photo-sequencing batch reactor under varying influent inorganic carbon (IC) concentrations (15, 10, 5 and 2.5 mmol L^-1) and different nitrogen loading rate (NLR) conditions (270 and 540 mg-N L^-1 d^-1). The IC/N ratios provided were 2.33, 1.56, 0.78 and 0.39, respectively, for an influent NH₄⁺-N concentration of 90 mg-N L^-1 (6.43 mmol L^-1). The results confirmed that both ammonium removal and N₂O production were positively related with IC concentration. Satisfactory ammonium removal efficiencies (>98 %) and rates (29-34 mg-N gVSS^-1 h^-1) were achieved regardless of NLR levels under sufficient IC of 10 and 15 mmol L^-1, while insufficient IC at 2.5 mmol L^-1 led to the lowest ammonium removal rates of 0 mg-N gVSS^-1 h^-1. The ammonia oxidation process by ammonia oxidizing bacteria (AOB) played a predominant role over the algae assimilation process in ammonium removal. Long-time IC deficiency also resulted in the decrease in biomass and pigments of algae and nitrifying bacteria. IC limitation led to the decreasing N₂O production, probably due to its negative effect on ammonia oxidation by AOB. The optimal IC concentration was determined to be 10 mmol L^-1 (i.e., IC/N of 1.56, alkalinity of 500 mg CaCO₃ L^-1) in the algae-bacteria symbiosis reactor, corresponding to higher ammonia oxidation rate of ∼41 mg-N gVSS^-1 h^-1 and lower N₂O emission factor of 0.13 %. This suggests regulating IC concentrations to achieve high ammonium removal and low carbon emission simultaneously in the algae-bacteria symbiosis wastewater treatment process.

Keywords: Algae assimilation; Algae-nitrifying bacteria symbiosis system; Ammonia oxidation; Inorganic carbon; N(2)O production.