BACKGROUND: Sodium oxalate is a key organic contaminant in alumina industry, which diminishes process yields and product quality. Given that Bayer process liquor is typically deficient in nitrogen (N), there is external supplementation of N in current full-scale biological treatment processes. This study, for the first time, examines oxalate degradation under N deficient conditions using two parallel biofilm-reactors, one N-supplemented and the other under N-deficient conditions. Oxalate degradation rates and oxygen uptake rates (OUR) were determined at different bulk water dissolved oxygen (DO) set-points. RESULTS: The results revealed that oxalate removal rates (33-111 mg h(-1) g(-1) biomass) linearly correlate with OUR (0-70 mg O-2 h-1 g(-1) biomass) in the N-supplemented reactor. However, in the N-deficient reactor, a linear increase of oxalate removal was recorded only with DO up to 3 mg L-1. Surprisingly, anaerobic oxalate removal was evident even in the presence of DO (up to 8 mg L-1) in both reactors. Further elucidation revealed formate, acetate and methane by-products during anaerobic oxalate removal in both reactors. CONCLUSION: This study revealed the feasibility of aerobic oxalate oxidation and fermentation under alkaline and N-deficient conditions. Further, this study confirms the critical role of DO in aerobic oxalate biodegradation. (C) 2017 Society of Chemical Industry