Biological nitrogen and organic matter removal from tannery wastewater in pilot plant operations in Ethiopia

The objective of this study was to set-up a pilot plant and to evaluate its effectiveness for biological nitrogen and organic matter removal from tannery wastewater in Ethiopia. A pilot wastewater treatment plant consisting of a predenitrification-nitrification process was constructed and operated for 6 months. This was fed with a raw tannery wastewater obtained from the Modjo Tannery located 70 km south of the capital, Addis Ababa. Up to 98% total nitrogen and chemical oxygen demand, and 95% ammonium nitrogen removal efficiencies were achieved in the system. The average effluent ammonium nitrogen ranged from 8.4 mg l−1 to 86.0 mg l−1, whereas the average effluent for nitrate nitrogen ranged from 2.9 mg l−1 to 4.4 mg l−1. The average values of denitrification and nitrification rates determined by nitrate and ammonium uptake rates (NUR and AUR) were 8.0 mg NO3-N [g volatile suspended solids (VSS)]−1 h−1 and 5.4 mg NH4-N (g VSS)−1 h−1, respectively, demonstrating that the treatment processes of the pilot plant were effective. Further studies of the effect of chromium III on AUR showed 50% inhibition at a concentration of 85 mg l−1, indicating that this metal was not causing process inhibition during performance operations. Thus, the predenitrification-nitrification process was found to be efficient for simultaneous removal of nitrogen and organic substrates from tannery wastewaters.