Efficiency of a multi-stage anoxic/oxic process treating tannery wastewater following a biochemical pretreatment

Ammonia nitrogen (NH₄⁺-N) removal is not satisfactory in the existing tannery wastewater treatment processes. In order to solve the problem, a multi-stage anoxic/oxic process (MAOP) was introduced as a secondary biological treatment unit to treat tannery wastewater deeply. The MAOP was a four-stage anoxic/oxic process and the influence of step-feeding, hydraulic retention time (HRT) and sludge return rate (R) on its simultaneous removal efficiency of chemical oxygen demand (COD) and NH₄⁺-N was investigated. The results showed that the MAOP was effective in COD removal. Whatever the step-feeding was carried out, the effluent COD was less than 300 mg/L with HRT 24 h and solid residence time (SRT) 18 d, coming up to the secondary emission standards of GB8978-1996. With the same HRT and SRT, the NH₄⁺-N removal was up to 97.7% when the inflow distribution ratio and R were 4:3:2:1 and 100% respectively. The effluent NH₄⁺-N about 3.6 mg/L met the primary emission standards of GB8978-1996. A multiple mechanism of denitrification, shortcut nitrification, simultaneous nitrification and denitrification could be occur in the MAOP. It was suggested that MAOP was a promising biological denitrification process for tannery wastewater treatment.