Optimization of main factors associated with nitrogen removal in hybrid sludge sequencing batch reactor with step-feeding of swine wastewater

Abstract

To attain a high nitrogen removal efficiency and good sludge settleability in a step-fed sequencing batch reactor (SFSBR) treating swine wastewater, L9(34) orthogonal experiments were carried out to optimize main factors associated with nitrogen removal, namely, the influent C/N ratio, feeding volume ratio, nitrogen loading rate and aeration intensity. Results showed that nitrogen loading rate contributed most for the build-up of NO2 −-N, NO3 −-N and NH4 +-N in the effluent, while aeration intensity was the most important factor for net nitrogen removal efficiency based on the initial and final nitrogen concentrations in the SFSBR cycle. Additionally, the periodic starvation created by stepwise feeding was the major inducing force for granulation in the SFBSR process and the influent C/N ratio had a profound influence on sludge settleability and granular sludge stability in terms of sludge volume index (SVI) and the fraction of granular sludge with diameter over 0.5 mm (ƒ0.5 mm ), respectively. Considering the most and secondary important control factor for individual response index, the optimal operating condition for nitrogen removal of SFSBR treating swine wastewater was determined as A3B3C1D2, i.e., influent C/N ratio 7.0 mg COD/mg NH4 +-N, feeding volume ratio 3:1, nitrogen loading rate 0.026 g NH4 +-N/gVSS · d and aeration intensity 4.2 L/m3 · s, respectively. Under the optimal operating conditions, inorganic nitrogen concentration in the effluent, net nitrogen removal efficiency, SVI and ƒ0.5 mm reached 21 mg/L, 72 %, 40.7 mL/g and 4.3 %, respectively.