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 (ƒ_{0.5 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 ƒ_{0.5 mm} reached 21 mg/L, 72 %, 40.7 mL/g and 4.3 %, respectively.

Keywords:

• Granular sludge
• swine wastewater
• step-fed sequencing batch reactor

Acknowledgments

The authors wish to thank the Minnesota Legislature Rapid Response fund, National Key Basic Research Project of China (2002CB410807), and the Key Project of Science and Technology of Zhejiang Province (021103723) for funding this project.

Notes

^a ,

^b ,

^c The number in the bracket denotes the level of a certain factor.

*, significant;

**, highly significant.