Electroflotation clarifier to enhance nitrogen removal in a two-stage alternating aeration bioreactor

Abstract

Stringent water treatment criteria and rapidly growing pollutant loads provoke the demand for retrofitting wastewater treatment plants towards a higher capacity. In this study, we assess a two stage alternating aeration (AA) bioreactor equipped with electroflotation (EF) clarifier, for nitrogen removal within a short hydraulic reTENTion time (HRT). The EF under steady solids loading required a minimum unit height and gas:solids ratio of 0.006 for efficient clarification. The separated sludge blanket was further thickened with retaining stability when the cyclic solids loading was smaller than 1.0 kg m⁻². In the continuous operation of the bioreactor, the returned activated sludge concentration increased to more than 18,000 mg L⁻¹, while the effluent suspended solids concentration was lowered below 5 mg L⁻¹. Under influent chemical oxygen demand (COD)/total inorganic nitrogen (TIN) concentration of 300/30 mg L⁻¹, the TIN removal efficiency was near 70% with cycle time ratios of 0.17 and 0.27. Under higher influent COD concentration of 500 mg L⁻¹, TIN removal efficiency was found to be 73.4% at a carbon:nitrogen (C:N) ratio of 10 and even higher (80.4%) at a C:N ratio of 16.6. The increased mixed liquor suspended solids concentrations (>6000 mg L⁻¹) under the high COD loading were efficiently maintained by using the EF clarifier. The results of this study demonstrate that an EF clarifier with a HRT of less than 1 h can support reliable nitrogen removal in the AA process that has a HRT of 6 h, even under increasing influent loadings.

Keywords:

• wastewater treatment
• alternating aeration bioreactor
• electroflotation
• activated sludge
• nitrogen removal

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Erratum

Acknowledgements

The authors would like to acknowledge the financial supports by the Korea Institute of Science and Technology (KIST), Bill and Melinda Gates Foundation (Reinvent the Toilet Challenge Grant OPP1037491) and SNU SIR Group of the BK21 research programme funded by Ministry of Education and Human Resources Development.