Development of a hybrid treatment system for combined sewer overflows using a hydrocyclone and a dissolved air flotation system

Abstract

Combined sewer overflows (CSOs) contain a mixture of untreated, contaminated sanitary, and storm water. CSOs include both precipitated and suspended matters. In this study, a novel system is proposed to treat CSOs using a combined hydrocyclone and dissolved air flotation (DAF) system for removal of both precipitated and suspended matter. The optimum operating conditions were determined by changing the internal flow rate and the split ratio (R_f) in the hydrocyclone and by changing the angle of the nozzle to determine the floc size, flow rate, and recycle ratio in the DAF system. The types of organic matter contained in prepared synthetic water samples are categorized as soluble COD_cr (SCOD) and particulated COD_cr (PCOD) in the characterization of the removal rate of the precipitated and suspended matter. The suspended solids (SS) were classified by their specific gravity to determine the removal rate of both precipitated and suspended matter. In a pretreatment process, the system employed a cationic polymer to obtain a low gradient time (G × t) value. The results showed that poly-aluminum chloride (PAC) used with the cationic polymer had a 9.2% higher turbidity removal rate than PAC alone, due to a stable floc being achieved in the hydrocyclone. The optimum operating conditions of the hydrocyclone showed the highest efficiency at R_f = 10% at a flow rate of 2.8 m/s; the DAF-treated solutions showed a higher removal rate for SCOD solutions vs. PCOD solutions, with the highest turbidity removal rate of 65%. In contrast, when the hydrocyclone-treated solutions had a higher PCOD than SCOD, they showed a higher removal efficiency. The removal rates of SS, COD_cr, turbidity, T-P, and T-N of the total process were 77, 80, 98, 98, and 45%, respectively.

Keywords:

• CSOs
• Dissolved air flotation
• Floc size
• Hydrocyclone
• In-line static mixer
• Strength factor

Acknowledgment

This research was supported by Korea Ministry of Environment (MOE) as “Advanced Technology Program for Environmental Industry” (E314-00015-0412-1).

Notes

Presented at the 7th International Conference on Challenges in Environmental Science and Engineering (CESE 2014) 12–16 October 2014, Johor Bahru, Malaysia