Water Environment Federation (WEF)

Biologically and Chemically Enhanced Clarification for Improved Treatment of Wet-Weather Flows


The Milwaukee Metropolitan Sewerage District (MMSD) conducted demonstration testing in 2005 to evaluate high-rate chemically enhanced clarification (CEC) followed by ultraviolet (UV) disinfection, for the treatment of wet-weather flows at MMSD’s two wastewater treatment plants (WWTPs). High-rate CEC—surface overflow rates greater than 20 gallons per minute (gpm) per square foot (ft2) [85 cubic meters per square meters per hour (m3/m2-hr)]—was shown to remove high levels of total suspended solids (TSS) during the demonstration tests but was less effective in removing 5-day biochemical oxygen demand (BOD5) given its inherent inability for removing soluble organic matter. One promising approach to improve the removal of BOD5 with CEC is biologically and chemically enhanced clarification (Bio CEC). In this process, activated sludge from a plant’s secondary process is routed to a short-duration contact basin where it blends with excess wet-weather flows to achieve rapid uptake of soluble organic matter into the biomass. This mixture of biomass and influent wastewater is then treated through a chemically enhanced clarification step.

Bench-scale testing of the South Shore WWTP primary influent was conducted in April and May of 2006 to assess the merits of incorporating a biological conditioning step before CEC. The testing objectives included assessing the effect of biological contact on the removal of TSS, total and soluble BOD5, and phosphorus; measuring its effect on UV transmittance (UVT); and determining key design criteria for the full-scale application of short-duration biological contact coupled with CEC. Key design criteria included biological contact time, target activated sludge concentration in the contact reactor, and coagulant and polymer dosage requirements. Bench-scale testing showed improved soluble BOD5 removal and improved UVT. Construction costs were estimated for two different Bio CEC configurations: BioACTIFLO® and biological chemically enhanced primary treatment (Bio CEPT).

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