Bench-scale testing was conducted to quantify the effectiveness of BOD5 removal utilizing an aerobic biological contact reactor and ballasted flocculation/settling. Tests were conducted at activated sludge plants. Samples were collected and mixed with return activated sludge (RAS) targeting a mixed liquor suspended solids concentration of 1 g/L. This mixture was aerated and true soluble and total soluble CBOD5 concentrations were measured over 25 minutes. Results demonstrated that a majority of the true soluble CBOD5 is removed immediately upon contact with RAS. This removal was assumed to be sorption by biomass. True soluble CBOD5 continued to decline over the remaining test period. This was attributed to utilization by biomass for growth under first-order kinetics. Following aeration, samples were jar-tested to simulate coagulation/ballasted flocculation/settling. This removed 90% of suspended solids resulting in removal of the majority of the particulate CBOD5, sorbed soluble CBOD5 and a large fraction of colloidal CBOD5. Effluent total CBOD5 concentrations were between 6 and 29 mg/L, demonstrating this treatment configuration can meet CBOD5 limits during wet weather events.
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