ALTERNATIVE CONSIDERATIONS IN SIZING AERATION BASINS

With greater emphasis on water conservation, more industries are discharging more concentrated wastewaters. This has caused organic concentrations of the wastewaters to increase. This paper will address activated sludge treatment of a high strength pharmaceutical wastewaters (up to 14,000 mg/L COD).Historically, activated sludge aeration basins have been sized based on BOD and COD removal requirements. However, for high strength and readily degradable wastewaters, BOD and COD removal kinetics do not define the required aeration basin volumes. Oxygen transfer limitations govern the size of the basins. Practical limitations for coarse bubble, fine bubble, surface and jet aeration systems will be compared in selecting aeration basin requirements. Furthermore, a comparison with a conventional oxygen dissolution system will be compared. The relative capital, operating and present worth costs will be included in this comparison for a pharmaceutical wastewater application.When aeration basins are sized on oxygen transfer requirements, the power levels in these basins typically exceed 300 hp/MG (0.059 kw/m3). At these elevated power levels, excessive floc shear and excessive mixed liquor temperatures for mesophilic operations are experienced. The excessive floc shear can promote improved sludge settleability (e.g. sludge volume index and zone settling velocity) and endogenous respiration due to improved dissolved oxygen penetration. However, carefully designed re-flocculation may be required prior to secondary clarification to avoid excessive effluent total suspended solids (TSS) due to dispersed floc. The careful integration of surface aeration or coolers in aeration system design can be used to control mixed liquor temperatures. Practical guidance regarding design measures to address floc shear and elevated temperature impacts is provided.