How Important is the EDI in Final Clarifiers

It has become widely accepted that it is necessary for the mixed liquor entering a final clarifier to be passed through jets of one kind or another to create energy for flocculation, then to dissipate the energy to ensure that these jets do not cause stray currents that might upset the quiescent conditions in the body of the final clarifier. This paper will look at alternatives in which the flow is entering the stilling well vertically from the bottom and is then dispersed in a way to prevent floc breakup without passing through a high-velocity area. Side-by-side comparisons will be made of these two types of clarifiers over a period of more than 10 years at the City of Kelowna, WWTP in British Columbia. A side-by-side comparison will also be made of two types of final clarifiers at the Darvill plant of Umgeni Water in Pietermaritzburg, South Africa. Two examples of retrofitting rectangular final clarifiers with inlet flocculation wells, one at the Mill Creek plant in Cincinnati and the other at the West Haven plant in Connecticut, will also be discussed. The main conclusion is that similar logic should be used in the design and retrofitting of all secondary clarifiers.