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DAF THICKENERS ENHANCING PLANT
Author: Tracy Stigers, Perry Schafer, and Bill McCarthy
Date: 11/05
From WEFTEC Conference 2005

Dissolved air flotation (DAF) units have been a traditional thickening device for decades. In recent years, devices such as gravity belt thickeners and rotary drum thickeners have gained popularity as alternative process choices. These units are typically smaller in foot print, have fewer components, and consume less power than DAFs. However, these devices are more limited than DAFs in their ability to accommodate scum and high hydraulic loading. With a DAFs high hydraulic capacity and significantly lower potential for blinding or clogging, many plants have found that DAFs provide opportunities for improving plant performance or gaining operational flexibility. One example is that with a DAF, the plant can choose to waste mixed liquor upstream of the clarifiers or sludge downstream of the secondary clarifiers. Wasting mixed liquor significantly simplifies process control of the activated sludge system. Another example is the ability to continuously waste from primary clarifiers. The high hydraulic capacity of DAFs allows for continuous, low concentration removal of solids from a primary clarifier when co-thickening primary sludge with secondary sludge. With continuous removal, sludge blankets do not build up and there is less likelihood of solids washout from the clarifier. Also, by not holding sludge in a primary sedimentation tank to thicken it, there is less potential for resolubilzation of BOD, reducing the loading on the secondary process. Many plants pump unthickened scum directly to their digesters and the amount of water associated with it can significantly impact digester residence times. By diverting primary and/or secondary scum to a DAFT, some facilities have found that the scum becomes better mixed with the other materials before digestion, and they reduce the amount of water pumped to the digesters. In these examples, the dissolved air flotation thickener enhances the performance and flexibility of operation of primary sedimentation tanks, activated sludge systems, scum treatment, secondary sludge wasting and digester residence time.