Sludge secondary flotation systems

Kinetics and Case Histories of Activated Sludge Secondary Flotation Systems... 

In Section 27.2, the principles of a DAF unit and the entire improved activated sludge wastewater treatment system are disclosed in detail. The economic use of secondary flotation in the improved system requires only a relatively inexpensive high-rate DAF cell that is commercially available. The consulting engineers should understand such principles for the selection of an appropriate DAF unit and for the optimization of the entire improved wastewater treatment system. 

Section 27.3 introduces the improved activated sludge systems involving the use of either a DAF clarifier or a dissolved air flotation-filtration (DAFF) clarifier as the secondary flotation clarification unit. 

FIGURE 27.12 Improved activated sludge system with the installation of a secondary flotation clarifier (example). Note 1MGD = 0.0438 m3/s lppm-lmg/L 1 TPD = 37.8 kg/h. 

A typical overloaded conventional complete-mix activated sludge treatment plant (shown in Figure 27.11) has been described in Section 27.3.3. The same conventional treatment plant can be improved by the addition of a secondary flotation clarifier (shown in Figure 27.12). Some advantages of the improved activated sludge system are mathematically presented below. 

Dissolved air flotation (DAF) is a process commonly used in refineries to enhance oil and suspended solids from gravity-separator effluent. In some refineries it is used as a secondary clarifier for activated sludge systems and as a sludge thickener. The process involves pressurizing the influent or recycled wastewater at 3-5 atm (40-70 psig) then releasing the pressure, which creates minute bubbles that float the suspended and oily particulates to the surface. The float solids are removed by a mechanical surface collector. 

A relatively new design of a high-rate DAF unit uses a shallow bed system (Supracell) with only 3 minutes of retention time and operated at an overflow rate of 140Lpm/sqm (3.5 gpm/sq ft) [42]. This unit has been used for industrial and municipal wastewater treatment and offers lower capital cost and headroom requirements. It was installed at a petrochemical complex in Texas as a secondary clarifier to improve the operation and the capacity of an existing activated sludge system [43]. In recent years, nitrogen has replaced air in covered DAF systems because of the potential for explosion. These systems are called dissolved nitrogen flotation (DNF) systems. The operations of DAF and DNF are similar. 

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