DAF system

FIGURE 27.1 Single-cell high-rate DAF system (Krofta Supracell). 

Dissolved air flotation equipment is available from a number of manufacturers. Packaged units of steel construction are available with capacities to 7.6 cu m/min (2000 gpm). The essential elements of the DAF system are the pressurizing pump, air injection facilities, pressurization tank or contact vessel, back-pressure regulating device, and the flotation chamber [40]. 

Three principal variations in the process design of DAF systems are full-flow, split-flow, and recycle operation (Fig. 14). Full-flow operation consists of pressurizing the entire waste... 

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. 

The results illustrating dependence of surface concentration of Cu(0H)2 on its initial concentration obtained for different DBS concentrations both in the DAF and DIS system have been given in Fig. 1. It is evident that an increase in DBS concentration is accompanied by an enhancement of Cu(0H)2 surface concentration. This is a logical consequence since at higher DBS concentrations the Cu(0H)2 particles are made more hydrophobic, and thus, easier Rotated. It can also be seen that on the adsorption curves corresponding to an initial Cu(0H)2 concentration of 8 x 10 mol/dm, a saturation level is achieved. Values for the surface concentrations of Cu(0H)2 obtained in the DAF system are about 10 times higher if compared to those obtained... 

The values for ratio are lower for the DAF system. This is imderstandable... 

The dependence of the values for surface concentrations of DBS in the presence > " in the absence ( dbS Cu(OH)2, on the initial DBS concentrations is depicted in Fig. 3. The figure actually shows the DBS separation in precipitate flotation in the presence of Cu(OH)2, as well as its recovery in the foam flotation. Surface concentrations of DBS obtained in the DAF system in the presence of Cu(0H)9 are about 10—30 times higher if compared to the DIS system. This is again the consequence of a smaller amount of air involved in the DAF system, which causes the total bubble surface to be lower. It is evident that the Tjjgg values in both systems are about 2—10 times lower from same time the recoveries of pure DBS are lower and... 

A comparison of the results obtained for DIS and DAF system indicate that in the latter case a small number of air bubbles is sufficient for a successful recovery of both the precipitate and collector. 

When PAC is dosed to a DAF system for both adsorption and flotation, the combined process is called adsorption flotation (30,36). 

Dissolved air flotation (DAF) has been widely used over the last 40 yr for removal of oil and grease, suspended solids, and biochemical oxygen demand (BOD) from wastewater and other industrial process stream. The reliability and performance of DAF systems have been improved along with the increasing use of this technology, as shown in Table 7. 

Continuous DAF system may not need the air saturation tank. The compressed air is injected in the recycle stream of the treated effluent. In this case, the control valve settings are done on basis of an almost constant characteristic of incoming effluent. This needs a sufficiently large equalisation tank and may also need chemical pretreatment by flocculants. 

Most of the discretization methods for time-dependent PDFs are extensions of the well-known method of lines based on first space and then time discretization. Discretizing first in space, the initial problem (1.1) is transformed into an ODF- or DAF-system which is nonlinear, stiff, sparse, large and block-structured in industrial applications (see Figure 1.1). 

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