Flotation Thickening

A variety of pressurized, air flotation units are commercially available as complete packages. The basic units are either rectangular or round, with rectangular units more common in wastewater treatment systems (see Figure 7.46.3). Both direct pressurization and pressurized recycling systems are available. Wastewater treatment facilities use the later when the liquid contains flocculant or other fragile particles that disperse or suffer attrition when subjected to the high shear forces in the pressurization process.

The performance of pressure flotation units is related to the ratio of the weight of the air used to the weight of the solids supplied to the unit, A/S. However, this approach neglects both the number of bubbles into which the air is divided after the pressure release and the number of particles into which the weight of incoming solids is divided.

No ready means exists to assess the potential impact of these variables. The degree of flocculation of an activated sludge can vary in response to changes in influent waste characteristics or process management. Therefore, waste-water treatment facilities should use conservative designs. Alternatively, they can add flocculating chemicals to the unit to insure a constant number of particles per unit weight of applied solids. Polymers are usually selected as the flocculating chemicals. Facilities can substantially increase the particle rise rate with low polymer dose rates.

Wastewater treatment facilities can use the following equation to select pressurized recycling flotation units:

where:

A/S = ratio of lb of air to lb of incoming solids C = A conversion factor, 0.834 sa = Solubility of air in water, 18.68 cc/l at 20°C f = Percent saturation achieved (decimal fraction)

A/S = ratio of lb of air to lb of incoming solids C = A conversion factor, 0.834 sa = Solubility of air in water, 18.68 cc/l at 20°C f = Percent saturation achieved (decimal fraction)

FIG. 7.46.3 Flotation thickeners. A, Pressurized recycle system; B, Direct pressurization system.

p _ Operating pressure, atmospheres

R _ Pressurized recycling rate, mgd

Cta _ Solids concentration in thickener influent, mg/l

Qw _ Waste sludge influent flow rate, mgd

Wastewater treatment facilities can use the following equation to select direct pressurization flotation units:

S Cin

Environmental engineers usually select flotation units that provide a surface overflow rate of 2500 to 5000 gal/ft2/day and a hydraulic detention time of about 20 min. They should also make provisions for dual units, bypassing, de-watering, and flocculant addition.

Environmental engineers develop design data by pressurizing either the waste influent (direct pressurization) or a simulated recycled sample, such as settled or filtered waste or sludge supernatant, and releasing it into an open graduated cylinder. For the pressurized recycling system, they release various amounts of recycled liquid into the required amount of waste or sludge to achieve the required recycling ratio (see Figure 7.46.4). Both the time of pres-surization and the pressure are based on the latitude of design permitted by the commercial equipment. For custom designs, environmental engineers should study a range of values. The effect of temperature should also be studied, especially if the flotation units are not housed or influent temperatures vary.

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