Gravity thickening in a deep, circular, open tank is frequently used before anaerobic digestion. Some wastewater treatment facilities also use polyelectrolytes to improve gravity thickening, but their use for this purpose is not widespread.
Air flotation thickening of activated sludge is an alternative. This process uses dissolved or diffused air and sometimes a coagulant to float the sludge to the surface where it is removed by a mechanically driven skimmer. The effluent drawn near the bottom of the flotation tank
is combined with incoming plant influent and passes through the treatment cycle. Most flotation units recycle a portion of the effluent through the flocculation-air chamber because the recycled liquid enhances flocculation.
Although flotation is effective on activated sludge because of its density, it is not used on clarifier sludges because they are more difficult to float.
The primary purpose of mixing tanks is to provide a satisfactory mixture of clarifier sludge and activated sludge when a single thickening process is used. A mixing-storage tank also permits the thickener to operate continuously with a uniform inflow rate.
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