A study on electroflotation as a secondary clarification of an activated sludge process

A study on electroflotation as a secondary clarification of an activated sludge process
Activated sludge; Electroflotation; Clarification; Thickening; Wastewater Treatment
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Desalination for the environment - clean water and energy
An essential aspect of successful operation of a biological wastewater treatment process is the efficient separation of biological solids from mixed liquor. A reliable solids/liquid separation provides clear effluent, highly concentrated return sludge, and independent sludge wasting to control SRT (solids retention time). The flotation must be an alternative to the gravity settling. Flotation of the fine and light solids has been applied to water and wastewater treatment mostly as dissolved air flotation (DAF) and less frequently as electroflotation (EF). The EF is known to produce very fine bubbles, and has a strong potential to separate fine solids in the mixed liquor. Furthermore, it is relatively easy to generate bubbles of different concentration simply by varying current density and electrode materials. The EF operation, therefore, seems to be adequate for flexible operation under various operating conditions such as influent loading fluctuation and variation of sludge settleability. This study assessed the feasiblity of electroflotation as a final clarification of an activated sludge process. Clarification and thickening characteristics of the mixed liquor were investigated through lab scale EF experiments combined with a biological nutrient removal (BNR) process. A reliable EF operation was possible, when the gas to solids (G/S) ratio was 0.005~0.01 and the total accumulated solids in flotation unit was smaller than 1 kg/m2. A scale deposition on the cathodes periodically deteriorated the clarification efficiency of the EF. However, proper current switching could before lateral growth of scale layer could alleviate the effects of the scale deposition. The solids concentration in the BNR reactor was maintained in the range of 5,500 to 6,500 mg/L by efficient thickening in the EF clarifier. The final effluent from EF was highly purified with the turbidity lower than 2 NTU.
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