Continuous clarification and thickening of activated sludge by electrolytic bubbles under control of scale deposition

Electroflotation (EF) was investigated as a final clarification of an activated sludge process, to intensify its novel clarification and thickening efficiency. During operation of a biological reactor combined with an EF clarifier, deterioration of clarification efficiency was observed. Scale deposition on electrodes caused a coarse electrode surface, significantly increasing the size of the electrolytic bubbles. The average bubble size was initially 34 μm and increased to 80 μm after bulk cell electrolysis for 150 h. X-ray diffractometry and scanning electron microscopy further characterized the scale deposition as a cluster of calcite (CaCO3) and brucite (Mg(OH)2). Switching the polarity of electrical current clearly alleviated the increase of bubble size, when applied before scale growth. Under the control of scale deposition, excellent clarification was observed, with the effluent turbidity consistently lower than 2 NTU. An efficient thickening, with the concentration of return activated sludge higher than 15 g L−1, was additional advantage of the EF clarifier.