AbstractThis study investigated the role of the gas-to-solids (G/S) ratio in electroflotation (EF) for activated sludge, which has not been significantly considered in prior studies. G/S ratio was quantified by measuring bubble generation rates under current densities of 30 to 270 A m−2. When the current density was greater than 100 A m−2, the bubble generation rate had a linear relationship with the electric current. Sets of batch and continuous EF experiments were performed using several different kinds of activated sludge in terms of settleability. Clarification and thickening efficiency distinctly increased with an increase of the G/S ratio in the batch EF. Influence of sludge properties, including extracellular polymeric substances (EPS) contents and floc mobility, was negligible, with G/S ratios greater than 0.01. In the continuous EF, clarification efficiency was also significantly enhanced at the G/S ratio of 0.01. However, an increase of bubble generation had little influence on thickening. Effects of solids loading rate and flotation time were not apparent, except when an excessive buildup of sludge blanket deteriorated the float stability. Operational range of the G/S ratio, found in this study, would be valuable for EF application to solids–liquid separation of activated sludge.