A quantitative comparison between chemical dosing and electrocoagulation

Abstract A renewed interest in electrocoagulation has been spurred by the search for reliable, cost-effective water treatment processes. This technology delivers the coagulant in situ as the sacrifcial anode corrodes, due to an applied potential, while the simultaneous evolution of hydrogen at the cathode allows for pollutant removal by flotation. By comparison, conventional chemical dosing typically adds a salt of the coagulant, with settling providing the primary pollutant removal path. This paper provides a quantitative comparison of these two approaches based on turbidity removal associated with a clay pollutant. Chemical coagulation was evaluated via jar tests using aluminium sulphate (alum). This proved more effective than electrocoagulation under acidic conditions (pH ∼4) and low coagulant levels (4 mg-Al l −1 being the minimum able to effectively destabilise the colloidal clay particles). Highly effective coagulation was observed at intermediate alum dosage levels (4–20 mg-Al l −1 ), where the isoelectric point occurred at pH ∼7.8. Three operating stages (lag, reactive and stable) were identified in a batch electrocoagulation reactor with the operating current determining the pollutant removal rate. At the isoelectric point, which occurs during the reactive stage, the greatest turbidity reduction occurs, indicating aggregation by a sorption mechanism (compared to the charge neutralisation as in the case of chemical coagulation). During the stable stage, continued precipitation of aluminium hydroxide and a decrease in turbidity indicated a sweep coagulation mechanism. The highest current (2 A) reduced the pollutant level in the shortest time, 1% residual turbidity after 30 min, though the highest efficiency (in terms of pollutant removed per unit of aluminium added) was achieved at the lowest current (0.25 A).