An electrocoagulation-flotation (ECF) pilot plant was designed, built and operated to evaluate its performance in comparison with a pilot scale conventional dissolved air flotation (DAF) system for treating lowland surface water in the UK. Treatment performance was assessed principally in terms of changes in dissolved organic content (DOC) and system running cost. On average, the decrease in DOC, UV-254 and colour was 10-15% with ECF at pH ca. 7.8 and current densities of 2-5 A m-2 (equivalent dose range of 1-5 gAl(III) m-3). Acidification of the influent (pH 8.4 down to 7.5) decreased only DOC and increased apparent current efficiencies for Al dissolution above unity. Passivation of the aluminium electrodes by Al(III) (hydrous) oxides caused difficulties in maintaining a steady dissolved Al(III) concentration and also increased specific electrical energy consumptions. Treated water quality with ECF was as good as that with DAF at lower equivalent coagulant doses (< 5 gAl(III) m-3), but was less so at higher coagulant doses. The pilot-plant DAF system appeared to be more expensive in terms of its running cost for the conditions used; the cost ratios of DAF to ECF were between 3:1 and 5:1, respectively, for an equivalent treatment performance.

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