Significance of flocculation for NOM removal by coagulation–ceramic membrane microfiltration

Potable water treatment with coupling coagulation–microfiltration processes are still rarely applied in commercial treatment plants. Raw water with a high content of organic matter, typical for Norwegian surface water sources, was treated in this study using a ceramic microfiltration membrane system. Three different pre-treatment options were investigated, a classical two-stage flocculation, a simplified one-stage fast mixing step and an inline flocculation treatment, using an iron chloride coagulant. DOC removal was similar (76–81%, 5.5 mg C/L in raw water) in all compared setups. The more compact, energy efficient inline configuration was investigated further, varying flux (140 and 220 LMH), pH (4.5, 5.5 & 6.5), G-value (60 and 300 s⁻¹) and HRT in the pipe (7 and 30 s), while monitoring DOC removal, fouling rate and residual iron concentrations. DOC removal was strongly pH dependent; 70% at pH 4.5, and 47% at pH 6.5. At high flux of 220 LMH the membrane fouled quickly and sustainable operation was not possible. At 140 LMH fouling was much less and no severe fouling was observed during the experimental period. Residual metal concentration was found to be the limiting parameter in the design and operation of the process configuration. Metal concentrations below the regulation limits (200 μgFe/L) were only achieved at pH 6.5. Reversible fouling was only observed at higher pH values.