Effect of pH on floc properties and membrane fouling in coagulation - ultrafiltration process with ferric chloride and polyferric chloride

Impact of pH on coagulation-ultrafiltration (C-UF) process was investigated with respect to coagulation efficiency, floc characteristics and membrane fouling in this study. Ferric chloride (FeCl₃) and polyferric chloride with basicity of 1.0 and 2.2 (denoted as PFC₁₀ and PFC₂₂) were used as coagulants and Fe (III) species in them was measured by a timed complexation spectroscopy method. Floc properties under four pH conditions were evaluated using a laser diffraction particle sizing device. Ultrafiltration experiments were conducted by a dead-end batch unit. The results showed that organic matter removal efficiency was higher under acidic conditions than under other pH conditions and turbidity removal efficiency was higher under alkaline condition. At same pH, FeCl₃ containing higher monomeric and polymeric species (Fe_a and Fe_b) had better organic matter removal and higher turbidity removal efficiency was obtained by coagulants with larger percentage of polymer or colloidal species (Fe_c). Flocs formed under acidic ranges were larger, weaker and looser. At pH 4.0, 7.0 and 9.0, flocs by FeCl₃ were larger and weaker than these by PFC₁₀, followed by PFC₂₂. In case of FeCl₃ and PFC₁₀, acidic pH conditions were helpful to reduce membrane fouling. For PFC₂₂, permeate fluxes were less sensitive to pH variations.