Wastewater reuse often requires particle destabilization and removal to protect water transport systems and membranes from clogging. Flocculation process of activated sludge effluent applying ferric chloride is examined and comparison with alum (aluminium sulfate) application is made in this work. Optimum flocculation conditions are determined based on the removal efficiency of different particle size groups and on turbidity as a function of coagulant dosage and pH. Results show that the best removal for ferric chloride coagulant occurs at pH 4-5 and dosage of 20-30 mgl−1. Settled water total particle count (TPC) of particle size ≥2μm was reduced by more than 99%, while turbidity removal reached 86%. Zeta potential measurements and visual observations indicate domination of adsorption and charge neutralization mechanisms. Best removal with alum occurred at pH 6-7 while dosing 30 mgl−1 and higher. Destabilization mechanism of adsorption and sweep coagulation is proposed. Generally alum performed somewhat better than iron for turbidity removal and worse for TPC removal. CMD (count mean diameter) is proposed for particle removal index: the higher the TPC removal, the lower the CMD.
Membrane filtration is adequate for producing disinfected clear water suitable for various kinds of applications. However, fouling of membranes is the main limitation. This study has focused on the ability of flocculation to remove from wastewater the organic colloids which play an important role in the fouling phenomena. First, flocculation was optimized for high-efficiency removal of suspended solids and organic material, and then, at a selected ferric chloride dose and pH, was used as a first step before filtration on membranes ranging from UF (50 KDa) to NF. This resulted in an improvement of the filtration flux. Fouling increased when high molecular weight cut-off (MWCO) membranes were used. The foulingmechanism seems to be blocking, by internal clogging and cake formation becoming preponderant with time. pH 5.5 (charge neutralization zone) provided better removal and lower fouling intensity than pH 7.8 (sweep coagulation zone). Ultrafiltration of 4 KDa at acidic pH 5.5 and 150 mg/L ferric chloride could reduce DOC by 70% and UV-254 nm by 60%. The quality of the filtrate was better than that obtained with nanofiltration at basic pH 7.8 with the same dose of flocculant and the fouling was lower with a 4 KDa membrane. Thus coupling of flocculation with a UF membrane might be the best compromise for producing, in very compact units, very clear water for possible reuse in industrial areas.
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