Pre-flocculation of secondary treated wastewater in enhancing the performance of microfiltration

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Journal Article
Desalination, 2002, 146 (1-3), pp. 367 - 372
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In this study, a new design of static floating medium flocculator was examined for its ability to remove suspended solids, organics and phosphorus. The floating medium flocculator also produced uniform microflocs, which could be removed easily by cross flow microfiltration. Flocs formed enhanced the permeate flux of microfiltration. The floating medium flocculator was able to be operated at high velocities of up to 40 m/h, producing filterable flocs of around 20 μm. Using a filter bed depth of 1m, the floating medium flocculator was able to remove 45% of suspended solids, to as low as 1.3 mg/L; 83% of turbidity, achieving values <1 NTU; 97% of phosphorus, reducing orthophosphate to 0.07 mg/L and 45% of organics, to as low as 1.02 ppm C total organic carbon (TOC). A periodic backwash for duration of 1 min every 90 min enabled the floating medium flocculator to run continuously without termination. A series of experiments was conducted with a combined system of flocculation-microfiltration to assess its capability in removing solids and organics. The improvement of flux by the pre-flocculation was also investigated. The purpose of the floating medium flocculator was to produce filterable flocs and also removal of solids and organics. The critical flux of kaolin clay suspension of 10-100 mg/L was measured with a 0.2 μm membrane, in the presence of 0-4 mg/L of fulvic acid. The pretreatment of flocculation leads to 50% removal of organics, while producing uniform microflocs of 13-16 μm. It enhanced the critical flux by 70% and resulted in a further 30-70% removal of organics by microfiltration.
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