On the effect of flocculation as pretreatment process and particle size distribution for membrane fouling reduction

摘要

This work deals with the feasibility of an olive vegetation wastewater purification process consisting of two consecutive steps: coagulation followed by membrane separation. The target of this work was to reach a purification grade of the wastewater compatible with the limits for the discharge into a municipal sewer that is a COD value less than 500mg L~(-1). A feedstock with 55 g L~(-1) of COD was used for the experimental work. In a first step, the feedstock was pretreated by a flocculation process using as coagulant aluminium sulphate or aluminium hydroxide. Then, the pretreated wastewater stream was purified by four consecutive batch membrane processes consisting of microfiltration, ultrafiltration, nanofiltration and reverse osmosis, each with a recovery rate of 90%. For each membrane, at different recovery values, the critical flux values were measured. Finally, all the streams were analyzed by means of a nanosizer, capable to measure the suspended particle size distribution in the samples from 0.8 nm up to 6.5 μm. Flocculation changes the fouling outcome on the membranes by changing sensibly the particle size distributions of the solution. The membrane's performances are strictly dependant on the coagulant type used during the pretreatment operation. In order to develop full advantages on fouling issues, the particle size shift effect of the flocculation process should be carefully studied and evaluated, and needs fine-tuned optimization.