Investigation of geometric properties of media particles for floating media filter

[摘要] ENGLISH ABSTRACT: In a floating medium filter, polymeric beads with a density less than that of water form afloating bed which removes suspended material. Polyolefinic beads (polypropylene andpolyethylene) are commonly used as filter media in this application. The geometric propertiesof the beads, and to a lesser extent the surface properties, strongly influence the performanceof the filter. In the case of water treatment, the primary performance requirement is theproduction of a filtrate with turbidity ≤ 1.0 NTU. The influence of geometric properties on theperformance of existing upflow filtration systems has not been extensively researched. Theaim of this thesis was therefore to investigate the effects of floating medium granule size andshape on the performance of the floating medium filter (FMF). Towards this goal a pilot plantconsisting of a dosing and flocculation unit and a clear PVC column with an inner diameter of0.3 m and height of 2.8 m was designed and constructed, allowing the effect of media type,bed depth and filtration conditions to be investigated.Artificial feed water for use during the experimental work was made up by dissolving 250mg/L of bentonite in tap water (≈ 60 NTU). Four median grain sizes (d50 = 2.28, 3.03, 3.30,and 4.07 mm) of polypropylene plastic granules were used. Two media shapes (cubic anddisc) were evaluated. The effect of filtration rising velocity, medium depth, and coagulantchemical dosage were investigated using a complete 23 full factorial experimental design.Filter performance was evaluated in terms of filtrate turbidity and headloss development. Thedirection of filtration was upward in all the experiments.It was found that optimal conditions for turbidity removal were low filtration rate (36.8 L/m2·min), longer media depth (0.6 m) and optimum coagulant dose (23 mg/L). At these conditionsthe best medium was the one with d50 = 2.28 mm, for which a minimum turbidity of 0.4 NTUwas achieved, and which was able to provide 624 L of filtrate of ˂ 1.0 NTU using a bed of0.014 m3. For this medium headloss was 109 mm H2O at breakthrough, while the other threemedia showed a headloss of 42 mm H2O at breakthrough. Visual observation indicated thatremoval of solids took place primarily in the first 0.3 m of the floating bed in the case of thesmallest medium, but that solids removal took place over the full depth of the bed for the other three media. It was found that bed depth had the strongest influence on performance fora given medium type.Experimental observation showed that coagulant dosage played an important role in floc size.A higher coagulant dosage (23 mg/L) resulted in a larger floc size which gave betterperformance. A lower velocity gradient was favourable for the formation of larger flocs.Some effect of media shape was noted, although it appeared that media size was dominant.It is concluded that FMF show promise for application in the water treatment. FMF, however,can be applied successfully as pre-filtration unit for treatment of high turbid water. Propermedium selection in conjunction with operating conditions can enhance performance of thefilter. Smaller medium would give better turbidity removal but high headloss developmentand more frequent backwashing becomes necessary than with larger medium.

[发布机构] Stellenbosch University