Investigation of geometric properties of media particles for floating media filter

Brika, Bashir (2010-12)

Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2010.

Thesis

ENGLISH ABSTRACT: In a floating medium filter, polymeric beads with a density less than that of water form a floating bed which removes suspended material. Polyolefinic beads (polypropylene and polyethylene) are commonly used as filter media in this application. The geometric properties of the beads, and to a lesser extent the surface properties, strongly influence the performance of the filter. In the case of water treatment, the primary performance requirement is the production of a filtrate with turbidity ≤ 1.0 NTU. The influence of geometric properties on the performance of existing upflow filtration systems has not been extensively researched. The aim of this thesis was therefore to investigate the effects of floating medium granule size and shape on the performance of the floating medium filter (FMF). Towards this goal a pilot plant consisting of a dosing and flocculation unit and a clear PVC column with an inner diameter of 0.3 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 250 mg/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 and disc) were evaluated. The effect of filtration rising velocity, medium depth, and coagulant chemical dosage were investigated using a complete 23 full factorial experimental design. Filter performance was evaluated in terms of filtrate turbidity and headloss development. The direction 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 conditions the best medium was the one with d50 = 2.28 mm, for which a minimum turbidity of 0.4 NTU was achieved, and which was able to provide 624 L of filtrate of ˂ 1.0 NTU using a bed of 0.014 m3. For this medium headloss was 109 mm H2O at breakthrough, while the other three media showed a headloss of 42 mm H2O at breakthrough. Visual observation indicated that removal of solids took place primarily in the first 0.3 m of the floating bed in the case of the smallest 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 for a 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 better performance. 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. Proper medium selection in conjunction with operating conditions can enhance performance of the filter. Smaller medium would give better turbidity removal but high headloss development and more frequent backwashing becomes necessary than with larger medium.

AFRIKAANSE OPSOMMING: In ʼn dryfmediumfilter vorm polimeriese korrels met ʼn laer digtheid as dié van water ʼn dryfbedding wat swewende materiaal verwyder. Poli-olefiniese korrels (polipropileen en poliëtileen) word algemeen in hierdie toepassing as filtermedia aangewend. Die geometriese kenmerke, en in ʼn mindere mate die oppervlakkenmerke, van die korrels het ʼn groot invloed op die funksionering van die filter. In geval van waterbehandeling is die hooffunksioneringsvereiste die produksie van ʼn filtraat met ʼn troebelheid van ≤ 1.0 NTU (“nephelometric turbidity units”). Die invloed van die geometriese kenmerke van filtermedia op die funksionering van bestaande stroomop-filtreerstelsels is nog nie omvattend nagevors nie. Die doel van hierdie tesis is dus om ondersoek in te stel na die uitwerking van die korrelgrootte en -vorm van ʼn dryfmedium op die funksionering van die dryfmediumfilter (DMF). Hiervoor is ʼn proefaanleg met ʼn doseer- en uitvlokkingseenheid sowel as ʼn deursigtige pilaar van polivinielchloried (PVC) met ʼn binnedeursnee van 0.3 m en ʼn hoogte van 2.8 m ontwerp en gebou, met behulp waarvan verskillende mediumtipes, beddingdieptes en filtreeromstandighede ondersoek kon word. ʼn Kunsmatige watertoevoer vir die proefneming is vervaardig deur 250 mg/L bentoniet in kraanwater op te los (≈ 60 NTU). Polipropileenplastiekkorrels met vier verskillende deursneë (d50 = 2.28; 3.03; 3.30 en 4.07 mm) is gebruik, en twee mediumvorms (kubus- en skyfvormig) is beoordeel. Die uitwerking van filtrasiestygsnelheid, mediumdiepte en die dosis koaguleermiddel is met behulp van ʼn volledige 23-faktoriaalontwerp ondersoek. Filterfunksionering is aan die hand van filtraattroebelheid en verlies aan drukhoogte beoordeel. Alle proefnemings is teen ʼn opwaartse filtrasierigting uitgevoer. Daar is bevind dat die beste omstandighede vir die verwydering van troebelheid ʼn lae filtrasiekoers (36.8 L/m2 per minuut), ʼn groter mediumdiepte (0.6 m) en ʼn optimale dosis koaguleermiddel (23 mg/L) is. In hierdie omstandighede was die beste medium die een met ʼn d50 van 2.28 mm, waarvoor ʼn minimum troebelheid van 0.4 NTU verkry is, en wat 624 L filtraat van 1.0 NTU met behulp van ʼn bedding van 0.014 m3 kon lewer. By deurbraak het hierdie medium egter ʼn drukhoogteverlies van 109 mm H2O getoon, teenoor die ander drie media se 42 mm H2O op dieselfde punt. Visuele waarneming dui daarop dat, met die kleinste medium, vaste stowwe hoofsaaklik oor die eerste 0.3 m van die dryfbedding verwyder is, teenoor die volle diepte van die bedding vir die ander drie media. Beddingdiepte blyk dus die grootste invloed te hê op funksionering wat enige bepaalde mediumtipe betref. Proefwaarneming toon dat die dosis koaguleermiddel ʼn belangrike rol in vlokgrootte speel. ʼn Hoër dosis koaguleermiddel (23 mg/L) het ʼn groter vlokgrootte en dus beter funksionering tot gevolg. ʼn Laer stygsnelheid blyk ook die beste te wees vir die vorming van groter vlokke. Hoewel mediumvorm oënskynlik ʼn mate van ʼn rol speel, is mediumgrootte eerder die dominante faktor. Volgens die studie blyk DMF belowend vir aanwending in waterbehandeling te wees, veral as voorfiltreereenheid vir die behandeling van baie troebel water. Behoorlike mediumkeuse saam met die regte bedryfsomstandighede kan die funksionering van die filter verder verbeter. Kleiner media sal troebelheid beter verwyder, maar het ʼn groot verlies aan drukhoogte tot gevolg, en sal dus meer gereelde terugspoeling as groter media verg.

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