This paper discusses the impacts of common design and operation parameters on direct filtration (DF) performance. These tests were carried out in a 45-m3/d pilot plant. The raw waters used were obtained by spiking a natural water with a kaolin suspension. The DF treatment comprises coagulation, flocculation, and filtration on a sand or anthracite bed. Seven parameters were studied over a wide range of design and operation conditions: effective size of the media (ES) 0.4 or 2.0 mm, depth of media (HMED) 50 or 300 cm, water head (WH) 50 or 300 cm, filtration rate (FR) 5 or 30 m/h, uniformity coefficient (UC) 1.3 or 1.5, raw water turbidity (TURB_RW) 1 or 5 nephelometric turbidity units (NTU), type of filtering media (MEDIA) sand or anthracite. The analysis of filter performance was based on the quantity and quality of filtered water produced and the cost of the filtration plant relative to each configuration. The results show that four of the parameters studied (ES, HMED, FR, and WH) have the most significant impacts on the filter performance and govern more than 77% of the performance criteria examined. This study also showed that the use of high-rate filtration increases the net productivity of the filters. However, to comply with filtered water quality requirements, the other three parameters should be set to appropriate values, since interactions among all four parameters are very high. The global cost analysis showed that a well designed and properly operated high-rate filtration process is more cost-effective than low-rate filtration. Key words: direct filtration, high-rate filters, cost analysis, statistical experimental design, Pareto chart.
The tests conducted in this study have made it possible to propose a rapid and simple laboratory method for determining the appropriate dosages of chemicals according to filtering materials effective sizes (ES), for direct filtration applications, and for adjusting the dosages according to raw water quality changes. Application of the proposed procedure requires but simple laboratory equipment: an Ives' filterability index measuring device, a filtration system operating under a constant vacuum, and 0.45 and 8 µm membrane filters. The use of 0.45 µm membrane filtration allows one to determine the best dosages for fine material with an effective size of 0.4 mm, whereas Ives' filterability index and 8 µm membrane filtration help determine the best dosage applicable to the 1.2 mm ES. The results obtained showed that for other effective sizes in the 0.4-1.2 mm range, the best dosages of alum and polymer can be estimated by linear interpolation. This laboratory procedure is a useful tool for quickly determining the best chemical dosages versus filtering media ES for a given water quality. It should be applied to raw waters with unknown characteristics prior to carrying out a more accurate full-scale validation, if necessary.Résumé : L'étude décrite dans cet article propose une procédure simple et rapide de détermination des meilleures doses de produits chimiques en filtration directe, selon le diamètre effectif du matériau filtrant (DE) et la qualité de l'eau brute à traiter. En effet, cette méthode requiert des équipements de laboratoire très simples dont un système de filtration sous vide avec deux types de papier filtre de 0,45 µm et de 8 µm et un dispositif de mesure de l'indice de filtrabilité de Ives. La filtration sur membrane de 0,45 µm s'est avérée utile pour déterminer le meilleur dosage pour un DE de 0,4 mm alors que l'indice de filtrabilité de Ives et la filtration sur 8 µm permettent de déterminer la meilleure dose pour un DE de 1,2 mm. Les résultats des essais ont montré que pour les valeurs intermédiaires de DE, les meilleures doses peuvent être déterminées par simple interpolation linéaire. Cette procédure devrait permettre de définir rapidement en laboratoire les doses appropriées de coagulants et de polymères requises pour une filière de filtration directe. Appliquée à une eau de caractéristiques inconnues, elle devrait permettre une estimation rapide de la dose de coagulant avant de procéder, si nécessaire, à des ajustements plus poussés en usine réelle.Mots clés : filtration directe, coagulation, floculation, meilleure dose, alun, diamètre effectif, indice de filtrabilité de Ives, filtration sur papier filtre.Tchio et al. 757
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