Performance Evaluation of Fabric Aided Slow Sand Filter in Drinking Water Treatment

Mondal, Pulin Kumar, Seth, Rajesh, Biswas, Nihar, Journal of Environmental Engineering and Science

Abstract: In this study, an assessment of the performance of slow sand filter (SSF) aided with non-woven fabric (NWF) was carried out. Several laboratory-scale SSF columns were tested with simulated raw water containing varying levels of turbidity and total organic carbon (TOC). The results show that in filters with NWF, the fabric layers captured most of the incoming solids and extended the filter run time for the sand bed. The run time for the sand bed increased with the increasing of the fabric thickness from 8.9 to 44.5 mm. Turbidity, TOC, and bacterial removal efficiencies of the filters with fabric were comparable to that without fabric and representing conventional SSF. The study thus demonstrates that operation of SSF with NWF can be a feasible option for simplifying the operation of and extending the viability of the SSF process to a wider range of raw water turbidity values than that considered economical for conventional SSF.

Key words: slow sand filter, non-woven fabric, filter run time, sand bed protection, filter cleaning, drinking water treatment, filter head loss.

Resume : Cette etude evalue le rendement d'un filtre a sable lent (SSF) aides d'un textile non-tisse. Plusieurs colonnes de SSF a l'echelle du laboratoire ont ete mises a l'epreuve en utilisant de l'eau brute simulee presentant divers niveaux de turbidite et de carbone organique total (COT). Les resultats montrent que, dans les filtres avec textile non-tisse, les couches de textile captaient la majorite des solides entrant et prolongeaient le temps d'utilisation de la portion lit de sable du filtre. Le temps d'utilisation du lit de sable augmente avec l'augmentation de l'epaisseur du textile de 8,9 mm a 44,5 mm. La turbidite, le COT et l'efficacite des filtres avec textiles a eliminer les bacteries etaient similaires a ceux des filtres sans textile representant les SSF conventionnels. L'etude demontre que le fonctionnement du SSF avec textile non-tisse peut etre une option viable pour simplifier le fonctionnement et, ainsi, accroitre la viabilite du procede SSF a une plus grande gamme de valeurs de turbidite d'eaux brutes que celles qui peuvent etre considerees rentables pour les SSF conventionnels.

Mots-cles : filtre a sable lent, textile non-tisse, temps d'utilisation du filtre, protection du lit de sable, nettoyage de filtre, traitement de l'eau potable, perte de charge des filtres.

[Traduit par la Redaction]

Introduction

Slow sand filter (SSF), being simple in technology and operation, is considered one of the more suitable low cost treatment technologies in treating drinking water, particularly for small community water supplies. Slow sand filter is the earliest technology in municipal water treatment and has been used since its introduction by James Simpson and Robert Thom in 1829 (Ellis 1985). Due to the occurrence of both physical and biological treatment processes, SSF is effective in removal of both turbidity and microorganisms.

Conventional SSFs typically achieve effluent turbidity of <1 nephelometric turbidity unit (NTU) (Collins et al. 1991). Researchers over the last few decades have also shown that SSF can achieve Giardia cysts and Cryptosporidium oocysts removal of 2 to 4 log (Rachwal et al. 1996; Palmateer et al. 1999). The United Nations recommends slow sand filtration as an important, reliable and cost-effective process for drinking water treatment in developing countries, especially for smaller water systems (Hendricks et al. 1991).

Even though SSF is considered suitable and economical, its application is recommended only for source waters with low turbidity (<5 NTU) (Cleasby 1991). Higher turbidity in source water and excessive proliferation of algae during the summer time increase the filter cleaning frequency by clogging the filter quickly, and thus the operation cost is increased (Montiel et al. 1988; Hendricks et al. 1991). Moreover, frequent filter cleaning by scraping the top sand layer removes a large part of the active microorganisms population from the schmutzdecke and also disturbs the sand bed below the schmutzdecke. Schmutzdecke is defined as a layer of material, both deposited and synthesized, on top of the filter bed that causes head loss disproportionate to its thickness and it is characterized usually as a gelatinous mat in which microorganisms thrive and cause a major portion of the removal that occurs (AWWA 1991). The sand bed disturbance during scraping has been shown to have a damaging effect on the performance of SSF (Huisman and Wood 1974; Bellamy et al. 1985; Ellis 1985).

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Several modifications have been developed and implemented with SSF to deal with water quality issues not adequately handled by conventional SSF. Roughing filters and filter harrowing are two such modifications that allow SSF to be used for waters with higher than acceptable turbidity or algal content (Collins et al. 1991). Roughing filters are used as pretreatment for the SSF influent waters and can achieve significant reduction in turbidity, coliform bacteria, and algal content. Filter harrowing is a method of cleaning SSF where the surface of the clogged sand bed is harrowed with comb-tooth harrow and simultaneously the surface sumps are kept open for the drainage of supernatant water and loosened debris. However, both of these modifications increase the complexity and the cost of construction and operation of the filters (Collins et al. 1991; Tanner and Ongerth 1990). The use of synthetic NWF as filter mat above the sand bed may be another option that may allow SSF to deal with high raw water turbidity, with possibly little increase in complexity or cost of the treatment process. The feasibility of the use of NWF in SSF has been examined in a few limited studies (e.g., Graham and Mbwette 1991; Mbwette 1989). These studies have indicated that NWF can capture the incoming solids and reduce the solid loadings to the sand bed. The use of NWF was shown to increase the filter run time to a factor of 1.5 to 2 as compared to the control filter (without NWF), with no significant effect on the overall removal efficiencies of turbidity, total organic carbon (TOC), and total coliform. Klein and Berger (1994) have reported the application of NWF at an artificial groundwater recharge SSF plant in Zurich for the protection of sand bed against sunlight and algae. They reported that NWF increased the filter run time by a factor of 10 when compared to a similar facility with no NWF. Despite these studies, the …

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Publication information: Article title: Performance Evaluation of Fabric Aided Slow Sand Filter in Drinking Water Treatment. Contributors: Mondal, Pulin Kumar - Author, Seth, Rajesh - Author, Biswas, Nihar - Author. Journal title: Journal of Environmental Engineering and Science. Volume: 6. Issue: 6 Publication date: November 2007. Page number: 703+. © 2008 NRC Research Press. COPYRIGHT 2007 Gale Group.

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