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

Article excerpt

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. …