Abstract

In the operation of both rapid and slow sand filters an initial ripening interval is often observed in which particle removal is less than desired. Alum, ferric chloride, and polyaluminum chloride are commonly used in drinking and wastewater treatment to modify the surface properties of the particles being filtered. In the research described in this paper, these three coagulants were utilized to artificially ripen a sand media before challenging the filter media with an otherwise untreated kaolin suspension. After modification of the filter medium, high particle removal efficiencies were achieved using a short (7.5 cm) sand column with relatively large grained sand (1 mm in diameter). The best observed particle removal (96 %) using alum and PAC occurred at the surface loading of 550 mmol Al/m2, but the filter performance deteriorated at higher surface loadings. In comparison, the untreated sand achieved a baseline particle removal of  60%. Fractional colloid removal increased with increased ferric chloride surface loading over the entire range of dosages tested (up to 97.5 % at 2200 mmol Fe/m2). The experimental results suggest that pretreatment of filter medium by coating with Fe or Al hydroxides can eliminate the poor performance otherwise experienced when a ripening period is needed.

Keywords: Filter ripening, enhanced filtration, coagulants

Introduction

Deep bed filtration through particulate media is a commonly used method for removal of solids present in surface waters, precipitated hardness from lime-softened water, and precipitated iron and manganese present in many well water supplies (Weber 1972). The removal of suspended particles within a filter is considered to involve at least two sequential steps: transport and attachment. In the first step, the particles are transported from the bulk fluid to the immediate vicinity of solid-liquid interface presented by the filter (i.e., to a grain of the media or to another particle previously retained in the filter bed). The transport of particles to the filter medium may occur through three mechanisms: Brownian diffusion (molecular effects), interception (contact as a result of fluid flow near the surface of the porous medium), and sedimentation (gravity effects). Particle attachment to the media surface is dominated by electrical and chemical interactions such as electrostatic attraction or repulsion within the electrical double layer and van der Waals attractive forces that act between particles and surfaces at short distances (O'Melia 1980, Yao 1971, Elimelech 1990).
Sand media is a key, if not the sole, component of most filters. However, sand media may not be efficient in the removal of fine or sub-micron particles including colloids, bacteria, and viruses because of electrostatic repulsion arising from the fact that both the particles and sand media are negatively charged at circumneutral pH values. Fe and Al are commonly added as coagulants to modify colloid surfaces (Weber 1972, Tchobanoglous 2003) and improve particle removal. Polyaluminum chloride (PAC) has recently been reported to be superior to alum in removal of particles with advantages of reduced alkalinity consumption, less sludge production, decreased temperature and pH dependence, and reduction of cost (Hu 2006). In some cases iron and aluminum hydroxides have also been used to modify filtration media to improve the particle removal efficiency. (Edwards 1989, Lukasik 1999, Ahammed 2006). In these studies Fe and Al surface modification was achieved by soaking the sand media in aluminum chloride or ferric chloride solutions and followed by treatment with an ammonium hydroxide solution.
In the research described in this paper, three common coagulants, alum, ferric chloride (FeCl3), and PAC were chosen to modify sand medium prior to challenging the filter media with a colloidal suspension of kaolin. A novel process was used for modification of the sand medium in which the sand media in the filter was "pretreated" in situ before challenging the filter. The resulting particle removal efficiencies are compared as a function of Al and Fe surface loading. Head loss was also monitored during pretreatment and during the challenge to the filter.

Materials and Methods

Apparatus. An automated filtration test apparatus described by Weber-Shirk (Weber-Shirk 2008) was used to conduct parametric tests of the use of alum, ferric chloride (Fisher Scientific), and polyaluminum chloride, PAC (Holland Company Inc.) to modify (artificially ripen) the media in a sand filter. A schematic of the apparatus is shown in Figure 1.

Some random text referring to the results from the Comparison of the Ability of Three Coagulants to Enhance Filter Performance.

[Sand Filtration]

Procedure

We used [process controller] to perform these experiments.