Performance Evaluation of Different Filter Media in Turbidity Removal from Water by Application of Modified Qualitative Indices

Article excerpt

Abstract

Background: Water filtration units have been faced problems in water turbidity removal related to their media, which is determined by qualitative indices. Moreover, Current qualitative indices such as turbidity and escaping particle number could not precisely determine the efficiency of the media in water filtration, so defining new indices is essential. In this study, the efficiency of Anthracite-Silica and LECA-Silica media in turbidity removal were compared in different operating condition by using modified qualitative indices.

Methods: The pilot consisted of a filter column (one meter depth) which consisted of a layer of LECA (450 mm depth) and a layer of Silica sand (350 mm depth. Turbidities of 10, 20, and 30 NTU, coagulant concentrations of 4, 8, and 12 ppm and filtration rates of 10, 15, and 20 m/h were considered as variables.

Results: The LECA-Silica media is suitable media for water filtration. Averages of turbidity removal efficiencies in different condition for the LECA-Silica media were 85.8±5.37 percent in stable phase and 69.75±3.37 percent in whole operation phase, while the efficiency of total system were 98.31±0.63 and 94.49±2.97 percent, respectively.

Conclusion: The LECA layer efficiency in turbidity removal was independent from filtration rates and due to its low head loss; LECA can be used as a proper medium for treatment plants. Results also showed that the particle index (PI) was a suitable index as a substitute for turbidity and EPN indices.

Keywords: Dual media filter, Water turbidity, Filtration, LECA, Silica

Introduction

Filtration is a separation process that consists in passing a solid-liquid mixture through a porous material (filter) which retains the solids and allows the liquid (filtrate) to pass through (1). Removing suspended solids by high-rate granular media filtration is a complex process involving a number of phenomena. Attempts to develop theories that quantitatively predict solids removal performance with sufficient precision and versatility to be of use in practical filter design have met with relatively little success. Consequently, filter media selection is often an empirical process. Pilot investigations are common tools for assessing the performance of a particular filter design (2-4).

The common types of media used in granular bed filters are Silica sand, Anthracite coal, and Garnet or Ilmenite. These may be used alone or in dualor triple-media combinations a number of properties of a filter medium are important in affecting filtration performance and also in defining the medium. These properties include size, shape, density, and hardness. The porosity of the granular bed formed by the grains is also important (2).

Although the selection of filter media type and characteristics is the heart of any filtration system, selection is usually based on arbitrary decisions, tradition, or a standard approach. Pilot plant studies using alternative filter media and filtration rates can determine the most effective and efficient media for a particular water (5).

Turbidity is considered as a common index for filtration efficiency determination; however, this index is weak for particles with diameter of 1-10 µm including pathogen microorganisms. Along with the EPN (escaped particle number), turbidity is used to control filtration efficiency too, but these indices are not satisfactory (5).

Some studies have been conducted to evaluate the performance of filter media on removal of different pollutants from water and wastewater (6-10). In these studies, however, no efficient indices have been applied to evaluate the performance of filter media and to compare the efficiencies of different media.

LECA (Light Expanded Clay Aggregates) has been considered as a media for water filtration due to its characteristics such as high adsorption capacity, abundant resources, inexpensive in compare to other media and etc, however most of studies were applied LECA as an adsorbent(11, 12). …