Depth filtration using novel fiber-ball filter media for the treatment of high-turbidity surface water

Abstract

Performance of the novel fiber-ball filter was investigated for the treatment of high-turbidity surface water. Impact of different operating parameters, such as coagulant type, coagulant dose and filtration velocity on the effluent turbidity, dissolved organic carbon (DOC) and head loss, as well as the particle removal characteristics were studied. The clean-bed filtration results showed that the fiber-ball filters had a great potential for the removal of turbidity matters. Addition of polymeric aluminum ferric chloride (PAFC) enhanced the removal of effluent turbidity and DOC, while the head loss was also increased. The effluent turbidity could be <0.5 NTU and the removal efficiency of DOC was ∼16% when the PAFC was dosed at 12 mg/L. The effluent turbidity and DOC increased slightly with increasing filtration velocities (30–50 m/h) due to the greater drag forces resulted from higher filtration velocities. However, the head loss was also increased. The particle removal characteristics indicated that large particles were primarily removed in the top 30 cm of the filter bed and the deep depth in the bottom 20 cm played an important role in preventing the occurrence of turbidity breakthrough. It was also found that particles in the size larger than 2 μm could be well removed by the fiber-ball filters, implying the potential application for the removal of protozoa like Cryptosporidium (3–15 μm) to meet the desired requirement for drinking water treatment during depth filtration.

Introduction

The removal of particulate matters present in raw water has been increasingly concerned due to high-regulation for the safe drinking water and high-purity water demanded in industry. Depth filtration is one of the most favorable particle separation means used universally in water and wastewater treatment processes. In this method, the particles are removed from its carrying fluid through a packed bed of granular or fibrous filter media by different mechanisms, such as transport, interception, aggregation, sedimentation and diffusion [1], [2]. Normally, the deposition of particles inside the filter depends on its transport by streams to the surface of filter material and/or previously attached particles. When they arrive there, the attachment can occur due to the interactions between particles and inner filter surface [3], [4]. Meanwhile, there still can be some individual particles remaining in streams due to the detachment from filter surface and/or previously captured particles.

Traditionally, the depth filtration media include two main types, the “slow” and “rapid” granular media, like sand, anthracite, diatomaceous earth, etc., which have been shown to be capable of efficiently removing many particulate type contaminants from water and wastewater [5], [6], [7], [8]. However, the major limitation in the use of granular filter media in solid–liquid separation is the capacity for retention of the suspended and colloidal particulate pollutants within the pore spaces of the filter media [9]. Another disadvantage is that little or no stratification by size occurs due to the mixing of filters during backwashing process, resulting in shorter filtration run period and worse effluent quality. Furthermore, although the slow granular filter media can effectively intercept the suspended particles and remove microorganisms, largely by straining through a biological layer that forms on the surface of filters, the filtration rate only can be of 0.1–0.3 m³/m² h passing through the filters. By contrast, the filtration rate of rapid granular single-, dual- or multi-medium filters can generally reach up to 4–20 m³/m² h, but their particle entrapped efficiency cannot preferably maintain due to the combined influence of their size and density followed by backwashing to release the trapped particles. In addition, it is unsuitable for granular filter media during the filtration of high-turbidity water, which is defined as those waters with turbidity higher than 50 nephelometric turbidity units (NTU) by World Health Organization (WHO) [10], because the filters will be clogged frequently and thus require extensive cleaning.

In recent years, fiber filter media which are man-made materials, have been developed and successfully applied for the tertiary treatment of wastewater and purification of drinking water [11], [12]. Compared with the conventional granular media, superior performance in removing particulate matters from water can be achieved by fiber filter media even under 10-fold higher filtration velocities [13]. Nevertheless, these studies are all with respect to the treatment of low-turbidity water. As far as the authors know, there has not been any research focusing on the treatment of high-turbidity water with fibrous filter media.

The object of this study is to investigate the filtration performance of the novel fiber-ball filter media developed by special preheat process for the treatment of high-turbidity surface water. In-line coagulation was applied to enhance the removal of particulate matters. Polymeric aluminum ferric chloride (PAFC) was chosen as the coagulant due to the synergistic flocculation reactions of both aluminous and ferric salts. The effects of operating parameters such as coagulant dose and filtration velocity on the effluent turbidity, dissolved organic carbon (DOC) and head loss were studied. The particle removal characteristics throughout the filter bed were also evaluated.