Development of filtration technologies for effective, cost-efficient and robust water treatment
Grace, Maebh A.
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The provision of high quality, potable water in a sustainable and effective manner is a key challenge for water engineers, scientists, and policy makers. Urbanisation and industrial development, along with population growth, intensification of agriculture and climate change, has increased the strain on current potable water supplies in the developed world, not only in terms of supply, but also by way of introducing new contaminants into the abstraction waters. Emerging contaminants, alongside increasingly stringent environmental legislation, make the maximum allowable concentrations for various contaminants within potable water more difficult to achieve. Although the situation is improving, over 660 million people worldwide still do not have access to improved water supplies. Sustainable and robust solutions that ensure effective water treatment for a variety of contaminants, while also being low cost and low maintenance, are required. Metals and nutrients continue to enter source water supplies by anthropogenic and natural sources. While metals can be damaging to human health, nutrients can lead to excessive microbial activity. The presence of organic matter in source waters also challenges water treatment plants (WTPs), as disinfectants used post-treatment can trigger a reaction with organic matter to create toxic by-products. These can develop both in the WTP and along the distribution network, and long-term exposure can be detrimental to human health. This is a major problem in Ireland and elsewhere, and to date, no cost-effective and sustainable solution has been found. Various technologies are being developed to target problem contaminants, including coagulation systems, gas transfer systems, oxidation technologies, and membranes. Although these may be effective, they may also require high capital investment, be subject to fouling, and require expert maintenance. In addition, costly treatment systems are not feasible for smaller water treatment facilities and developing countries. Instead, a move towards more traditional contaminant removal and retention mechanisms, such as adsorption and filtration, may be more sustainable. Such technologies, in tandem with the use of alternative filter media, such as waste products and locally sourced material, can improve the sustainability of WTPs while not negatively impacting performance. This study aimed to address the issue of contaminant removal, by designing a filter that could fit into the current model of a WTP, and which could remove a variety of contaminants including metals, nutrients, and disinfection by-product precursors. The technology incorporated the use of waste materials to reduce cost and maintain sustainability, and was comprehensively tested at laboratory-scale and pilot-scale. A number of waste products and local materials were first tested at bench-scale, using a variety of common water contaminants to assess their adsorptive capacities. The media tested included coarse sand, zeolite, granular activated carbon (GAC), pyritic fill, Bayer residue, bottom ash, fly ash, and granular blast furnace slag. Following this, laboratory-scale stratified filters were constructed, comprising some of the most successful media from the bench-scale study: fly ash, Bayer residue, zeolite, sand, and GAC. The filters were evaluated for treatment performance and media clogging potential. The results obtained from the laboratory-scale study led to a re-design for the pilotscale study, which was operated at a WTP that used lake water as its potable water source. The WTP was chosen as it had a history of formation of disinfection byproducts. Two filter configurations were examined under intermittent and constant loading rates, and comprised combinations of sand, Bayer residue, GAC, and pyritic fill. While each of the alternative designs proved more successful than a standard sand filter, a filter configuration comprising sand, GAC, and pyritic fill, proved most effective in dissolved organic carbon removal under a continuous loading regime. These studies show that waste products can be used in filtration technologies, where adsorption is a key mechanism, thereby reducing overall capital and maintenance requirements. Adsorption isotherms are instrumental in the design of bespoke filters, and this study found that it is possible to target the removal of specific contaminants, depending on the constituents of the source water. This study presents a simple, lowmaintenance design to reduce the concentration of key contaminants in potable water, and addresses a major problem for WTPs.