A novel filtration configuration for targeted humic acid removal from drinking water

Transcription

1 A novel filtration configuration for targeted humic acid removal from drinking water M. A. Grace, E. Clifford,2, and M.G. Healy,2 Civil Engineering, College of Engineering and Informatics, National University of Ireland, Galway 2 Ryan Institute for Environment, Marine and Energy Research, National University of Ireland, Galway ( m.grace@nuigalway.ie; eoghan.clifford@nuigalway.ie; mark.healy@nuigalway.ie) Abstract Natural organic matter (NOM) has been identified as a precursor to disinfection by-product (DBP) formation in a potable water purification system, and can be measured on site as dissolved organic carbon (DOC). Disinfection by-products are harmful to human health and have been linked to cancer and genetic mutations. To eliminate the potential for DBP formation in the plant, it is necessary to design a treatment system targeting DOC removal prior to disinfection. This study investigates the use of filtration as a mechanism to remove DOC. Based on a bench-scale adsorption study of a variety of media, two filter configurations, each containing layers of novel adsorptive media, were designed to target the removal of DOC. The filters contained a variety of media, including natural materials and industrial waste products. The results of the novel configurations were compared to those from a conventional sand filter (the study control), constructed as per Irish Environmental Protection Agency (EPA) guidelines. With percentage removals as high as 7% in some instances, the new filters, developed in this study, have the potential to remove DOC from drinking water. Keywords Filtration, drinking water, humic acid, natural organic matter INTRODUCTION Raw sources for drinking water generally comprise ground or surface waters. The very nature of this means that there will be a microbial community present, resulting from agriculture, anthropogenic and environmental sources (Pandey 26). Disinfection of potable water is used to inactivate any pathogens that may be present in the microbial community (EPA 2). Chemical disinfectants are used more often than physical disinfectants, with chlorine being the most commonly used worldwide (Sincero & Sincero 22; de la Rubia et al. 28). Disinfection normally occurs at the last stage of treatment within the water treatment plant, and can also be followed up with a boost along the distribution network, ensuring that the required residual remains at the tap (EPA 2). However, a problem associated with all types of chemical disinfectants, is the formation of disinfection by-products (DBPs) following disinfection. The occurrence of trihalomethanes (THM), following chlorination, was first reported in 974 (Minear & Amy 996). Trihalomethanes, like other DBPs, are toxic compounds, and long-term exposure has been linked to elevated cancer risk (Wang et al. 27) and reproductive and developmental defects in people and animals (Wang et al. 27). The maximum allowable concentration of THM in Ireland, and the European Union, is currently µg L - (European Communities 998; SI No 278 of 27), however, this will likely be reduced to align with the United States Environmental Protection Agency (USEPA) MAC of 8 µg L - (USEPA 29). Natural organic matter (NOM), the precursor to the formation of DBPs, is present in all raw waters from various sources and can be due to impacts of plant and animal decomposition leachates (Chen et al. 22). Natural organic matter is usually measured by the concentration of total organic carbon (TOC) in water and primarily comprises humic substances, which are classed as humic and fulvic acids (Chen et al. 22). The ratio of humic to fulvic acids depends on a variety of environmental

2 conditions including temperature, soil-type and rainfall (Minear & Amy 996). Seasonal changes to this ratio in raw water are also likely to affect DBP formation (Jacangelo et al. 995). Humic substances can be particularly problematic where much of the raw water for treatment is sourced from peatland areas (for example in Ireland) (Gough et al. 24). Humic substances are difficult to remove at the filtration stage of a conventional water treatment plant (WTP), without resorting to expensive ultrafiltration membranes, due to the small particle size, which falls in the molecular to macromolecular range (<. µm) (Jacangelo et al. 995). Disinfection by-product formation is a growing concern for the authorities, with exceedances in WTPs being reported every year (Water_Team 22). Much of the current research proposes methods of removing THM and other DBPs post-disinfection, using techniques such as ozonation reactors, and submerged membrane photocatalytic reactors (Reguero et al. 23). Many of these are capital-rich, difficult to maintain, and are unsustainable technologies. Other studies suggest changing the form of disinfection. However, this may result in the formation of other DBPs, not yet discovered, as it is thought that all disinfectants will produce some form of DBP (Ivančev-Tumbas 24). Such technologies may not provide residual effects, and technology upgrades can incur more capital expense. With removal of TOC in conventional WTPs reported to be as low as 8 48 % (Jacangelo et al. 995), targeting TOC removal appears to be a more sustainable approach. This study investigates the potential of redesigning filtration systems in WTPs, by replacing sand with a combination of novel media, to remove specific contaminants, such as TOC, and to compare its performance against filters constructed as per EPA guidelines (EPA 995). Such a technology would be adaptable to use in small and large-scale WTPs, would be sustainable, and would require little maintenance. Moreover, depending on the media and distance to the plant from source, capital expenditure would be greatly reduced. MATERIALS AND METHODS Media selection In a preliminary study (Grace et al. 25), the adsorption of humic acid on a selection of novel media was modelled using Langmuir and Freundlich adsorption isotherms (Table ). Based on the results in Table, laboratory filters were constructed using stratified layers of sand, zeolite, fly ash, granular activated carbon (GAC), or Bayer residue as filter media. Table Results of Langmuir and Freundlich adsorption models for TOC adsorption on a variety of media (adapted from (Grace et al. 25)). Media Isotherm R 2 Q max (µg g - ) /n K Sand Desorption Zeolite Langmuir.7 37 Fly ash Freundlich GAC Langmuir Bayer residue Freundlich Filter construction Filter columns, each with m depth of media, were constructed in triplicate. Filters (also at n=3) containing one layer of unstratified sand, with an effective size (d ) of.8 mm and a uniformity

3 coefficient of 2.9, were also constructed as per Irish EPA guidelines (EPA 995), and acted as the study control ( Control ). Config consisted of three equal layers of Bayer residue, zeolite and sand, and Config 2 consisted of four equal layers of fly ash, GAC, zeolite and sand (Figure ). The filter columns were fitted with sample ports at the media interfaces (at 25 mm or 33 mm intervals, depending on the configuration), in order to gain a clear understanding of what was happening within each layer of media. The Control was instrumented in a similar way to Config, with sample ports at 33 and 66 mm below the surface of the filter. Figure Profile design of Control, Config and Config 2. Filter operation The filters were operated under two loading regimes: constantly loaded and intermittently loaded. The constantly loaded filters had a head of 5 mm of water maintained above the filter surface at all times. The intermittently loaded filters were dosed with a peristaltic pump (7528-, Masterflex L/S Variable-Speed Drive) for min every 2 hr, at an average loading rate of 64 L m -2 d -. The filters were dosed with a synthetic water mix, containing, among other water contaminants, TOC at the concentrations stated in Table 2. To properly assess the efficacy of the novel filter configuration to remove DBP formation potential, laboratory-grade humic acid (Sigma Aldrich) was used as the TOC source in the synthetic water. Humic acid was prepared using a method adapted from Abdul et al. (99), where humic acid was added to water and stirred for 25 min before being centrifuged at RPM for 3 min. The mixture was then filtered through a.45 µm filter to remove as much of the non-water soluble fraction as possible, and to conform to standard specification for dissolved organic carbon (DOC) (de la Rubia et al. 28). The influent and effluent DOC concentrations were measured using a BioTector TOC analyser (BioTector Analytical Systems, Ireland). RESULTS AND DISCUSSION

4 The influent and effluent DOC concentrations from each filter configuration and loading regime are shown in Table 2. The four-layer filter (Config 2) had the best DOC removal, although removal was also evident in Config. However, both removals were greater than the Control. The continuous loading regime was more effective than an intermittent loading regime, particularly in the case of Config 2. The average removal of 7% of Config 2 was comparable to the bench-scale membrane photocatalytic reactor of Reguero et al. (23), yet Config 2 would be a less expensive technology. Towards the end of the experiment, clogging was observed across all filter configurations. This can be attributed to the high suspended solids in the humic acid mix (approx. 2 mg L - ), which was significantly higher than would normally be found at the filtration stage in a standard WTP, and also to the small particle size at the uppermost layer in Configs and 2, both of which clogged more quickly than the sand. Table 2 Influent and effluent DOC removal concentrations for three configurations*, loaded continuously and intermittently. Loading Regime Average concentrations mg DOC L - St. Dev Continuous Influent Concentration Effluent Concentration Control Config Config Intermittent Influent Concentration Effluent Concentration Control Config Config *Control = sand filter; Config = three-layer filter containing Bayer residue, zeolite and sand, and Config 2 = fourlayer filter containing flyash, granular activated carbon, zeolite and sand. Figure 2 shows the percentage removal of DOC at different depths within the filters when operated continuously and intermittently, at day, 45 and 9 of operation. Sampling at depths in intermittently loaded Config 2 filters was not possible. The uppermost section of the Control filters (measured from the filter surface to the 33 mm sample port) had a lower uptake rate at the end of the experiment than at the start, indicating that the media in the layer may have become saturated. The DOC removal in Config and 2 generally decreased over time, which would be expected as the adsorptive potential of the media may have been greatly reduced and much of the chemical removal would have lessened. The 9-day removal data for both Config and 2 suggest that there was less removal at the lower layers than at the top layers, indicating that there may have been some leaching of DOC back into the water by the time it reached the base. This may have been due to media saturation, with more specific surface area available in the uppermost layer, because of the fine particles.

5 Continuous Intermittent Control Control,4,4,8, Depth from surface (m),4,8 Config 5 5,4,8 Config -5 5 Config 2 Config 2,4,4,8,8 5 5 % Removal Figure 2 Profile results for each configuration in both loading regimes, showing percentage removal at depths throughout the columns at day, day 45 and day 9 of the experiment.

6 Outlook for adoption of technology These results show that alternative filter media may be employed in WTPs to remove DOC. Dissolved organic carbon removal at filtration stage is essential to reduce the potential for DBP formation post-disinfection in a WTP. The DOC removals in Config and Config 2 were greater than a standard sand filter, subjected to high suspended solids and high humic acid concentrations. Profile results show that media can become saturated with contaminants, though this can be fixed by replacing the clogged layer within the filter, before the DOC breaks through and releases back into the water from the saturated layer. Further investigation into the design is necessary as these configurations are not sustainable, given the potential for clogging (due to the low saturated hydraulic conductivity of the fly ash). However, this study showed that novel media can be used, and repositioning these within the filter may be an option. Future work will involve testing an alternative configuration to assess the potential of a successful design. This can be used to inform future developments in filtration technologies that use specific media to remove targeted compounds. CONCLUSION This study showed that Bayer residue, fly ash, zeolite, GAC and sand, in layered filter configurations, can remove DOC from drinking water sources. The most effective configuration was fly ash, GAC, zeolite and sand (Config 2). However, there is a potential issue with clogging of the filters, although this may be solved by removing the clogged layer of the filter, provided that clogging is restricted to the uppermost layer. To create a realistically viable filter configuration, it is imperative to redesign the filter to mitigate the effects of clogging. It was also shown that Bayer residue, zeolite and sand (Config ) were more effective that the control study at removing DOC, although it was not as successful as Config 2. REFERENCES Abdul, A.S., Gibson, T.L. & Rai, D.N., 99. Use of Humic Acid Solution To Remove Organic Contaminants from Hydrogeologic Systems. Environmental science & technology, 333(24), pp Chen, J. et al., 22. Spectroscopic characterization of the structural and functional properties of natural organic matter fractions. Chemosphere, 48(), pp EPA, 2. Water Treatment Manual : Disinfection, EPA, 995. Water Treatment Manuals: Filtration, European Communities, 998. EU Drinking Water Directive, Council Directive 98/83/EC, Gough, R. et al., 24. Dissolved organic carbon and trihalomethane precursor removal at a UK upland water treatment works. The Science of the total environment, , pp Grace, M.A., Healy, M.G. & Clifford, E., 25. Use of industrial by-products and natural media to adsorb nutrients, metals and organic carbon from drinking water. Science of The Total Environment, 58-59(278), pp

7 Ivančev-Tumbas, I., 24. The fate and importance of organics in drinking water treatment: a review. Environmental science and pollution research international, 2(2), pp Jacangelo, J.G. et al., 995. Selected processes for removing NOM: an overview. American Water Works Association, 87(), pp De la Rubia, A. et al., 28. Removal of natural organic matter and THM formation potential by ultra- and nanofiltration of surface water. Water research, 42(3), pp Minear, R.A. & Amy, G.L., 996. Disinfection by-products in water treatment: the chemistry of their formation and control, FL: CRC Press. Pandey, S., 26. Water pollution and health. Kathmandu University medical journal (KUMJ), 4(), pp Reguero, V. et al., 23. Comparison of conventional technologies and a Submerged Membrane Photocatalytic Reactor (SMPR) for removing trihalomethanes (THM) precursors in drinking water treatment plants. Desalination, 33, pp SI No 278 of, 27. European Communities (Drinking Water) (No. 2) Regulations 27, Sincero, A.P. & Sincero, G.A., 22. Physical-chemical treatment of water and wastewater st ed., Florida: CRC Press. USEPA, 29. National Primary Drinking Water Regulations C. O. F. Regulations, ed., Environmental Protection Agency. Wang, G.-S., Deng, Y.-C. & Lin, T.-F., 27. Cancer risk assessment from trihalomethanes in drinking water. The Science of the total environment, 387(-3), pp Wang, W. et al., 27. Risk assessment on disinfection by-products of drinking water of different water sources and disinfection processes. Environment international, 33(2), pp Water_Team, E., 22. Secure Archive for Environmental Research Data. Drinking Water Monitoring Results and Water Supply Details for Ireland. Available at: