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What accumulates in our drinking water storage tanks? Doronina, A. V., Husband, P. S., Boxall, J. B., Speight, V. L. Stream The Industrial Doctorate Centre for the Water Sector www.stream-idc.net For further information: [email protected] Postal Address: Department of Civil and Structural Engineering, Sir Frederick Mappin Building, Mappin St, Sheffield, S1 3JD Drinking water storage tanks are essential assets used to maintain our drinking water supply 1 . Overtime, material (organic and inorganic particulates) accumulates in these vessels, either from internal (e.g. corrosion) or external (e.g. ineffective removal at treatment) sources 2 . These deposits can provide nutrients for microbial growth including opportunistic pathogens, and impact water quality, for instance, by decreasing disinfection efficacy 1,3 . If re-suspended, this material poses a risk to water safety and regulatory compliance 4,5 . To address this, storage tanks are regularly cleaned, yet the collection and analysis of accumulated material is never considered, making it difficult to determine optimal cleaning frequencies by assessing deterioration rates, and also quantify any risks involved in material remobilising and entering the downstream network 6 . To investigate these missed opportunities, material has been collected from UK storage tanks and analysed for its composition and water quality impacts. Method Results & Discussion Conclusions References Based on typical TOC levels in finished drinking water (0.05-12.2 mg/l 7 ), results are not indicative of a high organic content and thus microbiological presence. • Material that accumulates in storage tanks is not high in organic content, which suggests a low microbial presence. • This same material, however, exerts a high chlorine demand, thus posing a safety risk to the surrounding water quality. Results suggest that material is predominantly humic-like across all tanks, which is not associated with microbial sources. Irrespective of the supposed low organic content of samples, they exert a high chlorine demand in that average hourly decay rate was 0.02 mg/l (0.48 mg/l across 24 h). Typical residual chlorine levels leaving UK water treatment works are 0.05 mg/l 8 , which, based on our results, would rapidly drop to 0 at storage tank outlets if material re-suspends. Accumulated storage tank material was collected from the bottom of 3 storage tanks following their drainage in preparation for an inspection and clean (Fig. 1). Tank description: • HH – 5.2 ML, rectangular, concrete, underground, chlorinated system • K - 0.4 ML, circular, concrete, underground, chloraminated system • R – 0.8 ML, concrete, underground, chloraminated system There are no guidelines on how to sample storage tank material, so a full storage tank material sampling protocol was created by project staff, and can be found in the ‘About Project/Findings’ section at: https ://materialdestiny.group.shef.ac.uk/ Material was collected from different locations within each tank; mainly around the inlet, outlet, and middle of the tank to get a representative sample. Samples were then shipped to the University of Colorado, Boulder, where they underwent different analyses, including for organic carbon (Fig.2), fluorescence (Fig. 3), and chlorine demand (Fig. 4). 1. Brandt, M. J., et al. 2016. Twort’s Water Supply. Butterworth-Heinemann. 2. Lytle, D. A., et al. 2012. Water Research 50:396–407. 3. van der Kooij, D. 2003. Heterotrophic plate counts and drinking-water safety. IWA Publishing. 199-232. 4. NRC. 2007. Drinking water distribution systems: assessing and reducing risks. National Academy Press. 5. Kirmeyer, G. J., et al. 1999. Maintaining Water Quality in Finished Water Storage Facilities. AWWA. 6. AWWA. 2011. Water Quality & Treatment: A Handbook on Drinking Water. McGraw-Hill. 7. Symons, J.M., et al. 1975. AWWA 67(11):634-647. 8. DWI. 2010. Chlorine. London: Drinking Water Inspectorate. Chlorine Demand Fluorescence Total Organic Carbon Figure 1 - The process of material sampling from tank to the lab. Figure 2 – TOC levels in the accumulated material storage tank samples. Figure 3 – EEMs for accumulated material storage tank samples. Figure 4 – Chlorine decay rates for the accumulated material storage tank samples. Introduction
