What accumulates in our drinking water storage tanks?Doronina, A. V., Husband, P. S., Boxall, J. B., Speight, V. L.
StreamThe Industrial Doctorate Centre for the
Water Sector
www.stream-idc.netFor further information: [email protected] 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 ourdrinking water supply1. Overtime, material (organic and inorganicparticulates) accumulates in these vessels, either from internal (e.g.corrosion) or external (e.g. ineffective removal at treatment) sources2.These deposits can provide nutrients for microbial growth includingopportunistic pathogens, and impact water quality, for instance, bydecreasing disinfection efficacy1,3. If re-suspended, this material poses arisk to water safety and regulatory compliance4,5. To address this, storagetanks are regularly cleaned, yet the collection and analysis of accumulatedmaterial is never considered, making it difficult to determine optimalcleaning frequencies by assessing deterioration rates, and also quantify anyrisks involved in material remobilising and entering the downstreamnetwork6. To investigate these missed opportunities, material has beencollected from UK storage tanks and analysed for its composition andwater quality impacts.
Method
Results & Discussion
Conclusions
References
Based on typical TOC levels in finished drinking water (0.05-12.2 mg/l7),results are not indicative of a high organic content and thus microbiologicalpresence.
• Material that accumulates in storage tanks is not high in organiccontent, which suggests a low microbial presence.
• This same material, however, exerts a high chlorine demand, thusposing 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 ahigh chlorine demand in that average hourly decay rate was 0.02 mg/l (0.48mg/l across 24 h). Typical residual chlorine levels leaving UK water treatmentworks are 0.05 mg/l8, which, based on our results, would rapidly drop to 0 atstorage tank outlets if material re-suspends.
Accumulated storage tank material was collected from the bottom of 3storage tanks following their drainage in preparation for an inspection andclean (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 systemThere are no guidelines on how to sample storage tank material, so a fullstorage tank material sampling protocol was created by project staff, andcan be found in the ‘About Project/Findings’ section at:https://materialdestiny.group.shef.ac.uk/Material was collected from different locations within each tank; mainlyaround the inlet, outlet, and middle of the tank to get a representativesample.Samples were then shipped to the University of Colorado, Boulder, wherethey 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
