Microbial risk and removal—a utility perspective M. Stanger*, P.A. Agutter*, R.C. Lake*, N.J. Ashbolt** and D.J. Roser** *Veolia Water Partnership, Blackwell House, Three Valleys Way, Bushey, Herts WD23 2LG, UK (E-mail: michael.stanger@veoliawater.co.uk) **Centre for Water and Waste Technology, School of Civil and Environmental Engineering, University of New South Wales, Australia Abstract In this paper the results of a sampling programme, undertaken as part of the EU MicroRisk project, are described. This project was undertaken to ascertain the occurrence of pathogens and indicators in the River Thames and their subsequent removal through a treatment works. Appropriate physico-chemical surrogates, as determined by statistical correlation are proposed for the microorganisms identified in the raw water. This study shows that under normal raw water conditions the treatment works is able to remove microbial contamination with a significant margin of safety. Keywords Bacteria; indicator species; Microrisk; removal; water treatment Introduction Egham Water Treatment Works (WTW) is located in the south of the UK, approximately 20 miles west of London, and its water treatment plant produces drinking water for approximately 225,000 people. It is designed to take raw water from the River Thames to produce potable water in accordance with company and statutory standards. The Thames contains significant levels of many pollutants necessitating a multi-stage treatment process at Egham designed for removal of solids, pathogens, bacteria, nutrients and organic material. The plant is in constant operation and is designed to cope with large changes in the pollutant and pathogen load. It is consistently able to supply water meeting the requirements of the UK Water Supply (Water Quality) Regulations (2000) throughout normal conditions and peak events. In the 3rd edition of the Drinking Water Guidelines published in 2004, the World Health Organisation recommended a risk based approach to the management of water quality from catchment to consumer. This approach has been supported by the Drinking Water Inspectorate and water utilities in the UK are developing their own systems for the effective management and protection of water quality in the catchment, treatment works and distribution system, to ensure a supply of safe drinking water that has the trust of the customer. As part of its methodology for developing Drinking Water Safety Plans, Veolia Water has been involved in the European Union project Microrisk. Twelve catchment-to-tap systems have been examined for Microrisk. These were chosen to provide a wide variety of raw waters, treatment, storage and distribution systems. Egham was chosen as a polluted surface water with a multi stage treatment process. To assist in identifying and quantifying microbial risk from source to tap in these catchments, a catchment survey has taken place alongside extensive sampling in the source water, through the water treatment works and distribution system over a 12 month period. As the distribution sampling indicated there was no ingress of faecal material into the distribution, this paper Water Science & Technology Vol 54 No 3 pp 253–259 Q IWA Publishing 2006 253 doi: 10.2166/wst.2006.477 Downloaded from https://iwaponline.com/wst/article-pdf/54/3/253/431640/253.pdf by guest on 17 November 2018