AWWA WQTC69483

The objectives of this project were to: quantify the level of contamination of filters by various organisms (amoebae, aerobic spores, E. coli, nematodes, etc.) through a survey performed on several production sites, with various resources and associated water treatment works; optimize filter washing to prevent or minimize health risks; and, define cleaning protocols in order to allow operators to decontaminate filter media. Protocols have been established from tests at laboratory and pilot scale before application on industrial sites. The first part of the study focuses on a worldwide survey of more than 15 filters from water treatment plants (WTPs) in Europe (France, Hungary, United Kingdom) to develop detailed information on filter contamination. The information collected includes process design, water resource characterization, and contamination level of filter media and filtered water quality with details on biological particles. Results of this survey revealed a wide variety of organisms causing the contamination of the filters. The mode of contamination of the filter beds (1st stage and 2nd stage filter) is primarily due to the accumulation of biological particles coming from the resources, and secondarily a consequence of direct development within the filters. Backwashing applied under standard operating conditions (in terms of air, water flow rates and frequency) efficiently removed organisms retained in the inter-granular space of the filter media during the first two steps; however, backwashing does not efficiently release the organisms (particularly amoebae and zooplankton) protected by the biofilm occurring on the media surface. Regarding the media cleaning, the most appropriate reagents for obtaining elimination of organisms, particularly amoebae fixed on a media, were chosen through batch tests with various chemicals (salts, oxidants, acid, base). Salts and oxidants were found to be the most efficient reagents for removal of organisms from the media. Includes 18 references, tables, figures.