Filter backwash and start-up strategies for enhanced particulate removal |
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Affiliation: | 1. Key Laboratory for Water Quality and Conservation of the Pearl Stream Delta, Ministry of Education, School of Environmental Science and Engineering, Guangzhou University, Guangzhou 510006, China;2. State Key Laboratory of Marine Pollution and Department of Chemistry, City University of Hong Kong, Hong Kong, China;3. School of Science and Technology, Open University of Hong Kong, Hong Kong, China;4. School of Environmental Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China;1. State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China;2. College of Water Sciences, Beijing Normal University, Beijing 100875, China |
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Abstract: | A significant consideration in forward planning for water treatment works design and operation concerns the effectiveness of a filtration plant in providing a barrier to particulates in the low micrometre size range, including Cryptosporidium oocysts. The performance of rapid gravity filtration plants is believed to be dependent on backwash and start-up regimes. It was the aim of this study to optimize direct sand filtration by identifying optimum filter backwash and start-up conditions which minimized the passage of particulates into the filtrate. The filter ripening period has long been identified as a cause for concern with respect to particulate passage into the filtrate; this work has shown that up to 40% of all particles that pass into supply during a 48 h run, do so in the first hour of operation. Optimum combined air water “collapse-pulsing” backwash durations were identified that reduced the number of 2–5 μm particles entering the filtrate, especially during the ripening period. Slow start-up was also found to reduce the number of 2–5 μm particles in the filtrate during the ripening period. The reductions in particulate passage resulting from a slow start was found to be media dependent, with smaller media requiring a longer slow start duration than coarser media. |
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