Removing Natural Organic Matter by Conventional Slow Sand Filtration

This article evaluates the capacity of slow sand filters to remove natural organic matter (NOM) and organic precursor materials as quantified by dissolved organic carbon, UV absorbance, and trihalomethane formation potential from source waters with varying levels of NOM. Municipal facilities were sampled to provide baseline data and to evaluate the effects of two ditTerent filter cleaning techniques, normal scraping and harrowing, on NOM and particulate removal. Pilot-scale filter studies were conducted to evaluate the influence of two different flow rates on NOM removal, particulate removal, and filter biomass development. A persistent misconception in the percolation of water through the porous water industry is that slow sand filtration sand media providing the only treatment. is an old-fashioned technology that has The filter bed consists of an underdrain been rendered obsolete by advances in system to collect filtered water, a layer of high-rate filtration techniques. Slow sand graded support gravel, and a thick layer or biological filtration has been in contin- of tine sand, which is usually 1 m deep in uous use since the 1800s and has proved a new filter bed. Filters in northern clieffective under widely varying circum- mates are usually covered for protection stances. According to the World Health against freezing; filters in southern cliOrganization,’ slow sand filtration is sim- mates are sometimes covered to prevent ple, inexpensive, and reliable and is still algal blooms and contamination by water the chosen method of treating water sup- fowl.‘,’ An outlet control structure is replies for many major European cities. quired to control flow rates and to mainA conventional slow sand filter lacks tain submergence of the sand media to pretreatment of any sort, with the slow minimize air-binding problems. Slow sand filtration is an ideal treatment technology for developing countries and rural communities where low cost, ease of operation and maintenance, and removal of pathogenic microorganisms are primary considerations.‘-’

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