AWWA ACE65409

Modified slow sand filtration is being successfully applied throughout North America to treat raw water that would not normally be considered suitable for slow sand filtration. This technology involves the combination of one or more unit processes with slow sand filtration. These unit processes may include pre-treatment ozonation, pre-treatment advanced oxidation, roughing filtration, post-treatment limestone contact or post-treatment biologically-active carbon filtration. The advantages of modified slow sand, including the relatively low operations and maintenance requirements, are of benefit to many small systems applications. The ability of modified slow sand filtration, and biological filtration in general, to deal with high levels of natural organic matter (NOM) and emerging contaminants has been a topic of recent research. This paper presents the results of analyses of several emerging contaminants from finished water produced by operating conventional and modified slow sand filtration plants, along with pilot scale research studies conducted in the US northeast and in Ontario, Canada. A summary of available information on the removal of emerging contaminants and pathogens using biological processes typical of slow sand filtration is also presented. The ability of biological filtration to effectively treat a wide range of parameters, particularly the more exotic compounds, while retaining a practical and sustainable operation for small systems is reviewed, with an emphasis on the effective application of this technology. The pathogens examined include Aeromonas sp., Cryptosporidium parvum and Giardia lamblia. Indicator microorganisms such as Bacillus spores and MS2 bacteriophage were also studied. Emerging contaminants included in these studies consist of endocrine disrupting compounds such as bisphenol-A (BPA), pharmaceuticals and the cyanobacterial toxin microcystin-LR. A brief discussion of the capacity of biological filtration to reduce geosmin and 2-MIB with and without ozone application is also included. Includes 29 references, figure.