Full Description

Quantification of Cryptosporidium removal in the subsurface is critical for utilities considering implementation of in situ riverbank filtration systems and/or faced with quantifying contaminant removals at sites where Ground Water Under Direct Influence (GWUDI) of surface water exists. Subsurface attenuation of pathogens occurs through filtration, dilution, and degradation processes. Detailed investigations of these processes in the field have not been reported in the literature. Field assessments of Cryptosporidium in riverbank filtrate are either unable to detect oocysts in filtrate or, when oocysts are detected, report bulk removals without assessing the true filtration of their riverbank by accounting for dilution and degradation effects. Laboratory and column investigations have characterized the effects of ionic strength and pH on particle zeta potential and hydrophobicity, two important factors that influence particle depositions in porous media. These effects have not been adequately assessed in the field. A comparison of contact efficiency between a rapid sand filter and a riverbank filter system in Kitchener, Ontario suggests that the riverbank filter may achieve the same or better particle contact with collectors. Ionic strength of the same riverbank well indicates favorable chemical conditions for particle attachment to collectors. Although a theoretical riverbank contact efficacy was calculated for a range of particle sizes, predicting Cryptosporidium removal using conventional physico-chemical filtration models may not be possible because of non-ideal deposition conditions in the subsurface, namely, surface charge and physical heterogeneity of the collectors, time-dependent deposition processes, and uncertainty in particle attenuation resulting from organic sorption and physical straining. An examination of subsurface filtration conditions indicates that in situ experimentation is most appropriate for assessing riverbank filtration efficacy for pathogen removal and that optimizing RBF design for low velocity may be beneficial. Includes 26 references, tables, figures.

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Edition: Vol. - No. Published: 11/01/2002 Number of Pages: 13File Size: 1 file , 480 KB