Enhanced Coagulation and Removal of Cryptosporidium

Abstract

In an effort to better control formation of disinfection by‐products (DBPs), the US Environmental Protection Agency's recently promulgated Disinfectants/DBP Rule requires that US surface water treatment plants whose influent and effluent waters meet certain criteria practice enhanced coagulation. Although this treatment technique has been shown to effectively reduce natural organic matter, and consequently DBPs, questions have been raised concerning its effect on other aspects of water treatment such as particle and pathogen removal. The current study was designed to investigate the influence of decreased coagulation pH levels (an integral component of enhanced coagulation) on removal of Cryptosporidium oocysts as well as on reduction of total organic carbon (TOC), turbidity, and particle counts. A series of pilot‐plant trials was conducted in which commonly used coagulants (ferric chloride, alum, and polyaluminum chloride) were used at various pH levels to treat river water spiked with large numbers of Cryptosporidium oocysts. The results showed that TOC removal is significantly enhanced by coagulation at lower pH levels and that all three coagulants are effective in removing Cryptosporidium oocysts during conventional treatment (mean removal = 4.3 log units). However, turbidity and particle counts appear to be unreliable indicators of oocyst removal. Finally, the investigation suggested that lowering coagulation pH does not interfere with removal of Cryptosporidium. However, questions remain concerning the use of alum at pH 5.