Alternate Indicator Organisms for Reclaimed Water in North Carolina

New reclaimed water rules that include limits for multiple microbial indicator organisms went into effect in North Carolina in 2011. One of the primary updates in these new rules is the establishment of two classes of reclaimed water: Type 1 is the less stringent class that is analogous to the older reclaimed water standard, and Type 2 is the higher quality reclaimed water, which requires the use of dual disinfection and which has additional permissible uses and stricter treatment requirements to protect public health. For Type 2 reclaimed water, the rules require treatment plants to provide a minimum 6 log reduction for E. coli, 5 log for coliphage, and 4 log for Clostridium perfringens in addition to single-digit concentration limits. To the author's knowledge, no other US state currently uses coliphage and C. perfringens as indicator organisms for reclaimed water regulatory compliance.This paper reviews published data from various plants in the United States on the effectiveness of municipal wastewater treatment for removing or inactivating these alternate indicator organisms, as well as new data from one of the larger producers of reclaimed water in the state of North Carolina, the City of Raleigh. Data from the published studies suggest that relatively high disinfection doses may be required to meet Type 2 reclaimed water standards; however, the data from the City of Raleigh show that the Raleigh treatment plant would likely meet Type 2 microbial concentration standards with their current reuse regime of UV followed by sodium hypochlorite.The published data suggest that an MS2 Reduction Equivalent Dose (RED) of 100 mJ/cm2 may be required to meet the C. perfringens requirements for Type 2 reclaimed water, if the 4-log overall reduction requirement is met with 2-log inactivation by UV and 2-log removal by upstream processes. In this example, the energy required for Type 1 reclaimed water assuming basic UV disinfection with an MS2 RED of 30 mJ/cm2, is less than one third the energy required to treat Type 2 reclaimed water with an MS2 RED of 100 mJ/cm2.