Disinfection For Water Reuse
Disinfection and Pathogen Inactivation - Emerging Trends and Implications for Water Reuse
Disinfection for water reuse has long employed chlorine-based disinfection. This practice has been used successfully over a long period, with no known public health impact. Despite this impressive track record with chlorination, concerns regarding health and safety, disinfection by-products (DBPs) and, more recently, disinfection performance with respect to pathogen inactivation, are driving the conversion from chlorine disinfection to other disinfection methods.
Carollo is working closely with the research community to help improve the understanding of these issues, and in integrating this knowledge into technology and process decisions for our clients.
Current Pathogen Inactivation Requirements
Title 22 of the California Code of Regulations (Title 22) governs recycled water treatment in California and provides a model for recycled water regulation nationwide. Specific to virus disinfection, Title 22 requires that recycled water disinfection practices must result in a minimum of 5-log (99.999%) reduction in viruses, with poliovirus being the standard. The 5-log standard can be directly traced back to a series of studies performed nearly thirty years ago by the County Sanitation Districts of Los Angeles County (LACSD). The LACSD work was titled the “Pomona Virus Study ” (PVS).
The PVS concluded, among other things, that a chlorination CT (combination of chlorine residual and modal contact time) of 450 mg-min/L resulted in 4 logs of poliovirus kill without media filtration and 5 logs of poliovirus kill with media filtration. Based on the disinfection performance demonstrated by LACSD, Title 22 requires the following treatment scheme for recycled water disinfection:
- Approved media filtration of clarified secondary effluent followed by chlorination (free or combined chlorine) at a CT of 450 mg-min/L; or
- An alternative filtration/disinfection process that results in 5-log inactivation or kill of MS2 or poliovirus.
Pathogen Inactivation Effectiveness of Disinfectants
For chlorine disinfection, the important item to note related to the specified level of treatment is that it currently is the same for free and combined chlorine. The EPA ’s Surface Water Treatment Rule (SWTR) has long espoused the difference in virus disinfection that results from the use of combined chlorine when compared to free chlorine. The EPA Guidance Manual for Alternative Disinfects and Oxidants indicates that a CT of up 1000 mg-min/L is required to achieve 4-log virus reduction using chloramines (EPA, 1999). Recent research by Carollo and others (Cooper et al.2000)1 clearly substantiates the SWTR conclusions, showing minimal viral disinfection by chloraminated wastewater, far below the 5-log reduction required by Title 22.Whether the chlorine residual is in the free or combined chlorine form will depend on the nitrogen speciation in the water.
UV disinfection, as documented extensively in recent literature, and referenced in the "Ultraviolet Disinfection Guidelines for Drinking Water and Water Reuse2,"provides for much greater disinfection, independent of nitrogen speciation in the wastewater matrix. UV disinfection can result in approximately 8 logs of poliovirus inactivation (projected) and 4 to 5 logs of MS2 inactivation at the currently required UV dose of 100 mJ/cm2 for UV systems following media filtration (adapted from Cooper et al.2000)1.
Ongoing Critical Research
The work previously discussed, along with findings from other research, has resulted in a WateReuse Foundation-sponsored research investigation into the applicable disinfection standards for various commercially available disinfection technologies. Carollo is a key participant in this research project, along with the University of California, Davis, public health risk assessors, and several key reclamation utilities. This work is expected to redefine recycled water regulations as they relate to pathogen inactivation. As the work is in the preliminary stages, the exact impact to recycled water facilities is not clear. However, based upon the work cited here, it is clear that disinfection with combined chlorine (chloramines) is not meeting the virus disinfection standard set by CDHS. It is also clear that UV disinfection exceeds virus disinfection standards (based upon poliovirus) at the current required recycled water dose.
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