UV Disinfection Limits and…

UV Disinfection Limits and the Need for Redundancy

The proposal includes UV disinfection for pathogen removal, which is a well established and standard technology widely used in Ontario. The proposed effluent limit of 200 CFU/100 mL is consistent with many ECAs; however, the expert review noted that several comparable facilities in Ontario operate under more stringent limits, typically in the range of 100–150 CFU/100 mL. Based on the expert’s experience, an objective of 100 CFU/100 mL would be more consistent with similar ECAs, although the final limit should reflect the sensitivity of the receiving watercourse.

The expert also noted that well operated SBR + filtration + UV systems routinely achieve far better performance than the proposed limit. For example:

• Havelock WWTP and Teeswater Formosa WWTP, both using similar Napier Reid SBR systems with chemical precipitation and filtration, regularly achieve <10 CFU/100 mL.
These examples demonstrate that significantly lower pathogen levels are achievable with this type of treatment process.

However, the proposal includes only a single UV disinfection train sized for maximum flow. This creates a single point of failure. If the UV unit requires maintenance or experiences an equipment issue, the plant would have no ability to disinfect effluent, which could result in non compliance and increased pathogen loading to the receiving river.

To improve reliability and ensure continuous disinfection, the expert recommended considering two parallel UV units. This would allow one unit to remain in operation while the other is offline for maintenance or repair.

Given the sensitivity of the receiving watercourse and the importance of reliable pathogen control, I respectfully request that MECP require the proponent to:

1. Re evaluate the proposed effluent E. coli objective and limit, considering that similar facilities routinely achieve <10 CFU/100 mL.

2. Assess whether a lower objective (e.g., 100 CFU/100 mL) is more appropriate for this site.

3. Provide redundancy in the UV system, such as two parallel UV trains, to ensure continuous disinfection during maintenance or equipment downtime.

These steps are necessary to ensure the WWTP can reliably meet pathogen removal requirements and protect the receiving watercourse under all operating conditions.