Selecting a UV Disinfection System Technology for Metro Vancouver's Coquitlam UV Disinfection Project

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Metro Vancouver (MV) is moving forward with the design of a new UV Disinfection Facility for the Coquitlam water supply to meet Health Canada's new requirements for 3-log Cryptosporidium inactivation. To select a UV system that will provide high disinfection performance cost effectively, medium-pressure (MP) and low-pressure high output (LPHO) UV systems were evaluated based on the following criteria: life-cycle cost (LCC), disinfection performance, head loss, and fouling risk.

Three UV equipment manufacturers submitted technical and cost information for two MP and two LPHO UV systems in response to a technical request for information (T-RFI). UV equipment manufacturer A (MFG-A) proposed a LPHO UV system design that is based on T1 phage reduced equivalent dose (RED) and utilizes an off-line chemical cleaning system. UV equipment manufacturer B (MFG-B) proposed a MP UV system design that is based on MS2 phage RED and a LPHO UV system design based on T1 RED phage and utilizes an on-line mechanical cleaning system. UV equipment manufacturer C (MFG-C) proposed a MP UV system design based on T1 RED.

Life-cycle costs (LCC) of the proposed UV systems were calculated based on UV equipment and operation and maintenance (O&M) costs for 10- and 12-unit train configurations. The results of LCC indicated that an MP UV system designed based on T1 RED had the lowest net present value (NPV), which was 40 percent lower than an MP UV system designed based on MS2 RED design. The impact of more frequent off-line chemical cleaning of an LPHO UV system (proposed by MFG-A) on O&M cost was significant. The NPV of LPHO UV system was 40 percent higher than an MP UV system based on T1 RED due to higher chemical cleaning and disposal costs. Although an LPHO UV system with on-line cleaning system had low O&M costs, its NPV was 90 percent higher than an MP UV system based on T1 RED due to high equipment cost.

Disinfection performance of the proposed UV systems was evaluated using a UV Cost Analysis Tool (UVCAT) computer model. UVCAT analysis included predictions of the number of operating unit-trains, power consumption, dose-pacing efficiency, and Cryptosporidium log inactivation as a function of design flow and UV transmittance (UVT). UVCAT results confirmed that all proposed UV systems could meet UV dose delivery requirement for 3-log Cryptosporidium inactivation for the full range of design flow and UVT and with a 10-unit train configuration. UVCAT results also confirmed expectations that LPHO UV systems have lower power consumption than MP UV systems. In addition, UVCAT model simulation indicated that LPHO UV systems proposed by MFG-A and B, which have large number of low wattage lamps, have more efficient power turndown and they can provide more efficient dose-pacing compared to an MP UV system proposed by MFG-C, which has smaller number of higher wattage lamps.

Head loss across proposed UV reactors was evaluated at maximum design flow of 1,200 ML/day. A UV reactor with a low head loss would help maintain the hydraulic grade line of the Coquitlam UV system above the minimum desired operating level at the maximum flow of 1,200 ML/day. An MP UV system proposed by MFG-C had the lowest head loss across its reactor at 429 mm while an LPHO UV system proposed MFG-A had the highest head loss at 803 mm.

Fouling risk of lamp sleeve, which has significant impact on O&M cost of the UV system associated with frequent chemical cleaning and disposal costs, was evaluated for the Coquitlam UV system by performing pilot-scale tests on LPHO and MP reactors. Pilot test results, which will be discussed in a future paper, indicated significant fouling of LPHO UV lamp sleeves. The annual O&M cost of an LPHO UV system proposed by MFG-A was approximately $315,000 higher than that of MP UV systems and LPHO UV system with on-line mechanical cleaning system.

Based on the findings of the UV systems evaluation, MV will allow both MP and LPHO (with on-line cleaning system) in the UV equipment procurement documents, which will include a base bid restricted to MP UV systems only and an alternate bid allowing LPHO UV systems (with on-line cleaning system). This approach will allow MV to receive “hard” capital and lifecycle costs for both types of UV systems and get the latest information on LPHO UV systems with on-line cleaning system. In addition, UV equipment procurement documents will allow offsite validation of UV reactors using T1 phage as the validation test organism. This should result in significant capital and O&M cost savings to MV while maintaining public health protection.

Keywords: Cryptosporidium; LPHO; MP; UV disinfection; UV transmittance; sleeve fouling; validation

Document Type: Research Article


Publication date: January 1, 2009

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