Cortez UV Disinfection

1
Cortez UV Disinfection

• Steven M. Ravel, P.E.
• Travis E. Meyer, P.E.
• Richard P. Arber Associates

2
Introduction

• Background
• Selection of UV Lamp Type
• Specifying UV Dose
• Verification of UV Dose

3
Background

• New WWTP for Cortez Sanitation District
• Treatment Train
• Headworks
• Extended Aeration
• Secondary Clarification
• UV Disinfection

4
Background

• Design Flows
• Average 1.8 mgd
• Peak Month 2.1 mgd
• Peak Hour 7 mgd

5
UV Lamp Types Considered

• Medium Pressure
• Low Pressure / Low Intensity
• Low Pressure / High Intensity

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Medium Pressure

• Advantages
• Lowest Number of Lamps Required
• UV Intensity Adjustable
• Sleeve Wipers Available
• Low Space Requirement
• Open Channel and Closed Pipe Systems Available

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Medium Pressure

• Disadvantages
• Low Efficiency (Approximately 10 - 20
• Higher Capital Cost (for these flow rates)
• Require warm-up period on start-up

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Low Pressure / Low Intensity

• Advantages
• Low Capital Cost
• High Efficiency (Approximately 40)
• Simple Design / Simple Operation

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Low Pressure / Low Intensity

• Disadvantages
• Highest Number of Lamps Required
• UV Intensity Not Adjustable
• Sleeve Wipers Not Available
• High Maintenance (Lamp Replacement and Cleaning)
• High Space Requirement

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Low Pressure / High Intensity(Selected)

• Advantages
• Low Capital Cost
• High Efficiency (Approximately 40)
• Less Lamps Required than Low/Low (Approximately
  1/4)
• UV Intensity Adjustable
• Sleeve Wipers Available

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Low Pressure / High Intensity(Selected)

• Disadvantages
• Relatively Short Track Record Compared to Low/Low
  and Medium
• More Lamps Required than Medium

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Specifying UV Dose

• EPA UVDIS Computer Model
• BioAssay
• Performance Based

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UVDIS Computer Model

• Advantages
• Computer program easy and fast to run
• Theoretical model developed by EPA

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UVDIS Computer Model

• Disadvantages
• Program designed for Low/Low systems
• May not be as applicable for Low/High or Medium
  Systems
• Model results must be adjusted using high safety
  factors to ensure system will perform as required

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BioAssay

• Advantages
• System sized based on dose response curve for
  actual equipment to be installed
• Dose response curve developed for site specific
  wastewater effluent quality

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BioAssay

• Disadvantages
• No Standard Protocol for BioAssay
• Detailed protocol must be developed to compare
  results for different systems
• Expensive and Time-Consuming
• If MS-2 Phage used for bioassay, correlation must
  be developed to translate results for fecal
  coliform

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Performance Based

• Advantages
• Specify what you want the system to do, let
  manufacturers design their system to do it
• Most direct method to specify

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Performance Based

• Disadvantages
• Relying on manufacturers to design adequate
  system
• Must determine a method to check performance in
  the field

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Cortez Sanitation District

• Pre-selection of UV System based on
• Primary UV Dose Criteria Performance for Fecal
  Coliform
• In 500,000/100 mL
• Out 200/100 mL
• Secondary UV Dose Criteria
• Min UV Dose of 30,000 µw/cm2 based on UVDIS

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Cortez Sanitation District

• Other Design Criteria
• Minimum Flow 1 mgd
• Average Flow 1.8 mgd
• Peak Hydraulic Flow 7 mgd
• Peak Process Flow 5 mgd
• UV Transmittance 65
• TSS 30 mg/l
• Number of Channels 2
• Number of Banks/Channel 2

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Comparison of UV Systems
Mfr A Mfr B of Lamps 48 64 Capital
Cost 100,000 110,000 Annual OM
Costs Electricity 1,400 3,000 Lamps 1,900 3,
200 Total 3,300 6,200 NPV (20 yrs,
6) 140,000 180,000 Includes Separate
Sunshade Structure for Ballast and Controls
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Verifying UV Dose

• During Performance Test, Need to Account for
  Differences (Design vs. Actual) in
• Flow
• UV Transmittance
• Lamp Age (70 output)
• Fecal Coliforms

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Flow Adjustment

• Adjust UV system output in proportion to
  difference in flow
• Example Design flow 5 MGD, Actual flow 1
  mgdAdjust UV system output to 20 of maximum

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Transmissivity Adjustment

• Adjust UV system output based on correlation
  between design UVT and actual UVT

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Fecal Coliform Adjustment

• Two Options
• Spike UV influent to bring fecal coliform up to
  design number
• Lower effluent fecal coliform requirement to
  achieve same log reduction as design

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Cortez Sanitation District

• Turn down UV system output to min during
  performance test.
• Schedule test for highest flow, lowest UVT,
  highest Fecal
• Record Flow Rate
• Test Fecal Coliforms (In vs.Out)
• Test UVT
• Make determination of performance based on
  available data

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Conclusions

• Low Pressure / High Intensity best solution for
  Cortez
• Performance based spec on fecal coliform with min
  UV dose based on EPA UVDIS
• Verification of UV Dose using best available data
  at time of performance test