A List of Considerations For UV Disinfection

Recently, have started working on a project that investigates adding UV disinfection to the existing process within a Water Treatment Plant (WTP). Since this is a drinking water application project, am looking at closed vessel chambers with the UV lamps either parallel or perpendicular to the flow in a horizontal or vertical orientation with presence of additional baffles and mixers (which increases head loss).

Courtesy USEPA UV Disinfection Manual

This article will be mostly a list of things that an engineer must keep in mind while designing a UV disinfection system. I accumulated them from various sources over a period of time and thought of putting them here.

In a survey done among various stakeholders (utility, manufacturer, academia) in USA, the main drivers for employing UV technology were the following:

1. Reduced overall time or complexity of operation
2. Reduced chemical handling
3. Straightforward automation
4. Simple maintenance

UV treatment brings three complex disciplines together:

1. Hydraulics
2. Photosciences
3. Microbiology

The six fundamental elements of UV disinfection equipment are:

1. UV lamp (Low Pressure, Low Pressure High Output and Medium Pressure)
2. Ballast (electromagnetic and electronic)
3. UV reactor design (an hydraulically optimized reactor means less power consumption and a smaller footprint)
4. Cleaning device (chemical using citric / phosphoric acid and mechanical using wipers)
5. Control system (specialised control and PLC)
6. Type of validation system used for certification (offsite and onsite)

From Journal AWWA (2005) ‘Essential Criteria For Selecting An UV Disinfection System’, 97(7)

When it comes to feed water quality, manufacturers typically will ask for the following parameters:

Feed Water Parameters That Affect UV Disinfection Equipment Design

In the AWWA Water Treatment Plant Design, the following factors have been mentioned that a design engineer must account for:

1. Hydraulic requirements
2. Upstream and downstream piping
3. Reactor isolation
4. Flow splitting and / or control
5. Flow measurement
6. Reactors / lamps staying full of water
7. Sample and drain lines
8. Cooling water system (mostly for medium pressure UV lamps)
9. Air relief
10. Space and access for reactor and ancillary equipment maintenance
11. Facilities for management of off spec water

The governing parameters of UV disinfection design are:

1. Required dose
2. UV transmissivity (UVT)
3. Flow
4. Level of redundancy

On the operation side, it is recommended that the following be monitored:

1. Flow rates
2. Lamp ON / OFF status
3. Lamp ON / OFF cycle
4. UV intensity
5. UVT (if part of the control strategy)
6. Calculated UV dose (if part of the control strategy)
7. UV sensor calibration (weekly basis)
8. UVT analyser calibration (monthly basis)
9. Lamp age
10. Off specification events

Some of the common mistakes that are made in UV disinfection design are:

1. Sizing on average UVT and flow (the correct design approach is considering lowest expected UVT and peak flow)
2. Ignoring particle shielding (good filtration system required upstream)
3. Extrapolating kinetics past the tail (rate constant used for 2 LRV will over predict system performance at 4 LRV)
4. Omitting UV lamp’s ageing and fouling factor (typically 0.7 or 0.8 used for each factor; has a say on number of lamps and the footprint)
5. Skipping validation (leads to unknown UV dose, short circuiting)

Whenever a UV disinfection system is designed, it will operate over within an envelope of operating conditions. The validation testing is a means to verify the claims made by the OEM on the UV reactor performance (with a focus on UV dose and monitoring). Typically, a validation protocol will include:

1. A set of documents of the UV system components supplied by vendor
2. Measurement of the UV dose delivery using a bioassay testing system
3. Correlation of the bioassay-measured UV performance with monitoring devices
4. Assessment of the uncertainty and bias in the data analysis

A validation takes place over a range of water quality UV transmittance values, flow rates, and UV lamp intensities. To account for uncertainties, there is a validation factor that will be used (similar to how safety factor is used for determining target SRT in biological wastewater treatment).

From Page 35 of the USEPA UV Disinfection Manual

During offline validation testing (that occurs in Germany, Austria, Oregon or New York), the following parameters need to be accounted for:

1. Lamp aging
2. Lamp sleeve fouling
3. UVT
4. Inlet and outlet piping or channel configurations of the UV reactor
5. UV dose (fluence) distributions resulting from the velocity profile of the water passing through the UV reactor
6. Failure of UV lamps or other critical system components
7. Measurement uncertainty of on-line sensors (both intensity and UVT)

Honestly, the terms such as required dose, validated dose, calculated dose, Reduction Equivalent Dose (RED) associated with UV design confused me. But I found the explanation on page 58 and 59 of the USEPA report here. I highly recommend reading these two pages so that the terminologies or the steps involved in validating the UV disinfection system become understandable.

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At the project site, the UV dose delivered should be either equal to or better than during validation and that can happen by paying attention to the upstream and downstream piping configuration as it affects the flow characteristics through the UV reactor. AWWA recommends the following measures:

1. A minimum straight pipe run length equal to the length of straight-run pipe used during validation testing, plus an additional five pipe diameters
2. Identical inlet and outlet conditions at the treatment plant and validation testing facility for at least 10 upstream pipe diameters and 5 downstream pipe diameters
3. Velocity profile measurements at the installation site and validation facility such that velocity is shown to be within 20% of the theoretical velocity at both locations

When it comes to health and safety, the following issues need to be factored in while designing UV system for the client:

1. UV light exposure
2. Electrical safety
3. Burn safety
4. Lamp breakage issue owing to cuts, abrasion and mercury exposure

Monitoring the performance of UV disinfection system involves the following:

1. UV intensity setpoint — In this approach, measurements made by the UV intensity sensor are used to control the UV reactor. The sensor is located in a position that allows it to properly respond to both changes in UV output of the lamps and UVT of the water. The sensor output and the flow rate are used to monitor the dose delivery. The setpoint values to be used for the UV intensity are determined during validation testing done by the manufacturer (over a range of flow rates).
2. UV intensity and UVT setpoint — This approach is similar to the above but the only difference is that the UV sensor is placed close to the lamp, so that it only responds to changes in UV lamp output and UVT is monitored separately (with an analyser). For a specific flow rate, the UV intensity and UVT measurements are used to monitor the
   dose/delivery. The setpoints to be used for the UV intensity and UVT over a range of flowrates are determined during validation testing.
3. UV dose — Similar to UV intensity and UVT setpoint approach, the UV
   intensity sensor is placed close to the lamp. The flow rate, UVT, and UV intensity are monitored, and their output values are used to calculate the UV dose via a validated computational algorithm (during validation testing) developed by the manufacturer.

The reliability and minimizing UV disinfection system downtime measures include the following:

1. Redundant equipment
2. Standby power with automatic transfer switch
3. Active series compensators
4. Uninterruptible Power Supply (UPS)

In a WTP, electrical loads attributed to the UV equipment are on the higher side. Power quality events can lead to UV lamps shutting off, which translates to off specification operation. The mitigation options are:

1. If power quality events are expected but infrequent, a generator may be sufficient
2. If the power quality events are expected to be more frequent and the client has off specification requirements in place, then UPS (online and offline type) systems provide continuous power if there are voltage sag events or power interruptions
3. Though active series compensators are not a substitute for handling power quality problems, they do help boost voltage supplied to the UV equipment during sags and interrupted conditions

UV LED has emerged as an alternative to the current offering in the UV market and offers the following advantages:

1. Absence of mercury
2. Unlimited ON / OFF cycle
3. Expanded dose modulation capability
4. Potentially less O&M

However, lack of UV LED specific validation standards, high costs, low wall plug efficiency (5–10% for UV LEDs vs 35–40% for LPHO UV lamps) and lack of full scale installations are proving to be a hurdle for the LED variant but am sure the economics and the efficiency will improve over time.