Small Public Water System Technology Guide - UV Disinfection Educational Module

Operational Strategy

Advantages

Disadvantages

Single Operation Setpoint

Simplest operational strategy

Not as energy efficient because the UV reactor is over-dosing at low flows

Variable

Setpoint Operation

Increased energy efficiency over the single setpoint approach

More complex operation compared to single setpoint approach and may necessitate more advanced controls for the UV reactor

Setpoint Interpolation

The most energy efficient operation and may reduce operational hours needed if operated automatically

Potentially more validation data is needed (which may increase validation costs) and necessitates advanced reactor controls

Along with the selection of an operational strategy, reactor control strategy must also be selected. A control strategy defines what parameter(s) are monitored in order to adjust UV intensity to achieve a particular operational strategy and/or determine if the UV reactor is operating within validation. Currently, there are three UV reactor control strategies: UV intensity setpoint, UVT and UV intensity setpoints, and calculated dose. These control strategies and their required monitoring parameters are listed in the table below. A control strategy setpoint is a limit established during validation for a monitored parameter, e.g., flowrate, UV intensity, or UVT, such that if the UV reactor operates outside these limits then it is operating outside of validated conditions. This is called operating off-specification and examples are shown in the table below. Off-specification operation is an indication of a UV reactor’s need for adjustment or maintenance, and state regulators may establish limits on off specification operation.

Control Strategy

Parameters Monitored

Off-Specification

Examples

UV intensity setpoint

UV intensity, flowrate, lamp status

Anytime these values are outside of the validated limits for these parameters

1)      UV intensity below setpoint

2)      Flowrate outside validated limits

3)      UV lamp failure

4)      UV intensity sensor failure

UVT and UV

intensity

setpoints

UV intensity,

flowrate, UVT,

lamp status

Anytime these values are

outside of the validated limits

for these parameters

1)      UV intensity below setpoint

2)      Flowrate outside validated limits

3)      UV lamp failure

4)      UV intensity sensor failure

5)      UVT below setpoint

Calculated

dose

Calculated

dose, flowrate,

UVT,lamp

status

Anytime the calculated dose is below the validated setpoint (if validation certifies that the calculated dose can be used to control the UV reactor )¹

1)      Calculated dose below setpoint

2)      Flowrate outside validated limits

3)      UV lamp failure

4)      UV intensity sensor failure

5)      UVT below setpoint

¹ If validation deems that the calculated dose control is not acceptable, the UV reactor should use the UVT and UV intensity setpoint control strategy

The three operational strategies and three different control strategies allow for eight different combinations of operation. There are eight rather than nine combinations because a UVT and UV intensity setpoint control strategy would not be used for a setpoint interpolation operational strategy since it only uses flowrate to adjust UV intensity and not UVT.

Depending upon the operational and control strategy, either a bench top spectrophotometer or more often an online UVT monitor may be used to monitor UVT. In the UV intensity setpoint control strategy, UVT does not need to be monitored.

An example of a UV intensity setpoint control strategy is where a UV reactor is validated at a UVT in the range of 85 to 97%, and with flowrates from 200 to 700 gallons per minute. If validation showed that a minimum UV intensity sensor reading of 5.0 mW/cm² was needed at all UVT’s from 85-97% in order to meet a required 2-log inactivation of Cryptosporidium, then the UV reactor could operate in the flow range from 200 to 700 gallons per minute and a UV intensity of at least 5.0 mW/cm² without requiring routine UVT monitoring. This approach would waste energy at low flowrates and the facility could opt to save energy by determining appropriate UV intensity setpoints for narrower ranges of flow and UVT and switch from a single setpoint operational strategy to a variable setpoint operational strategy. The UV intensity is adjusted simply by adjusting the lamp power.

In comparison with the UV intensity setpoint control strategy, measuring UVT along with UV intensity can provide higher energy efficiency. An example of a variable setpoint operational strategy using a UVT and UV intensity setpoints control is where the UV reactor is required to maintain a minimum UV intensity based on a lookup table of flowrate and UVT ranges.

In the calculated dose setpoint control strategy the calculated ultraviolet doses are determined for a variety of combined flowrates and UVT's. The doses are determined from UVT, UV intensity and flow measurements that create a matrix from which doses across a range of conditions are determined.

It is proposed in the LT2ESWTR that all UV reactors be monitored to show that they are operating within a range of conditions in which they have been validated to achieve their required UV dose. Also, for unfiltered water, it is proposed that the percentage of flow that was treated within validation limits be determined to receive inactivation credit. Recommended monitoring parameters and recording frequencies are given in the following table.

Parameter

Recommended

Recording Frequency

Notes

UV intensity

Every 4 hours

The UV intensity must be above the validated setpoint²

UVT¹

Every 4 hours

The UVT must be above the validated setpoint. If not required to be monitored, this information will assist in determining if low UV intensity readings are because of low UVT²

Calculated dose¹

Every 4 hours

The calculated dose must be above the validated setpoint²

Lamp status

Every 4 hours

The lamps should be energized if water is flowing through the UV reactor

Calibration of UV intensity sensors

Monthly

The UV intensity sensor calibration must be checked, using UVDGM sensor calibration check protocol²

¹ Only required if necessary for the control strategy

²proposed requirement of the LT2ESWTR and subject to change

Even though the minimum recording frequency for most parameters in the table above is every four hours, it is suggested it be continuous if possible.

It is suggested that all control strategies incorporate the recommend operational tasks at the scheduled frequency as seen in the following table.

Frequency

Recommended Tasks

Daily

·   Perform overall visual inspection of the all UV reactors. Ensure system control is on automatic mode (if applicable).

·   Check control panel display for status of system components and alarm status and history.

·   Ensure all on-line analyzers, flowmeters, and data recording equipment are operating normally.

·   Review 24-hour monitoring data to ensure that the reactor has been operating within validated limits during that period.

Weekly

·   Initiate manual operation of wipers (if provided) to ensure proper operation.

Monthly

·   Check lamp run time values. Consider changing lamps if operating hours exceed design life or UV intensity is low.

Semi-annually

·   Check ballast cooling fans for unusual noise. Check operation of automatic and manual valves.

The operation of a UV reactor ranges from manual to fully automatic. However, normal operation of a UV reactor, as described above, is most often automatically operated through a Programmable Logic Controller (PLC). A PLC controls the operation of the UV reactor in response to certain input signals. An example of first stage automation is when a PLC is used in the sequencing of lamp startup and valve actuation for bringing individual UV reactors online. An example of a fully automated system is where a PLC would control startup of UV reactors, activation of lamps, control of lamp intensity, and reactor shutdown.

A fully automated system can determine and perform an automatic reactor shutdown as indicated for certain alarms or notify the operator for other alarms. Reactor alarms can be categorized as minor, major, or critical. A minor alarm might only indicate scheduled maintenance. A major alarm would indicate validation and setpoint limits were exceeded and immediate attention would be required. A critical alarm would indicate a major malfunction of the operating system and initiate an automatic shutdown. Automatic reactor shutdown is necessary for all control strategies during critical alarm conditions. High temperature, reduction in flow, lamp or sleeve failure, would all indicate a critical alarm condition. Typical alarm conditions are given in the following table.

Alarm/Sensor

Purpose/Descriptions

Lamp Age

· Minor alarm occurs when run-time for lamp indicates end of defined operational lamp life.

Calibrate UV Intensity Sensor

· Minor alarm occurs when UV intensity sensor needs calibration based on operating time.

Differential Pressure Out of Range (When Differential Pressure is Used for Flow Split Confirmation)

· Necessary only if a single master flow meter is used. Minor alarm occurs if pressure drop across parallel, identical UV reactors indicates unequal flow split. Major alarm occurs if differential pressure across a given UV reactor indicates flow outside of the validated range.

Low UV Dose

· Major alarm occurs when dose condition falls below required dose

· Triggered by signals gathered by control system and compared to validated UV reactor dose requirements.

Low UV Intensity

· Major alarm occurs when intensity falls below design conditions.

Low UV Transmittance

· Major alarm occurs when UVT falls below design conditions.

High/Low Flow

· Major alarm occurs when flowrate falls outside of validated range

· Based on measurement from dedicated flow meters or calculated based on total flowrate divided by number of units operating.

Lamp/Ballast Failure

· Major alarm occurs when a single lamp/ballast failure is identified.

· Critical alarm occurs when multiple lamp/ballast failures are identified.

Low Liquid Level

· Critical alarm occurs when liquid level within the UV reactor drops and potential for overheating increases.

High Temperature

· Critical alarm occurs when the temperature within the UV reactor or ballast exceeds a setpoint.

Mechanical Wiper Function Failure

· Needed only if a mechanical wiper system is used. Critical alarm occurs if wiper function fails.

Note: Alarm conditions and relative severity shown above may vary dependent on specific conditions under which the UV reactor is validated, the type of UV reactor, the control strategy, and the disinfection objectives of the utility.

Operator interface to the PLC is accessed through a control panel providing diagnostic and operating condition information. Operational functions such as the adjustment of lamp intensity, startup, cool down, shutdown, manual override, safety interlocks, report generation, and operation and shutdown during power failure mode are directed through the control panel. Shutdown during power failure mode is accomplished with the assistance of a backup power supply.

Maintenance

The maintenance of UV reactors is necessary to ensure continual operation within desired operational, validation, and disinfection parameters. For example, if UVT monitors, UV intensity sensors, and flow meters are out of calibration, there may be a risk of pathogenic release (Cotton et al, 2003). General guidelines for recommended maintenance tasks and their frequency and procedure are listed in the following table.

Frequency

Task

Action

Weekly

Check on-line UVT monitor calibration

Calibrate UVT monitor when manufacturer’s guaranteed measurement uncertainty is exceeded.

Monthly

Check reactor housing, sleeves, and wiper seals for leaks

Replace housing, sleeve, or wiper seals if damaged or leaking.

Monthly

UV intensity sensor calibration check protocol

Check the sensor calibration at the lamp power utilized during

routine operating conditions (e.g., the majority of operation).

When UV

intensity

sensor fails

calibration

check

Replace duty sensor

with calibrated backup

sensor

·       Check the reference sensor with second reference sensor or two other duty sensors to ensure the first reference sensor is calibrated.

·       If reference sensor is properly calibrated, replace the duty sensor with calibrated sensor, and send the duty sensor that failed calibration to the manufacturer.

·       Check the replaced sensor one hour later.

Monthly

(OCC)

Semi-annually

(OMC)

Check cleaning

efficiency

·       Record UV intensity sensor reading.

·       Extract one sleeve per reactor (or bank of lamps for low pressure high output (LPHO) reactors) for inspection.

·       Check remaining sleeves if fouling is observed on the first sleeve.

·       Manually clean sleeve(s) if fouling is seen on the sleeves.

·       Record UV intensity sensor reading and compare to original reading after cleaning.

·       Replace sleeve if UV intensity is not restored to validated level.

Semi-annually (OMC)

Check cleaning fluid reservoir (if provided)

Replenish solution if the reservoir level is low. Drain and replace solution if the solution is discolored.

Annually

Calibrate reference sensor

Send the reference sensor to the manufacturer for calibration.

Annually

Test-trip GFI

Maintain ground fault interrupt (GFI) breakers in accordance with the manufacturer’s recommendations.

Manufacturer’s recommended frequency

Check flowmeter calibration

Calibrate flowmeter when manufacturer’s guaranteed measurement uncertainty is exceeded.

Lamp/

manufacturer

specific

Replace lamp

Replace lamps when any one of the following conditions occur:

·    Initiation of low UV intensity alarm (UV intensity equal to or less than set point value) after verifying that this condition is caused by low lamp output.

·    Initiation of lamp failure alarm.

When lamps are replaced

Properly dispose of lamps

Send spent lamps to a mercury recycling facility or back to the manufacturer.

Sleeve/ Manufacturer specific

Replace sleeve

Replace sleeve every 3 to 5 years or when damage, cracks, or excessive fouling significantly decreases UV intensity of an otherwise acceptable lamp to the minimum validated intensity level. The replacement frequency should be adjusted based on operational experience.

Pressure gauge

manufacturer

specific

Check operation of

the pressure gauges

that are used to

confirm flow split (if

applicable)

Replace the pressure gauge if deemed faulty by

manufacturer’s evaluation procedure.

Manufacturer specific

Clean UVT monitor

Clean according to manufacturer’s recommended procedure.

Manufacturer specific

Inspect OMC drive mechanism

Inspect and maintain OMC drive routinely as recommended by the manufacturer.

Manufacturer specific

Inspect ballast cooling fan

Check the ballast cooling fans for dust buildup and damage. Replace if necessary.

UV reactor performance is affected by lamp sleeve solarization, sleeve fouling, UV intensity sensor window fouling, and lamp aging. Lamp sleeves degrade over time due to solarization resulting in loss of UV transmittance; therefore, necessitating their replacement. Sleeve fouling is the formation of substances on the sleeve surface that reduce transmittance. In order to prevent sleeve fouling, either automatic sleeve wipers or flush and rinse systems are generally used. UV intensity sensor window fouling could create a condition of underestimating output, but is typically controlled by the cleaning system.

Lamp output decreases over time as a function of its age. The replacement of lamps can be determined on an individual basis if there is one sensor per lamp. However, in UV reactors where one sensor monitors multiple lamps, replacement is usually determined by the operational life of the lamp. Alternatively, if the oldest lamp has been placed next to the intensity sensor, as it should be, then the sensor readings could be used to help determine the need for lamp replacement. Additional consideration should be given to the replacement of certain components due to limits of their life expectancy. In the following table various component lives are shown. Replacement at the end of guaranteed life should be considered when forming a maintenance schedule.

Component

Design Life¹

Guaranteed Life ²

Low pressure lamps (LP and LPHO)

12,000 hours

8,000 - 12,000 hours

MP lamps

10,000 hours

4,000 - 8,000 hours

Sleeve

8 to 10 years

1 to 3 years

UV Intensity Sensor

3 to 10 years

1 year

UVT monitor

3 to 5 years

1 year

Cleaning systems

3 to 5 years

1 to 3 years

Ballasts

10 to 15 years

1 to 3 years

¹Expected duration of operation

² Accounts for variability of material quality, production, and operating conditions.

OPERATION & MAINTENANCE:

Operation

Control Strategies (USEPA, 2003)

Operational Strategy	Advantages	Disadvantages
Single Operation Setpoint	Simplest operational strategy	Not as energy efficient because the UV reactor is over-dosing at low flows
Variable Setpoint Operation	Increased energy efficiency over the single setpoint approach	More complex operation compared to single setpoint approach and may necessitate more advanced controls for the UV reactor
Setpoint Interpolation	The most energy efficient operation and may reduce operational hours needed if operated automatically	Potentially more validation data is needed (which may increase validation costs) and necessitates advanced reactor controls

Control Strategy	Parameters Monitored	Off-Specification	Examples
UV intensity setpoint	UV intensity, flowrate, lamp status	Anytime these values are outside of the validated limits for these parameters	1) UV intensity below setpoint 2) Flowrate outside validated limits 3) UV lamp failure 4) UV intensity sensor failure
UVT and UV intensity setpoints	UV intensity, flowrate, UVT, lamp status	Anytime these values are outside of the validated limits for these parameters	1) UV intensity below setpoint 2) Flowrate outside validated limits 3) UV lamp failure 4) UV intensity sensor failure 5) UVT below setpoint
Calculated dose	Calculated dose, flowrate, UVT,lamp status	Anytime the calculated dose is below the validated setpoint (if validation certifies that the calculated dose can be used to control the UV reactor )¹	1) Calculated dose below setpoint 2) Flowrate outside validated limits 3) UV lamp failure 4) UV intensity sensor failure 5) UVT below setpoint

Parameter	Recommended Recording Frequency	Notes
UV intensity	Every 4 hours	The UV intensity must be above the validated setpoint²
UVT¹	Every 4 hours	The UVT must be above the validated setpoint. If not required to be monitored, this information will assist in determining if low UV intensity readings are because of low UVT²
Calculated dose¹	Every 4 hours	The calculated dose must be above the validated setpoint²
Lamp status	Every 4 hours	The lamps should be energized if water is flowing through the UV reactor
Calibration of UV intensity sensors	Monthly	The UV intensity sensor calibration must be checked, using UVDGM sensor calibration check protocol²

Frequency	Recommended Tasks
Daily	· Perform overall visual inspection of the all UV reactors. Ensure system control is on automatic mode (if applicable). · Check control panel display for status of system components and alarm status and history. · Ensure all on-line analyzers, flowmeters, and data recording equipment are operating normally. · Review 24-hour monitoring data to ensure that the reactor has been operating within validated limits during that period.
Weekly	· Initiate manual operation of wipers (if provided) to ensure proper operation.
Monthly	· Check lamp run time values. Consider changing lamps if operating hours exceed design life or UV intensity is low.
Semi-annually	· Check ballast cooling fans for unusual noise. Check operation of automatic and manual valves.

Alarm/Sensor	Purpose/Descriptions
Lamp Age	· Minor alarm occurs when run-time for lamp indicates end of defined operational lamp life.
Calibrate UV Intensity Sensor	· Minor alarm occurs when UV intensity sensor needs calibration based on operating time.
Differential Pressure Out of Range (When Differential Pressure is Used for Flow Split Confirmation)	· Necessary only if a single master flow meter is used. Minor alarm occurs if pressure drop across parallel, identical UV reactors indicates unequal flow split. Major alarm occurs if differential pressure across a given UV reactor indicates flow outside of the validated range.
Low UV Dose	· Major alarm occurs when dose condition falls below required dose · Triggered by signals gathered by control system and compared to validated UV reactor dose requirements.
Low UV Intensity	· Major alarm occurs when intensity falls below design conditions.
Low UV Transmittance	· Major alarm occurs when UVT falls below design conditions.
High/Low Flow	· Major alarm occurs when flowrate falls outside of validated range · Based on measurement from dedicated flow meters or calculated based on total flowrate divided by number of units operating.
Lamp/Ballast Failure	· Major alarm occurs when a single lamp/ballast failure is identified. · Critical alarm occurs when multiple lamp/ballast failures are identified.
Low Liquid Level	· Critical alarm occurs when liquid level within the UV reactor drops and potential for overheating increases.
High Temperature	· Critical alarm occurs when the temperature within the UV reactor or ballast exceeds a setpoint.
Mechanical Wiper Function Failure	· Needed only if a mechanical wiper system is used. Critical alarm occurs if wiper function fails.

Frequency	Task	Action
Weekly	Check on-line UVT monitor calibration	Calibrate UVT monitor when manufacturer’s guaranteed measurement uncertainty is exceeded.
Monthly	Check reactor housing, sleeves, and wiper seals for leaks	Replace housing, sleeve, or wiper seals if damaged or leaking.
Monthly	UV intensity sensor calibration check protocol	Check the sensor calibration at the lamp power utilized during routine operating conditions (e.g., the majority of operation).
When UV intensity sensor fails calibration check	Replace duty sensor with calibrated backup sensor	· Check the reference sensor with second reference sensor or two other duty sensors to ensure the first reference sensor is calibrated. · If reference sensor is properly calibrated, replace the duty sensor with calibrated sensor, and send the duty sensor that failed calibration to the manufacturer. · Check the replaced sensor one hour later.
Monthly (OCC) Semi-annually (OMC)	Check cleaning efficiency	· Record UV intensity sensor reading. · Extract one sleeve per reactor (or bank of lamps for low pressure high output (LPHO) reactors) for inspection. · Check remaining sleeves if fouling is observed on the first sleeve. · Manually clean sleeve(s) if fouling is seen on the sleeves. · Record UV intensity sensor reading and compare to original reading after cleaning. · Replace sleeve if UV intensity is not restored to validated level.
Semi-annually (OMC)	Check cleaning fluid reservoir (if provided)	Replenish solution if the reservoir level is low. Drain and replace solution if the solution is discolored.
Annually	Calibrate reference sensor	Send the reference sensor to the manufacturer for calibration.
Annually	Test-trip GFI	Maintain ground fault interrupt (GFI) breakers in accordance with the manufacturer’s recommendations.
Manufacturer’s recommended frequency	Check flowmeter calibration		Calibrate flowmeter when manufacturer’s guaranteed measurement uncertainty is exceeded.
Lamp/ manufacturer specific	Replace lamp		Replace lamps when any one of the following conditions occur: · Initiation of low UV intensity alarm (UV intensity equal to or less than set point value) after verifying that this condition is caused by low lamp output. · Initiation of lamp failure alarm.
When lamps are replaced	Properly dispose of lamps		Send spent lamps to a mercury recycling facility or back to the manufacturer.
Sleeve/ Manufacturer specific	Replace sleeve		Replace sleeve every 3 to 5 years or when damage, cracks, or excessive fouling significantly decreases UV intensity of an otherwise acceptable lamp to the minimum validated intensity level. The replacement frequency should be adjusted based on operational experience.
Pressure gauge manufacturer specific	Check operation of the pressure gauges that are used to confirm flow split (if applicable)		Replace the pressure gauge if deemed faulty by manufacturer’s evaluation procedure.
Manufacturer specific	Clean UVT monitor		Clean according to manufacturer’s recommended procedure.
Manufacturer specific	Inspect OMC drive mechanism		Inspect and maintain OMC drive routinely as recommended by the manufacturer.
Manufacturer specific	Inspect ballast cooling fan		Check the ballast cooling fans for dust buildup and damage. Replace if necessary.

Component	Design Life¹	Guaranteed Life ²
Low pressure lamps (LP and LPHO)	12,000 hours	8,000 - 12,000 hours
MP lamps	10,000 hours	4,000 - 8,000 hours
Sleeve	8 to 10 years	1 to 3 years
UV Intensity Sensor	3 to 10 years	1 year
UVT monitor	3 to 5 years	1 year
Cleaning systems	3 to 5 years	1 to 3 years
Ballasts	10 to 15 years	1 to 3 years