14.3 Ultraviolet Disinfection Systems (Subclass 13)

The Validated Envelope

The central regulatory idea in ultraviolet disinfection is validation. A reactor does not receive credit because a calculation says it should; it receives credit because that specific reactor model was tested at a validation facility using challenge microorganisms, producing a validated range of flow rate, ultraviolet transmittance and sensor reading within which a stated dose is delivered.

The operating consequence is that the reactor must run inside that envelope. Water treated at a flow above the validated maximum, or at a transmittance below the validated minimum, is off specification even if the reactor is energized and the display looks normal. Off-specification water is tracked and reported, and a system that exceeds its allowance has a treatment technique problem.

Lamp Technologies

All of these are mercury vapor lamps, which is what makes a lamp break a hazardous materials event.

Controlling Delivered Dose

Delivered dose, sometimes called fluence, depends on lamp output, water transmittance and residence time. Three measured inputs drive control:

1. Ultraviolet intensity sensors mounted in the reactor wall, which must be the validated sensor type and must be checked against a reference sensor on a defined schedule.
2. Ultraviolet transmittance (UVT), measured by an online analyzer or grab sample, expressed as the percent of light passing through one centimeter of water. Typical treated surface water runs 85 to 95 percent; iron, manganese, natural organic matter and turbidity all drive it down.
3. Flow rate, since residence time falls as flow rises.

The two accepted control approaches are the calculated dose approach, which combines sensor reading, transmittance and flow through the validated algorithm, and the ultraviolet intensity setpoint approach, which simply requires the sensor to stay above a fixed value for a given flow. The setpoint approach is simpler and is common at smaller systems; it is also more conservative, since it does not credit high transmittance.

Fouling and Cleaning

Quartz sleeves separate the lamps from the water, and anything deposited on a sleeve blocks light before it ever reaches an organism. Fouling comes from calcium and magnesium hardness precipitating on the warm sleeve surface, and from iron and manganese oxides.

• Mechanical wipers travel along the sleeves on a timed cycle and are the first line of defense.
• Chemical-mechanical wipers carry a mild acid in the wiper collar for hard-to-remove deposits.
• Offline chemical cleaning with citric or phosphoric acid is scheduled when sensor readings decline despite wiping.
• The diagnostic distinction matters: falling sensor reading with normal transmittance points to sleeve fouling or lamp aging, while falling sensor reading with falling transmittance points to water quality.

Lamp Aging, Ballasts and Power

Lamp output declines with operating hours and with the number of on-off cycles, and the validation includes an end-of-lamp-life factor. Lamps are replaced on hours rather than on failure, and rated life varies widely by technology, commonly around 5,000 hours for medium pressure lamps and 12,000 hours or more for low pressure and low pressure high output lamps. Ballasts regulate lamp current; a failed ballast takes its lamp or bank offline and shows as an alarm and a drop in sensor reading. Because a power interruption extinguishes the lamps and they need several minutes to restrike and stabilize, reactors are configured to close an outlet valve or divert until dose is re-established, and the plant needs a defined restart sequence.

Safety and the Mercury Break Response

• Never look at an energized lamp. Ultraviolet light causes painful photokeratitis and skin burns; use the view port and interlocks provided.
• Lamp breakage inside the reactor releases mercury into the water. The prescribed response is to remove the unit from service, isolate and drain it to waste rather than to the distribution system, recover the debris, decontaminate the reactor, and notify DEP consistent with the system's approved plan.
• Handle spent lamps as universal or hazardous waste for mercury recycling; they never go in the trash.

Why Chlorine Is Still Fed

Ultraviolet light inactivates Cryptosporidium and Giardia at very low doses and bacteria readily, but adenovirus is unusually resistant, requiring a very high dose, and ultraviolet light provides no residual. Systems therefore continue to feed a chemical disinfectant downstream for distribution protection and for any required virus inactivation credit. A UV installation reduces the chlorine CT burden; it does not eliminate chemical disinfection.