12.3 Ultraviolet & Ozone Disinfection of Wastewater

12.3 Ultraviolet & Ozone Disinfection of Wastewater

Ultraviolet disinfection has displaced chlorine at many Arizona wastewater plants for one decisive reason: it leaves no residual, so no dechlorination is required and there is no toxic discharge to receiving water.

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How UV Works

Ultraviolet light near 254 nanometers is absorbed by nucleic acids, forming thymine dimers that prevent the organism from replicating. The organism is not killed in the chemical sense — it is rendered unable to reproduce, which is what matters for infection.

[!NOTE] The contrast with chlorine is a favorite exam point. Cryptosporidium is extraordinarily resistant to chlorine but readily inactivated by UV. This complementarity is why many advanced treatment trains use both.

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UV Dose

$\text{Dose (mJ/cm}^2\text{)} = \text{Intensity (mW/cm}^2\text{)} \times \text{Exposure Time (s)}$

Typical wastewater design doses run from about 30 mJ/cm² for secondary effluent coliform limits up to 80 to 100 mJ/cm² or more for reclaimed water and virus credit.

Everything that reduces intensity or exposure time reduces dose:

Photoreactivation and Dark Repair

Some bacteria possess enzymes that can repair UV-induced DNA damage. Photoreactivation occurs when the organism is subsequently exposed to visible light in the 310 to 490 nm range; dark repair occurs without light. Both are more likely after a marginal dose. Design doses therefore include a margin above the minimum inactivation threshold, and this is a genuine reason not to operate a UV system at the edge of its rating.

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UV Equipment and Maintenance

Lamp types:

• Low pressure, low intensity: nearly monochromatic at 254 nm; the most energy efficient; requires many lamps.
• Low pressure, high output (LPHO): higher output per lamp; fewer lamps; the common modern choice.
• Medium pressure: polychromatic, very high output, far fewer lamps, but much higher energy use.

Configurations are open channel (modules of lamps in a gravity channel with a level control weir) and closed vessel (pressurized pipe reactor).

[!WARNING] Never look at an energized UV lamp without eye protection. UV-C exposure causes photokeratitis, an intensely painful corneal burn that typically appears hours later, and it causes skin burns. Lock out the system before entering a channel, and use UV-blocking face protection and gloves for any inspection. Lamps also contain mercury, so breakage requires spill cleanup and the lamps require proper disposal.

Validation establishes what dose a specific reactor actually delivers at a given flow, UVT, and lamp condition. It is done by bioassay with a challenge organism, and it is the basis for the credit a regulator will grant. Operating outside the validated range voids the credit.

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Ozone

Ozone (O₃) is the most powerful disinfectant and oxidant in common water and wastewater use.

Generation

Ozone is unstable and must be generated on site, by passing dry oxygen or dry air through a corona discharge — a high-voltage field that splits O₂ into atomic oxygen, which recombines with O₂ to form O₃.

• Feed gas must be very dry. Moisture in the feed gas produces nitric acid in air-fed systems, which corrodes the generator, and it sharply reduces ozone yield. Air preparation with desiccant dryers is essential.
• Liquid oxygen (LOX) feed produces higher ozone concentrations than air feed and is common at larger facilities.
• Generation is energy intensive, and the dielectric elements require cooling.

Contacting and Off-Gas

Ozone is transferred into water in fine-bubble diffuser contactors, turbine mixers, or side-stream injectors with venturis. Because transfer is never complete, an off-gas ozone destruct unit — thermal, catalytic, or both — is mandatory before venting.

What Ozone Does Well

Limitations

• No lasting residual — ozone decomposes within minutes, so a secondary disinfectant is required for distribution systems.
• Bromate formation. Where bromide is present in the source water, ozone oxidizes it to bromate, a regulated carcinogen with a maximum contaminant level of 0.010 mg/L. This is the principal constraint on ozone use and is managed by pH depression, ammonia addition, or limiting the ozone dose.
• Assimilable organic carbon increases as ozone breaks large organics into smaller biodegradable fragments, which can promote biological regrowth downstream. Biologically active filtration after ozonation is the standard answer.
• Ozone is toxic. It is detectable by odor at very low concentrations, and continuous ambient monitoring with alarms is required in the generator and contactor areas. Materials must be ozone-resistant — stainless steel, PTFE, and specific elastomers; ordinary rubber and many plastics fail rapidly.