6.2 UV, Ozone, Chlorine Dioxide, Chloramines and Disinfection Byproduct Control

Matching the Disinfectant to the Job

Ultraviolet (UV) Disinfection

UV lamps emit light near 254 nm that damages microbial DNA so organisms cannot reproduce.

$$\text{UV dose (mJ/cm}^2\text{)} = \text{Intensity (mW/cm}^2\text{)} \times \text{Exposure time (s)}$$

Required doses come from the federal UV dose table that OHA uses (OAR 333-061-0036, Table 32):

Protozoa are easy to inactivate with UV, while viruses (especially adenovirus) need much higher doses. A UV-only groundwater system seeking 4-log virus credit needs a dose near 186 mJ/sq cm.

Oregon design and operating requirements (OAR 333-061-0050(5)(k)) include:

• reactors validated for flow, intensity, UV transmittance and lamp status;
• an upstream valve or device that keeps flow below the rated maximum;
• a UV intensity sensor with visual and audible alarms and a visual check that every lamp is lit;
• shut-off valves on the inlet and outlet, and no bypass piping; and
• lamp replacement on the manufacturer's schedule, with regular cleaning of sleeves and sensor windows.

Small groundwater units may need NSF/ANSI 55 Class A certification. Watch UV transmittance (UVT): iron, manganese, organic color and turbidity absorb UV and lower the delivered dose.

Ozone

Ozone (O3) is generated on site by passing dry air or oxygen through a corona discharge. It is dissolved in contact chambers through fine-bubble diffusers or side-stream injection. Leftover gas must pass through an off-gas destruct unit because ozone is a toxic respiratory hazard.

• Ozone oxidizes geosmin, MIB, iron, manganese and many organics, and inactivates Cryptosporidium with enough CT.
• It leaves no residual, so chlorine or chloramine is still needed for distribution.
• It oxidizes bromide to bromate (MCL 0.010 mg/L). It also breaks organics into biodegradable fragments, so ozone is often followed by biologically active filters to keep distribution water stable.

Chlorine Dioxide

Chlorine dioxide (ClO2) is generated on site, usually by reacting sodium chlorite with chlorine or acid. It does not form THMs or HAAs and works well for iron, manganese and taste-and-odor control.

• MRDL: 0.8 mg/L.
• Byproduct: chlorite (MCL 1.0 mg/L), with daily chlorite monitoring at the entrance to distribution. A ClO2 MRDL exceedance at the entry point can require Tier 1 public notice.
• Generators must be tuned so unreacted chlorite stays low.

Chloramines

Chloramination adds ammonia to chlorinated water, usually after primary disinfection with free chlorine.

• Target about 4.5 to 5 parts Cl2 to 1 part NH3-N by weight and pH above about 7.5 to favor monochloramine. Lower ratios leave free ammonia that feeds nitrifying bacteria; higher ratios move toward breakpoint and odorous dichloramine.
• Oregon's groundwater design standard calls for at least 2.0 mg/L combined chlorine after 3 hours of contact when chloramine is the primary disinfectant (OAR 333-061-0050(5)(d)).
• Customers need notice before conversion. Dialysis centers must remove chloramines, and aquarium and fish-pond owners must neutralize them.

Controlling Disinfection Byproducts

THMs and HAAs form when chlorine reacts with natural organic matter. Under Stage 2, compliance is a locational running annual average (LRAA) at each monitoring site (TTHM 0.080 mg/L, HAA5 0.060 mg/L), and operational evaluation levels warn of coming problems. Control tools, roughly in order:

1. Remove precursors. Conventional plants on surface water must practice enhanced coagulation under the Stage 1 rule. Required TOC removal depends on source TOC and alkalinity. For example, source TOC of 4-8 mg/L with alkalinity of 0-60 mg/L calls for 45 percent removal; TOC over 8 mg/L at that alkalinity calls for 50 percent.
2. Move the chlorination point downstream of clarification, so chlorine meets less organic carbon.
3. Lower dose and contact with precursors while still meeting CT.
4. Reduce water age through tank cycling, flushing and closing dead ends.
5. Switch the residual disinfectant to chloramines, or add GAC or tank aeration for THMs.