8.5 Wastewater Disinfection: Chlorination, Dechlorination & UV
Key Takeaways
- Disinfection is named in 23 CCR 3701(c)(1) as Grade I wastewater exam content and therefore appears at every grade.
- Chlorine residual toxic to aquatic life means most California NPDES permits require dechlorination, typically with sulfur dioxide, sodium bisulfite, or sodium metabisulfite.
- Sulfur dioxide requires about 1.0 mg/L per mg/L of chlorine residual; sodium bisulfite about 1.5 mg/L and sodium metabisulfite about 1.4 mg/L per mg/L of residual.
- UV disinfection has no residual and no byproducts, but its performance collapses with high total suspended solids, low UV transmittance, and quartz sleeve fouling.
- Title 22 disinfected tertiary recycled water disinfected with chlorine requires a CT of at least 450 mg-min/L with a modal contact time of at least 90 minutes.
Regulatory Drivers
Disinfection appears in the Grade I statutory blueprint (23 CCR 3701(c)(1)) and again in the Grade II list, so it is tested on every wastewater exam. What the plant must achieve depends on where the effluent goes:
| Effluent destination | Typical requirement |
|---|---|
| Ocean outfall under the Ocean Plan | Bacterial standards at the edge of the zone of initial dilution; often 30-day geometric mean limits |
| Inland surface water (NPDES) | Fecal coliform or E. coli limits, plus a total residual chlorine limit that is often non-detect |
| Title 22 disinfected secondary-23 recycled water | 7-day median total coliform ≤ 23 MPN/100 mL, no sample above 240 MPN/100 mL |
| Title 22 disinfected secondary-2.2 recycled water | 7-day median total coliform ≤ 2.2 MPN/100 mL |
| Title 22 disinfected tertiary recycled water | Filtered, then disinfected to a 7-day median total coliform ≤ 2.2 MPN/100 mL, ≤ 23 MPN/100 mL in no more than one sample in any 30-day period, never above 240 MPN/100 mL; with chlorine, CT ≥ 450 mg-min/L at a modal contact time of at least 90 minutes |
[!IMPORTANT] The Title 22 tertiary chlorine criterion is a CT and a modal contact time, both of which must be met. A plant can hit 450 mg-min/L with a high dose and a short basin and still fail, because the 90-minute modal contact time is a separate requirement. UV is an approved alternative to the chlorine CT pathway for tertiary recycled water, validated against a required dose.
Chlorine in Wastewater
Wastewater chlorine chemistry differs from drinking water in one decisive way: wastewater contains ammonia. Secondary effluent from a non-nitrifying plant typically carries 15 to 30 mg/L of ammonia-nitrogen, so added chlorine immediately forms chloramines rather than free chlorine.
Practical consequences:
- A non-nitrified effluent is disinfected by combined chlorine, which is weaker, so doses run higher (5 to 20 mg/L) and contact times longer.
- A fully nitrified effluent has little ammonia, so the same dose produces free chlorine - a much stronger disinfectant. Operators who nitrify seasonally must retune the chlorine dose or they will badly overdose in summer.
- Nitrite exerts a very heavy chlorine demand (about 5 mg Cl₂ per mg NO₂-N). A partially nitrified effluent that is stalled at nitrite is the worst case: high demand, poor disinfection, unstable residual.
- Total residual chlorine (TRC) is what a permit limits, and it includes both free and combined forms.
Contact Basin Design
Chlorine contact basins are serpentine to force plug flow. The same baffling factor logic used in drinking water applies:
Worked example. A contact basin holds 260,000 gallons, flow is 3.2 MGD (2,222 gpm), superior baffling gives a factor of 0.7, and the chlorine residual at the outlet is 4.5 mg/L. Theoretical detention = 260,000 / 2,222 = 117 minutes T10 = 117 x 0.7 = 81.9 minutes CT = 4.5 x 81.9 = 369 mg-min/L - short of the 450 mg-min/L needed for Title 22 tertiary, and the 81.9-minute modal time is also below 90 minutes. This plant would need a higher residual and more contact volume or better baffling.
Basins accumulate solids at the turns; sludge deposits reduce effective volume, short-circuit flow, and exert chlorine demand, so periodic draining and cleaning is a real compliance activity, not housekeeping.
Dechlorination
Chlorine residual is acutely toxic to fish and invertebrates at concentrations far below what is needed for disinfection, so most California NPDES permits require dechlorination to non-detect before discharge.
| Dechlorinating agent | Approximate dose per mg/L of chlorine residual | Notes |
|---|---|---|
| Sulfur dioxide (SO₂ gas) | 1.0 mg/L | Cheapest at scale; same handling hazards as chlorine gas; requires the same containment, scrubber, and SCBA program |
| Sodium bisulfite (NaHSO₃) | ~1.5 mg/L | Liquid, safer to handle, common at small and mid-size plants |
| Sodium metabisulfite (Na₂S₂O₅) | ~1.4 mg/L | Dry or solution |
| Sodium sulfite (Na₂SO₃) | ~1.8 mg/L | Less common |
| Sodium thiosulfate | ~1.5-2.0 mg/L | Common for grab-sample quenching and small applications |
Three control issues dominate:
- Overdose consumes dissolved oxygen. Excess sulfite scavenges DO in the effluent and in the receiving water. A permit with both a TRC limit and a minimum DO limit forces a narrow operating window.
- Underdose is a permit violation the moment the analyzer sees residual. Because the reaction is essentially instantaneous, control is by feed-forward from the chlorine residual analyzer with feedback trim from a post-dechlorination analyzer.
- Analyzer maintenance is the whole game. A drifting amperometric analyzer produces either a violation or a DO sag. Calibrate on schedule and verify against a bench DPD or amperometric titration.
Worked example. Effluent flow is 4.0 MGD carrying a 3.8 mg/L total chlorine residual, dechlorinated with sodium bisulfite at a 1.5:1 ratio. Chlorine to be neutralized = 4.0 x 3.8 x 8.34 = 126.8 lb/day Bisulfite required = 126.8 x 1.5 = 190 lb/day of bisulfite as 100 percent; divide by the solution strength and density to get gallons per day.
Ultraviolet Disinfection
UV inactivates organisms by dimerizing thymine bases in DNA and RNA at germicidal wavelengths near 254 nm. It has become the default for California recycled water and for many NPDES plants because it produces no residual to dechlorinate and no chlorinated byproducts.
| Factor | Effect on performance |
|---|---|
| Total suspended solids | Particles shield organisms from UV. This is the single biggest failure mode. Tertiary filtration ahead of UV is essential for recycled water. |
| UV transmittance (UVT) at 254 nm | The fraction of light passing through 1 cm of water. Secondary effluent commonly runs 55-65 percent; filtered tertiary 65-75 percent. Falling UVT means falling delivered dose. |
| Lamp age | Output declines over lamp life; systems apply an end-of-lamp-life factor. |
| Quartz sleeve fouling | Iron, manganese, hardness, and organic film on the sleeve block light. Mechanical wipers plus chemical cleaning are required maintenance. |
| Hydraulics | Short-circuiting and poor approach conditions reduce actual exposure. |
| Photoreactivation and dark repair | Some organisms repair UV damage; validated dose requirements account for this. |
Validation matters in California: recycled water UV systems must be validated (NWRI/AwwaRF guidelines or equivalent) and operated within the validated envelope of flow, UVT, and lamp status. Operating outside that envelope is a compliance failure even if the coliform results look acceptable.
| Chlorine + dechlorination | Ultraviolet | |
|---|---|---|
| Residual | Yes, must be removed | None |
| Byproducts | THMs, chlorinated organics, NDMA precursor reactions | None |
| Effective against Cryptosporidium | Poor | Excellent |
| Sensitive to TSS | Moderately | Severely |
| Sensitive to ammonia | Yes (forms chloramines) | No |
| Safety hazard | Toxic gas or corrosive liquids | Electrical, UV exposure, mercury in lamps |
| Operating cost driver | Chemical | Electricity and lamp replacement |
[!WARNING] A broken UV lamp releases mercury into the channel and into the effluent. Have a written lamp-breakage response: isolate the bank, stop flow through the affected channel if possible, notify the supervisor, follow the mercury spill procedure, and evaluate whether an effluent notification is required under the plant's waste discharge requirements.
A wastewater plant that normally does not nitrify begins fully nitrifying in warm weather. What happens to chlorine disinfection if the dose is left unchanged?
An effluent flow of 5.0 MGD carries a total chlorine residual of 3.0 mg/L and is dechlorinated with sulfur dioxide at a 1.0 to 1 weight ratio. What is the required sulfur dioxide feed rate?
A tertiary recycled water plant reports rising coliform in the UV-disinfected effluent while lamp output and flow are unchanged. What should the operator investigate first?