3.10 Wastewater Disinfection & Dechlorination Systems
Key Takeaways
- Wastewater chlorine demand is far higher than drinking water demand because effluent contains ammonia, nitrite, sulfide, and organic nitrogen, so most effluent chlorination produces combined rather than free residual.
- Chlorine contact chambers are designed for at least 15 to 30 minutes of contact at peak flow, and serpentine baffling is used to raise the baffling factor toward plug flow.
- Ultraviolet dose equals intensity multiplied by exposure time and is degraded by low ultraviolet transmittance, lamp aging, quartz sleeve fouling, and particle shielding.
- Sulfur dioxide requires approximately 1.0 pound per pound of chlorine residual removed, sodium bisulfite approximately 1.46 pounds, and sodium metabisulfite approximately 1.34 pounds.
- Overdosing a sulfite dechlorination agent consumes dissolved oxygen in the effluent and can cause a dissolved oxygen permit violation immediately downstream of the outfall.
Wastewater Disinfection & Dechlorination Systems
The ABC Wastewater outline lists disinfection equipment and disinfection treatment processes separately, each covering chlorination, dechlorination, ultraviolet, and ozonation. Virginia VPDES permits commonly require seasonal or year-round disinfection with an E. coli limit, and almost every chlorinating plant in the Commonwealth also dechlorinates.
1. Why Wastewater Chlorination Is Different
Drinking water chlorination usually produces a free chlorine residual. Effluent chlorination usually does not, because secondary effluent contains ammonia, nitrite, organic nitrogen, and sulfide - all of which react with chlorine.
- Ammonia converts free chlorine to chloramines almost instantly. At a non-nitrifying plant with 20 mg/L ammonia, reaching breakpoint would require roughly 10 times the ammonia concentration in chlorine - about 200 mg/L - which is neither affordable nor permittable. So non-nitrifying plants disinfect with combined chlorine, accepting slower kill and longer contact time.
- A nitrifying plant behaves oppositely. With ammonia near zero, chlorine goes straight to free residual, kill is much faster, and the same dose that was barely adequate before nitrification started can now overshoot and produce a total residual chlorine violation.
- Nitrite is a strong chlorine consumer - roughly 5 mg of chlorine per mg of nitrite-N. A partially nitrifying plant that accumulates nitrite sees chlorine demand jump without any change in flow or ammonia.
Chlorine dose = demand + desired residual, and effluent demand typically runs 5 to 15 mg/L. Typical applied doses are 5 to 20 mg/L at non-nitrifying plants and 2 to 6 mg/L at nitrifying plants.
2. Contact Chambers
Disinfection is a function of concentration multiplied by time, so the chamber matters as much as the dose.
- Design contact time is typically 15 to 30 minutes at peak hourly flow, measured as actual detention.
- Baffling converts a mixed tank toward plug flow. A serpentine, over-and-under, or around-the-end chamber achieves a baffling factor of 0.7 or better, while an unbaffled rectangular tank may achieve only 0.3.
- Effective contact time = theoretical detention x baffling factor. A 60,000-gallon chamber at 1.5 MGD has a theoretical detention of 60,000 / (1,500,000 / 1,440) = 57.6 minutes; at a baffling factor of 0.7 the effective time is 40 minutes.
- Solids accumulation destroys contact time. Chambers must be drained and cleaned on a schedule; a chamber a third full of settled solids has lost a third of its detention and is also generating a chlorine demand from the decomposing sludge.
- Rapid initial mixing at the injection point matters more than most operators expect, because chlorine's reactions are fast relative to the mixing time of a poorly designed inlet.
3. Ultraviolet Disinfection
UV inactivates organisms by dimerizing thymine in their DNA, preventing replication. It leaves no residual, forms no halogenated byproducts, and requires no dechlorination - which is why it has become the default for new Virginia plants.
UV dose (mJ/cm2) = Intensity (mW/cm2) x Exposure time (s)
Typical design doses for secondary effluent are 30 mJ/cm2 for coliform limits and higher where the permit is tight or where reuse standards apply.
The four variables that degrade delivered dose
| Variable | Effect | Control |
|---|---|---|
| UV transmittance (UVT) at 254 nm | The single largest driver. A drop from 65 percent to 55 percent can cut delivered dose by a third | Monitor UVT continuously; investigate upstream color and dissolved organics; industrial dye discharges are a classic cause |
| Lamp aging | Output declines to roughly 70 to 85 percent of new by end of rated life (typically 9,000 to 15,000 hours) | Track lamp hours; replace on schedule; the ballast tells you nothing about output |
| Quartz sleeve fouling | Scale (calcium, iron, manganese) and biofilm block light | Mechanical wipers, chemical-mechanical wipers, or periodic acid cleaning |
| Particle shielding (TSS) | Organisms embedded in particles are physically shaded | Upstream filtration; UV performance is tightly coupled to effluent TSS |
Photoreactivation and dark repair. Some bacteria repair UV damage, and coliforms exposed to sunlight can partially reactivate. This is why UV systems are sized with a safety factor and why a sample taken well downstream may show higher counts than one taken at the channel.
Safety. UV-C causes photokeratitis ("welder's flash") and skin burns. Never look at an energized lamp, never operate with a channel cover removed, and use interlocks. Lamps contain mercury; breakage requires a spill procedure and lamps must be recycled as universal waste.
4. Ozone
Ozone is generated on site from air or oxygen by corona discharge and is the most powerful of the common disinfectants.
- Very effective against bacteria, viruses, and protozoa, and it simultaneously oxidizes color, odor, and many micropollutants.
- No halogenated byproducts, but ozone forms bromate from bromide.
- No residual, so no dechlorination and no downstream protection.
- Energy intensive, and off-gas must be destroyed catalytically or thermally before venting.
- Ambient ozone monitoring is required in the generator room; the OSHA permissible exposure limit is 0.1 ppm as an 8-hour TWA.
5. Dechlorination
Virginia VPDES permits generally require total residual chlorine to be reduced to the permit's quantification level, because chlorine is acutely toxic to aquatic life at concentrations well below 0.1 mg/L.
Reagents and stoichiometry
| Reagent | Form | lb per lb Cl2 removed |
|---|---|---|
| Sulfur dioxide (SO2) | Compressed gas, fed through a sulfonator identical in principle to a chlorinator | 1.0 (often applied at 1.0 to 1.1) |
| Sodium bisulfite (NaHSO3) | Liquid solution | 1.46 |
| Sodium metabisulfite (Na2S2O5) | Solid or solution | 1.34 |
| Sodium sulfite (Na2SO3) | Solid | 1.78 |
| Sodium thiosulfate | Solution | ~0.56 to 1.0 depending on reaction path; used mainly for sample dechlorination |
The dechlorination reaction is essentially instantaneous - complete within seconds given good mixing - so a dechlorination contact chamber is not required, but a rapid mix at the injection point is.
Worked example
A plant discharges 3.0 MGD with a total residual chlorine of 1.8 mg/L and dechlorinates with sodium bisulfite.
- Chlorine to remove = 3.0 x 1.8 x 8.34 = 45.0 lb/day
- Bisulfite required = 45.0 x 1.46 = 65.7 lb/day of pure NaHSO3
- If the product is a 38 percent solution weighing 10.7 lb/gal: 65.7 / (10.7 x 0.38) = 16.2 gallons per day
The overdose problem
Sulfite reagents are reducing agents. Once the chlorine is gone, excess sulfite consumes dissolved oxygen:
Approximately 1.0 mg/L of SO2 (or its bisulfite equivalent) consumes about 1.0 mg/L of dissolved oxygen once chlorine demand is satisfied.
A plant that overdoses to guarantee zero chlorine can create a dissolved oxygen violation at the outfall instead. The correct control scheme is a flow-paced feed trimmed by a total residual chlorine analyzer on the dechlorinated effluent, with an alarm on excessive feed and a downstream DO check. Some permits contain both a TRC limit and a minimum DO limit precisely because the two are coupled.
Analytical caution
Total residual chlorine at permit levels is measured near the detection floor of the DPD method. Amperometric titration or a low-range analyzer is normally required, sample handling must avoid agitation and sunlight, and the analysis must be performed immediately - chlorine residual carries a 15-minute holding time.
6. Choosing Between the Technologies
| Chlorine + dechlor | UV | Ozone | |
|---|---|---|---|
| Capital cost | Low | Moderate | High |
| Operating cost | Moderate | Moderate (lamps, power) | High (power) |
| Byproducts | THMs, chlorinated organics | None | Bromate |
| Residual toxicity | Requires dechlorination | None | None |
| Sensitivity to TSS | Moderate | High | Moderate |
| Sensitivity to ammonia | High | None | Low |
| Safety hazard | Gas release, chemical handling | UV exposure, mercury | Ozone gas exposure |
| Effective against protozoa | Poor (chloramines) | Excellent | Excellent |
The decision usually turns on two site facts: whether the plant nitrifies (which transforms chlorine chemistry) and how clean the effluent is (which controls UV feasibility).
A wastewater plant discharges 2.5 MGD with a total residual chlorine of 2.2 mg/L and dechlorinates with sulfur dioxide at a ratio of 1.0 pound of sulfur dioxide per pound of chlorine removed. How much sulfur dioxide is required per day?
A plant begins nitrifying fully in late spring, and shortly afterward its effluent total residual chlorine results rise sharply even though the chlorine feed setting was never changed. What is the chemical explanation?
An ultraviolet disinfection system that has met its E. coli limit for two years begins failing, although lamp hours are low and effluent TSS is unchanged. Continuous monitoring shows ultraviolet transmittance has fallen from 66 percent to 53 percent. What does this indicate and where should the operator look?