Wastewater Disinfection: Chlorine vs. UV

By the time wastewater reaches the end of the plant, you've removed the solids and the BOD and (maybe) the nutrients. One job is left: kill the pathogens before the effluent goes back to the receiving water. That's disinfection, and your permit usually measures it as a fecal coliform or E. coli limit — an indicator that says, in effect, "show me the bugs that signal sewage are gone."

Two technologies dominate: chlorination (the old workhorse) and ultraviolet light (UV) (the increasingly popular alternative). They get to the same place by completely different routes, and the trade-offs between them drive a lot of plant decisions. Here's how each one actually works and what it takes to run it.

Key takeaways

• Chlorination adds a chemical that kills microbes; you control it by holding a residual (commonly ≥1.0 mg/L) for enough contact time (≥30 minutes at design flow).
• In wastewater, chlorine reacts with the ammonia that's present to form chloramines — a weaker "combined" residual that still disinfects with enough contact time.
• Chlorine residual is toxic to fish and aquatic life, so almost every chlorinating plant must dechlorinate (sulfur dioxide or sodium bisulfite) to near zero before discharge.
• UV uses light to scramble microbes' DNA so they can't reproduce — no chemical, no residual, no dechlorination, no disinfection byproducts — but the water has to be clear for the light to reach the bugs.
• UV is better against chlorine-resistant protozoa (Giardia, Cryptosporidium); chlorine leaves a protective residual that UV can't.
• Drill it with the wastewater disinfection practice test and pair this with how a wastewater treatment plant works.

Chlorination: the chemical route

Add chlorine to water and it forms hypochlorous acid (HOCl) and hypochlorite ion (OCl⁻). HOCl is the stronger disinfectant of the two, and the balance between them shifts with pH — more of the potent HOCl at lower pH. You can deliver the chlorine as gas (Cl₂), sodium hypochlorite (liquid bleach, usually 12.5%), or calcium hypochlorite (tablets/granules). They all end up as the same active chlorine in water.

Here's the wrinkle that makes wastewater different from drinking water: secondary effluent still carries ammonia, and chlorine reacts with ammonia to form chloramines. In drinking water you'd push past that "breakpoint" to get free chlorine; in wastewater you often rely on the combined (chloramine) residual to do the disinfecting. It's weaker than free chlorine, so it leans harder on contact time to finish the job. (For the underlying chemistry, see breakpoint chlorination explained.)

Two things drive performance, and they're the two things you control:

• Dose and residual. You feed enough chlorine to satisfy the water's demand and still leave a measurable residual. A common control target is at least ~1.0 mg/L of total residual chlorine carried through the contact tank — but the real test is whether you're meeting your coliform limit, so dose to the bug count, not just the residual number.
• Contact time. Disinfection is concentration × time. The chlorine contact chamber is sized to give the water enough minutes with the chlorine — commonly 30 minutes or more at design (peak) flow. Just as important is baffling: a long, snaking, plug-flow path so water actually spends its full detention time in the tank instead of short-circuiting straight to the outlet. Poor baffling is a classic reason a plant "doses enough" but still fails coliform.

The downsides are real: chlorine handling carries safety hazards (gas chlorine especially), it can form chlorinated disinfection byproducts, and — the big one — the residual itself is toxic to aquatic life.

Dechlorination: the step you can't skip

Because residual chlorine kills fish and invertebrates, you can't just send chlorinated effluent to the river. You have to remove the residual first — dechlorination — to protect the receiving water. (This surprises new operators: you spend money adding chlorine, then spend more money taking it back out.)

The common reagents are sulfur dioxide (SO₂) gas or, increasingly, sodium bisulfite / sodium metabisulfite (liquid sulfite salts). They react almost instantly with chlorine and neutralize it. Rough dosing: about 0.9 mg of SO₂ per 1 mg of total chlorine residual to be removed, applied with good mixing and roughly a minute of contact. Many plants have switched from SO₂ gas to sodium bisulfite purely for safety and ease of handling — it avoids keeping another toxic gas on site.

The operator's job here is to drive the residual to essentially zero at the discharge without overshooting (excess sulfite has its own oxygen demand). It's a balancing act between two chemical feeds — enough chlorine to disinfect, then just enough sulfite to erase the residual.

UV: the light route

UV disinfection skips chemistry entirely. Low-pressure (or medium-pressure) lamps bathe the flowing effluent in ultraviolet light around 254 nm, which penetrates the microbes and damages their DNA/RNA so they can't reproduce. A bug that can't replicate can't infect — it's effectively dead.

UV "dose" is intensity × exposure time (measured in mJ/cm²); secondary-effluent systems are often designed around the low tens of mJ/cm², with much higher doses for water reuse. But the dose that matters is the dose that actually reaches the organisms, and that's where UV's one big requirement comes in:

• The water has to be clear. Suspended solids and turbidity both absorb UV and physically shield bacteria hiding inside particles. UV systems live and die by UV transmittance (UVT) — a measure of how much light gets through the water. If your upstream TSS climbs, your effective UV dose drops even if the lamps are blazing. Good UV performance starts with good clarifier and filter performance.
• Lamps and sleeves foul and age. The quartz sleeves around the lamps build up a film (minerals, grease) that blocks light, so systems need regular cleaning — mechanical wipers or chemical cleaning — and lamps lose output as they age and must be replaced on schedule.
• There's no residual. The instant the water leaves the UV channel, the protection is over. UV disinfects what passes through and nothing after.

The payoffs are why so many plants are converting: no toxic residual (so no dechlorination), no chlorinated byproducts, no chemical storage/handling hazards, and notably better inactivation of chlorine-resistant protozoa like Giardia and Cryptosporidium — a real advantage for plants headed toward water reuse.

Chlorine vs. UV: how to think about the trade-off

Neither is "better" in the abstract — they fit different situations:

The short version: chlorine is cheaper to install and gives you a residual, but you're running two chemical systems (disinfect, then dechlorinate) with the safety and byproduct baggage that comes with them. UV trades the chemicals away for clean simplicity at discharge, but demands clear water and diligent lamp maintenance. The industry trend has been toward UV, largely to escape the chlorine-residual toxicity and chemical handling — but a plant with high or variable TSS may find chlorine more forgiving.

What operators watch and troubleshoot

Whichever system you run, the permit is the scoreboard: fecal coliform / E. coli (and, on chlorinating plants, total residual chlorine at the discharge, which usually has to be near zero).

On a chlorination/dechlorination plant: - Hold your disinfection residual and contact time; if coliform fails, suspect low dose, high chlorine demand (a slug of ammonia or BOD), or short-circuiting in a poorly baffled contact tank. - Confirm the dechlorination is complete — a toxic chlorine residual sneaking to the river is a serious permit violation; if residual breaks through, check the sulfite feed and mixing. - Watch the interplay: more ammonia means more chlorine demand and more chloramine, which means you may need more dose or contact time.

On a UV plant: - Watch UV intensity sensors and UVT — falling transmittance (rising upstream TSS) is the usual reason for a coliform exceedance, so disinfection problems often trace back to the clarifiers or filters. - Keep the sleeves clean and lamps within their service life; fouled sleeves quietly starve the dose. - Watch flow — overload shortens exposure time and drops the delivered dose.

In both cases, the most common lesson is the same: disinfection failures usually start upstream. Clean, low-solids effluent is easy to disinfect; dirty effluent defeats either technology.

Practice it

Disinfection shows up on every wastewater exam — expect questions on chlorine contact time and residual, why dechlorination is required, chloramines, and the pros and cons of UV. Drill them with the wastewater disinfection practice test, and pair this with how a wastewater treatment plant works, breakpoint chlorination explained, and CT calculations. For coliform sampling, see wastewater sampling and lab. More on the wastewater hub.

This guide is a free study aid for wastewater operators and reflects general disinfection practice. Permit limits, target residuals, doses, and contact times vary by facility and state — always follow your plant's NPDES permit and process-control program, and confirm specifics with your supervisor. Reviewed June 2026.

Sources

• Disinfection study guide (Wisconsin DNR)
• Chlorination & dechlorination (Mountain Empire CC)
• Chlorine vs. UV for Giardia & Cryptosporidium (NIH/PMC)