Disinfection and dechlorination

# Disinfection and dechlorination

> Disinfection is the last thing you do to wastewater before it leaves the plant. Get the dose and the timing wrong and you either pass pathogens through or poison the stream with chlorine instead.

Why the exam tests this

Most days you run the same chlorinator or the same UV bank at the same setting. The math behind it goes stale. The exam expects you to work the dose and contact-time math cold, not read it off a panel.

It also tests chlorine gas safety directly, because a leak can kill an operator in minutes. Expect questions that mix process math, like dose, feed rate, and contact time, with safety judgment: what to do first in a leak, why you never enter alone.

Treatment Process is the single heaviest duty on the exam, worth 38 percent. Disinfection is one of seven process topics inside it, so expect it to come up more than once.

Class I and II exams stay mostly at recall and basic application. By Class III and IV you troubleshoot a disinfection system that is missing its target, not just calculate a single dose.

What you need to know

Why you still need to disinfect

Secondary treatment already knocks out 95 to 98 percent of the microbes in the wastewater. That still leaves enough pathogens that most permits require disinfection before you can discharge.

Four things decide how well disinfection works: the disinfectant, the dose or intensity, and the contact time. Turbidity, color, and solids in the water interfere with all three, and temperature and the organisms' own tolerance matter too.

Disinfection is not optional if the plant discharges to a stream. Surface-water discharge is the most common path, and it needs an NPDES permit requiring at least secondary treatment plus disinfection. Reuse and land application carry their own treatment minimums, but every route back to the environment needs this step first.

Chlorination

Chlorination is the most common method because it is cheap and works at a low dose. You will see it in four forms: chlorine gas in cylinders, sodium hypochlorite (liquid bleach), calcium hypochlorite (HTH, a solid), and chlorine dioxide.

Typical dose for activated-sludge effluent runs 2 to 8 mg/L, aimed at holding a residual around 0.5 mg/L once demand is met. The contact tank has to give a minimum of 30 minutes of detention at the design annual average daily flow, not whatever flow happens to be running that hour.

Whatever method you use, the federal minimum secondary standard also holds effluent pH between 6.0 and 9.0 at all times. Any chemical you add for disinfection or dechlorination has to leave the effluent inside that band too.

Dechlorination

Chlorination must be followed by dechlorination before the water reaches the receiving stream. Residual chlorine is toxic to aquatic life, and it reacts with organic matter in the water to form disinfection byproducts that are carcinogenic.

The most common dechlorination method is injecting sulfur dioxide. Smaller systems often use sodium metabisulfite or sodium bisulfate instead, because they are easier and safer to handle than running a second gas feed.

This is why a permit that requires disinfection almost always requires dechlorination too. A plant can hit its chlorine residual perfectly and still fail on aquatic toxicity if dechlorination is skipped.

UV and ozone

Ultraviolet light disinfects by damaging the microbes' genetic material so they cannot reproduce. It leaves no chemical residual and forms no disinfection byproducts, and it does not need the 30-minute contact tank that chlorine does. A UV system is sized for the plant's maximum flow instead, and the bulbs need regular cleaning and replacement.

Ozone is a strong oxidant generated on-site. It works well but costs more to build and run, which is why small systems rarely choose it over chlorine or UV.

At higher grades, expect to weigh these three methods against each other on cost, byproduct risk, and reliability, not just describe how each one works.

Chlorine gas safety

Chlorine gas is the most dangerous chemical at most wastewater plants. It is greenish-yellow and heavier than air, so a leak sinks to the floor and to low ground outside instead of dispersing. Exposure causes burns, pulmonary edema, and can kill.

Cylinders must be secured and labeled, and the building and transport route posted. Handling a leak means wearing a self-contained breathing apparatus with a properly fitted mask and working in pairs, with a partner standing by outside on SCBA. If a leak happens, keep your head high, since the gas sinks low, and leave fast.

Worked examples

Chlorine feed rate. A plant discharges 0.5 MGD and needs a chlorine dose of 6 mg/L to hit its target residual. Feed rate in pounds per day is dose (mg/L) times flow (MGD) times 8.34. That is 6 x 0.5 x 8.34, or about 25 lbs of chlorine per day. Rework this feed-rate math yourself with the calculator at /tools/practice so the setup, not just the answer, sticks.

Contact time check. A contact tank holds 15,000 gallons and the plant runs at a design average flow of 0.6 MGD, or about 417 gallons per minute. Detention time is tank volume divided by flow: 15,000 gallons divided by 417 gpm comes out to about 36 minutes. That clears the 30-minute minimum, with a few minutes of margin if flow creeps up.

Reading a low residual. An operator doses at 3 mg/L and measures only 0.1 mg/L residual after contact, well under the 0.5 mg/L target. The dose is already inside the normal 2 to 8 mg/L range, so the fix is not more chlorine on faith. It is checking why demand jumped: a slug of ammonia or organic load, or high turbidity carrying extra solids into the contact tank, both of which eat up chlorine before it can leave a residual. On the exam, rank the likely causes before you reach for more chlorine.

Common traps

• Mixing up the effluent disinfection dose (2 to 8 mg/L, aimed at a 0.5 mg/L residual) with the much smaller chlorine dose used elsewhere in the plant to control filamentous bulking on the return sludge. They solve different problems and use different reference units.
• Treating "meets residual" as the finish line and skipping dechlorination. If you used chlorine, dechlorination is required before discharge, not optional.
• Assuming UV needs the same 30-minute contact tank as chlorine. It does not; UV is sized to the plant's maximum flow instead.
• Forgetting that turbidity, color, and solids in the wastewater cut into disinfection effectiveness, even when the dose and contact time look correct on paper.
• Working a leak alone. Chlorine gas response calls for a partner standing by on SCBA, not a single operator improvising.
• Reading the federal secondary standard (BOD5 and TSS at 30 mg/L, pH 6.0 to 9.0) as the disinfection limit. It sets the baseline the whole secondary process has to meet; your disinfection limit is usually a separate residual or bacteria number layered on top.

Practice

Filter the wwt-1 practice test to disinfection and dechlorination at /tools/practice. Work the chlorine feed-rate and contact-time problems until the 8.34 factor and the 30-minute check are automatic, then run a few of the safety and method-comparison questions cold.

If you miss a math item, redo it by hand before you move on, since the exam rewards operators who can rebuild the number, not just recognize the answer. Class III and IV candidates should also drill the method-comparison questions, chlorine versus UV versus ozone, since those show up more as the exam shifts from recall to analysis.

Quick reference

• Secondary treatment removes 95 to 98 percent of microbes; disinfection handles what is left before discharge.
• Chlorination: cheap, low dose, comes as gas, liquid hypochlorite, HTH solid, or chlorine dioxide.
• Typical effluent chlorine dose: 2 to 8 mg/L, targeting about a 0.5 mg/L residual.
• Chlorine contact tank: minimum 30 minutes detention at design annual average daily flow.
• Chlorination must always be followed by dechlorination before discharge.
• Common dechlorination chemical: sulfur dioxide (gas injection); sodium metabisulfite or sodium bisulfate for smaller systems.
• UV: no residual, no byproducts, no 30-minute contact tank; sized for maximum flow instead.
• Ozone: strong on-site oxidant, effective, expensive, uncommon at small plants.
• Chlorine gas is heavier than air and sinks; leak response needs SCBA, a partner standing by, and a fast exit with your head high.