6.5 Wastewater Disinfection & Dechlorination: Chlorine, UV and Ozone

6.5 Wastewater Disinfection & Dechlorination: Chlorine, UV and Ozone

Secondary treatment removes most organic matter but leaves large numbers of bacteria and viruses. Disinfection reduces pathogens before discharge so that the receiving stream can be used for swimming, fishing or water supply. Every WPI Wastewater Treatment outline lists chlorination (chlorine, hypochlorite, chloramine), dechlorination (sodium bisulfite, sodium thiosulfate, sulfur dioxide) and ultraviolet (UV) equipment and processes; the Class 4 outline adds ozonation.

Permit Targets

ADEM NPDES permits set bacterial limits, commonly E. coli in fresh waters (enterococci are used for coastal waters), as monthly geometric means and single-sample maximums based on the stream's use classification. Permits for chlorinating plants also set total residual chlorine (TRC) limits, often very low because chlorine is toxic to aquatic life; EPA's aquatic-life criteria for TRC are 0.011 mg/L (chronic) and 0.019 mg/L (acute). That is why most chlorinating plants also dechlorinate.

Chlorination of Wastewater

Feed and mixing. Chlorine gas or sodium hypochlorite is added at a point of rapid mixing, then the water flows through a baffled, plug-flow contact tank. Ten States Standards call for at least 15 minutes of contact at design peak hourly flow.

Residual chemistry is different from drinking water. Wastewater usually contains ammonia, so chlorine forms combined chlorine (chloramines) rather than free chlorine. Wastewater plants therefore control on total chlorine residual measured by DPD or amperometric titration. In a fully nitrified effluent with little ammonia, free chlorine can form, which disinfects faster but also reacts more with organics.

Nitrite demand. Partial nitrification, common during start-up or cold weather, leaves nitrite, which consumes chlorine rapidly:

$$\text{NO}_2^- + \text{HOCl} \rightarrow \text{NO}_3^- + \text{HCl}$$

Each mg/L of nitrite-nitrogen consumes about 5 mg/L of chlorine ($70.9 \div 14.0 \approx 5.1$). A sudden jump in chlorine demand with an unchanged flow often points to nitrite.

Control. Most plants pace chlorine feed to flow and trim it with a residual analyzer (compound-loop control). Operators verify analyzers against manual tests and check contact tanks for short-circuiting, solids accumulation and algae.

Contact time check. A 50,000-gallon contact tank at a peak hourly flow of 3.0 MGD has a theoretical detention time of:

$$\frac{50{,}000\text{ gal}}{3{,}000{,}000\text{ gal/day} \div 1{,}440\text{ min/day}} = \frac{50{,}000}{2{,}083} = 24\text{ minutes}$$

That exceeds the 15-minute guideline, although baffling and short-circuiting reduce the effective time.

Dechlorination

Dechlorination chemicals react almost instantly with chlorine residual. The theoretical amounts needed per mg/L of chlorine are:

Field doses are usually a little above theoretical. Overdosing wastes chemical, depresses dissolved oxygen and can lower pH, so feed is paced to the chlorine residual with a small excess.

Example. A 2.0 MGD effluent carries 1.2 mg/L total chlorine. Sodium bisulfite needed is $1.2 \times 1.46 = 1.75$ mg/L, or $2.0 \times 1.75 \times 8.34 = 29.2$ lb/day of 100 percent sodium bisulfite. For a 38 percent solution with a specific gravity of 1.36, the solution holds $1.36 \times 8.34 \times 0.38 = 4.31$ lb per gallon, so the feed is about $29.2 \div 4.31 = 6.8$ gallons per day.

Ultraviolet Disinfection

UV light at about 254 nm damages microbial DNA so organisms cannot reproduce. UV adds no chemical, forms no regulated byproducts and needs no dechlorination, but it leaves no residual.

• Dose: $\text{Dose (mJ/cm}^2) = \text{Intensity (mW/cm}^2) \times \text{Exposure time (s)}$.
• UV transmittance (UVT) is the percent of UV light that passes through 1 cm of water. Secondary effluent commonly ranges from about 55 to 75 percent; lower UVT means less dose.
• TSS and particle size matter because bacteria inside particles are shielded.
• Lamp types: low-pressure, low-pressure high-output (amalgam) and medium-pressure lamps. Output declines with age, so lamps are replaced on the manufacturer's schedule.
• Sleeve fouling by iron, calcium, grease and biofilm lowers intensity. Automatic wipers and periodic chemical cleaning keep quartz sleeves clear.
• Keep the water level over the lamps constant and avoid exposing lamps to air. Monitor intensity sensors, UVT, flow pacing and lamp-failure alarms.
• Never look at an operating lamp; UV burns eyes and skin.

Ozone

Ozone is generated on site from air or oxygen by corona discharge and dissolved through diffusers or injectors. It is a powerful disinfectant and oxidant, decays quickly and leaves no lasting residual, and is toxic. Contactor off-gas passes through an ozone destruct unit, and ambient monitors protect staff; OSHA's permissible exposure limit for ozone is 0.1 ppm as an 8-hour average.