12.2 Wastewater Disinfection and Dechlorination

Why Disinfection Limits Are Strict in Oregon

Wastewater effluent carries fecal bacteria, viruses and protozoa. Oregon's bacteria standard for freshwater contact recreation (OAR 340-041-0009) is a 90-day geometric mean of 126 E. coli per 100 mL (at least five samples), with no single sample over 406. Permits translate this into effluent limits and sampling frequencies. Many permits also set total residual chlorine (TRC) limits near the analytical detection level, because chlorine is acutely toxic to juvenile salmonids. Most plants that chlorinate must therefore also dechlorinate.

Chlorination

$$\text{Dose (mg/L)} = \text{Demand (mg/L)} + \text{Residual (mg/L)}$$ $$\text{Feed (lb/day)} = \text{Flow (MGD)} \times \text{Dose (mg/L)} \times 8.34$$

• Wastewater contains ammonia, so most chlorine forms combined residual (chloramines). These are slower disinfectants than free chlorine, so effective mixing at the injection point and adequate contact time are essential.
• Contact chambers are baffled for plug flow. Design standards typically call for at least 15 minutes of contact at peak hourly flow. Short-circuiting from missing baffles or sludge deposits lets organisms escape.
• Suspended solids shield bacteria and exert chlorine demand. Poor clarifier performance leads straight to disinfection failures.
• Sodium hypochlorite (about 12.5 percent trade strength) has largely replaced chlorine gas at small plants. It loses strength in heat and sunlight and forms chlorate as it ages. Store it cool and use it within weeks.

Example. A 3.0 MGD plant has a chlorine demand of 6.0 mg/L and targets a 1.5 mg/L residual before dechlorination.

• Dose = 7.5 mg/L
• Feed = 3.0 × 7.5 × 8.34 = 188 lb/day of available chlorine

Dechlorination

These reactions are nearly instantaneous, but the chemical must be well mixed before the sampling point. The feed is paced to flow and to an upstream residual analyzer. Practical rules:

• Overdose effects: excess sulfite consumes dissolved oxygen and lowers pH, which can violate DO or pH limits. Keep a small, controlled excess only.
• Monitoring: TRC is measured after dechlorination with methods sensitive enough for the permit limit. Analyzers need frequent calibration checks.
• Redundancy: a failed dechlorination pump can kill fish within minutes, so plants use alarms, standby pumps and storage for the chemical.

Example. Dechlorinating 188 lb/day of chlorine residual with sodium bisulfite takes about 188 × 1.46 = 275 lb/day of NaHSO3 as 100 percent product, before adjusting for solution strength.

UV Disinfection of Wastewater

Many Oregon plants now use UV to avoid chlorine residuals and disinfection byproducts entirely.

• Dose = intensity × exposure time. It depends on lamp output, the water's UV transmittance (UVT), flow rate and the cleanliness of the quartz sleeves.
• Water quality: secondary effluent UVT is much lower than drinking water. Higher TSS, iron, humic color and industrial dyes lower UVT and shield organisms inside particles. Particle-associated coliforms are the usual cause of UV failures.
• Lamp types: low-pressure, low-pressure high-output and medium-pressure lamps. Lamps age and lose output, so they are replaced on hour-meter schedules.
• Fouling: iron, calcium and grease coat sleeves. Mechanical or chemical wipers and periodic acid cleaning restore transmittance.
• Hydraulics: open-channel systems need a level control gate so lamps stay submerged; exposed lamps overheat and waste dose. Channels must be free of short-circuiting.
• Seasonal disinfection: some permits require disinfection only during the recreation season or year-round depending on the receiving water. Follow the permit, not habit.

Troubleshooting Bacterial Exceedances

1. Check the sample: proper sterile bottle, dechlorinating agent for chlorinated samples, holding time (8 hours for wastewater E. coli) and lab QC.
2. Check upstream solids: clarifier washout or high effluent TSS.
3. Check the disinfectant: residual before dechlorination, hypochlorite strength, UV intensity readings, lamp failures, sleeve fouling and channel level.
4. Check hydraulics: storm flows above contact capacity, or baffles missing.
5. Report permit exceedances as the permit requires, including 24-hour reporting where noncompliance may endanger health or the environment.

Worked Example: Chlorine Contact Time

A chlorine contact basin holds 40,000 gallons, and peak flow is 2.0 MGD.

• Flow = 2,000,000 gal/day ÷ 1,440 min/day ≈ 1,389 gpm
• Theoretical detention time = 40,000 gal ÷ 1,389 gpm ≈ 28.8 minutes

Real contact time is shorter, because water short-circuits through any basin. Baffling and a long, narrow serpentine path bring actual contact time closer to the theoretical value. Check permit contact-time assumptions at peak flow, when detention is shortest.