Disinfection and Disinfection Byproducts
This domain addresses primary and secondary disinfection, CT concepts, DBP formation and control, and regulatory limits. The exam tests your ability to calculate CT, select disinfectants, and balance microbial risk against DBP formation.
Disinfectants and Their Properties
- Free chlorine is the most common primary disinfectant; it is effective against most pathogens but forms THMs and HAAs. It requires a CT factor that varies with pH and temperature.
- Chloramines (monochloramine) are used as secondary disinfectants to maintain residual; they form fewer regulated DBPs but are weaker primary disinfectants and can form nitrosamines.
- Ozone is a powerful primary disinfectant and oxidant; it does not leave a residual. It can form bromate in bromide-containing waters.
- Chlorine dioxide is effective but forms chlorite and chlorate; it is limited to 1.0 mg/L as ClO2.
- UV provides inactivation of Cryptosporidium and Giardia but no residual; it is often used with chlorine or chloramines.
CT Concept
- CT is the product of disinfectant concentration (C, mg/L) and contact time (T, minutes). Required CT values depend on pathogen (Giardia, viruses, Cryptosporidium), disinfectant, pH, and temperature. CT is determined from tables in EPA guidance.
- Baffling factor adjusts theoretical detention time to actual contact time; typical factors range from 0.1 (unbaffled) to 0.7 (well-baffled).
DBP Regulations
- Stage 2 Disinfectants and Disinfection Byproducts Rule (Stage 2 DBPR) sets MCLs: total THMs 80 µg/L, HAAs 60 µg/L, bromate 10 µg/L, chlorite 1.0 mg/L. Compliance is based on locational running annual averages (LRAA).
- Initial Distribution System Evaluation (IDSE) identifies monitoring locations for Stage 2 DBPR.
- Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) requires treatment for Cryptosporidium based on source water monitoring.
Worked Example
A plant uses free chlorine at pH 7.5 and 20°C. The disinfectant residual is 1.5 mg/L after 30 minutes of contact time. The baffling factor is 0.6. Calculate the effective CT.
- Effective contact time = 30 min × 0.6 = 18 min.
- CT = 1.5 mg/L × 18 min = 27 mg·min/L.
- Compare to required CT for Giardia inactivation (e.g., 3-log) from tables; if required CT is 25 mg·min/L, then 27 exceeds it, so sufficient.
Common Traps
- Forgetting to apply the baffling factor to detention time.
- Using the wrong CT table for the disinfectant or pathogen.
- Assuming chloramines are as strong as free chlorine for primary disinfection.
- Overlooking that ozone and UV provide no residual.
- Mixing up MCL values for THMs (80 µg/L) and HAAs (60 µg/L).