Disinfection and Disinfection Byproducts
What to know
Disinfection protects public health but creates disinfection byproducts (DBPs) when disinfectants react with natural organic matter (NOM) and bromide. Know the major disinfectants - free chlorine, chloramines, chlorine dioxide, ozone, UV - and their strengths and weaknesses. Understand CT (concentration × contact time) and that CT credit depends on disinfectant, pH, temperature, and pathogen target (Giardia, viruses, Crypto). Learn the four Stage 2 DBP rule groups: TTHM and HAA5 (regulated), plus bromate and chlorite. Know residual maintenance in distribution and that chloramines require careful ammonia control and can cause nitrification. Understand DBP precursor removal (enhanced coagulation, TOC removal) and the tradeoff between pathogen control and DBP formation.
Traps
Questions often imply UV provides a residual - it does not. Another trap: assuming chloramines are stronger than free chlorine; they are weaker but more stable. Watch for CT questions that ignore temperature or pH corrections, and for confusing TTHM with HAA5 limits.
Worked example / rule of thumb
If a system switches to chloramines to reduce TTHM/HAA5, it must maintain an adequate residual and monitor for nitrification in storage and dead ends. Rule of thumb: reduce DBP precursors (TOC) before the disinfectant contacts them; post-treatment fixes are harder.
Study tip
Build a table of disinfectants vs. residual, DBP risk, and pathogen effectiveness.