Disinfection, Residuals, and Byproducts
Killing organisms while controlling byproducts
Disinfection is a two-front job: inactivate pathogens and keep a residual, while holding regulated byproducts low. Chlorine chemistry is central. In water, chlorine forms hypochlorous acid (HOCl), the strong disinfectant, and hypochlorite ion (OCl⁻). The pKa is about 7.5, so at pH 6.5 most chlorine is HOCl; at pH 8.5 most is the weaker OCl⁻.
Options and trade-offs
- Free chlorine - strong residual, but reacts with natural organic matter to form TTHM and HAA5.
- Chloramines - weaker and longer-lasting, fewer TTHMs, but can nitrify in the distribution system and need careful chlorine-to-ammonia ratio.
- Ozone - powerful, no residual, forms bromate where bromide is present.
- UV - excellent against Cryptosporidium, no residual, needs low-turbidity water.
- Chlorine dioxide - forms chlorite and chlorate.
Concepts
Demand is what the water consumes before free residual appears. At the breakpoint, combined chlorine is oxidized and free residual climbs. CT = residual (mg/L) × contact time (min); required CT comes from tables based on disinfectant, pH, temperature, and log inactivation. Dechlorination uses sulfur dioxide or bisulfite.
Traps
Free versus total chlorine; assuming UV leaves a residual; forgetting that nitrification destroys chloramine residual and raises nitrite.
Worked example
Dose 2.5 mg/L; after 30 minutes the free residual is 1.2 mg/L, so demand is 1.3 mg/L. With 20 minutes of contact time and a required CT of 20 mg·min/L at your pH and temperature, you need 1.0 mg/L - the measured 1.2 mg/L passes, but barely.