Disinfection
Chlorination
Free chlorine is the sum of HOCl and OCl⁻; the pKa is about 7.5. HOCl dominates below pH 7.5 and is far more powerful. Chlorine demand consumes chlorine before any residual persists. Combined chlorine (chloramines) is weaker and slower, so a total residual may be mostly monochloramine.
CT is disinfectant residual (mg/L) multiplied by contact time (minutes), using t10 - the time for 10% of a tracer to pass - not theoretical detention time. Required CT depends on target organism, log inactivation, temperature, pH, and demand; you compare achieved CT with the table value for those conditions.
Dechlorination uses sulfur dioxide, sodium bisulfite or metabisulfite, or ascorbic acid, at roughly 1 mg/L of reductant per mg/L of chlorine. Traps: over-dechlorination and low DO; chlorine continuing to react in a sample bottle (add a reducing agent for bacteria samples); assuming a residual is free chlorine when it is combined.
UV
Dose equals intensity times time, reported in mJ/cm². UV damages DNA and leaves no residual. Effectiveness depends on UV transmittance - high TSS or color blocks it - plus sleeve fouling and accurate dose monitoring. UV is strong against Cryptosporidium.
Ozone
A powerful oxidant generated on site with no lasting residual. It can form bromate from bromide, especially at high dose and pH, and requires off-gas destruction.
Worked example
A contact chamber has t10 of 20 min and a measured residual of 1.5 mg/L. CT = 20 × 1.5 = 30 min·mg/L; compare that to the required value for the organism, temperature, and pH.