Disinfection, Water Quality, and Sampling
Core concepts
Disinfection is the primary barrier against microbial contamination. Know chlorine chemistry: free chlorine (HOCl + OCl⁻), combined chlorine (chloramines), and CT (disinfectant concentration × contact time). Understand the breakpoint curve, chlorine demand, and how pH shifts the HOCl/OCl⁻ balance (HOCl is the stronger disinfectant). Recognize that chloramination reduces disinfection byproducts (DBPs) but requires careful ammonia control.
Water quality and sampling
Learn the meaning of turbidity, pH, temperature, alkalinity, hardness, and DBP precursors. Know bacteriological sampling basics: proper bottle, dechlorinating agent, holding time, chain of custody, and total coliform / E. coli significance. Understand residual chlorine monitoring, heterotrophic plate counts, and lead and copper sampling at customer taps.
Traps to watch
- Confusing free and total chlorine; total includes combined.
- Forgetting that high pH shifts chlorine toward the weaker OCl⁻ form.
- Assuming more chlorine is always better - DBPs and taste/odor increase.
- Mishandling sample bottles (no preservative, exceeded holding time) invalidates results.
Rule of thumb / worked example
CT = residual concentration × contact time. If residual is 1.0 mg/L and contact time is 30 minutes, CT = 30 mg·min/L. Compare to the required CT for the target pathogen and conditions; if short, increase residual or contact time. Always verify the sample's chlorine form and pH before judging disinfection effectiveness.