Disinfection, Effluent Quality, and Residuals
What you must know
Disinfection reduces pathogens; chlorine, UV, ozone, and peracetic acid are common. Chlorine dosing depends on demand, contact time (CT), pH, and ammonia (which forms chloramines). Free chlorine is more effective than combined; breakpoint chlorination describes the transition. Dechlorination (sodium bisulfite) protects aquatic life. UV effectiveness depends on transmittance, dose (mJ/cm²), and lamp maintenance. Ozone is powerful but costly and requires off-gas destruction.
Effluent quality parameters include BOD, TSS, ammonia, phosphorus, pH, and toxicity. Permits set limits and monitoring frequency. Residuals include waste sludge, screenings, grit, and spent chemicals; disposal options include land application, landfill, and incineration. Biosolids regulations govern pathogen and vector attraction reduction.
Traps in questions
- Assuming chlorine dose equals residual - demand consumes chlorine first.
- Forgetting that chloramines are weaker disinfectants than free chlorine.
- Overlooking UV transmittance reduction by TSS.
- Ignoring dechlorination requirements.
Worked example / rule of thumb
If chlorine demand is 3 mg/L and target free residual is 1 mg/L, dose ≈ 4 mg/L. Rule of thumb: ensure CT (residual × contact time) meets permit; monitor ammonia to avoid chloramine formation.