Disinfection and Effluent Quality
Core concepts
Chlorine chemistry dominates. Free chlorine exists as HOCl and OCl⁻; HOCl is the far stronger disinfectant, and the HOCl fraction increases as pH drops (best around pH 6.5–7.5). Combined chlorine (chloramines) is weaker and slower. Know breakpoint chlorination: initial demand is satisfied, chloramines form, then a breakpoint drops residual before free chlorine rises.
CT concept: disinfection depends on concentration × contact time, modified by pH, temperature, and demand. Cold water slows kill; high ammonia consumes chlorine and shifts to chloramines. Know required residual (often 0.5–1.0 mg/L free chlorine after contact), typical coliform limits (e.g., 200 fecal coliform/100 mL monthly geometric mean), and that dechlorination protects receiving streams.
Alternatives: UV (needs low turbidity and transmittance, no residual), ozone (power-intensive, no residual), peracetic acid. Effluent quality parameters include BOD, TSS, ammonia, phosphorus, and toxicity.
Traps
Questions ask what happens when pH rises - free chlorine shifts to OCl⁻, weakening disinfection. Another trap: adding chlorine to fix high ammonia without recognizing demand will spike combined chlorine. Don't confuse contact time (minutes) with detention time in the whole plant. Watch for UV questions where high TSS defeats the system - the answer is filtration, not more lamps.
Worked example / rule of thumb
If effluent ammonia is 2 mg/L, expect roughly 5× that mass of chlorine to satisfy the ammonia demand (stoichiometrically about 7.6 lb Cl2 per lb NH3-N for breakpoint). Rule of thumb: measure demand with a bench test before adjusting feed; chasing residual with the feed pump alone often creates chloramine taste and odor complaints downstream.