Disinfection, CT Rule & Chlorination

Quick Answer: CT = C × t for surface water pathogen credits. Lower pH favors HOCl. Reach breakpoint to get free chlorine residual. Balance DBPs (THMs) vs. disinfection power.

Disinfection is the final barrier in drinking water and essential in wastewater reuse. CT Rule math and chlorine speciation are high-yield FE topics.

Disinfection vs. Sterilization

Disinfection — destroys most pathogens; sterilization — all life. Practical systems target log inactivation of Giardia, viruses, Cryptosporidium (chlorine-resistant — UV/filtration).

Chlorine Species

[ \text{Cl}_2 + \text{H}_2\text{O} \rightleftharpoons \text{HOCl} + \text{H}^+ + \text{Cl}^- ]

[ \text{HOCl} \rightleftharpoons \text{H}^+ + \text{OCl}^- ]

pH 7.5–8.0 still disinfects but needs higher CT than pH 6.5.

Chlorine Demand and Residual

Demand — reacts with organics, Fe²⁺, H₂S, ammonia. Residual — remaining for continued protection.

Breakpoint curve: as dose increases, combined chlorine (chloramines) forms, then breaks through to free chlorine residual.

CT Concept

[ CT = \int C(t),dt \approx C_{avg} \times t_{contact} ]

Units: mg·min/L (check problem). SWTR/LT2 tables give required CT for temperature, pH, organism.

Worked example: C_avg = 0.8 mg/L free Cl₂; t = 45 min → CT = 36 mg·min/L. Compare to table requirement for 0.5-log Giardia credit at stated pH/T.

Baffling factor reduces effective t — plug flow CT basins better than mixed.

Contact Basin Design

Length:width ratio 20:40:1 for plug flow; baffles increase T10/T (fraction of theoretical detention). Dead zones waste volume.

[ t = \frac{V}{Q} ]

Use effective volume, not total if dead zone fraction given.

Disinfection Byproducts (DBPs)

Chlorine + NOM → THMs, HAAs. Regulated at LRAA in distribution. Strategies:

• Reduce precursor (coagulation, GAC)
• Use chloramines in distribution
• UV primary disinfection, low chlorine residual

Wastewater Chlorination

Higher doses and CT for fecal coliform permit limits. Dechlorination with SO₂ or bisulfite before discharge to chlorine-sensitive streams.

Alternative Disinfectants

Ozone/UV CT

UV uses dose = intensity × time (fluence), not mg/L chlorine CT.

Hypochlorite and Gas Chlorine

Sodium hypochlorite — liquid feed safety. Gas Cl₂ — leak scrubbers, confined space protocols (EHS chapter).

FE Calculation Flow

1. Identify organism and required log inactivation.
2. Look up CT requirement (given).
3. Compute actual CT from C and t (effective).
4. Adjust for pH, temperature, baffling if stated.

Exam trap: Using total chlorine when table specifies free chlorine residual.

Exam trap: Cryptosporidium needs UV or filtration — chlorine CT alone is insufficient at practical doses.

Disinfection ties microbiology to hydraulic design — practice one CT problem and one breakpoint graph interpretation before the exam.

SWTR and LT1/LT2 Framework (Conceptual)

Surface Water Treatment Rule requires filtration and disinfection credits for Giardia, viruses, and Cryptosporidium. Inactivation ratio = required log ÷ achieved log per disinfectant.

Chloramine vs. Free Chlorine

Chloramines persist in distribution with lower DBP formation but weaker primary disinfection — often used as secondary residual.

Ozone and AOP

Advanced oxidation (O₃ + H₂O₂, UV + peroxide) destroys micropollutants (pharmaceuticals, PFAS context) — emerging topic, conceptual on FE.

CT Basin Hydraulics

T10/T ratio from tracer studies — baffled basins approach plug flow, maximizing CT for given volume.

Disinfection Byproduct Rule

Stage 1/2 DBPR limits THMs and HAAs in distribution — drives enhanced coagulation and alternative disinfectants at surface water plants.

Sodium Hypochlorite Degradation

Hypochlorite strength decays with temperature and age — dose calculations use available chlorine assay on day of use. On the FE Environmental exam, confirm units before substituting into handbook equations; distractors often differ only by conversion factors. Practice locating handbook sections by keyword during timed drills so lookup takes under one minute. When a stem gives excess data, identify the governing regulation or equation first, then ignore unrelated values. Document assumptions on scratch paper: stability class, waste classification, exposure pathway, or discount rate as applicable. Cross-check magnitude: TCLP thresholds in milligrams per liter, emissions in pounds per hour, and cancer risk slopes in consistent units. NCEES items may supply all required constants in the stem; do not assume every value appears in the FE Reference Handbook. Flag uncertain items and return after a first pass; steady pacing across twelve knowledge areas improves pass probability. Distinguish whether the question tests federal acronyms (RCRA, CAA, EPCRA) or quantitative engineering before selecting an approach. On the FE Environmental exam, confirm units before substituting into handbook equations; distractors often differ only by conversion factors.

Multi-Baffled CT Example

Basin volume 2 MG, Q = 20 MGD, baffling factor = 0.5, T10/T = 0.3. Theoretical t = 2/20 = 0.1 day = 144 min. Effective contact T10 ≈ 0.3 × 144 = 43 min. Free Cl₂ residual C = 1.2 mg/L.

[ CT = 1.2 \times 43 = 51.6\text{ mg·min/L} ]

Compare to table requirement (often 30–60 for virus credit depending on pH and temperature). Increasing baffles raises T10/T without larger basin volume.

HOCl Fraction from pH

At pH 7.5, chlorine fraction as HOCl ≈ 50% (Handbook chart). Same total residual at pH 6.5 might be ~90% HOCl — same mg/L residual, stronger disinfection at lower pH.

Chlorine Demand Calculation

Raw water: dose 4.0 mg/L, residual 0.5 mg/L → demand = 3.5 mg/L. If ammonia 0.8 mg/L present, first forms monochloramine before breakpoint — combined chlorine may read on DPD test until breakpoint exceeded.