Wastewater Chlorine Disinfection & CT Calculator
Determine chlorine feed rate in lbs/day and gallons/day, contact chamber detention time, chlorine demand, and regulatory CT values for pathogen inactivation.
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Wastewater Chlorine Disinfection, Chemical Feed & CT Values
Chlorination is the most widely deployed chemical disinfection process in municipal wastewater treatment. To eliminate pathogenic bacteria (fecal coliform, E. coli, Salmonella) and viruses, the disinfect process requires maintaining both an adequate concentration of active free/combined chlorine and sufficient contact time.
Governing Disinfection Formulations
- Chlorine Mass Feed Rate (lbs / day):
Lbs Cl2 / Day = Flow (MGD) × Dosage (mg/L) × 8.34 - Chemical Bulk Delivery Rate:
For 12.5% Sodium Hypochlorite (trade percent, contains 1.25 lbs available chlorine per gallon):Gallons / Day = Lbs Cl2 / 1.25 - Chlorine Demand:
Organic matter, ammonia, and reduced metals react and consume chlorine:Chlorine Demand (mg/L) = Applied Dosage - Residual Concentration - Contact Time & Baffling Factor:
Theoretical detention time:T (min) = [ Basin Volume (Gal) / Flow (GPM) ]
Effective contact time (T10):T10 = T × Baffling Factor (BF) - Disinfection Concentration-Time (CT) Value:
CT = Residual (mg/L) × T10 (minutes)
Frequently Asked Questions
What is a CT value in water disinfection?
CT is the mathematical product of the residual disinfectant concentration (C, in mg/L) and the effective contact time (T10, in minutes). Regulatory agencies (EPA) prescribe minimum CT values required to achieve 3-log (99.9%) or 4-log (99.99%) inactivation of specific microbial pathogens.
What is the Baffling Factor (BF) and why is T10 used?
In a real contact basin, water does not travel in uniform plug flow; friction and eddies cause short-circuiting where 10% of the water exits faster than the theoretical retention time. T10 represents the time it takes for 10% of the water to pass through. Baffling factors range from 0.1 for unbaffled circular tanks to 0.7 for long multi-pass serpentine channels.
Why must treated wastewater be dechlorinated before discharge?
Even trace amounts of residual chlorine (0.01 to 0.05 mg/L) are highly toxic to fish and aquatic life in receiving streams. Plants inject sulfur dioxide gas or sodium bisulfite (SBS) at the outfall to neutralize residual chlorine to non-detectable levels.
How many gallons of 12.5% bleach equals 100 lbs of chlorine gas?
One gallon of 12.5% sodium hypochlorite contains approximately 1.25 lbs of pure chlorine equivalent. Therefore, 100 lbs of chlorine gas is equivalent to 100 / 1.25 = 80 gallons of commercial 12.5% bleach.