Disinfection Contact Time Calculator

Calculator Inputs

The page uses a single main content column. Inside the form, the fields adapt to three columns on large screens, two on medium screens, and one on mobile.

Disinfectant

Target Organism

Target Log Inactivation

Basin Volume (m³)

Flow Rate (m³/h)

Baffling Factor (0.01 to 1.00)

Measured Residual Concentration (mg/L)

Conservative Decay Allowance (%)

Water Temperature (°C)

Safety Factor (%)

Custom Target CT (mg·min/L)

Reset

Formula Used

1) Nominal detention time
Detention Time = (Basin Volume / Flow Rate) × 60

2) Effective contact time
T10 = Nominal Detention Time × Baffling Factor

3) Effective residual concentration
Effective Residual = Measured Residual × (1 − Decay Allowance / 100)

4) Adjusted required CT
Required CT = Base CT × Log Factor × Temperature Factor × pH Factor × Safety Factor

5) Delivered CT and required contact time
Delivered CT = Effective Residual × T10
Required Contact Time = Required CT / Effective Residual

Default CT values in this tool are illustrative engineering benchmarks for screening. They are not a substitute for approved regulatory tables, pilot testing, or site-specific validation.

How to Use This Calculator

Select the disinfectant and target organism.
Enter the desired log inactivation level.
Add basin volume, flow rate, and baffling factor.
Enter measured residual concentration and a conservative decay allowance.
Provide temperature, pH, and any added safety factor.
Use a custom CT only when you already have an approved design target.
Click the calculate button to display the result above the form.
Review delivered CT, required CT, T10, volume need, and the chart.
Download the result as CSV or PDF if needed.

Default Benchmark Matrix

Disinfectant	Giardia CT	Virus CT	Bacteria CT	Unit
Free Chlorine	45.00	6.00	1.00	mg·min/L
Chloramine	1200.00	700.00	50.00	mg·min/L
Chlorine Dioxide	25.00	8.00	2.00	mg·min/L
Ozone	1.50	1.00	0.50	mg·min/L

Example Data Table

Disinfectant	Organism	Volume (m³)	Flow (m³/h)	Residual (mg/L)	Baffling	Decay (%)	T10 (min)	Delivered CT	Required CT	Status
Free Chlorine	Giardia	900	180	1.40	0.50	10	150.00	189.00	50.69	Adequate margin

FAQs

1) What does CT mean in disinfection design?

CT is the product of disinfectant concentration and contact time. Engineers use it to estimate whether a process can deliver enough exposure for microbial inactivation.

2) Why does the calculator use T10 instead of nominal detention?

T10 is a conservative estimate of effective contact time. It accounts for non-ideal hydraulics and usually better represents real basin performance than nominal detention time alone.

3) What does the baffling factor change?

The baffling factor reduces nominal detention time to a more realistic T10 value. Better baffling increases usable contact time and usually improves delivered CT performance.

4) Why is there a decay allowance input?

Decay allowance applies a conservative reduction to measured residual. This helps account for uncertainty, short-term variation, and residual loss before final compliance assessment.

5) Are the default CT values official compliance targets?

No. They are screening benchmarks for preliminary engineering work. Always verify final CT requirements with the applicable regulations, approved guidance, and plant-specific validation data.

6) Why does pH matter more for free chlorine?

Free chlorine effectiveness changes with pH because hypochlorous acid and hypochlorite ion fractions shift. Higher pH generally lowers disinfection strength and increases required CT.

7) What should I do if delivered CT is below target?

You can increase residual, improve baffling, reduce flow, increase volume, or revise the process design. The calculator also estimates additional contact time and basin volume needed.

8) Can this calculator help size a new contact basin?

Yes. It estimates required basin volume from flow, baffling, and effective residual. It is useful for conceptual sizing before detailed hydraulic modeling and design review.