Design the disinfection step of a drinking-water or wastewater treatment train using CT (concentration × time) or IT (UV dose). Pick a pathogen and a method, set the reactor geometry, and the required exposure and achieved log inactivation update instantly.
Parameters
Disinfection method
Chemical (CT) or UV physical (IT)
Target pathogen
Sets pathogen sensitivity (CT₉₉%) automatically
Target log inactivation
log
3-log = 99.9%, 4-log = 99.99% inactivation
Residual conc. C / UV intensity I
mg/L or mW/cm²
mg/L for ozone & chlorine, mW/cm² for UV
Treatment flow rate Q
m³/h
Reactor volume V
m³
Theoretical HRT = V/Q used as the actual retention time
Water temperature T
°C
Lower temperature reduces effectiveness (~2% per °C)
Results
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CT/IT required (mg·min/L or mJ/cm²)
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Required retention time (min)
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Actual retention time (min)
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Achieved CT/IT (mg·min/L or mJ/cm²)
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Achieved log inactivation
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Design verdict
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Disinfection reactor — pathogen-removal animation
Pathogen particles in the influent are exposed to ozone / UV / chlorine inside the contactor and removed at the outlet. The bar below shows the achieved CT/IT value.
EPA LT2-style temperature correction (linear in °C), scaled by the target log inactivation N_log.
Ozone & UV Disinfection — CT and IT Dose Design
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On a water-plant tour I kept hearing about "CT values" with units of mg·min/L. It looks like a product of two things — what does it actually mean?
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Exactly that — CT is literally the product of C (residual disinfectant concentration in mg/L) and T (contact time in minutes). For example, ozone at 1 mg/L held for 10 minutes gives CT = 10 mg·min/L. Chick-Watson first-order kinetics says the kill is set by this total exposure, so halving the dose and doubling the time gives the same CT in theory. For UV we use intensity instead of concentration and call it IT = I·t in mJ/cm².
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Got it. So each pathogen has a required CT and we just have to clear it? The default settings say Cryptosporidium needs 27 mg·min/L.
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Yes — the US EPA LT2ESWTR guidance lists CT₉₉% (the CT for 2-log inactivation) per pathogen, and this tool uses those numbers. Crypto oocysts have a tough wall, so even ozone needs 9 mg·min/L per log — that is 27 mg·min/L for 3-log. E. coli, by contrast, takes only 0.16 mg·min/L per log. And chlorine is essentially useless against Crypto: it would need CT ≈ 7200 (about 1 mg/L for five days!), so UV or ozone is required.
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Wait — chlorine doesn't kill Crypto? Is that what caused the 1993 Milwaukee outbreak?
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Exactly. In April 1993 a Milwaukee plant leaked Cryptosporidium during high raw-water turbidity; coagulation-sedimentation was overwhelmed and the chlorine-only disinfection couldn't touch the oocysts. About 400,000 people fell ill and 69 died. That outbreak triggered the EPA LT2 rule, requiring UV or membrane treatment as an extra Crypto barrier on surface-water systems. Japan added similar requirements to the Water Supply Act after the 1996 Ogose outbreak in Saitama.
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With the defaults (O₃ 2 mg/L, 500 m³/h, 30 m³ tank), actual retention is 3.6 min × 2 mg/L = CT 7.2 — way short of the 27 we need, so the verdict is NG. Should I just enlarge the tank or raise the dose?
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Right idea. Going from 30 to 120 m³ pushes HRT to 14.4 min and CT to 28.8, just clearing the bar. Alternatively raising ozone from 2 to 8 mg/L gives the same CT in the same 30 m³. In practice, bigger tanks are cheaper than higher ozone — pushing the dose drives up bromate formation (BrO₃⁻) and off-gas treatment costs. And don't forget short-circuiting: only the T₁₀ portion of HRT really counts for disinfection (about 0.5–0.7 of HRT even with baffles), so designers usually target 1.5–3× the calculated CT as safety margin.
Frequently Asked Questions
CT is the product of residual disinfectant concentration C (mg/L) and contact time t (min): CT = C·t [mg·min/L]. It is the standard exposure metric for chemical disinfectants such as ozone and chlorine. IT is the product of UV intensity I (mW/cm²) and time t (s): IT = I·t [mJ/cm²], the UV dose. Both follow Chick-Watson first-order kinetics, so log inactivation = CT / CT99% links the required dose to the pathogen's sensitivity.
Cryptosporidium oocysts have a thick wall that is essentially immune to chlorine (CT of several thousand mg·min/L needed) and very resistant to ozone (CT≈18 for 2-log, ≈27 for 3-log). After the 1993 Milwaukee outbreak that infected 400,000 people, the US EPA LT2ESWTR rule made Cryptosporidium control mandatory for surface-water plants. Japan followed via the Water Supply Act, and UV (5.8 mJ/cm² for 2-log) is now the de-facto standard barrier.
Ozone is a powerful oxidant effective against protozoa, viruses and bacteria, and also removes taste, odour and colour. However, it forms carcinogenic bromate (BrO₃⁻) in bromide-containing water and leaves no residual, so it cannot prevent regrowth in the distribution network. UV produces almost no disinfection by-products and is ideal for low-dose Cryptosporidium control, but its effectiveness drops sharply when raw-water UV transmittance is low. In practice, multi-barrier trains such as 'UV → chlorine residual' or 'ozone → BAC → chlorine residual' are typical.
This tool uses HRT = V/Q (volume ÷ flow) as the actual retention time, but real reactors have short-circuiting and dead zones, so the effective contact time is the T₁₀ measured by tracer studies — the time at which 10% of the tracer mass has reached the outlet. T₁₀/HRT is typically 0.3–0.7, even baffled reactors only reach 0.5–0.7. EPA guidance therefore uses T₁₀ rather than HRT for the CT calculation, and designers add 1.5–3× safety margin on the required CT.
Real-World Applications
Drinking-water utilities: Mid-to-large surface-water plants increasingly use UV (5.8 mJ/cm² for 2-log Crypto) followed by chlorine for distribution residual. Where raw-water quality is poor, ozone combined with biological activated carbon (BAC) addresses taste, odour and disinfection by-product (THM) precursors. Japan's Kanamachi plant in Tokyo is a textbook example of the ozone+BAC train.
Wastewater effluent disinfection: Sewage treatment plants must inactivate coliforms before discharging to receiving waters. UV is widely used for marine discharges; chlorine (sodium hypochlorite) is still common but is being replaced by UV or ozone due to chlorination by-product concerns. Suspended solids absorb UV, so SS must be reduced below ~10 mg/L upstream of the UV reactor.
Food & beverage CIP sanitation: Beverage lines combine 80°C hot-water rinse with ozonated water (0.5–2 mg/L) or UV. Ozone leaves no residue (so no extra rinse needed); UV is fast enough to install in-line. Use this tool's CT/IT calculation to derive HACCP critical-control-point design conditions with the required safety margin.
Pools, spas and cooling towers: Legionella control requires 0.2–0.4 mg/L free chlorine residual in cooling towers and bathing facilities. Pool design also needs CT-based management, but cyanuric acid (chlorine stabiliser) reduces effective biocidal strength. Adding ozone as a secondary barrier is increasingly common — the CT concept is universal, not limited to drinking-water plants.
Common Misconceptions and Pitfalls
The biggest pitfall is treating theoretical HRT as the actual contact time. This tool simplifies by using V/Q, but real reactors are neither perfect plug-flow nor perfect CSTR — short-circuiting and dead zones are unavoidable. Tracer T₁₀ is typically 0.3–0.7 of HRT; even heavily baffled tanks max out at 0.5–0.7. EPA disinfection guidance therefore uses T₁₀, not HRT, and design CT must include a 1.5–3× safety margin on top.
Next, ignoring UV transmittance when computing UV dose. The intensity I is the value near the lamp surface, but it decays exponentially with depth following Lambert-Beer's law I(z) = I₀·exp(−α·z). A 90% UVT₂₅₄ raw water is fine, but secondary effluent or coloured surface water can drop to 50–70% UVT and double or triple the required lamp output. Flow non-uniformity inside the reactor also disperses the effective dose, so the proper validation is a biodosimetry test (e.g. MS2 phage) that reports the reduction equivalent dose (RED).
Finally, choosing a method without weighing disinfection by-products (DBPs). Chlorine forms trihalomethanes (THMs) and haloacetic acids (HAAs); ozone forms carcinogenic bromate (BrO₃⁻, IARC 2B) when bromide is present; UV produces essentially no DBPs. Japan's drinking-water standards cap total THM at 0.1 mg/L and bromate at 0.01 mg/L — a low bar that ozone systems easily exceed if Br⁻ in the raw water is not accounted for. Always pair the CT check with a DBP risk evaluation based on the source-water bromide and organic carbon levels.
How to Use
Select disinfection method: enter ozone concentration (mg/L) and contact time, or UV intensity (mJ/cm²) and exposure duration.
Define treatment target: specify required log reduction (e.g., 3-log for Giardia, 4-log for viruses) and influent flow rate (m³/hr).
Input reactor geometry: enter contact chamber volume (m³) to calculate actual retention time and verify CT or IT achievement against regulatory standards (EPA LT2R, WHO guidelines).
Review design verdict: simulator compares achieved CT/IT versus required CT/IT; flag undersizing or recommend operational adjustments (increase residence time or disinfectant dose).
Worked Example
Municipal wastewater plant treating 500 m³/hr secondary effluent for 3-log Cryptosporidium inactivation using ozone. Target CT = 6 mg·min/L. Operator sets ozone dosage 2 mg/L into a 50 m³ contact basin. Actual retention time = 50 m³ ÷ (500 m³/hr ÷ 60) = 6 minutes. Achieved CT = 2 mg/L × 6 min = 12 mg·min/L. Result: design exceeds requirement (12 > 6); log inactivation achieved = 3.2-log. Alternatively, UV system: 80 mJ/cm² dose, flow 300 m³/hr, reactor 25 m³ yields 5-minute IT window; if UV intensity = 16 mJ/cm²·s, actual IT = 16 × 300 = 4,800 mJ/cm², exceeding 2,400 mJ/cm² target for 2-log virus reduction.
Practical Notes
Ozone decays rapidly (half-life ~20 min at 20°C); account for residual depletion in long contact chambers—use baffles or staged injection to maintain CT.
UV efficacy depends on water turbidity (NTU) and UV transmittance (%T); clarity <0.1 NTU required; pre-filtration is mandatory for surface water.
CT values scale with temperature and pH; ozone effectiveness increases at lower pH (<7) and cooler water; recalculate design if seasonal temperature swings exceed 10°C.
Validate against local rules: Safe Drinking Water Act (SDWA) mandates 0.5–1 mg/L ozone residual for secondary disinfection; wastewater discharge may require 1–2 mg/L ozone demand + residual.
Monitor headloss across diffusers/reactors; excessive pressure drop reduces gas transfer efficiency and increases operational cost.