CT Achieved (Disinfectant Residual × Contact Time)

Disinfection obeys a dose law, and the dose is concentration multiplied by time. That is the whole of it. A regulator writing a drinking-water rule needs one number that says "this water has been disinfected enough", and $C \times T$ is the number: the residual a pathogen swam through, times how long it swam through it, in milligrams per litre times minutes. Hold 1.2 mg/L of free chlorine for 45 minutes of real contact and you have delivered 54 mg·min/L.

This page computes the CT you achieved. It does not tell you whether you passed. I want to be very plain about that, because it is the thing people most want a calculator to do and the thing a calculator has no business doing. The required CT depends on four things at once — the pathogen, the disinfectant, the water temperature and the pH — and it is a table published by your regulator, revised on their schedule, keyed to conditions you have to measure rather than assume. Look it up. There is no table on this site and there never will be, because a stale table is worse than no table: it looks authoritative and it is wrong, and the person who trusts it is running a plant.

Three things sink real plants, and I have watched all three.

The C is the residual at the END of the contact zone, not the dose you fed. Demand eats the difference on the way through, and on a coloured surface water in the spring it can eat most of it. Feeding 2.5 mg/L and measuring 0.6 at the outlet means your CT is built on 0.6. Where the residual is falling noticeably along the contact chamber, the honest practice is to break the chamber into segments and add up the CT of each, using the residual measured at the end of each one — that is what the EPA guidance describes, and it is more work and it is more nearly true.

The T is T₁₀, not the detention time. This is the big one and it gets its own page. Volume over flow gives you θ, the time an average drop spends in the tank if the tank behaves; T₁₀ is the time by which only a tenth of a tracer has come out the other end, and real basins short-circuit badly enough that T₁₀ can be a tenth of θ. Using θ where the rule asks for T₁₀ overstates your disinfection by up to a factor of ten, and it does it silently, on a spreadsheet that adds up correctly.

And the credit was earned at a temperature. The same 3-log Giardia credit costs roughly twice the CT at 5 °C as at 15 °C, because everything about the reaction slows in cold water. A plant that passes comfortably every August can be failing every February on identical operation, and the way it usually gets found is an inspection rather than a violation. Take your credit at the worst temperature of the day, not the average, and certainly not the design value.

The disinfectant matters as much as the temperature. Ozone and chlorine dioxide need far less CT than free chlorine for the same credit; chloramine needs vastly more, which is exactly why it is a distribution-system residual and not a primary disinfectant. And free chlorine loses potency as pH rises, because the species doing the killing is hypochlorous acid, and above about pH 7.5 an increasing share of it has dissociated into the far weaker hypochlorite ion. That is why the CT tables have a pH axis at all.

Cryptosporidium is the reason the whole framework looks the way it does. Its oocysts are effectively immune to free chlorine — the CT for a single log runs into thousands of mg·min/L, which is to say hours at residuals no plant would ever feed. You cannot chlorinate your way past it. That single fact is why UV and ozone appear in treatment trains, why filtration performance is regulated so hard, and why the rules that came after the Surface Water Treatment Rule were written the way they were. If Cryptosporidium is your concern, nothing on this page addresses it.

One note on units, because water operators will notice. The residual here is typed as a mass concentration, and this site deliberately will not convert mg/L to ppm. In fresh water they are numerically identical, and operators use them interchangeably every day and are entirely right to — a litre of water weighs a kilogram, so a milligram of solute in it is a part per million by mass. But that equality is a property of the solvent, not a conversion. Put the same measurement in brine, in a glycol loop, or in anything denser than water and the two numbers separate. The site keeps them apart so that the one time it matters, it does not quietly get it wrong.