EngiMetric Arena / Quick Challenge
How long must the water stay in contact?
What you should take away: Compute a required contact time from a residual and a required CT, and recognise that the arithmetic result is not a compliance demonstration.
The situation
A contact tank has a measured free-chlorine residual at the outlet of the baffled section. The design requires a stated CT. Given the residual, the contact time needed to reach that CT follows directly. Compute it.
Given
| Quantity | Value | Unit | Note |
|---|---|---|---|
| Free chlorine residual | 2.5 | mg/L | Measured at the outlet of the baffled section. |
| Required CT | 80 | mg·min/L | Stated by the design. Organism, pH and temperature dependent. |
| Water temperature | 15 | °C | The condition the CT applies at. |
Constraints
- Membrane elements are supplied whole. A fraction of an element cannot be ordered.
- Pressure vessels are loaded with a whole number of elements — 6 per vessel here.
- Every vessel in the array is filled to the same element count.
Your answer
Hints
- CT is the product of concentration and time. The relationship is linear in time, so rearrange it rather than searching.
- Watch the units. CT is in milligram-minutes per litre, and the residual is already in mg/L, so the time you get out is in minutes.
- A higher residual buys proportionally shorter contact time. That is the whole point of using CT rather than quoting one or the other.
Where that comes from
Each limit, checked
Why
- CT is the product of disinfectant concentration and contact time: CT = C × T, in mg·min/L.
- Solving for time gives T = CT ÷ C. At a 2.5 mg/L residual, the 80 mg·min/L requirement needs 32 minutes.
- Doubling the residual halves the required contact time, which is exactly why CT — and not either factor alone — is the basis of a disinfection credit.
- The important limit: **this is arithmetic, not compliance.** A CT figure says what the product of the two numbers is. Demonstrating compliance additionally requires showing that the stated contact time is genuinely achieved, and that the required CT corresponds to the organism, pH and temperature in question.
- In particular the contact time here must be a T10 — the period during which 90 % of the water is retained. Theoretical detention time from volume over flow is optimistic, because tanks short-circuit.
What this challenge does not establish
- The required CT is an engineer input, not a built-in value. It depends on the target organism, the pH, the temperature and the jurisdiction, so Arena does not supply one.
- A calculated CT is not a compliance demonstration and must not be presented as one.
- The stated contact time must be a T10. Entering a theoretical detention time from volume over flow overstates the real contact and produces a CT that looks compliant and is not.
- Required CT varies strongly with pH for free chlorine, and this tool carries no pH input.
- UV has no CT value; a UV dose is a separate calculation.