CT-710Chlorine Contact Tank

How it works

1. The idea — Chlorine does not act instantly. Inactivation follows the product of concentration and time — CT — so a high residual held briefly and a low one held long can be worth the same. What a tank is for is supplying the time half of that product.
2. The water does not arrive together — Parcels that enter at the same moment leave spread over many minutes. Some cut corners, some sit in a dead zone and some go almost straight through. Watch the parcels leaving the far end: they are not the same colour, because they have not had the same contact.
3. T₁₀, not volume over flow — The regulator does not care about the average parcel. It cares about the fastest tenth — T₁₀, the time by which ten per cent of the water has already left. Volume over flow describes a parcel that may not exist.
4. Baffling is the whole game — The baffling factor is simply T₁₀ over the theoretical time, and it is set by how many lengths of travel the walls force. An unbaffled tank gives back a tenth. Change the baffling above and watch both the path and the volume you would need.
5. The pH quietly halves it — Only hypochlorous acid disinfects properly — the hypochlorite ion is perhaps eighty times weaker. The split sits on a pKa near 7.5, so at pH 8.5 most of a perfectly good measured residual is doing very little. Push the pH slider and watch the requirement climb: on the USEPA basis it roughly doubles between pH 7 and pH 9.
6. Average — A tank designed by someone who knew what it was for. Inlet baffling and some internal walls. At this classification the tank is worth a baffling factor of 0.5, which is what every number on the right is built on.

Design parameters

The panel opposite runs the same correlations as the CT-710 design sheet, so the animation and the calculator cannot disagree. Every parameter below is live — drag one and the picture responds.

The full CT-710 design calculation — governing equations, accepted envelopes and the worked sizing — is in the design calculator workbook. Everything on this page stays free.