Chlorine needs at least 30 minutes of contact at room temperature, or 60 minutes in water colder than 10 degrees C, and the minutes count only for water that actually spends that long in the tank. A contact tank must therefore hold the peak outflow times the contact time divided by a baffling factor: for two taps running at 0.66 liters per second and a factor of 0.5, that is 0.66 × 1,800 ÷ 0.5 = 2,376 liters, not the 1,188 liters the simple product suggests.
Part of Water Treatment at Scale
Contact time is the period that treated water must stand in contact with a disinfectant, here chlorine, before it can be drunk, and the tank volume and layout needed to guarantee it.
Contact Is Counted For The Fastest Water, Not The Average
The carrier gives the chlorine requirement as at least 30 minutes at room temperature and 60 minutes in cold water below 10 °C, with a residual of 0.5 mg/L after 30 minutes and 0.2-0.5 mg/L at the point of consumption. Those numbers are what the water must experience. A tank with a through-flow does not give every drop the same time: some takes a short path and leaves early. The retention time page explains how to measure it (Retention Time).
The product of concentration and time, C × t, is sometimes used to compare treatments: 0.5 mg/L for 30 minutes is 15 mg·min/L, and for 60 minutes it is 30. I am recalling this convention from drinking-water practice. It is a bookkeeping device, not permission to trade a higher dose for a shorter wait: this wiki fixes the time and the residual separately, and shortening the time below 30 minutes is not supported by anything here.
Contact is only effective in clear water. Turbidity shields organisms and consumes chlorine, and the WHO guideline I am recalling for the 0.5 mg/L after 30 minutes figure assumes turbidity below 1 NTU and pH below 8. Cloudy or alkaline water needs the problem fixed upstream (Sedimentation).
What You Need
- A tank used only for contact, covered, with an inlet at one end and an outlet at the other
- The peak outflow in liters per second, from the taps that can run together
- A baffling factor for that tank, preferably measured
- A way to dose the inflow evenly (Dosing Calculation)
- A residual test at the tank outlet
Method: Size For The Peak And The Cold
Step 1: Fix the required time: 30 minutes, or 60 if water falls below 10 °C in any season.
Step 2: Fix the peak outflow Q. A tap running freely draws about 0.33 L/s (20 L in 60 s), so a stand with two taps open asks for 0.66 L/s (Standpipe Design).
Step 3: Choose the baffling factor BF. Typical values I am recalling from drinking-water practice are about 0.1 for an unbaffled tank, 0.3 for poor baffling, 0.5 for average and 0.7 for well baffled. Measure it if you can.
Step 4: Volume V = Q × t ÷ BF.
Step 5: Check at the outlet: the residual should be at least the target when water leaves the tank.
Worked Example: Two Taps, Three Tanks
Inputs: Q = 0.66 L/s; t = 30 min = 1,800 s.
- Simple product: 0.66 × 1,800 = 1,188 L.
- With BF 0.5: 1,188 ÷ 0.5 = 2,376 L.
- With BF 0.3: 1,188 ÷ 0.3 = 3,960 L.
- With BF 0.7: 1,188 ÷ 0.7 = 1,697 L.
- In cold water, 60 minutes doubles each figure: 2,376 L becomes 4,752 L.
Baffles that raise BF from 0.3 to 0.7 cut the required volume from 3,960 L to 1,697 L, a saving of 2,263 L, nearly 60%. Good internal layout is cheaper than a larger tank.
Why a batch tank is the safe alternative. In a batch scheme, a tank is filled, dosed, held for the full time with no water entering or leaving, and then drawn down while a second tank is filling. Every drop then gets the full time, whatever the baffling. Take 2,000 L/day, which is 2,000 ÷ 24 = 83.3 L/h, and two 1,000 L tanks. While tank A fills, tank B supplies; when A is full it must stand 30 minutes, and B must still have water for 30 minutes of that time: 83.3 × 0.5 = 41.7 L, or 83.3 L in the cold 60 minutes. Make each tank at least about 1,100 L, and size to the peak hourly draw, which is 2.5-3 times the average hourly draw (Distribution Network).
Failure Modes
- Residual at the outlet is zero: the dose is low, the demand is high or the contact is incomplete; do not release the water.
- Water at the tap smells strongly at one time and not at another: flow is uneven or the tank is short-circuiting.
- Untreated top-ups enter during the hold: this mixes raw water with water that has been waiting and defeats the time. Keep the inlet shut during contact in a batch tank.
- Contact time is met in summer and not in winter: the cold-water rule was ignored.
Common Mistakes
- Sizing on average flow. The peak draws water through faster.
- Using V ÷ Q as the contact time. Short-circuiting shortens the real time.
- Counting the distribution pipe as contact time. It cannot be checked.
- Trading time for a higher dose. Not supported here.
Safety
A contact tank holds strong chlorine water and is a confined space
Never enter a tank that has held chlorinated water without full ventilation, a harness on a line held by a person outside, and a rule that the person outside never goes in after a collapsed worker; chlorine gas is released if the chemical meets acid. Do not test the air with a flame. Keep the tank covered and vented, and never mix hypochlorite with acids, vinegar or ammonia products.
What’s Next
- Learn next: Chlorine Residual — proving that the contact worked
- Combine with: System Monitoring — watching the residual day by day