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Ideally you would want:
1) a flow meter,
2) a metering pump dosing just before a static mixer,
3) an online dual validated chlorine monitor monitoring the chlorine level after the static mixer but before the reservoir,
4) a controler system that adjusts the dose based on the flow and chlorine level,
5) a second dual validated chlorine monitor after your reservoir that can adjust the setpoint of the control system to take account for slower chlorine demand across the reservoir,
6) the reservoir should have some sort of weir that ensures a minimum residence time, and baffles, so all water spends the same amount of time passing through it - you want at least 30 minutes residence time.
You then would use this system to ensure a minimum Ct value (chlorine concentration x time) - that would typically be around 15 mg.min/l for groundwater or 30 mg.min/l for surface water.
However, budget may be an issue...
As a minimum you need to know the flow rate. From that and an understanding of your reservoir you need to determine the shortest time that water can spend in the reservoir. Hopefully that value is at least 30 minutes.
Take that time in minutes and divide 15 or 30 by it to determine the chlorine residual that you need to achieve at the outlet of the reservoir. So if your reservoir is holding water for 30 minutes, you would want 0.5 mg/l chlorine residual if you are treating groundwater or 1 mg/l if surface water (from a river or lake).
Does that make sense?
Chlorination is pretty serious stuff, so you I would suggest that you engage a competent consultant/contractor with lots of experience of similar projects. They should already know everything I've mentioned above, and more...