Gravity chlorination system

The problem you will experience with a gravity fed chlorinator is that dosing will diminish as a function of tank level and chlorine residuals will not be on target. The problem could be overcome by pressurising the feed tank but this opens a new set of issues. Your constraint of not using any power supply is self defeating. If it is because the spring is in a remote area with no utility power you could rig a small solar panel to drive a dosing pump, thus assuring chlorine residuals remain on target.

place the tank at a great height, in a tree, so that as the level falls in the tank it will just be a small % of the operating head.

what about winter freeze?

Hopefully there will be a tree conveniently close to the spring. The solution must address the issue of keeping free-chlorine residuals on target . As tank head decreases so will flow rate. You may start feeding at a calibrated tank height but flow rate and thus chlorine residuals at mid and low levels will be changing. For continous, in-line feeding no steady state is attainable by gravity feed. I believe the regulation free-chlorine in drinking water is 2 ppm max. which requires precise dosing control.

The chlorine solution which is mostly water will freeze along with the spring.

I remember using a constant head device that was easy to set up.

http://lawr.ucdavis.edu/classes/ssc100/mariotte_bottle1.pdf

I haven't thought about a setup like this since highschool...

I used constant head devices like that in the lab all the time as you want the rate of air/N2/CO2 to stay constant in many applications and the pressure in the system would vary a lot. Sometimes we used a bottle and a regulator, but with 20-30 users the traffic in cylinders gets oppressive and cylinders have this habit of going to zero

Most of my chemical delivery systems that I deal with require variable injection speeds based off of flow or other parameters so LMI's or other mechanical pumps tend to be the dosing method of choice in my applications.

In any event this is a decent discussion of the basics.

The WHO Guideline Value for free chlorine is 5 ppm (max) and not because of health considerations, rather for objectionable taste. The US EPA stndard is the same. Personally,I wouldnt drink water with more han 1 ppm free chlorine.

Quite. One thinks of municipal swimming pools, where chlorine levels are typically above this figure, and one wouldn't drink much before objecting to it...

In addition, if you have any power source (even a solar/deep draw battery combination) you can use a simple DC electrolytic cell to generate sodium hypochlorite (NaOCl) bleach from salt and water for <<less than the cost of calcium hypochlorite. It is easier to handle NaOCl too.

I spend more time and effort removing chlorine from water than putting it in but I definitely agree with coffeebean's assement.

A little more background information on the system would be helpfull. If this is for a cabin out in the woods that you go to on the weekends you just may be beter off boiling the water before using it to cook with or drink. Or some form of batch proccessing may work or you may not even need to chlorinate. Most springs are very clean and require little treatment. Now mind you that is at the source not 5 miles down stream. Talk to your local health dept about water testing, they (and I think this is by law in many area's) will walk you throught the whole process of getting the water tested. Last time I sent a water sample out for bacterial testing, TOC and Si the cost was just over $15 USD.

Interesting prospect. With no power supply how is mechanical mixing accomplished and complete reagent distribution assured. How is "level compensation" accomplished? Is reagent injection rate flow dependent?