Chlorine Testing Your Water
By Dave Skogberg

A 1500 gallon cistern requires about 1 cup of regular bleach (regular bleach is 5.25% chlorine) to reach an active chlorine level of 1-2 parts per million (ppm).  (This assumes that the system and the water are clean to begin with.)  Be careful if you use the newer "ultra" formulations as the concentration is higher so the amount should be reduced proportionally.  Do not use bleach with scent or gel added.  One to two ppm is the recommended amount of chlorine for municipal and residential water to keep bacterial growth in check.

The only way to know how much active chlorine is present in your water is to draw a sample from your own tap or cistern and test it.  A simple color comparison test is available at pool and spa store or Lowes, Home Depot, and seasonally at larger Wal-Mart or K-Mart stores where they have a pool and spa isle.

Start first with the cistern and check the active chorine level there.  If it is not between one to two ppm and the cistern is full add one cup of bleach (decrease the bleach dosage amount proportionally if the cistern is less than full, that is ½ cup for a half full cistern).  Run a garden hose into the top of the cistern and to mix the bleach with the water.  Mixing may take 15-30 minutes or more.  Re-test several times until you continue to get the same measurement.  CAUTION:  If the level is above 1-2 ppm it is probably because the chlorine is not adequately mixed into the system and your pump is pulling water from an area in the cistern that is temporarily higher in concentration and more mixing is required.  If adding one cup of bleach does not result in 1-2ppm it is most likely that contaminates in the water or the cistern are using up the active chlorine that was added.  If this is the case continue adding bleach one cup at a time, mixing and re-testing until you reach a level of 1-2ppm residual active chlorine.

Depending on the volume of water in the lines to your house it may take considerable time (days or weeks) to reach a concentration of 1-2ppm at you tap.  If a 1-2ppm concentration is maintained in the cistern over time the lines to your house should eventually be sanitized and a reading of 1-2ppm should be attained at the tap.  If it took several cups of bleach to get to 1-2ppm in the cistern then it will probably take longer to get to 1-2ppm on the house because the same contamination in the cistern and water must be destroyed in the delivery lines to the house.  This may take several days or weeks depending on level of contamination, volume of delivery lines and water usage.

Monitoring of the chlorine level should be an ongoing process over time.  This is not a one-shot deal.  Over time the chorine added to your cistern may be used up or evaporate into the air above the water and the concentration may fall and require re-dosing.  If you monitor and record how much bleach is required and the time it takes for the level of chlorine to drop over time you can establish an idea of the frequency at which you need to test and the amount of chlorine bleach required.  Keep in mind that this will vary with the amount of water you use and may be very different in the winter than in the summer so you may want to monitor for a whole year to get a pattern and always continue to check periodically thereafter.

Every water system is different so do not go by what your neighbor does or rely on any rule-of-thumb that someone has given you.  The only way to know how much active chlorine is present in your water is to draw a sample from your own tap or cistern and test it.  If you are on the metro water system it is still a good idea to check your water from your tap in your house periodically to be sure that you are not introducing contamination from your cistern and that the chlorine added is still present in high enough concentration to protect from bacterial growth when you receive your water as chlorine is lost over time by evaporation and handling of the water.