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How Much Chlorine To Add to Storage Tank To Kill Bacteria

Chlorine Bleach Disinfect WaterChlorine is an effective and low-cost chemical for disinfecting water storage tanks. We are often asked, how much chlorine is needed to treat water? How much chlorine bleach do I put in my water tank?

Using the charts below you can find out how much Chlorine Bleach is needed to disinfect and purify specific quantities of water, for example 1,000 gallons.  On this page we will talk more about the chlorine to water ratio and how much chlorine to put in drinking water.

If your tank is new, has been worked on recently, or you know it is contaminated, perform a “shock chlorination” with 50 to 100 parts per million (PPM) and let sit for 12 to 24 hours.

Test the chlorine residual after 24 hours and if the chlorine levels are 10 PPM or less, repeat the procedure.  If you are storing water and want to keep a chlorine residual to be safe, use a maintenance residual of 1 – 2 PPM.

Two Methods:  Shock Chlorination Or Chlorine Maintenance Residuals

NOTE: Shock chlorination will make the water unusable for potable use until the chlorine levels drop below 2 – 4 PPM, which typically occurs within a few days to a few weeks depending on temperature and water chemistry.

Don’t want to use shock chlorination?  It is best to use an automatic chlorinator if you wish to automatically maintain a chlorine residual to keep your storage tank disinfected as new water flows into it.

However if your storage tank is for long term storage or you wish to periodically chlorinate see the chart for adding 1 – 2 PPM.

A chlorine residual of 1 to 2 PPM is recommended if you plan to maintain a chlorine residual for potable water use.  Follow these steps and see the chart below to find out how much chlorine bleach to add to achieve these residuals.

Step 1: Clean the storage tank or reservoir. Remove debris and scrub or hose off any dirt or other deposits or interior surfaces. Pump to remove any suspended solids or foreign matter in the water if possible.

Step 2:  If possible, scrub interior surfaces of storage or reservoir if applicable with a strong chlorine solution containing ½ gallon household bleach, or ¼ gallon of pool chlorine to each 5 gallons of water.  Make sure there is adequate ventilation!

Step 3: Inspect the storage tank for cracks, leaks around the lid or man-way, or vents. Make sure no insects, rodents or other debris can enter the tank during normal operation of the tank and water system, by making sure the lid is tight fitting, and any vents are properly screened.

Use the chart below to decide how much chlorine bleach to put in water tank, to bring up the chlorine residual in the tank to the desired level. For example, you can use the chart below to find out how much bleach to disinfect 1,000 gallons of water and what chlorine to water ratio is needed to treat water. 

NOTE: If you need to use the water in the tank immediately after chlorination, consider adding enough chlorine to bring the levels up to 5 or 10 ppm and let sit for 12 hours or more.  Use 50 to 100 PPM chart only if you are doing a shock chlorination for new or heavily contaminated storage tanks.

Storage tanks: Disinfecting with liquid household bleach (5.25% Sodium Hypochlorite)

Step 4:  Add bleach. If using pool chlorine (12% sodium hypochlorite) use half the amounts below.

Storage Tank Gallons Approx. parts per million of chlorine residual achieved by adding 5% chlorine bleach, in the amounts below.
1 PPM 5 PPM 50 PPM 100 PPM 200 PPM
10,000 25 oz 1.0 gallon 10 gallons 20 gallons 40 gallons
5,000 12.5 oz. 1/2 gallon 5 gallons 10 gallons 20 gallons
2,500 6.75 oz 1.0 qt. (32 oz) 2.5 gallons 5 gallons 10 gallons
1500 1.5 oz (3 tb) 19 oz (2-1/2 cups) 1.5 gallons 3 gallons 6 gallons
1000 2.5 oz. (5 tb) 12.8 oz. (1-1/2 cups) 1 gallon 2 gallons 4 gallons
500 1.25 oz (2.5 tb)  6.4 oz (3/4 cup) 1/2 gallon 1 gallon 2 gallons
250  3 – 4 teaspoons 3 oz (6 tb) 1 qt. 1/2 gallon 1 gallon
100 1.5 teaspoons 1.3 oz (2.6 tb) 1 pint or 2 cups 1 qt. 1/2 gallon

Step 5: Drain and flush tank if using these higher levels of chlorine. Don’t put water with chlorine residual into drains leading to septic tanks, and avoid discharge into creeks, rivers or lakes.

Be careful if you have a steel storage tank, as corrosion may have occurred over time, and once the tank is cleaned it may develop leaks.

A general rule of thumb to shock chlorinate and disinfect a storage tank is to mix non-scented NSF-approved household bleach (5.25% chlorine) in the reservoir at the ratio of 1 gallon of bleach for every 1,000 gallons of water (i.e., 1 quart for every 250 gallons of water).

This will give a chlorine concentration of 50 ppm, far higher than the 0.5 to 2.0 PPM found in treated city water and make the water unusable for potable water use until residual drops down to less than 4.0 PPM.

For storage tanks or cisterns fed by well water: Add bleach directly to the storage tank at the same time you are disinfecting the well. Let the storage tank drain into the distribution system.

After sitting for 12 to 24 hours, drain the storage tank through a drain valve or through the distribution system.

Do not dispose of chlorinated water into septic tank or on vegetation or into surface water.

If you cannot find NSF certified chlorine bleach use NSF certified chlorine pellets or powder.

Click to see Well Safe Sanitizer Kit.

Using Calcium Hypochlorite Granular or Pellets

Storage tanks: disinfecting with dry 1 gram chlorine pellets, or chlorine granules.  Do not use pool bleach.  Use calcium hypochlorite for potable water.

1 cup pellets = 200 pellets = 1/2 lb 2 cups pellets = 400 pellets = 1 lb
25 pellets in 100 gallons = 50ppm 50 pellets in 100 gallons = 100ppm
Storage Tank Gallons Approx. parts per million of chlorine residual achieved by adding dry chlorine pellets in the amounts below.
1 PPM 5 PPM 50 PPM 100 PPM 200 PPM
10,000 1 cup 2.5 cups 5 lbs 10 lbs 30 lbs
5,000 1/2 cup 1.3 cups 5 cups 5 lbs 15 lbs
2500 30 pellets 2/3 cup 2.5 cups 5 cups 15 cups
1500 18 pellets 1/4 cup 1.5 cups 3 cups 9 cups
1000 12 pellets 32 pellets 1 cup 2 cups 6 cups
500 6 pellets 16 pellets 1/2 cup 1 cup 3 cups
250 3 pellets 8 pellets 1/4 cup 1/2 cup 1.2 cups
100 1 pellets 3 pellets 25 pellets 1/4 cup 1/2 cup
Useful Measurements:
1 cup = 8 fluid oz. 1 pint = 16 fluid oz.
1 quart = 32 fluid oz. 1 gallon = 128 fluid oz.
1 teaspoon = 1/6 fluid oz. 3 teaspoons = 1 tablespoon
1 fluid oz. = 29.6 milliliter (ml) 20 drops = 1 ml.


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  • Clean Water Systems & Stores, Inc., Water Treatment Equipment,Service & Supplies, Santa Cruz, CA

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