Hard Water Chemistry

Hard Water Chemistry

For swimming pools located in hard water regions, where the tap water has Calcium Hardness levels of over 400 ppm, managing water chemistry takes a specific focus.

The ideal range for Calcium Hardness in swimming pools is 180 – 220 ppm, although others may use 200-400 as an acceptable range of hardness.

Maintaining a Hard Water Pool

There are many pools that operate in the 400-800 ppm range, with special care of pH and alkalinity, and use of a Stain & Scale chemical, to keep the water clear and prevent precipitation and scale formation on plaster, steel and tile surfaces.

The most important thing with hard water pools is to maintain low pH and low Alkalinity levels, especially as the water temperature increases each summer. Hard water pools are less likely to become cloudy or produce scale when the pH level is kept around 7.2 and Alkalinity is below 100 ppm.

The second most important thing with a hard water pool is to make regular use of a good Stain & Scale chemical, aka sequestrants or chelators. These chemicals bind up minerals and metals to keep them dissolved, to prevent staining and scaling, but they deplete rapidly. Add an initial dose each spring, and maintenance dosages every week or two, for continuous protection from cloudy water, stains and scale.

Thirdly, for pools with high calcium hardness levels, you may want to discontinue use of calcium hypochlorite pool shock, which adds a small amount of calcium to the pool when used. You can use Dichlor, Lithium, Non-Chlorine Shock or bleach as alternatives to Cal Hypo pool shock.

To more accurately determine your pool water’s propensity for scaling, which is the main concern with hard water pools, you can run a Langelier Saturation Index, using the Pentair LSI calculator.

Reducing Pool Calcium Hardness Levels

DILUTION: Lowering calcium hardness by dilution (draining and refilling with softer water) is often the easiest method, and for many it’s the only solution that is recommended. If your tap water or treated and softened house water is much lower in calcium than your pool water, and you are allowed to drain and refill in your area (no water restrictions), lowering by dilution can be the best method to reduce water hardness in swimming pools.

HOME WATER SOFTENER: Many homes that draw their water from wells are equipped with a water softening system, limestone filters that exchange calcium ions for sodium ions. But most outside hose bib spigots aren’t connected to the water softener system. You can connect a hose to fill the pool with filtered, softened water by connecting a hose after the sediment tank or in the kitchen or laundry room. You may not want to fill continuously, but most systems can handle 8-12 hrs per day of filling the pool.

DELIVERED POOL WATER: In areas where pools and water wells are common, there are companies who deliver pool water taken from nearby municipal water supplies (fire hydrants), after treatment and softening. It’s not a cheap option, often costing $250 per 5000 gallons, but sure is fast and easy way to refill the pool with softer water.

POOL WATER RECYCLING: Available in limited areas with hard water and drought restrictions, there are companies who bring a truck mounted reverse osmosis filter system to your house, and pump the pool water through a series of filters and treatments, to deliver perfectly balanced pool water back to your pool in just an hour or two.

PRECIPITATION: Raising the pH to 10.0 causes dissolved calcium carbonate to come out of solution, or precipitate, clouding the pool. Allow it to settle, and then vacuum to waste. To use this method, you must have a means to bypass the filter by setting multiport valves to Recirculate, or removing internal cartridges/grids, to prevent clogging the filter with carbonate scale. You must also have a way to vacuum to waste, by setting multiport valves to Waste, or other ways to vacuum to waste, so a dusty layer of calcium scale can be vacuumed out of the pool.

  1. Place the filter system on Recirculate or remove filter media.
  2. Raise the pH to 10.0 by adding ph Increaser. Check poolcalculator for dosage amounts.
  3. Circulate pool for 8-12 hours, water should be cloudy, retest pH using diluted sample*.
  4. Shut off filter system for 8-12 hours, allowing precipitate to sink to the bottom.
  5. Add water to fill the pool high, and vacuum to waste to remove precipitate.
  6. Lower the pH back to 7.2-7.4, with a pH Decreaser+.
  7. Test and adjust Total Alkalinity reading back down to 80-120 ppm.
  8. Test your Calcium Hardness level – it should be lower.
  9. Add a Stain & Scale chemical like Scale Free to bind remaining calcium in solution.

Steps 7 and 8 may take a few times to get right, you may need to test and adjust several times to bring pH and Alkalinity back in line. If pool is still cloudy after the pH and Alkalinity have been lowered, use a clarifier or flocculent to assist the filter. Warm water can help remove more calcium as it’s more soluble in colder water. The pool should be clean and algae free with a good chlorine level before beginning.

*When testing pool water that is off the charts, or off the scale, you can use a diluted test sample. A diluted sample is where you are diluting the test sample with water of known chemistry, from another source. For testing pH levels of 10 on a test kit that only goes to 8.4, you can fill the pH test vial 50% full of pool water, then add water from the kitchen sink (tested to a known pH level) to fill the remaining 50%. Test the combined sample pH and double the difference to approximate pool pH.

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