One lesson we learned involved cyanuric acid, outdoor pools, and chlorine.  The accepted dogma is that cyanuric acid is required for outdoor pools and spas to stabilize the chlorine against UV degradation.  In fact, most granular or solid chlorine sold in stores is stabilized with cyanuric acid.  Dichlor and Trichlor have cyanuric acid in the formula.

When cyanuric acid interferes with chlorine

We started to try and understand the chemistry and science of cyanuric acid because of its side effects.  Cyanuric acid above a certain concentration (which is dependent on pH) inhibits chlorine’s (hypochlorous acid to be precise) ability to oxidize bacteria.  Failure to oxidize means no killing.

We also found that cyanuric acid is denser than water so it sinks to the bottom of a body of water.  Therefore, the level of cyanuric acid on the surface of the pool or spa is the lowest level in the pool and it increases from there to the bottom.  It will be the highest in the deepest part of the pool.

We tested this at the Olympic-sized pool.  We sampled water at the bottom, middle and top of the pool.  The cyanuric acid was set for 40 ppm.  At the surface the level was 30-40 ppm, in the middle it was 60-70 ppm and at the bottom it was 100 ppm.  From the middle of the pool to the bottom hypochlorous acid was essentially ineffective.

The other fact about cyanuric is that it is nonvolatile.  That means as you add more and more to your pool or spa the concentration continues to increase.  The only way to decrease the concentration is to empty some water and replace it with fresh water without cyanuric acid so you dilute out the chemical.  In places where the spa or pool is full all year long, the concentration of cyanuric acid can increase to the point where the pool has no effective chlorine.  I think this is why most pools have algae outbreaks starting in the bottom of the pool.  The high cyanuric acid levels inhibit hypochlorous acid so no killing of algae occurs.

The experiment

So, after we learned this, I decided to decrease the cyanuric acid level in the pools gradually to see if it is really needed.  The pool engineers told me “if you do that there will be no free chlorine in this pool in the morning.” We agreed to decrease cyanuric acid by 10 ppm each week and monitor the results.  The free chlorine levels never decreased and the combined chlorine remained at 0.  We decreased the cyanuric acid to zero and never added any more for the rest of the summer.  The levels slowly decreased to zero as makeup water diluted out the cyanuric acid.  The children’s activity pool behaved exactly the same.

In another pool we treated we were able to manage the large pool all summer without any cyanuric acid and maintained free chlorine levels from 1-3 ppm with no combined chlorine all summer.

Water treated with moss doesn’t need cyanuric acid

The bottom line is that with moss treated water, cyanuric acid is not needed.  The mechanism for this probably centers around biofilm.  I don’t think that cyanuric acid prevents chlorine from UV degradation or the free chlorine levels would have decreased in the outdoor pools we treated.  We know the moss inhibits biofilm formation in the laboratory and know that biofilm absorbs chlorine.  We know that free chlorine levels skyrocket when moss is added to the pool and to maintain a level of 1-3 ppm free chlorine, the chlorine added to the pool decreases by over half.  So a pool with moss doesn’t need cyanuric acid.  That allows the chlorine added to the pool to remain active providing effective microbial control.

“It’s a completely different pool!  My eyes and the kids’ eyes didn’t get red, the water didn’t smell of chlorine and it was a lot softer feeling in general,” said 19-year-old Mary Schmidt, a regular weekly visitor to the pool as a summer nanny.  “I love it!”

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