Do I Need To Shock?

[Dietitian]

After installing an ozonator, I have no combined chlorine in my water for any of my last five tests. I assume the ozone is burning up the chloramines and will continue to do so. If this is the case, would I ever need to shock the water to 10ppm?

Is there some other benefit to doing so?

What should I shock to with CYA at 30?

[chem geek]

Even without an ozonator and properly maintaining and using enough chlorine you would normally not need to shock. Shocking is most often a "catch-up" when people aren't using enough chlorine in the first place.

[Dietitian]

Even without an ozonator and properly maintaining and using enough chlorine you would normally not need to shock. Shocking is most often a "catch-up" when people aren't using enough chlorine in the first place.

What happens to the chloramines created by the oxidation of waste? My understanding is that they remain in the water until 7.6x dose of chlorine is added, or UV burns them off, or an ozonator destroys them, or perhaps MPS kills them.

With daily small dosing of bleach, where do the chloramines go?

[chem geek]

What happens to the chloramines created by the oxidation of waste? My understanding is that they remain in the water until 7.6x dose of chlorine is added, or UV burns them off, or an ozonator destroys them, or perhaps MPS kills them.

With daily small dosing of bleach, where do the chloramines go?

This is more B.S. from the pool/spa industry. The 7.6x rule, which is often quoted as the 10x rule, is based on chlorine oxidation of ammonia where chlorine is measured in units of ppm Cl₂ (i.e. chlorine gas) while ammonia is measured in units of ppm N (atomic nitrogen). There is roughly a factor of 5 difference in these two units of measure. Combined Chlorine (CC), on the other hand, is measured just as Free Chlorine (FC) in units of ppm Cl₂. Also, monochloramine already has 1 of the 1.5 chlorine used in oxidizing ammonia. So it takes no where near 7.6x or 10x of FC to oxidize CC. Even for urea, which is the largest component of sweat and urine, it takes no more than 3x the CC level in the worst case. See this post for more technical details.

Also, the whole point of adding chlorine to oxidize bather waste is to destroy the chloramines. When a proper amount of chlorine is added after a soak, then it oxidizes that bather waste (mostly ammonia and urea plus some amino acids; also dead skin, etc.). You don't need to shock because the oxidation is continuous. Shocking is normally recommended by the industry (but not by us) because most people simply don't use enough chlorine in the first place so they get behind and shocking is a way of catching up.

Your dosing of bleach daily is not small if you are using enough to have 1-2 ppm FC measured by the next time you soak.

When chlorine combines with ammonia, urea or other nitrogenous organics, it forms combined chlorine (CC). With ammonia in particular it forms chloramines. Further chlorine oxidizes these chloramines to nitrogen gas; with urea it also produces carbon dioxide. The only significant, though relatively minor, leftover from these that doesn't get outgassed is some nitrates.

[Dietitian]

The only significant, though relatively minor, leftover from these that doesn't get outgassed is some nitrates.

Are the nitrates oxidized by the ozone?

PS you are a Godsend chemgeek, thank you.

[chem geek]

The nitrates are not oxidized by ozone. They are at the end of the oxidation states of nitrogen meaning they can be reduced, but not oxidized. Nitrogen goes from oxidation states of -3 to +5. Take a look at the table of nitrogen species and oxidation states in this link. The oxidation state of nitrate ion (or nitric acid) at +5 is the end of the line and cannot be further oxidized (it is, itself, an oxidizer, though at E₀ = +0.934V, not as strong as chlorine. The EPA drinking water limit for nitrate is 10 ppm, mostly to prevent problems with infants drinking the water. From what I can tell from breakpoint chlorination models, it is unlikely to get to this level of nitrate increase in the time a residential spa is used before the water is changed.

There are a large number of possible disinfection by-products depending on the types of organic precursors available. I talk about some of these in this post. They are in relatively small quantities, but are the primary focus areas for research into disinfection by-products due to some of them being potentially harmful to health. Note, however, that the quantities in high bather-load pools is generally in the cancer risk increase category of 1 in 1 million to 1 in 100,000 though some get into 1 in 10,000 though there were individual pools getting to risk areas of 1 in 4000 (remember, however, that these risk estimates are based on drinking water at daily 2 liter (roughly 8 cup) quantities every day). Adding the chlorine after a soak may minimize exposure to the volatile disinfection by-products that are then allowed to outgas before the next soak. Spas also have lower by-products than pools, probably due to more frequent water replacement and hotter temperatures outgassing (thus not accumulating) more volatile products.

[Dietitian]

The nitrates are not oxidized by ozone. They are at the end of the oxidation states of nitrogen meaning they can be reduced, but not oxidized. Nitrogen goes from oxidation states of -3 to +5. Take a look at the table of nitrogen species and oxidation states in this link. The oxidation state of nitrate ion (or nitric acid) at +5 is the end of the line and cannot be further oxidized (it is, itself, an oxidizer, though at E₀ = +0.934V, not as strong as chlorine.

Could the nitrates be removed by giving them something to oxidize, or is the only way to remove them via replacing the water?

[chem geek]

Pretty much the only practical option is to change the water, but the nitrates are not normally a problem and as I said, their quantity is quite low in a spa since the water is normally changed regularly. In a pool, the accumulation of nitrates can (along with accumulation of phosphates) promote algae growth, though chlorine alone can inhibit/prevent such growth (that is, one does not have to use an algicide).