This post is in response to this post from piku.
Either chlorine or MPS can be used to get rid of ammonia. [EDIT] Not true -- monochloramine is only very slowly oxidized by MPS so ammonia is also unlikely to be oxidized by MPS especially in the presence of chlorine that VERY quickly reacts with ammonia to form monochloramine. [END-EDIT] I disagree that using chlorine to do so creates problems that are harder to get rid of. I've heard that before and yet the breakpoint chlorination models don't support that except with extreme excessive chlorination. I've walked through several pool owners opening their pool to lower CYA and high ammonia and have had them get rid of it using chlorine with no leftover Combined Chlorine (CC), but they had 3-6 ppm ammonia, not 15 ppm that you had.
Basically, 4.2 pounds of non-chlorine shock in 10,000 gallons is equivalent to 10 ppm FC of chlorine (1.6 gallons or 2.1 96-ounce jugs of 6% bleach). Given that non-chlorine shock costs $30 for 12 pounds for the least expensive one I could find (more typical is $95 for 20 pounds), that's $2.50 per pound or a little over $10 to be equivalent to 2 jugs of 6% bleach that are usually around $2.50 or so for the pair. It is more economical to use chlorinating liquid or bleach instead of MPS.
When closing a pool over the winter and letting it go by not adding chlorine to it regularly or having sufficient algaecide, then soil bacteria that gets into the pool can grow and convert Cyanuric Acid (CYA) into ammonia via the degradation pathway shown in this link. The net equation is as follows:
C3H3N3O3 + 4H2O --> H+ + HCO3- + 3NH3 + 2CO2
Cyanuric Acid + Water --> Hydrogen Ion + Bicarbonate Ion + Ammonia + Carbon Dioxide
So the net result is a lowering of pH with some release of carbon dioxide and increase of carbonates in the water (the TA remains unchanged for technical reasons I won't get into here). For every 10 ppm CYA that is decomposed, it produces around 3.3 ppm ammonia (measured as ppm Nitrogen) and would require around 30 or so ppm FC to get rid of it. Fortunately, ammonia is a little volatile so some of it may outgas.
When you first add the chlorine, it converts the ammonia to monochloramine which registers as Combined Chlorine (CC) and this happens in seconds with no CYA or about 1 minute with 30 ppm CYA.
HOCl + NH3 --> NH2Cl + H2O
Hypochlorous Acid + Ammonia --> Monochloramine + Water
Then, additional chlorine oxidizes the monochloramine to the products shown below and with no CYA in the water that takes around 10 minutes for 90% completion. With 30 ppm CYA, it can take several hours. So generally speaking it is best to not add more CYA until one gets rid of the ammonia. There will be more nitrogen trichloride produced, but that is easily removed through outgassing and breakdown from sunlight (I do not show the more complex reactions below).
HOCl + 2NH2Cl --> N2(g) + 3H+ + 3Cl-+ H2O
Hypochlorous Acid + Monochloramine --> Nitrogen Gas + Hydrogen Ion + Chloride Ion + Water
Note that the above reaction is acidic, but it exactly compensates for the alkalinity (high pH) of hypochlorite sources of chlorine. It takes three hypochlorous acid to oxidize two ammonia. On a ppm basis, this is a chlorine to ammonia ratio of 7.6, but greater efficiency to completion is achieved using a ratio of 8-10 so that is where the 10x rule comes from and it only applies to ammonia measured as ppm Nitrogen, NOT to monochloramine which is measured as ppm Cl2. You can see that it takes one hypochlorous acid to oxidize two monochloramine so that's a little more (for efficiency) then an FC level of half the CC level.
The net reaction of oxidizing ammonia using a hypochlorite source of chlorine may be written as:
3OCl- + 2NH3 --> N2(g) + 3Cl-+ 3H2O
Hypochlorite Ion + Ammonia --> Nitrogen Gas + Chloride Ion + Water