Extreme ammonia levels

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Extreme ammonia levels

Postby Larry » Sat 27 Mar, 2010 04:08

I have just taken over the maintenance of an indoor heated pool. The water has a recent history of high ammonia levels, with levels reaching 4.5ppm at times over the last 3 months. The pool is filled from an artesian well which has not been analyzed, so the water quality is unknown.

The water quality at present is:

FC: 0 - 3 ppm
TC: 0 - 3 ppm
pH: 7.2
TA: 100
CH: 250ppm
CYA: 40ppm

The pool: 75,000gallons, tiled, indoor heated (80 degrees F) with exposure to sunlight
Pool chemicals: sodium hypo, trichlor, chlorine dioxide

Since my first visit to the pool 2 days ago I have added in total about 15lbs trichlor and 5 gallons chlorine dioxide (3%). I added the trichlor in 5lbs increments. Prior to chlorinating, the FC is zero and Combined Chlorine either zero or around 3ppm. The FC and CC increase for about 6 hours, then CC falls to zero after a further 6 hours, leaving an FC level of 1-1.5ppm. 6 hours later the FC is again zero and CC climbs to 3ppm.

I am at a loss as to what is happening and would appreciate some insights into the underlying processes. I appear to be reaching breakpoint chlorine levels but the FC rapidly degrades to CC each time. I added 5gallons of chlorine dioxide about 12 hours ago and FC is now 1.5ppm, CC is zero. I have no doubt that this will change at my next test.

Update: Yup! 5 hours later and FC is 0.5ppm, CC is up to 3ppm again. At this rate I won't have any hair left by the end of the week. 5 gallons of sodium hypo (12%) going in at the end of the day.


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Extreme ammonia levels

Postby mas985 » Sat 27 Mar, 2010 10:34

I'm sure Richard, aka chemgeek, will post soon but until he does, from many of his posts in the past, he has mentioned that ammonia uses a lot of chlorine. How much I'm not sure but to break it down requires quite a bit of chlorine.

Also, sometimes the source of ammonia is CYA and a bacteria which coverts the CYA to ammonia. Has the CYA level dropped recently?
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Extreme ammonia levels

Postby chem geek » Sat 27 Mar, 2010 11:41

Most ammonia test kits actually measure the sum of monochloramine and ammonia. The Combined Chlorine (CC) test measures monochloramine and other combined chlorines such as chlorourea. So you need both measurements to know the true ammonia level and note that ammonia is measured in ppm N units while FC and CC are measured in ppm Cl2 units. That means that a given amount of monochloramine will measure 5 times higher as CC than as ammonia.

It takes around 8 times the ammonia level as cumulatively added FC to oxidize it. So with 4.5 ppm ammonia, that's 36 ppm FC. However, it may require more than that, perhaps double, if bacteria converted CYA into ammonia since there may be intermediate products of that conversion not showing up in any test. See this link for more technical details. When you add FC, it very quickly becomes monochloramine so CC. Eventually, added FC will start to reduce CC and the FC may slowly drop over hours. At some point (after adding a lot of chlorine), adding FC will hold and the CC will be minimal.

You should not add any more stabilized chlorine since you've already got a CYA of 40 ppm and for indoor pools I usually recommend only 20 ppm since you generally want faster oxidation (i.e. a higher FC/CYA ratio of around 20%) since there is no sunlight available. Also, you really should be using a FAS-DPD chlorine test so you can get FC and CC readings better than 0-3 ppm (or are these varied readings over time?).

As Mark noted, did the FC in the pool get to zero at some point and did the CYA level drop? If so and if you measured ammonia, then it's likely bacteria converted some or all of the CYA into ammonia. Keep adding unstabilized chlorine until the FC starts to hold. You can read about my own personal experience with this in this thread where you can see that it takes a whole lot of chlorine before it starts to hold.

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