What's in your tap water

[chem geek]

This is actually quite common and we already have monochloramine in our tap water where I live. The reason for the switch is that chlorine produces disinfection by-products and some of these may cause cancer (don't worry -- it doesn't happen very much at all in pools due to the CYA that makes effective chlorine concentrations MUCH lower and therefore DPB formation MUCH slower so it doesn't build up). For drinking water, that can be a problem. Also, the chlorine does not tend to last very long in the piping system since it combines with organics along the way and may get used up thereby letting bacteria grow down the line.

Monochloramine is only introduced at the rather low rate of 1 ppm and it reacts much more slowly (about 50 times slower) so lasts a lot longer. It is still powerful enough to kill bacteria since exposure times are still long and remember that chlorine in pools with CYA is really only equivalent to 0.1 ppm FC with no CYA so the net effect is that monochloramine at 1 ppm kills bacteria around 5 times more slowly than in a pool with 0.1 ppm FC and no CYA (3.5 ppm FC with 30 ppm CYA) -- so instead of 7 seconds to kill half the bacteria, we're talking 35 seconds (so instead of 1.6 minutes to kill 99% of the bacteria, we're talking 8 minutes). The other main advantage is that monochloramine does not form disinfection by-products.

As for the effect in a pool, it is minor unless you've got LOTS of evaporation. But if you've got lots of evaporation, then that means you've got lots of sunlight so odds are the monochloramine will break down quickly so long as you have chlorine in the water along with that sunlight. Yes, this does mean there's another source of chlorine consumption, but it only takes 0.5 ppm of chlorine to break down 1 ppm of monochloramine (I won't go into the 10x rule here and how that doesn't apply) and even with lots of evaporation you're only talking about a fraction of your total pool water volume. So perhaps with a 6" evaporation in a day out of 5 feet of depth that's 10% so that means a chlorine consumption per day of 0.05 ppm. That's not even measurable, so I wouldn't be concerned.

Richard