What's in your tap water

[fcfrey]

A local BIG water company announced that they were going to switch from using chlorine to using chloramines (they call it a mixture of Chlorine and Ammonia).

WHY WOULD THEY DO THAT???

I can't help wondering if this is something different than what we, as pool owners, try so desperately to keep out of our pools. If not, if you were to top off your pool from the garden hose on city water, up goes the combined chlorine.

There are a bunch of pool owners in the greater Harrisburg, PA area that will be affected. Unfortunately they don't know about this site and will be going crazy shocking the pool every time they top off and take a sample to the pool store.

Maybe it's a conspiracy

I sure am glad I have a well.

[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

[aquarium]

Huh, I thought this change was mandated by law to go into effect a long time ago. For all the reasons Richard says, it's just a better way to treat tapwater.

If you have any friends with fishtanks, you might warn them. Back when there was a higher frequency of water utilities making this change, the aquarium forums were full of the tragic consequences.

[fcfrey]

Thanks for the explanation.

Since I fill my water bottles at home from my well we almost never drink city tap water, and the pool gets topped off from me geothermal heat pump water discharge, I really don't care much. Just thought it might affect others.

I will warn my friends with the fish tank.