Cities do that because chlorine dissipates quickly. Adding the ammonia to the chlorine, creates chloramine.
Chloramine isn't as powerful for sterilization, but it does not evaporate from standing water like chlorine does.
People who have chlorine in their city water, would often just fill up drums or tubs, outside in the sun, and let it sit for a few days, and the chlorine evaporates. They then water their plants.
That trick doesn't work with chloramine.
But a carbon filter that works for chlorine, will also work for chloramine.
Other tricks to get rid of chlorine:
-shock with chlorine, which frees up the previously bonded chlorine, then chlorine will evaporate using normal methods (sun and air).
So again, chlorine and ammonia make chloramine. Which is also referred to as "bonded chlorine". Cause it's bonded to the ammonia.
Anyone who has a pool and cares for it themselves, knows about unbonded vs bonded chlorine.
When your pool has plenty of unbonded chlorine, the water is blue and clear.
When your pool has almost no unbonded chlorine, it's all bonded, the water is tinted gray/green, and is clear.
Chloramine is a weak sterilizer, but stays in the water for a long time.
Large cities switch, because they can add the chlorine/ammonia to the water at fewer distribution points, because the bonded chlorine (aka chloramine) isn't going to dissipate over long distances. With unbonded chlorine (what most people know as regular chlorine), they have to setup more distribution points so that even the outskirts of the city get disinfected water.
Sorry for the longwindedness. It's the end of the day, and my brain is fuzzy.
Another thing about chloramine, is that it stinks. Unbonded chlorined diluted in water is difficult to smell. Most people who think they smell "chlorine" in their swimming pool, are actually smelling "chloramine" (aka bonded chlorine) instead. Which means the free chlorine has been doing it's job and neutralizing organic matter in the water, by bonding with it. It essentially means the pool does not have enough free chlorine, and needs more. (of course if your pool water PH is off, you'll never the chlorine levels right..., but that's another story.)
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