How do water treatment plants neutralize acid rain?
Don't think they do to any appreciable extent.
Water treatment plants are built to clean the water for domestic consumption. They usually draw water from rivers, lakes or streams and then kill bacteria, filter out sediment and other techniques to make it safe to drink. The intent is not to neutralize acid rain. However, having said that if the water is very acidic, they might add a chemical to bring the ph up a bit.
Acid rain tends to fall everywhere and causes damage to both land and aquatic ecosystems. The solution is not through water treatment plants, but though reducing emissions of SO2 and NOx that cause the problem in the first place. North America and Europe have made solid progress in this regard, but acid rain remains a problem in many developing countries.