Don't fill your pool with sewage! (or, Using rainwater for recreational water bodies)
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With a cold front blowing in a couple months ago, Task 1 was getting the hot tub in working order. As part of this annual maintenance, we drain the tub and replace filters. After draining the tub, I thought "Why not fill it up with rainwater?" An acquaintance has a full-blown swimming pool and uses rainwater to top it off (not to mention Mother Nature's topping off of open water bodies).
I went to the pool/tub store for a new filter, chemicals, and some advice on using rainwater. When I asked about rainwater, you would of thought I had told them I was filling our tub with raw sewage. They advised against it. "Why?" I asked.
First, I was told that the total dissolved solids of the water was too high and that I would have to use city water to dilute the rainwater and, if I was doing that, why not just use city water in the first place? "Really?!?!" I replied, and then explained that rainwater has very low total dissolved solids (about 10 parts per million) as compared to city water (about 200 parts per million). They then winced and mentioned that rainwater has high metals. I restated the low total dissolved solids number. "Does that include metals?" they asked. "Yes," I replied. and noted that metals. They then mentioned that the pH of rainwater is not right. The pH of rainwater is about 6; the desired pH of water in a hot tub needs to be between 7.2 and 7.6. City water? 9.7, so it's more out of whack than rainwater.
After seeing that they weren't going to be able to talk me out of using rainwater, I got my chemicals, and went on my merry way. Using rainwater for recreational water bodies was more difficult than I thought.