Well water & Chlorine contact tank time?

[HarryCW]

I'm a spa newbie, zero experience, preparing to fill my new-to-me (old) spa for the first time. From my measurements, I'm guessing the tub's got about a 250 gal capacity.

We have well water, with a chlorine injector system along with a 120gal contact tank, which deals with the iron bacteria and sulfur, followed by a whole-house carbon filter (w/ backwash capability), and then the softener. I do understand it's best to bypass the softener to fill the hot-tub to avoid "softened water" problems.

Using a Taylor kit, I've measured the free chlorine in the water between the contact tank and the carbon filter, it reads between 1.5-2.0ppm at that point, and I've measured the free chlorine after the carbon filter, using my (more precise?) test kit for household water, and I get zero-ppm. No chlorine smell and no sulfur at that point, after the filter.

My question is for people who understand these kinds of contact-tank systems.... how long does the treated water need to stay in the contact tank, in order for the chlorine to 'do its thing' at killing the iron bacteria? ie: How much, and how often, can I drain out of this 120gal tank in one go, before I'd simply be pulling untreated water through?

Would it be best to only take out 75 gallons or so at a time, so the next filling of the contact tank has time to work? And then should I let it sit overnight, run the carbon-filter backwash, and then tap out another 75 gallons?

Or does the contact tank need more time to treat the water than that?

I did try filling the tub all in one shot a few months back (a "test fill"), and by the time it was full the water that was coming into the tub was smelling like sulfur.

Is there a better way to do it?

Thank you for any help!

--

Harry

Edited March 11, 2023 by HarryCW
to clarify

[HarryCW]

Now that I think about it, I wonder if the sulphur smell at the end of my "test fill" was only due to the carbon-filter needing a backwash.

Is the time in the contact-tank really that important, or is simply mixing the Clorox with the iron bacteria enough to kill it? 

Thanks for any input.