Chlorine contact tank contact time, for iron bacteria?

[Hcw3]

I'm a hot-tub/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 will bypass the softener to fill the hot-tub to avoid "softened water" problems.

Using a Taylor pool/spa 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 about the contact-tank system.... how long does the chlorine-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 pull 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?

Is overnight long enough 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?

Thanks much for any help!

--
Harry

 

[Reach4]

Where does your chlorine get injected -- before or after the pressure tank?

What controls when the injection pump runs?

 

[Sponsor]

 

[Hcw3]

Thanks @Reach4.

The chlorine injector is between the pressure tank and the contact tank. The injector pump runs whenever the well pump is running.

 

[Hcw3]

Now that I think about it, I'm wondering if the sulphur smell at the end of my "test fill" was possibly only due to the carbon-filter needing a backwash, after so many gallons being run through it at once.

Is the time in the contact-tank important for the function, or is simply mixing the Clorox into the water with the iron bacteria enough to kill it? There is a turbulator type of mixer at the injection point.

Thanks for any input.

 

[Bannerman]

And then should I let it sit overnight, run the carbon-filter backwash, and then tap out another 75 gallons?

Chlorine when fully mixed into the water can sanitize in as little as 3-minutes. Longer contact time will assist to ensure the chlorine will be thoroughly mixed with the water and will allow time for bacteria neutralization and for ferrous iron & suphur to become oxidized into a ferric state so the resulting iron/sulphur solids can begin to precipitate out from the water, to accumulate on the bottom of the tank, where they may be periodically flushed out to drain.

While this is a simplified description of how a contact tank functions, if your hot tub is being continuously filled @ 5 GPM, assuming the incoming chlorine/water is thoroughly mixed before entering your 120 gallon contact tank, there will be ~24 minutes contact time before that water exits the tank to flow to your hot tub. If flowing @4 GPM, 30-minutes, 6 GPM, 20-minutes, 7 GPM, 17-minutes etc.

 

[Hcw3]

@Bannerman, thanks! I should have realized the obvious, The reason there's a tank rather than just a pipe, is to extend the exposure time...

I found a site that describes contact tank 'science' and actually gives me a formula to use to calculate - I haven't looked it over thoroughly but I'm hoping I have the numbers that I'll need to input. https://www.highlandtank.com/chlorine-contact-tank-how-it-works/

I guess another way I could do it would be trial and error flow-rate into the hot tub, stopping at intervals for a filter-backwash.

Thanks again.

 

[Reach4]

Whatever else, it is good to do a sanitizing of well and plumbing periodically. For me, the period is about 3 years, but others need it more often.

https://terrylove.com/forums/index....izing-extra-attention-to-4-inch-casing.65845/ is my sanitizing write-up.

You will also want to do a flush of your pressure tank. Based on what you see, you can decide how often to do that.

Precharged Pressure tank flush:
1. Connect a hose to the sediment drain valve, and run that to where you plan to drain the water. I suggest filtering the output through a cloth if you suspect the sediment may include sand.
2. Turn off the pump.
3. Open the drain valve, and let it drain until the water stops. It would be possibly interesting to watch the first water that comes out.
4. Close the valve, and turn the pump back on, and let pressure build.
5. Repeat steps 2, 3 and 4 as needed.

 

[Hcw3]

Whatever else, it is good to do a sanitizing of well and plumbing periodically. For me, the period is about 3 years, but others need it more often.

https://terrylove.com/forums/index....izing-extra-attention-to-4-inch-casing.65845/ is my sanitizing write-up.

You will also want to do a flush of your pressure tank. Based on what you see, you can decide how often to do that.

Precharged Pressure tank flush:
1. Connect a hose to the sediment drain valve, and run that to where you plan to drain the water. I suggest filtering the output through a cloth if you suspect the sediment may include sand.
2. Turn off the pump.
3. Open the drain valve, and let it drain until the water stops. It would be possibly interesting to watch the first water that comes out.
4. Close the valve, and turn the pump back on, and let pressure build.
5. Repeat steps 2, 3 and 4 as needed.

Thanks @Reach4 ... I haven't sanitized the casing in years. I appreciate your reminder. Will check your write up, thanks in advance for that.

I don't think I've ever flushed my pressure tank, except years ago with an old galvanized one that was getting plugged at the outlet, if I recall. It was a rental house, and the landlord was absentee... The whole system should have been replaced, but instead we moved out, and bought our own old house.... (This one has a modern water system)

Thanks again.