This may be a bit of an odd question, but where I live, the well water is extremely bad. The amount of sulfer in the water makes it unusable for anything other than washing dishes and laundry. I had a chlorine injection system installed shortly after buying my home and it does a great job removing the odor. In 6 years I have not drank the water or cooked with the water, or let my pets drink it. It's disgusting to be honest.

I installed a reverse osmosis (RO) system that is now tapped in (using a saddle valve on the PVC) before my softener and my chlorine system, whereas up until recently, was AFTER both the softener and chlorine systems. This RO system will soon fill (I am still working on this) two 55 gallon potable water barrels to have on hand to top of my tanks after cleaning/partial water changes.

Previously(when the RO was installed after the softener and chlorine), I would disable my softener and chlorine system, and run the water until it stinks to make sure all the chlorine and treated water passes. I would then turn on my RO system to top off my tanks (125g and 180g tanks). This takes an incredibly long period of time. The RO system does roughly 5 gallons an hour optimally. I could need 30-50 gallons of water to top of the tank(s) after a quick cleaning, which means ~10 hours of having my softener and chlorine systems disabled. During this time, I can't take a shower, flush the toilet, run any water during this time, or else my house will smell of rotten flesh and rotten eggs. It's VERY inconvenient.

With the old setup (RO after the softener and chlorine), If I leave the softener and chlorine systems enabled, the RO water is very high in nitrates, ammonia, and basically the levels are bad enough to kill my fish. This just happened recently. I had 10+ freshwater fish die from a water change.

I want to know if my treated water could backflow into my RO system. I only thought of this after I tapped into the water just after the well pressure tank.

My water system is currently setup like this:

  1. Well to 27 gal pressure tank,
  2. pressure tank to chlorine system (pressure tank injected by a stenner pump),
  3. chlorine pressure tank to softener,
  4. then onto the house plumbing and hot water tank

The softener is after the chlorine system because previously, when the softener would run, my whole house would smell INCREDIBLY horrid(softener was before the chlorine). The place that installed a NEW softener(and also did my chlorine system) said it would have to be installed after the chlorine system to avoid this.

So now, for my RO system's water supply, I tapped into the main water line just after the well pressure tank thinking that since it was before the softener, chlorine, etc, it would be feeding my RO system with un-treated water. But after thinking about it, I am concerned that treated water could backflow into my RO system's intake.

I hope this makes sense. I've searched and searched and haven't been able to decide one way or another.

Sorry if my terminology is off, but I am not a plumber (you guys are WAY underpaid).

  • It doesn't sound likely it would backflow, because there would be pressure on it at all times. Don't have time for a full answer right now but -- ack, chlorination in a home system. When chlorine interacts with organic particles it can form byproducts that are linked to cancer (most notably THM): some futher info here. There's also dealing with a dangerous chemical. Anyway, you don't drink the water, which is good, but thought I'd post anyway for anyone else that comes across this.
    – gregmac
    Jan 6, 2015 at 8:09
  • 1
    Install check valves, and you'll never have to worry about backflow.
    – Tester101
    Jan 6, 2015 at 11:06
  • I'm not familiar with chlorine injection of this type. So forgive the question.... Does the water have to be softened after being chlorinated? Could you do it in the reverse order? If so, you could extend the life of the RO membrane considerably by using softened water instead of well water directly. Not all fish, however, are going to like the extra sodium... African cichlids and the like would be completely fine, but Amazon species are not likely to tolerate it too well. Jan 6, 2015 at 20:28
  • My water is unbearable without the chlorine system, believe me. I almost backed out of the home purchase until I was assured it could be "fixed". I am not happy with the "fix" because it's an on-going cost to treat my water, and maintain the injection system. I already posted about purchasing a check valve, that will give me peace of mind, along with testing my RO water before using to top off the tanks. My softener has to be AFTER the chlorine system because of the HORRID stench that fills my house when the softener "charges". It used to be setup the opposite way and I had to disable it. Jan 8, 2015 at 5:50

1 Answer 1


It can backflow if you lose pressure. Perhaps from a power outage, or if you draw water faster than the pump can replenish it.

Backflow valves (check valves) are pretty cheap (about $5 - $10). If the consequences are as severe as you make them out to be, I'd put one in.

Also I would add a carbon filter after the RO system, that would remove ammonia and other things, just in case. If the water was clean the filter would not get worn, so it's cheap insurance that you can leave there for a long time without replacing the filter.

PS. It's good you are not trying to drink softened water - the extra salt is not good for you.

  • Upvote on check valve suggestion... Another possible consequence of backflow: If your RO unit carbon pre-filter is near end of life, or your chlorine concentration is really high, then you could destroy the RO membrane. The RO membranes I'm familiar with are destroyed by chlorine... hence the carbon pre-filters. Jan 6, 2015 at 20:23
  • Thanks guys, I did order a check valve tonight, I think it will (as noted by more than one of you) be an easy preventative solution. My RO system currently has a pre-carbon filter, and post-carbon filter, in addition to the RO system's included filters. I Jan 8, 2015 at 5:42

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.