CYA and effects on ORP

Read something the other day that said CYA greater than 30 ppm renders ORP readings nearly useless, and it got me to wondering. We watch ORP closely on our trailer to protect our membranes, and the system shuts down if we exceed about 500 ORP.

We definitely do not want to damage the membranes, but I tire of getting a low (1-2 ppm) reading of the chlorine (which our injection pumps can easily handle) yet still see the rig shut down. With adjustment we can make this work, but I wonder if we may just be getting an elevated ORP reading due to the high CYA (although I would think that would be the opposite).

We use sodium metabisulfite to neutralize the chlorine in the injection pump. I can see a quick result in chlorine drop when adding directly to the pool, but it seems to "lag" at the trailer and the sensor there.

Any ideas?

Replies

Clemente Rivera April 1, 2014 at 11:15am

Bruce:

Old post...

Question, you mention membranes...are you referring to RO?

if so, lowering chlorine with Carbon Filters does the trick.

Otherwise, explain your process, and I can tell you what Id advise.
Bruce Wettstein March 30, 2012 at 9:52am

But it may be a good choice, Al!

I'd really just like to find the best product (whatever that might be!) to address this issue. I am not opposed to anything that is safe and would speed up reaction time!
- John > Bruce Wettstein May 7, 2012 at 10:14am
  
  How about an inline static mixer, that would mix the two in far less time, doesn't add much to the operating head and wouldn't require heating.
Al Neumann March 30, 2012 at 9:48am

Bruce,

I wasn't recommending using thiosulfate over sodium metabisulfite. I just know that thiosulfate lowers the pH, and thought the sodium metabisufite might be doing the same thing.
Bruce Wettstein March 30, 2012 at 9:16am

Great comments, Al; thank you! I would like to say that we have thought through all of the scenarios, but that would be a lie!

I have had some conversations (with Dave Rockwell) about using thiosulfate, and I need to check with my RO man as to any adverse conditions that might arise from this. We use a food grade SMBS just to be safe, and I would like to make sure that we could get something similar if we go that route (even though we know thiosulfate is acceptable for swimming pool usage).

I will look further at your suggestions to see what I can learn. Some of this has been discussed/considered, but I have no problem revisiting or learning more!
Al Neumann March 30, 2012 at 8:49am
Bruce,

Some things come to mind in this discussion that may or may not be applicable. However, they may be food for thought. It could be that it’s not just a high cyanuric issue.
1. If I remember right, ORP sensors sometimes have a lag time in their readings to immediate changes in chlorine. This is seen with heavy bather loads, that show an ORP change, but not an so maybe that is what is happening.
2. Also, am wondering on the pH of the sodium metabisulfite, which if low, like in thiosulfate, will give a higher ORP for any given ppm of chlorine. In your case, this would be a false measure of the real chlorine/orp relationship, meaning very low levels of chlorine maybe over your 500 mV threshold due to lower pH.
3. Many controller manufacturers warn against doing any calibrations right after additions of chlorine neutralizers because of false readings that maybe occurring at the time. I'm not exactly sure what specific reasons they are referring to here, but maybe affecting the sensor .
Bruce Wettstein March 29, 2012 at 8:02am

Good point! I honestly can't remember when we saw this start occurring, but it may very well have been since the season ended and we just did not note that.

I think I might start a log with water temps and graph that. We keep water readings so the we know what to expect for run times, but we have not done the same with water temp. If nothing else, it will be interesting to see what other changes we note from cooler to warmer water.
- Wes Burdine > Bruce Wettstein April 21, 2012 at 3:05pm
  
  Would simply heating the water, prior to injection solve the time issue, or I am just missing the point? If it does, maybe there is an ideal temperature for the reaction to occur.
  
  If so, I would think this could be done with a small Coates heater put in line on your trailer and powered by the generator..
Bruce Wettstein March 29, 2012 at 7:41am

You are correct again, Richard, on the injection point. I would also agree that there does not appear to be enough time between the point of injection and the time the water gets to the ORP sensor (although I did not know the "burn time" on SMBS effect on chlorine).

We have occasionally treated the entire pool (one of the nice things about our system is that the pool can be used during the process) when we have high FC and have had to deplete it quickly. This has always been in pools that are not being used, as that obviously leaves no sanitizer in the pool. We prefer to let the injection pump do its job whenever possible and control this in the trailer, but we seem to be getting high ORP readings on every pool lately, and we didn't see this some time back (we have checked the sensors).

Perhaps we need to look for a better (faster) neutralizing agent that would not take quite as long, and would not affect the membranes or the bathers if the pool was being used. We have been able to work around the high reading with some diligence, but it would be nice to minimize the set up time and get to the next job, and currently the time to deal with this impedes that.
- Richard A. Falk > Bruce Wettstein March 29, 2012 at 7:54am
  
  If the problem is more recent, then perhaps it is due to the colder temperature of the water in the pools you are dealing with this time of year. Colder temperatures slow down all reaction times so would slow down how quickly chlorine gets released from CYA and would slow down how quickly it reacts with the chlorine neutralizer.