Planning for RO treatment

[flynwill]

I have our local reverse-osmosis provider scheduled for later this month. My goal with the process is to get the CH number down from it's current ~725 number to somewhere in the 250-300 range. With that plus adding borates I hope that I can allow the pH to stabilize in the 7.8-8.0 range and still have the CSI remain in the acceptable -0.3 to +0.3 range. That in turn will hopefully reduce the frequency of acid additions from the current pace of every 3-4 days. (I have 15 lbs of boric acid from Duda Diesel sitting in the shed ready to go.)

My working assumption is that in reducing CH, the TA, CYA, Salt numbers will also come down proportionately. I am also assuming that pH should not be particularly effected by the process or perhaps drop a bit. So mapping that out:

	CH	TA	CYA	Borate	Salt	Temp	pH	CSI
Pre RO	725	80	40	0	3200	85	7.5	0.125
Post RO	300	33	17	0	1324	85	7.5	-0.514
After all adjustments	300	80	40	50	2800	85	7.8	-0.014

Assuming this is true, do I need to be concerned about raising the TA before proceeding with adding the boric acid?

My inclination is "no", but to add back the needed CYA (which will raise the TA a bit), get the salt level back to where my SWG is happy (probably 2800) and proceed with adding the boric acid. And the once that it settled add baking soda if necessary to raise TA to get the CSI above -0.3.

The RO folks say that "re-balancing the water is part of the service", but I plan to just waive that off unless the pH does drop too low , in which case I will ask them to add either Sodium Carbonate or Borax to raise it back to ~7.5.

Aside to @JoyfulNoise I did try bringing up the CYA to 60 over the summer, but I can't say that made a major difference in the amount time needed on the SWG. It appeared to be pretty much the same except for a short period where the demand rose which corresponded to the hot spell where the pool was ~90F.

 

[JoyfulNoise]

RO should remove all of the TA (or most of it) and lower the pH of the returning water to below 6.5. The high TA pool water that mixes with the post-RO filtered water will buffer any sudden drop in pH but I would expect the final pool volume to have a low pH, likely around 7.0 or slightly lower. You should have some baking soda on hand to be able to immediately raise the TA to at least 50ppm. They sell 13lbs bags of Arm & Hammer in Costco for cheap. Getting CYA and FC back up to normal levels is your primary objective. You can worry about TA and pH after that. Borates should be the last thing you add.

 

[flynwill]

I do have baking soda on hand. I've got a 5 lb box that I've had for ages which should be more than enough to get the TA back to 50 from the expected 33 or so. It occurs to me that I could also skip the periodic acid addition in the days before the treatment and let the pH rise to 8.2 or better.

I'm not worried about the FC levels. There should be no problem adding salt and getting the SWG back on line within an hour or so of shutting down the RO process. I've also got a several month old gallon of liquid chlorine on hand. Even if it is down to 1/2 strength it's about 8 ppm worth. The CYA will take a day or two to get back up, but rhe pool is nearly always covered so the loss is not very great even without the CYA present.

I'll post a follow up to this thread and let you know how it comes out.

 

[JoyfulNoise]

Generally speaking, when you add salt to a pool you really need to give it 24-36 hours to mix and homogenize. Salt water is more dense than fresh and it will sink and not mix well. TFP gets posts every year of people putting salt in their pool and then posting that they see an unexpected salt level (high or low) and/or their SWG is screaming HIGH SALT at them. It’s usually a mixing/homogenization problem and, after a day or two, it goes away. You can brush like a madman but it’s not as effective as you think. Even running a robot doesn’t do as much mixing as you would expect.

So raising FC immediately using liquid chlorine is typically the best approach and then let the SWG run when the water is mixed. CYA can be added via a bunch of socks at the returns or in the skimmer as long as the pump is running. I would just deploy the CYA in socks and then run the pool as if it were all dissolved. It will eventually get there.

 

[Newdude]

*said high concentrations of salt can wreck the SWG. Its less likely in pools with only skimmers, and more likely in pools with main drains, but evenso, uing the SWG is just not worth it for a day or two.

And HEY !!!! You know how everybody is always obsessed with stretching the SWG life ? Yours will get TWO whole days on the back end.

 

[flynwill]

Generally speaking, when you add salt to a pool you really need to give it 24-36 hours to mix and homogenize. Salt water is more dense than fresh and it will sink and not mix well. TFP gets posts every year of people putting salt in their pool and then posting that they see an unexpected salt level (high or low) and/or their SWG is screaming HIGH SALT at them. It’s usually a mixing/homogenization problem and, after a day or two, it goes away. You can brush like a madman but it’s not as effective as you think. Even running a robot doesn’t do as much mixing as you would expect.

Good point, but our pool has a giant mixer -- the Riverflow swim current generator. Run it for 10-15 minutes and I can pretty much guarantee that the main part of the pool is 100% mixed. On the other hand I do have this aging jug of liquid chlorine that I might as well use up and leave starting the SWG to the next day.

 

[flynwill]

Follow up:
Had the RO process done on my pool yesterday. Nick arrived with his trailer about 10:15 AM and was packed up and gone by 4:15 PM. The actual time spent running the machine on my small pool was about 3 1/2 hours. The before and after number numbers (before any re-balancing) were:

​	CH​	TA​	CYA​	pH​	Salt​
Pre-RO​	750​	80​	30​	7.6​	3200​
Post-RO​	175​	40​	0​	< 6.8​	800​

I checked my water meter before and after treatment, and can say that ~1,600 gallons were used to do my 6,100 gallon pool, so roughly 25%. Nick also over-shot the mark a bit as I had asked him to target getting the CH in the 200-250 range. Through out the process he was monitoring TDS (presumably a continuity type sensor) on the water coming in to the trailer from the suction hose in the deep end. He was also periodically checking the water from various depths to try and gauge how the process was going.

Fairly early in the process I took a sample of the water being returned to the pool and tested it and got CH 50, TA 50. @JoyfulNoise the non-zero TA number was a bit of a surprise, and leads me to conclude that the 50ppm of CH that is getting past the RO filters is almost entirely Calcium Carbonate. I also tested pH, and the normal Phenol Red indicator was bright yellow, indicating something well less that 6.8. I also tried checking pH with Bromothymol Blue that I have on hand, and got a very clear green color which from the various charts I find would indicate a pH of ~7.0. I expect that given the sample is only weakly buffered this variation in pH indications is not surprising.

Once the RO system was shutdown I fired up the swim current and put the filter system in spa-overflow mode for a while to thoroughly mix the pool up before pulling the samples for the "after" testing. I used Borax and Baking soda to get the pH back to 7.5 and the TA up to 60. Had Nick add Calcium Chloride sufficient to bring the CH back up to 225, plus some liquid chlorine.

Today's task will be to clean the filter (not directly connected to the RO but due to be done), and then add back in the salt and CYA. Assuming that all stabilizes I plan to add the Boric Acid later this week.

 

[JoyfulNoise]

What comes out of the pipe really depends on the internal plumbing setup of the RO system. Membrane filters typically work at a specific pressure and TDS set point for optimal efficiency and to avoid fouling of the membrane. To achieve that, there are often multiple membranes that are hooked up in a bypass configuration with a split parallel flow of treated and partially treated water being sent to the output stream. Also, some of the reject water stream is sent back to the input. This ensures that the membranes have a specific pressure across them and that scaling is avoided inside the membrane filter. Calcium scaling inside the membrane needs to be avoided at all costs or else they will be irreparably damaged. And they ain’t cheap!!

So I’m not surprised that your output had a small amount of alkalinity in it. Some of that is the natural hydroxide alkalinity of water and some or maybe a minor amount of bicarbonate alkalinity. What you really care about is mineral hardness and the membranes were clearly reducing that as well as the salt and CYA as efficiently as one would expect from a portable RO unit.

Sounds like your RO professional did a good job for you. Other pool owners in your area could avail themselves of his service.

 

[Newdude]

What was the cost for members considering it ?

 

[flynwill]

Indeed the system had a small pump dispensing some sort of anti-scaling agent from a barrel in the truck into the line running to the membranes, but I don't know exactly what it was. And yes the plumbing was fairly complicated, more that I could follow by casual observation. It appeared that two of the 3 membrane cells were just plumbed in parallel but exactly how the third was connected wasn't clear. There was also a vertical cylindrical pre-filter who's filtering element Nick describes as being just a cloth bag.

Cost was $700, but I believe that was lower than typical as my small pool could be treated in a single day, vs having to run the equipment overnight and come back for it the next day. I think the charge for a larger pool is likely closer to $1000. Plus $18 for the city water used. A drain an refill would have been $78 worth of water (assuming I ignored the ban on refilling a pool). So not cheap, but arguably a better result than a simple drain and refill.

If there are any SoCAL readers here that would like Nick's contact info send me a PM. He is apparently currently booked well into January of next year. He also said he is spending a couple of week in Phoenix in January doing the pools for one of the hotel chains there.

 

[JoyfulNoise]

The descaling agent is nothing more than HEDP which is found in most pool grade scale and stain inhibitors. The HEDP is pumped into the input stream and the primary RO membrane simply rejects it so it goes out into the waste stream. It’s purpose is to simply hold calcium and magnesium in solution so it doesn’t precipitate at the membrane surface.