Differential Draining of Pool Water

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
15,307
Tucson, AZ
So I thought I would start this thread for the purposes of logging useful information and, perhaps, teaching others. I am currently doing a differential drain of my swimming pool whereby I am removing water from my deep end well using a submersible pump and some PVC plumbing while I am adding fill water to the skimmer. As this process is running, my pool pumps and automation system is completely shutdown for safety and water flow reasons. Note - this idea is NOT new nor do I claim any privilege over this process, it has been discussed in passing many times here on TFP.

The idea is simple - by removing colder and higher TDS water from the deep end, one can then add warmer, lower TDS water to the skimmer and the two layers should remain separated enough to effect an efficient drain of the water and closely approximate the results of a complete drain and refill. There are many reasons why a pool owner might want to do it this way but a significant reason is that here, in HOT Arizona, you cannot fully drain a plaster pool when the air temps are above 86F as damage can occur to the plaster surface. And, in Arizona, air temps consistently can get above 80F starting as early as March....heck, there have random days in January where the mercury has reached 75-80F.

Here's some shots of the setup

Deep end removal -


And flow meter -



Shallow end skimmer fill -



Skimmer fill flow meter -



Some info -

Deep end pump out flow - 22-23GPM

Skimmer fill flow rate - 5GPM

Water temperature of deep end well - 69F
Input Water temp from fill hose - 77.5F

You can look at my signature for my latest test results. I have a salt water pool with a nominal salinity of 3400ppm and currently my CH is around 1100ppm. So the pool water has very high TDS and it's about 8 deg F cooler than the input water. My hose fill water (softener shut off) has a pH of around 7.6-7.8, TA 100ppm and a CH of 180ppm (TH 200-220ppm). My end goal is to get about half the water out of my pool and lower my CH to less than 800ppm or better. While that's still about 2X the recommended level, I'll happily take that over where I was last year at an eye-popping 1500ppm CH.
 

kimkats

Mod Squad
LifeTime Supporter
Jul 10, 2012
39,199
Tallahassee, FL
Matt will you please check the deep end water temp off and on just to see if the waters do mix and how much they do. We are all going to learn a lot from this one!!
 
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setsailsoon

LifeTime Supporter
TFP Guide
Oct 25, 2015
1,170
Stuart/FL
So I thought I would start this thread for the purposes of logging useful information and, perhaps, teaching others. I am currently doing a differential drain of my swimming pool whereby I am removing water from my deep end well using a submersible pump and some PVC plumbing while I am adding fill water to the skimmer. As this process is running, my pool pumps and automation system is completely shutdown for safety and water flow reasons. Note - this idea is NOT new nor do I claim any privilege over this process, it has been discussed in passing many times here on TFP.

The idea is simple - by removing colder and higher TDS water from the deep end, one can then add warmer, lower TDS water to the skimmer and the two layers should remain separated enough to effect an efficient drain of the water and closely approximate the results of a complete drain and refill. There are many reasons why a pool owner might want to do it this way but a significant reason is that here, in HOT Arizona, you cannot fully drain a plaster pool when the air temps are above 86F as damage can occur to the plaster surface. And, in Arizona, air temps consistently can get above 80F starting as early as March....heck, there have random days in January where the mercury has reached 75-80F.

Here's some shots of the setup

Deep end removal -


And flow meter -



Shallow end skimmer fill -



Skimmer fill flow meter -



Some info -

Deep end pump out flow - 22-23GPM

Skimmer fill flow rate - 5GPM

Water temperature of deep end well - 69F
Input Water temp from fill hose - 77.5F

You can look at my signature for my latest test results. I have a salt water pool with a nominal salinity of 3400ppm and currently my CH is around 1100ppm. So the pool water has very high TDS and it's about 8 deg F cooler than the input water. My hose fill water (softener shut off) has a pH of around 7.6-7.8, TA 100ppm and a CH of 180ppm (TH 200-220ppm). My end goal is to get about half the water out of my pool and lower my CH to less than 800ppm or better. While that's still about 2X the recommended level, I'll happily take that over where I was last year at an eye-popping 1500ppm CH.
Matt,

No surprise you've converted your pool into a full-scale lab! Very cool!

Any thoughts about filling with softened water? Seems like you could cycle to get the softener to produce what it can then regenerate that night and you'd be able to get all the way down to normal CH.

Chris
 

JamesW

TFP Expert
Mar 2, 2011
14,597
Don't you want to balance the input and output so that the water level stays the same?
 

borjis

LifeTime Supporter
Aug 19, 2014
2,809
Pacific NW
Excellent.
I had to do this recently as well.
I have a very odd phenomena whereby there is a slight iron build up after winter, every other year. I find myself with
light green tint after adding the first chlorine of the year. It makes ZERO sense as I don't get green prior to that during swim season
and my fill water is municipal (not well) and does not have high metals content.

I basically do the opposite of what Matt does. My drain pump is not rated for being under water for extended periods,
so I drain FROM the skimmer and have the garden hose in the very deep end bottom, on the other side of the pool.
Ran it twice. Saturday for around 8 hrs then on Sunday for 8 hrs.

The green was gone by monday.

The reason I don't just drain down is that my liner was a very tight fit and I noticed it getting stressed whenever I have
drained a significant amount of water. This prevents the straining / any potential for the liner popping out of the track.
 
Last edited:

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
15,307
Tucson, AZ
Matt will you please check the deep end water temp off and on just to see if the waters do mix and how much they do. We are all going to learn a lot from this one!!
Kim,

It's been a bit overcast today so not super bright or hot. I just remeasured the discharge water and it's up to 71F from the deep end well. The fill water is still holding at 77F. So the temperature differential has decreased a bit but still significant.

Matt,

No surprise you've converted your pool into a full-scale lab! Very cool!

Any thoughts about filling with softened water? Seems like you could cycle to get the softener to produce what it can then regenerate that night and you'd be able to get all the way down to normal CH.

Chris
I thought about this and I decided I did not want to stress out my water softener. That, and all my spigots are plumbed with hard water. I have one outdoor spigot that supplies soft water but it is too far away to be useful. In the end, my fill water CH is 180ppm so it's fine for refilling a pool. You are quite correct though that the process could be made more efficient by using soft water.

I actually put my water softener in manual bypass at the Clack valve and I turned my fill pot back on (in bypass, the autofill goes back to hard water). So now there's the garden hose AND the fill pot adding water.

Don't you want to balance the input and output so that the water level stays the same?
Ideally, yes. In practice, it's much harder to do.

In my case, the best I can get out of a garden hose is around 6GPM. I have two submersible pumps to choose from, a 1/4HP and a 3/4HP. The 1/4HP pump produces a flow rate of about 8GPM with a standard garden hose. The 3/4HP pump that's in use now generates about 22-23GPM using 1" PVC that eventually attaches to 1-1/4" blue discharge hose. I'm sending the water about 200ft away to the front of my property so a standard garden hose just won't do. I could choke the 3/4HP pump down by using a garden hose but it will still push water at 14GPM (I've tested that out in the past). Because of the imbalance, I wind up discharging water for about 30-40mins, then I shut the sub pump off and let the garden hose do the refill. The refill takes over an hour or so. I'm draining to just below the tile line/bottom of the skimmer. I don't want to expose the PebbleTec.

If I wanted to balance the flow rates, then I'd have to balance them on the slow end because I only have one spigot that I like to use (home improvement project item #6,230,874 - replace the leaky spigot nearest the pool). On the low end, I'd have to try to achieve a 6GPM flow on both ends. That's incredibly slow and with the volumes I'm trying to move it would require days of continuous pumping to make it happen. I really don't want to leave my filtration system off for that long. As it is, I'm a little worried about having had no filtration today and I put a puck floater in the pool (It's ok, my CYA could use it's usually spring time bump up). I'm likely going to cut my experiment short and let the pool pump run overnight to homogenize everything and filter then see where I have landed. By the time I wrap things up tonight, I'll probably only have exchanged about 3,000 gallons, maybe less...
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
15,307
Tucson, AZ
Thanks Mark, I remember reading that thread a long time ago.

Ideally I would love to use my equipment to do some of this but I have no discharge into my sewer, only onto my property. I'm limited in how much water I discharge near my equipment pad but the front of my property has a very large open area to discharge into so it's easier. It also has several million miles of ground squirrel tunnels and burrows that magically make the water disappear underground....I'm amazed at how hundreds of gallons of water can drain down these holes without filling up....the squirrels must have a very nice underground pool to swim in 🐿🐿🐿

As fortune would have put, we might actually get some rain tonight. It probably won't be more than 1/4" of water or so but I'll happily take some free refill water...and calcium free to boot :kim:

Looks like I'll get about 2,800 gallons out of the pool for today. I plan to run the pumps and mix the water. I'll do some tests tomorrow to see where I'm at.
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
15,307
Tucson, AZ
Actually the rain has been on and off all night and pretty heavy at times with thunder & lightening .... rain in April in Tucson ??? And there were bright lights spotted above the Catalina’s ... hmmmmm .... :alien::alien::alien:
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
15,307
Tucson, AZ
Conclusion - Worked pretty darn good!

Total Volume Drained = 2880 gallons
Final CH = 950ppm (using the 10mL, 25ppm/drop CH test)

So, if you look at the starting CH level (1100ppm) and the final CH level (950ppm) and note that my pool volume is ~16,000 gallons with fill water that is 180ppm CH, you can do the math and rearrange the terms to figure out what volume of water exchange would yield that drop in CH. The instructor leaves that exercise up to the students to perform on their own ( :laughblue::laughblue:I loved saying that to undergrads when I was a grad student TA....) -

Theoretical volume exchange = 2608 gallons
Actual Volume Exchanged = 2880 gallons

So the efficiency is about 90%. Because of the large difference in flow rates, I did three runs of my sub pump each running about 40 mins followed by over 90 mins of refilling with the sub pump off. So even though the process was not continuous, there was still adequate enough separation of the water layers to make it act as though it were a continuous process.

Note that all of the measurements today were taken after the pool had filter had been running all night and after 2 cycles of running the robot cleaner (with some brushing on my part). So I feel the water column was adequately mixed. My salt levels are also down to 2800ppm which is pretty much the lower limit of the cell so I will have to add a bag or two of salt to the pool. I have not measured my CYA yet but I expect it to be too low for an SWG pool. As well, I haven't added borates since last year so I expect those are below 50ppm as well. I'll take my time on rebalancing the chemicals but I'm glad to at least be below 1000ppm CH.
 

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