Managing CH While Using Sequestrant

[geekgranny]

I have to backwash very frequently and to get the (way undersized) filter clean enough so that I can go more than two days between backwashing I have to use a lot of pool water. I would say about 4" of water per backwash; ~500 sq ft pool. So most weeks I have to use 8" of water from pool for the average two backwashes per week. I think I will be able to put the new, much bigger Pentair Quad 80 in next week so will be backwashing much less often.

Anyway, I'm having to use so much CH increaser because of so much loss weekly. [EDIT] The savings in not having to purchase so much CH increaser will pay for the new filter in no time. :-D [end edit]
Right now it needs about 25-30 lbs to bring it up to acceptable level. I just purchased only 15 lbs at Leslie's this week thinking it was a 25 lb bucket.

A couple of months ago I did the AA treatment to rid the pool of iron staining that comes from the silt that is dragged into the pool by dogs, with lots blown in also. I'm adding more than maintenance dose of sequestrate every couple of weeks. BTW... keeping pH a bit less than 7.5 and sequestrate is slowly releasing the scale on horizontal surfaces which I like.

Dilema ..... hope there is a solution. Advice please. What affects will there be on the CH levels with continuous sequestrate application? Will the added CH increaser be locked up by the sequestrate? How do I manage this? Right now my CH is 175. I can't seem to get it higher because every time I purchase it I don't get enough along with the loss from backwashing. So if I add the 25 lbs of CH increaser this weekend (backwashing today first), which, in normal circumstances would bring CH up close to 288 or add more to bring it up to 300+ will the sequestrate lock up a lot of the calcium?

With the CH being depleted so fast do I need to panic each time it test below 250 and instantly add more CH increaser or do I have a bit of time to bring it back up a bit without damaging my 13 yr old plaster any more than my years of "abuse" prior to TFP? Lets say it goes down steadily for a few weeks, which it does. CYA is also being depleted. TA not such a problem as baking soda is so cheap.

I have some 3" trichlor tabs in a floater right now, but with the water being so cold they are hardly dissolving at all.

How do I manage this?

Many thanks, gg=alice

 

[PaulR]

500 sq ft x 4" = 170 cu ft = 1270 gal = 5% of the pool

So, you're replacing about 5% of the water per backwash, and want to maintain the pH down around 7.5. Are you worrying about plaster damage because you see you have a low CSI? There is some debate about this point but low CSI may not be a significant problem, plaster damage is more associated with low pH than low CSI. Look through the Deep End for CSI discussions, there are a couple. I'm in a rush right now or I'd find them for you.

If you want to be doubly safe, then increasing either TA or CH would increase CSI, given the same pH and temperature. Play around with the Pool Calculator to see what you can do to influence CSI.

Sorry I don't have a clue how the sequestrant would interact with CH.
--paulr

 

[chem geek]

The difference between a CH of 288 vs. 175 is a saturation index difference of around 0.2 so not a big deal, especially if the 288 ppm results in close to 0 for the saturation index. If you are worried about it, target the saturation index slightly higher, say to 0.1 for the 288 ppm CH.

 

[JasonLion]

To get the water level changes you describe with a typical pool pump would take on the order of 30 minutes per backwash. Even a high volume spa pump would take on the order of 10 minutes to lower the water level by 4 inches in a 500 square foot surface area pool.

If you need to backwash for that long, something is wrong with the filter (other than just being too small).

 

[geekgranny]

To get the water level changes you describe with a typical pool pump would take on the order of 30 minutes per backwash. Even a high volume spa pump would take on the order of 10 minutes to lower the water level by 4 inches in a 500 square foot surface area pool.

If you need to backwash for that long, something is wrong with the filter (other than just being too small).

Thanks all. I'm backwashing today, hopefully. I've been down with a cold past couple of days but I think I'm up to it today. I'm probably exaggerating the amount of water used. I'll mark the levels today and record times. I do have a vary powerful pump 1.5, SF 1.5, and in backwashing, it does not have the head to deal with as when it is filtering with the reduction of return 2" out of filter reduced to one 1.5" feeding four pool returns, with two returns restricted way down to allow more water to the two PoolSkims, each attached to the remaining two returns. So it comes out of the backwash pipe pretty strong.

There is one problem with the filter that I know is affecting the backwashing (and filtering) ability. I identified it when I dragged the filter out from under deck to inspect, clean, soak, replace grids several months ago. I replaced 4 or 5 because of weakening plastic but no tears. The top manifold is getting old. But because I have been planning on getting a new Quad 80 put in for months, I didn't want to spend the money on a new manifold. The nibs, on top manifold, that hang down to keep the grids in proper position, were cracking (the outside ones) and two actually broken off half way. So that is allowing one or more grids to "wobble" at the top and press either out or in against another grid or side of filter, depending on whether filtering or backwashing. At the same time, after the cleaning/replacing, I switched to cellulose, which does seem to capture our extremely fine silt even better than DE. At the same time I added a second PoolSkim, reducing the flow down, on a second return, so the pressure at the clean filter, raised about 5 psi, with starting pressure of 18-20 psi, depending on how well I get it backwashed before adding the new cellulose. With my very powerful pump and large head on return side of filter, I'm really expecting a lot from this 36 sq ft, almost 30 yr old filter's parts and grids. BTW.... over the years, of course, I have replaced parts, but the push/pull valve is on it's last leg along with the screw collars on unions to filter. The collars are held together with steel band clamps.

I'm hoping to get the new filter in in next week or so. Unluckily, it is in high 60's for next few days, but I can't get out to pick it up till Wed or Thur when the temps will then be dropping below freezing again. But this is Texas so temps will get back up in 60's in no time.

The technique I use for best cleaning of filter is as follows: (This give me an additional 3-5 starting psi over just backwashing several times per session and an extra day between backwashing.)

(Pumping Station below grade)

Backwash for 1 minute. Filter for 1-2 minutes. Backwash for 30 seconds. Filter for 1-2 minutes. Backwash for 15-30 seconds. Close suction side valves. Let filter drain on its own with push/pull valve in backwash position (no need to close off return side valves).

Backwash valve back to filter position. Open suction side valves to let filter partially fill with water. (My pressure relief valve is so old I don't mess with opening it so some air will come through the returns when pump is turned on.)

Run on filter for about 2-3 minutes.

Repeat whole process.

Add cellulose slowly through skimmer.

As stated earlier, without doing this process thoroughly my starting pressure is higher and I lose between-backwashing-cycle time.

I'll put our extremely fine silt (and tons of it) up against any one's for a "smallest" contest. It is, in and of itself, essentially Diatomaceous Earth; with some of it being finer than some grades of DE. So when my pool is filtering it is essentially like adding additional DE continuously.

I've certainly been anxious to get the Quad 80 in but life has gotten in the way. One of the things that has held me up is digging the hole, in 90% + rock, where I can't stand up, under deck, hole about 30" deep X 24" X 24", to accommodate the vertical clearance needed for the Quad. I just came up with a temporary solution. I'm going to, temporarily, locate the Quad 80 on a landing step, out from under the deck, about 4 ' from the pump and raised vertically about 30" instead of lowered 30" when it will finally be when I get the hole dug for it. Either way should be about the same head. It and the flex pipes won't get full, blazing, TX sun till late spring and I should have the hole dug with drain and concrete reinforcement, by then, to move it to its permanent position under the deck.

gg=alice