Filter Sand

[Matt 633]

As the dog days are upon us, many are reluctantly thinking of closing their pools. So, to keep us on our toes I have a sand filter question.

I was reading a PF post that talked about adding pea gravel into the filter first. This got me to thinking - has anyone tried a progressive sand addition. Pea gravel, size 1/2 sand, 12x20, 20x40, topped off with a little DE??

In my filter, I have 100# of 12x20 and 100# of 20x40 on top of it. If I would have read the suggestion to put pea gravel in first, I probably would have. Perhaps on the next one! :-D

I would like to hear some thoughts concerning this.

TIA

 

[duraleigh]

Pentair suggested the option of partial pea gravel in my filter. Checking with several PB's, no one used it so I chose to follow their lead.

I am VERY curious to find where you found the different grades of sand for your filter (never heard of them anywhere before) and what the philosophy behind it is.

My thinking has always been that progressive filtration's only advantage is it may require less backflushing but no other real reason. In any event it makes for interesting discussion.

 

[Matt 633]

Hi Dave,

As I recall, you were one of the participants in the prrevious discussion and I'm glad you've chimed in.

I was given my Hayward 210 by a lady that wasn't happy with how it filtered. Is was about a year old and was full of algae. I emptied it out and cleaned it. Upon looking for a replacement sand, I found that Hayward recommended #20 silica sand between .16-.21 mm. My local PS only carried size 1/2 sand. I checked with another PS about 30 miles away and they are an authorized Hayward warranty center. They made the recommendation of the 50/50 mix. So far I've been very happy with it. In service for 6 weeks and just under a 1 psi increase. I only run on low speed, but do so 24/7. I expected not to backwash until the end of the season.

 

[TFPJules111]

My new filter arrives on Friday, and I'd like to have the sand on hand to fill it.

The manual specifies 300 lbs of .45-.55mm filter sand (No. 20 Silica Sand).

Where do you buy your sand? Are there any benefits to additives like Zeolite or DE in a sand filter?

 

[JasonLion]

Zeilite is used instead of sand, improving filtration. But zeolite dust can sometimes get into the pool (annoyingly) and it is somewhat more expensive. Some people absolutely love it, I prefer regular sand with DE added.

Small amounts of DE are added to a sand filter after each backwash to improve the filtration. It is slightly more work, additions after each backwash. I use DE added to my sand filter.

It is difficult to find the right grade of sand anywhere but a pool store. Make sure you are really getting #20, other sizes won't work at all well.

 

[Matt 633]

From what I can discern, the #20 signifies pool filter grade. For your particular size, .45-.55mm, according to Hayward, this would translate into size 1/2 (not one half). Rule of thumb is to make sure it comes in a bag labeled as swimming pool filter sand. Some pool stores will sell blasting sand because it is more common and some what less expensive for them to buy.

I tried to elicit some discussion about sand, but as you can tell from the lack of responses, it didn't get much attention . I do believe that the pea gravel addition (25%) as the first media in does merit some consideration. This is not the norm - but hey, most of us here don't use the "norm" as a basis for our pool care. :-D

As I recall, you will be pushing a lot of water through this. So size 1/2 would be the best sand for your application. Here in East Texas the price is around $10 for 50 lbs.

 

[matt4x4]

In my opinion, adding pea gravel to the lower portion (Or any portion) of your filter will only do one thing - lessen filtration.
Adding anything larger than the sand specified is really just adding something to take up space, it will not aid filtration, the spaces between it will fill up with sand, where the sand is, water will flow, where there are rocks or gravel or bricks or anything else too large to actually filter, no water will go, essentially, you're actually reducing the surface area that sand can flow through, this speeds the rate of travel of the water up in those crevices between the stone, faster water does not get filtered as well as slower moving water, this is why lower HP pumps do a better job with filtration - because the flow rate is lower.

You would also have less sand in use because some of your space available is now being used up by the pea gravel, less sand also equals less filtration.

 

[Matt 633]

Very good points 4x4. However, there are those on this site that make their livings servicing pools and pool equipment that think the addition of the pea gravel is a very good idea and may even increase the efficiency of filtration. The belief is that the pea gravel will help protect the laterals and decrease the chance of sand entering them. One believes that the sand will remain on it's own layer and not infiltrate the gravel to a great extent. Just an option for those who want choices.

How much filtering is actually being done at this level (bottom) anyway? I suppose that it would be hard to tell without cutting a dirty filter in two and analyzing the sand. Pretty drastic measure. :-D

 

[matt4x4]

Unless you separate the two layers using a good filter cloth - I really don't think so - I live on clay (very fine stuff) - if I put 6" gravel down, within 3 years, every crevice between the gravel has been filled with the clay beneath (even in an upward direction - working against gravity)- I can stop or slow this down by putting filter cloth down.
Small material WILL fill the gaps between the larger pieces - gravity and water flowing through from small to large particles will assure that this happens almost immediately

 

[waste]

Hi guys! :-D

For some reason, I'm partial to adding the pea gravel, the guy on the other forum who suggested it must be really smart! The difference in filter performance is negligible (let's face it - what's that extra 2-3" of fine filter material really gonna do?) But I like the fact that the laterals don't get clogged up nor the lateral on the drain which keeps the water 'moving right along'. Another benefit is it makes changing the sand a good bit easier/ quicker (no having to use a dentists pick to get the sand out of the laterals) I look at it this way: the pea gravel filters the sand (prevents it from clogging the laterals), just like the sand filters the water. I will also add that if I had a sand filter, I'd certainly use DE in it!

 

[The Mermaid Queen]

ted,
how does the sand NOT fall or get pushed into the spaces around the pea gravel??

 

[matt4x4]

my sentiments EXACTLY - mermaid Queen, UNLESS they're using a filter cloth on top of the gravel - also, removing 3" of sand from a filter WILL have an effect on performance - why else would the manufacturer suggest adding 150 lbs of sand (or whatever is called for) and not 120 lbs, you may as well just buy one size smaller filter then what was spec'd for the pool.....
Anyways, I don't run gravel in mine and my laterals don't have debris in them, probably because the openings in the laterals are made smaller than the sand particles by design otherwise, where's the point???
And yes, the spaces between the gravel will fill with sand almost immediately, and to go one further, the finest sand (that stuff that got ground up really small during shipping) will still get through all the way to your pool where you will continue to pick it up/push it through again - UNLESS you vac to waste to dump it.

 

[waste]

Grace, Matt - I'm certainly not any sort of engineer, I only know what I've seen. What I've seen is that filters with pea gravel NEVER have sand going back into the pool (unless the multi is broken) and they drain properly when closing the pool.

Evan was the one who supported my suggestion on PF about having a 'coarse' layer to improve filtration, and his info sounded right. I've said it before and I'll say it again - IT'S YOUR POOL AND YOU CAN DO WHAT YOU WANT TO WITH IT -- I'm just trying to save people some headaches.

Let's do a 'thought experiment'. -- with small pea gravel (river stone), the sand will try to migrate into the gravel layer, however the space between the pea gravel will 'catch' the sand within the first 1" or so (the sand will catch the sand and the other sand will be caught up on that and not pass further), therefore - no sand can pass further than 1" (and that's an exaggeration)

As I've said, most of the filtration occures in the top 18 - 24" of the filter, if it weren't that way the DE wouldn't work.

That's my story and I'm sticking to it :lol:

 

[matt4x4]

Waste - I have also NEVER seen sand go back into a pool and none of the filters I know of have gravel in them - Immediate neighbourhood - every house on our street has a pool - some as old as 20 years - no sand, so I believe that mostly all filters are designed properly from the get go - that being the lateral's openings are smaller than the sand particles, any manufacturer that doesn't design this specail little feature onto their laterals should not be in business.

I'd love to get an actual explanation HOW pea gravel would stop sand from flowing into the spaces between the gravel pieces and keep a barrier between the laterals and the sand, and HOW pea gravel attracts the really fine silt to make it stick and stay in the filter - you know the silt I'm talking about here - the really fine stuff that comes in every new bag of filter sand - it's not much, but every new pool will blow int out on startup - once disposed you don't see it again because it's gone.

PLEASE someone actually explain this to me, I'm really curious, because my Son's sandbox sits on a 6" bed of pea gravel - under the gravel, I have filter cloth that covers the clay, when the sandbox was in use, the kids would continually spill sand onto the gravel, the sand would disappear, if I removed some gravel in this area - guess where the sand was - you guessed right - sitting on top of the filter cloth - and this happened without 15lbs of downward pressure on the sand - anyone got any ideas how I can stop this from happening - and please don't tell me a bed of pea gravel - I already tried that.

Editing here - just thinking about your theory of sand not migrating past 1" deep - can't be true unless the spaces between the pea gravel are smaller than the sand particles - this will never be because the pea gravel is more or less round and the spaces will ALWAYS be larger than sand particles, if you'd want to test it in a relatively visible environment use a container with the bottom cut out, set it on a splatter screen, fill the container with gravel, put sand on top, run or sprinkle water through the top....tell me what you find - probably the same results as my son's sandbox spills.

 

[JasonLion]

Various manufacturers of larger commercial sand filters suggest the use of pea gravel around the laterals. Astral Pool Products says:

"Gravel helps support the laterals from the weight of the sand
Gravel will filter out the sand from the water before reaching the lateral system
Gravel will help eliminate anaerobic bacteria from forming underneath the lateral system
Gravel provides more open area for a smother flow of water after it has passed through the sand.
Warranty could be voided if the required amount of gravel is neglected. Please be sure to read your owners & operations manual before attempting any work."

I assume that pea gravel is not used in residential filters because that would make them more complex and the manufacturers want to keep things simple for their residential customers.

 

[duraleigh]

Interesting discussion. My tendency is to think more in terms of what Matt is saying.

Pea gravel, or a coarser layer of anything, may keep the sand from migrating downward to surround the laterals for a period of time but it seems logical to me the sand will work it's way to the laterals sooner or later and the coarse layer will be negated.

On the other hand, it also seems possible to me that backwashing pushes the sand back upwards off the laterals thereby cleansing the pea gravel and, to an extent, recreating the layered condition that previously existed. While this sounds pretty good in theory, I question whether the upward momentum of backwashing would actually lift much sand out of the gravel.

If you accept the idea that the two media will stay separated (layered) then that would offer credence to the practice of adding DE to a sand filter. While I'm not completely convinced that adding DE is all that beneficial, it apparently does maintain some degree of separation from the sand in that the majority of it seems to stay on top and is quickly backwashed out.

However, that may be more a function of the specific gravity of DE compared to sand (I assume it (DE) is significantly less).
My guess is that the specific gravity of sand, however would be at least equal to and most likely greater than pea gravel, thereby giving it a natural tendency to migrate to the laterals....again negating the effectiveness of the pea gravel.

In my pool, by far the most influential factor in having really crystal water is pump run-time. My water is noticeably cleaner when I extend the pump run-time and it has noticeably more particles suspended in the water when I dont run the pump enough. My thinking is all other factors pale in comparison to that issue.

 

[matt4x4]

Simple Physics and a little test like the one above would dismiss #1, #2, and dismissing #2 will Dismiss #4 - as for #3 - I have no comment on since it's not up for discussion here.

Look at what they are stating on the first line - "helps support laterals from weight of sand" - I would hazard to guess that portions of the laterals ARE protected since by their theory there would be spaces between the gravel, so the areas where gravel does not touch, there should be no pressure since there's supposedly only air - but think about this - you still have 150 lbs of sand in the filter, this weight does and will push down, so the remaining pieces of gravel that DO touch the laterals MUST exert more pressure per sq inch that the sand would have done on the full area of the laterals.
Now the laterals have pressure spots on them - I don't think that's as beneficial as having an even pressure all along them.

Once the sand works it's way through (and it will), you will once again have even pressure on the laterals.

Since this is a long weekend - I'd like everyone to try my little experiment above and report back Tuesday morning.


Edit - just read Duraleigh's reply - and just want to add, once sand works it's way down between the gravel, the remaining 150lb mass of sand will move down a small amount to replace the spaces left above the sand that traveled down, when backwashing, I don't think that the pump would have enough power to move that 150lb sandmass upwards again.
Really, if it did have the power - would our laterals not crush relatively fast because of the massive "Bump" we would give the sand every time we backwash? (Push up, shut off pump - gravity pulls down)

DE does penetrate into the sand, quite a bit, every fall when I close my pool, I stick my garden hose down the vertical tube in the filter - basically a lower pressure backwash, while doing this, I stir up the sand on top, teh first 3" will be full of DE, once I clear all that out, I dig further into the lower untouched portions of sand - probably at least 6", white colored water once again starts flushing up - I only use a few cups of DE a year (early in the season, so the remnants that don't backwash out have all summer to push into the sand - so, further proof that if DE can penetrate, sand can penetrate gravel....

As for length of pump time - totally agree - probably the #1 contributor to crystal clear water.

 

[duraleigh]

"Gravel helps support the laterals from the weight of the sand

- being submersed in fluid and pressure equal in every direction, that makes no sense

Gravel will filter out the sand from the water before reaching the lateral system

- poorly worded, but I think theyre saying the media will remain separated.....I dont think so.

Gravel will help eliminate anaerobic bacteria from forming underneath the lateral system

- Hmmmm...never thought about that but I don't understand how it would be more effective than sand

Gravel provides more open area for a smother flow of water after it has passed through the sand.

- yes, assuming the two media will remain separated

Warranty could be voided if the required amount of gravel is neglected. Please be sure to read your owners & operations manual before attempting any work

. - Well, my only thinking here is that's a typical manufacturer absolving himself from any product liability every chance he gets.

As I mentioned in an earlier post on this forum, My filter (Triton TR-140) instructions offer the 1/3 pea gravel-2/3 sand as an option but they make no mention whether they, as the manufacturer, have an opinion as to which is best

 

[waste]

I don't want this to be sent to 'Agree to disagree' but, perhaps by continuing this discussion, we can find the ~ truth of the matter. As said, I don't know the 'mechanics' behind it, but the sand seems to stay ~above the gravel. Logic tells us that the paths through the gravel, while small, are not as small as the grains of sand, therefore the gravel should be full of sand. However, when changing the sand in a 'gravel filter' there is little in the gravel and ~ none in the laterals :?: (as opposed to, in a just sand filter, there's a lot of sand enbeded in the laterals - and I'm assuming that when too much gets stuck in the laterals, the pressure makes them break, leading to sand in the pool)

Matt, I don't really know but, in the kids sandbox you have horizontal motion (from walking, etc) as well as the vertical motion, which would facilitate the merging of the 2 layers - that's the best I can come up with

Another thought experiment: tennis balls and BBs - fill a large tank with tennis balls and then pour BBs on top of them. The BBs will pass through the first few feet of tennis balls, but at some point, they will congest and not be able to pass further ( some of the BBs will get caught and cause a 'back-up' preventing the other BBs from getting through) Now, like at rush hour, there will eventually be so much congestion that none can travel further.

I'm more than willing to discuss this further, because I haven't fully captured what I'm trying to express, but does it make any sense to anyone??