Using DE in a sand filter to (a) test function or (b) boost filtration

DE powder is a white fine granular material, made of algae fossils (literally!) that is used as the filter media in DE or diatomaceous earth filters. In DE filters, water-flow holds the DE powder against the synthetic fabric filter 'membranes', allowing the powder to act as filter media, sort of like sand in a sand filter does.

Obviously, you normally use DE powder in DE filters.

But, it can be used in sand filters for TWO purposes:

1. To TEST and verify function of the sand filter, AND
2. To temporarily BOOST a sand filter's ability to remove fine particles . . . like dead algae.

I'll cover both uses.


The DE Filter Function Test

DE testing is very simple: a properly operating sand filter will TRAP DE added to a skimmer, and will NOT return it to the pool. So, if you add DE to your skimmer while the filter is running, and then watch emerging water flow re-entering the pool from a return . . . if the water remains clear, your filter passes a basic function test. BUT, if you see white powder emerging in the return flow, your filter has problems.

Step-by-step:

First:
Get some pool-grade DE powder. Do NOT get -- or use -- "food grade"; it's too fine! You can check Lowes, Home Depot, Walmart or your pool store. Or you can get DE powder from Amazon for about $27 (2018) for 25 pounds. It's difficult to buy smaller quantities, but it keeps forever and is not damaged by getting wet.

Next:
Check your pool filter size, and match the 'test' amount with your filter:
=> Tiny filters -- 2 cups
=> 12" - 18" -- 3 cups
=> 18" - 24" -- 1 quart
=> 30" -- 2 quarts
=> 36" -- 3 quarts

THEN get ready:
1. Backwash your filter. Note the pressure when you restore flow.
2. During backwashing, Have a helper check the backwash discharge for sand. If sand is coming out during backwash, you have a problem.
3. Put the DE in a container that allows you to dump it in all at once.
4. Locate the pool return or 'eyeball' with the greatest amount of flow.
(If you only have 1 eyeball . . .use that one!)
5. Make sure your pump is ON, and the water is circulating.
6. Make sure water is flowing through your skimmer.

Test the filter, part 1:
7. Dump the all DE into the skimmer at one time.
8. Move to the eyeball, so you can watch closely
(Usually, you can see 'leaking' DE at the eyeball, even in a cloudy pool IF you are right there)
9. Watch the eyeball for 180 seconds -- 3 full minutes. (On IG pools with LONG piping runs, watch for 300 seconds or 5 minutes)
10. If the flow remains clear . . . the filter passes. Passing does NOT mean your filter is working WELL, but it does mean it is working at least somewhat.
11. BUT, if you see a 'shot' of white emerging from your return, your filter FAILS and needs service. The most COMMON reason is that your filter has been losing sand during backwash, and needs to opened and refilled.
12. Leave the filter RUNNING.

Test the filter, part 2:
13. Note your filter pressure and how much water is flowing (hand in front of the return)
14. Do NOT backwash, but check the filter again in 12 hours. If your pool is cloudy, the PRESSURE should have increased. If not, your gauge may be broken. Allow the filter to CONTINUE to operate, till the pressure rises at least 5 pounds above the level you recorded in step 1 above.
15. When the pressure increases 5 - 10 pounds, backwash. Have your filter helper watch the backwash for SAND and for WHITE DE powder.
(DE powder but NO sand is what you want.)

Results:
  • If your filter does NOT 'leak' SAND (steps 1 & 15) or DE (steps 10 & 11), it is probably working acceptably.
  • If your filter leaks DE, but no SAND, the sand level is TOO LOW to work well.
  • If your filter leaks some sand but NO DE, you don't have problem YET, but you will. Leaking sand MAY mean broken or cracked internals, but USUALLY means your pump is TOO BIG for the filter.

Boosting SAND filter function with DE

Sand filters are BY FAR the easiest and most reliable pool filters, but they do not filter small particles (like dead algae) as well as DE filters. And sometimes, if you add a bunch of pool store chemicals, you can create a mix that actually makes small particles NEARLY IMPOSSIBLE TO FILTER with sand.

Adding DE to a PROPERLY WORKING SAND filter, just like your were testing it, will TEMPORARILY increase it's ability to remove fine particles. But this increase comes at a price: instead of going days or weeks between back-washing, you may have to backwash every day, or even every 12 hours.

Still this can be a tremendous help on pools filled with dead algae.

BUT . . . LIVE algae stops up DE almost instantly. So, if you want to try boosting filter function with DE, KILL ALL THE ALGAE FIRST. Then, when the pool is CLOUDY BLUE or GRAY, instead of CLOUDY GREEN, add the DE . .. and get ready to backwash!