new(ish) sand filter returning fine debris to pool

[in-two]

Not sure if this is a pure filter issue or treating/balancing water, or both, but here goes; I have the usual 'greyish/milky water after shocking' issue. I know it can take time to clear but I've been at it for 12 days. I can see the bottom (8feet) just, but every morning there is a lot of fine greyish dust on the pool bottom, particularly at the floor/wall joins. When vacuuming through the filter there is a plume of the same stuff coming out of the returns (which are in the bottom of my pool). Clearly the material is too fine for the filter, or something is wrong. Vacuuming to waste is not a great option as fresh water here is scarce and expensive and a bit unfair on my neighbours whose taps run dry when I'm filling my pool... I have a conventional sand filter fitted new two years ago which seems to be functioning correctly in that there seem to be no obvious plumbing issues, air or water leaks, ditto pump. My questions are:
How can I check that the filter is filtering correctly, or as well as it can?
Would adding DE help?
Would trying flocculent likely work, and what's the best procedure?

Thanks to anybody who can help, and generally to everybody that contributes to this site, which is a Godsend.

 

[woodyp]

Easiest, cheapest thing you can do is to add some DE and see if it helps any.

my-sand-is-channeled-how-to-fix-it-t7626.html

.............and you can check that too!

 

[MassNerd]

1) When is the last time you backwashed? Backwash and rinse your filter if you haven't in the past 24-48 hours.
2) Let the debris settle for at least a couple hours and then vacuum worst areas of the floor to waste
3) Add DE to your sand filter following the instructions in Pool School. Keep an eye on it, if you pool is really dirty you may need to backwash in <2 hours the first few times.

 

[in-two]

many thanks for responses, just off to try and find 'Terra Di Diatomee' as they say here in Italy.

ciao

 

[in-two]

quick progress report and dumb question, found some DE, my filter took 2.2lbs (1kilo) to increase filter pressure by 1 psi, from 18 to 19. Seems to be working really well, no visible plume of debris from the returns during vacuuming. Pool seems to be clearing but still a way to go.
Dumb question; filter pressure is slowly rising (a good sign I think), when I eventually backwash will I need to re-dose with DE, i.e does backwashing tend to wash out the DE as well. If so is the dosing procedure the same, enough to increase pressure by 1psi or should I aim for the previous value, 19 psi?
Thanks for helping.

 

[JohnN]

Yes, backwashing removes DE. You repeat the process aiming for a 1 psi increase.