Without getting into it, the ONLY reason for cellulose is as a replacement for DE in DE filters. You have a sand filter; we're using the DE primarily as a test. The cellulose can NOT be used for that purpose (and it's not all that good as a DE replacement, either!).

We would have told you not to use the monopersulfate . . . but the result you got is very surprising. MPS would have had NO effect on algae. I am completely puzzled by what could make your pool green, be cleared up by MPS, and then return to green. I'm going to see if Chem_Geek has any ideas.

Meanwhile, please take your kit and do a COMPLETE set of tests on your pool: FC, CC, pH, TA, CH, and CYA and report those.

Also, turn your pump on and leave it on.

Finally, when you need to backwash, please note the color of the backwash discharge.

Thanks,