Welcome to the Forum!

I wrote the "use and care for DE filters' and it was based on the pools I saw as a service tech and was meant to tell folks how to avoid the problems I had to go fix.

You most certainly have to replace DE after you backwash the filter!!! (it's in the stickie)

As for frequency, I'll stick to what I said, "when the PSI rises 8, it's time to backwash and then add ~80% of DE that you would, when the filter is new or manually cleaned" - however, if you want to backwash more frequently, you'd need to increase the % (of a fully clean filter) up from 80%.

I'm sorry that I don't know anything about 'food grade' DE, perhaps someone else does and will chime in.

I hope this post helps you a little, if you need more info - I'm here on weekends and there are a number of other folks that will help you