Where I live in the DFW area, the City requirement is that you must put your DE waste output into the sewage drain. Putting it in the storm drain is not allowed. The reason that I have a cartridge filter, on my new pool, is because the City would not allow a DE filter unless it was plumbed into the house waste drain system. My two other pools have DE filters, and I find that I don't backwash them much, if at all. I find it easier to just break the filter apart about twice a year and that seems to keep everything in check. When I break the filter down, I just hose the old DE right on the lawn. I've been doing this for several years and have seen no ill effects to the grass, both Bermuda and St. Augustine.
Jim R.