Hi Molly, is it for a domestic dwelling, or a visitor centre etc.? The solutions will vary with the loading rate. For a domestic system it
should be easy enough to get an off-the-shelf unit in a
local hardware shop.
These should have an insert that lifts out. What I used to do was dig a shallow hole in the soil and put the sludge from the insert into it. Then I'd
bucket out the base sludge if it was very murky, into the same hole; and then backfill. This was an area of the garden that was fenced off from
gardening etc. so it was easy to just let the soil organisms get to work on the sludge and deal with it in their own time. We use ecofriendly
cleaners, soaps, shampoos etc. so there was no toxic load on the garden.
If you want to avoid emptying anything, then what most people do here is plumb the grey
water directly to the septic tank with no trap. Then it gets emptied annually during the septic tank desludging process (if you follow the regs...)