I'm not sure I know much more, but we had a passive Klargester in the last house (where we lived for over 20 years) and have a package treatment plant (a Klargester biodisk) in this one.
Don't have one of those.
I've been rinsing the coffee pot into our private drains for over
20 years (admittedly I scrape the bulk of the grounds into the bin first).No grease, oil, fat or anything similar. Indeed, this is a good rule of thumb;
Those of us with septic tanks, Would like to give a word of thanks, For putting nothing in the pot, That isn't guaranteed to rot. Kleenex is bad, cigarettes too, Feminine products are taboo, No hair combings please - use the basket. There's a very good reason why we ask it. With your kind cooperation We'll keep our tank in operation!
Don't put anything down the drains that hasn't been through a human being, other than toilet paper.
We get our desludged annually.
Neither of our systems have a drainage field, so I can't say. They both drain into ditches in the adjacent fields. I planted reeds in the ditch in the last house, and will likely do the same here.
There has been a lot of fuss (and misinformation) going round about the requirement to register systems with the EA. You might want to start reading here;