Goedjn wrote:

That ought to work out to around
10 square feet of trench per person. Why are leach fields typically
so much bigger than that? There's obviously some constraint
other than liquid gallons per day driving this, but what?

Expected lifetime. It's not just how fast the soil percs, it's how long it will
do before plugging up. Eventually the suspended particulate blocks the most
porus soil and the field has to be replaced.

I don't know what the standard is these days, but I suspect it's at least 10
years and maybe even 20. In thes part of the country, the local enviro health
people want to see room for a second field on the lot if at all possible, so you
don't have to dig up the first one when it fails.