Yes, 250 gpd is probably too large. I was thinking about municipal water demand. The Civil Engineering Reference Manual says about 75-130 gpcd (gal per cap per day). With that, I'm looking at 26,000 gpd.
Has anybody had experience designing on-site septic for mining/oil man-camps with about 200 people in the Dakotas. One issue is there is no receiving stream, as the area drains to a local depression/wetland. I'm trying figure out what is typically done in this area. Obviously, a leach field...
The EPA Green Book for on site septic: http://www.epa.state.oh.us/dsw/documents/greenbook.pdf
This site has some good design bulletins: http://ohioline.osu.edu/lines/bulls.html
Bulletin's 829 & 813 guide how to design a mound system.