Here in ND, you need min. 1000 gal. working septic tank, and 110 ft. of trench per bedroom of the house. Any more than three bathrooms, add 50' per. More than one laundry, add another 50'. Garbage disposal? Must have 1500 gal. septic and extra 100'. Any trench system (gravelless, rock bed, infiltrator) must be no more than 30" deep to bottom of trench. If using rock bed, must be 18" of rock. Bottom of any trench must be minimum 2' above oxidation layer in the soil. If oxidation layer is less than 4½' down, rock bed may not be used; must be gravelless with clean, sandy topsoil brought in to get minimum 12" cover over top of pipe. If oxidation layer is less than 2', a sand bed mound must be used. Average size for a mound is around 60' by 120' at base, minimum 1' washed sand, 6" rock under leach pipe, foot of soil over top of entire mound.
I don't get into the septic work since I went on my own 3 yrs ago, but did it for 20 yrs. with my Dad. The gravelless systems were the cheapest, about $2500 for tank/pump chamber install, then around $10/foot for the gravelless pipe installed. Washed sewer rock around here goes about $30/yd³ or more delivered, so rock beds are about $12/foot. I personally have never liked the look of the domes or chamber pipe so I don't have any opinion on those. Mounds have cost close to $20,000 here. They take on average 350 to 500 yds³ of washed sand at $15/yd³. Plus, about 100-200 yds³ of good topsoil.