I just installed a new system also here in northern MN. I could still go with a drain-field system. $4100 installed. We have sand. In fact, the new well we also had installed has sand all the way to 60ft.
The original system had to be inspected if...there was a change of title, or if any improvements, ie.. building permits were to be issued. I'm putting up a 30x40 building.
The system failed because...1)the baffle from the holding tank to the drainfield was broken, for who knows how long, and 2)The tank hadn't been pump for probably 15yrs. My bride had purchased the house "as is" in the early 90's, and I moved in two yrs ago. When I had the tank pump two yrs ago, it was like clay, and it took the pumper a few hours to backwash it to loosen it enough to pump. At that time, I was not there to check the baffle before it all got covered back up.
I decided to go with the newer "chamber" system for the drainfield. The drench doesn't need to be as deep (only 3ft)and also does not require rock. The chamber system cannot plug up with hair or other non bio waste, and provides a much larger bio mass area.
Here's is a sample of the chamber system....
http://www.infiltratorsystems.com/productline/quick4_infiltrator.asp
I'd like to also add that the new tank is a two chamber tank with a washable filter (big brush) installed in the drainfild baffle. It's added protection to trap hair and lint. What I've read is washing machine lint is the number one killer of drainfields. Put a filter on your washer, as I did. You will be surprised!!
Doug