Paul,
I can't answer those questions, but I think the right person to ask would be a civil or environmental engineer. I suspect most municipalities and/or counties probably employ them.
Also, with regard to winter wastewater treatment, about 12 years ago in Wisconsin I helped install an engineered wetland for wastewater treatment. It was the first of it's kind so far north. We installed it at Riveredge Nature Center (description at
http://riveredgenaturecenter.org/default.aspx?id=40). The reason it was experimental at the time was because they didn't know how the system would react to extreme cold (zone 4-5). As it turns out in that climate the temperature of the water moving through combined with the decomposition activity to keep it functioning year round. The system could have two feet of snow on it and it would still function. Several have now been installed in the area.
I would guess that a drain field would work similarly to a
compost pile, where the heat of decomposition would prevent it from freezing and keep it functional (I assume this is what you're getting at).
Dave