A drain field is 18 inches deep.  The idea is that there are gobs of micro critters that are hanging out right there that love to eat poopy water. 

How deep do these critters go?  How much poopy water do they eat?  Do they eat the pathogens?  Do they let most of the N go down, down, down to our drinking water?

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

So a standard septic system will work just fine?  I wonder if there are reports of how much waste actually does not get handled.

I would imagine how much N makes it into the groundwater depends on the soil type and the depth of the water table as well as the vegetation growing over the drain feild. so I don't think there is a single answer to that question. different areas require different methods of waste managment. the bacteria do "eat" the pathogens in a system functioning correctly jsut as they would if you left your poo in the ground but because the conditions are purposely created to foster the enviroment for the micro organisms (moisture/depth/ some time in the tank first..... it should be a faster more reliable method. the old place required an aerobic system where all the microbial action took place before the water left the system. we were told that eventually standard drain feild systems are to be phased out due to the many variables affecting their proper functioning and its relationship to ground water quality.   

I wonder if there is some data somewhere that has PPM info on NPK and pathogens under septic systems and for city sewer systems.  And then if we could extrapolate that on a per person per year basis.

So, in summary, we could have:

pathogen PPM per person per year

nitrogen PPM per person per year

phosphorus PPM per person per year

potassium PPM per person per year

And then we could have a basis for a threshold for alternative systems.

Paul talks about this in this podcast: http://www.richsoil.com/permaculture/435-podcast-077-creating-an-oasis-with-grey-water-2/

More about wastewater and why normal septic systems might not work all that well:  http://www.solviva.com/wastewater.htm

Looking just for N reduction doesn't really have any bearing on pathogens. Highly soluble N is the canary in the coal mine for other nasties, prescription drugs, persistent chemical pollutants. Testing for nitrates is cheap and easy as compared to others.
One can easily treat for N with Alternative Treatment Technology ( ATT's) yet not treat for those other possibly more dangerous things.
Nitrate contamination if that is the only fear is much easier and afforadbly addressed with filtration. The Feds indicate 1.5% of gross income should be what is normally set aside for clean drinking water. Of systems approved within Oregon the most effective for nitrate reduction cost app $39,000.00 far above 1.5% of my income.
Composting toilets are an efective path for reduction of nitrates temperatures within the composted material addressing pathogens. I would still have problems in disposing of the material on food plants. In Oregon the material is prohibited from that use. 
De-nitrification naturally occurs through plant uptake in riparian areas and as it travels to the sub-oxic layer of groundwater.
Nitrates an effective and easily detected precursor to how septic effluent passes or infiltrates the groundwater but not necessarily any indication of E-coli or human pathogens within the aquifer.

Paul, I'd be interested in your views on Mike Reynolds work (the earthship guy). His buildings address all of the issues you discuss in your podcasts. They collect rainwater for all water usage (no wells, pumps, softening, very little filtration needed), they process their own grey and black water separately, they have at least one self contained greenhouse for the processing of greywater and allow for the addition of another for the black water system, the homes are designed and built from New Mexico, Haiti, Canada without conventional heating or cooling systems (solar gain, thermal mass and convection only), and are designed to be powered completely off grid. His videos may be seen here:
Garbage Warrior
http://www.youtube.com/playlist?list=PL2F4C0B3123189EC8&feature=mh_lolz
Earthship Seminar 1,2
http://www.youtube.com/playlist?list=PL7C8F40E6B3BBDFAE&feature=mh_lolz

Nitrogen in a standard septic system is generally considered 61 mg/l for  what enters the tank it is generally determined that it is reduced to 46-41 mg/l after it leaves the tank and encounters ground water.
A grey water system which in Oregon 2/3rds the size of what is required for a standard system is about 17.5 mg/l of nitrates as it enters the tank and it is commonly estimated that 3-4 as it reaches the groundwater.
Now there are a lot of different factors but this is generally accepted as initial figures for nitrates.