(2nd try - sure hard to post messages on this site!)
In my state you'd have real issues getting such a thing approved. You don't mix _anything_ with a leach field.
In Minnesota we worry more about heating than cooling, and you would _not_ want to cool your septic system down that much - plenty are freezing up right now... Septic laterals are put in at 12" deep these days, while the frost line can go to 4-5', so I don't understand how combining the dig helps anything. Earth tubes should be in deep, to use the earth's natural equilibrium temp. If you are only going with a foot or 2 of cover, air temp will affect it, you'll be in 70 degree summer/ 20 degree winter topsoil (here) instead of 55-60 degree stable subsoil.
I don't understand how the closed lateral lines are going to get any air to circulate? Or is each of those tubes open to the surface? Then the closest one will pull the most air, the far one won't draw much at all. You want a really long tube (or 2), with slow air movement to do the heat exchange. Your layout looks like short lines, less time for the air to exchange heat, plus a short path overused, a long path underused.
Just too much chance for yuck to happen with that layout, I would not want it. I would not want curculated house air coming from within the leach field. Would walk away from that house in 5 minutes. In my state septic systems have vents, are you going to pull your septic vent air through your pipes?
The closed loop water system would really interest me. If deep enough, that would work well. No chance to pull in nasties, either from the leach field or the condensing musty water in the tube. No critters living in the tube. No smoke or vapors floating by getting sucked into the house. To me the water system is far simpler & foolproof - bury long enough hose deep enough, add a pump & a water to air exchanger in your furnace chamber, and you are done. Can't really goof it up.
In any event, if you are serious about building a heating system, just build _it_. Cutting corners by combining it with a septic system seems like it will be designed to fail? Two long tubes connected to one header by the house, with the pipe burried deep, one or 2 at most air inputs is what you want. This gives you slow moving air in a long tunnel with few restrictive sharp corners, and a good chance for success if you can control the moisture. (Oh, and you know it is actually more efficient to push air through the pipe, than to try to suck it into the house - couple lbs of pressure difference makes a difference on heat transfer?)
Just my opinion, my climate is a lot different than yours as well as my state laws for septic systems so who knows?
--->Paul
