About 2 years ago I built a new house. It is an all electric house and I'm trying to get the lowest energy footprint possible. I put a coil of 3/4 pex in my septic tank and circulate a closed loop antifreeze solution through it. I use this loop to bring heat from the septic tank to a tube in tube heat exchanger which is my evaporator. I then used a small airconditioning compressor to move this heat into another tube in tube heat exchanger which then sends the heat into my water heater. It runs about normal airconditioning pressures..... 40 degree low side and 130 degree high side.
If I had it to do over the only change I would make would be to insulate my septic tank before back filling, as I'm sure I'm loosing heat into the ground before I reclaim it.
It was a fun project, and has been working great. Please feel free to copy my idea.