Heat pumps, sewage and sewage lines

So a heat pump works better when the gass or liquid it is stealing heat from is warmer.
A brief search shows that the average temperature of  municipal sewage is 50° to 70  ° F.
I'm gonna guess that the air in the system is a similar temperature.
This gives me some crazy ideas.

-An air sourced heat pump that gets its air source from sewage vents.
To avoid breaking the  liquid seals in the traps we would put in as much air as we extract.
-A liquid sourced heat pump that draws heat from grey water heals  in an insulated tank.
Held greywater tends to turn into to black water, but aeration can  prevent that.
-A liquid sourced heat pump that draws from a counterflow heat exchanger.This could be the least efficient.

I think the air sourced pump could be better because it will get cooler temps overall which will help for cooling, plus no tank of dirty water to deal with.

In a house that has city sewage but doesn't use it, there is way more leeway for such a system.

In house with a septic system, there is already a giant underground container of dirty water.
This could be a place to put a coil.

William Bronson wrote: So a heat pump works better when the gass or liquid it is stealing heat from is warmer.
A brief search shows that the average temperature of  municipal sewage is 50° to 70  ° F.
I'm gonna guess that the air in the system is a similar temperature.
This gives me some crazy ideas.

-An air sourced heat pump that gets its air source from sewage vents.
To avoid breaking the  liquid seals in the traps we would put in as much air as we extract.
-A liquid sourced heat pump that draws heat from grey water heals  in an insulated tank.
Held greywater tends to turn into to black water, but aeration can  prevent that.
-A liquid sourced heat pump that draws from a counterflow heat exchanger.This could be the least efficient.

I think the air sourced pump could be better because it will get cooler temps overall which will help for cooling, plus no tank of dirty water to deal with.

In a house that has city sewage but doesn't use it, there is way more leeway for such a system.

In house with a septic system, there is already a giant underground container of dirty water.
This could be a place to put a coil.

William,
If you steal heat from a septic system the bacteria that break down the solids into liquids that can easily flow through a tile bed will slow down or stop. They need the heat to stay productive.

You are both right...

What could be done is embed pex tubing around the OUTSIDE of the septic tank to collect any heat and route it back to the economizer of the heat pump. In that way you are only collecting the heat that the septic tank is already losing.

But it need not be a heat pump. Just circulating the warm water back to a geothermal, or radiant heat type heating system would net you some gains on the Delta T. I am not sure if the return on investment would be worth it, but in quick thought, I think so.

1. Dig dirt out around the septic tank as much as possible
2. Place coils of pex up against the concrete tank
3. Put sheets of 2 inch stryofoam, perhaps several layers to sandwich the pex between the concrete tank and the insulation to allow heat to migrate to the pex coils and dwell there.
4. Add a pump to get the warmed water back in the pex to the manifold loop of the radiant/geothermal heating system

As an HVAC tech who has worked with ground-source heat pumps, I see some potential, but let me also give you some points to ponder:

For a closed loop liquid ground-source system, I see design numbers in the range of 300-500+ feet of heat exchange pipe in the ground per ton of AC/heating. Heat moves relatively slowly through earth, so if the volume where you're pulling/dumping heat is too small, by seasons end you'll have a too high/low earth temperature and lose efficiency, or even not be able to run the unit if temps get too extreme.

This seems like an easy situation to get in if you're pulling air from the sewer, the main air source is likely to be your neighbors' vent stacks (assuming you can pull air from your sewer line, and still let your sewage through without pulling air from your own vent stacks), so your effective pipe in the ground distance will be the length of yours and your closest neighbors' sewer feeds plus the distance between feeds of the main sewer line. This has the potential to work, especially if you have some distance the air has to travel down the main sewer line (i.e. not another sewer feed just across from yours). The "fresh" water flowing down the line will greatly expand how much heat you can pull/dump.

That is the big issue I see with using a septic tank: the relatively small volume around the tank, and the relatively small inlet of "fresh" water seems to me to be way too small a source for household heating/cooling. A ton of AC capacity is called that because it's equivalent to the heat absorbed by melting 1 ton of ice in a 24hr period, and (at least here in Texas) a household AC system is often 2-4 tons (and similar sizing is needed for heating). You'd have to calculate the water flow going into the septic system and if you have enough flow to dump/extract heat with a reasonable temperature drop/rise. If not, in a very short time of heating a house in winter weather, the septic system might become a block of ice!

(To do the above calculation a ton of AC capacity is 12,000 BTU's/Hour; the definition of a BTU (British Thermal Unit) is the amount of heat needed to raise 1 pound of water by 1 Deg F).

Another potential issue I see is flowing enough air through the bottleneck of piping designed only to flow water/sewage.

The best option would be getting a heat exchanger in the main sewer line of the neighborhood. The decently large non-recirculating flow of water would be ideal for pulling/dumping heat. A pump and dump ground-source system usually involves multiple gallons per minute of water flow through the heat pump, often sourced from a well; waste water would be a much more sustainable source.