Just wondering does anyone have an opinion on the practicalities of this, or if any research has been done on it? You have a heat source (tank) at probably around 15 - 20 degrees celsius year round, would it be possible to put a heat exchanger into the tank and connect it on to a heat pump to source a highly sustainable thermal energy source?

Problems might occur with a layer of slime forming over a heat exchanger however. Any opinions would be appreciated.

Also, along similar lines - I know of a few people who have combined their horizontal ground loops (geothermal system) with the leach field/percolation area of their wastwewater treatment systems. I think in the long run,they're asking for trouble. I think a biomat will form around the ground loop piping, that will hinder thermal transfer. However, they will in the short term invariably gain improved heat transfer rates due to the additional moisture in the soil surrounding the ground loop. Anyone have any horror stories?

You could wrap your ground coil round the outside of the tank - I did that in ooo 1981 but never connected it up. However, I did wonder, does the biological process happen properly if it's cooled? Once it's out of the tank, in the soakaway zone, then no harm in cooling that.

There are some excellent pros, such as a huge surface area with a relatively good thermal conductor. Calculate the amount of pipe you need to make that surface area. The down side is that the heat you are extracting is actually solar energy, so if your horizontal area (for solar gain) is 3m X 1.5m that determines the amount of heat you can extract. As your tap water is at ground temperature, the only real heat gain would be in recycling the heat in the dishwasher, washine machine, bath outlets etc. So if your enegy usuage for the above equals the energy demand of your house for heating, you have won!! I would have thought that heating would account for 80%+ of a houses energy demands in the winter, so I reckon that you could replace 20% of you slinkies with a septic tank loop. I reckon a better compromise would be to use the slinkies trench as your soak-a-way, or vice versa. So you have the benefit of recovered heat and better thermal conduction from the (wet) soil.
Frank

Posted By: galway83as for a heat exchanger in a septic tank, i don't think you could categorically state that it would be detrimental to the breakdown of waste. i don't think sufficient amount of heat would need to be extracted to cause worry in this area, especially if combined with a heat pump.

Sorry, but it's time for a reality check as there is a lot of confusion here between "heat" and "temperature". Where do people think the heat in a septic tank comes from? Obviously its primary source is the excrement that goes in there in the first place. So the maximum amount of heat that could be generated is equal to the food intake of the occupants that "feed" the tank in the first place. Let's assume there are four somewhat lardy adults who consume 3000 Calories per day each. And, for the sake of argument, let's assume that their excrement also contains 3000 Calories each (i.e. they each digest about half of what's available in the food). 4*3000 Calories is 13.96kWh. Not a huge amount of heat. A more realistic figure is probably half this. Since a heating system probably requires 5-10x this amount, the septic tank would be rendered pretty cold pretty quickly and this would, of course, slow down the waste digestion. As for combination with a heatpump, this doesn't magically make more heat available, it just makes it usable - the heat still has to come from somewhere.

To cut a long story short, it's a waste (pardon the pun) of time to try and use the heat from a domestic septic tank and running refrigeration loops through such a tank would quickly degrade its performance - just in the way stuff in a fridge rots more slowly when it's cold.

Paul in Montreal.

To disagree with you Paul, I'd have thought myself that the majority of the heat in a septic tank came from energised wastewater. I'm aware of a study that found that the typical daily hydraulic loading rate to such domestic on-site wastewater treatment units was circa 180 litres per person.

Don't know about yourself Paul, but I don't produce 180 litres of faeces a day, even though with the cantankerous response left by yourself above, maybe you might have the disadvantage of emitting supplementary volumes from additional orifices to the ordinary person.

Also the majority of treatment of WW takes place in the percolation field anyhow, with relatively little breakdown occurring in the septic tank in the first place.

Good point about septic tanks not doing much in the way of biological breakdown, the septic tank mostly just separates solids from the liquor (lovely word).
But I would agree that a tiny little domestic septic tank wouldn't be much use for anything.

We have a domestic sewage treatment plant (not a septic tank) it has a big wheel that goes round and trickles the water through a bio medium. I wonder if we could put a heat pump into the bottom of this (the wheel goes round in a section at the top). You could even put the heat pump down the house waste pipe so you wouldn't have to dig any holes. ???

I thought the heat was produced by the decomposition of the waste, NOT by the warmth of the waste as it leaves the human body. I agree with paulfrom montreal, you don't want to remove that heat from the rotting matter.