Free Heat from Septic Tank

Hi! I copied this conversation from https://permies.com/t/172751/Tonight#1730614

There are  also some related points in https://permies.com/t/187778/Septic-System

"What I am Doing Tonight"?

Well, I'm setting the tone for the new year. It's not quite midnight. I need to melt snow for DW's houseplants and to keep my fish alive. But it bothers me to bring pails of snow inside -- that's another dollop of natural gas, and we burn more than I like already.

But: I noticed a free heat source. Our septic tank is right beside the house. I always cover the liquid side with plywood or something and bury with snow for insulation, because there's an anti-siphon loop close to the top and I don't want it to freeze. This year, even at -35C, I noticed there was enough heat coming off to melt holes in the snow I piled on.

So tonight, I took an old enamel canner full of ice and snow, added a litre of water for heat transfer, and set it on top of the septic tank lid. It was literally warm to the touch. Still, I had to cover it -- with a broken plastic tub that was headed for the dump eventually. And then I shovelled on a big pile of good old prairie fluffy snow for insulation.

Yes, I have pics, but this beater laptop can't pull them off my phone. Stay tuned. BTW I illuminated them with a Pelican headlamp that I recycled from the dump yesterday.

Top that, permie peeps! And a Happy New Year to you!  

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Jay Angler wrote:

Jane Mulberry wrote:Douglas, your creative use of the septic tank made me laugh till I teared up! Love it!  

My thoughts immediately went to, "I wonder if I could start seeds over ours... I'd need deer fencing..." Unfortunately, ours doesn't get the hours of sun that young plants would appreciate - but it might work for some people!

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Douglas Alpenstock wrote:Mine is in full shade too, so no growing. But I may have found my spot for winter composting.

It usually takes 24 hours to melt a big pail of snow in the bathtub. So I'll check my magic hob tonight and report back.

Got a thermometer you can use to get outside air readings, and soil readings? I'm curious what kind of rise you are getting.

:D

Like Jay, mine is in full shade. But I am also wondering whether I can make some use of any extra warmth that might be there. When I'm next at that house I will use the compost thermometer to check.

I put a raised bed with cold frame over ours at the old house. It made a big difference, probably another two zones.

Thinking about this, I recall that the septic tank at my old place didn't push out a lot of heat at all. This was in deep clay, and likely a lower volume. And that pure clay would be an infinite thermal mass, sucking heat away. I never noticed snow melting in 20 years.

But in our new digs it's a bigger, deeper tank. Buried in a fine sand/silt that packs tight but dries out quickly and becomes a reasonably effective insulator. Lots of residence time for the anaerobic compostables; pumpout is maybe once a week. There is real heat on the heavy concrete lids. Hmm.

I always wondered why people with septic tanks did not do this? On a new tank, where they come in halves, right before putting them together, a coil could be inserted on the inside of it.

I can see the gross factor being at play, perhaps some fouling of the overall flow inside the tank, and possibly drawing off enough heat to stop human manure composting, but it would certainly provide some heat for domestic needs.

Steve Zoma wrote:I always wondered why people with septic tanks did not do this? On a new tank, where they come in halves, right before putting them together, a coil could be inserted on the inside of it.

All of them have top access, so a retrofit is possible. But it's a pretty low grade heat source. You'd need to use electricity to pump water down through the coil. On a residential scale, I'm not sure the net energy gain would be worth it.

A passive arrangement seems more viable to me.

It worked! Not quite a home run, but definitely a qualified success.

From a canner packed heaping full of ice and packed snow, I ended up with a canner of 60% meltwater with a raft of loose slush floating on top. I would estimate 4 gal/ 16 litres.

Some refinements are needed for the next round. Better insulation on the inside, mostly.

Temperatures as I opened it up after 36 hours:
-13 C  / 8 F on the ground outside
-6 C / 21 F  on the side wall of the tub
-2 C  / 28 F on the side of the canner
+6 C  / 42 F on the concrete lid ("hob") of the septic tank

Well done! Can't wait to see the next iteration...will it be a stacked "oven" of straw bales?

I have seen people surround the sides of their tank and the top with foam to keep heat in. I would be worried that if you drew out too much heat you tank or bed would freeze. Not at the scale you are talking but if you tried to do the heat exchanger thing.

David Baillie wrote:I have seen people surround the sides of their tank and the top with foam to keep heat in. I would be worried that if you drew out too much heat you tank or bed would freeze. Not at the scale you are talking but if you tried to do the heat exchanger thing.

I think that's a valid concern, particularly for a septic field/bed in our cold climate. A freeze-up would be a royal PITA. My setup wouldn't be affected, but it's worth keeping in mind.

I'll stick to the lazy method of recapturing a little waste heat to solve the problem at hand.

This totally gives me some hope that my idea of starting seeds might work if Hubby actually gets a few trees cut down that are threatening our power lines.
Nissa started a thread here: https://permies.com/t/207198/Seed-Starting-electricity
specifically about finding ways to start seeds without using electricity. There are two factors - one is heat and the other is light. The heat is often the expensive one of the two needs.

I'm mulling over ideas of how to build a simple shelf system - maybe tiers to capitalize one whatever light we get - that would help plants that can't take freezing. I'd not actually be "removing" heat from the septic tank. I'd just be reusing any heat that escapes on it's own. I think it would need some sort of a cold frame over it to hang onto that heat, but unfortunately the frame I'd use had chickens in it at the moment.

Douglas Alpenstock wrote:

David Baillie wrote:I have seen people surround the sides of their tank and the top with foam to keep heat in. I would be worried that if you drew out too much heat you tank or bed would freeze. Not at the scale you are talking but if you tried to do the heat exchanger thing.

I think that's a valid concern, particularly for a septic field/bed in our cold climate. A freeze-up would be a royal PITA. My setup wouldn't be affected, but it's worth keeping in mind.

I'll stick to the lazy method of recapturing a little waste heat to solve the problem at hand.

I think you could totally scrape the surface of the tank and insulate everything except the two covers. You would create sort of water melting hotplates on your tank. Any loss of heat to melting would be made up by increased insulation on the rest of the tank and you would still have access to the covers for pump outs.

David Baillie wrote:I think you could totally scrape the surface of the tank and insulate everything except the two covers. You would create sort of water melting hotplates on your tank. Any loss of heat to melting would be made up by increased insulation on the rest of the tank and you would still have access to the covers for pump outs.

Cool idea. I'll let you know when it makes my Top 10 to-do list.

Just kidding of course. This tank seems quite deep compared to my old one, which is perhaps why it stays hotter. There's no way I'm digging it out unless there is evidence of failure -- that's a backhoe job.

But I wonder if I could run a coil right underneath the lid and use a small, low power consumption water fountain pump to push heat to a raised bed cold frame where the sun hits it in March. Backing onto a dark brown insulated shed. Maybe add a thermal break and some thermal mass underneath. Hmm. That would be cool, and confirm my neighbours' suspicions that I'm a madman. Haha.

Douglas Alpenstock wrote:

Steve Zoma wrote:I always wondered why people with septic tanks did not do this? On a new tank, where they come in halves, right before putting them together, a coil could be inserted on the inside of it.

All of them have top access, so a retrofit is possible. But it's a pretty low grade heat source. You'd need to use electricity to pump water down through the coil. On a residential scale, I'm not sure the net energy gain would be worth it.

A passive arrangement seems more viable to me.

Maybe???

I think there is a fair amount of heat in a septic tank though. Around here they are 1000 gallons, so to melt the snow on top of the grounds says how much heat they are putting out. That is a HUGE amount of mass already heated to a pretty high temperature, and that says nothing about the 57 degree ground around it that has also been heated up.

How to use it depends.

Passive heat does not work for me here in Maine, and especially so with radiant floor heat. That is a low and slow kind of heating system, but my system does not care how that warm water is delivered to it. I say warm because it is only 85 to 100 degree water that circulates through my floor. It is constantly flowing but is not all that hot. This is perfect for a Jean Pain Composting system or gleaning a bit of heat out of the septic tank.

Passive solar does not work because it defeats the heating system. It warms the room, but only when the sun is shining. When it goes out, the heating system runs double-time to try and keep up. On cloudy days, it does the opposite. It heats the floor to try and make the house the right temp, but when the sun comes back out, the house is suddenly roasting.

There is no way a 1000 gallon septic tank is going to heat a home all winter, but the right home, with the right heating system, would enable the house to be heated in the shoulder seasons (fall and spring) reducing the overall heating cost. I think anyway.

I've noticed that the working "high" level in my septic tanks is a lot lower than the maximum capacity -- less than 50%. I imagine that reduces the total recoverable heat by a fair bit.

I don't know if that setup is mandated somehow, but that's how the pro's do it. It certainly gives a wide safety margin in case of a pump failure or an extended power outage.

Round 2:

For better results, I put a lid on the enamel canner and loosely wrapped the sides and top with 1" polyester padding from the cushions of a dead couch. Same setup otherwise.

Result: 2 days later, the canner is 60% full of luke-cool meltwater. Perfect.

Douglas Alpenstock wrote:I've noticed that the working "high" level in my septic tanks is a lot lower than the maximum capacity -- less than 50%. I imagine that reduces the total recoverable heat by a fair bit.

I don't know if that setup is mandated somehow, but that's how the pro's do it. It certainly gives a wide safety margin in case of a pump failure or an extended power outage.

I never even thought of a pump scenario, but you are right, if the septic system is set-up with a pump system, then how full the tank is varying quite a bit. At work I deal with A LOT of sumps and pumps, so I can see why that would be a concern for people with that type of system. Around here, the terrain is rolling hills so almost everyone has a gravity feed system so the septic tank is always full and constantly leaching off without a pump.