I recently scooped up an industrial double-layer greenhouse, wood stove and blower for next to nothing provided that I disassemble it and remove the twenty or so years of crap surrounding it.

Done. Harder than I thought it was going to be, but it's here.

Now I have to wait until at least spring before I can rebuild it and I get lots of time to daydream about the details.
Every property where I live is on septic systems.
All of them.
My property is no exception other than there are like seven systems or so, (I haven't located them all yet as they are buried and exposing them is even less fun than it sounds).

One of these systems is a sani-dump for the campground. It's one of only a few in the area so we literally take every body's shit.
That shit gets hot, like 130 degrees or so. We don't have any papers on just how big it is but we can find no evidence of it having ever been pumped out yet.
The greenhouse is going to be located fairly close to this tank and I am toying with the idea of dropping a coiled copper water-line directly into the tank and plumbing the upright intake and outflow ends so that an enclosed loop then snakes back and forth beneath the floor of the greenhouse.

Hot water rises and cold water descends so the fresh water in the lines will circulate passively without any pump as long as the system remains filled and there are no low spots for cold water to pool.

Pack those floor pipes in sand and then cobbles to slow release the heat and there's a good chance I will need to vent heat even in the middle of winter.

If I plumb it so that fresh water can enter the convection loop and leave an up-stack with a tap to bleed off any air which might enter the system it would also provide hot water on demand.

The temperature differential between the tank and the outer air could also run a heat engine to provide power for winter lighting, pumps, and fans.

I'm going to see if I can try and figure out how to keep this little biosphere self sufficient year 'round and grow me some citrus fruit without a power bill.

Why?
Because I can of course.

Picture it. Dog-sledding my way from the igloo to the greenhouse every morning to toil shirtless in my little passive tropical oasis garden as another 20 cm of snow falls that day.

Glorious.
-- 2 fries shy of a happy meal, Dec 24 2021

You probably knew all this, but ... https://www.biocycl...naerobic-digestion/
[pertinax, Dec 25 2021]

Biocorrosion of copper https://www.ncbi.nl...rticles/PMC5615691/
[bs0u0155, Dec 27 2021]

Canadianized Ausie critters... https://kangaroocre...angaroo-creek-farm/
...or at least a few of the one's who aren't bent on killing you. [2 fries shy of a happy meal, Dec 28 2021]

// Not new... just new to me // Did you find any good info about other people doing this for small septic systems? I've thought about this before, but didn't find any good info. I didn't pursue it much because I didn't figure I'd get that much heat out of my little domestic septic tank. I also worried that if I cooled it too much I might make it less effective for breaking down waste, potentially damaging the drain field. I guess I was worried about making it too cold since I was planning a ground source heat pump which could actually end up chilling it significantly. I can't see how there would be much harm in cooling it to room temperature.

You might consider using pex pipe instead of copper. It's a whole lot cheaper. It doesn't conduct heat nearly as well, but should be good enough. I used it in my hydronic floor heat, and that seemed to be pretty standard.

Also, if you're serious about hot water on demand, make sure you use a proper double-wall heat exchanger. It's required by code around here when "mixing" hydronic heating and potable water, and probably a good idea since one small leak in the pipe through the septic tank could ruin your day.
-- scad mientist, Dec 25 2021


And good luck with the project. I'd love to hear if it works out.
-- scad mientist, Dec 25 2021


People have used compost heaps for heating (& cooking) so not really any reason this won't work. Check on the details of using a thermosiphon, especially the lift you want & pipe diameter required. Might need a little pump (solar?) to get everything to work the way you need. Calculations for heat exchangers can get messy (I gave up last time I tried; "proportional to this one I'm mostly copying" was good enough..).
-- neutrinos_shadow, Dec 25 2021


// Did you find any good info about other people doing this for small septic systems? I've thought about this before, but didn't find any good info. I didn't pursue it much because I didn't figure I'd get that much heat out of my little domestic septic tank. I also worried that if I cooled it too much I might make it less effective for breaking down waste, potentially damaging the drain field.//

I was just dreaming up ways to heat the greenhouse and came up with this. I knew about the thermosyphon principal but hadn't looked up anything about septic heat retrieval. I'm not too worried about cooling the sani-dump down, but the smaller tanks might suffer from heat loss.

You might consider using pex pipe instead of copper. It's a whole lot cheaper. It doesn't conduct heat nearly as well, but should be good enough.//

hmmm, I was wondering if copper would corrode if submerged in waste.

//Also, if you're serious about hot water on demand, make sure you use a proper double-wall heat exchanger. It's required by code around here when "mixing" hydronic heating and potable water, and probably a good idea since one small leak in the pipe through the septic tank could ruin your day.//

See? Brilliant. I doubt I would have gone as far as a secondary heat exchange, but I will now.

//And good luck with the project. I'd love to hear if it works out.//

Thanks, I will.

//People have used compost heaps for heating (& cooking) so not really any reason this won't work. Check on the details of using a thermosiphon, especially the lift you want & pipe diameter required. Might need a little pump (solar?) to get everything to work the way you need. Calculations for heat exchangers can get messy (I gave up last time I tried; "proportional to this one I'm mostly copying" was good enough..).//

According to the discussion a1 linked to pipe diameter and length are the only impediments to passive heat circulation without pumps but I will be sure to leave access points in case some augmentation is needed to get the whole thing up and running.
-- 2 fries shy of a happy meal, Dec 26 2021


//if you're serious about hot water on demand, make sure you use a proper double-wall heat exchanger.//

I'm glad this is part of code. The last thing you want is a single point of failure that allows raw sewage into the food/water supply. Especially if only corrosion is needed and that corrosion can occur on a component no one ever wants to inspect. Sewage like that has a near infinite variety of bugs, many of which probably require copper as a trace mineral and many that also enjoy performing complex oxidation/reduction chemistry on any available transition metals. Reading around, it seems I've just worked out "Biocorrosion" from the ground up <link>.

Before doing any major work, it might be worth figuring out how much energy is on hand. What you have is a tank that's likely large, therfore low surface-area volume ratio, and possibly well insulated, e.g. a plastic tank sitting on/surrounded by dry gravel acting as an air gap.

The exothermic decomposition might be able to reach the equilibrium temperatures you mention with only a few hundred Watts, which probably isn't worth trying to extract, especially since the rate of reaction in there is temperature linked, so if you extract too much heat, the temp drops and the heat generation drops of exponentially.

You could work it out by measuring the temp. while extracting a known amount of energy, the easiest way I can think of doing that is to dump snow in there. I think you're likely to have something in the order of kW, but who knows?

Do you also get burnable methane off-gas?
-- bs0u0155, Dec 27 2021


// Reading around, it seems I've just worked out "Biocorrosion" from the ground up <link>.//

Nice. Double walled it is.

//Before doing any major work, it might be worth figuring out how much energy is on hand. What you have is a tank that's likely large, therefore low surface-area volume ratio, and possibly well insulated, e.g. a plastic tank sitting on/surrounded by dry gravel acting as an air gap.

The tanks are old-school concrete and if the one's I've had replace so far are any indication they were just back filled with dirt.
The sani-dump tank is very deep and must be well insulated because the snow above it does not melt.

//The exothermic decomposition might be able to reach the equilibrium temperatures you mention with only a few hundred Watts, which probably isn't worth trying to extract, especially since the rate of reaction in there is temperature linked, so if you extract too much heat, the temp drops and the heat generation drops of exponentially.//

This tank holds the waste from fifty RV sights as well as the waste from every campground in the surrounding area which don't have full hook-ups.
It has been filling since the early eighties with no signs of it getting anywhere near full. There's so much biomass in there I doubt I will leach enough heat away to drop the overall temperature even a degree with a single heat exchanger.

//You could work it out by measuring the temp. while extracting a known amount of energy, the easiest way I can think of doing that is to dump snow in there. I think you're likely to have something in the order of kW, but who knows?//

From what I've read large septic tanks cook at a stable 130 degrees Fahrenheit or more.

//Do you also get burnable methane off-gas?//

Yes, but that will be a future project.
I will need to replace several more systems before this place is up and running.
If this first experiment is successful I think I will wrap the exterior of any new tanks in water lines and then insulate before back-filling and find out the cost of methane digesters just on spec.

My hope is to utilize as many passive energy sources as this property can provide to try getting our power bill down to zero.
When the sun shines my solar furnace turns sand molten.
Incoming water pressure turns Pelton generators as it fills a water tower, which again turns the turbines when released.
Vertical windmills at strategic locations provide power when the wind is blowing.

...and I'm considering flywheel storage of the energy so that I can give Fortis, (local power company), the constant sign wave they require to turn my meter backwards.

If I can push it past zero they will cut me a cheque for the wholesale difference between whatever power I provide over what I consume.
Then I plan to grow most of our own food.
I'm not allowed to keep livestock here but I found a loophole in the form of a petting zoo which I am allowed to run.
I imagine that minature milk cows would be very contented getting hand fed and petted daily...

<completely off topic now>

Any Kangaroos currently living in Canada are Grandfathered in but no new blood can enter the country.
As such they have started to become inbred and have albino babies and such.
They are looking for places equipped to expand the number of kangaroo farms.

We could have Polar-roos someday!
-- 2 fries shy of a happy meal, Dec 27 2021


I don't think they'd like snow much - not because of the temperature, but because of the way they travel; deer can trot, picking up their hooves so as not to have to move too much snow at each step, but I don't think roos can do that.

You'ld have to give them wide snow shoes so that, when they did their mighty downward thumps, they wouldn't just be digging themselves into a hole.
-- pertinax, Dec 28 2021


They don't seem to mind snow so much as long as they have a warm dry enclosure to duck into.
They are smaller than normal kangaroos but there are certainly kangaroos living in the snow belt not too far from here, and many other normally exotic critters, [link].

They get hundreds of visitors a day during the season but strictly refuse to market any secondary merchandise which I have a hard time wrapping my head around.

Who wouldn't want to get their kids pouched hoodies with little ears and tails that say;
You Will Roo The day!
on the back?

...and I mean, c'mon! Albino kangaroos? In Canada!...
I'm torn between calling them Polaroos or Arcticangaroos.

If this heat exchange thing works well enough I can make them a passively heated preserve with native plant species and such.

Oh we're going to have so much fun.
-- 2 fries shy of a happy meal, Dec 28 2021


Interesting; have you been able to observe how they walk in deep snow? Maybe they've come up with a good trick for it.
-- pertinax, Dec 28 2021


Anyway, back on topic ... there's a reason why I posted that link on aerobic vs. anaerobic sludge digestion. Many years ago, I was at a trade fair for the water and sewage industry, and I remember that one of the first strategic decisions you have to make when designing a new installation was whether to go aerobic or anaerobic.

It seems, from the linked article, that the aerobic process, which is substantially identical with composting, will give you more heat, but the anaerobic process will give you useful amounts of methane.

So, this would suggest, if you wanted to optimise for heat output, then you might want to think of a way to introduce more air into your feedstock, and maybe break it up into slightly drier chunks.

On the other hand, if you wanted to optimise for methane, then there's this; if, over the next few years, fossil fuels are phased out, and if there are still people who like cooking with gas, that might increase the market value of any bio-methane you produced. However, if I'm reading this right, that approach might call for an increase in the number of distinct sludge tanks you had to work with.
-- pertinax, Dec 28 2021


//Interesting; have you been able to observe how they walk in deep snow? Maybe they've come up with a good trick for it.//

Never seen it myself but the videos on line show them hopping around alright in the bit of snow they've seen recently down there.
Now I want to see how they deal with deep snow too. I'll drop them a line and see if they have any snow videos.

//one of the first strategic decisions you have to make when designing a new installation was whether to go aerobic or anaerobic.

It seems, from the linked article, that the aerobic process, which is substantially identical with composting, will give you more heat, but the anaerobic process will give you useful amounts of methane.

So, this would suggest, if you wanted to optimize for heat output, then you might want to think of a way to introduce more air into your feedstock, and maybe break it up into slightly drier chunks.

if you wanted to optimize for methane/
that approach might call for an increase in the number of distinct sludge tanks you had to work with.//

I'm just trying to work with what is already here. Not exactly sure what would happen if I dropped an aerator to the bottom of a cesspit that's been anaerobic for several decades.
I was not aware of the difference in either heat or gas production regarding the two systems so that gives me something to think about as we swap things out in the future. Thanks eh.
-- 2 fries shy of a happy meal, Dec 28 2021


//There's so much biomass in there I doubt I will leach enough heat away to drop the overall temperature even a degree with a single heat exchanger.

From what I've read large septic tanks cook at a stable 130 degrees Fahrenheit//

Huh, so it's at equilibrium but not one constrained by heat generated vs heat lost. It's an equilibrium constrained by the fact that many bacteria start to be inhibited by high temperatures, 130F/52 ish C is at the edge of conventional heat-tolerant bacteria, much above that, you're into extremophiles. That's interesting, because if you take heat out of the first one, you enter a positive (in a negative direction, makes sense, I promise) feedback and the temperature crashes. In the second scenario, I reckon if you take heat out and drop the temperature very slightly, you'll reach a new equilibrium where you approach a more comfortable temp. and the bugs work harder. A much more resilient feedback.

//hope is to utilize as many passive energy sources as this property can provide to try getting our power bill down to zero.//

very noble, and while septic tanks are low-grade heat, the commonest and least versatile energy source, if it's high capacity, it can certainly do the heavy lifting on all the low- grade heat needs, like floor heating and hot-water pre heating. If you tossed the cold end of a commercial ground-source heat pump in the tank, that would be a convenient solution that works out of the box - I imagine they design to a worst-case scenario where the cold end is in super corrosive/contaminated ground.

//when designing a new installation was whether to go aerobic or anaerobic//

At the municipal scale, perhaps. They're dealing with a lot of, er, stuff. They have a relatively small area of land and they have to deal with it quickly. The usual, aerobic system is to get as much O2 in contact with the flow as possible. Ultimately, that's done by bacteria growing, they waste a little energy all the time, they run out of food, start eating each other, all the time wasting energy (making CO2) and the left over nitrogen from protein is mineralized as soluble nitrate (NO3). The CO2 floats off and all you're left with is soluble minerals in the water and insoluble minerals (sludge).

Anaerobic is similar, but without O2, carbon is exported as methane, N2 & maybe some H2S. Now, nature doesn't choose between aerobic and anaerobic, and neither does a low-throughput septic tank.

Nature/a tank can run a hybrid. The top layer will have bugs making a conventional living oxidizing carbohydrate/lipid/polypeptide etc. to CO2 & NO3, and deeper (6"-1'+ depending on surface O2 + permeability/agitation of the stuff) you can have anaerobic bugs making a living by oxidizing carbon chains with the dissolved NO3 to make N2, along with a whole load of other chemistries, iron/sulphur etc. You can have a much more complete degradation with the hybrid system, with the trade off being speed/capacity. You will end up with a crude mix of minerals left over, or "soil" as nature calls it.
-- bs0u0155, Dec 28 2021


One more thought about using your 130 degree (F) septic to heat your greenhouse. That sounds good initially, but assuming you can extract enough BTUs to heat your greenhouse, you still need to be sure that you can transfer that heat to the greenhouse. When I was designing my radiant floor heating I was seeing a lot of examples where people set their boilers to 160 degrees. For my design, I purposely put the pipes in more densely and was somewhat careful with floor cover selection to try to minimize the required temperature to make it more efficient when using a heat pump. I can heat my house with 100 degree (F) water (probably lower, but I haven't experimented yet), but my house has very little air leakage (1.1 ACH) and R21 insulation, and I'm probably in a milder climate than you (a bit inland from Seattle). A greenhouse will be much less insulated and will likely leak more air, so I'm guessing it may need a lot of pipes in the floor to heat it with water somewhat cooler than 130. I used a program called LoopCad (free trial) to figure out how hot the water would need to be for various floor heat pipe configurations.
-- scad mientist, Dec 29 2021


//native plant species//

Just a note of caution there: plant species native to Australia tend to have a higher ratio of oil to water in their makeup than their peers on other continents.

The significance of this becomes apparent when they catch fire*.

On the positive side, some people have speculated about extracting terpenes from gum trees to make organic non-fossil jet fuel.

*I seem to remember that some of the recent wildfires in California have featured introduced eucalypts in prominent roles.
-- pertinax, Dec 29 2021


All very appreciated advice. I gotta lotta loinin ta duz.

Ooh wait!...

...Canadangaroos.
-- 2 fries shy of a happy meal, Dec 30 2021


I seem to remember reading about some stately home in/near Paris which had been re-engineered to scavenge heat from a large sewer-pipe which passed nearby.
Makes a lot of sense.
-- Loris, Dec 30 2021



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