Last summer I built the Methane Midden which was inspired by Jean Pain’s epic work in 1970 France. It was big, it was a little insane, and ultimately it didn’t work. Don’t get me wrong – it made a syck amount of compost, but it failed to meet my goals of also producing methane and hot water. Time to do better. The second Midden will focus on Hot Water (L’eau Chaude) with a dedicated heat exchanger and will be even larger. Not only that, but I wanted to learn how much material it truly takes to make one of these so I also sourced all the biomass myself from two local farms, dropping literally hundreds of invasive buckthorn and honeysuckles as well as aggressive “weed” trees such as red mulberry and box elder over a week and then renting a 27hp Vermeer chipper to grind them up. It was awesome!
This post is pretty epic itself – I opted for smaller photos in the post to keep the overall post length a bit shorter – click on them for a 600×800 shot. This really should have been 2-3 posts, but I spent more time working than writing. Skim or read it all the way through at your preference. Total work covered in the build is 3 weeks, with another 1.5 in the biomass acquisition. These are “puttering” time lengths – I am always working on 4-10 projects at a time :).
With the material on site it was time to get it soaking. One of the keys to Jean Pain style brushwood composting is to soak the material for 2+ days to ensure that the chips are saturated to supply the pile with enough water to sustain it for months. One of the problems I ran into early on is that of scale. I can only soak about 6 yards of material at a time, which limited me to 1.5 batches a week with my days off and the shorter daylight of Fall. All told it took over 3 weeks just to soak all the material.
Once the material was soaking it was time to prep the ground. I spent a day spreading the old Midden around the gardens. This was unreal – never before have I had 8+ yards of compost to spread all at once. EVERYTHING got mulched. Yes, spreading compost by the wheel barrow load (40 of them!) is as fun as it sounds. I will say it again and again – COMPOST is the true point of the Middens – gathering energy from them is merely function stacking an intrinsically useful activity.
As this pile was going to be 50% larger I had some concerns about it breathing well. At one point I had myself talked into laying out 4″ perforated drain tile for air circulation, but thought better of it for several reasons.
First – it added expense and resources. I like to keep the plastic to a minimum; while not afraid to use energy or resources for the Greater Good, I also like to use natural products whenever its feasible. Second, Jean Pain never did so I’m not even sure its necessary. My solution was simple – I cut down a half dozen Cupplants that were nearby in the prairie garden that serves a insectary habitat near my permaculture guilds. The thought is that these will allow air to come in slowly from the under the pile as they decompose themselves. Basically the same reason you are often told to put twigs under a normal passive compost pile.
Now, the observant amongst you will notice a piece of .5″ conduit stuck in the ground with a tape measure in front of it. Of course there is a very good reason for this. The Midden LC will have a 3′ (1 M) diameter core of brushwood that will be wrapped with 1″ tubing, but I am getting a bit ahead of myself. With the base down, it was time to get building.
The core diameter of 3′ was chosen for the simple reason that hot composting seems to benefit from a minimum dimension of at least a meter. Build a pile smaller than this, and you won’t get hot enough. One of the downfalls of the Methane Midden is that the layout forced a width of only 2′ in most cases. I thought that the straw bales insulation would be enough. I was wrong. Back to basics then. Getting wood chips to form a cylinder can take some doing. Luckily I had enough 2′ garden fencing in the garage to make a “mold”. The thought was to cut the fencing to the circumference of a 3′ diameter circle. Time for fun with math!
A circle’s circumference is Pi x the Diameter of a circle; 3.14 x 3′ = about 9.5′. Done. Now, when you cut the fencing – leave the “nubbins” on the cuts – this works well to fold around once you get the circle made – think giant velcro. The conduit I stuck in at ground zero and measured 18″ off each side to center the core column. Then it was simply a matter of schlepping in the soaked chips. About every 8″ (22 cm) or so I tamped the chips well with a 12# (5kg) sledge hammer. Once I had 2′ tamped in place, I unhooked the fencing and unwound it. Thanks to the tamping, the chips stay in place very well. I then wrapped the fencing on the top of the cylinder with about 2″ (5 cm) of overlap on the bottom. Then I started wrapping the heat exchanger around.
This progressed 2 times until the core got about 5′ tall. At this point I was becoming somewhat concerned that the column could topple, despite how stable it seemed. Redoing it at this point would SUCK, so I opted to switch gears and work on the outer “donut” of biomass. The thought here was to again use the 1 meter width of material to maximize the bio-reaction of the thermophilic bacteria. Having 3′ of material on each side of the exchanger giving the Midden L’eau Chaude a total diameter of a bit over 9′ (2.75 m). The other important reason to start building out the “donut” was that stacking, and especially tamping, the Core was getting difficult as it was over shoulder height for me at this point. With a 2′ tall rim around the Core I could bring it up to its goal of 6’+ bringing the total heat exchanger length to over 550′ (168 m). The heat exchanger needs a bit of explaining since it is critical to the Midden, so here goes.
The Heat Exchanger
This is the finished heat exchanger and I won’t even try to be modest. It’s GORGEOUS!. The water will come in the bottom, spiral up for 6′ and then take two larger loops back down to exit the pile. Why this way? It has a lot to do with temperature gradients. Heat transfers best the larger the temperature differential (delta T). The final goal is to that the water entering the pile at about 80 F (26 C) and exiting north of 120 F (49 C). The bottom of these piles are cooler (heat rises!), but will still be rather warm. As the water in the exchanger warms it is also climbing up through the pile, which is also getting hotter. The thought is that there will always be a delta T of 20+ degrees between the pile and the water in the tubing. Slick, huh? Now wth is up with the big looping spirals? The poly tubing kinks somewhat easily, so I chose to take 2 revolutions to expand the diameter of the spirals and slowly increase the degree of bend up to my final exit point. Now some final thoughts on the exchanger. First, I would shift the hole thing up a foot. Starting 4″ from ground level like I did will not add much heat to the water, whereas there is over a foot above the top of this column now that I am done. I had thought that the top would be cooler, but that is not the case – no matter how much I pile on the heat just comes right through it: put your exchanger to within 6″ of the top without fear as long as the pile is hot. Can you see I am already planning Midden #3? But for now we need to bury that exchanger. Fleshing out the Midden
To keep the pile nice a tidy, and also to ensure that I could still walk around it, I again used a garden fence mold for the 9′ (2.7 m) diameter “donut”. this time I opted for 3′ tall fencing. The outer section takes an immense amount of material to fill – it was almost 2 weeks to get it to this point as the pile at this point was over 12 cu yards and weighed over 6 tons due to the sopping wet chips. Remember that I could only do 5-6 yards at a time as the chips had to soak for several days. With the straw in place around the fencing, I then stacked a third tier of straw to form the mold for the final several yards of material. In the shot at right the pile is nearing 7′ (2.1 m) tall which was the goal. At this point I was nearing the end of the biomass and began to focus on rounding out the pile and maintaining the 3′ length from the heat exchanger as much as possible. Was incredibly pleased with how the Midden was literally shaping up. Sometimes a project just CLICKS. This was one of those!
To keep the pile nice a tidy, and also to ensure that I could still walk around it, I again used a garden fence mold for the 9′ (2.7 m) diameter “donut”. this time I opted for 3′ tall fencing. The outer section takes an immense amount of material to fill – it was almost 2 weeks to get it to this point as the pile at this point was over 12 cu yards and weighed over 6 tons due to the sopping wet chips. Remember that I could only do 5-6 yards at a time as the chips had to soak for several days.
With the straw in place around the fencing, I then stacked a third tier of straw to form the mold for the final several yards of material. In the shot at right the pile is nearing 7′ (2.1 m) tall which was the goal. At this point I was nearing the end of the biomass and began to focus on rounding out the pile and maintaining the 3′ length from the heat exchanger as much as possible. Was incredibly pleased with how the Midden was literally shaping up. Sometimes a project just CLICKS. This was one of those!