Aerating a compost bin

Elsewhere I started a thread about designing a forced-air compost heating system for my neighbour's studio but thought I'd make a new thread on this as it would apply to composting in general.

I want to make the composting bin as mainentance-free as possible. It will be about 10' long by 5' high and wide.

Knowing that it will need to be aerated, I'm thinking of framing in the bottom with heavy steel mesh (about 1.5" square openings) on top, and mounting the box about 6" off the ground to allow airflow beneath.  I'm in the Paciic NW and we get a fair bit of rain, so I'm also thinking of a metal-clad lid which would have about a 3" gap around the edges rather then closing tight, to also aid with airflow through the pile while keeping rain out. It could be opened to allow access for turning/poking/watering, as necessary.  I've heard of people using fans and perforated pipes for aeration but I'd like to avoid that and encourage natural convection.

Would it also be advisable to put slatted, rather than solid, sides on this? I've seen small home-made compst bins with slatted sides. If so, perhaps 1 x 6 (actually 5.5") with about a 1" gap between them? On the other hand, this might allow too much cooling around the edges?  Ultimately some of this will have to be determined by experiment but I thought I'd ask here to get opinions from people with some experience.

I definitely would not aerate the sides of the pile.
Maybe seal the whole thing, put a solar chimney on top and supply "preheated" air into the bottom?
The preheated air could come from an earth tube, a solar collector,  or a combination of the two.

William Bronson wrote:I definitely would not aerate the sides of the pile.
Maybe seal the whole thing, put a solar chimney on top and supply "preheated" air into the bottom?
The preheated air could come from an earth tube, a solar collector,  or a combination of the two.

Many thanks. Creating draft through the pile via a solar chimney + earth tube is interesting! I suspect the solar chimney would only work in a suitable climate and location with lots of sun, though. The purpose of this ezperiment (as it will be at this point)  is to heat the studio in winter and its location doesn't really take max. advantage of solar gain, plus we get too much dull, dreary weather over the winter to make a solar chimney practical.  Perhaps more practtical would be the earth tube and a fan at the top of the box that would be controlled via a moisture meter, as I've read about unregulated air circulation systems drying out the pile.  At this point the project is an experiment and I'm trying to keep it as simple as possible, but the earth tube + fan might be a possibility if the heating system works well enough and it becomes "a thing", ie a permanent installation.

The more heat it generates the more natural draft it would have. I’m wondering about mixing the pile so it’s hot? Ive got similar questions for the diy composting toilet I’m designing. How to vent it? How to contain it? How to regulate it? Is mixing necessary? Etc.

Jeremy Baker wrote:The more heat it generates the more natural draft it would have. I’m wondering about mixing the pile so it’s hot? Ive got similar questions for the diy composting toilet I’m designing. How to vent it? How to contain it? How to regulate it? Is mixing necessary? Etc.

I understand mixing is a "given" part of the process. The backyard rotary compost tumblers are designed to do this. I read where one fellow pokes hole in his compost pile with a length of 1/2" rebar to allow air to circulate and he can tell how hot it is by how warm the rebar gets. So I expect I'll have to do this from time to time and will design the box so it can be accessed from the top- and the location of the duct pipe inside clearly marked to avoid poking it!

I haven't researched composting toilets for a long time but as I recall, the Sun-Mar units don't require turning, just an additive to create the proper environment for composting. They do require that the drum be kept warm, though, and some models have a small fan. Check their website- it might give you some ideas.

I've attached a drawing, that's been around for a while and used by others, as a design to keep a compost pile working in a greenhouse.  Note that there are no pipes going through the middle of the pile, as that is where it needs to be hottest.  I got a compost tumbler in the '90s with a pole through the middle, and it was a total mistake.  And then the pole rusted out, leaving it unusable as a tumbler.  I spent way too much money on that tumbler to end up with a barrel that cost probably less than 1/4 of what I paid.  

Good old turning of the pile with a pitch fork is your best bet, although I know it can be a pain.  But visualizing what is going on in the pile is important.  Every season, and every time the ingredients change the pile will behave differently.  It's not just about heat with a compost pile, it's about the kind of mold/bacteria/fungi that grows there.  Unless you are looking at it, and learning about it, BTW, it's a guessing game.  Unless you can get it to be relatively consistent by knowing about what you are looking at, it's unlikely to give you enough heat in the long run.  It's easy to get a pile to heat up when you first pile it up.  But then everything changes, and knowing what to do at that point is what is necessary.

The plants in the greenhouse benefit from the carbon dioxide coming off the pile, not just the heat, and it needs to be down at the plant level so they can be exposed to it.

About the composting toilets, I highly recommend looking into a worm/woodchip box, vermiculture, to handle black water.  The day I took a reciprocating saw to the unusuable metal stirrers in my composting toilet was one of my happiest days.   I've had a worm box handling black water for 3 years now, and it's one of the most stunning and effective ways to do it.  The worms are the happiest I've ever seen them, and they make the absolute best castings that can then be used in the garden.  There are several threads at this site with diagrams.  Just be sure the box has at least a 1 1/2" to 2" drain pipe because the worms can drown, or freeze, or bake, but it's not hard to avoid these conditions.  It needs to be bigger than an inch in case some chunks of castings get past the exit location.

I've had two kinds of composting toilets, they all rely on a drain tube when the liquids get to be too much, and every single drain tube blocked up rather quickly.  Clearing that out is a real yucky job because the liquids are still in the container, so removing that tube makes a real mess.  Every time there are several people drinking coffee/water/tea/whatever and using that toilet the liquids will overwhelm the contents.

Also, for a composting toilet there needs to be someplace big enough to store the browns or sawdust for all of winter.  Then there has to be a rather good sized container inside to store them as well.  If using mowed weeds they inevitably will bring in bug eggs to hatch, ladybugs, spiders, weird dark worms, etc.   Gnats will get in there, most likely during the summer, and sticky traps won't be enough to stop them.

There is a constant sweeping of the floor around a composting toilet because the browns spill over on their way to the toilet.  Then at some point the finished compost has to be taken out of the container, which means getting down on your knees, lining the floor with a tarp, having a container to scoop the contents into.  I was never lucky enough to have it be "finished" compost.  In the winter it's much slower, so it needs emptying a lot more often, finished or not.  Then there needs to be a special compost pile somewhere to have it finish, and it's blackwater, so you don't want kids or pets or boots having access.

With worms there's a nice clean ceramic toilet with nice clean water inside.  Nothing drops onto the floor.  If anyone gets sick there's a decent place to run to.  Any work with the worms is outside where it's easy to handle.

Cristo, thanks for the detailed reply. (Pic is pretty small and I don't seem to be able to download it but I can see the basic details )  

I anticipate some maintenance to keep the pile hot over the winter and will design the box to allow access from the top for poking or turning. It will also have a thermocouple or two in the pile to be able to constantly monitor the temperature in the middle.  One end of the box will be removable so it can be cleared out in the late spring/summer when heat isn't needed.

The plan is to run a u-shaped loop of 6" spiral duct (about 18' in all) through the middle, with the tubes spaced about 20" apart, so except where the tubes enter the box, the tubes will have about 20" of material around them on all sides.  A fan will be installed on the outgoing duct from the studio to circulate air through the duct line, which will be a closed loop, pulling air from the studio into the box and returning it.  Controlling the trmperatire the studio (about 45' x 25' x 9') would be done by adjusting fan speed and putting 45º take-offs fitted with dampers on both the outgoing and incoming air lines, both of which will be insulated as the total run will be about 85-90'. A rather ambitious project..... (!)

I take my compact excavator over to my neighbour's every couple of weeks to muck out the barn, so we have a regular supply of poop and hay, and will spray it down before dumping into the box to get it properly moist.

It's all very much in the planning stage at this point as I wouldn't begin building it until next summer, hoping to fill it up in mid-Sept. so there might be some heat in October  :)

Yes, thanks Cristo. Will definitely take that vermicomposting toilet into consideration. Hey Colin, as you have a excavator and a source of poop have you considered a bio digester? They are so simple and the hugest garden vegetables I’ve ever seen were at the outflow of one I looked at.
 I’m wondering if a super hot composter could be designed then some of the heat removed so that it’s not so hot that it burns up as quickly. It seems to me if you are removing heat you want surplus heat. Ive dreamed about building some type of mechanical mixer so I don’t have to manually turn the compost. With the “windrow” method they just have long piles of compost and turn it with a loader. Ive only seen this outside. This is the opposite of the “mouldering” method where no turning is done and it takes longer and is cooler. The good old two-bin mouldering toilet is what I was going to build. One full toilet bin sits for a year or two while the other toilet bin is in use.
I thought about building a series of large steps into a hillside. Carbon/nitrogen material is dumped onto the first step. Then shoved onto the next step letting gravity do most of the work. Actually just dumping stuff at the top of a hill and collecting rich soil at the bottom of the hill works good and is a simple as it gets. So I thought of building a greenhouse lengthwise going up a hill. Is your site flat?

A cool design would be a greenhouse going up a hill with a composter going down one side or both sides. Fresh material could be dumped in the top and finished compost taken out the bottom. A “continuous” composter.  Also the greenhouse could create a warm draft at the top and that draft could be used for heating a house, shop, studio, drying food, etc. Ive long wanted to live on a site with a hill and use gravity and convection.

Just got reminded why turning compost in a greenhouse is necessary.....gnats and white files.  I've got a ton of transplants growing on top of hotbeds covered with sheer curtains, and gnats were starting to show up.  Turning the top hand's depth knocked around any bug eggs and it's improved in just a couple days since I did it.

Jeremy, search on New Alchemy Institute for a very similar drawing of how they did Bruce's 1984 setup.   The "exhaust" in the pipes at the roots of the plants is the carbon dioxide, which plants like.