Aerated Compost: the Composting Process

This post is the second in a series about composting stall waste, specifically with an aerated compost system by O2 Compost. For details on the construction of the compost bins, click here.

The idea behind aerated compost is that you can reduce composting time and the need to turn the pile (a major timesaver!) by introducing air into the system. O2 Compost says it better than I do so, in their words: "The oxygen stimulates the microorganisms that are already in the mix, and their by-product is heat. In a properly operated compost system, pile temperatures are sufficient to pasteurize the raw material and dramatically reduce offensive odors. High temperatures also destroy fly larvae and weed seeds. This means that the result of this process is safe, high-quality soil enhancement that supports healthy plant growth." O2 Compost advertises results in 30 to 60 days.

Quick turnaround time is important if, like me, you have limited space for your compost bins. You don't want to wait months for the waste to compost because your horses aren't going to stop eating and pooping if you run out of space.

I was worried that even a 6'x6'x4' bin would fill up faster than the other one could compost. I currently have one more horse than I ever planned for (ha, they do say not to build a bigger barn than you need or you will fill it with horses) and all three of my horses are pretty big. The extra horse is temporary but could be here for a year or more, so I needed to plan for that.

I talked the whole thing through ad nauseam with the people at O2 Compost before I decided to take the plunge. They assured me that although your compost will get better if it sits 30-60 days or longer, the critical temperatures needed to kill pathogens are typically reached within a week, so a shorter timeframe produces compost that is safe to spread on your own fields, though not something anyone else would want.

Let's fast forward now to late April, when my first bin filled up 29 days after it received its inaugural load. During that time, it was used for the stall waste of three large warmblood horses of about 1,500-1,700 lbs each and a mini-donkey. They were out on pasture from about 7 am to 5 pm (longer on nice days) and inside overnight, so the bin was used for 14 hours of waste per day.

O2 Compost recommends covering a full bin with six inches of clean material to insulate the waste from moisture and temperature variations, reduce odor, and reduce fly activity. I used two 8-cubic-foot bags of "premium pine shavings" from Tractor Supply, because they're less expensive than the fine shavings I like to use for stalls, and less likely to mix into the composted material. O2 Compost instructs you to keep this layer damp, so I sprayed it with the hose as I filled it.

I was impressed by how well the lids kept the top layer from drying out, something I was worried about as the heat of summer approached. We also got a lot of rain in the first few weeks after the bin filled, and I'm very glad I had the lids to keep most of that out. Apparently if your waste gets too waterlogged it will not compost and the only way to fix that is to remove it from the bin and spread it out to dry--what a hassle. No, thanks!

I was also pleasantly surprised by how the flies had zero interest in the full bin after I added the top layer of clean shavings.

After I filled it and covered it in a cozy blanket of pine shavings, I was a bad compost mom to this first load. I went out of town right after filling it, so I did not take temperature readings like I was supposed to. I also didn't feel like asking the farmsitter to water my compost in addition to all the other things she had to do for the living, breathing animals, so I just hoped for the best. Thankfully, as I've said, the lids did a great job maintaining the correct moisture content despite inches of rain.

The moment of truth came on June 13th, when the second bin was full and I had to empty the first one so I'd have somewhere to put new waste. I was pleased with how much the materials had "reduced" during composting. Here are some photos of what the materials--okay, let's go ahead and call it compost now--looked like after 51 days of composting:

The composting process has considerably reduced the volume of the waste.

The very top layer of the shavings looked dry, but just below the surface they were nice and damp.

Look, a mushroom! That's supposed to be a good sign. I found quite a few of these suckers in there. (No, I did not eat any of them.)

Here's the compost after I scraped off as much of the clean shavings as I could, to be reused on the second bin. (Partially because I hate waste, partially because I'm frugal, and partially because I didn't want to spread more uncomposted woody materials on my pasture than necessary.) I'd say there's about 60-70% of the original volume remaining. I was a little anxious about opening the doors but it held its shape and did not bury me in an avalanche of sh!t.

The innards of the pile. Some parts of it looked dark and uniform like compost is supposed to. Others just looked like well-aged stall waste. I suppose it wasn't the best batch of compost anyone has ever made but hey, it was my first attempt. On the plus side, almost none of it smelled like stall waste. It wasn't pleasantly earthy, but it wasn't offensive either so that must be a good sign.

Emptying the bin was kind of brutal, especially since I did it on the two hottest days of the year so far. The tractor bucket could only help so much because the bin was deeper than the front-loader could reach, and I couldn't scrape all the way to the ground because of the aeration tubes. I had to shovel two spreader loads out by hand (by back?). Still, it was very satisfying to get it all out of there and spread it on the field.

Speaking of spreading, stay tuned for more details on that!

You can read more about my experiences with the aerated compost system here:

Bin Construction
Spreading
Is It Right For You?
Pallet Floors for Air Pipes
3-Years of Composting (coming soon!)