Green Waste Composting Process

Our compost is derived from urban green waste, so it includes grass clippings, leaves, tree trimmings, shrub prunings — that type of material. It’s important that we inspect it and pull out any contamination, which mainly consists of things like plastic bags and plastic bottles. If we pull those out now, then they don’t get mixed into the material that we’re grinding and trying to compost. In Eugene, people are fairly conscientious and we’re fortunate that we have a pretty clean stream going into the process, which makes our jobs easier.

The green waste material will vary by season. In the fall, for instance, we’ll get apples and windfall items from fruit trees in people’s yards, along with a higher percentage of leaves. In the spring, we get a higher percentage of grass. All of that just becomes part of the mix and we have to adjust our parameters a little bit based on what those feed stocks do seasonally.

Generally, we’re in a great area of the country where our incoming materials are really pretty consistent — and relatively easy to compost. We don’t have to do a lot of manipulation; we just inspect it as we described above. After that, it goes through a grinder, which just reduces the size and makes it easier to break down.

After it’s ground up, we add the material to an aerated static pile. That means that we have air pipes beneath the pile that blow air up through the material, which keeps it aerobic. Aerobic composting is the most efficient way; it’s faster and it gives us a lot of control so that we can keep the pile’s temperature within our desired range.

It’s important that the pile maintains a minimum temperature of 132 degrees Fahrenheit for three days to make sure that we kill any pathogens. A lot of people don’t think about this, but because green waste often comes into contact with animals, it could contain fecal coliform, salmonella, or E. Coli, which are all kind of big, scary things. We need to take those seriously, so the time and temperature — 132 degrees for three days — ensures that we’ve heat-treated the material and it’s safe. That doesn’t mean that it’s done composting, but it’s an important step in the process and it’s also a requirement for our organic certification.

This composting technique really makes it easy to keep thorough documentation for pathogen control. Typically, what we’ll do is put the pile on air and monitor it. Once we get to 132 degrees and stay there for three days, we’ll document that. We’ll then keep it on the air for another 14 days before pulling it off. We keep it aerated a bit longer than is required because the beginning of the composting stage demands the most oxygen. That helps us get over the hump and then we don’t have to turn it as much.

Next, it’ll go into piles, where we’ll keep it for another three months. We’ll turn them as needed. After it goes through a curing phase, we’ll screen it. The end result is a compost that’s nice. It’s dark and it’s rich. It’s the goal of this whole process.

People sometimes think of compost as a fertilizer, and it does have some nutritional value for plants, but the biggest benefit to using compost is probably the biology that comes with it. We have a lot of bacteria, fungi, and other organisms that are present — and very, very active — in our compost. When we introduce that to a soil or landscape, we see all kinds of benefits thanks to the interactions between those microbes and the soil.

Our final step in quality control is a bioassay, which measures the compost’s potency and its effects on plants. It is like the canary in the coal mine. We take a plant that’s sensitive to certain compounds, and we grow it in this material, as well as in a control soil (we know that plant will be healthy). We can compare as they grow and make sure that we don’t have any issues with our material.

Finished compost should have a really earthy smell. It should smell fresh, like the forest — nothing like the compost that’s mid-process. There can be some kind of funky smells going on there, but that’s just what happens as the materials are consumed by the biology. A sweet, earthy smell is one of the indicators that the material is well composted; it’s ready to go.

Potting soils all contain a lot of similar ingredients. Maybe you’ll have some bark fines, pumice, perlite, coir fiber, peat moss, and then fertilizer packages. Some potting soils have some compost in them, but many don’t. We think that having compost in there is very, very important because of what it can do for the soil and the plants: it helps to regulate the fertilizer. Compost may not always account for a large percentage of the mix, but it’s a very important component.