The Anaerobic Digester

The Anaerobic Digester

Composting kitchen and garden waste is a big part of keeping a garden fertile and productive. It’s also an easy way to reduce household waste. The challenge with composting is that it doesn’t use all types of organic kitchen waste and takes some extra time and care to work well. Keeping a well run composter going should be a priority for any household with gardens, but you can also use an anaerobic digester.

The anaerobic digester is similar to a composter but it’s a different set of microbes breaking down the contents, working in the absence of oxygen to reduce organic materials into their essential elements. There are fewer rules with a digester and you can throw just about anything in, including meat and bones, citrus, and even small amounts of pet waste. This makes for a really smelly mix of decomposing waste, but the lid is airtight to keep oxygen out and odours in. Digesters are often promoted as an alternative for composters in bear country, though a well run composter doesn’t smell either.

The other difference that sets the digester apart from a composter is that you don’t necessarily harvest matured compost from a digester. If they’re left unopened for a long period then you could remove the contents for garden fertility. But typically, in constant use, part of the contents are matured but the newly added parts are not. But what happens is that as the oldest contents putrefy, they’re converted into a leachate, which is water rich in minerals.

When the digester is placed adjacent to or in the middle of a garden, the leachate leaches into the surrounding soil and fertilizes the garden. A digester can be perfect in the middle of a keyhole garden for example.

Probably the most attractive feature of the digester is that there’s virtually no maintenance. You just open the lid, try hard not to breathe through your nose, and throw your waste in. You will need to have more than one digester going if the contents don’t reduce as fast as you add more waste.

There are some downsides to the digester. One is that the contents become very acidic through the process and can bring the pH of your soil down. Adding something like limestone screenings around your digester can help counter the acidity. The other problem is that unlike a composter that emits carbon dioxide, the digester produces methane, which is a serious contributor to climate change. Of course, methane is also a fuel, and with a bit of innovation, we could use methane for fuel which converts it to carbon dioxide and water. Harvesting biogas is on our list of things to try out so we’ll let you know how it goes when we get there.