NUTRIENT-RICH DIGESTATE IS A VALUABLE ON-FARM RESOURCE

By Rebecca Hannam

BIOGAS AND RENEWABLE NATURAL GAS (RNG) ARE THE PRIMARY FOCUS OF ANAEROBIC DIGESTION, but digestate is another valuable output that should not be overlooked.

Digestate is the residual material leftover after organic matter – such as manure or food waste – and is processed through digestion. Composed of both liquid and solid portions, it’s a nutrient-rich product that contains virtually all the macro and micronutrients found in the original material before it went into the digester.

For farmers, digestate can be a sustainable resource that can help reduce input costs, improve soil health and build a circular economy.

Rob McKinlay, who operates Harcolm Farms with his family near Woodstock, Ont., installed North America’s first small, farm-scale anaerobic digester in 2017. The 20 kilowatt (kW) mini digester system is fed only on-farm material from the 85-cow dairy, including manure and small amounts of corn silage.

In addition to generating electricity to offset the farm’s needs and fulfil MicroFIT and Net Meter contracts (to sell excess energy produced back to the grid), his system produces digestate, which he separates for different uses.

“We chose to install a bedding system so when the digestate comes out of the digester, it goes through a screw press to separate the solids, which we then use for bedding the cattle,” he explains. The system is automated, meaning a cart of digestate- based bedding is delivered to the freestall barn daily.

After the separation process, McKinlay stores the liquid digestate in his manure pit and uses it as an organic fertilizer to spread on crops.

“We spread it on cover crops, try to get multiple applications through the growing season and have been able to offset quite a bit of commercial fertilizer by doing that,” he says, noting the nutrients in digestate, such as ammonium (NH4), are more readily available to the crop compared with dairy manure.

Digestate can also be used as a soil amendment because it has characteristics similar to compost. It supplies the soil with organic matter and many slow-release macro and micronutrients that will benefit the land for multiple years.

All the best management practices (BMPs) for managing crop nutrients still apply, McKinlay says. Like manure or other fertilizer, it’s important to know the characteristics of your digestate and soil before application. Farmers should collect representative samples for laboratory analysis and follow the “4Rs” of nutrient stewardship: right rate, right place, right time and right method.

Rebecca Hannam is an agricultural communications professional who grew up in a grain farming family near Guelph, Ont. She has a degree in agricultural business from the University of Guelph. Communications in agriculture is a passion.