By Chris Negus
Composting and anaerobic digestion (AD) both take organic material and break it down to produce a biologically valuable substance. Although the technologies use similar inputs to produce similar outputs, there is little direct competition between the sectors. This is because the type of feedstock available determines the most effective way of processing it.
Composting breaks down green waste in the presence of oxygen, making it an aerobic process. Although food waste can be used, composting at a large scale works best with green waste like grass and hedge clippings. The compost produced can be used for gardening, horticulture and agriculture. There are two main methods of large-scale composting; open windrow and in-vessel composting.
Open windrow composting
Open windrow composting is used to process garden waste by laying it in long lines on an impermeable surface, either outdoors or in a large indoor space. It is only suitable for garden waste. According to the Animal By-products Regulations, any food waste must be processed by in-vessel composting or AD.
In-vessel composting is commonly used to process a mix of different types of organic waste, such as garden and food waste, to produce a soil conditioner. Rather than being out in the open air, the process takes place inside a tunnel, container, silo or similar enclosed space.
Anaerobic digestion process
Unlike composting, AD occurs without oxygen present. It can be used to breakdown food waste and agricultural wastes like manure or slurry. It can produce biofertiliser digestate, a nutrient-rich substance which is used on agricultural land to replace demand for chemical fertiliser and improve soil quality by adding organic matter.
AD also produces renewable energy. This may be in the form of electricity, or biomethane, which can be used in home boilers instead of fossil fuel gas.
Opportunities to add value to compost and digestate biofertiliser
Composting and AD have a place in the development of a more circular economy, as both take organic waste and process it to make useful products.
However, there are now growing opportunities to add value to compost and digestate. For example, blending the two together produces a resource in which nutrients are more accessible to plants than in compost alone. This is an attractive product for gardeners, especially given that peat-containing compost is due to be phased out.
The processes can also be used together to tackle more challenging waste types, such as mixed household waste. In Portugal, a project at Amarsul Seixal AD plant is taking municipal black waste, sorting it, processing it using a dry AD system and then composting the resulting digestate. The composting is used as a secondary process to reduce the amount of contamination in the digestate. There is scope for innovative projects such as this to be used to process waste anywhere.
In the future, composting and AD will continue to play key roles in keeping waste out of landfill, generating products which add value to the soil and AD can also provide a source of renewable energy.