We have two main types of commercial composting in the UK.
The first is In Vessel Composter, or IVC for short. This is where the compostable packaging, food and garden waste is repeatedly super-heated, breaking the materials down and mixed together to create a nutrient rich compost, suitable for supporting our country’s crop production. This is where our packaging must go to get composted into compost!
The term ‘in-vessel composting’ covers a wide range of composting systems, all of which involve the enclosed composting of waste – allowin oxygen levels, temperature and moisture to be controlled.
IVC can be a one or two stage process: the Animal By-Products Regulations (ABPR), introduced in 2003 to reduce health and environmental risks, allow waste excluding meat to be treated in one stage but require waste including meat to undergo two stages.
In stage one, food and garden waste is firstly shredded to a uniform size and loaded into a bay or tunnel. Naturally-occurring micro-organisms, already present in the waste, start breaking it down. The process draws oxygen from the air and raises the temperature to the 60-70ºC, which is needed to kill weed seeds and pathogens. Compost is produced, with water vapour and carbon dioxide given off as by-products.
If needed (if the original waste includes meat), the material is transferred to a second ‘barrier’ to ensure the material is fully sanitised. The two stages each take between 7 days and 3 weeks.
The compost is then left to mature in an open windrow for approximately 10-14 weeks to ensure stabilisation. Screening takes place to produce different quality compost.
The second is Anaerobic Digestion, or AD for short. This is where the food and garden waste is broken down by microorganisms in an oxygen free environment. The biogas these little critters produce as they are munching through the food and garden waste is harvested as energy. The final product left over once these guys have worked their magic is the compost. However, it is really important to know that these microorganisms do not like packaging, and so our compostable packaging should not be sent to an AD composting facility.
n Anaerobic Digestion (AD) biodegradable waste breaks down in the absence of oxygen (in contrast to In-Vessel Composting where oxygen is present). It’s widely used to treat wastewater in the UK, and increasingly used for food waste and manure.
In the first stage, ‘pre-treatment’, the materials are mixed together to ensure the right consistency; water may be added. The material is then screened for
contaminants, such as plastic and grit. Packaged food often has its packaging removed at this stage as well (although in some AD plants, this ‘depackaging’ happens at the end).
Next, the materials are fed into a digestor. It’s here that they break down into digestate (a bio-fertiliser that can be used on the land for healthy plant growth and soil), as well as biogas (mostly methane and Co2).
Because of this biogas, AD is another Waste-to-energy (WtE) or energy-from-waste (EfW) technology like incineration (point 1. above). Waste-to-energy is all about ‘energy recovery’ from waste. That is, generating electricity and/or heat from waste – either directly from the combustion of waste like incineration, or by producing a combustible fuel like AD.
The biogas produced in AD can be used directly as fuel, in combined heat and power gas engines, or upgraded to biomethane (Co2 and other contaminants are removed leaving a gas that’s 95% methane). This biomethane is injected into the gas grid or used as vehicle fuel.