In-vessel Composting
In-vessel composting (IVC) is a means of treating Food Waste and Green Waste in an enclosed environment which has accurate temperature control and monitoring[1]. IVC uses naturally occurring microbes which feed on organic material and require oxygen. IVC is therefore a form of Aerobic Digestion and where the process utilises Animal By-product Material, it must comply with the Animal By-Products (Enforcement) (England) Regulations 2011 requirements.
Overview
The development of IVC facilities in the UK was primarily driven from the early 2000's by the collection of mixed Food Waste and Green Waste - driven by legislation to remove Biodegradable Municipal Waste from landfill and a need to comply with the treatment requirements that were implemented as a result of the foot and mouth outbreak in 2001[2] (the processing requirements are summarised in WikiWaste in the page Animal By-product Material).
There are two key differences between IVC and Open Air Windrow Composting:
- IVC can reach higher temperatures that can be more accurately controlled than in Open Air Windrow Composting- which results in the destruction of potentially harmful bacteria found in meat or other Food Waste which may have come in contact with meat[3].
- IVC occurs in a sealed unit rather than in the open. This means that materials can reach and consistently maintain high temperatures for the required minimum period of time and keeps the material away from outside influences such as weather and wildlife and complies with the management requirements of Animal By-product Material.[3]
Types of Systems
In-vessel Composting can occur in a variety of systems (pictures run top left to right) including:
- Containers
- Silos
- Agitated Bays
- Tunnels
- Rotating Drums
- Enclosed halls[1]
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Stages of an IVC[1]
Stage 1:
The Food Wasteis either collected separately or with Green Waste and is delivered to an enclosed reception area. Any contamination such as plastic bags, bits of metal etc are removed and the remaining material is shredded to a uniform size and consistency (Green Waste may be added to ensure the mixture has a structure to allow air to pass through the material during the treatment process). From there it is loaded into a first ‘barrier’ which will be a bay/tunnel etc depending on the system used. All the material delivered to a facility, plus the first barrier stage, is considered a ‘dirty area’ under ABPR. The regulations ensure that strict procedures are in place to prevent cross-contamination of ‘clean areas’ (where product is processed and stored) from ‘dirty areas’.
The composting process is kick-started by naturally occurring micro-organisms already in the waste. They break down the material, releasing the nutrients and in doing so they increase the temperature to the 60-70ºC needed to kill pathogens and weed seeds, and meet the regulations for processing Animal By-product Material.
Stage 2:
Stage two normally lasts between seven days and three weeks. The material is transferred to the second ‘barrier’, where the composting process continues. Processing in two stages ensures that all parts of the composting mass reaches the required temperature. The oxygen level, moisture and temperature are carefully monitored and controlled during both composting stages to ensure the material is fully sanitised.
Stage 3:
Once the sanitisation process is complete the compost is left to mature in an open windrow or an enclosed area for approximately 10-14 weeks to ensure stabilisation, often termed 'maturation'.
Screening:
Screening usually takes place pre or post maturation, to produce a range of product grades suitable for various end uses such as soil conditioning[3]. The resulting product to come out of the process is a compost/soil improvement material. Provided it meets the required Quality Standard PAS100 the compost can be used by farmers and others as a soil conditioner and is considered to have met an End of Waste Criteria.