What is Aerobic Digestion/Composting?

Currently, the majority of biodegradable waste (green waste, food waste, paper/card waste) we generate is sent to landfill for final disposal, which in turn, is having a negative effect on our environment. This is because when sent to landfill, biodegradable waste is broken down by bacteria and releases methane into the atmosphere, a harmful greenhouse gas that contributes to climate change.

Demanding targets have been set by the EU Landfill Directive (1999/31/EC) in order to reduce the amount of biodegradable waste being sent to landfill. These targets are: by 2010, to reduce the amount of landfilled biodegradable municipal waste to 75% of that arising in 1995; by 2013, to reduce the amount to 50% of that arising in 1995; and by 2020, to reduce the amount to 35% of that arising in 1995 (DEFRA).

Aerobic digestion, also known as aerobic composting, is the process whereby organic material is decomposed into carbon dioxide (CO₂), water (H₂O), and heat through microbial respiration in the presence of oxygen leaving a stabilised residual solid material, compost  (DEFRA). Aerobic composting is a controlled process that converts biodegradable waste into compost, instead of sending the waste to landfill where it will break down and release methane into the atmosphere. Ultimately, the aim of using aerobic composting to treat waste is to reduce the impacts residual waste has on the environment when sent to landfill. Home composting based on the same principles can be done easily at home with a small system

Aerobic Composting Processes

Open Windrow Composting

Open windrow composting is used for processing agricultural/garden waste such as grass cuttings, tree trimmings and leaves. The process begins with the bio waste (agricultural/garden waste) being shredded, mixed together, and placed in elongated, windrow heaps. This process takes around 16 weeks from start to finish, and throughout the duration of the process, the windrow heap is periodically turned in order to improve the oxygen content. The final part of the process involves screening the compost in order to remove any unwanted contaminants such as plastics and metals.

In Vessel Composting

In vessel composting is a process used to treat a mixture of both food and agricultural/garden waste. These systems ensure that composting takes place in an enclosed environment, with accurate temperature control and monitoring (WRAP). Bio waste (food waste and agricultural/garden waste) is brought to the in vessel composting facility where it is shredded into smaller particle sizes to facilitate the composting process. This shredded material is then placed in a tunnel/bay where it undergoes the first stage of the process. This initial stage (which can take anything from 7 days to 3 weeks) involves micro-organisms already present within the waste breaking down the material, which in turn increases the temperature to the 60-70ºC needed to kill pathogens and weed seeds, and meets the regulations for processing animal by product material. Following this stage, the material is then transferred to another tunnel/bay where the composting process continues for a similar duration. This is to ensure that all parts of the composting material reaches the required temperatures in order to be fully sanitised. Following the sanitisation process, the compost is left to mature in either an open windrow or an enclosed area for around 14 weeks to ensure stabilisation (WRAP).

Containers, silos, agitated bays, tunnels, rotating drums and enclosed halls are all different types of in vessel composting systems used to treat bio waste such as food waste and agricultural/garden waste, both mixed and unmixed.

Main Factors Affecting the Composting Process

Oxygen and Aeration - Aerobic composting is a biological process that requires a large amount of oxygen. If the oxygen supply is limited in the aerobic composting process, there is a chance that the process may turn from aerobic to anaerobic, which is a slower, odorous process.

Moisture - It has been proven that the aerobic composting process is much slower when the moisture content is below 40%. It has also been proven that the aerobic composting process can change to an anaerobic digestion process if the moisture content is above 65%. It is important to get the moisture content just right in order to achieve maximum results.

C:N Ratio – Carbon and nitrogen are the two main nutrients needed for the aerobic composting process to take place. Micro-organisms involved in composting use carbon for energy and growth, and nitrogen for protein production and reproduction.  Raw materials blended to provide a C:N ratio of 25:1 to 30:1 are ideal for active composting, although initial C:N ratios from 20:1 up to 40:1 consistently give good composting results (Pace et al, 1995).

Potential Advantages

• Aerobic composting reduces the biodegradable content of residual waste which in turn reduces the production of the harmful greenhouse gas methane when waste is sent to landfill.
• The aerobic composting process itself can treat biodegradable waste such as agricultural/garden waste and food waste without releasing harmful greenhouse gas emissions into the atmosphere.
• The process requires simple, durable, and cheap technology.
• Aerobic composting produces a stabilised residual material called compost that can be used as soil fertiliser.

Potential Disadvantages

• Waste must be separated into biodegradable and non biodegradable before it can be processed.
• Aerobic digestion plants that are mechanized have high capital and operational costs.
• In some cases the market for compost is a good distance away from its point of production. This can increase transportation costs.