:: Specific Design Brief

CAT 1:
Biodegradable Waste - further information


General background
Households in England and Wales produce 29.3 million tonnes of refuse every year and this is increasing by 3% a year. 82% of this goes into landfill sites (9% is recycled and 8% has energy recovered from it). The organic material in this refuse is 38%. When this organic material goes into a landfill site it decomposes anaerobically and produces methane which is a powerful greenhouse gas (20x worse than CO2 per unit of gas).
The organic material in refuse consists of food, grass cuttings, other garden waste, paper, cardboard and wood. These decompose at different rates. Woody wastes decompose very slowly and some plant material quite slowly.

The chemistry of composting
As plants grow, the process of photosynthesis uses CO2 and water to produce plant material and oxygen –
6CO 2 + 6H2O = C6H12O6 + 6O2

In a compost heap oxygen is absorbed as the organic material is broken down into the carbon dioxide and water that we started with –
C6H12O6 + 6O2 = 6CO2 + 6H2O

However, in a landfill site there is no oxygen around for that to happen and methane (CH4) is produced –
C6H12O6 = 3CH4 +3CO2

Issues to consider
• the type of materials to be composted
• the volume of material to be composted
• the optimal shape and size of the container
• where the container will be located
• the material used for the container itself
• removal of finished material to use on gardens
• avoiding smells
• dealing with rodents

The law and composting in the school
Food contamination in recent years has led to the introduction of regulations intended to minimise the risk of transmission of things such as BSE (ABPO – Animal by-products order). These apply to “catering” waste and therefore to the waste from school kitchens and refectories. The concern appears to be that animals or birds might pick up contaminated material, carry it somewhere else where it could be eaten by something that would carry it into the food chain. There is no evidence that compost heaps have transmitted problems in this way. The only materials that could cause problems are animal products.
The regulations say that “catering” waste can only be composted if it is in a closed vessel and the temperature reaches a high enough level for long enough. The composter has to demonstrate that the temperature levels have been reached.
If the vessel was completely closed the material inside would not get enough oxygen and would therefore produce methane. The smallest animal that would need to be kept out would be a small rodent.
The impact of this regulation is that food waste that was previously composted is now going into landfill sites. At the same time the Government have recently set the following targets –
at least 25% of household waste to be recycled or composted by 2005
at least 30% of household waste to be recycled or composted by 2010
at least 33% of household waste to be recycled or composted by 2015
Also the EC landfill directive (1999/31/EC) aims to phase out the landfilling of biodegradable waste.

Websites & other resources:
The government’s current strategy for dealing with waste in England and Wales.

Wastewatch run Schools Waste Action Clubs (see the “school” section on their website). Their website also has a list of educational resources (including the free pack Wise up to Waste) and information sheets on waste topics (both in the “information” section of their website)

Work at Waste at School is a Wastewatch practical guide available to download on the Waste on Line website, which is intended to be a one-stop source of information, resources and organisations useful to schools.

Search for Animal by-products order for details of the regulations.

The organisation in Wales co-ordinating work on waste.

Centre for Alternative Technology, Machynlleth SY20 9AZ

[email protected]

About Managing Waste
Book with a great deal of useful factual information and separate teachers notes. It contains sections on What is waste?, The history of waste, What happens to waste?, Designing to reduce waste, Conserving resources, Recycling in practice and What about the future? £5.99
Waste and Recycling
One of the Issues series. Contains short, illustrated articles by a number of different organisations, giving a broad perspective. Photocopiable.
£ 6.95 available from CAT

There are a great variety of compost bins on display and in use at CAT, including a number of commercial ones and ones made from waste materials, ranging from car tyres to old freezers. There are also a variety of “recipes” for the contents.