Organic waste and landfills
Organic waste is anything that was once living or made from something that was living eg food, garden and lawn clippings, animals and animal waste, paper, cardboard and timber.
When organic waste is put into a landfill, it breaks down by a process called anaerobic decomposition. The organic waste creates a liquid called leachate, and two main gases - methane (CH4) and carbon dioxide (CO2), both of which are greenhouse gases.
The carbon cycle
Carbon from the atmosphere gets locked up by plants (in photosynthesis) and released back into the atmosphere either quickly (by respiration) or slowly by decomposition (eg breakdown of organic materials) or burning of fossil fuels. Methane is one of the carriers of carbon from organic material to the atmosphere so it’s part of the natural carbon cycle.
Carbon naturally moves from one part of the Earth to another through the carbon cycle. However, human activity is adding carbon to the atmosphere (in the form of carbon dioxide and methane) faster than natural processes can remove it. That is why the amount of carbon dioxide and methane in the atmosphere is increasing, which is causing the earth to warm and the climate to change.
This diagram of the carbon cycle shows how carbon is cycled between the air, oceans, ground and living things:
Greenhouses gases and the greenhouse effect
Greenhouse gases are found naturally in the atmosphere and keep our Earth warm enough to support life. They include water vapour, CO2, methane, and some others.
The Earth gets energy from the Sun in the form of sunlight. The Earth's surface absorbs some of this energy and heats up. That is why the surface of a road can feel hot even after the sun has set, because it has absorbed heat from the sun.
The Earth cools down by giving off a different form of energy called infrared radiation. However, before all this radiation can escape to outer space, greenhouse gases in the atmosphere absorb some of it, which makes the atmosphere warmer. As the atmosphere becomes warmer, it also makes the Earth's surface warmer, a bit like a greenhouse.
Planets without greenhouse gases get very hot when the sun shines on them but extremely cold at night time.
The Earth needs greenhouse gases to trap heat but if these gases increase too much the atmosphere will continue to get warmer, resulting in ‘global warming’. Carbon dioxide is the greenhouse gas you hear people talking about the most. That is because we produce more carbon dioxide than any other greenhouse gas and it is responsible for most of the warming. Greenhouse gases come from all sorts of everyday activities, such as heating our homes and driving around town.
The graph below shows the major greenhouse gases that are produced from human activity:
Besides CO2, another important greenhouse gas is methane (CH4). Methane is produced by bacteria that breaks down organic matter. You would expect landfills to create lots of methane - and they do!
Methane is an interesting (but bad) gas:
- It is over 25 times more harmful as a greenhouse gas than carbon dioxide.
- It can kill you if you breathe it (because it stops you absorbing oxygen).
- It is very explosive.
Methane is, however, a valuable resource. Methane contains lots of energy:
- Energy from the sun is absorbed by plants to make food.
- Some of this food is eaten by animals.
- Plants and animals and products made from them (eg paper, cardboard, timber, woollen carpets) all becomes organic waste.
- When organic waste is broken down in a landfill they make leachate, CO2 and methane.
At Redvale the methane is collected through a network of wells and pipes then used to create electricity.
A system of gas wells is used to maintain the waste pile under vacuum to draw the gas into the collection system. Over 90% of the landfill gas generated at Redvale is collected. Pipes take the gas to 12 large generators. These generators provide beneficial use of the gas by supplying renewable electricity to the local grid. Current capacity would power approximately 12,000 homes, and ultimate capacity by 2023 is estimated at over 24,000 homes.
A pipeline has also been installed to deliver landfill gas to a neighbouring commercial greenhouse, fuelling a boiler used for space heating.
Closing the loop
Redvale landfill controls the gas that comes from the breakdown of organic waste. This is called a ’Gas Management System’. Not only does it stop harm to the atmosphere, it also generates electricity from waste, a renewable resource.
The electricity created by organic waste at the landfill is being used to fuel new electric vehicles that collect waste - how cool is that!
Redvale will produce gas for several decades from now.