Human activities produce many different kinds of waste:
- Household rubbish
- Industrial waste
- Sludge from effluent treatment plants
- Agricultural waste: straw, manure (a solid mixture of animal excrement and litter) and liquid manure or slurry (a liquid mixture of animal urine and excrement)
- Wood residues (residue from forestry operations, shavings, bark and sawdust).
We are all familiar with household rubbish. We are asked to sort it – glass to one side, organic waste to the other, the rest in a third bin (or sometimes, even several other bins for paper and cardboard, metal and plastic products).
Why are we asked to sort the contents of our rubbish bins? Simply because different types of waste do not go to the same destination. And because sorting at source enables significant savings in time and energy, making the recycling operation more profitable.
Some waste products are recycled. Recycling is not a source of energy in itself. But it is a way of making energy savings, by recovering and reusing materials whose manufacture used a lot of energy.
Other waste products are transformed into energy, in two main ways:
Incineration: the waste products are burnt, producing heat or electricity or both (co-generation),
Methanisation (anaerobic fermentation): waste of organic origin is transformed into methane (biogas). This methane can then be used in the same ways
as natural gas: as an industrial fuel to produce electricity and heat, as an automobile fuel, or as a fuel to be injected into the gas distribution network. |