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...energy for life. However, we cannot digest all of the food we eat and what remains undigested, ends up in the sewage system. About 10 billion litres of sewage are produced every day in England and Wales and this has to be treated to remove harmful substances and pathogenic microbes before the waste can be safely released into the environment. The main component of sewage...

...energy, labour and other direct and indirect costs. These costs are known as the private costs. It can be argued that, to calculate the true cost of an activity to society as a whole, costs should be apportioned to any damage caused to the commons and these damage costs should be taken into account. In this context, the commons could include people’s health, the general...

...energy, are transported via other organisms (e.g. hosts) or environmental processes (e.g. flowing water). It is possible some transported species (i.e. ‘alien’ to their new environment) will survive and colonise their new destination, unless limited by environmental factors or antagonistic organisms. When such introduced ‘alien’ species start to infest and...