Introduction

In our modern society, we’re continually pushing towards being greener, more sustainable, more eco-friendly. As a result, engineers and scientists are increasingly turning away from more traditional energy sources, such as fossil fuels, and starting to look in more unusual places. Consider the example of the Sheffield Energy Recovery Facility. Every day the plant burns 400te of the city’s municipal refuse to produce up to 60MW of heat and up to 19MW of electricity.

This is an example of taking a material that would have simply gone to landfill as waste, and making use of it to provide energy. Another example of where a potentially large source of free energy goes to waste is in the sewers. Consider the amount of hot water that is poured down the drain every day, not to mention organic matter. Just how much energy is down there?

Task

The city of Sheffield is looking to increase the amount of energy produced from non-traditional renewable sources. Following the example of the city of Brainerd in Minnesota, USA, the city council and the regional water provider wish to investigate the possibility of extracting energy from the city sewers. They want an estimate of how much energy is available, and how it could be extracted. All solutions should be based on distributed energy recovery; they should be installed along the length of the sewer network, rather than based at water treatment works.

Considerations

Although you do not have to make use of all of them, you should consider all sources of energy available in the sewer, including the kinetic energy of the water, the heat in the water/air, and biomass including gases. You will need to estimate the amount and quality of the energy available in the sewers below the city. You should determine possible methods of extracting this energy, and fully evaluate any costs/benefits of all options proposed.

You should consider both designing a system from scratch, as well as whether you can use or adapt any existing technologies for the work and what adaptations would need to be made in order to allow the use of these technologies in a sewer environment. In this era of government and city council spending cuts you should be mindful of the fact that if the costs of installing and running the technology exceed the benefits then it will not be viable. As such it will be necessary to provide a cost estimate for the installation of any systems, as well as their anticipated maintenance costs and the projected returns.

Keep in mind that any system implemented must not impact on the primary functions of the sewer system; protecting the public. As such it is required that your designs will not increase flooding, release of waste water, or risk of blockage.