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How anaerobic digestion can maximise the value of sewage

Wastewater and sewage aren’t the most appealing of subjects, but it’s certainly one where the old adage of ‘where there’s muck, there’s brass’ rings true when it comes to its suitability as a renewable energy feedstock, explains Tim Broadhurst, CCO of CooperOstlund.

For most people, wastewater and sewage is considered to be an unpleasant by-product of our human existence, hardly a cheery topic of conversation and certainly not something that’s associated with having value. But the reality couldn’t be more different – in terms of its worth at least. It’s actually a hugely valuable industry, providing jobs and sanitised water and, importantly, supplying a prime source of renewable energy.

This is because, as with any organic material, sewage and sludge releases a significant volume of methane during its decomposition. Using anaerobic digestion (AD) technology, this gas can be captured and stored. It’s then possible to transform this flammable gas into renewable, clean energy using combined heat and power (CHP) engines.

The economic and sustainable benefits

Sewage and sludge is particularly appealing as an energy source because its organic matter is fertile and contains almost 10 times the energy needed to treat it. During the processing of wastewater, the UK generates more than two million tonnes of sludge every year.

The financial benefits for its use are compelling – AD not only creates energy from the sludge, but it also reduces the solids content by up to 30%, reducing the energy costs involved in its transportation.  Water and wastewater AD facilities themselves can also rapidly translate into profit with a typical payback of 12-18 months, not bad for a process that is also kind to the environment – a crucial consideration given today’s appetite for sustainable practices.

How the process works

By treating sewage sludge with AD, it is possible to reduce the amount of organic matter and the number of disease-causing microorganisms present in the solids.

The sludge is fed into large tanks and held for a minimum of 12 days to allow the digestion process to perform the four stages necessary to digest the sludge. These are hydrolysis, acidogenesis, acetogenesis, and methanogenesis. In this process the complex proteins and sugars are broken down to form more simple compounds such as water, carbon dioxide, and methane.

Treated sewage produces biogas that can be cleaned and injected into the gas grid or combusted via CHP to generate low-carbon electricity. The raw methane that is produced can also be used for on-site processes, such as heating tanks or running machinery engines, boosting the circular economy.

Effective maintenance is crucial

Regular and expert maintenance is vital if AD sites are to operate at their maximum capacity and generate their true potential of renewable energy. Poorly maintained engines can see efficiencies fall by as much as 20%. Indeed, a well-managed 500kW engine could be generating an additional £35,000 in revenues every year. From poor system calibration and incorrect timing patterns, to the wrong fluid choice and inappropriate air/methane balance, missing the finer details has a considerable impact on engine performance. CHP maintenance is certainly not a ‘nice to have’ option – it could be the difference between profit and loss.

Obtaining maximum value

As CHP experts, we’re often asked to assess existing facilities with the aim of suggesting changes to improve site outputs. Advances in technology continue to provide solutions to meeting this challenge. One example can be seen in the use of state-of-the-art open protocol control panels that provide immediate digital access, allowing engineers to easily control each setup, via the internet, without the need for specialist support.

This means that the settings of each system can be flexibly modified to suit operational requirements, for example to meet increasing or decreasing feedstock levels.  Electric turbo compounding (ETC) products are also available for CHP systems, providing energy-efficient performance uplifts and reducing system emissions. Crucially, both control panels and turbo systems can be retrofitted to a wide variety of existing CHP engines, negating the need for substantial investment in brand new machinery.

The future

AD is widely considered the most effective solution for the long-term management of organic waste and, unsurprisingly, sewage and sludge is a feedstock that isn’t likely to become scarce anytime soon! Set against the backdrop of continually diversifying energy markets and a focus on renewable energy, its transformation into energy offers a number of financial and environmental benefits, resulting in an industry that can only continue to flourish in the future.

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