Where there's muck, there’s brass

Tim Broadhurst, CCO of CooperOstlund, on utilising sewage and sludge as a renewable energy feedstock and how to maximise the value

While for most people sewage might be considered to be an unpleasant by-product of our human existence, it is a hugely valuable industry, providing jobs, sanitised water and, importantly, a prime source of renewable energy.

As with any organic material, sewage and sludge release a significant volume of methane during decomposition.

Using anaerobic digestion (AD) technology, this gas can be captured and stored, and then transformed into renewable, clean energy using combined heat and power (CHP) engines.

The economic and sustainability benefits

Sewage and sludge are particularly appealing as an energy source because the 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 per cent, 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 to 18 months, which is 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 per cent. 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 meet 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 are 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.

This article was originally published in the July issue of WWT