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Biomethane from Sewage Sludge at Minworth STW Heats 3,000 UK Homes

This is the story of how one UK water utility company is making biomethane from sewage sludge and helping the UK meet emissions reduction targets. They are purifying biogas into pure methane (biomethane) renewable energy, and reducing carbon dioxide emissions substantially. They are doing this by using their own sewage sludge waste product at Minworth Sewage Treatment Works, where in addition to making electricity, a CHP Unit also heats 3,000 local homes.

Severn Trent Water is a major water and waste utility located in the middle of the UK. They operate from the Midlands, around Birmingham, out into Wales and a major part of their business that is growing at the moment, is renewable energy production from the biomethane from sewage sludge.

They have a number of sites, around about 35, across the Midlands where they treat sewage sludge and use it to generate electricity through the use of Combined Heat and Power engines.

Biomethane from Sewage Sludge – the Latest in a Sequence of CO2 Emissions Reducing Improvements since 2011.

Their site in Minworth is particularly interesting, because they have been making biogas from sewage sludge there and generating electricity for about 50 years. They were previously doing this using the anaerobic digestion process, on-site to generate electricity from the raw biogas. For many years they have fed both electricity and heat as well, using CHP (Combined Heat and Power) engines that have allowed them to generate electricity, power and heat the site, and feed the local electricity grid.

Since 2011 they've developed an alternative to this by upgrading their biogas quality into biomethane (natural gas quality) and injecting it out into the National Grid in the form of methane gas.

The site here was the first to upgrade biogas from sewage treatment works where they don't have a great control over their feedstock, or what's coming into the site.

To overcome this they have a number of process steps in place, to treat the biogas and generate the maximum efficiency.

The first step, is that they look at removing a lot of the volatile organic compounds that are in the raw biogas that's coming off their sludge digestion plant.

Image for page about Biomethane from Sewage SludgeThe digestion plant is quite old. It's a standard mesophilic digester, but it produces gas that can contain impurities of anything from siloxanes (which cause problems for combustion plants if they pass out into the network), or odorants that we might find in shampoos or shower gels.  Odorants could cause an impact on the gas as it flows out through the system.

The need to remove remove these, and other unwanted constituents, with an activated carbon treatment system. This is at the front-end of the process, so that they can get on with the real business of upgrading the biogas, as you might well have read about on other sites around the UK.

At Minworth, STW decided to utilise a water scrubbing plant. They made this choice this because they had seen it operating at a number of places on the continent. The STW experts were able to visit sludge upgrading at some of those plants. They were able to see the sewage sludge, and see it being upgraded at the flows and the quantities that they desired.

In principle, there are a number of technologies that people can be used to achieve the transformation of sewage sludge to biomethane, but because of the variability of the feedstock and the variability of the gas that they had, the water scrubber was found to be the most resilient.

They were also the first site in the UK to inject natural gas quality biomethane into the local transmission system. That means the National Grid's slightly higher pressure system between 15 and 20 bar gauge pressure, and what that meant was they had to introduce new biomethane compression plants. These were built on the site, and new monitoring equipment at the end. So once they've purified that gas, and got it purified to around 97/98% methane, they compress it, before we pushing it out into the UK's National Grid Distribution System.

This gas (biomethane) is now 100% ready to go into the grid, and STW have a system where they run some checks, and monitor continuously. They constantly collect data on the quality of their output of biomethane gas and the performance of that gas. They do that before they pass it under the road into the local transmission system. National Grid take it away from the sewage works, for use in the domestic network. They benefit of this way of working is considerably higher efficiency energy distribution than they say that they been able to achieve before.

“The biomethane upgrading process has been an excellent addition to our energy portfolio and to date we've run it for nine months with minimal problems since commissioning and we've injected around about 40GW hours of renewable gas.”

We are informed that this gas is approximately enough to heat 3,000 UK homes, but what they use it for is mitigating their gas use as a company. They now intend to be completely self sufficient in the production of biomethane from sewage sludge that they use to operate their business, on a daily basis. As found on Youtube

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