Severn Trent generated more than a third of its energy from renewable sources last year (35 per cent in the six months to September 2016) and has ambitions to increase that to 50 per cent by 2020.
The company, which serves eight million people across the Midlands and mid-Wales, has invested heavily in renewable energy in recent years and is now developing this expertise to generate both power and income.
At the forefront of this is a commitment to creating energy through anaerobic digestion (AD) using sewage sludge, and more recently food waste, as sources. Severn Trent now has 34 AD plants turning sludge into energy. In 2014/15 this generated 208 million kilowatt-hours of energy – enough to power more than 50,000 homes for a year.
The vast majority of this was used to offset the company’s own energy use. Because of its widespread geography it has a relatively high demand for electricity to move waste and water around. That means it has had to be really innovative in using its outputs from sewage treatment works.
As the company has gained knowledge and expertise in converting sludge into clean energy, it is now putting that to good use in food waste AD plants.
Severn Trent currently has one food waste AD plant at its site in Coleshill, a second is under construction at Roundhill near Stourbridge, and planning permission was granted for a third in Spondon in Derbyshire at the end of November.
The food waste plants take contaminated food from local businesses and waste management companies and digest it to produce biomethane, which can then be converted into gas or electricity. The Coleshill site has the capacity to process 48,500 tonnes a year of both packaged and unpackaged food waste, producing enough energy to power around 4,000 homes for a year.
Currently, most of that electrical energy goes back to run sewage works, but the new food waste digesters in Roundhill and Derby will output biomethane into the grid.
While Severn Trent has the ambitious target of generating the equivalent of 50 per cent of the energy it uses by 2020, the focus is increasingly turning towards “gas to grid”, which will generate income from the power it generates.
Severn Trent’s first gas-to-grid plant was at its Minworth Sewage Treatment Works, serving Birmingham and the Black Country. The company is able to arbitrage whether to use the gas to generate power for the treatment works (which, due to its size, is power hungry) or to inject it into the national grid.
The gas is made suitable for use in homes with a process that involves some very complex engineering techniques. The gas is “washed” at high pressure; it is then “squashed” or compressed so it is at the same pressure as natural gas; and then it is “tested” for quality and an odour is added so it smells like conventional gas.
Testing also includes a review of the energy composition of the gas. Once that has been done, it is injected into the gas supply network. Before it is injected there is an opportunity to stop the process, so the company can be 100 per cent certain it is perfect.
In addition to AD plants, the company also makes use of other assets for producing renewable energy. Solar panels are in place at 34 sites and a total of five wind turbines are in use at five other locations.
In the past its expertise was in dealing with delivering fresh water and taking away wastewater but the focus on renewable energy has seen the company grow huge amounts of knowledge and expertise in other areas.
It has been using AD for more than 60 years, which is why it made sense to concentrate on that area and develop its knowledge. With all of the company’s sludge now being processed through AD, Severn Trent has turned its attentions to developing a food and other organic waste business.
Clearly, we have limited capacity if we just concentrate on sludge, so we are very much looking at ways we can use our expertise across the waste industry.