Severn Trent Water, are a major water and waste utility situated in the middle of the UK and a leader when it comes to delivering Anaerobic Digestion gas to grid. They run their operation from the Midlands. They serve the area around Birmingham, and also out into Wales.
A major part of their business that they're growing at the moment is renewable energy production. They have a number of anaerobic digestion plant sites. In these total around about 35, and are installed across the UK Midlands where they treat sewage sludge. They utilize the sludge to create electrical power through making use of Combined Heat and Power (CHP) engines. The engines generate electricity from the biogas which the anaerobic digestion plants produce.
50 Years of Anaerobic Digestion
At their site in Minworth, they have been generating biogas for about 50 years from anaerobic digestion of their water treatment works sludge. However, nowadays they use the digestion processes on site to produce biogas that is further purified and compressed so that it can be directly injected into the UK's national natural gas grid. The technique is called “Anaerobic Digestion gas to grid”
For many years they have actually fed the biogas out into CHP engines that have actually allowed them to generate electrical power. This was previously only used to power the wastewater treatment site itself, and feed the regional electrical energy grid with the surplus power. However, since 2011 they have established a better (more efficient and more sustainable) alternative to this and started by looking at updating their biogas systems and injecting it out into the National Grid. The injection is done through the nearby natural gas main in the road outside the AD Plant.
Minworth UK First to Upgrade to Anaerobic Digestion Gas to Grid
The site at Minworth was the first to upgrade biogas from sewage treatment works, to provide Renewable Natural Gas (RNG) to the gas grid. They do not have a great level of control over their feedstock and exactly what's entering the Wastewater Treatment site. The result of that is that the biogas contains some substances which are impurities, and these cannot be injected into the natural gas main.
So they have a number of procedural actions that they've put in place to treat that gas and create the maximum effectiveness. In the primary step, they look at removing a lot of the unstable natural compounds that they have in the gas that's coming off their digestion plant. The anaerobic digestion plant is quite old. It's a basic mesophilic digester. It produces a biogas that can consist of impurities, and one unwanted compound is low quantities of siloxanes which trigger problems for combustion plants.
Impurities Must be Removed to Produce Anaerobic Digestion Gas to Grid
These cannot be allowed to go into the gas supply network, with other substances from deodorants that they might find in hair shampoos or shower gels that could cause an impact on the gas as it moves through the system and is used. Severn Trent Water engineers eliminate these impurities with an activated carbon treatment system at the front of the procedure. That means that they can proceed with the real task of upgrading the biogas.
This is done, as elsewhere at the growing number of other anaerobic digestion gas to grid (biomethane) wastewater treatment (sewage works) sites around the UK. At Minworth, they have decided to make use of a water scrubbing plant. They have done this since they've seen it running on a variety of anaerobic digestion gas to grid sites on the continent where they were able to see the sewage sludge being biodigested. They were also able to see it being upgraded in volume, at the flows and the quantities that they desired.
There are a variety of innovations that biogas digester operators can use, but because of the irregularity of the quality of their feedstock and the variability of the biogas that they produce, the water scrubber is the most resilient technology to use to clean up their biogas. It has worked well, and they have found it to be an excellent addition to their portfolio.
At Minworth Wastewater Treatment Works they were also the very first site in the UK that injected “anaerobic digestion gas to grid”. It is injected into the regional transmission system. That implies that the gas must be pressurised to the National Grid's somewhat higher pressure system of in-between 15 and 20 bar gauge pressure.
Upping the Pressure to Push the Biogas to Grid
What that needed was for them to introduce new compression plants onto the site and brand-new gas quality monitoring equipment was also added to keep a check on the biogas (biomethane) quality achieved. Once they've cleaned that biogas and they've got it to around 97/98% methane, they then compress it before they pump it out into the National Grid.
This gas is now 100% ready to enter into the grid and they have the necessary gas quality monitoring system where they run some checks and keep track of the quality of that gas continuously. They must be absolutely certain of the high quality of that biomethane prior to passing it under the roadway into the local transmission system.
National Grid take it away from there for usage in the domestic network, so they now get a considerably higher performance as far as energy efficiency and sustainability than they've had the ability to attain previously.
The biomethane upgrading process has actually been an exceptionally successful addition to their energy portfolio. At the time of writing (recording their video), they had run it for nine months with very few problems subsequent to completing commissioning. In fact, they report that they have injected around about 40GW hrs of eco-friendly biomethane gas.
This gas is roughly enough to heat 3,000 UK houses, but exactly what they utilize it for is reducing their gas usage as a company and they now intend to be entirely self-adequate in the gas that they utilize to run their entire utility service every day.