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Pressure cooker

Davyhulme wastewater treatment works in Manchester has recently undergone massive changes. This has not only had a huge impact on the way sewage is dealt with in the North West, but it also fits into wider government plans to try and break the UK’s dependence on fossil fuels. Katherine Smale reports on another British Construction Industry Awards winner.

Ever thought about what happens when you push the flush handle after that rather private daily ritual? It is a somewhat taboo subject, but the answer to the question of what happens to the waste we produce is an extremely important part of how we are able to live in a safe, clean and healthy environment in today’s congested world. And it is an area so ripe with technical innovation that it truly warrants further scrutiny.

United Utilities’ Davyhulme wastewater treatment works in Manchester is no stranger to new innovations and technical excellence.

Since it was built in 1890 its basic function of taking raw sewage, filtering it, separating the liquid from the semi-solid waste, known as primary “sludge”, and treating both parts has always been the same. However, in 1934 it pioneered the technique of processing the liquid part of the waste through “activated sludge”, a process which blows air through the liquid, creating a perfect environment for the pollutants (the secondary sludge) in the waterto be broken down, separated and added to the primary sludge for processing.

Davyhulme sewage treatment works

Big site: Aerial view of the Davyhulme site

And now today it is proudly boasting a brand new thermal hydrolysis plant. This enables the treatment works to generate its own electricity to power itself, saving its owner, United Utilities, millions of pounds in annual energy costs. The thermal hydrolysis plant has also tripled sludge handling capacity, enabling the treatment works to act as a regional centre for sludge treatment.

It is an impressive achievement, and it took some doing. In fact the new plant is the result of 12 years of planning, design and implementation.

Initially working with consultant MWH to produce the concept design, United Utilities appointed Black & Veatch as design and build contractor in December 2009.

Construction began in 2010 with the plant becoming fully operational in October 2013. Costing £100M it was, at the time of opening, the largest operational thermal hydrolysis facility in the world. It now treats the wastewater from 1M customers in the Manchester area - that is over 4M.t of sewage sludge a year, enough to fill Wembley Stadium two and a half times over.

“Thermal hydrolysis is there as a pre-treatment process to allow greater destruction of the volatile matter in the sludge ”

Richard Lancaster, United Utilities

Traditionally the process of treating the sludge required huge amounts of energy to be put in to enable the waste to be broken down and safely disposed of. Yet this “waste” is actually energy rich. Harnessing this embodied energy was the driver for the project at Davyhulme.

“In one day we all produce enough waste to power a 20W lightbulb for a day,” says Black & Veatch global director of water technology John Tattersall.

“One of the few huge questions within our industry is: can we get to energy neutrality or even better than that in terms of wastewater treatment? Can we become a resource generator and not a resource user?

Davyhulme sewage treatment works

New plant: Inside the new treatment thermal hydrolysis treatment works

“How do we redress that balance?” he asks.

It begins by rethinking the treatment process.

Once flushed, waste and other assorted items join the sewer network. When it arrives at a wastewater treatment works the first stage is to filter out everything that is not the desirable organic material.

Bras, wheel hubs, old phones and whatever else has mysteriously managed to negotiate the sewer network are removed by passing the raw sewage through a 6mm filter.

“The site today - one of Europe’s largest wastewater treatment works - is, at some points during the year, energy neutral”

Richard Lancaster, United Utilities

Grit from the roads, which gets into the sewers, also presents a huge problem to the plant and this is taken out by letting the sludge settle in small settlement tanks. From here the “good stuff” is put into larger settlement tanks and, by a pure gravity process, the solid and liquid parts are separated. The liquid part, which still contains a proportion of pollutants in solution, is taken to be processed in a parallel facility. The solid part, the proverbial gold dust, is now “sludge” and can be remediated. At this point it is around 2% to 4% solids.

In a conventional configuration, this sludge is transferred to digestion tanks where bacteria break it down over a period of two weeks.

But at Davyhulme it is at this point that the thermal hydrolysis plant steps in.

Davyhulme sewage treatment works

Intricate: Pipework feeding the thermal hydrolysis plant

“Think of the thermal hydrolysis process like a pressure cooker,” says United Utilities regional sludge manager Richard Lancaster. “It’s there as a pre-treatment process to allow greater destruction of the volatile matter in the sludge during the digestion process,” he explains

During the thermal hydrolysis process, the sludge is heated to a temperature of 165°C at a pressure of 610kN/m2 and cooked for half an hour. After this time, the sludge moves to a “flash tank” where the pressure is suddenly released, smashing open the complex organic substances the sludge contains. Harmful bacteria, the pathogens, are destroyed by the heat, and the sludge can now be much more easily digested due to the breakup of the cell structure.

The sludge then slots back into the original process and is transferred into the digestion tanks for digestion.

The benefits

It is once there that the benefits of the thermal hydrolysis are realised, the key one being biogas, a byproduct of the digestion process.

The rupture of the cell structure within the sludge material allows the bacteria in the digestion tanks to feast on a greater proportion of the sludge - generating much larger volumes of biogas than by conventional sludge digestion alone. This is the crux of it. The introduction of the thermal hydrolysis system has increased the volume of gas produced by 300%.

Davyhulme sewage treatment works

The biogas produced by the digesters is held in two giant gas holders, double membrane green “bags” with 9,000m3 capacity each, before it is turned into electricity in an onsite combined heat and power (CHP) plant. The energy produced is re-fed back around the plant in a closed loop system, powering the entire Davyhulme site.

“The site today - one of Europe’s largest wastewater treatment works - is, at some points during the year, energy neutral,” says Lancaster.

“That’s of huge, huge benefit.”

In fact the plant now produces 60GWh of energy per year - equivalent to the needs of 25,000 homes. With this United Utilities not only saves about £3M every year in energy bills, but it can also power a recently built regional data centre and still have some left over to sell back to the grid.

The future

Norwegian company Cambi originally designed and developed the technology behind thermal hydrolysis and the process has now become a tried and tested method.

The United States has even followed suit and a new facility has just been opened at the Blue Plains advanced wastewater treatment plant in the District of Columbia.

There, the scheme is so successful other states in the US are now following suit.

 

“We’ve reduced our customers’ bills by significantly reducing energy, which we need to import into our facilities,” notes Lancaster.

Energy generation is not the end of the benefits though,

The destruction of pathogens such as ecoli, coliform and salmonella in the thermal hydrolysis process is a big step forward in how the plant operators are able to dispose of the processed waste. Previously the product that was produced following the digestion process was 99% free of pathogens, which is compliant with conventional standards when disposing of it to agricultural land.

“That sludge is capable of being recycled to agriculture as long as it meets a number of stringent regulatory requirements and codes of practice,” explains Lancaster.

But the conditions on where the product can be placed are strictly controlled. There are limits on the pH, metals and pathogen content and there are time restraints before the land can be used for certain agricultural uses.

Davyhulme sewage treatment works

Storage: Two giant double membrane “eggs” hold biogas produced by the digesters before turning it into electricity

The new process takes this one step further. Because the pathogens are killed by the heat in the thermal hydrolysis, the end product now meets an enhanced standard and is a staggering 99.9999% pathogen free. As a result it can be used far more effectively.

“It is a very valuable source of nutrients,” enthuses Lancaster. “So things like phosphorus and nitrogen can be returned to the environment and used as opposed to being wasted.

“In fact if you went to Davyhulme and saw the product, you’d probably think it was a soil or a peat. It’s a low-odour, peaty-looking fertiliser which is applied to agriculture.

“We can return that product back to the environment in a safe manner which causes no harm to humans, animals or the environment when we apply it to agricultural land,” he explains

Having the sludge as a more desirable agricultural product has another knock-on benefit.

Land is in demand in the UK, and a growing population is not relieving the situation.

As a result the land bank available for recycling the increased amount of traditional outputted product was coming under pressure.

Now with the thermal hydrolysis plant in place, the improved quality of the product means a much broader land bank is available for its disposal.

Project team

Commissioning authority: United Utilities
Principal designer:
United Utilities/MWH
Principal contractor: Black & Veatch
Engineer: MWH/Black & Veatch

 

“It offers us an opportunity to return valuable nutrients back to the environment, as opposed to spending lots of money trying to destroy them and sending them to landfill,” says Lancaster.

“It’s exactly the right thing to do for our customers and exactly the right thing to do for the environment.”

The final benefit is to do with how waste is dealt with in the North West.

The upgrade works were part of a key strategic decision to centralise the treatment works rather than building satellite facilities around the region.

Davyhulme sewage treatment works

BIM: Model of the new thermal hydrolysis plant

As such, it is now able to treat triple the volume of sludge, the additional sludge being tankered in from seven different feeder sites around the north west of England.

Despite this, the improved digestion process means the volume of treated material, which needs to be disposed of, has remained the same. And perhaps more importantly than this, the treated product is now also in a much better condition.

Sludge that is not finding a home in agriculture is transferred via a pre-existing pipeline to United Utilities’ Mersey Valley incineration plant at Shell Green. The improved quality of the outputted material burns more efficiently than the conventionally treated sludge.

Planning, design and construction

The decision to focus the upgrade of the sewage treatment works on one central site was taken early in the planning process. This has significantly reduced the embodied carbon and made the process more efficient.

Situated in a densely populated area and close to the Trafford Centre shopping mall, the treatment works in Davyhulme had to undergo extensive work to control odours, an understandably contentious issue. Surveys and modelling exercises were required to satisfy the planners and obtain regulatory permits for the construction phase and the operational lifetime of the new plant.

During the design and construction phase of the plant, an area of the site was earmarked for the team to be co-located and facilities were built on site. This created a particularly “collaborative” team effort, says Tattersall.

“Particularly when you’re doing something like this on a live site, it means that you can immediately talk to people responsible for operations,” he says. “You can get issues addressed very quickly. I think it makes a huge difference.”

“One of the things they were keen to do, was not have a plant that was actually running on its maximum capacity all the time, so they wanted something with a little bit of room - so they could optimise it “

John Tattersall, Black & Veatch

Construction of the new plant was designed to have minimal impact on the day to day running of the existing site. The key components were manufactured off-site to minimise time on site. With this philosophy, 80% of the main thermal hydrolysis treatment plant was erected in just six days.

The team also adapted the plant’s existing eight huge digestion tanks, taking two off line at a time to clean out and retrofit the required new pipework.

“[The eight digestion tanks] are the original tanks, that’s a huge reuse of an asset,” explains Tattersall.

The plant is currently running at about three quarters of its capacity, allowing the process to be optimised and run as efficiently as possible.

“There is undoubtedly some growth headroom built in there as well, but one of the things they were keen to do, was not have a plant that was actually running on its maximum capacity all the time, so they wanted something with a little bit of room - so they could optimise it and get it running very efficiently, which is what they’ve done,” says Tattersall.

Lancaster says there is no doubt that the generation of biogas from the processing of wastewater has wider implications for the energy market in the UK and abroad.

“It’s a real industry and the UK government is trying to stimulate it,” says Lancaster.

“There’s a real drive out there to do more. Sludge is not on its own in having a calorific value, there are lots of other wastes out there that can be digested. It’s a good way of getting the energy out of what was in essence a waste and has now become a product and a valuable resource,” concludes Lancaster.

BCIA judges’ verdict

The judges said this was “a clearly united team, which constantly pushed the boundaries of innovation.
“A team that approached the challenge with the end and the end user in mind,” they said. Technologically it is a world first and it truly considered and applied sustainability to every stage of the contract.
This is an outstanding example of how a construction project can genuinely make a difference to people’s lives and environment. An excellent application of Early Contractor Involvement and also maintaining involvement in the design process.

  • The Davyhulme wastewater project won the major civil engineering project of the year award at the British Construction Industry Awards 2014.
  • Do you have a project like this? Entries for this year’s British Construction Industry Awards are now open at bcia.nce.co.uk

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