When Deephams plant, in Enfield north London, was constructed in the 1930s it was surrounded by farmland, but now the site has houses backing onto it as urban sprawl has consumed the once open space. As the population has boomed so has the need for water treatment – to the tune of one million customers.
Work began on upgrading Thames Water’s sewage works in 2015 and completed this summer. The plant continued to operate during the construction of the new site. It took more than a million man hours and up to 500 people working on site to deconstruct the old treatment lines and bring the replacements online with no interruption in service.
As well as population growth and aging equipment, the upgrade was driven by changes to effluent quality consent, which were lowered for flow into the adjacent River Lee. The upgraded plant had to achieve the more stringent discharge consent.
Under the Freshwater Fish Directive and the Water Framework Directive, levels of ammonia, suspended solids and biochemical oxygen demand allowed in treated effluent were lowered from 31 March 2017.
Failure to meet it would have led to prosecution from the Environment Agency and reputational damage, so the discharge permit team had to be confident the plants could work in a reduced footprint. It was tasked with delivering consent standards that were three times more stringent, working with one third less of the plant while it was under construction.
The site was divided into three parts with two parts of the plant “live” at each time. Martin Hoff, head of major construction at Thames Water, said this meant the team had to make the plant work harder in a smaller space.
To achieve this an IFFAS (integrated fixed fill activated sludge) system was used in the water treatment tanks to aerate the water and strip out ammonia as it is treated.
The site includes 250 stainless steel structures with what look like curtains hanging down. Instead of the bacteria floating around in the water, they cling to and grow on these “curtains” and accelerate the rate of treatment. It’s the equivalent of adding extra treatment space in a smaller footprint so the reduced size stream is the equivalent size of a full-size treatment works.
This technology has not been used at this scale anywhere else. IFFAS has been used in other treatment plants in the UK but on a much smaller scale.
Hoff said: “The biggest challenge about new technology in the water industry particularly is we’re dealing with biology, so when you put a treatment process in, it’s not like a chemical or electrical process. Therefore, you need experience in it in order to know that it will work. So, introducing new technology into the water and wastewater treatment business you’ve got to be careful.”
These IFFAS curtains hold the biology, suspended in the liquor, but as soon as it’s out of the water that biology starts to die off. Hoff explained that it’s as much about understanding how the technology works as it is about trying it out.
“We were working on a much bigger scale than had been tested anywhere else so there was a lot of learning about how to control the oxygen that sits within the water. If we don’t get the oxygen distribution levels right the biology changes from good to bad. Bad biology generates ammonia instead of stripping it out.”
Reducing the site’s footprint by one third meant space was left for expansion for future resilience plans.
“The economics of this is where there is insufficient space to expand, existing tanks can be used to put in this technology and change the way aeration is controlled to get 50 per cent more out of it.”
As well as increasing the efficiency of the ammonia treatment to meet the discharge consent rules, efficiency was central to the plant’s thinking on energy with two-thirds of the electricity required on-site self-generated from methane. This is extracted in the treatment process and used to drive turbines.
With urban encroachment came close neighbours so odour emissions and the nuisance to neighbours was a central concern in a way it hadn’t mattered when the original site was built.
An odour reduction target of 85 per cent was set. The finished works reached 99 per cent, which was achieved with partial covering of the inlets.
“We dealt with the odour nuisance in the first stage of the new plant. In terms of engagement with neighbours this worked well because they saw an immediate benefit of the upfront work,” Hoff explained. “Therefore, during the construction works they were happier neighbours.”
Traffic management on and off the site, which sits next to an industrial park as well as being surrounded by residential areas, was always going to be a big issue.
Ashley Weyell AMK project director, the company contracted to undertake the building works, explained there was a commitment not to take material offsite so 100 per cent of the deconstructed old works were recycled and reused on the build.
He said concrete was crushed and re-used, while the new tanks sit in the footprints of their predecessors. He said the foundations of old works are effectively under the new works.
To reduce the amount of concrete meant the team maximised the use of pre-casting, which Weyell said also resulted in better quality products. “This is increasingly common for projects like this, but the team learnt that working pre-casting into the planning stage made a difference and retrofitting precast was not as successful.”
As well as keeping community and residents on side for the build, the Thames Water team fostered a close relationship with the London Borough of Enfield throughout the work.
Hoff said: “If we hadn’t got that relationship right it would have been a real stopper. Working as a team to make sure we got the right output and working with Enfield meant we got the right output in a timely manner and didn’t incur extra costs.”
The site had 31 planning permissions to meet including achieving an ‘excellent’ CEEQUAL score for how it balances environmental concerns with planning and construction.
“Planning applications always have naysayers. The planning decision had officers on board to champion what we submitted. In everything we do as a business, we must work closely with the local authorities. Street work and projects like this, we need to involve the community.”
The community was invited to take an interest in the works with 14 drop-in sessions for the project ahead of the planning process to ensure local support. The site has hosted open days with public tours around the plant and a large education centre included in the new build.
This will welcome school groups and students to learn more about how water is treated and what a water company does.
These open days have already sparked an interest in a different career with a number of jobseekers who have joined a back to work scheme supported by Job Centre Plus to provide support and training to three cohorts and one even working full-time at Deephams after the scheme.
For something that had the potential to be a bugbear to neighbours, Thames and AMK got the community on board and, even more surprising, enthusiastic about the work being done.
The company has incorporated technology to process waste efficiently and to environmental standards while future proofing Deephams for the inevitable population growth. The reduced footprint means there is space for a third processing lane so Deephams can grow with London.