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Water: Filter formula

An innovative temporary solution installed during refurbishment of a wastewater treatment works could indicate the shape of things to come during the next funding period. Report by Margo Cole.

siltbuster tank

Bypass: A temporary filter system was brought in while upgrade work took place

During the current water industry funding period, AMP5, Dŵr Cymru Welsh Water is spending around £1.45bn upgrading its water and sewerage network. As with many AMP5 programmes around the UK, the water company is spending its money on a mix of new water infrastructure, mains replacement and on refurbishing and upgrading existing treatment works.

One scheme within this programme is the £4.2M upgrade of Kingstone & Madley wastewater treatment works in Herefordshire. The problem is fairly typical - an ageing works that needs modernising to meet consent levels, coupled with a need to increase capacity to cope with a growing population - but the solutions being introduced on this small project are less typical. And the innovations introduced in this quiet corner of rural Herefordshire may set the tone for projects in the next AMP cycle, when the emphasis is set to shift away from building major new infrastructure, in favour of optimising existing assets.

Kingstone & Madley wastewater treatment works was originally built during World War II, and has had a number of improvements and additions in the subsequent 50 years. It consists of five very strong rectangular concrete tanks that contain an array of filters and settling media to clean the wastewater before it discharges into the nearby river. Four reed beds were added at the site during the 1980s, to treat overflow from the storm tank.

The benefits of aeration are the far smaller area needed to achieve the same level of treatment, and the robustness and low maintenance

With the ageing tanks becoming difficult to maintain, and the local population forecast to increase significantly over the next 20 years, Dŵr CymruWelsh Water decided to give the treatment works a major overhaul, replacing much of the existing civils infrastructure, and adding more modern treatment processes. Various options were considered - including a total rebuild of the works - with the water company ultimately choosing to re-use some of the existing infrastructure within a more modern treatment regime.

The solution has been designed to accommodate wastewater from a population equivalent to that anticipated in 2040, and includes a new inlet works with screens, a compactor and lift pumping station.

Once the flow has been through this screening process, it will discharge to a new circular primary settlement tank.

If the amount of wastewater coming into the works exceeds the 33l/s design flow, it will be diverted into a storm tank, with overflow from the tank discharging to the reed beds.

“The key thing is about the surface area for the bugs to grow on”

Richard Coulton, Siltbuster

The two existing mineral media trickling filter tanks are being refurbished so they can be used for biological treatment, with the walls rebuilt to accommodate more biological media. Final settlement will be provided by a new circular humus tank, with tertiary treatment of the settled filter effluent provided by two new aerated reed beds.

Dŵr Cymru Welsh Water says it chose this option because it included reusing the existing trickling filters, while adding the radial primary and humus tanks, which are far easier to maintain than the old rectangular tanks.

“Some of [the existing tanks] we didn’t want to keep at all,” says Richard Codd, senior mechanical engineer for Imtech, which is delivering the project as part of its AMP5 framework with the water company. “The rectangular tanks are very difficult to de-sludge, but using them for storm storage with overflow is OK.

Reed beds

“The filter beds were in good order from a process perspective,” he adds. “When we came here there were four reed beds - all standard with normal horizontal flow and without any gravel medium.”

And this is where the first of the innovations comes in. Two of the original horizontal flow beds are being replaced with aerated vertical flow beds.

Vertical flow allows the effluent to be nitrified, while the aeration greatly improves the transfer of oxygen through the effluent, making the beds far more efficient.

Adding aeration to reed beds was developed in North America, and has been used in Germany and Japan for several years, but is still quite rare in the UK, and this is the first such installation in Wales.

“Severn Trent and Anglian [Water] have got some, but they’re not commonplace,” says Codd.

Aeration benefits

The benefits of aeration, says Codd, are the far smaller area needed to achieve the same level of treatment, and the robustness and low maintenance compared with mechanical nitrification plants.

The new beds, covering an area of around 1,500m2, were built by UK firm Eco-tech Systems. The contract included placing 2,000t of aggregate on top of a geotextile membrane before installing the effluent distribution pipework.

As is typical in wastewater treatment projects, the works has been fully operational throughout the project. But the existing trickling filter tanks had to be taken out of use so they could be refurbished before being brought back into service for biological treatment.

“We needed to find a way to refurbish the filters and keep this plant running,” says Imtech senior process engineer Jon
Ovens, who explains that the project team considered refurbishing the two tanks one at a time, but the timescales were
too tight.

“We could almost get away from the construction perspective, but to get the media in as well would have been getting very tight.”

The “media” he refers to is the material that is home to the “bugs” that provide the biological treatment.

Time to seed

This media needs time to “seed” - that is for the bugs to grow and develop before they become active. So, while it may have been possible to run the works with just one tank while the other was being
refurbished, there would not have been enough time to get the new media up and running in time to meet the ultimate project deadline - meeting new consent levels in March 2015.

siltbuster tank

Temporay tank: Siltbuster supplied a protable moving bed bio film reactor to treat sewage while the treatment system was being upgraded

Instead, the team went for an innovative solution - diverting the effluent through a temporary secondary treatment system in the form of a portable moving bed bio-film reactor (MBBR), provided by Siltbuster Process Solutions. MBBRs are well established in Scandinavia, but there are only a couple of permanent plants in the UK, one of which is in Swansea. Siltbuster installed its first MBBR at the Glen Keith whisky distillery in Scotland at the end of 2013, but the installation at Kingstone & Madley is the first time it has been used to temporarily replace an existing treatment process.

The company has a framework with Dŵr Cymru Welsh Water for temporary wastewater treatment plant, so was well known to client and main contractor. It was keen to collect performance data and operational experience on its new MBBR, so all parties agreed to try it at Kingstone & Madley.

“It’s got to fulfil same role as biological filters - remove most of the biochemical oxygen demand (BOD) and take out a portion of ammoniacal nitrogen,” explains Ovens.

Three compartments

Siltbuster’s MBBR has three compartments, each three quarters filled with media that biomass grows on. This
biomass removes BOD in the first two chambers, while the ammonia is removed in the third, with the addition of nitrogen.

An aeration system distributes air throughout the three chambers, and all three chambers are moving all the time.

“The key thing is about the surface area for the bugs to grow on,” explains Siltbuster managing director Richard Coulton, who says the media in these tanks - thousands of multi-sided plastic pieces - has a surface area of 600m2/m3, compared with around 200m2/m3 for the material that is usually used for biological treatment. “The bugs are growing on the surface area, so the greater surface area the larger the biological mass,” he says.

Again the media had to be given time to seed - before the old secondary treatment tanks were taken off line and all the effluent started going through the Siltbuster MBBRs. The company has had two of the units on site over the summer, with the plan that they will operate for around 18 weeks in total.

“We see the primary market for these MBBRs as this sort of application - temporary treatment systems - but I’m sure that will lead us to some primary plants as well,” says Coulton, who explains that the traditional solution to the problem at Kingstone & Madely would probably have been temporary aerated filters. “They don’t have the same treatment capacity for the same footprint,” he explains. “We would’ve needed three or four tanks rather than two, and space is constrained here.”

Coulton believes the next water industry funding cycle, AMP6, will see even more demand for this type of temporary installation.

“AMP6 is all about refurbishment of small waste water treatment works, where the existing plant needs to be taken offline and a temporary treatment system installed for the duration of the works,” he says. “On these, space is limited and access can be problematical.”

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