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FLO uses new type of water treatment unit on Northern Line scheme

A new type of water treatment unit from Siltbuster is being deployed on the Northern Line extension in London.

The iHB20R is being used by FLO, the joint venture between Ferrovial Agroman and Laing O’Rourke, at two separates sites to remove solids from, and adjust the pH of, water generated during the tunnelling and groundworks so that it meets Thames Water discharge consent before it is released. 

The Northern Line extension involves two shafts in Kennington, 3.2km of tunnels and two new underground stations at Nine Elms and Battersea. To build this, high volumes of concrete are needed. Furthermore, groundwater runs high in London, and the cementitious water created during the construction work contains large amounts of solids and has a very high pH of 12-13 so the water cannot not be safely discharged without treatment.

Siltbuster was chosen for the project, following a competitive tender, as the iHB20R which it developed specifically for the task can perform both the solids removal and pH adjustment in one, single integrated unit. The easy to operate unit works at high and low flow rates and is capable of handling 20m3 of water per hour, well above the specified level of 10m3 per hour – giving FLO plenty of headroom.

FLO also chose the Siltbuster solution as with the iHB20R, the pH adjustment is done using carbon dioxide (CO2) on a fully automated basis. Using CO2 to adjust pH offers several advantages over rival systems which use citric acid. CO2 is cheaper to use and it is more controllable; so, with CO2 there’s no risk of over-dosing and creating acidic water, which is equally polluting. Furthermore, CO2 is safer for workers to handle than an acid and with the Siltbuster system no manual dosing is required. Lastly there are no disposal considerations with CO2, whereas with citric acid any unused acid must be disposed of as hazardous waste, which is costly.

Alejandro Vazquez, tunnel agent at FLO, said: “When we weighed up all the benefits of the Siltbuster solution it seemed by far the best option for us. Siltbuster not only understood our needs and came up with a bespoke solution very quickly, but they also gave us a lot of great advice along the way which has saved us money.”

James Baylis, technical sales at Siltbuster, said: “When initially talking to the team at FLO they specified that the treated water attain such a high standard that it could be released to the river. However, in London 90% of projects have access to the sewer, and this was one of them. So, we advised them that a lower specification was possible. By changing the discharge quality treatment parameters from 50mg/l total suspended solids (TSS) to 2,000mg/l, we delivered a saving of around £12,000 for each plant.”

The two iHB20R, which are now on site on a semi-permanent basis for 24 months, are also quiet and easy to move around; making them ideal for use in central London where construction work must be completed near business and residential properties. FLO’s workers can leave the units operating 24 hours a day, separating out the solids, adjusting the remaining water’s pH and releasing it to the sewer.

The iHB20R’s internal rake ensures that the concrete-laden sludge, which has been separated out from the water, doesn’t solidify, so it’s easy to remove at the end of each day at the push of a button. 

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