Siltbuster has worked with the Porr Skanska A Hak joint venture on a significant National Grid project to build a new gas pipeline under the River Humber. The water treatment specialists were called in to provide a temporary, high-flow dissolved metals removal system ensuring clean water was available for a key part of the construction.
The location of the new 5km pipeline under the Humber between Goxhill and Paull, just outside Hull, is a Special Area of Conservation (SAC), a Special Protection Area (SPA), a Ramsar Site, and has numerous Sites of Special Scientific Interest (SSSI). The 4m diameter tunnel through chalk geology has taken many months to build; Siltbuster has been on site since early 2017 treating water runoff and alkaline waters from the tunnelling operation, ensuring high value habitat is protected.James Baylis, Regional Technical Sales Engineer at Siltbuster, explains: “We had an excellent ongoing relationship with the site during the tunnelling phase of the project, keeping the works compliant with their discharge permit. The next stage of the project was the installation of an impressed current cathodic protection (CP) system to prevent corrosion of the new pipeline.”
George Zonda, the joint venture (JV) Project Director, details the challenge faced: “For the CP system we need to flood the tunnel, which requires 56,000 cubic metres of ultra-pure water. The water source was the local aquifer and ultrafiltration and reverse osmosis techniques were to be used to highly purify the water before it enters the tunnel.
“Unfortunately, it was soon discovered that the source water, although visibly crystal clear when extracted, quickly became a rusty orange colour on exposure to the air. Naturally occurring dissolved iron was being oxidised causing the discolouration. Both the dissolved and precipitate forms of the iron caused problems, blinding the filters and fouling osmosis membranes. A quick response to treat the iron-rich waters was needed to get the project back on track.”
Siltbuster designed and rapidly mobilised a high-flow three-stage iron removal water treatment system capable of treating up to 120m3/hr. The oxidised form of iron has a much lower solubility at neutral pH than the reduced form and above pH8.5 is virtually insoluble. This fact was used in the system design with the first stage being aeration to convert ferrous iron (Fe2+) to ferric iron (Fe3+). Air sparger units were used on the feed water leading to two large 30m3 MT30 mix tanks arranged in parallel. Dosing of reagents was then carried out via two containerised systems, firstly the pH-controlled addition of caustic soda, raising the pH to accelerate the iron precipitation, and then the flow proportional dosing of a flocculant to agglomerate tiny iron hydroxide particles into bigger ‘clumps’. In total six Siltbuster HB40R lamella clarifiers with integral rakes were used for effective recovery of the settling particles with the sludge being transferred into a final holding tank prior to disposal.
James continues: “The Siltbuster inhouse laboratory once again proved invaluable. It rapidly analysed water samples from the project allowing us to refine our process design in response to dynamic realtime changes on site. We ran several trials to demonstrate the process treatment system comprising aeration and reagent dosing, followed by settlement, would achieve the Total Suspended Solids (TSS) and very low dissolved iron quality criteria.”
George concludes: “When complete, this project will be the longest hydraulically inserted pipeline in the world and will supply 25 percent of Britain’s gas supplies. Siltbuster’s help was crucial to resume filling the tunnel and get this high profile and essential critical infrastructure project going again.”
To find out more about Siltbuster or any of its projects, visit www.siltbuster.com