Siltbuster has worked with Porr Skanska A Hak joint venture on a significant National Grid project in the North to build a new gas pipeline under the River Humber. The water treatment specialists were called 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 5 km 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 have been on site since early 2017 treating water runoff and then 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.”
The Overall Objective
George Zonda, the JV Project Director, details the challenge faced: “For the CP system we need to flood the tunnel and altogether this 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 the source water, although visually 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 120 cubic metres an hour. 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 Fe2+ to 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 pH-controlled addition of caustic soda (raising the pH) to accelerate the iron precipitation and flow proportional dosing of a flocculant to agglomerate tiny iron hydroxide particles into bigger ‘clumps’. A total of 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.