Tata Steel treats dissolved iron in North Wales groundwater
Monmouth-based construction water treatment specialist, Siltbuster, has just successfully completed a 10 week long project to treat iron-laden groundwater recovered by a series of well points at the Tata Steel site in Shotton, North Wales.
“The client was part-way through construction work required to accommodate a new production facility in one of its existing buildings,” said Siltbuster’s Project Manager, Clwyd Jones. “This involved the construction of a reinforced concrete box some 10m below ground level in running sand. The proposed method of construction involved the installation of sheet piles to form a box within which the reinforced concrete chamber could be constructed. A series of well points had been installed around the periphery of the sheet piles to lower the water table and prevent the base of the excavation from heaving.”
Soon after dewatering commenced, the dissolved iron concentration in the groundwater started to increase rapidly. Had it continued, this would have resulted in the site exceeding the limit stipulated on its discharge permit to the environmentally sensitive River Dee. At that point Tata stopped the dewatering, and put the project on hold whilst a suitable method of removing the dissolved iron was sourced.
Tata’s Senior Project Engineer Simon McCormick said: “Tata Steel believes that all our activities can be carried out safely and with due care to the environment. We manage pollution from our activities by implementing and maintaining effective environmental systems. We take this responsibility very seriously. Therefore we had procedures and control measures in place to identify high iron contamination during the dewatering process that had been endorsed by Natural Resources Wales prior to operation.”
Upon reaching the system control alarm, the process was stopped to prevent a breach of the site’s Environmental Operating Permit. Tata then consulted a number of companies for help. After careful evaluation of the quotations submitted, the Siltbuster proposal was deemed the most attractive from a technical and financial viewpoint.
McCormick continued: “We were confident that they had both the expertise and equipment available to set up a temporary treatment plant on site within a short time frame.”
The process deployed on site involved a combination of pH adjustment and aeration to precipitate the dissolved iron and then removal of the iron using dissolved air flotation from 120m3/hr of ground water. Because of the large volume water to be treated, the solids recovered by the dissolved air flotation plant were then dewatered using a mobile filter press mounted on an articulated lorry.
“It’s the sort of project we have undertaken several times before when treating contaminated water from abandoned mines both in the UK and worldwide,” said Clwyd Jones. “Normally we have weeks or months to mobilise the equipment rather than the few days Tata Steel required in order to minimise delays to the project.”
With the treatment solution in place, pumping recommenced, and the iron concentrations rapidly rose from an initial value of 9mg/l to a maximum of 70mg/l and averaged 59mg/l. During the whole 10-week project duration some 180,000 cubic metres of groundwater was treated and over 10 tonnes of dissolved metal prevented from contaminating the River Dee.
Clwyd Jones added: “What also made the project slightly more challenging was the fact that once the box had been excavated, the pumping and associated treatment could not be stopped until completion of the last concrete pours had cured, since failure of the system could have caused uplift of the partially completed structure, resulting in it floating like a boat.”
To eliminate this Siltbuster made sure it had both duty and standby components available for key parts of the process plant and provided around the clock technical support to the Tata Steel team operating the plant. Siltbuster experienced good teamwork with all Tata Steel’s staff throughout the project.
Simon McCormick concluded: “Without the expertise and swift response from Siltbuster, the project could have been delayed for several months with significant additional costs incurred whilst an alternative method of construction was developed and implemented.”
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