Construction wastewater specialist Siltbuster has partnered with France-based pump rental company Telstar to treat waste water on the €23bn (£20.5bn) Grand Paris Express rail scheme.
Telstar recognised the challenge of dealing with waste water on the project while working with Soletanche Bachy on tunnel shafts at Vitry-Sur-Seine in Paris. The project involves construction of 68 stations and over 200km of new track so Telstar realised the challenge at one site would be repeated elsewhere and brought in Siltbuster to help.
The partnership is working to deal with alkaline and grout particle polluted waste water from management of excess grout.
“Historically when faced with waters like these, French construction sites have relied on conventional methods such as baffle tanks, but that old-fashioned approach doesn’t properly limit the pollution potential of such waters,” said Siltbuster CEO Richard Coulton. “In today’s more highly regulated, environmentally aware construction industry, companies are looking for a much more effective solution.”
Siltbuster’s team advised that the water be collected within a storage tank where a small submersible transfer pump forwards them to a Siltbuster modular chemical dosing system comprising an MT8 mixing tank and a lamella clarifier designed for the export market (the HB50E). The mix tank has an integrated pH probe and controller which monitors the incoming pH level of the water as it enters the system. When the alkaline level detected exceeds an upper user-defined limit, the system automatically introduces carbon dioxide into a stirred reaction tank to neutralise the waters. The dissolving of carbon dioxide (CO2) into water forms a mild carbonic acid which steadily reduces the pH level of the waters until pH 7 neutral is reached.
The Siltbuster system favours using CO2 rather than mineral these are dangerous to handle and must be securely stored. Surplus acid must be disposed of as hazardous waste, and with acid it is easy to overshoot the target resulting in acidic water - which is equally polluting. Using the acids creates “secondary pollutants” - sulphate and chloride - which are harmful for the environment. Citric acid is and alternative but this also increases the biochemical oxygen demand in the water.
Coulton added: “In contrast, cylinders of carbon dioxide can simply be returned to the supplier at the end of the job and CO2 is between 50% to 80% cheaper than mineral or citric acids. It’s important we communicate these benefits to the French construction industry as we don’t want to see companies who are keen to change their approach, actually taking a backward step by adopting acids as a method of water treatment.”
Once the pH of the waters has been safely neutralised using CO2, the Siltbuster system adds both a coagulant and flocculant to the wastewater. These ensure any non, or slow, settling solids form rapidly settling clumps which can then be removed. The remaining pH-neutral water is then directly discharged to an adjacent foul sewer.
This first Siltbuster modular three-stage chemical dosing system was installed in August this year with a further eight similar systems on order for this massive project. With four new lines and extension of two existing lines, the scheme is planned to take another six years until total completion.
“Right across the global construction industry we’re seeing a drive to improve environmental standards, with site waters being an obvious area of focus. It’s a real step-change moment, and one we’re really proud to be a part of,” said Coulton.