GPE includes the construction of 68 stations and over 120 miles of new track to improve the public transport network across Paris and alleviate heavy road congestion. Telstar’s team is supplying pumps to Bachy Soletanche for use on a section that involves the construction of new tunnel shafts at Vitry-Sur-Seine. Telstar observed that there were potentially going to be issues with wastewater on the site and recommended bringing in Siltbuster’s technology.
During the grouting of the GPE shafts and tunnel faces, excess waste grout is pumped to an above-ground centrifuge and filter press. Here it is dewatered, and changed from a liquid slurry to a solid waste, enabling it to be disposed in landfill, at a reduced cost. However, the wastewater, created as a by-product of the dewatering process, contains very small grout particulates, causing the waters to appear grey and discoloured. It also has an elevated alkaline pH level of circa 12-13, which is akin to oven cleaner and thus extremely harmful to aquatic life.
“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 Dr Richard Coulton, Siltbuster’s CEO. “In today’s more highly regulated, environmentally aware construction industry, companies are looking for a much more effective solution.”
With up to 40m3 per hour of such alkaline, sediment-rich waters to deal with, Bachy Soletanche asked Telstar for advice. It recommended calling in Siltbuster, which advised that the waters be collected within a storage tank where a small submersible transfer pump forwards them to a Siltbuster modular chemical dosing system. This comprises an MT8 mixing tank and a lamella clarifier. 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 into water forms a mild carbonic acid that steadily reduces the pH level of the waters until pH 7 neutral is reached.
The Siltbuster system favours using CO2 rather than mineral or fruit acids for many reasons. Mineral acids 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 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 with a further eight similar systems on order for the project.