Project Focus: An innovative solution to the metaldehyde challenge
The installation of CarboPlus technology at a treatment plant near Nantes in France has demonstrated the ability to hit tightened European limits on pesticides
by Frederic Carbonnier, Plant Director, Saur Glasgow
It will be well known to readers of this magazine that the issue of micropollutants and the challenges of drug residues in water are regularly in the news. The toxic effect of these persistent compounds on the environment, even in low concentrations, and the contamination of underground water resources, is becoming a key issue for water operators to solve.
Following improved ability to detect trace compounds, regulation in the form of the October 2015 European Commission directive 98/83/EC has tightened water quality standards. The updated list of monitored substances in the directive defined a limit concentration on drinking water of 0.1 µg/L for individual pesticides and of 0.5 µg/L for total pesticides. This, together with increasing public health concerns, drove Saur Group - the international utility group which operates a wastewater treatment plant in Glasgow for Scottish Water - to push hard to deliver an innovative solution which met both the regulatory limits and client concerns. The resultant CarboPlus technology has been introduced in over 20 sites, providing excellent quality of water while meeting rising needs.
The most recent installation is at one of Saur’s largest drinking water sites at Basse-Goulaine, near Nantes in France. The site, operated by Saur since 1952, currently handles 72,000 m3/day running continuously to provide drinking water to 405,000 inhabitants. The raw water pumping station at the plant is positioned on a tributary of the Loire river and the catchment area contains significant areas of intensively farmed land. As seen in many similar catchments, evolution and changes in agricultural practices has seen increased levels of pesticides entering the ageing plant. The surface water is also characterised by low organic matter, and high concentration of metals. The scope of the project at Basse-Goulaine was to achieve new tightened limits of individual pesticides and of total pesticides, while increasing water production to supply the growing population in the area.
Basse-Goulaine is a significant water plant in the region and while a key project objective is to reduce pollution for local drinking water, continuity of operation is an equally critical requirement for the client due to the constant water demands. This presented the project team with the challenge to install and commission the CarboPlus technology without compromising the running of the plant.
Work started in January 2014, with the site clearance and earthworks for the installation of seven CarboPlus tanks, each with a capacity of 500 m3 per hour. The project presented a number of construction challenges as a result of heavy clay soils which impacted on the site works but the simplicity of the CarboPlus process allowed the technology to be fully constructed and tested within the original construction programme of 17 months.
The improvement works saw the building of a new process stream and the rehabilitation of the old one by replacing the existing GAC filters with three CarboPlus filters. The building of the new process stream (2 Delreb, 4 CarboPlus and 6 sand filters) resulted in an increased flow from 52000m3/d to 75000m3/d. The improvement works also included provision of a new UV plant for the new and refurbished streams.
There was no interruption to production during the construction period and all client performance and quality objectives were met. The total project investment, including 2 Delreb (high rate clarifiers used to remove the suspended solids), 7 CarboPlus, 6 sand filters and 4 UV reactors was in the region of 25 million euros.
The technology has delivered efficiency improvements as well as pollution reduction.
The CarboPlus technology used was selected primarily for its effectiveness in pesticide removal. However the process was also cost efficient due to its small total footprint of only 300 m². The technology used at Basse-Goulaine has been refined through the use of the micro granular Activated Carbon. This has generated additional efficiencies through the ability to capture and reactivate the media to reduce operating costs further, as well as negating the need for any chemicals dosage. Other efficiencies realised on the project when building the complete process stream was the construction as a single connected process with clarifier, CarboPlus and final sand filter within a single building.
Stephanie Piel, Saur’s R&D engineer on site, appreciates the benefits of the new CarboPlus process.
“CarboPlus works by ensuring a continuous dosage of activated carbon, so that an adequate quantity of new and fresh activated carbon always exists in the reactor, which in turn ensures consistent efficiency of the carbon fluidised bed,” says Piel. “As the reactor now contains a larger amount of activated carbon compared to other solutions, it means the process is more efficient because there is a higher concentration and larger exchange surface to capture the pollutants.”
The technology also brings added efficiency as doses of carbon can be adjusted for peak periods. In Basse-Goulaine, where quantities of metaldehyde peak in the winter with more intensive farming practices, the treatment dose is adjusted every season, to follow this trend.
In less than six months since its introduction, the technology has also produced some encouraging results.
The CarboPlus technology was introduced to treat all water flow through the plant. Early results show a decrease in pesticide levels to well below regulatory limits of 0.5µG/L for total pesticides, and below 0.1µG/L for metaldehyde, from a raw water metaldehyde concentration of 0.32 µG/L observed during winter pollution peak before the installation.
In-depth results and analysis of the CarboPlus patented technology have been published in scientific articles over the last two years, including by INRA (France’s top agricultural research institute), the CNRS (French National Centre for Scientific Research) and the École des Ponts-ParisTech graduate school. This independent research confirms the assertion of Saur Group’s Head of R&D Fabrice Nauleau, who says: “Adsorption using activated carbon is the only technique that removes micropollutants. We have a duty, as a professional provider of water services, to respond to the questions asked by our clients and the trends emerging as a result of customer needs. That pushes us to find solutions that don’t just work but that go beyond current legislation so they stand the test of time”.
-More information is available at saurglasgow.com/saur-worldwide/carboplus
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