Historic site overcomes environmental challenge

A combination of manmade and natural forces has resulted in much of the bay silting up and the sea has receded, almost marooning the abbey in a salt marsh. This is now being addressed through a major project including a new pumping station.

The abbey of Mont St Michel.

Such was the extent of the task at hand, it was 10 years ago when the Mont St Michel project started. It has taken in the construction of a new dam on the Couesnon river close to the estuary in order to increase hydraulic capacity, the removal of the 130-year-old causeway between the mainland and the abbey and its replacement with a foot bridge, removal of sand surrounding the base of the rock to create improved hydraulic seawater flows and the development of enhanced visitor facilities and infrastructure.

One particular aspect has been to address the removal of wastewater and effluent in a safe, effective and environmentally acceptable manner to a treatment plant located 2.5 km along the coast. Instigated by Synicat Mixte Baie du Mont St Michel, the project involved the construction of a pumping station (Fig.2) that would accommodate the considerable fluctuations in effluent flow associated with visitor numbers at different times of year.

The compact arrangement of the pumps reduces the footprint area.

In addition, it had to be insulated from the effects of high spring tides. The contract for designing, equipping and constructing the pumping station was awarded to Sogea North West Construction (Vinci Group) and KSB.

In order to satisfy the many technical, environmental, performance, economic and security challenges, both contractors had to come up with an almost unique solution. The result was a dry pit on-line lift pumping station discretely positioned below sea level at the approaches to the main entrance to Mont St Michel, equipped with four KSB Amarex KRT pumps.

Influencing factors

Several factors influenced the choice of a submersible dry pit pumping station, not least of which was the minimum impact that it would have on the environment of Mont St Michel. Design and operating requirements KSB had to consider included the non-emission of odors and gases, variable flow demands on the pumps, potential storm conditions, discharging wastewater and effluent over a long distance, space and access constraints, absence of water storage, high performance, energy efficiency, low environmental impact and easy maintenance.

“The design that we proposed involved the installation of four Amarex KRT pumps in a 3 + 1 arrangement, with frequency inverters for speed control,” said Stéphane Quertain, product manager Pumping Station KSB France. “The Amarex KRT (Fig.3) is a dedicated wastewater pump and is more than capable of handling the type of solids laden fluids that are discharged from toilets, kitchens and other facilities used both by residents and tourists. Also, there was the safe removal of rainwater to be considered. The design of the Amarex KRT impeller and pump housing ensure a free flow of solids-laden liquid (Fig.3A) directly to the treatment works without the need for a holding tank ahead of the pumps.”

A significant factor in the selection of KSB’s Amarex KRT pump for the project is that it is IP68 rated for operation when fully submerged in water. The ability to comply with this requirement was a stipulation of the contract because during periods of exceptionally high tides, seawater could enter the pumping station.

Each of the Amarex KRT pumps uses high efficiency IE3 motors and can provide flows of up to 540 m³ at pressures of 10 bar. The ability for the Amarex KRT to deliver on-demand pumping is of major importance for the Mont St Michel pump station as it removes the need for liquid storage and screening facilities, thereby reducing the foot print area (Fig.4). A further benefit of this arrangement is that odors and gases are contained in the effluent pipes and never linger in the pump station. This eliminates any unpleasantness for visitors and residents and removes the need for maintenance personnel to work with dirty water.

The design of the Amarex KRT impeller and pump housing ensure a free flow of solids-laden liquid.

Variable pump speed

Because of the variations in flow thought the day and year due to visitor numbers, it makes economic sense to vary pump speeds to meet demand. Flow monitors positioned in the inflow pipes measure the incoming flow and when a set level is reached the frequency flow controller activates the required number of pumps and the speed at which they need to run. By equipping all four pumps with frequency flow control, the pump speed of each pump is matched to the incoming flow so power consumption is optimised and greatly reduced. Savings of up to 30% can be attained over an equivalent fixed speed drive. Each pump has its own dedicated control panel with an easy to operate, touch-sensitive screen contained in robust, water-resistance cabinets. The purpose-designed software program enables operators to switch the operation of all four pumps, record and interrogate historical events, monitor pump performance and provide external communications.

The communications package offers double protection for service continuity and flexibility of use between peak and off-peak hours. During peak hours, electronic control ensures that pumps run in accordance with fluctuating flow rates and discharge effluent and wastewater in the same proportion. During off-peak hours and seasons, pumps are automatically adjusted to meet reduced flow demand and are constantly monitored to detect any problems or unauthorized operation by untrained personnel.

Accessed by a manhole immediately in front of the walls leading into the abbey, the pump station has been constructed in a specially designed subterranean chamber (Fig.5). Given the space and access constraints, KSB and SOGEA North West have delivered a robust and energy efficient pump solution that meets both the existing usage demand, and can accommodate future growth in demand.

“Pump stations are often oversized to handle the possibility of increased demand, or are dimensioned to accommodate exceptional flows that may occur only every 10 to 20 years,” said Mr Quertain. “As a result, in normal operation they are too large and too powerful resulting in high energy costs. Because fixed speed pumps only operate at peak flow, the load losses are constant. Variable speed pumps operate on instant flow so the load losses are proportional to the instant flow, leading to a reduction in consumption of between VF and VV: 10-50%.”

At Mont St Michel, the Amarex KRT pumps only operate on demand and, by varying the speed, each pump adapts to the actual flow rate which can often be less than the peak flow. Load losses are proportional to the flow so if the flow drops below peak demand then the load losses are reduced. As a result of load losses between instant flows and peak flows, variations in pump speeds contribute to energy gains. The economic impact in the case of Mont St Michel over 84% of the time that the pumps are running delivers an instant flow rate of less than 50m³/h, or 62% of the peak flow.

By undertaking a comparative study to estimate the potential savings between a traditional fixed speed Amarex KRT and the variable speed instant flow version recommended for Mont St Michel, KSB was able to show the energy saving could reach 30%. The result of the combined efforts of KSB and SOGEA North West is an energy efficient, compact, discrete and robust wastewater pump station designed for long term continuous operation and capable of transferring waste matter to a water treatment plant 2.5kms away.