Mont St Michel gets new wastewater pumping station in environmental restoration programme

The project: Constructing a pumping station for effective wastewater removal in Mont St Michel

Built on a rocky outcrop one kilometre offshore between the mouth of the Couesnon River and the English Channel, the 1,000-year-old abbey overlooks one of the largest bays in the world where the tidal rise of up 15 meters is one of the highest in Europe.

Over many decades the maritime character surrounding the abbey has changed dramatically. By 2005 a combination of manmade and natural forces had resulted in much of the bay silting up and the abbey was now almost marooned in a salt marsh. This regrettable situation has been addressed through a major project with the aim of restoring the degraded environment and renewing tourist appeal.

The Mont St Michel restoration project, which commenced in 2005, 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 at the base of the rock to create improved hydraulic seawater flows and the development of enhanced visitor facilities and infrastructure.

Incorporated in this programme of works were improvements to the infrastructure for the removal of wastewater and effluent in a safe, effective and environmentally acceptable manner from Mont St Michel to a treatment plant 2.5km away.

Instigated by Syndicat Mixte Baie du Mont St Michel, the project involved the construction of a pumping station that would accommodate the considerable fluctuations in effluent flow that are associated with times of the year when visitor numbers can be both high and low. 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.

The challenge: Sourcing environmentally-friendly options for effluent discharge and wastewater discharge

In order to satisfy the many technical, environmental, performance, economic and security challenges, contractors Sogea North West Construction (Vinci Group) and KSB S.A.S. had to come up with an almost unique solution. It should be considered that the structure has minimal impact on the environment of Mont St. Michel. The pumping station with a dry pump shaft is below sea level, so it is not noticeable when approaching the mountain.

Design and operating requirements that KSB had to consider were:

• the non-emission of odours and gases
• variable flow demands on the pumps
• potential storm conditions
• discharging wastewater and effluent over a 2.5km
• space and access constraints
• absence of water storage 
• high performance, energy efficiency  
• low environmental impact
• easy maintenance

The solution: Installing Amarex KRT pumps in submersible dry pit pumping station

The design proposed by KSB involved the installation of four Amarex KRT pumps in a 3+1-arrangement, with frequency inverters for speed control.

“The Amarex KRT 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.

In addition, 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 directly to the treatment works without the need for a holding tank ahead of the pumps,” reports Stéphane Quertain, Product Manager Pumping Stations KSB S.A.S.

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 80m³ 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.

 A further benefit of this arrangement is that odours 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.

Variable pump speed

Because of the variations in flow thoughout the day and year due to visitor numbers, it makes economical 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. 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. 

Data I Facts I Figures

• Location: 
  Mont St Michel, Normany France
• Client: Synicat Mixte Baie du Mont St Michel
• Application: Wastewater and effluent
• Scope of supply: 

  Four Amarex KRT pumps in a 3+1-arrangement, with frequency inverters for speed control.