When gravity isn’t enough

Moving huge amounts of soil, re-positioning drainpipes and sewer lines as well as cutting through concrete, are all very expensive activities that make traditional wastewater gravity drainage one of the more challenging aspects for commercial building services professionals. However a simpler solution to this problem can be achieved via the installation of a lifting station as they offer a very flexible solution to the problems that wastewater management can pose either in a refurbished, re-configured or a new building project.

Today’s new generation of lifting stations such as the Grundfos Multilift offer significant advantages in connection with many different types of building projects because their simple installation can help to cut costs. Take, as an example, a shop being converted into a restaurant with basement level sanitary facilities. Here, a lifting station would collect wastewater from all the new sanitary appliances below ground and discharge it into the existing gravity pipe under the basement ceiling. This saves the cost of cutting through concrete or digging a well outside the building to install new drains.

The tank sizes and pump performances of lifting stations can be tailored to fit any building. Designed for internal use in buildings, they come completely pre-assembled with a connected controller, which makes installation significantly easier, as by simply positioning the unit, attaching inlet and outlet pipes and mounting the controller on the wall, the installation is complete and lifting station operation is fully operational. In a double-pump unit (which is recommended for all commercial building applications) the controller ensures that the pumps operate alternately.

Sizing factors

When sizing a lifting station, you need to know the wastewater flow rate, pressure pipe size and duty point. The first thing is to determine the volume of wastewater that your lifting station will have to drain bearing in mind that wastewater discharge is not constant but varies over the course of a day so the system needs to be designed so it is able to cope with peak flows.

A lifting station has a limited tank volume and pump motors that are designed for intermittent duty, since they are not always submersed in a cooling medium. Pump size is influenced by the tanks effective volume between start and stop levels and the pump’s intermittent operation time. If the intermittent operation time of a pump is given as 1 minute, this means the pump is designed for 60 starts/stops per hour and 30 second operation followed by 30 second pause.

The pump must be able to discharge the hourly volume of wastewater generated by the system. To avoid over-sizing and thermal cut-outs caused by too frequent pump starts and stops, the tank volume needs to be calculated correctly.

This tank volume should match the wastewater flow rate and the pump performance. By avoiding over-sized tanks, you can ensure that the wastewater is stored in the tank for the shortest time possible and thus minimise wastewater fouling.

Double does not mean double

A double pump station does not provide double drain capacity – each of the pumps must be able to discharge all wastewater at peak flow in order to provide 100% backup in case of breakdown. During sizing, parallel pump operation should only be considered for extreme situations. Parallel operation does not necessarily mean a double drain capacity since the resulting higher flow increases friction loss in the discharge pipe significantly.

For example the smallest Grundfos lifting station for commercial application, the Multilift MD double station with 92l effective volume and a footprint of just 0.6sq m, can drain up to 5,500l of wastewater per hour if just one of the two pumps is sized for the full load. This ensures that there is full backup if one pump switches off. But the small lifting station could discharge up to 11,000l/h in peak flow situations (the equivalent of 72 bathtub drains) when its two pumps are operating together.

An alternative solution can also be found in the Grundfos PUST range, which is a complete pressurised pumping system designed for easy transport that is self-cleaning, with smooth surfaces. Essential for these prefabricated pumping stations they also offer low maintenance and easy service.

These systems are simple to install and maintain and once in situ will give decades of trouble-free operation. So when the need arises to deliver your next wastewater solution, you may find that a lifting station will offer a cheaper and more flexible solution to the problem, whether the demand is part of a new or refurbishment project.