When gravity drainage from a property is not possible, a pumping station is usually required to pump wastewater and sewage to the public sewer. David Johnson of Pump Technology explores the correct specifications for pumping stations
For single or multiple dwellings that are not going to be adopted by a water company, a private sewage pumping station can be used. Selecting the right size pumping station, chamber sizes and pump/s are critical for the future reliability and long life of the station. Because of this, it is good practice to seek advice from a pump expert.
The chamber should be sized to provide 24-hour storage for the property. This calculation is based on 120 litres per person, per day. The pump is sized based on the differential head and friction losses to the discharge point. Additionally, the internal pipe diameter, and the number of bends and distances also needs to be taken into consideration.
The location for any pumping station and its associated groundworks needs careful planning to account for pipe runs, on-going access routes to discharge points, service access and traffic flow (vehicle or pedestrian) over any covers which are fitted.
The best pump station manufacturers and suppliers offer stations with a wide variety of tank sizes and pump options to allow the exact matching of the pumping performance to the application. Tanks can range from relatively small (e.g. for a single dwelling) to much larger (e.g. for multi property developments). Ideally tanks should be supplied with customisable invert and discharge options to ensure that groundwork needs can be met.
Pump options range from those fitted with shrouded impellers for wastewater, rainwater or final effluent applications, to systems with free-flow vortex impellers or a cutter/impeller design for sewage pumping. The latter type is suitable for high head applications, or for small-bore piping, nominally 50 mm diameter.
In addition to the pump performance matching the required lift, the flow should be capable of generating a velocity through the pipe of a minimum 2.5 litres per second. This ensures debris does not attach itself to the side of the pipe, and therefore blockages are avoided.
If the whole property or properties are relying on the pump station, then the pump station should be fitted with two pumps. These pumps will operate as ‘duty standby,’ each pump capable of the full pumping duty requirement. The pumps are located within the pump station via a duck foot/pedestal with guiderails to allow them to be lifted easily and then lowered and re-seated accurately for inspection and maintenance.
When groundworks and a simpler pump station is required for a house extension, kitchen, toilet or utility room for example, and the property has toilet facilities that are gravity fed to the public sewer, a single pump can be used.
The ideal pump selection for this is one which features a free-flow vortex design, with a free flow passage of 60 mm or greater, and the discharge pipework being 80/100 mm diameter. For small house extensions, a smaller free flow passage pump is often used. This is purely a price saving exercise as the small diameter free flow passage means a more efficient and hence smaller motor pump. In practice the larger free flow diameter allows the easier passage of foreign items, like sanitary items for example. These items should not be flushed, but commonly are.
Floats commonly determine the control of the pump station (four floats for a two-pump station) – start, stop, duty assist and high-level alarm. The control panel will automatically configure a new duty pump every cycle. If the high-level alarm is activated, or if a pump should trip due to clogging or electrical fault, then an alarm will be activated. This alarm can be any, or a combination of: a local beacon, buzzer, connected to a building management system or telemetry remote monitoring.
Again, the size of the pumping chamber – just like the pump – can be configured to suit the site conditions; depth of the inlet, or inlets and position of inlets relative to the discharge. This means that the best practice pipe runs can be used, and installation is made easier.
The installation, like the pump station, should follow strict guidelines, in essence the chamber should be considered as a waterproof liner, any hydraulic water pressure on the chamber being prevented by the concrete surround.
David Johnson is business development manager at Pump Technology