The PIA’s Australian Pump Technical Handbook is a cornerstone text for the Australian pump industry. In this ongoing series, we feature chapters to showcase the various areas covered in the book and to reacquaint readers with the technical aspects of pumps.
In this issue we look at the Wastewater Pump Station chapter – previously known as Sewage Pumps Stations.
Wet and dry well wastewater pump stations
General
Since trouble free operation of wastewater pumping stations is paramount, the selection of pumping equipment for performance and construction is a critical factor when planning and designing.
It is important that sumps are designed to collect and pump raw sewage. Topics such as inflow, pumping rates, system pressure and storage times are all considered in designing a correct well – topics which have been covered in the series of articles on Intake Design in previous Pump Industry editions. In practice the number of pumps and the arrangement thereof is decisive in dimensioning a sump.
In central municipal wastewater pumping stations, where several incoming sewers are brought together to discharge into a common large station, wet well/separate dry pit installations (Figure 2) are more commonly used because they are more easily controlled and maintained. Wet well pump stations are commonly called transfer stations or low lift stations where mostly the wet wells contain the pumping equipment (Figure 1).
Occupational health and safety
Occupational health and safety play an important role for operating and maintenance staff due to contact with microbiological organisms and gasses from the sewage and the dangers of working from heights and potential falls into the various designs of pump stations. Operators and maintenance staff should be trained for working in confined spaces, working at heights and in fall prevention as a normal part of training.
Pumps
In modern wastewater pump stations, both for wet and dry well pump stations, pumps are normally submersible. These pumps have centrifugal liquid ends with the appropriate impeller design connected to an electric motor, designed to run either partially or fully submerged in fluid which cools the motor.
For smaller stations, cutter (shredder or grinder) pumps can be used, which cut the sewage into small particles to pass through the impeller and down small-bore discharge pipes.
In the case of submersible pumps used in dry well stations, the fluid pumped is circulated around the motor for cooling. Modern submersible sewage pumps incorporate double mechanical seals with alarms to detect seal failure for motor protection.
Selection of pumps for efficient reliable operation should ensure that blockages are avoided by the correct selection of impeller design. There are various designs available including:
- Single vane
- Dual vane
- Vortex (semi vortex)
- Screw
- Macerator
- Propeller
- Cutter (shredder, grinder)
Australian plumbing standard AS 3500.2 refers to the requirements for submersible pumps for both small domestic and municipal pumping stations, including pump flow rate determination, number of pumps, levels and alarms, pit sizes etc.
The inlet to a pump station should be designed to avoid backflow into the incoming sewer pipe (settling of solids is prevented in this way) and to ensure that correct baffles are installed to prevent the formation of bubbles and to avoid
air entrapment in the pump suction. Consideration should be given to the elimination of dead zones in the wet well. Adequate benching/profiles needs to be formed on the walls with a slant of 45-60 degrees depending on the solids loading. For smaller pump stations operating intermittently, the base of the well should be kept to a minimum.
Level controls
Pumps are switched on using either float switches, level probes or hydrostatic transmitters. Hydrostatic transmitters are considered the most accurate and serviceable. The pressure applied minutely deflects the diaphragm in a hydrostatic transmitter (as little as 10 microns), which is highly stable to pressure and temperature shocks. The expansion of metal due to temperature can be as much as the deflection of the diaphragm. This deflection is then converted into an output signal which provides for a very accurate instrument.
The ‘switch off ’ level should be in such a position that the pump volute is flooded at a position that prevents vortexing at the pump suction. The ‘switch on’ level should be determined according to the pump/motor design and incoming flows, therefore variable time trialling should be carried out prior to setting the optimum level for switching pumps into operating mode.
Pipe selection
Pipe sizing in pump station design is important to avoid the settling of solids in the pipe and to prevent blockages.
The recommended maximum flow rates for lengths of pipelines of up to 500m depends on the diameter of the pipelines as shown in following table:
Sewage pipes should be positioned to feed into the main from the side and not from below. Larger pump stations or longer rising mains to the eventual point of discharge should be engineered to suit the application.
Valves
Valves are generally only installed in small sewage pump stations and in cases where access into the well is not prohibited for safety reasons. It is recommended that a separate valve chamber be constructed with due consideration to correct pipeline velocities and an air release valve installed for evacuation of gases from the flow. Valves should have an internal full pipe size bore to avoid clogging or an area for the settling of foreign particles when the pumps are not in operation.
VFD operation of sewage pumps
During start-up of systems that include variable frequency drives (VFDs), a pump should be run up to full speed and then adjusted downwards to the required operating speed. If pumps have been operated at low speeds for long periods of time, a regular increase to full speed for a short time is recommended to flush the system/rising main of solids that may have settled in a pump well or pipeline. Shortly before a pump switches off, the pump should be allowed to return to full speed by means of the programmable logic controller.
Packaged wastewater pump stations
Pre-packaged pump stations offer a convenient and safe alternative to stations poured and assembled onsite.
Packaged pump stations are a cost-effective solution when compared with higher cost cast in-situ pump stations. In some cases, these can be installed within a day because labour onsite is limited to placing them into excavated
holes with connections only to the necessary electrical connections, incoming sewage pipes and to sewer rising mains to pump from the station.
Packaged pump station sizes can be designed to individual system requirements from 1200mm to 4000mm diameter in concrete, fibreglass or polyethylene construction.
Packaged pump stations can be a single chamber including pipework and valves, or there are options to have the pumps and pipework in one well and a separate valve chamber.
All packaged wastewater pump stations can be equipped with optional additional equipment, as follows:
- Switchboard
- Light or heavy-duty access covers to suit foot or vehicular traffic
- Well washers
- Odour control filters
- Flow metering
- Lifting gantry
- Access ladders
Packaged wastewater stations mainly come with dual pumps, sometimes more, however they can be provided with only one pump installed in the pit thus allowing for smaller pits catering for lower system demands.
Pressure wastewater pump stations
Pressure sewer systems are networks of sealed pipes and below ground pump stations and are designed for domestic and commercial applications, particularly in rural areas
where the use of gravity fed sewer pump stations may be uneconomical to install. Pressure sewer systems utilise either helical rotor or centrifugal grinder pumps designed for low flows to suit smaller dwellings.
Pressure sewer systems are best suited for the following applications:
- Hilly or difficult terrain
- Areas and municipalities with low population
- Sites with high water tables
- Remote and environmentally sensitive locations
Pressure sewer systems commonly pump into a gravity sewer which takes the sewage either to a treatment plant or gravity pump station for onward pumping.
Pressure wastewater pump stations are equipped with pump well for underground installation with ground level access lids, pump level controls for operation according to liquid levels and a control box with an alarm in the event of a system failure.
Summary
This chapter provides general information on specific fundamentals and other important information for the planning of wastewater pumping stations, however it is not possible to include all recommendations for individual cases, as engineering would be specific to each application.
