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LIFT STATION

 

 

 

 

FUNCTION

 

Lift stations (also known as pump stations) are facilities designed

to collect and then move wastewater or storm water from lower

elevations to higher, where gravity flow conveyance is not possible/

desirable to reach its destination. The latter may be due to technical

or cost reasons. The lift station often consists of a collector basin

known as a wet well (or just well) that may be custom built or

pre-fabricated and is located below ground. The sanitary sewer system and/or the storm drainage network is connected to it.  Pumps are located within the facility that then pump (lift) the sewage or storm water to a piping system at a higher level called a rising main.

   

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Wastewater sources include:
 

  • Sanitary wastewater, called sewage.  Sewage includes gray water from sinks, showers, and washers, and black water from toilets which has been in contact with fecal material. Sanitary wastewater can be pumped fairly readily with the correct pumping equipment but some items that are referred to as non-flushables, in particular baby wipes, diapers and personal care products (PPCP) may prove a challenge to pump, leading to clogs and blockages that can lead to overflows that impact the environment.

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  • Industrial wastewater includes used water from manufacturing processes.  Industrial wastewater may contain components that are aggressive in nature (pH, sand or grits) that can attack the construction materials of the pumping system causing premature failure. Hardened materials and coatings improve the resistance to such contaminants.

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  • Infiltration and Inflow (I&I), two types:

    • ​Infiltration is ground water which enters the sanitary sewer system through cracks and joints;

    • Inflow is rain and melted precipitation which enters the sewer system through legal and illegal cross connections with the storm sewer system.

 

 


Lift Station Components
 
The lift station is located below ground to collect the sanitary sewer or storm drainage network. These typically range from 6 feet (1.8 meters) to 30 feet (10 meters) below ground but can be even deeper for specialised applications. Lift Stations can be constructed of concrete, glass reinforced plastic (GRP), plastic or steel and may come as pre-built units or may be custom built on site.  The receiving basin called a wet well (or well) collects the wastewater or storm water inflows.  Level measurement equipment in the wet well provides control equipment with the switching on and off points for the pumps.The pumps can be located in the wet well as either submersible units, long shaft units or may be located in an adjoining dry well chamber that has piping connected to the wet well. 

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In large lift stations there can be as many as three wet wells. Different pump sets may be installed in each wet well.  Where large varying flow ranges are being accommodated, there may be up to four, six or even nine pumps installed of varying sizes that switch on in a scaled fashion based on the flow needs. Depending on the nature of pumps installed, piping may be installed in the wet well along with guide rails (in the case of submersible pumps) to facilitate easy removal of pumps.  Additional valves for isolation / shut off or to prevent back flow when the pumps stop, called check valves or non-return valves (NRVs) may be installed on the outlet piping within the wet well or more commonly in a valve chamber outside the wet well for ease of access. 

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Dry wells house the pumps adjacent to the wet well to facilitate maintenance.  Isolation valves allow the pumps to be taken out of service. Check valves or non-return valves (NRVs) may be installed in the dry well also. Pump sizes can range from small 1.5 Hp (1kW) pump units right up to 1300 Hp (1000 kW) units. Controls are usually installed in a control cabinet at the surface adjacent to the lift station but can sometimes be installed below ground in the dry well.
      


Level Measurement Devices

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  • Ultrasonic – This device has a small transducer that looks down on the surface of the liquid. It has a transmitter and receiver housed in the same unit.  It sends an ultrasonic ‘ping’ downwards towards the surface of the liquid and times the echo that reflects back. It compensates for air temperature and then converts the time to a distance that is then translated to a level. It usually outputs this as a 4-20mA signal or can present it as a communications packet.

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  • Pressure Transducer – This consists of a transducer with a semi flexible or rigid diaphragm that converts hydrostatic pressure to a scaled electrical 4-20mA signal. It may have an atmospheric compensation air tube that returns to the surface within the signal cable to compensate for atmospheric pressure variations or it may be electronically compensated.

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  • Bubbler System - These use compressed air (or nitrogen) introduced through a bubble or sensing tube that is inserted below the liquid level. As the surrounding liquid level changes more or less bubbles escape from the end of the tube. As the air or nitrogen escapes more is required to maintain the pressure. These pressure differences are measured by a pressure gauge and converted into a scaled level output, usually presented as a 4-20mA signal.

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  • Conductivity Probe – This consists of a tube (usually PVC) that has one or more stainless steel sensors located at uniform distances from each other. A low AC voltage from the sensing circuit is applied to the sensor(s).  If liquid is present, i.e. if the sensor is covered, a tiny current (<1mA) flows back through the liquid via ground (earth) to the sensing circuit.  The resistance to this electrical path is measured and if it is low enough it results in a positive ‘ON’ signal.

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  • Float Switch / Ball Float / Level Regulator – This consists of an electrical circuit that travels down a signal cable and then passes through a ‘tilt’ switch’ inside a plastic ball or teardrop shaped housing. The entire housing is water proof and is partially buoyant to the liquid it sits in. As the liquid level rises, the housing rises and then tilts causing the internal switch circuit to close or break the circuit depending on the mode of operation, thus registering an on or an off point.        

 

 


Lift Station Pumps
   

  • Subersible Pump – This consists of an electrical motor in a hermetically sealed housing that is close coupled to a pump body.  Within the pump body is an impeller that pushes the liquid to the surface via a pipe. Typically sensors are located in the electrical housing to measure the motor temperature and also to measure moisture that could occur due to a leak.  Seals on the shaft and electrical couplings prevent water ingress.  

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  • Long Shaft Vertical Pump – This consists of a ‘dry-mounted’ electrical motor that is located vertically above a long shaft that is connected to the pump body (the latter described as above).  

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  • Drywell Pump - This consists of an electrical motor (sometimes also in a hermetically sealed housing) that is close coupled to a pump body but is located in the dry well, adjacent to the wet well of a lift station. Submersible pumps can often be installed in this environment but frequently require a cooling jacket to provide adequate cooling. This facilitates ease of maintenance on the unit. The pump body is as described earlier.

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  • Impellers – Many different impeller choices are available for lift station pumps depending on the application. Popular choices include the following:

    • Channel Impeller

    • Vortex Impeller

    • Screw Impeller

    • Grinder Impeller

    • Chopper Impeller

 

 


Pump Control Systems

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  • Pump control systems range from simple to complex;

  • Systems can be completely standalone or can be connected to alarm dialers, GSM/GPRS/3G based annunciator systems or dedicated SCADA systems;

  • Relay Logic systems tied to float switches provide the most basic level of control;

  • Programmable Logic Controller (PLC) or micro-processor-based control systems provide more advanced control over operating needs;

  • Intelligent Pump Station Management (iPSM) systems provide the most advanced form of control by providing integrated real time control of stations that can adapt intelligently to changing environmental needs in a highly dynamic way.  Often these are connected to the Cloud and react to weather data or other events to provide better centralised catchment management.

 

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BENEFITS

 

Sanitation systems including wastewater collection are generally regarded as one of the greatest engineering achievements of the 20th century. Wastewater pumping systems convey contaminated water and storm water away from residences, commercial and industrial facilities to treatment facilities before returning to water courses.  They reduce the significant capital costs for excavation that would otherwise be needed to maintain gravity flows throughout the sanitary/storm network. 

 

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 SYSTEM REQUIREMENTS

  

A control system consists of field instruments, final controlling equipment, controllers such as dedicated pump controllers, programmable logic controllers (PLC), Intelligent Pump Station Managers (iPSM) and a communication system. Field instruments (measuring devices), such as level sensors, gather the level data. A supervisory control and data acquisition (SCADA) acquires, analyses, and displays the data.

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