FUNCTION

A Pressure Sewer System (or PSS for short) is a network of small

waste water lift stations (also known as pump stations) connected

to individual or sometimes a small collection of houses or buildings,

that are designed to collect and then move wastewater 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 PSS station usually consists of a small collector basin known as a wet well (or just well) that is often pre-fabricated and is located below ground. Typically, a single grinder pump is used for individual houses to macerate the waste and allow conveyance through smaller diameter pipes. Where a number of houses / or small commercial buildings are connected to a single PSS station, a second pump is often deployed for redundancy.

 

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.

 

 

  • 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 pre-mature failure.  Hardened materials and coatings improve the resistance to such contaminants.

 

 

  • 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.

 

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Pressure Sewer System Station Components

PSS Stations are typically constructed from glass reinforced plastic (GRP), plastic or even concrete.  These are often pre-built and just dropped into the excavated hole at each site. These typically range from 4 to 6 feet (1.2 - 1.8 meters) below ground but can be deeper for commercial applications.  The receiving basin called a wet well (or well) collects the wastewater.  Level measurement equipment in the wet well provides control equipment with the switching on and off points for the pump(s).

 

The pump(s) is usually located in the wet well as a submersible unit. 

 

For small, single house stations, the pump is connected to a piping arrangement that includes an isolation valve along with a check valve or non-return valve that prevents backflow from the sewer to the station.  This usually has just a simple coupling to allow pump removal.  Larger pump installations may be installed in the wet well and attached to guide rails to facilitate easy removal of the pumps. From here the outlet pipe(s) connects to the sewer main.

  

The sewer piping normally follows the land contour. The pipes are installed below the frost line. New pipe systems are developed that can be installed above the frost line. This system includes pipes integrated with isolation and heating cable (used during the winter time). 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.

Typical pump sizes are in the range of range from 1.5 Hp (1kW) pump units to 3 Hp (2 kW) units. Controls are usually installed in a small control cabinet at the surface adjacent to the PSS station, often on the outside of the building it serves.

     

  • Level Measurement Devices (Small to Larger PSS):

    • 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.

  • 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.  A variant of this type of system that is used in small single house stations, is a simple pressure switch that uses pre-set trigger points to start and stop the pump.

  • 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.

  • 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.        

Pressure Sewer System Station Pumps

  • ​Submersible 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. 

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

    • Channel Impeller

    • Grinder Impeller

 

  • Pump Control Systems:

    • Pump control systems are generally simple in this market sector using basic, Relay Logic control or simple microprocessor based control.

    • Systems are generally standalone with beacons and sounders for alarm annunciation. Stations can be connected to alarm dialers using GSM/GPRS/3G that give simple SMS messages or can be connected via mesh or point to point radio links that can show runtimes and some other basic parameters.   

 

 

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. 

PRESSURE SEWER SYSTEM