FSP0004 – Lift Stations – Facility Science Podcast #4

By | May 21, 2019
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Notes for FSP0004 – Lift Stations
What is a Lift Station?
  • Collects wastewater from a building or group of buildings. The wastewater is everything that goes down the drain. This could be a commercial building or apartment complex or residential neighborhood.
  • Pumps the wastewater into a force main (that’s a pressurized sewer main), and that force main might be part of a continuous pressurized sewer system or it might just carry the wastewater to a gravity main across some elevation or distance.
  • Lift stations are everywhere. Edge of property, control panel with manhole or hatch on the ground, red light and buzzer, sign saying who to call if the alarm goes off.
Why is a lift station?
  • The wastewater from our buildings needs to get to…wherever the wastewater goes in our community, probably some type of treatment plant before discharge into somewhere.
  • The simple thing is to put the treatment plant at the lowest elevation and let gravity take the wastewater downhill.
  • In many places there isn’t enough “downhill” to reliably move wastewater away from our buildings to the treatment or discharge point.
  • So we use lift stations and force mains to move wastewater when there is insufficient elevation to take advantage of gravity or when using gravity would be prohibitively expensive (eg: you have to dig through a mountain).
Terminology Lift Station vs Pump Station
  • Sometimes you’ll hear a lift station called a pump station or a pumping station or something similar.
  • Some people will differentiate between the 2
    • A lift station is used to pump wastewater to a gravity sewer main at a higher elevation (because “lift”) and a pump station pumps wastewater into a force main.
    • or, a lift station is the name for a thing used to pump wastewater and a pumping station is the name for a thing used to pump clean water.
  • Some people use the terms interchangeably.
  • Some people only use one or the other term.
  • Refer to context, local norms, etc.
Parts of a Lift Station
  • Lift station has wet well, invert, pumps, valves, controls.
  • Wet well is the sump for collecting wastewater from the buildings. This is basically a big tank. The wet well is fed by gravity, so it’s downhill from the buildings it services, and almost always underground because it needs to have the majority of it holding volume below the level where the wastewater enters.
  • Invert is the connection where the gravity sewer (the sewer lines from the buildings serviced by the lift station) enters the lift station.
    • This is generally high up on the side of the wet well. Ideally water level stays below this, but sometimes gravity sewer pipe under buildings can be used as emergency storage volume.
  • Pumps. There are many types of pumps and pump configurations.
    • Submersible pumps in the wet well. Sit near the bottom of the wet well so they are always under water). These are often on rails to allow them to be pulled out for service without needing to enter the wet well. Gasketed break-away fitting makes seal when pump is lowered on rail.
      • Very common especially in newer construction, smaller stations
      • Benefits: Simple, small footprint, pumps don’t require a lot of suction so relatively small pumps can be used, little risk of cavitation damage to pump or heat damage to motor because pump is under water at all times, pretty much no worry about pump priming
      • Drawbacks: Submersible pumps require more specialized (more expensive) parts and maintenance than non submersible, pumps have to be hoisted out of wet well for service, electrical pump underwater risks damage to electrical equipment or electrocution hazard if pump seals fail.
    • Pumps outside the wet well in a separate “dry well.” The dry well is a chamber next to the wet well.
      • Dry well allows access to pumps without having to hoist them up or enter the wet well.
      • Pumps normally installed at a level near the bottom of the wet well with the inlet extending into the wet well.
      • Dry well is subject to flooding both from storm water and from the lift station wet well. Often the dry well can act as emergency overflow volume if the pumps fail.
        • Sometimes electric motors that drive the pumps will be installed above ground with linkage to the pumps below to prevent damage from flooding.
        • Another solution is to install submersible pumps in the dry well so occasional flooding won’t damage the pumps.
      • Benefits: Easier access to pumps, lower electrocution risk, less obstruction in wet well.
      • Drawbacks: Some suction/priming required to pull wastewater out of wet well, possibly larger pumps and motors required, larger lift station footprint to accommodate dry well.
    • Pumps at ground level above the wet well.
      • Suction pulls wastewater out of wet well.
      • Often a feature of prefabricated lift stations.
      • Benefits: Easy, safe access to pumps and motors, simple, low installation cost.
      • Drawbacks: High suction required, pump priming a concern, requires pump house/structure above ground.
    • Any combination of these things and probably others too
    • Station usually has at least 2 pumps, but could be one or 3 or more, depending on requirements. See controls section.
    • Lift stations have to deal with solids, so pumps need to handle solids either by being capable of passing them through or by grinding or otherwise breaking them down. in some situations, separate grinders are installed or large solids are screened out and discarded as solid waste
  • Valves
    • Check valves – device that only allows flow in one direction, usually spring loaded.
      • Prevents back flow from force main into wet well
      • One for each pump
        • discharge piping from each pump merges together into the force main
        • check valve after each pump but before the main prevents the pumps from pumping back into the wet well through the piping of a different pump
      • Check valves can be in the wet well or dry well (if present) or in a separate valve vault (for easier access in a station without a dry well), or buried in the ground.
    • Isolation valves – Usually a gate valve that can be closed to block the flow of water, used to isolate a component (or the whole station) from water flow to allow servicing of the component
      • Often installed just after each check valve so check valves can be isolated and serviced.
      • Almost always installed outside the wet well in a valve vault (with the check valves) or in the dry well (if present). Sometimes just buried in the ground or installed above ground.
    • Bypass port
      • This is a flange to connect a hose or pipe from another pump, and valve to isolate the connection.
      • Installed in the force main (so after the check valves and isolation valves).
      • Allows connection of a bypass pump so lift station pumps and valves can be completely bypassed. Pump could be portable suction pump above ground or submersible pump lowered into wet well.
      • Used in case of lift station pump failure or, with an engine (diesel/gasoline) driven pump in the case of power failure.
    • Air/Vacuum release – only where applicable
      • Air release valves allow air that might collect and get trapped at high points in piping to escape.
      • Vacuum release valves (or vacuum breakers) allow air to enter the force main to prevent negative pressure in the force. Negative pressure can be caused if, for example, the force main discharges into an unpressurized system at a lower elevation than the pumps. This type of negative pressure can cause siphoning of the wet well.
  • Controls – The purpose of the control system is to turn the pumps on and off based on the level of wastewater in the wet well. Also to warn of failure situation to prevent overflow.
    • Monitor multiple levels
      • Lowest level – no pumps should run
      • Lead pump level – start the lead pump, pump turns off when lowest level reached.
      • Lag pump level – if water continues to rise with one pump running, start the second pump (lag pump)
      • Alarm level – If water reaches this level, pumps might not be able to prevent overflow, sound alarm.
    • Level Sensing Mechanisms
      • float switches – device, size something like a baseball or softball, hangs down into the wet well. gives discrete levels, one level for each float
      • bubbler – tube extends to bottom of wet well, compressed air system forces air into the tube, sensor detects how much pressure is required to make bubbles come out of the bottom of the tube (hence bubbler), pressure can be used to calculate water depth.
      • ultrasonic – transmitter bounces ultrasonic waves off water surface, travel time of reflected waves allows calculation of water depth.
      • submersible level sensor – drop to bottom of wet well, measures pressure to determine water depth, like bubbler but no air supply or tube needed.
    • Pump alternation Multiple pumps in a lead-lag (this is terminology you might hear) configuration (this means…one pump runs, other pump runs if necessary….lead pump alternated)
    • Alarm notification system – audible/visual alarm, remote messaging system
    • Pump health monitoring
      • simplest is pump run hours, can be used to detect problems with the pumps
      • More advanced pump monitoring mechanisms available.
Safety
  • Inside of lift station is a dangerous place, including dry well. Confined space, poisonous/flammable gases, electric motors in contact with water, proper training, equipment, safety protocols, proper ventilation in wet wells and dry wells
  • Motors and pump, moving parts, lock out/tag out important.
  • Fall risk, engulf/drowning risk.
Maintenance
  • Main objective – don’t let it overflow
    • Overflow can cause health hazard to people, contamination of fresh water sources, environmental contamination
    • Overflow as result of negligence/neglect can incur fines or other penalty from government.
  • Absolute minimum (if you do nothing else, not saying you should do nothing else, but if you do nothing else)
    • monitor the top float or alarm level so you know when the station has failed
    • have a relationship with an on call lift station service company that will come help when your lift station fails
    • Make sure you and your staff know how to manually pump down the lift station
  • Monitor pump run hours – record daily or weekly and calculate daily or weekly pump hours
    • Drastic change in weekly or daily hours can indicate many problems, pump wear or clogging, damaged or clogged check valves, faulty switches, change in volume entering station, etc.
  • Pump down and clean occasionally
    • Occasionally depends on your lift station details, consult local utility and lift station service company
    • remove build up of debris, solids, grease from pumps, valves, level sensing mechanisms.
    • Vacuum solids that may have settled at bottom of wet well.
  • Odor control
    • Maybe only need proper pumping rate to prevent sewage from sitting in wet well too long.
    • Might need chemical treatment or other methods, consult local utility and service company.
  • Power failure plan? backup power, pump truck, engine driven bypass pump Does wastewater still flow into your station when the power is out?
People Service Lift Stations
  • Lift Stations essential to functioning of modern civilization
  • Thousands of people around the world do the dangerous, dirty work
  • Thanks to those people.
Further Reading