11.2 Lift Stations, Pumps & Level Control Systems

11.2 Lift Stations, Pumps & Level Control Systems

Topographical constraints often prevent continuous gravity sewer conveyance from collection basins to treatment facilities. When sewer lines reach excessive depths (typically greater than $20 - 25\text{ feet}$), encounter uphill terrain, or cross river valleys, wastewater lift stations (pumping stations) are required to lift sewage to a higher elevation gravity main or pump it under pressure through a force main directly to the treatment plant. Because raw wastewater contains high concentrations of solids, abrasive grit, rags, and corrosive gases, lift station design demands specialized pumps, explosion-proof level sensors, and robust hydraulic surge controls.

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Classification of Wastewater Lift Stations

Municipal lift stations are broadly categorized into two structural configurations based on pump location relative to the raw sewage:

          SUBMERSIBLE LIFT STATION               DRY-WELL / WET-WELL LIFT STATION

          Control Panel & Alarm                     Control Room & Vent Blower
             ┌──────────────┐                          ┌──────────────┐
             │  PLC / SCADA │                          │  Motor Center│
             └──────┬───────┘                          └──────┬───────┘
                    │                                         │
        Grade ══════╪═════════════                 Grade ═════╪═══════════════
             ┌──────┴──────┐                            ┌─────┴────┐┌────────┐
             │  WET WELL   │                            │ WET WELL ││DRY WELL│
             │             │                            │          ││        │
             │  Guide      │                            │          ││ Non-   │
  Incoming   │  Rails (SS) │                 Incoming   │          ││ Clog   │
  Gravity ──►│    ║   ║    │                 Gravity ──►│          ││ Pump   │
  Sewer      │    ║   ║    │                 Sewer      │          ││ ┌────┐ │
             │    ▼   ▼    │                            │  Suction ││ │(M) │ │
             │  ┌──────┐   │ Force                      │  Pipe    ││ └─┬──┘ │
             │  │Submer│===╪═════►                      │  ════════╪╪═══┘    │
             │  │ Pump │   │ Main                       │          ││ Pump   │
             │  └──────┘   │                            │          ││ Sump   │
             │  Auto-Coupler                            └──────────┘└────────┘
             └─────────────┘

1. Submersible Lift Stations

In a submersible lift station, the non-clog pump and its sealed, explosion-proof electric motor form a single integrated unit that operates fully submerged at the bottom of the wet well.

• Guide Rail & Auto-Coupling System: The pump is mounted on dual stainless steel guide rails and lowers onto a stationary discharge elbow base bolted to the wet well floor. A precision-machined metal-to-metal or rubber gasket auto-coupler creates a leak-tight seal under the pump's own weight without requiring bolts.
• Key Advantages: Lower capital construction cost (single wet well basin), compact footprint, aesthetic integration, and safe above-ground retrieval for service using an overhead crane or hoist truck—eliminating the need for operators to enter the hazardous wet well (Class I, Division 1 hazardous atmosphere).

2. Dry-Well / Wet-Well Lift Stations

In a dry-well / wet-well station, the facility is split by an impermeable concrete dividing wall into two distinct chambers:

• Wet Well: An open receiving sump where incoming raw sewage collects.
• Dry Well (Dry Pit): An adjacent underground room housing the pumps, suction and discharge piping, valves, and electrical instrumentation completely isolated from liquid sewage.
• Key Advantages: Easy physical access to pumps, seals, bearings, and motors for maintenance without pulling pumps; operators can inspect mechanical seals, monitor vibration, and adjust packing under clean conditions.
• Disadvantages: Substantially higher construction cost, requires continuous mechanical ventilation (standardly $12 - 30\text{ air changes per hour}$) and sump dewatering pumps to handle seal leakage and condensation.

3. Package & Suction Lift Stations

For shallow depths ($< 15 - 20\text{ ft}$) and low flows, self-priming suction lift stations locate pumps above the wet well at ground level. However, maximum practical suction lift is limited by atmospheric pressure and cavitation risks (Net Positive Suction Head, NPSH).

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Non-Clog Wastewater Pumps & Impeller Mechanics

Standard clear-water pumps fail instantly in raw sewage due to rag binding and solids clogging. Municipal wastewater pumps must be specifically designed to handle large solids, fibrous materials, and stringy plastics:

The 3-Inch Solids Requirement

PUB2754 says pumps other than grinder pumps handling raw wastewater should pass a 3-inch ($76\text{ mm}$) solid sphere. A case-by-case exception may be approved when the piping is at least 1 inch larger than the sphere and equivalent protection from clogging or damage—such as grinding or screening—is provided.

Impeller Designs & Operational Characteristics

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Wet Well Design & Operating Volume Mathematics

Proper wet well sizing requires balancing two competing engineering constraints: motor thermal protection vs. wastewater septicity control.

1. Motor Cycle Time & Maximum Starts Per Hour

Electric pump motors draw massive inrush current ($500% - 800%$ of full-load running amps) across the line during starting. Starting a motor too frequently causes severe overheating of stator windings, degrading insulation and triggering premature motor burnout.

• Standard Motor Limit: Standard NEMA-rated wastewater pump motors should be limited to a maximum of 6 to 10 starts per hour (equating to a minimum cycle time $T_{\text{cycle}}$ of 6 to 10 minutes between successive starts of the same pump).

2. Maximum Detention Time (Septicity Control)

If a wet well is sized too large, wastewater sits stagnant during low nighttime flows.

• Maximum Retention Time: Wet well detention time during minimum flow periods should not exceed $10\text{ to }30\text{ minutes}$.
• Stagnant sewage becomes anaerobic, releasing foul odors, generating explosive methane ($\text{CH}_4$), accumulating thick surface grease crusts, and producing $\text{H}_2\text{S}$.

3. Wet Well Operating Volume Formula

The operating volume ($V_{\text{operating}}$) is the active liquid volume contained between the pump Lead Start elevation and the pump Stop elevation. The minimum cycle time occurs when incoming sewage inflow ($Q_{\text{in}}$) is exactly half of the pump's discharge capacity ($Q_{\text{pump}} / 2$):

$V_{\text{operating}} = \frac{T_{\text{cycle}} \times Q_{\text{pump}}}{4}$

Where:

• $V_{\text{operating}}$ = Effective active wet well volume in gallons
• $T_{\text{cycle}}$ = Minimum allowable cycle time in minutes ($T_{\text{cycle}} = \frac{60\text{ minutes}}{\text{Max Starts Per Hour}}$)
• $Q_{\text{pump}}$ = Single pump discharge capacity in gallons per minute ($\text{gpm}$)

Worked Wet Well Sizing Calculation

Problem: A new submersible lift station is equipped with two alternating $600\text{ gpm}$ pumps. The motor manufacturer specifies that the motors must not exceed 6 starts per hour. Calculate the minimum required wet well operating volume ($V$) in gallons and the corresponding liquid depth in a circular wet well with a diameter of $8.0\text{ feet}$.

1. Calculate minimum cycle time ($T_{\text{cycle}}$) in minutes: $T_{\text{cycle}} = \frac{60\text{ minutes}}{6\text{ starts}} = 10\text{ minutes per cycle}$

2. Calculate required operating volume ($V_{\text{operating}}$) using the formula: $V_{\text{operating}} = \frac{10\text{ min} \times 600\text{ gpm}}{4} = \frac{6,000}{4} = 1,500\text{ gallons}$

3. Calculate the required operating liquid depth ($H$ in feet):

   • Convert gallons to cubic feet: $\text{Volume} = \frac{1,500\text{ gal}}{7.48\text{ gal/ft}^3} = 200.53\text{ ft}^3$
   • Calculate cross-sectional area of the 8-ft diameter wet well: $A = \frac{\pi \times (8.0\text{ ft})^2}{4} = 50.27\text{ ft}^2$
   • Calculate vertical operating depth between Pump Stop and Pump Start: $H = \frac{\text{Volume}}{A} = \frac{200.53\text{ ft}^3}{50.27\text{ ft}^2} = 3.99\text{ ft} \approx 4.0\text{ feet}$

Wet Well Floor Geometry

To prevent the accumulation and compaction of heavy grit, sand, and organic sludge in dead zones, wet well floors must be engineered with steeply sloped hopper bottoms having a minimum slope of 1:1 ($45^\circ$) or steeper directed toward the pump suction intakes.

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Level Sensing Instrumentation & Control Automation

Automated control systems start and stop pumps based on liquid level in the wet well. Because wet wells contain turbulent foam, grease caps, and humid corrosive vapors, instrument selection is vital:

                      WET WELL CONTROL LEVEL STAGES

   Elevation
      ▲
      │  ┌───────────────────────────────────────────────┐
      ├──┤ HIGH WATER ALARM SENSOR (Independent / Approved)  │  ◄── Alarm Siren / SCADA Dial
      │  ├───────────────────────────────────────────────┤
      ├──┤ LAG PUMP START (Second Pump Energizes)        │  ◄── Heavy Storm Inflow
      │  ├───────────────────────────────────────────────┤
      ├──┤ LEAD PUMP START (First Pump Energizes)        │  ◄── Normal Operating High
      │  ├───────────────────────────────────────────────┤
      │  │                                               │
      │  │        ACTIVE OPERATING VOLUME (V)            │
      │  │                                               │
      │  ├───────────────────────────────────────────────┤
      ├──┤ ALL PUMPS STOP (Both Pumps De-energize)       │  ◄── Minimum Submergence Level
      │  ├───────────────────────────────────────────────┤
      │  │ Submergence Depth (Motor Cooling & Anti-Vortex)│
      └──┴───────────────────────────────────────────────┘

Level Sensing Technologies

1. Mechanical Tilt Float Switches: Polypropylene sealed bulbs containing mercury or micro-switches suspended on weighted cables. Simple and inexpensive, but prone to grease crust fouling, rag entrainment, and cable tangling.
2. Bubbler Tube Systems: A continuous stream of compressed air is purged down an open-ended dip tube submerged near the wet well floor. The backpressure required to force air bubbles against the water column is measured by a pressure sensor ($P = \gamma h$), which is directly proportional to water depth. Completely isolates sensitive electronics from sewage, but requires a continuous air compressor, desiccators, and line purge valves.
3. Ultrasonic Level Transducers: Non-contact sensors mounted in the headspace that transmit ultrasonic sound waves downward and calculate distance based on the time-of-flight echo returned from the liquid surface. Completely non-contact, but can produce false echo readings from thick grease blankets, dense surface foam, or heavy steam condensation.
4. Submersible Hydrostatic Pressure Transducers: A piezoresistive pressure sensor encased in stainless steel/titanium suspended near the bottom of the wet well. Measures hydrostatic head pressure and converts it into a continuous $4 - 20\text{ mA}$ analog signal. Highly accurate, but requires regular cleaning to remove grease accumulation from the sensing diaphragm.
5. High-Water Alarm Channel: Provide the alarm functions required by PUB2754 and the approved design, including high wet-well level and pump/power failures as applicable. Independence from the primary control can improve resilience, but the standard does not mandate one universal sensor type. Alarm transmission, local audio-visual indication, holding capacity, and on-call notification depend on the approved station arrangement; the alarm system needs an uninterrupted power source.

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Force Mains, Appurtenances & Surge Control

A force main is a pressurized discharge pipeline conveying wastewater from a lift station pump to a higher gravity discharge manhole.

Force Main Hydraulic Velocities

• Minimum velocity: 10 CSR 20-8.130 and PUB2754 require at least $2.0\text{ ft/s}$ at the force-main operating point.
• Maximum recommendation: PUB2754 recommends no more than $8.0\text{ ft/s}$ to limit head loss and protect valves. Verify the complete pump/system curve and transient analysis.

Pump Discharge Valves & Access

PUB2754 prohibits valves in the wet well unless a valve is integral to a pump or its housing. Arrange required shutoff/check valves for safe inspection, maintenance, and removal under the approved design:

1. Check Valves: Swing check valves with external weighted levers or cushioned rubber flapper check valves installed on each pump discharge line to prevent pressurized backflow from draining the force main back into the wet well when the pump de-energizes.
2. Isolation Plug Valves: Quarter-turn eccentric plug valves installed downstream of each check valve to allow complete isolation of individual pumps for maintenance without draining the force main. Resilient plug valves provide a smooth, unobstructed port that prevents solids trapping.
3. Air Release / Vacuum Relief:
   • Place air-release valves at force-main high points to prevent air locking.
   • Evaluate the profile and head conditions to determine whether and where vacuum-relief capacity is necessary; it is not automatically required at every slope break.
   • Provide isolation, vault access, ventilation, drainage, and maintenance features under the approved design.

Water Hammer Mitigation

Water hammer is a violent hydraulic pressure transient caused by the sudden change in fluid momentum when a pump stops abruptly or a check valve slams shut.

• Mitigation Methods: Variable Frequency Drives (VFDs) providing soft-start and ramped soft-stop decelerations, cushioned non-slam check valves, surge relief valves, and hydropneumatic surge tanks.