Pump Stations, Wet Wells & Force Mains

Wastewater pump stations are used when sewage cannot continue to flow by gravity. They collect wastewater in a wet well and pump it through a force main to a higher elevation or another part of the collection system.

Reliable pump-station operation depends on pumps, motors, level controls, valves, alarms, electrical power, wet-well condition, and force-main hydraulics working together. A failure can quickly lead to backups, sanitary sewer overflows, equipment damage, or loss of service.

Purpose of a Pump Station

A pump station lifts wastewater so it can continue moving through the collection system.

Pump stations may be used where:

• terrain prevents continuous gravity flow;
• sewers become too deep for practical construction;
• wastewater must cross an elevation barrier;
• a force main is needed to reach downstream gravity sewers or treatment.

Major Pump-Station Components

Common components include:

• wet well;
• wastewater pumps;
• motors;
• level controls;
• check valves;
• isolation valves;
• force main;
• electrical controls;
• alarms;
• backup power.

Wet Well

The wet well temporarily stores incoming wastewater before pumping.

Its operation affects:

• pump starts and stops;
• pump run time;
• wastewater detention time;
• odor;
• septicity.

Wet-Well Level

Wastewater level rises as flow enters the wet well.

When level reaches a control point, a pump starts.

When level falls to the stop point, the pump shuts off.

Start and Stop Levels

Control levels should provide enough operating volume to avoid:

• excessive pump cycling;
• unnecessary long detention time;
• loss of emergency storage.

Excessive Pump Cycling

Frequent starts and stops can:

• increase motor wear;
• increase electrical stress;
• reduce equipment life.

Long Wet-Well Detention

If wastewater remains in the wet well too long, it can become septic.

Possible results include:

• odor;
• hydrogen sulfide;
• corrosion;
• poor downstream wastewater quality.

Lead and Lag Pumps

Many pump stations have at least two pumps.

A common arrangement includes:

• lead pump;
• lag pump.

Lead Pump

The lead pump starts first during a normal pumping cycle.

Lag Pump

The lag pump starts if:

• wet-well level continues rising;
• incoming flow exceeds lead-pump capacity;
• additional pumping is required.

Pump Alternation

Pump stations often alternate lead duty so operating hours are distributed between pumps.

Why Alternation Matters

Alternation can:

• balance run time;
• reduce uneven wear;
• confirm both pumps remain operational.

Pump Run Time

Run time is an important operating indicator.

A change in run time can indicate:

• flow changes;
• pump wear;
• force-main problems;
• changing wet-well controls.

Increasing Run Time

If a pump takes longer to lower the wet well under similar incoming-flow conditions, possible causes include:

• reduced pump capacity;
• impeller wear;
• ragging;
• force-main restriction;
• valve problem.

Shorter Run Time

Shorter run time can result from:

• lower incoming flow;
• changed start and stop levels;
• inaccurate level measurement;
• increased pump output.

Pump Capacity

Pump capacity is commonly expressed as a flow rate such as:

• gallons per minute;
• million gallons per day.

Flow Conversion

A useful conversion is:

MGD = gpm × 1,440 ÷ 1,000,000

Pump-Flow Example

A pump delivers 700 gpm.

MGD = 700 × 1,440 ÷ 1,000,000

MGD = 1.008 MGD

The pump capacity is approximately 1.01 MGD if it operates continuously at that flow.

Wet-Well Drawdown Test

A drawdown test can estimate pump output by observing how quickly a known wet-well volume decreases while the pump operates.

Drawdown Concept

A simplified calculation is:

Pump Flow = Wet-Well Volume Change ÷ Pumping Time

Drawdown Example

A pump lowers the wet well by a volume of 6,000 gallons in 8 minutes.

Pump Flow = 6,000 ÷ 8

Pump Flow = 750 gpm

This simplified example assumes incoming flow during the test is negligible or appropriately accounted for.

Correct for Incoming Flow

If wastewater continues entering the wet well during a drawdown test, actual pump discharge is greater than the observed net drawdown rate.

Pump Performance Trend

Repeated drawdown testing under similar conditions can help identify gradual loss of pump performance.

Centrifugal Wastewater Pumps

Many pump stations use centrifugal pumps.

Important components can include:

• impeller;
• volute;
• shaft;
• bearings;
• seals;
• motor.

Impeller Problems

Wastewater pump impellers can be affected by:

• rags;
• wipes;
• debris;
• wear;
• corrosion.

Ragging

Fibrous material can wrap around pump components and reduce flow.

Symptoms can include:

• longer run time;
• lower discharge flow;
• higher motor load;
• repeated clogging.

Pump Suction Conditions

Submersible wastewater pumps normally operate with the pump submerged in the wet well.

Poor suction conditions can still result from:

• vortex formation;
• insufficient submergence;
• debris;
• poor wet-well geometry.

Vortex Formation

A vortex can draw air toward the pump inlet and reduce stable pump performance.

Motor Current

Motor current can help operators evaluate pump condition.

Unexpected current changes can indicate:

• mechanical loading changes;
• pump blockage;
• electrical problems;
• dry or abnormal operation.

Check Valve

A check valve prevents wastewater from flowing backward through a pump after the pump stops.

Check-Valve Failure

A leaking or failed check valve can cause:

• reverse flow;
• rapid wet-well refill;
• excess pump cycling;
• loss of pumping efficiency.

Check-Valve Slam

Rapid valve closure can create pressure transients and mechanical stress.

Isolation Valve

Isolation valves allow sections of piping or equipment to be taken out of service for maintenance.

Incorrect Valve Position

A partially closed discharge valve can:

• reduce pump flow;
• increase discharge pressure;
• increase pump run time.

Force Main

A force main carries pumped wastewater under pressure.

Unlike a normal gravity sewer, it usually flows full while the pump operates.

Force-Main Pressure

Pressure depends on:

• elevation difference;
• friction loss;
• flow rate;
• pipe diameter;
• valve condition.

Static Head

Static head is the elevation difference the pump must overcome between the suction and discharge hydraulic levels.

Friction Head

Friction head is the energy required to overcome friction through:

• pipe;
• fittings;
• valves.

Total Dynamic Head

In simplified terms:

Total Dynamic Head = Static Head + Friction Head + Other Relevant Losses

Higher Flow Creates More Friction Loss

As flow increases, friction loss generally increases.

This affects how much flow a pump can deliver.

Force-Main Restriction

A restriction can cause:

• higher discharge pressure;
• lower pump flow;
• longer pump run time.

Possible Force-Main Restrictions

Causes can include:

• debris;
• closed or partially closed valve;
• solids accumulation;
• air binding;
• pipe deformation.

Air in Force Mains

Air can collect at high points in a force main.

Accumulated air can:

• reduce effective flow area;
• increase head loss;
• interfere with pump performance.

Air-Release Valves

Air-release equipment may be installed at appropriate locations to remove trapped air.

Air-Release Valve Maintenance

These valves can foul because they are exposed to wastewater and corrosive gases.

Force-Main Velocity

Force-main velocity affects:

• solids transport;
• friction loss;
• wastewater detention.

Low Force-Main Velocity

Low velocity can contribute to:

• solids deposition;
• long detention time;
• septic conditions.

High Force-Main Velocity

Very high velocity increases:

• friction loss;
• energy use;
• potential pressure transients.

Velocity Calculation

A basic relationship is:

Velocity = Flow ÷ Area

Pipe Area

For a full circular force main:

Area = π × Diameter² ÷ 4

Force-Main Velocity Example

A full force main has an inside diameter of 12 inches, or 1 foot.

Area:

Area = 3.1416 × 1² ÷ 4

Area ≈ 0.785 ft²

If flow is 2 ft³/s:

Velocity = 2 ÷ 0.785

Velocity ≈ 2.55 ft/s

Force-Main Detention Time

Travel time can be estimated by:

Travel Time = Pipe Length ÷ Velocity

Travel-Time Example

A force main is 10,000 feet long and wastewater velocity is 2.5 ft/s.

Time = 10,000 ÷ 2.5

Time = 4,000 seconds

4,000 ÷ 60 ≈ 66.7 minutes

Long Detention and Septicity

Long force-main detention can increase the likelihood of:

• anaerobic conditions;
• hydrogen sulfide;
• odor;
• corrosion.

Hydrogen Sulfide

Hydrogen sulfide is especially important in pump stations and force mains because it can create:

• worker toxicity;
• odor complaints;
• corrosion.

Corrosion

Corrosion can affect:

• wet-well structures;
• piping;
• valves;
• electrical components;
• downstream manholes.

Odor Complaints

Odors near pump stations or force-main discharge points can indicate:

• long detention;
• septic wastewater;
• hydrogen sulfide generation.

Level Controls

Wet-well level may be measured using:

• floats;
• pressure devices;
• ultrasonic sensors;
• radar sensors;
• other instruments.

Level-Control Failure

A failed level sensor can cause:

• pump failure to start;
• pump failure to stop;
• high wet-well level;
• unnecessary pump cycling.

Verify Level Instruments

If indicated level does not match actual wet-well conditions, verify:

• sensor cleanliness;
• calibration;
• signal wiring;
• SCADA display.

High-Level Alarm

A high-level alarm warns that wet-well level has risen above normal operating conditions.

Possible Causes of High Level

Possible causes include:

• pump failure;
• power failure;
• excessive incoming flow;
• force-main restriction;
• closed valve;
• control failure.

Respond to High Level Immediately

Operators should determine:

• whether pumps are running;
• whether discharge flow is occurring;
• whether backup power is available;
• how much wet-well storage remains;
• whether overflow is imminent.

Backup Power

Backup generators or other emergency power sources can allow pumping during utility outages.

Generator Testing

Backup equipment should be tested according to facility procedures rather than assumed to work because it worked previously.

Automatic Transfer Equipment

Automatic transfer equipment can switch the station from utility power to emergency power.

Power Failure

During a power outage, incoming wastewater usually continues entering the wet well.

Operators need to understand available storage time.

Emergency Storage Time

A simplified relationship is:

Remaining Time = Available Wet-Well Volume ÷ Incoming Flow

Emergency-Time Example

A wet well has 12,000 gallons of usable storage above the current level.

Incoming flow is 400 gpm.

Remaining Time = 12,000 ÷ 400

Remaining Time = 30 minutes

This calculation assumes no pumping occurs and incoming flow remains constant.

Portable Pumping

Some facilities maintain portable pumps for emergency bypass or station failure.

Bypass Pumping

A bypass system must be capable of safely conveying expected incoming flow.

Alarm Systems

Important alarms can include:

• high wet-well level;
• pump failure;
• power failure;
• generator failure;
• high motor temperature;
• communication failure.

SCADA

Supervisory control and data acquisition systems can provide remote information such as:

• wet-well level;
• pump status;
• run time;
• alarms;
• flow;
• pressure.

SCADA Does Not Replace Field Verification

If data are unusual or an emergency condition exists, field verification may be necessary.

Preventive Maintenance

Pump-station preventive maintenance can include:

• pump inspection;
• motor inspection;
• valve exercising;
• level-control testing;
• alarm testing;
• generator testing;
• wet-well cleaning.

Wet-Well Cleaning

Wet wells can accumulate:

• grease;
• rags;
• grit;
• floating debris.

Grease Accumulation

Grease can interfere with:

• floats;
• level sensors;
• pump suction;
• odor control.

Wet-Well Safety

Wet wells can present:

• confined-space hazards;
• hydrogen sulfide;
• methane;
• low oxygen;
• biological exposure.

Do Not Rely on Odor

Odor cannot be used to determine whether a wet-well atmosphere is safe.

Electrical Safety

Pump stations contain electrical equipment in wet environments.

Operators should follow applicable:

• lockout procedures;
• electrical safety practices;
• equipment-specific procedures.

Example: Wet-Well Level Rises While Pump Runs

Possible causes include:

• incoming flow exceeds pump output;
• pump capacity has declined;
• force main is restricted;
• discharge valve is partially closed.

Example: Pump Runs Longer Than Normal

Review:

• incoming flow;
• pump output;
• impeller condition;
• force-main pressure;
• valve position.

Example: Pump Cycles Rapidly

Possible causes include:

• start and stop levels too close;
• check-valve leakage;
• incorrect level signal;
• small effective wet-well volume.

Example: Wet Well Refills Immediately After Pump Stops

A leaking check valve may be allowing wastewater to flow backward from the force main.

Example: Discharge Pressure High and Flow Low

Possible causes include:

• force-main restriction;
• partially closed valve;
• air accumulation;
• downstream pressure increase.

Example: Discharge Pressure Low and Flow Low

Possible causes include:

• worn pump;
• damaged impeller;
• ragging;
• motor-speed problem.

Example: Pump Current Rises

Investigate:

• mechanical blockage;
• ragging;
• bearing problems;
• changing hydraulic load.

Example: Pump Current Falls and Flow Falls

Possible causes include:

• damaged impeller;
• air entering the pump;
• low hydraulic load;
• electrical or speed problem.

Example: High-Level Alarm During Heavy Rain

Review:

• incoming wet-weather flow;
• lead and lag pump operation;
• force-main capacity;
• available emergency storage.

Example: High-Level Alarm During Dry Weather

A mechanical, electrical, valve, control, or force-main problem is more likely than storm-related hydraulic overload.

Example: Strong Odor at Force-Main Discharge

Review:

• wastewater travel time;
• pump cycling;
• force-main velocity;
• septicity.

Example: One Pump Has Much More Run Time

Review:

• pump alternation;
• availability of the other pump;
• control logic;
• individual pump capacity.

Example: Both Pumps Run but Wet-Well Level Still Rises

Possible causes include:

• incoming flow greater than total station capacity;
• major pump-performance loss;
• force-main restriction;
• downstream hydraulic problem.

Example: Station Flow Declines Gradually Over Months

Review trends in:

• pump run time;
• drawdown test results;
• discharge pressure;
• motor current;
• maintenance history.

Force-Main Failure

A force-main leak or break can cause:

• wastewater release;
• loss of discharge pressure;
• pump-station operational problems;
• environmental impact.

Possible Force-Main Leak Indicators

Signs can include:

• unexpected pressure loss;
• unusual wet ground;
• wastewater surfacing;
• reduced downstream flow.

Pressure Transients

Rapid changes in flow can create pressure transients in force mains.

Possible causes include:

• pump start;
• pump stop;
• rapid valve operation;
• check-valve closure.

Operating Records

Useful pump-station records include:

• pump run hours;
• number of starts;
• flow;
• wet-well level;
• discharge pressure;
• motor current;
• alarms;
• maintenance history.

Trend Run Hours

Increasing run hours at similar wastewater flow can indicate deteriorating pump performance.

Trend Starts

Increasing starts can indicate:

• control-setting changes;
• check-valve leakage;
• changing incoming flow.

Common Pump-Station Mistakes

• Responding to a high-level alarm without checking whether pumps are actually moving wastewater.
• Using pump run time without considering incoming flow.
• Ignoring check-valve leakage.
• Ignoring gradual pump-capacity loss.
• Assuming force-main problems always produce visible leaks.
• Ignoring air accumulation in force mains.
• Allowing long wet-well detention that promotes septicity.
• Assuming backup power will work without regular testing.
• Relying only on SCADA without verifying abnormal field conditions.
• Ignoring run-hour, pressure, current, and drawdown trends.

A Practical Pump-Station Review

1. Review wet-well level.
2. Review lead and lag pump status.
3. Review pump run time and starts.
4. Review flow and discharge pressure.
5. Review motor current.
6. Review check and isolation valves.
7. Review force-main condition.
8. Review alarms and backup power.
9. Compare current performance with historical trends.

A Practical High-Level Alarm Review

1. Verify actual wet-well level.
2. Confirm which pumps are running.
3. Confirm wastewater is being discharged.
4. Review incoming flow.
5. Review discharge pressure.
6. Check valves and force-main condition.
7. Start backup pumping or power as needed.
8. Estimate time remaining before overflow.

A Practical Low-Pump-Capacity Review

1. Verify the flow measurement.
2. Perform or review a drawdown test.
3. Review discharge pressure.
4. Review motor current.
5. Inspect for ragging or impeller wear.
6. Review valve positions.
7. Review force-main restrictions.
8. Compare results with historical pump performance.

A Practical Force-Main Review

1. Review discharge flow.
2. Review pressure trends.
3. Review air-release equipment.
4. Review valve position.
5. Check for visible or suspected leaks.
6. Review wastewater detention and odor conditions.
7. Compare hydraulic performance with normal operation.

What to Remember for the Exam

• Pump stations lift wastewater where gravity flow alone is insufficient.
• Major components include wet wells, pumps, motors, controls, valves, alarms, force mains, and backup power.
• Wet-well level controls determine pump start and stop operation.
• Excessive cycling increases equipment wear, while excessive detention can promote septicity.
• Lead and lag pumps provide capacity and operational redundancy.
• Pump alternation helps balance operating hours.
• Increasing pump run time can indicate higher flow or declining pump performance.
• A wet-well drawdown test can estimate pump output.
• Check valves prevent reverse flow after a pump stops.
• A leaking check valve can cause rapid wet-well refill and excessive cycling.
• Force mains carry wastewater under pump pressure and normally flow full while operating.
• Total dynamic head includes static head, friction head, and other relevant losses.
• High discharge pressure with low flow can indicate a force-main restriction or partially closed valve.
• Air accumulation in a force main can increase head loss and reduce flow.
• Force-main velocity can be calculated from flow divided by cross-sectional area.
• Long wet-well or force-main detention can contribute to hydrogen sulfide, odor, and corrosion.
• High-level alarms require rapid evaluation of pumps, power, incoming flow, discharge, and remaining storage.
• Backup generators and alarm systems should be tested routinely.
• Pump stations and wet wells can contain hazardous atmospheres and electrical hazards.
• Good pump-station troubleshooting combines level, flow, pressure, pump run time, motor current, valve condition, force-main condition, alarms, and historical trends.