A force main is the pressurized pipe that carries wastewater away from a lift station. Unlike a gravity sewer, it flows completely full under pump pressure — and that difference creates a specific set of problems operators need to know how to handle.
In a gravity wastewater collection system, pipes are sloped so wastewater flows downhill on its own. But gravity only works when the terrain cooperates. When it doesn't — when the land is too flat, the sewer would need to be buried too deep, or a low-lying area needs to drain to a higher point — a lift station takes over. The force main is the pipe the lift station discharges into.
Force mains are pressurized. They flow completely full under the pressure generated by the lift station pumps. That single difference — pressurized vs. gravity flow — changes nearly everything about how they behave, what problems they develop, and how operators maintain them.
A force main is the pressurized discharge pipe from a lift station or pump station that conveys wastewater under pressure to a point where it can resume gravity flow. The discharge lines from the lift station pumps come together at a manifold and then enter the force main as a single pressurized pipe.
At the downstream end — typically at a manhole where the terrain drops enough to restore gravity — the force main discharges and wastewater continues by gravity toward the treatment plant or the next collection point. The force main itself doesn't treat wastewater or filter it in any way. Its only job is to move it from a low point to a high point under pump pressure.
A gravity sewer flows partially full by design, relying on slope. A force main flows completely full under pressure, relying on pumps. Everything else — materials, problems, maintenance — follows from that difference.
| Characteristic | Gravity Sewer | Force Main |
|---|---|---|
| What moves the flow | Gravity — pipe is sloped | Pump pressure from the lift station |
| Pipe fullness | Designed to flow half-full at average flow — air space above the flow is intentional | Flows completely full under pressure at all times when pumps are running |
| Minimum slope required | Yes — minimum velocity of 2 fps to prevent solids settling | No slope required — can run uphill, downhill, or level |
| Solids settling | A real concern if velocity drops below 2 fps | Pressure keeps solids in suspension while pumps are running |
| Air management | Air space above the flow handles venting naturally | Air accumulates at high points and must be vented through air release valves |
| Pressure | Atmospheric — open to air above the flow | Above atmospheric — sealed and pressurized by pump operation |
| Flow when pumps are off | Continues — gravity keeps it moving | Stops completely — no pump, no flow |
| Septic conditions | Can develop if flow is too slow | Can develop when pumps are off for extended periods — wastewater sits stagnant |
Force mains are built to handle pressure and resist the corrosive environment of wastewater under pressure. The most common materials are:
The most widely used force main material for smaller diameter applications. PVC is inexpensive, easy to work with, and resistant to most substances in wastewater. It's available in pressure-rated grades specifically for force main service. The main limitation is susceptibility to damage from heavy external loads, so the pipe grade must be matched carefully to burial depth and traffic loading conditions.
HDPE is increasingly common for force mains, particularly in larger diameters. It's flexible, durable, and highly resistant to the corrosive environment inside a force main. The key installation advantage of HDPE is that sections are thermally butt-welded together in the field — creating a virtually seamless, leak-free joint. Fewer joints mean fewer potential failure points, which matters on a pressurized pipe. HDPE is also the material of choice when the force main route requires directional drilling or trenchless installation.
Used where high pressure, heavy loads, or difficult installation conditions demand a rigid, strong pipe. Bridge crossings, shallow installations under roads, and areas with heavy traffic loads are common applications. More expensive than PVC or HDPE but offers superior strength and resistance to external loads. Joints are typically mechanical push-on or flanged type.
Semi-flexible and corrosion-resistant, FRP sees use in force main applications but requires careful installation. There is some evidence of susceptibility to hydrogen sulfide damage over time, so long-term service in high-H2S environments warrants consideration of alternative materials.
This is the most important operational concept for force mains — and the one most likely to appear on the exam.
Because a force main flows completely full and is sealed under pressure, air has nowhere to go. Air naturally separates from the wastewater and migrates to the highest points in the pipe profile. Once trapped there, it forms a pocket that progressively reduces the effective cross-section of the pipe — restricting flow, increasing the head the pump must work against, and eventually causing problems that operators feel as pump performance issues before they ever trace the cause to a high point in the force main.
The most serious consequence of unmanaged air in a force main is water hammer — a high-pressure shock wave that travels up and down the pipe when pumps start or stop. Here's what happens: when a pump shuts off, the water column it was pushing continues moving momentarily by momentum, then decelerates and reverses. The check valve slams shut to stop backflow. If there's trapped air in the line, the pressure wave from that slamming valve reflects back and forth until it dissipates — sometimes violently enough to damage joints, crack fittings, bend piping, or cause check valves to repeatedly slam open and closed.
Water hammer announces itself. You hear it as a bang — sometimes a series of bangs — from the piping at or near the lift station when pumps cycle. A noisy or hard-closing check valve is the most reliable early warning sign that air is accumulating somewhere in the force main upstream.
The standard solution is to install air release valves (also called air relief valves or combination air valves) at every high point along the force main profile. These valves open automatically to vent accumulated air while the line is pressurized, then close again before liquid escapes. They're small — typically 1 to 2 inches — but they do critical work. A force main without properly functioning air release valves at its high points will develop air locks, reduced pumping efficiency, and eventually water hammer damage.
Maintenance of air release valves is frequently overlooked. A valve that has seized in the open position will continuously discharge liquid. A valve that has seized in the closed position provides no protection against air accumulation. Both failure modes are common. Inspecting and exercising air release valves should be part of any force main preventive maintenance program.
Banging or hammering sounds when pumps start or stop
Hard-closing or noisy check valves at the lift station
Reduced pump flow despite pumps running normally
Higher than normal pump discharge pressure
Wet well not drawing down at the expected rate
When the lift station pumps shut down — either intentionally during low-flow periods or due to a failure — wastewater stops moving in the force main and sits stagnant. Unlike a gravity sewer, which continues draining by gravity, the force main holds its contents in place.
Stagnant wastewater goes septic relatively quickly, especially in warm weather. Bacteria consume the dissolved oxygen and shift to anaerobic metabolism, producing hydrogen sulfide (H2S) gas. When the pumps restart and pressurize the line again, this septic wastewater and the H2S it contains get pushed to the downstream discharge point — often a manhole — where the H2S off-gasses into the atmosphere or the receiving gravity sewer.
Septic force main discharge is a common source of odor complaints downstream of lift stations. It's also a corrosion driver — H2S combines with moisture in the manhole atmosphere to form sulfuric acid, which attacks concrete and causes crown rot in the receiving sewer. Force main discharge manholes often show accelerated concrete deterioration compared to other manholes in the system for exactly this reason.
A force main break is one of the most urgent emergency responses in collection system operations. Unlike a broken gravity sewer, which loses flow gradually as the pipe deteriorates, a pressurized force main break can discharge large volumes of raw wastewater rapidly — potentially causing a significant sanitary sewer overflow and environmental damage before operators can respond.
Common causes of force main failures:
When a break is suspected — unexplained wet areas along the force main alignment, sinkholes, loss of pump efficiency, or visible discharge — isolate the lift station, notify the appropriate authorities, and begin emergency repairs. Document everything for regulatory reporting requirements.
Force mains can accumulate grit and debris, particularly in low-velocity sections or when pump runtime is intermittent and flow velocities drop. Cleanout access points — installed at intervals along the force main — allow operators to flush the line or access it for inspection and maintenance without excavation. Cleanout placement and spacing should be documented in the collection system as-built drawings.
When a force main becomes partially blocked by grit or grease, the signs are similar to air accumulation: reduced pump output, elevated discharge pressure, and slower wet well drawdown. The distinction is that flushing or pigging the line resolves a grit blockage, while air release valve service resolves an air accumulation problem.
Force main questions on the operator exam concentrate on four areas. Know all four well.
Definition and purpose. A force main is the pressurized discharge pipe from a lift station. It carries wastewater under pressure from a low point to a high point where gravity flow can resume. The exam may ask you to identify it from a system diagram or distinguish it from a gravity sewer.
Air accumulation and water hammer. This is the highest-frequency topic. Know that air collects at high points in force mains, that air release valves vent it, and that unmanaged air leads to water hammer — a pressure shock wave caused by check valves slamming. The exam commonly describes a noisy check valve or banging pipes and asks for the cause and solution.
Force main vs. gravity main. Know the comparison table: force mains flow full under pressure, gravity sewers flow partially full by gravity. Force mains can run uphill; gravity sewers require downhill slope. Force mains stop when pumps stop; gravity sewers continue draining.
Pipe materials. PVC and HDPE are the most common force main materials. Know that HDPE is butt-welded and preferred for its leak-resistant joints. Know that ductile iron is used for high-pressure or heavy-load applications like bridge crossings.
An operator hears banging sounds from the piping at a lift station every time a pump shuts off. The check valve is noisy and slams hard on shutdown. What is the most likely cause and what should be checked first?
Answer: Air accumulation in the force main. Trapped air at a high point is causing the water column to decelerate suddenly when the pump stops, slamming the check valve and producing water hammer. Inspect and service the air release valves at the high points in the force main profile.
Our Fundamentals study guide covers the collection system topics tested on Class I and Class II exams — including lift stations, force mains, gravity sewers, infiltration and inflow, and more. 150 questions with detailed answer explanations.
Get the Study Guide — $17Instant PDF download · 150 questions · Answer key included