Lift stations and force mains

# Lift stations and force mains

> Lift stations and force mains keep sewage moving where gravity can't do the job alone. Know how they're built and sized, and this part of the exam gets a lot easier.

Why the exam tests this

Lift Stations is its own duty area on the wastewater collections exam, not just background knowledge folded into gravity sewer topics. If you run mostly gravity sewer with one small station, you know your own equipment well. You may not have seen the design logic behind it, only the settings on your own equipment.

If you run several stations of different ages and types, the exam still expects you to reason from the general design rules. It tests the rules behind the equipment, not just your own site's settings. That means wet-well sizing, pump selection, force main hydraulics, and the alarms and standby systems that keep a station running when something fails.

What you need to know

A lift station moves wastewater from a lower elevation to a higher one. Utilities use them where gravity flow can't reach, or where a gravity line would need trenches too deep to dig affordably.

Every lift station shares the same core parts: a wet well, pumps and piping, motors and a power supply, and a control and alarm system. Most stations also need ventilation and odor control.

Pumping costs more to build and run than gravity flow. A lift station is usually the last choice, picked only when rerouting the sewer or digging a deeper gravity line isn't practical.

Stations come in two common configurations. A dry-well station keeps the pumps in a separate, walk-in pump room next to the wet well. That makes routine inspection and repair straightforward.

A submersible station has no pump room. The pumps sit sealed in the wet well itself and lift out on guide rails for service. The valves and meters sit in a small dry vault at grade.

Submersible stations usually cost less to build, need less routine pump maintenance, and take up less room. They also blend into a residential setting better than a dry-well structure. That is why the industry has been moving small and mid-size dry-well stations, roughly under 6,350 gpm, toward submersible designs.

Wet-well sizing has to split the difference. Big enough that pumps don't cycle on and off too fast. Small enough that sewage doesn't sit so long it goes septic and starts releasing odor.

With constant-speed pumps, the wet well is typically sized for 20 to 30 minutes of detention time at design flow. A station running variable-frequency drives can track incoming flow more closely, which tightens that window down to a 5-to-15-minute range.

A wet well should never run dry between pump cycles. Some sewage should always be present to hold odor down. The well floor is sloped at least 2:1 so solids don't settle out and accumulate.

Pump selection starts with the head-capacity curve. That is what head and flow the pumps need to handle across the full range of conditions, not just the average.

Smaller stations with a peak inflow under 700 gpm typically get two pumps. Each one is sized to handle the full peak flow alone, so one is always standing by.

Larger stations size and count pumps to track the flow range without starting and stopping too often, and without needing excessive wet-well storage. A design that uses multiple pumps running together in parallel to meet peak flow needs at least three: two duty pumps that together cover the peak, plus one standby for when one fails.

Where head losses are high, a single pump carrying the flow is usually the better design. Running two pumps together in that case barely adds flow over running one alone.

Straight-flow, angle-flow, and mixed-flow centrifugal pumps cover most of the field. They sort roughly by how much head versus how much flow the job needs.

Reliability comes from redundancy, not from any single part. Standby pumps and controls, backup power, non-clog pumps matched to the wastewater, and automatic alarms all cut the odds that one failure takes the station down.

Backup power can mean a second utility feed, an on-site generator, a portable generator with quick-connect, or a standby holding facility. A power outage is the biggest risk: without power, pumping stops, and sewage can back up and flood everything upstream of the station.

Ventilation matters anywhere personnel routinely enter. The dry-well requirement is 6 air changes an hour on continuous operation, or 30 if it only runs intermittently. Wet wells need double that baseline, 12 on continuous or 60 on intermittent, because that is where gases concentrate most.

Odor control usually starts with the simplest fix: cutting turbulence in the wet well. From there it escalates to scrubbers or biofilters on the vented air, or to chemical dosing added to the sewer upstream.

Chlorine, hydrogen peroxide, ferric chloride or ferrous sulfate, and potassium permanganate are the chemicals you will see used this way.

A force main is the pressurized pipe on the discharge side of the lift station's pumps. It carries sewage to a gravity sewer or treatment plant.

Ductile iron and PVC are the two materials you will see most. Ductile iron brings strength and tight joints. PVC brings lighter weight, corrosion resistance, and lower cost in smaller lines.

General force main design velocity runs 2 to 8 feet per second. Shorter force mains, under about 2,000 feet with less than 30 feet of lift, are usually designed toward the higher end, 6 to 9 feet per second. That allows a smaller pipe but adds friction loss and energy cost.

At peak flow, velocity should never exceed 10 feet per second. The pipe should stay full and above 10 psi at all points, to keep gases from coming out of solution.

Vertical alignment matters as much as velocity. High points trap air, reduce the pipe's effective flow area, and invite sulfide corrosion. Low points collect solids.

Where a high or low point can't be designed out, install an air and vacuum valve at the high point and a blowoff at the low point.

Pressure surges are sudden jumps in pressure when a pump starts or stops. They typically last 2 to 15 seconds and are managed with surge-control valves or surge tanks sized for the specific system.

Because a force main runs full and under pressure, dissolved oxygen in the sewage depletes quickly. The wastewater arrives septic, often carrying sulfides that drive odor and corrosion.

That is why force mains get periodic cleaning by pigging: running a scraper device through the line to clear grease and solids. It is usually launched near the lift station and retrieved near the discharge point.

At higher grades, expect the exam to push past the base rules into judgment calls. That includes choosing between a single large pump and multiple parallel pumps for a given head-capacity curve, and weighing whether variable-speed drives are worth their added cost and space for a station's specific friction losses.

It also includes classifying a station's complexity, based on factors like deep or congested excavation, dual power supply, or heads above 200 feet, to reason about its design and maintenance burden.

Worked examples

Pump count

A proposed station has a maximum inflow of 550 gpm. How many pumps does the base rule call for? How should each pump be sized?

Since 550 gpm is under the 700 gpm threshold, the design calls for two pumps, each sized to handle the full 550 gpm alone. One pump runs, one stands by.

Parallel peak-flow pumps

A station is designed with two duty pumps that together deliver a 1,200 gpm peak flow in parallel. How many total pumps does a code-compliant design need?

At least three. Two duty pumps sized so together they cover the 1,200 gpm peak. One standby pump means the station can still hit peak flow if a duty pump goes down.

Run pump-and-flow problems like this one against the calculator in the practice tool so you can check your own numbers.

Force main velocity check

A 10-inch force main carries 980 gpm. What velocity does that represent, and does it fall inside the acceptable design range?

980 gpm through a 10-inch main works out to about 4 feet per second. That's comfortably inside the general 2 to 8 fps range, and well under the 10 fps peak ceiling.

If that same main were a short, low-lift run instead, designed to the tighter 6 to 9 fps band, 4 fps would actually be too slow for that choice. Main length and lift change which range applies.

Common traps

• Assuming a dry-well station is always the cheaper or lower-maintenance choice. For small and mid-size flows, submersible stations usually cost less to build, need less routine pump maintenance, and take up less room.
• Applying the 20 to 30 minute wet-well detention rule to a station that actually runs variable-frequency drives. VFDs narrow the window to 5 to 15 minutes; using the constant-speed number oversizes the wet well against the design intent.
• Sizing a parallel-pump peak-flow station with only two pumps. The rule calls for at least three: two duty pumps covering peak flow together, plus a standby, not two pumps doing double duty as both the peak-flow team and each other's backup.
• Treating the general 2 to 8 fps force main velocity range as the only number to know. Short, low-lift force mains are designed toward a tighter 6 to 9 fps band, and no force main should exceed 10 fps at peak flow regardless of length.

Practice

Filter the wwc-1 test to Lift Stations at /tools/practice and work the pump-count and parallel-pump problems until the two-pump and three-pump rules are automatic.

Then run the force main velocity and pressure questions the same way, so you can check your own unit handling. The calculator in the practice tool catches the gallons-per-minute versus feet-per-second slips that cost the most points here.

Quick reference

• A lift station moves wastewater uphill when gravity flow isn't feasible; core parts are the wet well, pumps and piping, motors, a control and alarm system, and usually ventilation and odor control.
• Dry-well stations keep pumps in a walk-in room next to the wet well; submersible stations run sealed pumps inside the wet well itself. Submersible usually costs less and needs less routine maintenance for small to mid-size flows.
• Wet-well detention time: 20 to 30 minutes tops on constant-speed pumps, down to a 5-to-15-minute range when the station runs VFDs. Wet-well floor slope: at least 2:1.
• Stations with peak inflow under 700 gpm typically use two pumps, each sized for the full peak flow.
• Parallel peak-flow design needs at least three pumps: two duty pumps covering peak flow together, plus one standby.
• Ventilation: dry wells run 6 air changes hourly on continuous operation (30 if intermittent); wet wells need double that, 12 continuous (60 intermittent).
• Reliability comes from standby pumps and controls, emergency power, non-clog pumps, and automatic alarms, not from any single part of the station.
• Force main design velocity: 2 to 8 fps generally, 6 to 9 fps for short mains under about 2,000 feet with under 30 feet of lift, never over 10 fps at peak flow.
• Force mains must stay full and above 10 psi to keep gas from coming out of solution; high points need air and vacuum valves, low points need blowoffs.
• Force mains run septic and low in oxygen by the time they discharge, which drives odor and corrosion; pigging clears the grease and solids buildup that causes it.