14.2 Sewage Lift Stations & Force Mains

Sewage Lift Stations & Force Mains

Chapter 3 covered building sewage ejectors and sumps under FPC Section 712. This section scales that up to the site lift station and force main — the named part of Content Area E sub-topic 3 that moves wastewater from a low building or a low subdivision to a gravity main or a treatment plant somewhere uphill.

Gravity drainage is always preferable. A lift station is what you build when the geometry gives you no choice, and in Florida — where much of the developed coastline sits within a few feet of sea level and the water table is often within 3 feet of grade — the geometry frequently gives you no choice.

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1. Station Architecture

A duplex submersible station consists of a wet well (usually precast concrete or fiberglass), two pumps sized so either one alone can handle peak flow, guide rails and lifting chains, a level control system, a valve vault containing the check valves and isolation valves, a control panel with alternator and alarm, and the force main leaving the site.

Level Control and Alternation

Float switches, pressure transducers or ultrasonic sensors establish four elevations in the well:

1. Pump off (low level) — above the pump volute so the pump never runs dry.
2. Lead pump on — sets the working volume between starts.
3. Lag pump on — the second pump joins during peak inflow.
4. High-level alarm — annunciated locally and usually by telemetry.

The controller alternates lead and lag on each cycle to equalize wear. Working volume matters: too small and the pumps short-cycle and burn up motors; too large and sewage sits septic in the well generating hydrogen sulfide.

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2. The Valve Order on a Pumped Discharge

The Florida Plumbing Code fixes the sequence on building sumps and ejectors, and site stations follow the same logic:

FPC Section 712.3.2 — the discharge piping shall have a full open valve located on the discharge side of a backwater or check valve.

Reading it from the pump outward: pump, then check valve, then full open (isolation) valve, then the force main. The reason is maintenance. With the isolation valve downstream of the check, a technician can close the isolation valve, hold the head of the full force main behind it, and pull or service the check valve without draining the entire line back into the well.

FPC Section 712.3.3 governs the discharge pipe size, and Section 712.3.5 the connection of the pump discharge to the gravity drainage system. On a site station, the force main typically discharges into a manhole with an energy-dissipating arrangement so the incoming jet does not scour the invert or splash the bench.

One exception worth remembering: in one- and two-family dwellings, only a check valve is required, located on the discharge piping from the pump or ejector.

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3. Sizing the Force Main

Scouring Velocity Governs the Diameter

A force main is not sized like a water main. A water main is sized to hold velocity down; a force main is sized to hold velocity up, because raw sewage carries grit and organic solids that settle out the moment flow slows.

• Minimum scouring velocity: conventionally not less than 2 feet per second, and many Florida utility standards specify 2 to 3.5 fps.
• Maximum velocity: generally about 8 feet per second, above which friction loss, surge pressure and abrasion become the controlling problems.

Undersize the pump and the main silts; oversize the main and the same thing happens, because the velocity drops below scouring. A force main that sits stagnant also turns anaerobic, generating hydrogen sulfide that oxidizes to sulfuric acid on the crown of the receiving gravity sewer and destroys concrete pipe and manhole benches.

Total Dynamic Head

The pump is selected on total dynamic head (TDH):

$\mathbf{TDH = \text{Static Lift} + \text{Friction Loss} + \text{Residual Discharge Head}}$

• Static lift is the vertical distance from the pump-off water level in the wet well to the discharge point elevation — not to the top of the well, and not to average water level.
• Friction loss is the head lost in the force main and every fitting, computed over the total equivalent length exactly as in water distribution sizing.
• Residual discharge head is any pressure that must remain at the discharge, such as when a force main ties into a pressurized regional main rather than a free-discharging manhole.

The system curve is TDH plotted against flow. The pump's operating point is where the system curve crosses the manufacturer's pump curve, and a competent submittal shows that intersection falling inside the pump's best-efficiency range with the force main velocity still above 2 fps.

Worked Example

A duplex submersible station serves a small Florida commercial park at a peak flow of 150 gpm. The force main is 4-inch C900 PVC, 900 feet long, with fittings adding 120 feet of equivalent length. The wet well pump-off level is at elevation 4.50 feet; the receiving manhole invert is at elevation 31.00 feet. Friction loss for 150 gpm in 4-inch C900 is 1.9 feet per 100 feet.

Step 1 — Check the velocity. A 4-inch C900 main has an inside area of roughly 0.0884 square feet. $V = \frac{150\text{ gpm}}{448.8\text{ gpm per cfs}} \div 0.0884\text{ sq ft} = \frac{0.334\text{ cfs}}{0.0884} = \mathbf{3.8\text{ feet per second}}$ That sits comfortably between the 2 fps scouring minimum and the 8 fps maximum. A 6-inch main at the same flow would fall to about 1.7 fps and silt.

Step 2 — Static lift. $31.00 - 4.50 = \mathbf{26.50\text{ feet}}$

Step 3 — Friction loss over total equivalent length. $TEL = 900 + 120 = 1,020\text{ feet}$ $h_f = \frac{1,020}{100} \times 1.9 = \mathbf{19.38\text{ feet}}$

Step 4 — Residual head. The main free-discharges into a gravity manhole, so residual is 0 feet.

Step 5 — Total dynamic head. $TDH = 26.50 + 19.38 + 0 = \mathbf{45.88\text{ feet, say 46 feet}}$

Select a submersible pump delivering 150 gpm at 46 feet TDH, and confirm from the curve that the duty point is near best efficiency. Because the station is duplex, each pump alone must make that duty point — the second pump is redundancy, not additional capacity.

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4. Florida-Specific Design Problems

Buoyancy Is the Governing Load

A 6-foot-diameter, 20-foot-deep precast wet well displaces roughly 565 cubic feet of groundwater. At 62.4 pounds per cubic foot that is about 35,000 pounds of uplift on an empty structure — enough to float a wet well out of the ground during a wet season or a hurricane, shearing the incoming gravity sewer and the force main. The station must be checked empty, with the water table at its seasonal high, and ballasted with an anti-flotation collar, a poured base extension, or tie-downs to a mat. This is the same buoyancy analysis applied to interceptors in Chapter 3, scaled up.

Hydrogen Sulfide and Corrosion

Warm Florida sewage turns septic quickly. The countermeasures are: keep detention time in the wet well short by right-sizing the working volume, ventilate the wet well, line the receiving manhole with a corrosion-resistant coating, use corrosion-resistant valve vault components, and in chronic cases dose chemically at the station. Station hatches, guide rails and chains should be stainless steel, not galvanized, in a sulfide environment.

Standby Power

Florida loses grid power in every named storm. Critical stations carry a permanent standby generator with automatic transfer, or at minimum a quick-connect receptacle and manual transfer switch so a portable generator can be brought in. Storage volume in the wet well between the alarm level and the lowest gravity inlet buys the response time.

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5. Entering a Wet Well Is Confined Space Work

A sewage wet well is a textbook permit-required confined space under OSHA 29 CFR 1926 Subpart AA — it is large enough to enter, has limited means of entry and egress, is not designed for continuous occupancy, and contains a hazardous atmosphere. Hydrogen sulfide is heavier than air, deadens the sense of smell at the concentrations that kill, and pools at the bottom of the well; methane displaces oxygen and is explosive.

Before entry, the atmosphere is tested in this exact order under 29 CFR 1926.1204(e):

1. Oxygen content
2. Flammable gases and vapors
3. Potential toxic air contaminants

The order is not arbitrary: a combustible-gas meter gives a false reading in an oxygen-deficient atmosphere, so oxygen must be confirmed first. Entry then requires a permit, continuous forced-air ventilation, continuous monitoring, retrieval equipment, an attendant stationed outside, and rescue services summoned by non-entry means. Chapter 11 develops the full Subpart AA program; the point here is that no lift station maintenance task is ever a two-minute unattended climb.