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Low-pressure sewer systems explained

Short answer

A low-pressure sewer system replaces deep gravity pipes with a pump at every home and small pressurized mains, usually 2 inches and up, running at 40 psi or less. Each house grinds or settles its own waste and pushes it into the shared main. Utilities build them where ground is flat, wet or low.

A low-pressure sewer system is a public sewer in which every connected home has its own pump that pushes wastewater through a small-diameter pressurized main, instead of letting it flow downhill through large gravity pipes. The EPA describes the family as pressure sewers, with two types: grinder pump (GP) systems and septic tank effluent pump (STEP) systems.

This guide explains how the pieces fit together, what a force main is, where these systems make sense and where they do not, and compares the published low-pressure sewer rules for selected Baldwin and Mobile county utilities.

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How a pressure sewer differs from a gravity sewer

A conventional sewer, the EPA notes, moves sewage by gravity through buried piping to a treatment facility, consumes no power, and is usually reliable, but the slope needed to keep flow moving can require deep excavation in hilly or flat terrain plus sewage pump stations, and manholes add substantial cost. Pressure sewers use smaller pipes laid with a slight slope or following the surface contour of the land, which reduces excavation and construction cost.

The EPA gives two more reasons that matter on a coast. Pressure sewers break down large solids in the pumping station before transport, and their watertight design with no manholes eliminates extraneous flows into the system, so they are preferred where high groundwater could seep into a gravity sewer. The same tightness keeps sewage in the pipe, which protects groundwater.

Engraving of a gentleman pointing with a long pointer at a street plan on an unrolled scroll, rows of small houses joined by thin pipes.
Plate L. Every house on the street, joined by small pipes. His Grace finds it rather intimate.

The parts of a low-pressure sewer

From the house outward, a GP-type system has these components. Figures are from the EPA and the manufacturers.

  • A grinder pump station on each lot: tank, pump with cutter, level control, alarm panel. The EPA O&M study describes grinder pumps placed at each home, above or below ground, serving one or more residences.
  • A check valve and a shut-off (curb stop) near the property line, so the main cannot push back into the tank. Liberty's grinder manual tells installers on pressure sewers to verify a redundant check valve assembly, curb stop and check valve, as close to the right-of-way as possible; E-One requires its own stainless check valve between the station and the street main.
  • A service line, normally 1-1/4 inches, from the station to the main.
  • Collection mains from 1-1/2 to 6 inches; the EPA's fact sheet says pressure mains generally use 2-inch or larger PVC (SDR 21) or fused HDPE pipe.
  • Operating pressure of 40 psi or less in normal service.
  • Clean-outs and valve assemblies in place of manholes, spaced farther apart, and air release valves at high points, which the EPA says are often vented to soil beds to limit odor.
  • A discharge point: a gravity manhole, a treatment plant, or a lift station that forwards the flow.

What a force main is

A force main is any sewer pipe that carries flow under pump pressure rather than by gravity. The main that leaves a lift station is a force main; so is the shared main of a low-pressure sewer, which is why the terms overlap. The distinction that matters to a homeowner is who else is on it. MAWSS's Policy 2.19 splits its grinder pump rules on exactly that point: a grinder on a gravity sewer, or in a development whose low-pressure system is served by an adjacent gravity sewer, may be any pump from its accepted list; a grinder in a development whose low-pressure system discharges into a force main that also carries other developments' flow must be a low flow, high head pump. More pumps pushing into one pipe means more pressure at your discharge, and the pump has to overcome it.

The EPA's design note says the same thing in engineering terms: the number of pumps that can share the same downstream force main is limited, and GP systems must reach a scouring velocity of three to five feet per second at least once a day to keep solids from settling in the main.

GP versus STEP on a pressure sewer

Both types feed the same kind of main. In a GP system the grinder station on each lot does the grinding and sends the slurry straight into the pressurized main. In a STEP system, a septic tank captures the solids and an effluent pump sends the liquid on, which the EPA says better tolerates low flows in areas with highly fluctuating seasonal occupancy, at the cost of pumping the tank every two to three years. The EPA puts on-lot facilities, meaning the pumps and tanks at each home, at more than 75 percent and up to 90 percent of a pressure sewer's total cost, which is why the on-lot choice gets so much attention. The STEP guide takes the comparison further.

Where low-pressure sewers make sense, and their drawbacks

The EPA says pressure sewers are most cost effective where housing density is low, where terrain undulates with relatively high relief, where the outfall sits at the same or higher elevation than the service area, and where flat terrain is combined with high ground water or bedrock, making deep cuts or multiple lift stations excessively expensive. It also lists the costs: many mechanical components across the service area that need institutional involvement, higher operation and maintenance cost from the number of pumps, power outages that can cause overflows without standby generators, higher life-cycle replacement cost, and odor and corrosion from septic wastewater that require venting and non-corrosive parts.

For the homeowner, those bullets translate to one sentence: on a low-pressure sewer, part of the public sewer lives in your yard and runs on your electricity, and you are expected to know what to do when it alarms.

Who runs low-pressure sewers around Mobile Bay, and what each requires

The table summarizes published utility rules. Confirm responsibility for a particular address when it is inferred from the service-line boundary or the town has more than one provider.

  • Baldwin County Sewer Service (Spanish Fort, Summerdale, Elberta, Silverhill, Perdido Beach and much unincorporated Baldwin County, including Fort Morgan and Lillian): a grinder pump is required at every home or building that connects inside its low-pressure territory; progressive cavity design; owner's pump, repairs by BCSS or a plumber; starting January 1, 2026, BCSS supplies and installs every pump itself.
  • Gulf Shores UtilitiesOwner provides, constructs, operates and maintains a grinder system to GSU's specification, minimum 2 hp, 10 gpm at 90 feet of head, 115-gallon tank, 1-inch discharge with check valve and anti-siphon device; call GSU any time the red light shows.
  • City of Orange Beach (its collection system is low-pressure and gravity lines in the city plus low-pressure lines to Ono Island and the Miflin Creek area): owner buys and maintains the pump to the same 2 hp, 10 gpm at 90 feet, 115-gallon minimum; installation by the city only.
  • Riviera Utilities and the City of FoleyCustomer owns everything on the customer's side of the shut-off valve, pumps, floats, panels, tanks and power supply included; written approval before connection; grinder tanks of at least 150 gallons, 2 hp, generator receptacle; STEP accepted; the utility will not repair or replace.
  • North Baldwin Utilities (Bay Minette and the northern county)Customer-owned grinder delivering at least 7 gpm at 185 feet; NBU's pressure sewer responsibility ends at the customer's property line; STEP components need NBU approval.
  • City of LoxleyOwner's pump, at least 7 gpm at 185 feet.
  • Daphne UtilitiesLow-pressure systems with multiple grinder pumps are not acceptable in its service area unless Daphne Utilities approves; city code says neither the city nor the utilities board maintains such pumps.
  • Fairhope UtilitiesThe customer maintains the service line from the right-of-way connection into the premises; a State Licensed Master Plumber certifies each grinder installation on the city's form.
  • MAWSS (Mobile)A small percentage of customers connect by grinder pumps that MAWSS does not own; the customer obtains, installs and maintains the pump; low flow, high head pump required where the force main serves other developments; MAWSS maintains the tanks and pumps only in its decentralized subdivisions.
  • Mobile County Water, Sewer and Fire Protection Authority (south Mobile County)A Barnes low-pressure system of about 500 grinder stations, installed from December 2014 in a high-water-table area; stations owned by the homeowner but maintained by the Sewer Department, with possible charges for pump repair or replacement.
  • South Alabama Utilities (Semmes, Citronelle)Effluent (STEP) sewer, not grinder; SAU maintains the pump and panel and pumps the tank.

Questions people ask

What is a low pressure sanitary sewer system?

A sewer in which each connected home pumps its wastewater into a small pressurized main instead of draining by gravity. The pump at each home is a grinder pump (GP system) or a septic tank effluent pump (STEP system).

How much pressure is in a low pressure sewer?

The EPA's operations study puts GP sewers at 40 psi or less in normal service, with 1-1/4 inch service lines. The pressure at any one house rises when more pumps on the same main run at once.

What is the difference between a force main and a low pressure sewer?

A force main is any pressurized sewer pipe, including the one leaving a lift station. A low-pressure sewer is a collection system built from force mains fed by home pumps. MAWSS treats a low-pressure system fed from a shared force main as the harder case and requires low flow, high head pumps there.

Who maintains a low pressure sewer system?

The utility maintains the mains; the homeowner maintains the station on the lot under almost every local rule. The exceptions in our two counties are South Alabama Utilities' effluent systems, the Mobile County authority's Barnes stations, and MAWSS's decentralized subdivisions.

Are low pressure sewer design guidelines published locally?

Yes, by the utilities. Gulf Shores Utilities, the City of Orange Beach, Riviera, North Baldwin and Loxley publish minimum head, flow, horsepower and tank sizes for home stations, and Daphne Utilities publishes design criteria that restrict multi-pump low-pressure systems. The EPA fact sheet gives the general design approach.

  1. Who Owns Your Grinder Pump? Rules by Utility, Quoted
  2. BCSS
  3. MAWSS
  4. Lift Stations: What They Are, How They Work, Who Owns Them
  5. Grinder Pump vs Septic STEP System: Which Is Which
  6. What Is a Grinder Pump? Parts, Purpose and Who Owns It
  7. Grinder Pump Installation for New Homes and Additions
  8. Grinder Pump Discharge Line Repair (Pressure Service Line)

Sources and facts

  1. EPA: conventional gravity systems are reliable and consume no power but slope requirements may require deep excavation in hilly or flat terrain and pump stations; pressure sewers use smaller-diameter pipes with a slight slope or following the land's contour; two major types are STEP and GP; pressure sewers break down large solids in the pumping station and their watertight design with no manholes eliminates extraneous flows, so they may be preferred where high groundwater could seep in. www.epa.gov/… (EPA Wastewater Technology Fact Sheet: Sewers, Pressure, 832-F-02-006, Description) (read 2026-11-05)
  2. EPA applicability, advantages and disadvantages: most cost effective at low density, undulating terrain, outfall at same or higher elevation, or flat terrain with high ground water or bedrock; disadvantages include institutional involvement, higher O&M, overflows in power outages without standby generators, higher life-cycle replacement cost, odor and corrosion; the number of pumps sharing a downstream force main is limited; clean-outs and valve assemblies replace manholes; air release valves are often vented to soil beds. www.epa.gov/… (Applicability; Advantages and Disadvantages) (read 2026-11-05)
  3. EPA design criteria: GP systems must attain three to five feet per second at least once per day; pressure mains generally use 2-inch or larger PVC (SDR 21) or HDPE with fused joints; on-lot facilities account for more than 75 percent and up to 90 percent of total cost; STEP better tolerates fluctuating seasonal occupancy; STEP tanks pumped at two to three year intervals. www.epa.gov/… (Design Criteria; choice factors) (read 2026-11-05)
  4. EPA O&M study: the GP sewer is normally under low pressure, 40 psi or less; pipe sizes normally 1-1/4 inches for service lines and 1-1/2 to 6 inches for the collection system; grinder pumps placed at each home above or below ground serving one or more residences. nepis.epa.gov/… (read 2026-11-05)
  5. Liberty LSG manual: for pressure sewer applications verify a redundant check valve assembly (curb stop and check valve) between pump discharge and street main, as close to the right-of-way as possible. E-One requires its stainless check valve between the station and the street main. www.libertypumps.com/… p.1; eone.com/… p.1 (read 2026-11-04)
  6. MAWSS Policy 2.19: grinder on gravity sewer or on an LPS served by adjacent gravity sewer may be any accepted Appendix A Part III pump; on an LPS served by a force main that may serve other developments it must be a low flow/high head pump; the Board will not maintain the grinder station. MAWSS FAQ: a small percentage of customers connect by grinder pumps not owned by MAWSS; the customer obtains, installs and maintains it. Policy 2.21: MAWSS maintains septic tanks and pumps in its decentralized subdivisions. www.mawss.com/… p.20 and section 2.21; www.mawss.com/residential/service-support-faqs (read 2026-11-04)
  7. BCSS sewer territory: city limits of Spanish Fort, Summerdale, Elberta, Silverhill and Perdido Beach plus many unincorporated areas; each home or building connected within its low-pressure territory requires a grinder pump; progressive cavity required when BCSS does not supply the pump; BCSS not responsible for pump systems on private property; from January 1, 2026 all pumps bought from and installed by BCSS. www.baldwincountysewer.com/…; www.baldwincountysewer.com/…; www.baldwincountysewer.com/… (read 2026-11-05)
  8. Gulf Shores Utilities: applicant provides, constructs, operates and maintains the grinder system; minimum 2 hp, 10 gpm at 90 ft TDH, 115-gallon tank, check valve and anti-siphon device, 1-inch discharge; contact the Board any time the red light is on. www.gulfshoresutilities.com/grinder-specs; www.gulfshoresutilities.com/grinder-pump (read 2026-11-05)
  9. City of Orange Beach: the collection system is comprised of low-pressure and gravity lines, mostly in the city limits, plus low-pressure lines to Ono Island and the Miflin Creek region; Code 74-36: owner buys the pump, minimum 2 hp simplex, 10 gpm at 90 ft TDH, 115-gallon tank; installation by the city only; pump is the owner's sole responsibility. www.orangebeachal.gov/278/Utility-Billing; Orange Beach Code of Ordinances section 74-36 (read 2026-11-04)
  10. Riviera Utilities: the customer is responsible for the entirety of the low pressure sewer system on the customer's side including pumps, floats, control panels, pipes, fittings, valves, tanks and power supply; Riviera and the City of Foley will not repair, replace or be responsible for it; written approval required; tanks minimum 150 gallons; generator receptacle with manual transfer breaker; STEP systems accepted. www.rivierautilities.com/… sections 7.2-7.4; www.rivierautilities.com/… (read 2026-11-04)
  11. North Baldwin Utilities: customer-provided grinder pump with at least 7 gpm against 185 ft; NBU Pressure Sewer System responsibility terminates at the customer's property line; all STEP components approved by NBU before installation. City of Loxley: owner's pump, 7 gpm at 185 ft. www.northbaldwinutilities.com/… (read 2026-11-04)
  12. Daphne Utilities design criteria: low pressure (multiple grinder pumps) systems shall not be acceptable within the City of Daphne, its planning jurisdiction and the Daphne Utilities service area unless approved by Daphne Utilities; city code: neither the City of Daphne nor the utilities board is responsible for upkeep or maintenance of such pumps. daphneutilities.com/…; Daphne city code (read 2026-11-04)
  13. Fairhope Code section 21-52: the customer is responsible for all maintenance of the service line from the connection up to and within the premises; the Grinder Pump Installation form requires the State Licensed Master Plumber to certify the installation. library.municode.com/…; www.fairhopeal.gov/… (read 2026-11-04)
  14. Mobile County Water, Sewer and Fire Protection Authority: Barnes low pressure sewer system, about 500 grinder stations installed from December 2014 in a high-water-table area; stations owned by the homeowner but maintained by the Sewer Department; there may be service charges for pump repair and/or replacement. cranepumps.com/…; empoweringpumps.com/…; MCWS customer page (read 2026-11-04)
  15. South Alabama Utilities runs effluent (STEP) sewer systems that it services and maintains, including the pump and control panel, and pumps the tank when necessary. www.southalabamautilities.net/… (read 2026-11-04)

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