While gravity sewer systems are the municipal standard—relying on continuous downward slope and larger pipe diameters to carry raw sewage—engineers use four primary alternative wastewater collection systems when terrain, bedrock, high water tables, or low housing density make deep gravity trenching impractical or cost-prohibitive:

1. Low-Pressure Sewer Systems (LPSS) — Grinder vs. STEP
2. Force Mains & Lift Stations (Pumped Gravity Hybrids)
3. Vacuum Sewer Systems
4. Small-Diameter Gravity Sewers (SDGS / Effluent Sewers)

1. The Alternative Sewer Systems

Low-Pressure Sewer Systems (LPSS)

Instead of relying on a continuous downward grade, LPSS networks use small-diameter pressurized pipes ($1.25\text{ to }4\text{ inches}$), usually made of HDPE or PVC, buried just below the frost line following the natural contour of the land. Each property has an on-site unit that pressurizes the line:

• Grinder Pump Systems: Raw sewage flows from the building into a small buried wet well containing a heavy-duty grinder pump. The pump shreds solids into a fine slurry and pumps it directly into the pressurized main.
• STEP (Septic Tank Effluent Pump) Systems: Raw waste enters an on-site septic tank first. Solids settle and digest in the tank, while a high-head effluent pump inside a filter vault pushes only the clarified liquid (effluent) into the small-diameter pressure main.

Force Mains & Lift Stations (Hybrid Gravity-Pressure)

Even in standard municipal gravity networks, pipes eventually get too deep to excavate economically (often beyond $20\text{ to }30\text{ feet}$) or need to cross a valley or ridge. At a lift station (pump station), gravity flow terminates in a central wet well. High-capacity municipal pumps then push the raw sewage uphill through a pressurized force main until it reaches a high point where it discharges back into a gravity manhole.

Vacuum Sewer Systems

Ideal for flat, coastal, or high-water-table areas with moderate-to-high density, vacuum systems rely on a central vacuum station that maintains negative pressure ($-16\text{ to }-20\text{ inHg}$) throughout a network of shallow, saw-tooth profiled PVC pipes.

• Wastewater flows by gravity from a home into a sealed vacuum valve pit at the property line.
• Once roughly $10\text{ gallons}$ of sewage collects in the sump, a pneumatic interface valve opens for a few seconds. Atmospheric air rushes in behind the sewage, propelling it into the main as a high-velocity liquid plug ($15\text{ to }18\text{ ft/s}$).

Small-Diameter Gravity Sewers (SDGS) / STEG

Also known as Septic Tank Effluent Gravity (STEG) systems, these are a hybrid of septic and gravity design. Each home has an intercepting septic tank that captures grit, grease, and settleable solids. Because only liquid effluent enters the collection network, the municipal pipes do not need to maintain the $2.0\text{ ft/s}$ self-scouring velocity required for raw solids. Pipes can be much smaller ($3\text{ to }4\text{ inches}$), laid at flatter grades, and even run with slight inflections as long as the overall hydraulic grade line drops from the inlet tank to the discharge point.

2. Key Differences: Gravity vs. Alternative Systems

The diagram below shows the classic gravity sewer lateral and main profile, where every foot of run requires a steady downward pitch toward the public sewer main and manhole access points:

Design & Operational Factor	Conventional Gravity Sewer	Low-Pressure (Grinder / STEP)	Vacuum Sewer System	Small-Diameter Gravity (SDGS / STEG)
Driving Force	Gravity (open-channel flow)	Positive pressure (on-site pumps)	Negative pressure (central vacuum station)	Gravity (hydraulic head of liquid only)
Conveyed Material	Raw sewage (solids + liquids)	Ground slurry (Grinder) or liquid effluent (STEP)	Raw sewage + atmospheric air	Clarified septic effluent only
Pipe Size (Mains)	$8\text{” to }36\text{“+}$ diameter	$1.25\text{” to }6\text{“}$ diameter	$3\text{” to }10\text{“}$ diameter	$3\text{” to }6\text{“}$ diameter
Trench Depth & Alignment	Deep ($6\text{ to }25\text{+ ft}$); strict straight lines & constant slope	Shallow (frost depth, $2\text{ to }4\text{ ft}$); follows terrain contours	Shallow ($3\text{ to }6\text{ ft}$); laid in “saw-tooth” lifts	Shallow-to-moderate; flexible horizontal alignment
Access Structures	Manholes every $300\text{ to }500\text{ ft}$ and at all bends/junctions	Cleanout/flushing pig stations & air release valves (no manholes)	Inspection boxes & isolation valves (no manholes)	Cleanouts and vents (rarely requires manholes)
Inflow & Infiltration (I&I)	High risk via aging joints, cracks, and manhole covers	Virtually zero (sealed, positive-pressure pipe)	Very low (leaks trigger vacuum alarms immediately)	Low (sealed small-diameter PVC/HDPE)
Capital vs. O&M Cost	High upfront excavation cost; very low ongoing O&M and power cost	Low upfront trenching cost; higher O&M (pump repairs, electricity)	Moderate-high equipment cost; centralized O&M at vacuum station	Moderate upfront cost; periodic septic tank pumping required
Rehabilitation & Maintenance	Standard CCTV inspection, jetting, and trenchless CIPP lining	Pigging (foam swabs), air-release valve flushing, pump replacement	Valve diaphragm servicing, vacuum leak isolation	Periodic flushing and on-site septic solids pumping every $3\text{ to }7\text{ years}$

3. Core Engineering Trade-Offs

• Excavation vs. Mechanical Complexity: Gravity systems front-load costs into civil earthwork—deep trenching, shoring, dewatering, rock blasting, and precise laser-grade bedding. Once installed, a gravity sewer has zero moving parts and can run for $50\text{ to }100\text{ years}$ without electricity. Alternative systems trade deep excavation for mechanical hardware (pumps, valves, sensors) that sit just below frost depth via open-cut narrow trenching or horizontal directional drilling (HDD).
• Hydraulic Grade vs. Minimum Scouring Velocity: Gravity mains must be sloped precisely to maintain a minimum self-cleansing velocity of $2.0\text{ ft/s}$ at daily peak flow so solids don’t settle and cause blockages, without exceeding $10\text{ ft/s}$ where turbulent erosion and hydrogen sulfide ($H_2S$) off-gassing strip concrete and mortar. Pressure and vacuum systems generate high velocities artificially during pump/valve cycles, allowing pipes to travel uphill.
• Septicity and Odor Control: Because gravity sewers flow partially full with an air space at the crown of the pipe, wastewater stays partially aerated. In force mains and LPSS pipes, the pipe is $100\%$ full (surcharged) with zero oxygen exchange. Long retention times in pressure mains cause anaerobic bacteria to thrive, generating corrosive $H_2S$ gas that rapidly degrades concrete receiving manholes at the point of discharge unless chemical dosing or air-injection systems are installed.