Mound Septic System Service

Why this matters

A mound system is the engineered solution for sites where conventional septic cannot work: high water table, shallow soils, slow-percolating clay. The drainfield is built ABOVE grade as a constructed sand/gravel mound, with effluent pump-fed under pressure to perforated laterals on top. Mound systems serve about 15 percent of US residential septic installs in regions with poor soils (Northeast, Upper Midwest, parts of the South). They cost 2 to 4x conventional drainfields to install (per the application) and require specialized service because the components (pump, controls, dosing system) differ from gravity-fed systems. A failed mound is a major remediation event. The contractor who understands mound systems can service the 1 to 3 percent of customers with them at premium rates because the supply of mound-capable techs is small.

When to service

• Annual scheduled service
• Pump alarm activation
• Surface effluent on or near the mound (rare; serious indicator)
• Customer reports slow drains or odor
• After significant weather events (heavy rain, freezing)
• At sale of property (mound inspection often required)

Components of a mound system

A typical mound system includes:

1. Septic tank (conventional, often with effluent filter)
2. Pump tank (separate from septic tank typically)
3. Effluent pump (delivers effluent under pressure to the mound)
4. Pressure distribution system (laterals on top of the mound with small orifices)
5. The mound itself: constructed sand/gravel bed, typically 18 inches to 6 ft above grade
6. Pump control panel (timer-based dosing in most installs)
7. Float switches (pump-on, pump-off, alarm)
8. Effluent piping (force main from pump tank to mound)

Tools and materials

• Full septic PPE
• Multimeter (electrical diagnostics)
• Pressure gauge (for verifying distribution pressure)
• Squirt height measurement tool (verifies orifice flow)
• Hose for cleanout flushing
• Pump tank service tools (see pump-tank-maintenance article)
• Effluent filter cleaning supplies

Procedure

Step 1: Pump tank service

Service the pump tank first (see Septic Pump Tank Maintenance article for full procedure):

• Inspect effluent in tank (level, color, clarity)
• Verify pump operates and pumps correctly
• Inspect float switches
• Verify alarm panel

Step 2: Verify pump dosing

Mound systems use timed dosing: the pump runs for a programmed duration (typically 1 to 10 minutes), then waits a programmed off-time (typically 2 to 6 hours).

• Note the configured dosing schedule
• Verify the schedule matches the system's design (per the original installer or current technician)
• Calculate dose volume: pump flow rate (gpm) x run time (minutes) = gallons per dose
• Verify dose volume matches the mound's per-dose capacity (orifice count x orifice flow rate)

Step 3: Manual pump cycle

Initiate a pump cycle and observe:

1. Pump starts smoothly
2. Discharge flow visible in cleanout (some mounds have a cleanout / observation port at the mound)
3. Pump runs the configured duration
4. Pump stops at end of cycle
5. Verify residual pressure in the force main bleeds correctly

Step 4: Inspect the mound surface

Walk the mound:

• Surface should be dry (no surface effluent, no visible wet zones)
• Grass over the mound should look healthy (slight green; no dead patches; no abnormally lush)
• No erosion or settling of the mound material
• No visible holes or animal damage
• No surface evidence of pipe break (small wet area following a lateral line)

Step 5: Verify lateral distribution

The pressure distribution system has perforated laterals with small orifices on top of the mound. Each orifice should deliver equal flow.

Some installs have an observation port at the end of each lateral allowing direct verification:

• Open the cleanout cap on the lateral end
• Observe the pressure / flow during a dose
• Each orifice should have similar squirt height (typically 1 to 4 inches visible during dose)

If one lateral has very different squirt height than another, the distribution is uneven. Causes:

• Clogged orifices (the small holes plug with biofilm)
• Lateral pipe damage (root intrusion, freeze damage)
• Pressure regulator failure (some systems have one)

Step 6: Cleanout flushing

For chronic uneven distribution, flush the laterals:

• Open the cleanout caps
• Flush water through the laterals at moderate pressure
• Allow debris to discharge out the open end
• Close and verify normal operation

Some technicians use a sodium hypochlorite or hydrogen peroxide flush to clear biological buildup in orifices.

Step 7: Verify the mound's hydraulic capacity

If the customer reports slow drains or chronic alarm activation, the mound may be operating at maximum capacity:

• Customer water use within design (typically 100 to 200 gallons per person per day)?
• Recent water-use changes (new appliances, leaks, occupants)?
• Mound's actual capacity per the original design

For undersized or overloaded mounds, the only fix is reducing inflow (customer behavior) or expanding the mound. Mound expansion is significant; better to manage inflow if possible.

Step 8: Documentation

Record:

• Date and technician
• Pump status (operating, fault code if any)
• Float positions
• Dose schedule
• Mound surface condition
• Lateral distribution observations
• Service performed
• Next service due

Common mound system issues

Pump cavitation

If the pump tank's water level drops below the pump intake during a cycle, the pump pulls air and cavitates. Causes:

• Excessive cycle length (pump emptied the tank before stopping)
• Low water level (insufficient gravity flow from septic tank into pump tank)
• Pump positioning too high in the tank

Force main freeze (cold climate)

In cold climates, the force main between the pump tank and the mound may freeze if:

• Not buried below frost line
• Insufficient flow / dwell time to keep warm

Insulation, deeper burial, or heated lines may be needed.

Surface effluent on mound

Indicates failure of the mound's treatment / infiltration capacity:

• Hydraulic overload
• Mound material has degraded (over decades)
• Mound was undersized for the customer's actual use

Surface effluent is a public-health issue; address immediately with reduced inflow + diagnosis.

Pump tank flooding

If the pump tank fills above the alarm float:

• Pump failure (won't run)
• Float switch failure (pump-on float stuck)
• Downstream blockage (force main, distribution)

Address immediately to prevent backup into the home.

Mound erosion

A mound's grass cover prevents erosion. If grass dies (improper care, drought) or fails to establish, erosion can expose the underlying sand. Reseed; restore grass cover.

References

• EPA Onsite Wastewater Treatment Systems Manual, Chapter 4 (Mound Systems).
• Wisconsin Mound Soil Absorption System Manual (industry reference).
• State / county septic regulations.
• Manufacturer literature: Orenco AdvanTex, Bio-Microbics, Geoflow distribution systems.
• Manuall internal: Septic Pump Tank Maintenance, Septic Drainfield Diagnostic.