Septic System Types Reference

Overview

Not all septic systems are identical. Understanding the type informs maintenance, troubleshooting, + replacement decisions.

Conventional gravity septic

The most common residential system. ~80% of US septic systems.

Components:

• Septic tank (concrete or polyethylene)
• Distribution box (D-box)
• Drainfield (trenches in suitable soil)

Operation:

• Wastewater flows from house to tank
• Solids settle; liquid flows to D-box
• D-box distributes to multiple field lines
• Field lines distribute over soil; soil absorbs + treats

Best for: good soil percolation, level/sloped terrain, normal occupancy.

Maintenance: pumping every 3-5 years.

Failure modes: drainfield clogs, tank fills with solids, biological mat overgrowth.

Pumped (pressure dosing) septic

Conventional but with pump.

Differences:

• Pump tank or pumps within main tank
• Effluent pumped to drainfield at set intervals
• More even distribution to field lines
• Better for poor-slope sites

Best for: sites where gravity doesn't reach drainfield; sloped lots.

Aerobic Treatment Unit (ATU)

Higher treatment than conventional.

How it works:

• Compressor pumps air into tank
• Aerobic bacteria (vs anaerobic in conventional) treat waste
• Treated water cleaner before discharge
• Some include disinfection (chlorine, UV)

Best for: poor soil sites, sites with high water table, environmentally sensitive areas, smaller lot sizes.

Maintenance: more frequent service + checks (typically annual contract required).

Cost: 2-3x conventional.

Sand filter septic

Effluent passes through sand bed for additional treatment.

How it works:

• Tank → pump → sand filter bed → drainfield
• Sand removes organics, suspended solids
• Better effluent quality than conventional

Best for: poor soil, high water table.

Mound septic

Drainfield built above natural ground level.

How it works:

• Tank → pump → mound (raised drainfield)
• Native soil augmented with sand + gravel
• For high water tables OR poor native soil

Best for: wet soils, high water table, slopes.

Maintenance: similar to conventional but field is visible (mound).

At-grade septic

Compromise between conventional + mound.

How it works:

• Drainfield at ground level
• Less material than full mound
• Suited to moderate site limitations

Drip distribution septic

Modern, low-pressure systems.

How it works:

• Tank → pump → drip emitters distributed throughout field
• Slow, controlled dispersal
• Less concentrated flow

Best for: sites where conventional doesn't work but mound is overkill.

Holding tank (no drainfield)

For sites where no drainfield possible.

How it works:

• Wastewater held in large tank
• Pumped out by service when full
• No treatment occurs in tank

Best for: small lots without space; lake/river-adjacent (regulatory).

Drawback: expensive (must pump frequently); customer dependent on service.

Composting toilet (not septic, but related)

Some rural/remote homes.

How it works:

• Toilet composts waste in unit below
• Liquid drains to gravel pit
• Periodic compost removal

Best for: off-grid, remote, environmental concerns.

Cesspool (legacy)

Older systems; mostly being replaced.

How it works:

• Single tank/pit; no field
• Effluent absorbs through tank walls
• No real treatment

Status: illegal in most areas; replacement required at sale.

Tank materials

Concrete:

• Most common
• 30-50 year life
• Can crack over time

Polyethylene:

• Increasingly common
• Lightweight
• More resistant to cracking

Fiberglass:

• Used in some applications
• Lighter than concrete

Steel:

• Older systems
• Corrodes; rarely installed today

System sizing

For residential, by household size:

Code typically requires sizing based on bedroom count (not actual occupancy).

Drainfield sizing

Depends on:

• Tank capacity
• Number of bedrooms
• Soil percolation rate
• Local code

Standard residential field: 600-1,500 sq ft.

Local engineer/regulator determines actual size based on soil tests.

Lifespan expectations

When to recommend each type

Conventional gravity: normal residential, good soil, level/sloped lot.

Pumped: sloped lot where field is uphill from tank.

ATU: poor soil percolation, high water table, smaller lot.

Mound: wet soil, high water table, where below-ground field won't work.

Sand filter: poor soil + budget below mound.

Holding tank: no space for field (rare in residential).

Failure modes by type

Conventional:

• Drainfield aging + biological mat
• Tank failure (cracks)
• Outlet pipe break

ATU:

• Compressor failure
• Disinfection failure
• Sensor problems

Mound:

• Sand layer degradation
• Pump failure
• Settling of mound

Holding tank:

• Tank failure
• Pump truck dependency

Regulatory requirements

Often:

• Permit for installation
• Inspection at install
• Periodic re-inspection
• Pump records required
• Failure must be reported

States/localities vary; check local code.

Replacement decision

Replace when:

• System failure (multiple problems, recurring)
• Aging system + significant repair needed
• Site change (occupancy increase, new bedroom)
• Regulatory order

Customer often surprised by cost; financing options:

• Home equity loan
• HUD 203(k) (renovation loans)
• State revolving loan funds (some areas)
• Cash payment

References

• EPA decentralized wastewater management
• NEHA (National Environmental Health Association)
• State + local regulations
• Manuall internal: Septic Tank Pumping Service SOP, Perform a Septic System Inspection