Drain Field

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Drainfield – Septic System Components – Part II Restore

Drainfield Introduction

The Septic System Owners Manual / Septic System Components: Drainfields – Part II

  • Learn about the different types of drainfields such as leach fields, seepage pit, dry well, vertical leach field, drainage field, leach bed, seepage bed, mound.
  • Discover that the primary function of all drainfields is to return wastewater to the ground.
  • Learn that drainfields are designed to perform a filtering function, cleansing the wastewater before it returns to the water table.
  • Learn that the most common cause of septic system failure is the drainfield becoming clogged and impermeable.
  • Discover that Aero-Stream can save any type of failed or failing drainfield by executing a controlled aerobic septic system conversion for under $1700!

Many people with septic system problems such as odor, slow draining sinks and tubs, gurgling pipes, backups and sewage water ponding in their yard mistakenly assume the cause of these issues is their septic tank. This, however, is not true.

The most common cause of septic system problems and failure is their septic system absorption component, more commonly known as a drainfield, becoming impermeable so that the wastewater can no longer be absorbed into the soil.

Ponding from Flooded Drain Field

Variations of Drainfield

There are many design variations of drainfields. A drainfield design depends upon many variables such as the size and topographical features of the lot, depth of the water table, soil conditions and types and other criteria. The size of the drainfield can vary infinitely, but most have minimum size requirements based on state and municipal codes.

The designs reviewed in this chapter are:

  • Gravity Drainfields
  • Pressure Distribution Drainfields
  • Cesspool (or Cesspit)
  • Drywell (or Seepage Pit)
  • Mound Systems
  • Holding Tank
  • Aerobic Treatment Unit (ATU) & Aeration Systems

Gravity Drainfields:

As implied, gravity drainfields work by allowing the effluent to flow from the septic tank into a series of trenches by gravity. To allow this, the drainfield must be below the outlet level of the septic tank.

A conventional gravity system consists of a network of distribution components laid in gravel filled trenches. The trenches are dug in natural, undisturbed soil typically 3 feet above any natural occurring restrictive layer, such as a hardpan or the seasonal high water table.

in situ anaerobic – typical
Typical anaerobic gravity drainfield returning wastewater to the water table

The undisturbed soil filters out the remaining solids in the effluent which are then processed by anaerobic bacteria transported from the septic tank in the effluent.The anaerobic biochemical process cleans the effluent before it reaches the groundwater.

Maximum effectiveness of the final treatment occurs when the soil is dry and permeable. The size of the drainfield depends on the estimated daily wastewater flow (generally based on the number of occupants), soil type, and soil conditions.

Pressure Distribution Drainfields:

A pressure distribution drainfield is installed when the site conditions do not allow the drainfield to be installed at a lower elevation than the septic tank outlet, preventing the effluent to flow to the drainfield by gravity.Pressure distribution systems are similar in design to a gravity field and differ only by the use of a mechanical effluent pump to distribute the effluent from the pumping chamber or settling chamber to the drainfield. The pump is activated intermittently by use of a mechanical float device in the chamber where the pump is located.

Drywell (or Seepage Pit):

A drywell is similar in design to a cesspool with an added layer of crushed stone around the outside diameter to increase the absorption area.

The drywell is typically a secondary chamber added in series after the septic tank or cesspool which allows only the clarified effluent from a septic tank or the gray water to enter.

Stressed anaerobic drywell returning wastewater to the water table.
Drywells can also be used to return storm water to the ground or to relocate basement drainage water to another location above the water table.

Mound Systems:

A mound system is used when a site has inadequate soil depth between the ground water table and the drainfield preventing the use of a gravity or pressure distributed drainfield buried below grade. A mound system consists of a drainfield constructed above the natural soil surface by means of a manmade mound which contains a specific sand fill material.
Stressed anaerobic mound returning wastewater to the water table and leaching wastewater to ground level.

Within the sand fill is a gravel bed with a network of perforated distribution components, typically PVC pipes. Septic tank effluent is pumped in controlled intervals through the pipes to insure uniform distribution throughout the bed.

Final treatment of the effluent occurs as it filters through the sand and into the natural soil. Drainage around the mound site is critical if the system is to function properly. On sloping sites the down slope area below the mound must remain protected.

Holding Tank:

A holding tank is a self-contained watertight sewage tank which has no outlet for effluent. It is most commonly used as a temporary measure to allow continued occupancy of a house until a more permanent fix can be arranged. Holding tanks must be routinely pumped to prevent overflows or back-ups into the house. Some schools and other public properties use this system when sewers are not available and no suitable site exists for an on-site septic system. Homeowners must post a bond for potential spills and have to have a contract with an approved sludge hauler.

Aerobic Treatment Unit (ATU):

An aerobic treatment unit introduces oxygen into any anaerobic septic system, allowing aerobic bacteria to flourish and convert the system from anaerobic to aerobic. The aerobic bacteria are far more efficient, processing up to 90% of the solids in the septic tank versus an anaerobic system which processes only 40% of the solids in the tank and allows 60% to flow to the drainfield, resulting in biomat clogging over time.


The drywell and mound designs are subject to the same failure mode over time; they eventually become impermeable so that the sewage outflow from your septic tank cannot be absorbed into the ground.

Fortunately, you do not have to replace your drainfield to return your septic system to an operational condition. Aero-Stream® has developed a product and process that will fix and restore any type of failed drainfield.

Learn more about our product and septic systems, in general, by navigating through our website, reading the Septic System Owners Manual, downloading the Septic System Report and calling us toll free.

The Septic System Owners Manual – Drainfield

Nobody plans for the expense of having septic tank problems. Whether your septic system is new or failing, this manual is a must read for any homeowner.Understand the causes and discover the solutions to your septic system and septic tank problems.

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