Words You Need To Be Familiar With

Scum – substances lighter than water (oil, grease, fats) float to the top, where they form a scum layer that floats on top of the water surface in a septic tank.

Sludge – the “sinkable” solids (soil, grit, unconsumed food particles) settle to the bottom of the tank and form a sludge layer.  The sludge is denser than water, so it forms along the tank bottom.

Effluent – the clarified wastewater left over after the scum has floated to the top and sludge has settled to the bottom.  It is the clarified liquid between the scum and sludge.  It flows through the tank outlet into the absorption area.

Absorption area – natural way of purifying wastewater when it leaves the septic tank, includes drainfields (leachfield or disposal field), mounds, seepage bed, seepage pits, cesspools.

Biological mat (biomat) – is a black, jelly-like mat that forms along the bottom and sidewalls of a drainfield trench.  It is composed of anaerobic microorganisms that anchor themselves to soil and rock particles and eat the organic matter in the septic tank effluent.

Aerobic – requires oxygen in order for the microorganisms to live.

Anaerobic – does not need oxygen in order for the microorganisms to live.



Septic Tanks

A septic tank is a watertight tank that is made out of durable material that does not corrode or decay most are concrete.  In the 1990s, two compartment tanks became the norm.  (Although properly functioning one-compartment tanks can still be acceptable).  Most septic tanks in Pennsylvania are 1,000-gallon tanks.  You should never enter a septic tank.  Septic tanks contain hazardous gases and should only be entered by properly trained professionals with the correct oxygen breath equipment.


Cross Section of Septic Tank


The first compartment consists of:

  • Inlet pipe allows the wastewater to flow into the inlet baffle.
  • Inlet baffle (right), which forces the water downward as it enters the tank, letting solids settle out more effectively.  It also prevents the wastewater from skimming across the tank and exiting untreated through the outlet.
  • Majority of Scum and Sludge Layers, the sludge (solids) will settle to the bottom and the scum layer will form on the top with water being in the middle.
  • The water will flow through a hole or a pipe in the wall that separates the first compartment from the second compartment.


The second compartment consists of:

  • Outlet baffle, which prevents the floating scum from moving into the absorption area.
  • Gas deflector that prevents gas bubbles (that solid particles often attach to) from leaving the tank by deflecting them away from the outlet pipe and preventing them from entering the absorption area.
  • Effluent filter (right) restricts and limits the passage of any suspended solids in the effluent.
  • Outlet pipe allows the effluent to flow to the absorption area.


The 5 Main Functions of a Septic Tank

  • Receives all wastewater from the house
  • Separates solids from the wastewater flow
  • Causes reduction and decomposition of solids
  • Provides storage for the separated solids (sludge and scum)
  • Passes the clarified wastewater (effluent) out to the absorption area


Aerobic Treatment Tanks

This is a mechanical system, that uses an air compressor or a churning propeller, to maintain an aerobic (oxygenated) environment for the growth of microorganisms.  Aerobic microorganisms more efficiently break down sewage than do the anaerobic microorganisms that are found in septic tanks.  Compared to septic tanks, aerobic tanks have higher initial costs and greater maintenance costs, but more efficiently breaks down sewage and have a higher quality effluent and fewer solids, which reduces the chances of a clogged absorption area.




A seepage pit consists of a deep hole 4-12 feet in diameter and 10-40 feet deep with a porous-walled chamber and a filling of gravel that is between the chamber and the surrounding soil.  Septic tank effluent enters the chamber and is temporarily stored there till it gradually seeps out and into the surrounding sidewall of soil.  Over time these systems have become less popular.

Cross Section of Seepage Pit


The earliest version of a septic system is the cesspool.  A cesspool is typically a cylindrical hole deep in the soil and several feet in diameter.  In most there is a porous inner wall of stone, masonry or other materials.  The outer surface (between the stone wall and the outer soil wall) is filled with gravel.  There is a concrete lid on the top and then soil is backfilled on the top of the lid.  Raw wastewater flows into the inner chamber and where solids are retained and partially digested. The effluent seeps through the stones and the gravel-filled outer chamber and then into the soil.

Today public health officials view cesspools as undesirable, basically because the inner wall of the cesspool is a leaky septic tank and the outer soil wall is the disposal field.

Cross Section of Cesspool


Sources of Household Sewage

Almost 20 million homes, approximately 29% of US population has an on-lot septic system.  Household sewage is a combination of wastewater from toilets, showers, sinks, washing machines, dishwashers, water softeners, and garbage disposals.



  • Raw or cooked meat
  • Varnishes, paints, solvents
  • Diapers
  • Antifreeze or motor oil
  • Paper towels or unapproved toilet tissue
  • Harsh or caustic drain cleaners
  • Cigarette filters and buttes
  • High foaming laundry detergents
  • Plastic
  • Bleach
  • Eggshells, bones, food scraps
  • Herbicides and pesticides
  • Coffee grounds
  • Cat litter
  • Excessive oils and grease

Common Septic System Do’s and Do Not’s


  • Clean and inspect your septic system on a regular basis
  • Minimize or eliminate the use of garbage disposals.  Garbage disposals add extra solids that are not easy broken down in the septic system

Do Not’s

  • Do not connect roof drains and/or yard drains into your septic tank.  The extra water will flood the tank and absorption area
  • Tree roots will clog the pipes in the absorption area so, do not plant trees near the absorption area
  • Vehicles and heavy objects (such as swimming pools) should not be put on top of your septic tank or absorption area


Common Septic System Myths

1. If I flush yeast, buttermilk, or commercial products into my septic system I will not have to have it pumped.

WRONG, no scientific research has been able to find that these additives are helpful to your septic system.  However, it has been found that these additives will damage your septic system.  The additives agitate the solids in the septic tank instead of allowing them to float to the top or settle at the bottom of the tank.  This agitation suspends the solids.  They are then flushed into the absorption area and clog the pipes and soil pores.  This clogging of the pipes and soil pores can lead to failure of your septic system and costly replacement.

2.  If my septic tank is failing I should flush it with large amounts of water.

WRONG, your septic tank is probably failing because of too much solids in the septic tank or a clog in the absorption area.  Flushing the system will force the solids into the absorption area furthering the damage to the absorption area.

3. My septic system is healthy because the grass is bright green over the leach lines.

WRONG, this is a sign that the effluent is probably not flowing down into the soil the way it needs to.  Also, look for marshy areas and standing effluent.



What is involved in a septic tank inspection?

The inspector will do a visual inspection of your septic tank and absorption area.

The inspection of your septic tank will involve the following:

  • Measure and make a record of the scum and sludge levels
  • In most cases pump out your tank
  • Check the baffles in your tank (this is to make sure solids are not exiting your tank)
  • Check the tank for cracks, leaks, or infiltration
  • Make an assessment of the tank’s construction and installation (this will check for any sensitivities or future problems)

The inspection of your absorption area will involve the following:

  • Check for signs of system failure (such as odors, mushy spots, effluent at the surface)
  • Surface water (shows poor filtration)
  • Check for proper effluent distribution
  • Check for potentially harmful shrubs, trees, or any other hazards in the absorption area

The inspector will write up a report about the inspection and about your septic system.  This report is not a warranty it is to inform you that your septic system is in proper or improper working order at the time the inspector did the inspection.


When should I have an inspection done?

Your septic system should be inspected on a regular basis.  It is recommended that the septic system be inspected every 1 to 3 years.  It is always an opportune time and recommended to inspect your system when it is being cleaned out.  It is highly recommended that you have the septic system inspected before you purchase a home.

Septic tank pumping frequencies table



Where does the wastewater go when it leaves the tank?

After the septic tank the clarified wastewater (effluent) will flow into the soil absorption system.  The most common type of soil absorption area is called the drainfield or leachfield, but seepage beds are also used.



The drainfield is designed to discharge the septic tank effluent below ground into the natural soil for treatment and disposal.  Typically a drainfield will consist of several relatively small and shallow trenches with gravel in them.  A perforated pipe will run near the top of the gravel to distribute the effluent throughout the length of the trench.

Drain Field


How does the drainfield work?

            The effluent will come out of the septic tank through the outlet pipe and will continue to run through a watertight pipe to the drainfield trenches.  When it reaches the trenches it will flow through the perforated pipes in the gravel.  The effluent will flow out of the perforated pipes and through the gravel and seep into the soil beneath and beside the perforated pipes.  The main purification of the effluent takes place through filtration and the biological activity of microorganisms breaking down the remaining solids in the effluent as the effluent flows through the biological mat on the sidewalls and the bottom of the trench and then through unsaturated soil.  The purified liquid will then eventually evaporate, be taken up by plants, or work its way into the groundwater.



A seepage bed is generally one large excavation.  The bottom is covered in a layer of gravel and multiple pipes are laid on the gravel with 3-5 feet of spacing between the pipes.  The pipes are covered with gravel and a protective layer of fabric, straw or untreated building paper.

Seepage Beds



Most conventional systems use gravity to deliver the effluent from the treatment tank to and through the absorption area.  Even distribution of the effluent is key in order for the system to function properly.  Uneven effluent distribution leads to the over load of the absorption area and can eventually result in major problems and costs.  Most in ground trench or bed systems use gravity distribution.  In all trench systems and some bed systems, a distribution box is used to divide the flow equally.  Some systems require that the effluent be pumped to the absorption area.  This occurs when the effluent cannot use gravity to flow to the absorption area, usually because it needs to be pumped up hill.


What happens if you do not maintain your on-lot wastewater treatment system?

Effluent will start to back up into your yard.



Your filter can become clogged and will back up your septic tank causing serious damage or the need to replace your septic tank.




This septic tank went unpumped for 15 years and is now completely filled with solids.




Links to other helpful sites







www.psma.net  & The Homeowner’s Sewage Disposal System Handbook