PASEO Training Tutorial #2  — Septic Tanks

1.0    Introduction:

A septic tank is one of two types of treatment tanks used to treat on-lot sewage in Pennsylvania.  While the other type of treatment tank (aerobic tank or aerobic treatment unit) introduces the sewage to air, a septic tank works to decompose the sewage waste in an anaerobic, or septic, environment.

2.0   How Septic Tanks Work:

When sewage is discharged from a structure, it flows through the building sewer and into the septic tank.  The sewage hits a baffle and is deflected downward.  Components of the sewage waste that are lighter than water (e.g. fats, oils, and grease) float to the surface and form a slowly degrading mat that is called “scum”.  Components of the

sewage waste that are heavier than water sink to the bottom of the tank and become “sludge”.  Anaerobic bacteria work to decompose the sludge reducing its volume.  As new sewage is added to the septic tank and equal volume of effluent, or clarified liquid, is displaced and passes out of the tank. 

The anaerobic decomposition of the sludge produces gasses.  As the gasses rise, they create turbulence in the tank.  Smaller particles of sludge often become suspended in the effluent.  When new sewage is added to the tank, suspended solids may get flushed out of the septic tank.  For that reason, today’s regulations require a second septic tank or septic tank compartment to settle out and treat these suspended solids.

3.0   Types of Septic Tanks

Septic tanks are usually made of pre-cast concrete, steel, and various plastic materials.  They may be rectangular or cylindrical in shape.  Some are single compartment, others multiple compartment.  While in many cases the material or configuration is not important, the sewage enforcement officer must be aware that certain applications have specific tank requirements.  Furthermore, there are often installation instructions specific to the individual brands of septic tanks.

Important:  A septic tank is a complete unit having an inlet baffle, an outlet baffle and space to store the sewage waste.  It may be constructed as a single unit or as a multi-compartmented tank.  For purposes of this tutorial, unless otherwise specified, individual tanks and tank compartments will be considered the same.  

Two separate tanks making one septic tank (left pair)   One dual compartment tank making one septic tank (right pair)

4.0  What Do the Regulations Say:

Section 73.31 contains the regulations that address all septic tanks and specific regulations that are applicable to concrete and steel tanks.  Click here to read the regulations.

5.0 How the Regulations Apply to Septic Tanks

     5.1  Septic Tank Sizing

The total capacity of the septic tanks is based on the estimated flow of the structure.  The chart found in § 73.31 (a) (3) determines the minimum size of the total septic tank capacity. 

Design Flow in Gallons Per DayMinimum Tank Capacity in Gallons
0 – 500(3.5 x flow exceeding 400 g.p.d.) + 900
500 – 5000(1.5 x flow exceeding 500 g.p.d.) + 1250
5000 – 7000              (1.45 x flow exceeding 5000 g.p.d.) + 8000
7500 – 10000(1.35 x flow exceeding 7500 g.p.d.) + 11625
10,000+(1.5 x daily flow)

Example #1 — a 3 bedroom single family house would have a design flow of 400 gallons per day.

3.5 x the flow over 400 gallons per day + 900 = min. septic tank capacity

3.5 x 0 + 900 = 900 gallon minimum capacity

Therefore, the minimum capacity for the septic tanks in Example #1 would be 900 gallons.

Example #2 —  A 5 bedroom single family residence would have a design flow of 600 gallons per day.

1.5 x the flow exceeding 500 g.p.d. + 1250 gallons = septic tank capacity

1.5 x 100 = 150   150 +1250 = 1400 gallon minimum septic tank capacity

In Example #2, the larger estimated sewage flow for a 5 bedroom house would require a minimum septic tank capacity of 1400 gallons.

Important:  The first tank or compartment in a series shall be referred to as the primary tank or compartment.  The second tank or compartment will be referred to as the secondary tank or compartment.

§ 73.31 (b) (4) requires that the septic tank must comprise at least two tanks/compartments and no more than four.  The secondary tank (and subsequent tanks, if applicable) must be at least half the capacity of the primary tank. While there is no specified minimum size for a tank or compartment, the requirements for the construction of a tank/compartment creates a de facto minimum size of 168.5 gallons.  Clearly a tank this size would be too small to be practical and the 900 gallon could not be met even with four tanks. It is recommended that 450 gallons be the minimum sized tank used in residential systems.

Example #1 — a 3 bedroom single family house would require a 900 gallon minimum septic tank.  As such, the following configurations could be considered:

450 gallons + 450 gallons

600 gallons + 300 gallons

300 gallons + 300 gallons + 300 gallons

Example #2 — a 5 bedroom single family house would require a 1400 gallon minimum septic tank.  As such, the following configurations could be considered:

1000 gallons + 500 gallons

750 gallons + 750 gallons

500 gallons + 500 gallons + 500 gallons

5.2     Septic Tank Construction

There are four types of materials used to construct tanks: precast concrete, constructed on-site masonry, steel, and various plastic / fiber-glass materials.  First this tutorial will discuss the requirements for the structural construction of a septic tank.  Then it will discuss the non-structural requirements.

5.2.1  Precast Concrete Septic Tanks

Precast concrete septic tanks are constructed by pouring cement into a mold(s).  When the concrete has set, the tank is assembled and shipped to the job site.  In some cases, the septic tank will arrive in two pieces (top and bottom) which will be placed together during installation.

A typical precast concrete septic tank

§ 73.31 (b) (i) and (ii) require that the walls of a precast tank be a minimum of 2.5 inches thick and adequately reinforced.  If the tank is covered by a slab, it must be 3 inches thick and adequately reinforced.

The top tank drawing represents a monolithic cast tank with a slab top.  The bottom represents a two piece tank, joined in the center.  Areas colored blue must be 3 inches thick while areas in red may be 2.5 inches thick.  Cylindrical tanks that are made of precast concrete must also meet these requirements.

5.2.2  Constructed On-site Masonry Septic Tanks

With the popularity of large plastic and fiber-glass tanks, SEOs are not likely to encounter large septic tanks that are constructed on-site.  However, SEOs should be aware of the rules concerning their use.  § 73.31 (b) (iv) limits the use of these tanks to where the tank will exceed 5000 gallons and will be constructed on-site.  When such a tank is proposed, the SEO must ensure that the tank will meet the National Concrete Masonry Association standards for reinforcement and waterproofing.  The standards may be found in their publication, Concrete Masonry Foundation Walls.

5.2.3  Steel Septic Tanks

Another type of septic tank that is becoming more rare with the advent of plastic and fiber-glass tanks are those made of steel.  Steel tanks must meet the United States Department of Commerce Standard # 177-62.  As such, steel septic tanks may not be constructed from tanks that were created for another purpose. 

The corrosion and holes on this recently excavated septic tank demonstrate why steel tanks

are rarely used these days.

5.2.4  Plastic / Fiber-glass Septic Tanks

Septic tanks may also be made of various types of plastic, fiber-glass, and similar materials.  To simplify matters, this tutorial will refer to tanks made from such materials as “plastic” tanks, regardless of the actual material used in their manufacture.

A typical plastic septic tank

Steel tanks must meet the federal standard.  Precast concrete tanks are heavy and generally are only sold in a limited area so it is in the best interest of the tank manufacturer to design the tank to meet state regulations.  But being relatively light, plastic tanks may be manufactured in other states, or even foreign countries.  Installers have also been found converting plastic tanks that were constructed for other purposes into septic tanks.  It is very important that the SEO carefully examine plastic tanks during the inspection process.

The sewage regulations do not specifically address plastic tanks.  § 73.31 (b) (1) states, “ Tanks shall be watertight and constructed of sound and durable material not subject to excessive corrosion or decay.”   Plastic tanks can be made watertight and plastic materials are not subject to corrosion.  However, “sound and durable” may pose a problem.  Unlike precast concrete tanks where the minimum wall and slab thickness is specified, there are no regulatory specifications for plastic tanks.  As such, the SEO will need to use some discretion when approving or disapproving the use of a particular tank.  Beware, however, in using this discretion the SEO may be required to defend the determination in an appeal hearing or court of law.

Plastic tank manufacturers usually have a set of requirements that must be adhered to when setting their tank.  Some of the requirements could include:

  • The maximum depth below finished grade in which the tank may be set.
  • The required use and the minimum amount of a particular bedding material (e.g. sand) under the tank.
  • Specifications for the type of backfill that may be used around or over the tank.
  • Requirements that water be added to the tank when it is being backfilled.
  • Requirements that the bottom of the tank be placed higher than the seasonal high water table.

Failing to install the tank correctly, may result in the septic tank collapsing or floating out of the ground during periods or wet weather. In addition, incorrect installation will usually void the tank’s warrantee.  An SEO may obtain the installation requirements for a particular septic tank by visiting the supply house that sells the tank or by visiting the tank manufacturer’s website.

5.2.5  Non-structural Requirements for Septic Tanks

§ 73.31 (c) establishes the standards for the non-structural construction of the tank. 

§ 73.31 (c) states that the invert (bottom) of the septic tank’s inlet pipe shall be a minimum of 3 inches higher than the invert of the tank’s outlet pipe.

The inlet pipe must have a baffle that extends below the liquid level by at least 6 inches, but may not exceed the depth of the outlet baffle.  A 4 inch maximum diameter inspection port is required above the inlet baffle.  The inspection port must be brought above the final grade and be secured with a cap.  This is not a requirement in all states so the SEO should take special care to check this if the installer is using a plastic septic tank or if the septic tank is imported from another state.

The outlet baffle of each tank or compartment must extend at least to a depth of at least 40% of the liquid depth.  If the tank is cylindrical in shape and installed horizontally, the baffle need only penetrate 35%.

Both the inlet baffle and the outlet baffle shall extend at least 1 inch above the liquid depth.

Gas venting shall be provided between each tank or compartment.

As sewage waste decomposes, it produces gases.  Lighter than the effluent, the gasses bubble up to the top of the tank.  Gasses lighter than air (e.g. methane) backflow through the baffles into the building sewer and escape into the atmosphere via the plumbing’s roof vents.  Gasses heavier than air (e.g. hydrogen sulfide) pass through the baffles and are discharged into the absorption area or trapped in the dosing tank.

The outlet baffle of each tank must be equipped with a solids retainer.  A solids retainer is a device intended to deter particulate matter from escaping the when such solids are in suspension due to turbulence caused by decomposition gasses bubbling up from the bottom of the tank.  Because of this, solids retainers are sometimes referred to as gas deflectors.

The solids retainer may be as simple as a plastic plate under the baffle or as complex as an effluent filter. 

The first picture above shows a solids retainer that consists of a plastic plate below the baffle.  The second picture shows an effluent filter that may also serve as a solids retainer.

Access must be provided to each tank/compartment.  If the access hole is square the minimum dimensions shall be 20 inches by 20 inches.  If the access hole is round, it must be at least 20 inches in diameter.  If the access hole will be below ground, it may not be buried deeper than 12 inches.  The access may be extended to grade.  If so, the lid must be of sufficient weight or securely locked against unauthorized entry.  Lids that are above the surface shall have the ground graded to divert rain runoff away from the access.

5.2.6  Septic Tanks Used for Alternate Systems

Citing studies that show that rectangularly shaped tanks may perform better than cylindrically shaped tanks, DEP generally prohibits the use of cylindrically shaped tanks for installations classified as alternate or experimental systems.  SEOs should ensure that the alternate or experimental permits they issue specify that only rectangularly shaped septic tanks may be used and follow up on this during installation inspections.

5.2.7  Effluent Filters as Septic Tank Baffles

§ 73.31 (c) (v) requires the use of a solids retainer on the outlet baffle of the last septic tank/compartment.  An effluent filter may substitute for a solids retainer.  Some system manufacturers may go so far as to require particular makes and models of effluent filters be used.  Not all effluent filters are equal as filtering quality can vary. The SEO should be aware of the requirements for the particular type of sewage system that will be installed.

Effluent filters must be cleaned periodically.  Therefore, an appropriately sized access must be provided above the filter.  Do not allow a filter to be installed that can not be removed from outside the septic tank.

An effluent filter serving as an exit baffle.  Note the large tank opening to facilitate removal of the filter.

5.2.8  Installation of Piping

After the septic tank has been set, the piping must be installed.          § 73.21 (j) states that “Building sewers shall be connected to treatment tanks by means of water-tight mechanical seals or hydraulic grouting. [The] Use of Portland cement grouting is not permitted”.  Properly sealing the tank is necessary to prevent effluent from leaking out of the tank or groundwater seeping in.

Sealed with hydraulic grout 
Sealed with a mechanical seal

6.0   The Duties of the Sewage Enforcement Officer

The SEO is responsible to inspect the septic tank to ensure that it complies with the plan requirements and the regulatory specifications. In addition, the SEO should:

  • Check to see that the tank has been set level.  Tanks that are set unevenly could result in short circuiting (solids or scum escaping the tank) or effluent back up into the building sewer.
  • Check the septic tank interior for any cracking that may have occurred when the tank was shipped or set.  (Note:  Look from outside the tank.  Never enter the tank.)

7.0   How to Do it Better

The regulations provide the minimum standards for installing a septic tank.  There are other things that the installer can do to improve the quality of the job.  Remember, unless required by the local ordinance, the SEO may only suggest that the installer do the following:

  • Bedding the tank:  The SEO should encourage installers to place a 6 to 12 inch layer of sand or gravel in the hole before setting the septic tank.  This bedding will make it easier to level the tank.  The sand or gravel will also cushion the tank and lessen the chances of the tank cracking. 
Red represents gravel bedding the septic tank.

  • Tamping the over dig:  It is common for the backfill around a septic tank to settle over time. When the soil under the pipes settles, the weight of the soil over the pipe as well as extraneous weight such as people walking over the pipe may cause it to flex.  This may result in the pipe sagging and/or the septic tank grout cracking.  Pipe sagging may be avoided by tamping the fill at the ends of the tank to reduce the setting.  The pipes entering and leaving the tank should be bedded into the tamped soil.
Soil placed in the area marked in red should be tamped to reduce settling.

  • Water test the tank:  Septic tanks may leak.  The leaks may be the result of improper construction, cracks from transporting or setting the tank, insufficient caulking of the seals, or failure to properly seal the inlet and outlet pipes.  A leaking tank may allow sewage effluent to seep into the soil without the proper treatment.  It may also result in groundwater to enter the tank and disrupt the treatment process.  As such, DEP has suggested the following process to test tanks after they have been set and the inlet and outlet pipes have been installed:

1.  Set tank.

2.  Install inlet and outlet pipes.  Cap the ends.

3.  Install any risers.

4.  Fill with water to a point at least 2 inches above the highest joint on the risers.

5.  Allow to stand for 24 hours.

6.  Refill with water to the original level.

7.  Allow to stand for 1 hour.

8.  If the water level has lowered, the tank is not properly water-tight.

Again, these suggestions should not be enforced unless they are required by the local sewage ordinance. 

8.0   Formulas for Determining Tank Sizes

8.1  Calculating the Internal Capacity of a Rectangular Septic Tank per Inch of Depth

1.  Measure the length and width of the tank exterior

2.  Subtract the thickness of the tank walls.

3.  Use the chart to determine the gallons per inch.

4.  Measure depth of tank below the outset invert (bottom of outlet pipe).

5.  Multiply gallons per inch by the number of inches below the outlet invert.

6.  The result will be the internal capacity of the tank in gallons

8.1.1 Example

1.  Measure the length and width of the tank exterior

Tank length is 78 inches.  Tank width is 66 inches.

2.  Subtract the thickness of the tank walls.

Walls are 3 inches thick.  There are two walls per length.  Tank length is 78 inches less 6 inches for tank walls = internal length of 72 inches.

Walls are 3 inches thick.  There are two walls per width.  Tank width is 66 inches less 6 inches for tank walls = internal width of 60 inches.

3.  Use the chart to determine the gallons per inch.

Internal width of 60 inches and an internal length of 72 inches results in 18.7 gallons per inch.

4.  Measure depth of tank below the outset invert (bottom of outlet pipe).

The depth from the outlet invert is 48 inches.

5.  Multiply gallons per inch by the number of inches below the outlet invert.

48 inches (internal depth) times 18.7 (gallons per inch) = 897.6 gallons

6.  The result will be the internal capacity of the tank in gallons

The example tank would have an internal capacity of 897.6 gallons.

8.2  Formula for Calculating the Internal Capacity of a Cylindrical Septic Tank per Inch of Depth

1.  Measure the outside diameter of the tank.

2. Subtract the thickness of the tank walls to get internal diameter of the tank. Divide that by 2 to get the internal radius.

3.  Measure the depth of the tank below the outlet invert.

4.  Convert all measurements to feet.

5.  Use the following formulas to determine the volume of the tank in cubic feet:

Cubic Feet = p x (internal radius squared)

Cubic Feet x 7.48 = Gallons

8.2.1  Example

In the picture above, the tank has an exterior diameter of 66 inches and a 3 inch sidewall, the tank would have an internal diameter of 60 inches or 5 feet.  It has a depth below the outlet invert of 48 inches or 4 feetp = 3.14.

5 ÷ 2 = 2.5                         (internal radius x internal radius)

2.5  x 2.5  = 6.25.             (internal radius x internal radius)

3.14  x 6.25 = 19.62              (p x internal radius)

19.62  x 4 = 78.48            (times height = cubic feet)

78.48 x 7.48 = 587                 (times 7.48 = gallons

The volume of the tank is approximately 587 gallons.

Note:  Answers may vary due to rounding

9.0  SEO Field Charts

As an SEO your time is valuable.  As such, PASEO has prepared charts to allow the SEO to make quick field calculations.  The charts may be downloaded by clicking on the link. Print them out for use in the field

9.1  Field Chart for Determining Minimum Septic Tank Size Based on the Number of Bedrooms

9.2  Chart to Determine Tank Capacity in Gallons Per Inch of a Rectangular Tank

9.3  Chart to Determine Internal Capacity of a Cylindrical Tank in Gallons

10.0  Regulations Pertaining to Construction and Placement of Septic Tanks

DEP’s regulations for the construction and placement of septic tanks may be found in Chapter 73 and are reprinted here.

§ 73.31. Standards for septic tanks.

 (a) Capacity.

   (1)  The minimum liquid septic tank capacity for any installation is 900 gallons.

   (2)  For single-family dwelling units, not served by a community onlot system, a minimum daily flow of 400 gpd shall be used to determine required septic tank capacity. This figure shall be increased by 100 gallons for each additional bedroom over three. The daily flow indicated provides for use of garbage grinders, automatic washing machines, dishwashers and water softeners.

   (3)  The minimum septic tank capacity shall be calculated from the following table using estimated sewage flows from paragraph (2), or § 73.17(a)—(c) (relating to sewage flows):

Design flow g.p.d.   Tank capacity in gallons

0 – 500                        (3.5 x flow exceeding 400 g.p.d.) + 900

500 – 5000                  (1.5 x flow exceeding 500 g.p.d.) + 1250   

5000 – 7000                (1.45 x flow exceeding 5000 g.p.d.) + 8000

7500 – 10000              (1.35 x flow exceeding 7500 g.p.d.) + 11625

10000 +                     (1.5 x daily flow)

   Note: Septic tanks may be connected in series to attain required capacity.

 (b)   Construction.

   (1)  Tanks shall be watertight and constructed of sound and durable material not subject to excessive corrosion or decay.

     (i)   Precast concrete tanks shall have a minimum wall thickness of 2 1/2 inches and be adequately reinforced.

     (ii)   Precast slabs used as covers shall have a thickness of at least 3 inches and be adequately reinforced.

  (iii)   Tanks having a liquid capacity of 5,000 gallons or less may not be constructed of blocks, bricks or similar masonry construction.

     (iv)   Tanks having a capacity in excess of 5,000 gallons may be constructed onsite to meet the standards of the National Concrete Masonry Association for reinforcement and waterproofing as listed in the most recent edition of its publication ‘‘Concrete Masonry Foundation Walls,’’ copyright 1957 NCMA.

     (v)   Steel tanks shall meet United States Department of Commerce Standards 177-62.

   (2)  The depth of liquid in any tank or its compartments shall be:

     (i)   Not less than 2 1/2 nor more than 5 feet for tanks having a liquid capacity of 600 gallons or less.

     (ii)   Not less than 3 feet nor more than 7 feet for tanks having a liquid capacity of more than 600 gallons.

   (3)  No tank or compartment may have an inside horizontal dimension less than 36 inches.

   (4)  Septic tank installations shall consist of tanks with multiple compartments or multiple tanks. The first compartment or tank shall have at least the same capacity as the second but may not exceed twice the capacity of the second. Tanks or compartments shall be connected in series and may not exceed four in number in any one installation.

 (c)  Inlet and outlet connections.

   (1)  The bottom of the inlet shall be a minimum of 3 inches above the bottom of the outlet.

   (2)  Inlet baffles or vented tees shall extend below the liquid level at least 6 inches. Penetration of the inlet device may not exceed that of the outlet device.

   (3)  The outlet baffles or vented tees of each tank or compartment shall extend below the liquid surface to a distance equal to 40% of the liquid depth. Penetration of outlet baffles or tees in horizontal cylindrical tanks shall be equal to 35% of the liquid depth.

(4)  The inlet and outlet baffles or vented tees shall extend above liquid depth to approximately 1 inch from the top of the tank. Venting shall be provided between compartments and each tank.

   (5)  The outlet baffles or vented tees of the last compartment or tank shall be equipped with a solids retainer.

 (d)  Treatment tank access.

   (1)  Access to each tank or compartment of the tank shall be provided by a manhole with an inside dimension of at least 20 inches square (20 x 20) or in diameter, with a removable cover. The top of the tank containing the manhole or the top of a manhole extension may not be more than 12 inches below grade level. If access is extended to grade, the access cover shall be airtight. Grade level access covers shall be secured by bolts or locking mechanisms, or have sufficient weight to prevent unauthorized access.

   (2)  The ground shall slope away from any access extended to grade level.

 (e)  Inspection port. A maximum 4-inch diameter inspection port with sealed cover shall be installed to grade level above the inlet tee.

Sample Inspection Check Sheet for Septic Tank