PASEO Training Tutorial #3 – Conventional Gravity Distribution Seepage Beds

1.0    Introduction:

The quintessential on-lot sewage disposal system is a septic tank and a seepage bed in which gravity delivers the effluent.  They have been used for over 200 hundred years and are quite environmentally friendly when designed and operated correctly.  (For purposes of this tutorial, siphon dosed systems will not be discussed as they are considered pressure dosed.)

In this tutorial, we will assume that the SEO already knows the testing and permitting requirements for seepage beds.  Therefore, we will concentrate on the construction and inspection of an already permitted system.

2.0 Goal of this Tutorial

It is the goal of this tutorial to help a Sewage Enforcement Officer to understand:

  • What is a seepage bed.
  • When may a seepage bed be used.
  • The regulations that apply to seepage beds.
  • The proper installation of a seepage bed.
  • How to inspect a seepage bed.
  • Ways that the SEO may do the job in a better manner.

3.0 What is a seepage bed?

In its most basic form, a seepage bed is a shallow hole in the earth. The hole is filled with a layer of stone aggregate which contains a grid of two or more laterals (pipes) to disperse sewage effluent throughout aggregate for the disposal and renovation by the soil under and surrounding the bed. 

The seepage bed is the disposal option of choice for many property owners as it is relatively simple to install and maintain, much less expensive, and is more aesthetically attractive than many other sewage disposal options.

There are a number of variations of seepage beds.  Some use leaching chambers or other proprietary materials rather than stone aggregate.  There are also pressure dosed versions that utilize pumps and siphons.  These will not be discussed in this tutorial.  We will only discuss seepage beds that utilize what DEP calls “gravity distribution” and which are classified as conventional systems.

Alternatives to aggregate such as these leaching chambers may be used in seepage beds but they will not be discussed in this tutorial.

4.0  Components of a Seepage Bed

Seepage bed sewage disposal systems consist of the following components:

  • A building sewer – the pipe between house and treatment tank(s).
  • Treatment tank(s) – these may be either septic tanks or aerobic tanks depending on the system design.
  • A distribution system – this may be a distribution box or a header pipe.
  • Aggregate – usually crushed stone but may consist of other materials such as shredded tires.
  • Pipe – both solid wall and perforated, usually made of PVC .
  • Soil barrier material – usually hay, straw, or geo-textile fabric.
  • Cover soil.

5.0  How does a Seepage Bed Works

There are two types of distribution.  The first utilizes a distribution box.  The second utilizes a header pipe.

5.1 Systems utilizing a distribution box:

Effluent leaves the treatment tank(s) and flows into a distribution box.  In the distribution box, the effluent is divided equally and passes out through two or more orifices, the number being determined by the number of laterals in the seepage bed.  The effluent then moves via gravity into the seepage bed.  Once in the bed, the effluent fills the laterals and drizzles out into the aggregate where bacteria and other microorganisms consume some of the pathogens and nutrients. The effluent eventually settles onto the soil under the aggregate.  As the effluent soaks the soil, bacteria and other microorganisms continue to treat and renovate the effluent.

A schematic drawing of a two lateral seepage bed utilizing a distribution box.

5.2  Systems utilizing a header pipe:

Effluent leaves the treatment tanks and flows directly into the seepage bed.  Once in the bed, the effluent flows into a header pipe and the effluent is divided equally into 2 or more laterals.  In the laterals, the effluent fills the pipes and drizzles out into the aggregate where bacteria and other microorganisms consume some of the pathogens and nutrients. The effluent eventually settles onto the soil under the aggregate.  As the effluent soaks the soil, bacteria and other microorganisms continue to treat and renovate the effluent.

A schematic of a two lateral seepage bed utilizing a header.

IMPORTANT:  The regulations for the construction of seepage beds are found in four separate sections of Chapter 73:

  • Section 73.42 contains the Gravity Distribution regulations.
  • Section 73.51 contains the General regulations for constructing seepage beds, seepage trenches and other below ground systems.
  • Section 73.52 contains regulations pertaining to Standard Trenches
  • Section 73.53 contains regulations pertaining to Seepage Beds and refers back to 73.52 making some of the Standard Trench requirements applicable to Seepage beds.

If you can’t find a requirement in the Seepage Bed section, always refer back to the General, Standard Trench, and Gravity Distribution  requirements.

6.0  When May a Seepage Bed Be Used?

The regulations permit the use of gravity distribution seepage beds when the following conditions can be met:

Limiting Zone – 60 inches or greater   § 73.51

Percolation Rate – 6 to 60 Minutes Per Inch   § 73.16 (d)

Slope –  0 to 8%   § 73.53 (1)

7.0  Constructing a Seepage Bed

7.1    Pre-construction

Stake the corners of the absorption area

Stake out the bed and the location of the treatment tanks and distribution box (if applicable).  Remind everyone working on the site to remain away from these areas.  Although seepage beds may tolerate a little more soil disturbance than other types of absorption areas ( i.e. the soil will be excavated from the site) compaction around the bed area or ruts caused by trucks running over wet soils could result in permit revocation.

7.2 Site Preparation

Mow down any high grass.  Cut and remove any brush.  If there are any trees in the bed area, they should be cut as close to the ground as possible.  Any loose surface rock should be rolled off.

7.3 Excavation

Excavate the bed area to the proper depth

Working from the upslope side of the bed, the seepage bed should be excavated to the depth specified on the plans.  On a sloped site, there may be an appreciable difference in depth on the uphill side than the downhill side.     §  73.51 (b)

IMPORTANT:  It is very important to only excavate the bed to the depth specified on the plans.  Over digging the bed may be grounds for permit revocation.

Excavation should only take place when the soil is friable to dry.  Excavation when the soil is wet may result in permit revocation.     §73.51 (c)

Clearly, this site is too wet to begin construction of the sewage system.

7.4 Add the bottom layer of aggregate. 

Add the bottom layer of aggregate

It should be 6 inches deep across the entire bed and raked level.         § 73.52. (b) (8)

7.5 Install the piping

Add the piping

The pipes should be placed on the aggregate.  The pipes must be laid out according to the plans. While not a regulatory requirement, the pipe’s label should face up and be aligned with one another.  (More on this later.) All pipes should be level (+/- 4 inches in 100 feet).

While 4 inch diameter PVC is the most commonly used pipe in seepage beds, the pipe may be as little as 3 inches or as much as 6 inches in diameter depending on what is called for in the plans.    §  73.42 (f)

Seepage bed using header pipe for distribution.

7.5.1 Distribution box seepage beds

Install the distribution box

If a distribution box is called for in the plans, it should be installed at this time.  The top of the box should be level in all directions.  The laterals should then be connected to the distribution box and sealed by way of mechanical seals or hydraulic cement.     §  73.42 (e)

Test the distribution box

To ensure that the distribution box is set correctly and that all the laterals receive an equal amount of effluent, the SEO must slowly pour water into the box.  The SEO should note that the water enters each lateral at exactly the same time.  If some laterals receive water sooner than others, the installer must adjust the box’s mechanical levelers or, if the box does not have mechanical levelers, the box will need to be reset.      §  73.42 (e) (2) (iii)

This drawing shows the top view of a typical two lateral distribution box with a baffle on the inlet. A distribution box may have many more laterals which may also extend out of the sides of the box. Baffles are not required on gravity distribution systems but are recommended.

7.5.2  Header pipe seepage beds

The delivery line (from the treatment tank to the header pipe) should also be installed at this time.  §  73.42 (d)

7.6 Add the remainder of the aggregate

Add the remainder of the aggregate and level to a point 2 inches above the pipes

The remainder of the aggregate is then added to the bed.  Care must be taken to avoid disturbing the laterals.  The amount of aggregate must be leveled to a depth of at least 2 inches higher than the top of the laterals.  §  73.52 (a) (11) & (12)

7.7 Install the soil barrier

Install the soil barrier

The top of the aggregate must be covered by a soil barrier.  A soil barrier is a layer of material intended to keep the cover soil from migrating into the aggregate but will allow the transfer of gasses.  Soil barriers may be a layer of geo-textile material or a 2 inch thick layer of hay, straw or similar material.  §  73.52 (a) (13)

7.8  Cover the seepage bed

Place cover soil

The cover soil should be placed on the seepage bed.  The cover soil must be a minimum of 12 inches deep and brought at least to the original land surface.  §  73.52 (a) (14)

If the top of the aggregate is less than 12 inches below the original land surface, the cover soil must be brought the necessary height above grade to get a full 12 inches of cover depth. In such cases, the cover soil must extend a minimum of three feet outward from the seepage bed. 

In this drawing, the aggregate is less than 12 inches from the surface so that part of the cover soil that is above the surface is graded out 3 feet.

On a sloped site, the cover soil may be deeper on the upslope side than the down slope side.  At no point may the cover soil exceed 36 inches deep.

Example 1 shows a seepage bed on a slope. The cover soil on the left can not exceed 36 inches deep. The cover soil on the right must be at least 12 inches deep. Example 2 shows a seepage bed on a slope. While there is sufficient cover on the left hand of the bed, there is insufficient cover on the right. Therefore, cover soil is added to get the required 12 inch minimum. Examples are not to scale.

The soil used to cover the bed must be free of rock and contain no deleterious material (sticks, rubbish, etc.).  It must also be suitable for growing grass.

7.9   The cover soil

The cover soil must be seeded with grass seed.  It is recommended that the area also be mulched to protect the seed through germination.

8.0 Inspecting the Construction of a Seepage Bed

8.1  Pre-construction

Most SEOs don’t require pre-construction meetings unless the project is a large commercial or community system project.  However, SEOs do a lot of pre-construction inspecting when they issue the sewage permit.

The SEO must take the slope and limiting zone into consideration when reviewing the sewage design.  A 48 inch vertical separation is required between the bottom of the bed and the limiting zone.  This may be easy to maintain on a relatively flat site.  However, it might be quite difficult on a slope.  Care must be taken to ensure that the bed does not extend out of the ground or get buried by over 3 feet.

This illustration shows how a seepage bed may extend above the ground surface if slope is not taken into account during the design phase.
This illustration shows how a seepage bed on a slope could result in a violation of the 36” maximum cover.

While the system designer should be aware of this issue, many seepage beds are designed by the property owner or other parties not well versed in the design issues.  Therefore, it is of the utmost importance that the SEO be aware of this problem and ensure that it is addressed.

To determine the maximum width in which a seepage bed may be constructed, the SEO must know the following:

  • The depth of the limiting zone
  • The maximum slope on the site in which the bed is to be placed
  • The intended depth of the bed

With that known, the SEO can use the following formula:

[ Limiting Zone – (Depth + 48) x 8.3 ] / Slope

In this formula, 48 represents the vertical separation that must be maintained and 8.3 is a constant.

Example #1

Limiting zone = 80 inches

Slope = 4 %        

Proposed bed depth = 12 inches

[ 80 – (12 + 48 ) x 8.3 ] / 4

 Becomes: [ 80 – 60 x 8.3 ] / 4

  Becomes: [ 20 x 8.3 ] / 4

Becomes: 166 / 4

Which results in:     41.5

Therefore, in this particular example, the bed could be designed 41.5 feet wide without violating the chapter 73 requirements.

Of course, there are other factors that can affect the maximum width of the bed.

All seepage beds must be a MINIMUM of 8 feet wide.  If The bed can not be designed to be at least 8 feet wide, the designer must change to seepage trenches or another disposal option.

The depth of the percolation holes is another factor to be considered.  The percolation holes must be dug to the intended depth of the bottom of the seepage bed.  For example, if the percolation test is conducted at 24 inches, the bottom of the seepage bed must be located at 24 inches even if the limiting zone would allow a 36 inch deep bed.

Finally, at no point may the bottom of the bed be greater than 36 inches below the original or final grade, nor may the bottom of the bed be less than 12 inches from the original or final grade.

8.2  Excavation of the Bed

Most SEOs require an inspection to observe the excavation of the bed.  At this time the SEO should:

  • Check to see that the bed is located where the site was tested and exactly where indicated on the plans.
  • Check to see that the bed is dug to the proper depth as indicated on the plans.
  • Check to see that the bed is the proper length and width.
  • Check to see that the bottom of the bed is level.
  • Check to see that the soil at the bottom of the bed is not smeared.

8.3  Inspection of the piping

An inspection of the piping is required before the piping is covered.

  • Check to see that the pipes are configured correctly as shown on the plans.
  • Check to see that the pipe labels aligned indicating that the holes are all aligned.
Always keep the wording up and each pipe aligned with the others for the best effluent distribution.
  • (In distribution box systems) Check to see that the lateral ends are capped.
  • (In header pipe systems) Check to see that the header and tailer pipes made of non-perforated pipe.
  • (In header pipe systems)  Check to see that the delivery line enters the header between two laterals.
ACCEPTABLE –The delivery line enters the header pipe between two laterals. It would also be acceptable for the delivery line to enter between the other two laterals
NOT ACCEPTABLE – The delivery line is not permitted to enter the header pipe at a point in which it could flow directly into a lateral.
  • Check to see that the pipes are all level.
A bubble level is inexpensive yet accurate tool to determine if the laterals are level.
  • Certification that the aggregate meets the Section 73.51 (a) requirements should be collected at this time.

Coarse aggregate used in the distribution system shall meet the requirements of the Department of Transportation specifications, Publication #408 (1994) section 703 available from the Department of Transportation. The size and grading of the aggregate shall meet AASHTO No. 57 requirements from a PADOT certified stockpile and shall be of Type B quality requirements.

8.4  Distribution Box (when applicable)

If a distribution box is used in the system, it must be inspected and tested.

  • Is the distribution box set level in all directions?
  • Is the distribution box placed on an adequate base of compacted soil or gravel or properly anchored to the ground?
  • Is the distribution box outside the absorption area?
  • Are the inlets and outlets sealed with mechanical seals or hydraulic cement?
  • Does the distribution box have a removable lid?
  • Water test the box to ensure that each lateral receives water at the same time.
In this drawing we see an end view of a four lateral distribution box in which water was added to check the level. Laterals 1 and 2 both have water at the bottom of the invert. Lateral 3 is set too high. Lateral 4 is set too low and, if left as is, would take all the effluent.

8.5  Soil Barrier

Because most installers cover the bed right after the pipes are inspected, many SEOs stick around to observe the placement of the soil barrier.  If the SEO inspects the soil barrier, the SEO should observe the following:

  • Is the soil barrier a geo-textile material or other material that would hold back the soil but allow the bed to “breathe”?  Note: Plastic sheeting, tar paper, and similar materials are not acceptable for use as soil barriers.
An example of a geo-textile fabric
  • If hay or straw is used, make sure it is layered at least 2 inches thick.

8.6  Completed System

The SEO should observe the covered seepage bed prior to the use of the system.  The SEO should look for:

  • Evidence that the cover soil was sown with grass seed.
  • Rain gutters, surface swales, etc. are diverted away from the seepage bed.
  • There is no settling or erosion of the cover soil.
  • That the cover is equal to or exceeds the height of the original ground.  That is, the area over the bed can not be concaved which would allow rain water to collect over the bed.

9.0  Ways to do the Job Better

The regulations and the local ordinance provide the minimum standards that must be met.  However, there are some things that may be done to help the SEO do a better job and to improve the quality of the sewage disposal system.  SEOs should consider it part of their mission to educate installers in the means to improve their work.  The SEO should also discuss with the local agency officials the possibility of incorporating some of these suggestions into their local ordinance.

  • Installers complain that every local agency requires different inspections.  Write an inspection schedule and attach it to the permit at the time of issuance so that there can be no confusion about when an inspection is required.
  • Hay and straw soil barriers will eventually degrade and allow soil to migrate into the absorption area.  Geo-textile materials will last forever.  SEOs should instruct their installers about the benefits of using geo-textile fabric as a soil barrier.
The soil barrier in this system prematurely rotted and allowed the cover soil drop into the aggregate resulting in a malfunction.
  • It is recommended that the seepage bed be “crowned”.  That is, instead of the cover soil surface being flat, the surface should be slightly higher than the surrounding ground surface.  Crowning will help divert heavy rainfall from soaking into the seepage bed and will help prevent a depression from forming as the cover soil settles.
  • Many local agencies require the soil under the bed be scarified prior to placement of the aggregate.  This is done as excavation buckets often smear the soil.  Installers walking on the bottom of the bed while shooting grades will also compact the soil.  Scarifying the soil will help break up the compaction and will increase the surface area through which the effluent may infiltrate.  If the sidewalls appear smeared, many installers will also use a garden rake to scarify them to allow better sidewall permeability.
The installer scarified the bottom of this seepage bed.
  • Use the cleanest aggregate possible.  Fine material in “dirty” aggregate will eventually settle to the bottom of the bed resulting in reduced permeability. 
This aggregate was taken from a stockpile just minutes after the pile was tested and approved by PennDOT. Although it passed certification, it clearly has a large amount of fine material in it which could reduce the life expectancy of the system.
  • Always require certification that the aggregate meets the requirements of Section 73.51 (a).  The aggregate must have been obtained from a PennDOT certified stock pile.  Attach the certifications to the local agency copy of the permit.
  • Slope is very critical especially for seepage beds on less deep limiting zones.  Slopes must be shot with a level over the exact area in which the bed will be constructed.  Require that the designer stake out the seepage bed when submitting the permit application.  Otherwise the SEO must do it in order to verify that the slope is correct when determining the proper depth of the bed.
  • For best results, the laterals should be set with the holes facing 30 degrees downward off the center axis of the pipe.

This allows the lateral to fill along its length before the effluent flows out the holes.

When the laterals are placed with the holes facing downward, the effluent flows out the first few holes and saturates part of the bed.  However, by having the pipe fill first, the effluent is more evenly distributed allowing the entire bed to work.

In these drawings, the blue represents the effluent leaving the lateral. The red represents the seepage into the soil. As noted in the first drawing, the downward facing holes only distribute the effluent into a small part of the bed while the angled holes allow the effluent to flow though out the bed and creating less saturation of the soil.
  • Distribution boxes may heave when the ground freezes.  Distribution boxes should be placed on a 12 inch bed of aggregate to reduce the effect of frost heaving.  Plastic distribution boxes must be anchored in place.
  • Request that your installers use a quality grass seed mix of perennial rye grass, fescue and/or blue grass.  Inexpensive “contractor’s mixes” are mostly annual rye grass which germinates quickly but fails to live past the first year.  Instruct the property owner of the importance in getting a good cover of grass growing over the seepage bed.

10.0  Inspection Check List

Use this check list to document your inspections.  When complete, attach it to the local agency copy of the permit as a permanent record of your inspection.