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Drywells & Seepage Pits for Graywater Disposal & Septic System Design
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• What is a drywell or "septic drywell", seepage pit, and how is it different from a cesspool?
• What care is needed for a drywell?
• What is the failure critera for a drywell?

This document explains what a drywell (or seepage pit) is, gives safety and maintenance advice for drywells, and defines the criteria for drywell failure. What are Septic Drywells, What Drywell Maintenance is Needed? How do Drywells Fail? We warn readers that while installation of a drywell to receive graywater at a property may be a good idea, the use of drywells for this purpose may be a warning about the capacity and remaining life of the septic system. This material is a chapter of our Septic Systems Online Book: This document explains septic system inspection procedures, defects in onsite waste disposal systems, septic tank problems, septic drainfield problems, checklists of system components and things to ask. Septic system maintenance and pumping schedules.

Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted. Use of this information in electronic form, in books or pamphlets for sale is reserved to the author. Some technical review by industry experts has been completed-reviewers are listed at "References." Further review comments and content suggestions are welcome. Home buyers who want less technical advice should see the Home Buyer's Guide to Septic Systems. Also see The Septic Systems Home Page. © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use the links at page left to navigate this document or to go to Other Website Topics. Green links at left show where you are in our document & website.

DRYWELLS - or seepage pits for disposing of septic effluent or gray water onsite

I use the terms "drywell" and "seepage pit" differently as follows:

DRYWELLS for GRAYWATER - Drywells as Graywater Disposal Facilities

A drywell or seepage pit, sometimes called a leaching pit, leaching pool, or incorrectly a cesspool, is a covered pit with an open-jointed or perforated lining through which septic tank effluent seeps into the surrounding soil. In other words this is one of the oldest and simplest methods of disposing of gray water or in some cases septic effluent - a simple pit or hole in the ground, open to soil at its sides and bottoms, intended to receive and dispose of gray water (water from building non-sewage drains such as laundry, showers, sinks).

A drywell, or "seepage pit" is used at some building sites to receive "gray water" from a laundry, sink, or shower. The pit may be site-built of stone or dry-laid concrete block, rubble-filled, or constructed of (safer) pre-cast concrete. Modern drywells may also be sold as pre-designed units constructed of fiberglass or plastic. A drywell design may be similar to that of a cesspool, but only gray-water and not sewage is discharged into a drywell. The hole or absorption pit is typically 6-8 feet deep and 4-10 feet square (or round) depending on the amount of drainage to be handled and the percolation rate of the soil.

Some drywells were filled with large rubble. A top of concrete or stone slab is used to keep surface water and children out of the opening, and is usually covered and graded to be invisible in the yard. Older covers of wood or steel may be very unsafe and present a child or adult hazard, possibly including the risk of fatality. Mark off, prevent access to, and investigate such systems immediately.

What's the difference between a seepage pit or drywell and a cesspool?

The term "drywell" or "seepage pit" might be used by some people to describe a simple pit for disposing of septic effluent. Note that I said effluent not sewage. By contrast, a cesspool holds both solid waste and septic effluent. I use the term "seepage pit" to refer to a pit used to dispose of septic effluent which originated as blackwater, the solids having been retained in the septic tank. I use the term "drywell" to refer to a pit used to dispose of graywater (greywater) which originated in sinks, laundry facilities, or showers. While their uses and implications of their presence at a property are quite different, the actual construction details of a seepage pit or a drywell are about the same. Let's sum up the use of these terms being rather particular:

• Drywell: a hole in the ground intended to receive graywater from sinks, showers, or even roof or surface runoff. It may be site-built using stone or concrete block, or it may be constructed of pre-cast concrete buried in a pit and surrounded by gravel to increase its capacity.
• Seepage pit: a hole in the ground intended to receive septic effluent such as the outlet from a septic tank. Seepage pits may be permitted where site space or soil conditions do not permit a conventional leach field. However even if effluent is successfully "disposed-of" it is probably not being adequately treated if it's coming out of a conventional septic tank. A Seepage pit might be constructed just as the drywell above but it is likely to require different (larger) site clearance requirements.
• Cesspool: a hole in the ground intended to receive sewage or blackwater from a building. The cesspool may also receive graywater, or in order to reduce the loading of the cesspool, some buildings may direct their graywater to a separate drywell. The cesspool might be constructed just as the two systems above but may have different site clearance requirements.

Why are drywells installed?

A drywell is used at a property typically for these reasons

• To relieve the liquid load on an onsite waste disposal septic system leach field, particularly in an area where the leach field is in trouble, at or near the end of its life, or has very limited capacity to dispose of effluent
• To receive gray water from a building plumbing fixture (laundry or sink) which has been installed at a location so remote from the main house drain (to the septic system) that it is more convenient to construct a separate, nearby outside facility to receive this graywater than to route an indoor or outdoor buried rain to the existing septic system.
• (Readers are invited to send additional suggestions)

Extending Drywell Life

Some experts recommend installing a filter on water entering the drywell. Particularly for a drywell used to receive water from a clothes washing machine, installing a lint filter between the washing machine and the drywell can extend the life of the drywell by reducing the moment of soil-clogging particles of lint and debris into the system.

Intermittent dosing systems such as are used for some alternative septic system designs, can also be adapted to graywater systems. In simple terms, this means that multiple drywells are used, and graywater is routed intermittently among them, giving the unused drywell time to recover. A simple valve system on the graywater drain line can serve to route greywater (graywater) to alternating drywells.

More Reading

• Cesspools for more in-depth information about those systems.

Drywells as Seepage Pits to Receive and Dispose of Septic Effluent from a Septic Tank

Seepage pits: a "seepage pit" may be used as a septic effluent absorption system such as shown in the sketch at the top of the page and also immediately above. In this application, instead of receiving only graywater, the pit is receiving not just graywater but also blackwater-effluent or septic effluent which flows out of a septic tank. Unlike the drywell described just above, in this design waste, including black water and gray water, leaves the building through a main drain which enters a septic tank. The tank retains solids and grease/floating scum, allowing clarified effluent to enter the seepage pit for disposal into the soil.

Seepage pits come in a variety of capacities, sizes, shapes, and can be safe (against collapse) when a modern factory-built unit is shown, such as those in this photo.

I do not consider the use of traditional seepage pits a complete and satisfactory onsite waste disposal system design, though where space is limited or for other problem site reasons it may have been the choice of the septic system installer.

Disposal of septic effluent deep below ground in a seepage pit (typically four to six feet deep and placed another two or more feet below grade level) means that little or no aerobic bacteria will be present. Limiting the bacterial action to anaerobic may mean that the biomat that forms around the seepage pit will not adequately treat the effluent. The system may then be discharging contaminants into groundwater and the environment.

Fortunately there are alternative onsite wastewater disposal alternative designs which can handle limited or even zero-space sites, so a simple seepage pit as a destination for blackwater effluent is no longer the only choice for limited-space sites.

More Reading about Seepage Pits for Septic Effluent Disposal

Drywell and Seepage Pit Safety and Septic System Capacity Warnings

Drywell & Seepage Pit Safety

• Safety Warning: do not walk over the top of or close to the edges of a drywell or any other onsite pit or excavation because of the danger of fatal collapse. Keep pets and children away from such systems.
• Safety Warning: there is a risk of drywell collapse, risking injury and potential fatality if someone falls in to one of these systems, particularly for older site-built systems that were often made of dry-stacked stone or concrete block, and more-so if such systems are not protected by a very secure cover. Believe it or not, simple wood board covers were often used on home-made drywells. Such covers rot and ultimately collapse.

Drywell & Seepage Pit Capacity and Testing Limitations

• Limited septic system capacity is implied by the presence of a drywell. Wet soil conditions or limited space for a functioning drain field (for the septic system) often leads property owners to reduce the liquid load on the septic system by routing gray water to a separate drywell. Where such a system is installed owners/buyers should be alert for these conditions and should expect to face extra costs for system maintenance and repair as well as limited septic system capacity.

• An exception to the septic system capacity warning above at a large building where a sink or laundry are added in an area distant from piping connected to the septic system, an owner may add a remote drywell as an alternative to inconvenient and costly routing of a drain line from the laundry to the existing septic system.
• Drywells can obscure or prevent effective septic loading and dye testing: If a drywell system is installed, running water at the fixtures draining into this system (as may be done by some septic tests) are likely to fail to perform a loading test on the septic system even if such a test was attempted. First, the fixtures where water was run in attempt to test the septic system (tank and leach field) may not even be delivering water to those components if instead the fixtures drain to a separate drywell. It is critical to trace building drains as part of a septic test or to otherwise try to determine if the test water is actually entering the septic system.
• Even if we're trying to "test the drywell", if the drywell is "working" at all, it is a large empty hole in the ground. After a period of disuse it may appear to be working during a test but fail in a day or two when the no-longer functioning hole is filled with wastewater.
• Where a drywell is installed at a property and a septic inspection was attempted, more testing may be in order: in cases where only toilets empty into the septic system, it may be possible to make a more effective septic loading and dye test: put a test water load directly into the toilet by using a garden hose. This is not a typical/normal septic test procedure and has its own concerns such as overloading a system of unknown design that did not anticipate such volumes and possible back-contamination of water supply via hose in toilet (a cross connection). Whether or not such an additional test is performed, our warning about a system of limited capacity for which the owner may face significant costs to repair or expand capacity remains in effect.

Are "Drywells" really "Dry"?

Drywells in many areas are a misnomer since during wet weather as water tables rise, the "drywell" is not very dry and in fact may fill up with water and simply stop working. In wet areas of the Northeastern U.S., for example, I disagree with the practice of "solving" a roof drainage disposal problem at a flat site by building a "drywell" since in my experience these fail rather soon and in some cases even fill up and work backwards, sending water back to a building footing drain or roof drainage system where water then leaks into the building! Worse, if the "drywell" was buried close to the building foundation wall it may actually serve as a conduit, sending water against the building foundation and actually into the building.

More Reading::
Biomat Formation in the Septic System Drainfield Absorption System - what leads to drain field clogging and expensive drainfield repairs
Safety: Septic System, Septic Tank, & Cesspool Safety Warnings for Septic Inspectors, Septic Pumpers, and Homeowners
Cesspools for onsite wastewater disposal
Drywells for onsite wastewater or graywater disposal
Don't Flush these things into a septic system: a list of what's ok and what's not ok to put into septic tanks and building drains
Tank Location - How to Find the Septic Tank, how deep will the cover be, how to document its location - these methods also apply to locating a cesspool or drywell at a property.
Inspecting & Testing Septic Systems: Online Book, Inspection, Test, Diagnosis, Repair, & Maintenance: Our Online Septic Book: Septic Testing, Loading & Dye Tests, Septic Tank Pumping, Clearances, details of onsite waste disposal system inspection, testing, repair procedures.
Dye Tests: how to perform a Septic Loading and Dye Test - septic dye testing procedures in complete detail, a septic function test - a chapter in the above book.

(2) Site Requirements for septic seepage pits

(i) If soil and site conditions are adequate for absorption trenches, seepage pits shall not be used.

(ii) A minimum three foot vertical separation must exist between the bottom of any pit and the high groundwater level, bedrock, or other impervious layer.

(3) Design Criteria for seepage pits used to dispose of septic effluent

(i) The required "effective seepage pit area" is obtained from Tables 6 and 7 which are shown below.

(ii) No allowance for infiltration area is made for the bottom area of a pit or the surface area of impervious soil layers (percolation rate slower than 60 minutes/inch).

(iii) The effective diameter of a pit includes the diameter of the lining plus the added diameter provided by the annular ring of aggregate. Any area surrounding the liner with rock smaller than 2 1/2 inches in size shall not be included as part of the effective diameter.

(iv) Effective depth is measured from the invert of the seepage pit inlet to the floor of the pit, with the thickness of impervious layers deducted.

Site Distance Requirements for Drywells

This sketch (from New York's Wastewater Regulations) shows the recommended site clearances between seepage pit (used to receive septic tank effluent in this case) and other building and site features. A more detailed list of site clerances between all site features and all types of septic system components is listed at "More Reading" just below. Notice that absorption pits like this one not only need to be at a sufficient distance from the building, from wells, from property lines, but also if multiple absorption pits are installed, if you expect each pit to dispose of the graywater or effluent sent to it, they should be adequately separated from one another.

More Reading

Absorption Area Requirements for Seepage Pits

The table below shows the required soil absorption area for seepage pits as a function of soil percolation rate and anticipated daily wastewater flow in gallons.

TABLE 6 - SEEPAGE PITS - REQUIRED ABSORPTIVE AREA (IN SQUARE FEET) FOR HOUSEHOLD SYSTEMS

Seepage Pit Size Requirements to Obtain Necessary Absorption Area

The table below shows the size of seepage pit necessary in order to provide the required soil absorption area for seepage pits. First use the preceding table to determine the necessary seepage pit absorption area, then use this table (below) to determine the necessary seepage pit size to provide that absorption area.

Seepage Pit Construction Details: Linings, Separation, Strength, Connections

(v) Seepage pit linings may be precast concrete, cast-in-place concrete, or built in place with unmortared hollow cinder or concrete blocks. Concrete shall have a minimum compressive strength of 2,500 psi and 3,000 psi is recommended. Material with comparable structural strength, determined in accordance with commonly accepted sewage construction standards, principles or practices, may be allowed on an individual basis to prevent unreasonable hardship, provided public health is not prejudiced. (vi) The separation between the outside edges of seepage pits shall be three times the effective diameter of the largest pit. This separation is measured as the undisturbed soil between pit excavations. (vii) Pits shall be designed with sufficient structural stability to withstand lateral soil forces as well as vertical loads.

(4) Construction of seepage pits for septic effluent disposal

(i) Laterals leading to each seepage pit must be at least four inches in diameter with a minimum slope of 1/8 inch per foot.

(ii) Seepage pits shall not be connected in series. A distribution box shall be required where more than one seepage pit is installed.

(iii) The pit excavation is to be raked to minimize sidewall smearing that may occur and reduce infiltration capacity. If groundwater is encountered, the pit shall be backfilled with the original soil to a level at least three feet higher than maximum groundwater and adjustments made in the pit dimensions.

(iv) The linings are placed upon a concrete block, poured concrete, or precast footing and surrounded by a six inch minimum annular ring of large aggregate (2 1/12 - 4 inches in size).

(v) The rock is covered to prevent soil from filling the void spaces. Building paper, a four inch thick layer of hay or straw may be used.

(vi) The seepage pit cover shall be structurally sound and capable of supporting 300 pounds per square foot at the weakest point. Covers may be precast concrete or cast-in-place and shall be reinforced. A manhole with an opening of at least 20 inches in the shortest dimension shall be provided.

How Seepage Pits or Drywells Work, Fill-up, and Fail

Seepage pits, drywells, or cesspools can collapse and thus be dangerous. The photo shows a contractor's truck after it drove over and collapsed a seepage pit being used to receive effluent from a septic tank. This case is discussed later in this section. The general stages of pit life and failure are discussed first. Stages in the Life of a Drywell In a perfectly functioning, new seepage pit or drywell, wastewater enters the pit and seeps at first out of the pit bottom into the surrounding soils. The wastewater may be septic effluent if it's coming from a septic tank, or it may be graywater from other building drains if the pit is being used just for graywater disposal. Regardless of the effluent source, as wastewater enters the seepage pit or drywell, it contains some non-dissolved solid particles. This debris settles to the bottom of the pit. The bottom of a seepage pit, drywell becomes clogged with scum and debris fairly quickly (and in the case of cesspools, very very quickly.

As the pit bottom becomes sealed with settled debris, effluent entering the pit can no longer drain out immediately through the pit bottom surface. So as the seepage pit is used more and more, the level of liquid in the pit/drywell/cesspool rises. The rising wastewater then seeps out through the pit sides into the surrounding soil.

As the seepage pit ages, the soil around the seepage-out area of the pit bottom and lower sides becomes clogged and stops accepting wastewater. This causes wastewater inside the pit to rise still further, where it can exit the higher sides of the pit which are in contact with soil.

At the end of its life, the soil under the bottom and around the sides of a seepage pit has become clogged all the way up to near the very top of the pit. Then it's time to dig a new one.

Damaged or Collapsing Drywells, Cesspools, or Seepage Pits

When a contractor accidentally damages a seepage pit, such as when the dump truck shown in the photo above collapsed a seepage pit cover, the best repair would be to excavate the pit and repair any damaged components. If only the top were damaged it could be replaced. If the sides of the drywell were damaged on a site-built pit (such as one constructed of large stones or of concrete blocks), it may need to be rebuilt.

If a damaged drywell or seepage pit is also already near the end of its life (static liquid level near the pit top) it would make more sense to fill it in and relocate a new pit.

One correspondent wrote that after the truck (in the photo) collapsed her seepage pit cover, the contractor "repaired" the system by partially filling it in with gravel, saying that this would make no difference or would even improve the system. Now I've seen a few drywells that were filled with large rubble as insurance against having the sides collapse inwards. But I'm not sure I agree with the contractor.

If instead of a rebuild the contractor dumps gravel into the pit s/he may be simply covering up debris which fell into the pit when it was damaged - the broken cover or other components. By filling the pit and reducing its liquid volume capacity, we are forcing effluent to exit the pit higher on the pit's sidewalls and thus moving it more quickly to the end of its life.

So filling in a seepage pit with gravel both reduces its capacity to receive, buffer, and then dispose of effluent (you have less septic system capacity) and probably shortens its remaining life.

More Reading:

More Information About Septic Systems - Onsite Waste Disposal Design, Maintenance, Repair

• The Septic System Information Website - Extensive, Detailed Consumer and Industry Information on Septic System Inspection, Testing, Maintenance, Alternatives for on-site waste disposal

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