Drywells & Seepage Pits for Graywater Disposal & Septic System Design
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This document explains what a drywell (or seepage pit) is, describes how drywells are used, 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 BOOK, ONLINE: This article series 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.

Readers trying to diagnose and deal with sudden soil subsidence or yard collapses should see  CESSPOOL SAFETY WARNINGS as those hazards can also apply to drywells and septic tanks, and also see SINKHOLES, WARNING SIGNS. Additional septic system safety warnings are at SEPTIC & CESSPOOL SAFETY.

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.

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

We 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)

Tips for 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.

Cesspools for more in-depth information about those systems.

Questions & Answers about Drywells

Question: The drywell at my new home stays full of water and my house floods: who is responsible for the repair?

I bought a new construction home in Staten Island, NY. Unfortunately my garage and side apartment have flooded twice last month due to the heavy rainfall.

The gutter downspouts and sump pump are being led to the drywell however it is overflowing at all times and is not taking on any new water.

Even the catch basin is filled with water and will never completely drain into the drywell. to temporarily try to resolve this issue I have removed all downspouts and sump pumps out of drywell and are now draining onto my property which inevitably is flooding but better outside my home than in my home.

It is new construction and I am basically looking for a little advice on whether this is a structural defect and builder should be responsible for this or is it going to fall on me and how can this be properly fixed. - M.N. Staten Island, NY

Reply: For your site the drywell may have never been a workable solution to handling runoff; real estate attorney should review your contract of sale and advise about the new home warranty law

Was the Drywell a Usable Solution for your Building Site?

At ARE DRYWELLS DRY? we express the view that in some areas of high water table and wet soils, a drywell is not likely to be a useful means of collecting and disposing of surface or roof runoff in wet weather. That's because in those areas the "drywell" is in fact wet, or flooded, or at least partially flooded seasonally if not all year long.

From your description it sounds as if one or more of these problems underlies the flooding problem in your new home

The site is one that has a high water table, keeping the drywell flooded - this seems most likely from your description.

The drywell was improperly constructed, too small, or lacking proper site preparation, for example by setting a pre-cast drywell into a pit without adequate pit size or adequate gravel

Groundwater, surface runoff, or roof runoff directed into the drywell are of much larger volume than anticipated.

Who is Responsible for Groundwater, Roof Runoff, or Flooding in a New Home

This question is an ugly morass that is unfortunately all too common in new construction projects, and it is one that needs assistance from an attorney expert in real estate law and familiar with local building codes in your neighborhood.

Our photo ( left) shows a combination that is a near guarantee of a flooded basement: the builder constructed the deck before final site grading, leaving in-slope grade and a virtual pool under the deck and draining towards the home. And gutter improprieties send water running down the house wall during rain.

In general one would expect that proper construction of a new home would include taking whatever means were necessary to protect the home interior from water entry. And one would wish that initial site investigation (for questions such as where is the water table?) and building code approval would have reviewed any special water or drainage requirements that the builder needed to meet.

Some new home construction contracts excuse the builder from certain home waterproofing steps, explicitly or implicitly leaving those responsibilities up to the homeowner. And unfortunately some new homeowners are unaware that those additional steps are not optional if the home is to be protected from flooding.

When the home floods at the first rain, or when it floods three years later after footing drains have become clogged with silt, the owner and builder end up in a finger-pointing contest that might have originated in the these two basic questions, even if other site water problems are absent:

1. No gutter system? A common example of ambiguity that leads to home flooding that we encounter is the omission from the construction contract of any provision for roof drainage management: gutters and leaders. The builder leaves that "detail" up to the homeowner and may even excuse themselves from responsibility for future home flooding if that detail is not addressed by the owner. And it's fair to say that it is a homeowner responsibility to keep the gutters and downspouts clean and working.
2. Poor final site work? A second common source of new house flooding that crops up is a new home contract that is unclear about what constitutes rough grading versus final landscaping. The builder accepts responsibility for "rough grading" and explicitly leaves "landscaping" to the new owner. But the new owner and the builder may have different ideas about what those terms mean. A builder might leave the ground around the building very uneven, with local low areas or even in-slope grade that drains towards the building, while the buyer thought that they'd have a properly sloped and graded lawn that just needed a sprinkling of grass seed to be complete.

When a new home site combines both problems 1 and 2 above, there is a very high risk of basement or crawl space water entry and ultimately flooding. Just how quickly flooding appears depends further on other site conditions:

• Area water table level
• Local groundwater
• Degree to which surface runoff is directed onto this particular building site
• Adequacy of foundation drainage or even, as we find at some homes, its complete omission
• The overall terrain shape: is the home in a low spot or depression or at the bottom of a hill? Is the entire area dead flat with no place to send runoff? Is there a storm drain system into which surface and subsurface runoff may be directed?

Some possible solutions to the Flooded Drywell Problem

Your question does not include information about your neighborhood and site nor specifics about the home. But it may be useful to start by asking: Does an inspection of the drywell show that it is properly constructed? Is there a storm drain system into which the flooding drywell can be pumped? If so, is that pumping permitted in your municipality? Exactly where and how is water entering to flood the home? Is there a working roof drainage system that directs roof spillage well away from the home? Gutters and downspouts might be installed but not working properly, or a mistake like using perforated downspout extensions might be sending water back to the foundation. Do other homes in the immediate neighborhood also have flooding problems? If not, what's different in the dry homes? Interior basement de-watering system (photo at left) ? This last resort, if not properly installed, could still leave excessive interior moisture and contribute to a mold contamination problem. For details about diagnosing and curing building flooding and water entry, see WATER ENTRY in buildings and WET BASEMENT PREVENTION. Important safety warnings about collapsing drywells and cesspools are at CESSPOOL SAFETY WARNINGS .

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Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

BUILDING SAFETY HAZARDS GUIDE
BUILDING SETTLEMENT

SEPTIC SYSTEM INSPECT DIAGNOSE REPAIR

  DRYWELL DESIGN & USES
    DRYWELLS for GRAYWATER
    DRYWELLS as SEEPAGE PITS
    DRYWELL SAFETY CONCERNS
    ARE DRYWELLS DRY?
    PIT SITE REQUIREMENTS
    PIT AREA & SIZE
    PIT CONSTRUCTION DETAILS
    PIT COLLAPSE or FAILURE

WATER ENTRY in buildings
  ATTIC LEAKS Moisture or Mold
  BASEMENT De-Watering Systems
  BASEMENT CEILING VAPOR BARRIER
  BASEMENT HEAT LOSS
  BASEMENT LEAKS, INSPECT FOR
  BASEMENT WATERPROOFING
  BASEMENT WINDOW LEAKS
  CONDENSATION or SWEATING PIPES, TANKS
  CRAWL SPACE DRYOUT PROCEDURES
  DEW POINT CALCULATION for WALLS
  DEW POINT TABLE - CONDENSATION POINT GUIDE
  DRYER VENTING
  DRYWELL DESIGN & USES
  EFFLORESCENCE, Salts & White / Brown Deposits
  FLOOD Damage Assessment & Repairs
  FLOOD DAMAGED FOUNDATIONS
  FLOOD VENTS & FLOOD PORTS
  FLOODS IN buildings-priorities
  FOUNDATION WATERPROOFING
  GRADING & SITE WORK, EXTERIOR
  GUTTERS & DOWNSPOUTS
  HUMIDITY CONTROL & TARGETS INDOORS
  HOUSEWRAP AIR & VAPOR BARRIERS
  LOG HOME Leak Diagnosis & Cure
  LOG HOME Condensation &Moisture
  MOISTURE in BUILDING WALLS, EFFECTS
  MOISTURE in CELLULOSE INSULATION
  MOISTURE CALCULATIONS
  MOISTURE METER STUDY
  MOISTURE PROBLEMS: CAUSE & CURE
  MOLD in buildings
  MOLD PREVENTION AFTER FLOODING
  MOLD RESISTANT CONSTRUCTION
  SEWAGE CONTAMINATION
  SUMP PUMPS GUIDE
  SWEATING (CONDENSATION) on PIPES, TANKS
  WET BASEMENT PREVENTION

• Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. Thanks to Alan Carson and Bob Dunlop, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education including the ASHI-adopted Home Inspection Training Program (home study course), publications such as the Home Reference Book, report writing materials including the Horizon report writer, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors
  

How Big Should the Leach Field Be? includes a practical example using sample calculations and a table of soil percolation rate vs. field size

Design Basics for Septic Systems: Choosing Septic Tank Size, Leach Field Size - basic septic system volume and absorption system design guides.

Components of a Septic System- the Basic Parts of a Conventional Septic Tank and Leachfield, a chapter in the Home Buyers Guide to Septic Systems

Sketches of the Septic System Components Private Sewage Disposal Systems - Septic Drawing Library

Cesspools for more in-depth information about those systems.

DRYWELL DESIGN & USES in-depth information about drywells

The Septic System Information Website home page for this topic

Septic Systems Inspection, Testing, & Maintenance- online textbook. Detailed how to inspect, maintain, repair information

The Home Buyer's Guide to Septic Systems

Septic Tank Pumping Guide: When, Why, How to pump the septic tank

Home & Outdoor Living Water Requirements

Septic Tank Capacity vs Usage in Daily Gallons of Wastewater Flow, calculating required septic tank size, calculating septic tank volume from size measurements

Septic Tank/Soil-Absorption Systems: How to Operate & Maintain [ copy on file as /septic/Septic_Operation_USDA.pdf ] - , Equipment Tips, U.S. Department of Agriculture, 8271 1302, 7100 Engineering, 2300 Recreation, September 1982, web search 08/28/2010, original source: http://www.fs.fed.us/t-d/pubs/pdfimage/82711302.pdf.

How Big Should the Leach Field Be? - table of soil percolation rate vs. field size

Septic System Drainfield Absorption System Biomat Formation - what leads to drain field clogging and expensive drainfield repairs

Table of Required Septic & Well Clearances: Distances Between Septic System & Wells, Streams, Trees, etc.

Ten Steps to Keeping a Septic System Working, suggestions from the U.S. EPA, edits and additions by DJF

Pennsylvania State Fact Sheets relating to domestic wastewater treatment systems include

Pennsylvania State Wastewater Treatment Fact Sheet SW-161, Septic System Failure: Diagnosis and Treatment

Pennsylvania State Wastewater Treatment Fact Sheet SW-162, The Soil Media and the Percolation Test

Pennsylvania State Wastewater Treatment Fact Sheet SW-l64, Mound Systems for Wastewater Treatment

Pennsylvania State Wastewater Treatment Fact Sheet SW-165, Septic Tank-Soil Absorption Systems

Document Sources used for this web page include but are not limited to: Agricultural Fact Sheet #SW-161 "Septic Tank Pumping," by Paul D. Robillard and Kelli S. Martin. Penn State College of Agriculture - Cooperative Extension, edited and annotated by Dan Friedman (Thanks: to Bob Mackey for proofreading the original source material.)

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Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.

Inspecting 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.

Advanced Onsite Wastewater Systems Technologies, Anish R. Jantrania, Mark A. Gross. Anish Jantrania, Ph.D., P.E., M.B.A., is a Consulting Engineer, in Mechanicsville VA, 804-550-0389 (2006), Advanced Onsite Wastewater Systems Technologies. Outstanding technical reference especially on alternative septic system design alternatives. Written for designers and engineers, this book is not at all easy going for homeowners but is a text we recommend for professionals--DF.

AEROBIC SEPTIC SYSTEMS

Builder's Guide to Wells and Septic Systems, Woodson, R. Dodge: $ 24.95; MCGRAW HILL B; TP; Quoting from Amazon's description: For the homebuilder, one mistake in estimating or installing wells and septic systems can cost thousands of dollars. This comprehensive guide filled with case studies can prevent that. Master plumber R. Dodge Woodson packs this reader-friendly guide with guidance and information, including details on new techniques and materials that can economize and expedite jobs and advice on how to avoid mistakes in both estimating and construction. Chapters cover virtually every aspect of wells and septic systems, including on-site evaluations; site limitations; bidding; soil studies, septic designs, and code-related issues; drilled and dug wells, gravel and pipe, chamber-type, and gravity septic systems; pump stations; common problems with well installation; and remedies for poor septic situations. Woodson also discusses ways to increase profits by avoiding cost overruns.

Country Plumbing: Living with a Septic System, Hartigan, Gerry: $ 9.95; ALAN C HOOD & TP; Quoting an Amazon reviewer's comment, with which we agree--DF:This book is informative as far as it goes and might be most useful for someone with an older system. But it was written in the early 1980s. A lot has changed since then. In particular, the book doesn't cover any of the newer systems that are used more and more nowadays in some parts of the country -- sand mounds, aeration systems, lagoons, etc.

Design Manuals for Septic Systems

US EPA Onsite Wastewater Treatment Systems Manua [online copy, free] Top Reference: US EPA's Design Manual for Onsite Wastewater Treatment and Disposal, 1980, available from the US EPA, the US GPO Superintendent of Documents (Pueblo CO), and from the National Small Flows Clearinghouse. Original source http://www.epa.gov/ORD/NRMRL/Pubs/625R00008/625R00008.htm Onsite wastewater treatment and disposal systems, Richard J Otis, published by the US EPA. Although it's more than 20 years old, this book remains a useful reference for septic system designers. U.S. Environmental Protection Agency, Office of Water Program Operations; Office of Research and Development, Municipal Environmental Research Laboratory; (1980)

Eco John® Innovative Toilet Solutions, Global Inventive Industries, Fountain Valley CA, PDF, product brochure

"International Private Sewage Disposal Code," 1995, BOCA-708-799-2300, ICBO-310-699-0541, SBCCI 205-591-1853, available from those code associations.

"Manual of Policy, Procedures, and Guidelines for Onsite Sewage Systems," Ontario Reg. 374/81, Part VII of the Environmental Protection Act (Canada), ISBN 0-7743-7303-2, Ministry of the Environment,135 St. Clair Ave. West, Toronto Ontario M4V 1P5 Canada $24. CDN.

Manual of Septic Tank Practice, US Public Health Service's 1959.

Onsite Wastewater Disposal Books

Onsite Wastewater Disposal, R. J. Perkins; Quoting from Amazon: This practical book, co-published with the National Environmental Health Association, describes the step-by-step procedures needed to avoid common pitfalls in septic system technology. Valuable in matching the septic system to the site-specific conditions, this useful book will help you install a reliable system in both suitable and difficult environments. Septic tank installers, planners, state and local regulators, civil and sanitary engineers, consulting engineers, architects, homeowners, academics, and land developers will find this publication valuable.

Onsite Wastewater Treatment Systems, Bennette D. Burks, Mary Margaret Minnis, Hogarth House 1994 - one of the best septic system books around, suffering a bit from small fonts and a weak index. While it contains some material more technical than needed by homeowners, Burks/Minnis book on onsite wastewater treatment systems a very useful reference for both property owners and septic system designers.

Septic Tank/Soil-Absorption Systems: How to Operate & Maintain [ copy on file as /septic/Septic_Operation_USDA.pdf ] - , Equipment Tips, U.S. Department of Agriculture, 8271 1302, 7100 Engineering, 2300 Recreation, September 1982, web search 08/28/2010, original source: http://www.fs.fed.us/t-d/pubs/pdfimage/82711302.pdf

Soil Percolation Tests soil perc testing guide and instructions

Percolation Testing Manual, CNMI Division of Environmental Quality, PO Box 501304, Saipan, MP 96950

Planting Over Septic System Component", Daniel Friedman (author/editor, InspectAPedia.com), The Innovator, Winter/Spring 2008, BCOSSA, British Columbia OnSite Sewage Association, 201-3542 Blansard St., Victoria BC V8X 1W3 Canada

Save the Septic System - Do Not Flush These Items Down the Toilet, Daniel Friedman, InspectAPedia.com - PDF document, printable

SEPTIC STANDARDS