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SEPTIC SYSTEM INSPECT DIAGNOSE REPAIR
SEPTIC CARE INSTRUCTIONS
SEPTIC D-BOX INSPECTION
SEPTIC DRAINFIELD FAILURE DIAGNOSIS
SEPTIC DYE TEST PROCEDURE
SEPTIC FAILURE SIGNS
SEPTIC INSPECTION & TEST GUIDE
SEPTIC LIFE EXPECTANCY
SEPTIC SUPPLIES & PARTS
SEPTIC SYSTEM DESIGN ALTERNATIVES
SEPTIC SYSTEM DESIGN BASICS
SEPTIC SYSTEMS, HOME BUYERS GUIDE to
SEPTIC SYSTEM SAFETY WARNINGS
SEPTIC TREATMENTS & CHEMICALS
SEWAGE & SEPTIC CONTAMINANTS
SEWAGE BACKUP, WHAT TO DO
SEWAGE BACKUP TEST & CLEANUP
SEWAGE BACKUP PREVENTION
SEWAGE CONTAMINATION in buildings
SEWAGE CONTAMINANTS in FRUIT / VEGETABLES
SEWAGE EJECTOR / GRINDER PUMPS
SEWAGE LEVELS in SEPTIC TANKS
SEWAGE NITROGEN CONTAMINANTS
SEWAGE PATHOGENS in SEPTIC SLUDGE
SEWER GAS ODORS
SEWER LINE REPLACEMENT
SINKHOLES, WARNING SIGNS
SMELL PATCH TEST to Track Down Odors
SOAKAWAY BED FAILURE DIAGNOSIS
SULPHUR & SEWER GAS SMELL SOURCES
TOILETS, INSPECT, INSTALL, REPAIR
TOILETS, DON'T FLUSH LIST
TRAPS on PLUMBING FIXTURES
TREATMENTS & CHEMICALS, SEPTIC
VIDEO GUIDES: Septic Videos
WATER CONSERVATION MEASURES
WATER SOFTENERS & CONDITIONERS
WATER SUPPLY & DRAIN PIPING
WATER, WELLS, WATER TANKS: TESTING GUIDE
WASHING MACHINES & SEPTIC SYSTEMS
WASTEWATER TREATMENT BASICS
WINTERIZE A BUILDING
Here we list detailed "how to" articles on the design of septic systems of both conventional (tank and drainfield) and alternative (aerobic, mound, raised bed, sand filter, media filter, etc) septic system designs for difficult sites.
We give in-depth information about conventional septic tanks, drain fields, septic pipes, and septic waste handling. We include tables for septic tanks: pumping frequency, septic tank size, septic tank design, and clearances between septic systems, wells, and other site features and boundaries.
See basic SEPTIC SYSTEM DESIGN BASICS and alternative SEPTIC SYSTEM DESIGN ALTERNATIVES septic system design and product links below and at the left of this page. Also see our SEPTIC DRAWINGS and our SEPTIC SYSTEM DESIGN MANUAL - Online. This article series answers just about any question you might have about buying or owning a house with a septic system. If you do not quickly find here information you need, CONTACT US
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"Septic tanks" are what home owners or home buyers think of when buying or maintaining a home with a private septic system. But we should be thinking about the whole septic system - since the drain field or leach field makes up half of a typical septic system cost.
We are also quite concerned with septic system health and safety since there are potential collapse hazards which can be fatal, and there are bacterial and pathogen hazards for both site occupants and for the environment if a septic system is not working properly. The articles listed at this website form an extensive guide to septic systems care, inspection, testing, and installation. We welcome questions and suggestions for content.
Also see these Directory Lists of Septic System Parts & Supplies
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SEPTIC DESIGN BASICS - Septic System Design: Choosing Septic Tank Size, Absorption System Size, Basic Design Notes
The full Chapter on Design Basics is at SEPTIC DESIGN BASICS. Excerpts are below.
If you need basic septic system design parameters such as finding the recommended septic tank volume and conventional recommended leach field or drainfield size, along with some notes on how to calculate these from simple water usage and site conditions, here are links to that information. Also take a look at the septic publications, septic design manuals, and onsite waste disposal standards links and articles above and below this section. I am re-writing and reorganizing this design material into a separate guide for laymen (like myself). Septic engineers already know all this stuff. Meanwhile look at the articles listed below as they will allow you to answer basic septic system design questions.
Septic Tank Sizing: to determine the required septic tank size you would need to look at article #1 to see the required tank size based on daily water volume usage and perhaps article #2 to confirm that you're estimating the water volume reasonably.
Tank Retention Time if you just wanted to know our opinion about tank size as a function of effluent retention time, see item #3.
Septic field size planning: see article #4 which sizes the field based on its percolation rate and #9 which describes performing a perc test, with perhaps a look at #10 to understand biomats and why a leach field in a wet area won't treat the effluent.
A "septic system," also referred to as a private, on-site waste disposal system, receives waste water and solids from a Building's plumbing facilities (bathrooms, kitchens, shower, laundry), treats, and then disposes of the effluent from this waste, by permitting it to absorb into soils at the property. "Treatment" is accomplished by bacterial action in the "septic" or "treatment" tank and it is mostly accomplished by bacteria in the soil around and below the effluent absorption system, or "drain field." This bacterial action is needed to reduce the level of pathogens in the effluent discharges from the waste system into the soil. The principal components of a private on-site waste disposal system usually include the following:
Many variations on this general scheme are used, depending on local climate, soil conditions, available space, economy, and available materials. Special equipment and systems may be designed for problem or difficult sites such as rocky or wet ground, permafrost, or wet tropical marshlands. Readers who are unfamiliar with what a private septic system is and the types of systems installed should review articles and sketches of septic system components found at The Septic System Information Website
Types of treatment tanks, adsorption systems, pumps, and other special equipment are discussed in some further detail in this text, and are listed in the septic system inspection checklist data. For a more detailed introduction you may want to read our Lockwood article "What is a Septic System" and then return here to continue by using your browser's "BACK" button.
Soil Percolation Requirements and Soil Depth Requirements for Septic Absorption Systems / Septic System Drainfields
What is a septic system soil percolation test?
In specifying the size and type of absorption field (leach field, seepage pits, galleys, other) a septic engineer or health department official will require that a soil percolation test or "perc" test be performed. You may hear it described as a "deep hole test." The first time I participated in this procedure I found myself smiling with surprise at how low-tech the procedure actually was (in New York State.)
After identifying the most-likely location on the lot for placement of a septic drainfield, the excavator used a backhoe to dig a rough hole about 5 ft. deep. Happily no groundwater immediately filled in the hole (which would have been bad news).
Perhaps this is why builders try to have this test done in July which is the period of most-dry weather and lowest groundwater table levels. After digging this rough hole, the septic engineer poured a 5-gallon (joint compound) bucket of water into the hole.
In some cases a few buckets might be dumped therein. After that sophisticated move, the observers simply watched the rate at which the water disappeared. a one-inch drop in water level in this hole in three minutes was considered very good. If the water was found still in the hole at no drop in level the next morning, this was considered seriously bad and probably requiring some soil exchange or other special design measures.
What are the soil perc and other soil requirements for septic systems?
I like the Massachusetts Title 5 Septic Inspection criteria for defining a (at least possibly) functional drainfield, as the text explains the role of the biomass below the absorption bed, sets soil depth requirements, and recognizes the importance of keeping the bottom of the working biomass area in well drained soil sufficiently above the seasonal high water table.
Here is an example of soil requirements for a functional drainfield. This version is particularly clearly written and is for residents of Ohio but the principles apply anywhere. "In Ohio, soil absorption systems can be used in areas where the percolation rate of the soil is between 3 and 60 minutes per inch (soil permeability between 1 and 20 inches per hour). At least 4 feet of suitable soil is required under the soil absorption system to provide adequate treatment of the septic tank effluent. To accommodate the construction of the system and provide adequate soil cover to grade, a minimum of 5 1/2 to 6 1/2 feet of suitable soil is needed above the limiting layer.
A limiting layer may be bedrock, an impervious soil layer (hardpan, fragipan) or a seasonally high water table (gray soil or mottles). The soil absorption system must be at least 8 feet from any drain line on the lot, 50 feet from a water supply, and 10 feet from the property line, right-of-ways and the house. Septic systems cannot be placed on the flood plain and are limited to areas with less than a 15 percent slope." http://ohioline.osu.edu/aex-fact/0743.html Ohio State University Fact Sheet "Septic Tank - Soil Absorption Systems"
How large does the absorption field need to be?
The size of the absorption field needed (in square feet of area, presumably also unencumbered by trees, driveways, buildings, etc.) can range considerably depending on the soil percolation rate. A lot with a good percolation rate or "perc" of perhaps one inch of percolation in three minutes might require about 4500 square feet for a typical three bedroom home. If the same home were built where there was a poor a soil percolation rate of an hour per inch, 9000 square feet or more might be required for the absorption area.
Drainfield size and location also have to take into account local zoning - setback requirements from property borders, setbacks from streams, wetlands, wells, water supply lines, and other encumbrances.
Drainfield trench/line specifications
In the most common design of drainfield, perforated pipes are buried in gravel-filled trenches to form the drainfield. Pipes are placed across the slope line of sloped property (so that all of the effluent doesn't simply rush down to and leak out at the end of the drain line pipe). While some experts describe the bottom of these trenches as "level" in practice they are dug to slope slightly, perhaps 1/8" per foot or less. A typical trench is 18 to 30 inches in depth, and 8 to 12 inches wide.
The septic drainfield trenches are dug about 6 feet apart which allows, in good design, space for a set of replacement trenches to be placed between the original ones when the first set fails. The maximum length of a trench is typically about 150 feet but I've found installations that were three times that length.
Where lot space does not permit drainfield trenches such as I've just described, a septic engineer may specify that seepage pits or galleys are to be installed. These fit in a smaller space since a single pit may be 6' to 8' in diameter. But the depth to which effluent is being delivered (4' or more) means that the sewage effluent is unlikely to be fully treated by a biomass. These systems may successfully "dispose" of effluent but they are probably not adequately "treating" it.
See SEPTIC DRAINFIELD SIZE for design specifications for septic drainfields.
What destroys or shortens the life of drainfields?
It's easy to ruin or shorten the life of a drainfield:
Design Manuals for Septic Systems
Onsite Wastewater Disposal Books