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How to Test & Inspect Septic Systems for Signs of Failure

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Septic system inspection & test procedures:

Acomplete guide to septic system inspection & testing: this article series answers just about any question you might have about buying or owning a house with a septic system. In articles listed here we explain in complete detail how to buy, inspect, test, diagnose maintain and repair septic tanks and all other components of septic systems.

We give in-depth information about conventional septic tanks, drain fields, septic pipes, and septic waste handling.

This page organizes and links to our detailed septic system inspection, test, repair, and design articles, including our online septic systems book. Septic testing class presentations, photos, sketches, tables, links to products and consultants are provided. [Above photograph shows an open access to a steel septic tank.]

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Septic System Inspection & Testing - Topics List

Photograph of a septic cleaning contractor opening a septic tank for inspection and pumping.Inspecting a home or commercial septic system means finding evidence that the onsite wastewater disposal and treatment system is working properly.

The procedures used must cope with the difficulty that a septic system consists of buried components, involves major costs to replace, and may involve serious health and life safety risks as well. The major steps in septic system inspection and diagnosis are discussed below in narrative form.

Watch out: death by leaning over, falling-into or deliberately entering a septic tank is more common than many people realize.

Do not fail to observe and keep people away from septic tanks, cesspools, drywells, or other site features that are not protected by a safe, secure, cover of adequate strength.

Article Contents

This article provides a complete overview of septic system inspection & testing & common failures. At the end of this article we provide links to in-depth septic system design, inspection, testing & repair articles.

Home buyers of a property with a septic tank should be sure to read HOME BUYERS GUIDE to SEPTIC SYSTEMS - What to Do, Step by Step.

Inspecting, Testing, & Maintaining Residential Septic Systems - An Online Septic System Textbook

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.

Introduction to Septic Systems - Inspection, Testing, Maintenance - What is a Septic System

Photograph of  a conventional septic tank during installation.

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.

Basic Components of a Private Septic System

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


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 SEPTIC CARE INSTRUCTIONS and then return here to continue by using your browser's "BACK" button.

Safety Warnings for Septic System Inspectors

Septic tank collapsing © D Friedman at

Providing inspection and diagnosis of on-site waste disposal systems is an extremely valuable public service which helps protect people from expensive unanticipated septic system repair costs and helps protect public health by assuring sanitary disposal of sewage and gray water waste from buildings.

More importantly though, such inspections may detect and warn about serious safety hazards at some properties.

The strong warnings issued below intend to reduce septic system safety hazards for inspectors and property owners/occupants, but it is not the author's intention to dissuade inspectors from providing this valuable service.

Watch out: Danger lurks at cesspools, open covers, tanks or tank covers in poor condition, and from high levels of methane gas. These risk collapse, falling, asphyxiation, and other potentially fatal hazards as well as risks of unsanitary conditions.

Septic and Cesspool Inspection Safety:

Septic System Warnings to Home Owners and Home Buyers

Steel septic tank collapse © D Friedman at

Septic system concerns for a Building owner start with safety. Here are some red flags:

Septic Inspector Qualifications/Licensing

Requirements, including licensing and education requirements for septic system inspectors or test personnel vary among states & provinces. Here we list examples of qualifications for septic inspectors.

If you perform septic inspections you are obligated to do so with proper information, training, procedures, and in some communities a license is required. Some states (e.g. CA, CT, NJ, MA) have specific certification requirements for inspectors of septic systems, as well as specific regulations regarding the performance of the inspection itself. Be sure to obtain information pertinent to your own state, usually from the state health department or state department of environmental protection.

For example, Massachusetts septic inspectors will want to look at the links and the Title 5 regulations at our page on the Massachusetts Septic Testing Law.

Other links to septic system installation and inspection regulatory agencies are at our "Local, State, U.S. Federal Government, & International Agencies & Resources for Septic Systems Wastewater Treatment" page.

Watch out: too often we hear from readers that their "septic inspector" was a septic tank pumping company who did nothing but pump the septic tank, or that their inspector simply looked around the site and gave a generic septic inspection report that did not provide the information recommended in the article above.

How Does Each Septic System Component Fail? - What to Look For During a Septic Inspection

SEPTIC effluent BREAKOUT in the yard, or odors, is a sign of a problem with the soil absorption system

Regarding the photo above, we have seen effluent breakout in the yard soon after new owners occupied the home. Some careful gentle excavating explored the problem to confirm that the breakout was in the drainfield, not at a collapsing septic tank.

This article explains detailed "how to" steps instructing the investigator in how to inspect specific septic components for signs of failure. The following section will discuss types and causes of septic failure and will provide criteria that define "failure."

Before digging up your septic tank or calling a septic pumper, if you think the septic system is failed because of drain blockage or drains backing up into the building, you should

see CLOGGED DRAIN DIAGNOSIS & REPAIR Is it a blocked drain or the septic system? - A First Step for Homeowners".

If you link to that text, please return here using your browser's "BACK" button.

Inspecting Outside Waste Piping

Outside waste piping conducts sewage (black water and gray water) from the Building to the treatment tank or "septic tank," and from the treatment tank to the distribution box. These lines should be of solid, non-perforated material and need to be protected from mechanical damage (such as by vehicles).

Piping extending from the distribution box into drain fields is normally perforated, though solid lines might be used if effluent is being processed by more specialized devices such as seepage pits, galleys, or a sand-bed system.

House to septic tank

Steel septic tank © D Friedman at

This line may become blocked by waste, damaged by collapse of a section, or invaded by roots. Detection of these conditions is fairly easy by routing a snake or power snake from the Building drain to the septic tank.

An experienced power snake operator can often tell by "feel" that a drain line is collapsed, partially collapsed, or invaded by roots. While you may make a temporary "repair" of such a condition by drain-cleaning, if the line is broken or root-invaded, you should expect to have to excavate and replace it soon.

The septic tank and cleanout access covers are easy to spot next to this home in northern Maine.

Because this property is located close to the seashore, extra care to review clearance distances as well as condition of the septic system are in order.


Tank to Distribution Box

The same failures can occur on this line as from house to tank. But the distribution box may be more difficult to find.

By viewing the entire site to locate the drainfield or soakaway bed it is often possible to make a good guess at the D-box location between the septic tank and drainfield.

Drain field piping

In a conventional "drain field" of perforated pipes buried in gravel-filled trenches, a drain line may be invaded by tree roots. This is why experts advise keeping tree and shrub plantings away from drain fields. Vehicle traffic can also collapse this or any outdoor waste piping, which is why experts advise against ever driving over a drainfield or over any other septic system components.

Also see SEPTIC DRAINFIELD LOCATION: how to find the septic drain field or leaching bed.

Inspecting Septic Tank Condition

The purpose of the treatment tank or "septic tank" is to contain solid waste and to permit the beginning of bacterial action to process sewage into a combination of clarified effluent, settled sludge, or floating scum in the tank.

An intact, un-damaged septic tank is normally always filled with these materials.

Only by pumping and visual inspection can actual tank capacity and condition be completely determined. Probing in the area of a tank, without excavation, is not recommended as the probe may damage a steel or fiberglass tank.

Steel septic tanks

Steel septic tank © D Friedman at

Steel septic tanks typically last 20-25 years, then rust, and collapse. Before this time steel baffles may rust off (damaging the drain field with sludge) or the tank top may become rusty and unsafe.

Since steel tank tops can be replaced while leaving the old tank in place, the condition of the top itself is not a reliable indicator of tank condition. See STEEL SEPTIC TANKS for details about steel septic tanks.

In our photo at left we see a round steel septic tank cover - really too close to the building, but easy to find. The waste line between this building and the septic tank will be short but may be hard to access if it's accessible only through the crawl space.

Concrete septic tanks

Concrete tanks at an existing septic installation are usually viable, but might have damaged baffles or cracks that permit seepage of groundwater in or septic effluent out around the tank. Occasionally we've seen tanks made of poor-quality concrete (insufficient portland cement) which eroded badly.

If the tank outlet or absorption system have been blocked, examination of the tank interior may show that effluent is or has been above the top of the baffles (see "baffles" below) thus indicating a system failure discussed next.

See CONCRETE SEPTIC TANKS for details about concrete septic tanks.

For detailed guides to inspecting different types of septic tanks


Inspecting the Condition of Septic Tank Baffles

Septic tank baffles © D Friedman at

Baffles in a septic tank are provided to keep solids and floating scum and grease inside the tank. Baffles are provided at both the inlet to the tank (from the Building) and the outlet from the tank (to the absorption system).

Broken baffles or high sludge levels can cause solids to flow out of the tank and into the absorption system. The result is reduced absorption into surrounding soil and eventual failure of the system.

Floating scum thickness and settled solids thickness can be measured through access ports into the tank or cesspool. Finding solids at or covering the outlets or damaged baffles should result in report of a very questionable adsorption system and possible major repair cost.

If baffles are lost or damaged (rusted off on a steel tank or broken off on a concrete tank), they can be repaired or replaced. For example at a steel tank the contractor may simply insert a plastic piping "Tee" into the tank inlet or outlet to create a new baffle system.

However, depending on how long the tank was used without good baffles, the volume of solids and grease that moved from the tank to the absorption system will have begun clogging soils there and will have reduced the future life expectancy of the absorption system.

Baffle damage and repair, or even a complete tank replacement when the absorption system has been left alone always lead the author to warn the Building owner that the future life of the absorption system may be in doubt and that additional expense will be involved.

See  SEPTIC TANK BAFFLES for details.

Inspecting the Level of Accumulated Solids, Sludge and Floating Scum in Treatment Tanks

Solids entering a septic tank are intended to remain there until pumped out during tank service. A large portion of solids settle to the bottom of the tank as sludge.

Grease and floating scum remain at the top of the sewage in the tank. Baffles (discussed above) help keep solids, scum, and grease in the tank. Bacterial action in the tank make a modest reduction in the solids volume and begin the processing of sewage pathogens, a step later completed by soil bacteria in the absorption fields.

Net free area: If the sludge level becomes too high or the floating scum layer too thick, in addition to risking passage of solids out of the tank (damaging the absorption system), the remaining "net free area" of liquid in the tank is reduced. When the net free area becomes too small, there is insufficient time for waste entering the tank to settle out as bottom sludge or top floating scum.

That is, for an in-use septic tank with a small net free area, the frequent entry of solid and liquid waste will keep the tank debris agitated, thus forcing floating debris into the absorption system where the life of that component will be reduced (due to soil clogging).

The importance of keeping an adequate net free area in a septic tank is the reason that tanks need to be pumped at regular intervals. Building owners who never pump a tank until it is clogged have already damaged the absorption system.

See the Massachusetts Title 5 Septic Inspection procedure discussed below for details about determining the level of floating and settled scum and sludge in the tank.


Inspecting the Septic System Distribution Box

Photograph of a septic system distribution box

The distribution box (more than one may be in use) connects a single effluent line from the septic tank to a network of absorption system components such as drainfield leach lines or to a network of seepage pits or galleys.

In good system design the outlet openings from the distribution box to each drainfield line can be adjusted to regulate the flow among the various absorption lines.

If a distribution box becomes tipped (or clogged) effluent may be routed to only a portion of the absorption system, thus overloading it and leading to a "breakout" of effluent at the surface or to clogging and system backup.

An examination of the box interior may show flood lines in the box if the drain field has been clogged or saturated in the past even if at the time of inspection the box is not flooded. If the fields have been flooded you should be pessimistic about the remaining life of the absorption system.

If the distribution box or D-box is tipped and/or effluent has not been uniformly distributed among the drainfield lines (assuming they are of equal length and in equally good soils), only a simple adjustment of the outflow may be needed. Round plugs with eccentric openings may be present or can be inserted in the D-box outlet openings to regulate flow among the individual absorption lines.


Liquid level in the adsorption system

The absorption system or "drain field" has two jobs. First, it disposes of liquid effluent by permitting it to seep into the soil below. Second, a "bio-mat" of bacteria which forms in the soil below the drainage field processes pathogens in the septic effluent to make the effluent sufficiently sanitary as to avoid contaminating nearby ground water.

This distinction between successful "disposal" and successful "treatment" is important to avoid groundwater contamination but has not been addressed by regulation in every municipality. Municipalities which require a minimum distance between the bottom of the drain field trenches (or equivalent component) and the top of the seasonal high ground water table have recognized the importance of a working bio-mat and the need to provide adequate dry soil for it to function.

Even in a well-designed drainage field, eventually the soil surrounding the drainfield device (perforated pipe in gravel trench or other seepage system) becomes clogged with grease and debris.

Examining an excavated cross-section of a failed drainfield will often display a black or gray band of sludge and grease of about 1" thickness at the inside perimeter of the gravel trench. When this layer of soil becomes sufficiently clogged the passage of effluent into the soil below is slowed and eventually blocked, leading to the need for replacement. Keeping a tank pumped so as to reduce the passage of debris and grease into a drain field will extend its life.

This is the most expensive problem to correct. Look for septic effluent seepage to ground surface in area of equipment or downhill from such equipment.

Look for (illegal) drain field line extensions to nearby streams, storm drains, or adjoining properties where the temptation to "fix" a failing system by sending the effluent to an improper destination overwhelmed a previous owner or repair company. In some areas inspectors use septic loading and dye test. Seepage may be due to overloaded tank, failed absorption system, or blocked/broken piping (may be less costly).

An excavator or septic contractor will often explore one or more drain lines (or similar components) by excavating a portion of it to look for evidence of flooding or soil clogging. We've used a simple probe at the end and along a leach bed to check for flooding of that component. (Be careful not to break or collapse old piping.)

Types, Causes, and Failure Criteria for Septic Systems: Defining "Failed" Onsite Wastewater Treatment or Disposal

This article explains types of septic system failure lists causes of each type of septic component failure, and lists the septic component failure criteria or in other words what conditions are defined as "failure"?. The detailed "how to" steps instructing how to inspect specific septic components for signs of failure are discussed in the text above.

In simplest terms, there are two visible disposal failures:

Septic odors may also indicate a system failure or an imminent failure. But such odors may also be produced by defects in the plumbing vent system or other site conditions. Beware, sewer gas contains methane and is explosive if it reaches a dangerous concentration inside a Building.

Typical causes range from things that are easy and cheap to repair, to a need for complete system replacement:

However there can also be treatment failures. Effluent may not back up or appear on the surface, but if insufficiently treated effluent reaches a private well or any stream or waterway, the environment is being contaminated -- an unacceptable condition. Historically many people have just worried about disposal.

As the quality of drinking water deteriorates in many areas and as population grows in many previously thinly-populated areas, proper treatment has become the real concern for everyone's health.

For example, if there is not sufficient soil between the bottom of the soil absorption system trenches and the local groundwater, the local environment is being contaminated.

Other causes of onsite wastewater disposal system failure:

Onsite Waste Disposal System Failure Criteria

Massachusetts Title 5 lists specific failure criteria and serves as a good model for septic inspections anywhere.

Cesspool failures (MA)

Soil Absorption System Failures (leach fields, drain fields, seepage pits)

See these detailed articles describing

Types of Septic System Failures

Levels of Septic System Inspection and Testing

Photograph of septic dye test - You are at the (C)Copyright Protected 2006-1986 Septic System Information Website - Septic Tanks, Septic Fields, Septic System Repairs, Septic System Design - this sketch is a Septic tank

LEVEL-0 Septic Inspections - Basic Visual, Loading & Dye Test

The minimum quantities of water and septic dye needed are discussed in a separate chapter at SEPTIC DYE TEST PROCEDURE - a chapter of this text "Inspecting, Testing, & Maintaining Residential Septic Systems".

ADDITIONAL LEVELS OF INSPECTION for real estate transactions and for problem diagnosis and bidding prior to actual repair work.

Level 1 Septic System Inspections - Visual, Open Covers, Inspect Equipment, Loading & Dye Test

Also see Level 1 Septic System Inspections in our Inspecting, Testing, & Maintaining & Designing Residential Septic Systems - Septic Systems Online Book

Level 2 Septic System Inspection Includes - Visual, Open, D-Box, Pump & Inspect Septic Tank

Also see Level 2 Septic Inspections in our Inspecting, Testing, & Maintaining & Designing Residential Septic Systems - Septic Systems Online Book

Level 3 Septic System Inspection Includes - visual, open, pump, soil percolation, site engineering

Also see LEVEL 3 SEPTIC INSPECDTIONS in our free online book: Inspecting, Testing, & Maintaining & Designing Residential Septic Systems - Septic Systems Online Book

Absorption System Design & Maintenance Soil Perc Tests & Septic Drain Field Design & Maintenance

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." 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 for a septic drainfield is 18 to 30 inches in depth, and 8 to 12 inches wide. The 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.

What destroys or shortens the life of drainfields?

It's easy to ruin or shorten the life of a drainfield:


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