Chapter 10—On-site Wastewater Treatment

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-1

Treatment of Human Waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-1

On-site Wastewater Treatment Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-3

Septic Tank Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-3

Alternative Septic Tank Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-6

Maintaining the On-site Wastewater Treatment Systems . . . . . . . . . . . . . . . . . . . .10-8

Symptoms of Septic System Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-9

Septic Tank Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-9

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-11

Additional Sources of Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-12

Figure 10.1. Conventional On-site Septic System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-1

Figure 10.2. Straight Pipe Discharge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-2

Figure 10.3. Clear Creek Water Contaminated With Sewage. . . . . . . . . . . . . . . . . . . . . . . . . . .10-2

Figure 10.4. Septic Tank System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-3

Figure 10.5. Septic Tank. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-4

Figure 10.6. On-site Sewage Disposal System Site Evaluation Form . . . . . . . . . . . . . . . . . . . . .10-5

Figure 10.7. Cross-section of an Absorption Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-5

Figure 10.8. Mound System Cutaway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-7

Figure 10.9. Low Pressure On-site System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-7

Figure 10.10. Plant-rock Filter System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-8

Figure 10.11. Sludge and Scum in Multicompartment Septic Tank . . . . . . . . . . . . . . . . . . . . . . .10-10

Table 10.1. Mound System Advantages and Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . .10-6

Table 10.2. Low-pressure Pipe Systems Advantages and Disadvantages . . . . . . . . . . . . . . . . . .10-7

Table 10.3. Plant Rock Filter System Advantages and Disadvantages . . . . . . . . . . . . . . . . . . . .10-8

Table 10.4. Septic Tank System Troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-10

Healthy Housing Reference Manual 1

Contents
“Technology has made large populations possible; 

large populations now make technology indispensable.”

Joseph Wood Krutch, Author, 1932

Introduction

The French are considered the first to use an

underground septic tank system in the 1870s. By the

mid 1880s, two-chamber, automatic siphoning septic

tank systems, similar to those used today, were being

installed in the United States. Even now, more than a

century later, septic tank systems represent a major

household wastewater treatment option. Fully one-fourth

to one-third of the homes in the United States use such a

system [1]. 

On-site sewage disposal systems are used in rural areas

where houses are spaced so far apart that a sewer system

would be too expensive to install, or in areas around

cities where the city government has not yet provided

sewers to which the homes can connect. In these areas,

people install their own private sewage treatment plants.

As populations continue to expand beyond the reach of

municipal sewer systems, more families are relying on

individual on-site wastewater treatment systems and

private water supplies. The close proximity of on-site

water and wastewater systems in subdivisions and other

developed areas, reliance on marginal or poor soils for

on-site wastewater disposal, and a general lack of

understanding by homeowners about proper septic tank

system maintenance pose a significant threat to public

health. The expertise on inspecting, maintaining, and

installing these systems generally rests with the

environmental health staff of the local county or city

health departments. 

These private disposal systems are typically called septic

tank systems. A septic tank is a sewage holding device

made of concrete, steel, fiberglass, polyethylene, or other

approved material cistern, buried in a yard, which may

hold 1,000 gallons or more of wastewater. Wastewater

flows from the home into the tank at one end and leaves

the tank at the other (Figure 10.1) [2]. 

Proper maintenance of septic tanks is a public health

necessity. Enteric diseases such as cryptospordiosis,

giardiasis, salmonellosis, hepatitis A, and shigellosis may

be transmitted through human excrement. Historically,

major epidemics of cholera and typhoid fever were

primarily caused by improper disposal of wastewater. The

earliest epidemiology lesson learned was through the

effort of Dr. John Snow of England (1813–1858) during

a devastating cholera epidemic in London [3]. Dr. Snow,

known as the father of field epidemiology, discovered

that the city's water supply was being contaminated by

improper disposal of human waste. He published a brief

pamphlet, On the Mode of Communication of Cholera,

suggesting that cholera is a contagious disease caused by

a poison that reproduces in the human body and is

found in the vomitus and stools of cholera patients. He

believed that the main, although not only, means of

transmission was water contaminated with this poison.

This differed from the commonly accepted belief at the

time that diseases were transmitted by inhaling vapors.

Treatment of Human Waste

Safe, sanitary, nuisance-free disposal of wastewater is a

public health priority in all population groups, small and

large, rural or urban. Wastewater should be disposed of

in a manner that ensures that

• community or private drinking water supplies are

not threatened;

• direct human exposure is not possible;

• waste is inaccessible to vectors, insects, rodents, 

or other possible carriers;

• all environmental laws and regulations are 

complied with; and

• odor or aesthetic nuisances are not created.

Chapter 10: On-site Wastewater Treatment

Healthy Housing Reference Manual 10-1

Figure 10.1. Conventional On-site Septic System [2] 

Effluent leaves home through a pipe, enters a septic tank, travels through a

distribution box to a trench absorption system composed of perforated pipe.
In Figure 10.2, a straight pipe from a nearby home

discharges untreated sewage that flows from a shallow

drainage ditch to a roadside mountain creek in which

many children and some adults wade and fish. The clear

water (Figure 10.3) is quite deceptive in terms of the

health hazard presented. A 4-mile walk along the creek

revealed 12 additional pipes that were also releasing

untreated sewage. Some people in the area reportedly

regard this creek as a source of drinking water.

Raw or untreated domestic wastewater (sewage) is

primarily water, containing only 0.1% of impurities that

must be treated and removed. Domestic wastewater

contains biodegradable organic materials and, very likely,

pathogens. The primary purpose of wastewater treatment

is to remove impurities and release the treated effluent

into the ground or a stream. There are various processes

for accomplishing this:

• Centralized treatment—Publicly owned

treatment works (POTWs) that use primary

(physical) treatment and secondary (biologic)

treatment on a large scale to treat flows of up to

millions of gallons or liters per day,

• Treatment on-site—Septic tanks and absorption

fields or variations thereof, and

• Stabilization ponds (lagoons)—Centralized

treatment for populations of 10,000 or less when

soil conditions are marginal and land space is ample. 

10-2 On-site Wastewater Treatment

Figure 10.2. Straight Pipe Discharge

Source: Donald Johnson; used with permission.

Figure 10.3. Clear Creek Water Contaminated With Sewage

Source: Donald Johnson; used with permission.

John Snow, a London physician, was among the

first to use anesthesia. It is his work in

epidemiology, however, that earned him his

reputation as a prototype for epidemiologists.

Dr. Snow’s brief 1849 pamphlet, On the Mode of Communication of Cholera, caused no great stir, and his theory that the city’s water supply was

contaminated was only one of many proposed

during the epidemic. 

Snow, however, was able to prove his theory in

1854, when another severe epidemic of cholera

occurred in London. Through painstaking

documentation of cholera cases and correlation of

the comparative incidence of cholera among

subscribers to the city’s two water companies, he

showed that cholera occurred much more

frequently in customers of one water company.

This company drew its water from the lower

Thames, which become contaminated with

London sewage, whereas the other company

obtained water from the upper Thames. Snow’s

evidence soon gained many converts. 

A striking incident during this epidemic has become

legendary. In one neighborhood, the intersection of Cambridge Street and Broad Street, the

concentration of cholera cases was so great that the

number of deaths reached over 500 in 10 days.

Snow investigated the situation and concluded that

the cases were clustered around the Broad Street

pump. He advised an incredulous but panicked

assembly of officials to have the pump handle

removed, and when this was done, the epidemic

was contained. Snow was a skilled practitioner as

well as an epidemiologist, and his creative use of

the scientific information of his time is an

appropriate example for those interested in disease

prevention and control [3].

Epidemiology
Not included are pit privies and compost toilets. 

Historically, wastewater disposal systems are categorized as

water-carrying and nonwater-carrying. Nonwater-carried

human fecal waste can be contained and decomposed onsite, the primary examples being a pit privy or compost

toilet. These systems are not practical for individual

residences because they are inconvenient and they expose

users to inclement weather, biting insects, and odors.

Because of the depth of the disposal pit for privies, they

may introduce waste directly into groundwater. It should

be noted that these types of systems are often used and

may be acceptable in low-water-use conditions such as

small campsites or along nature trails [4–6].

On-site Wastewater Treatment Systems

As urban sprawl continues and the population increases in

rural areas, the cost of building additional sewage disposal

systems increases. One of the prime reasons for

annexation is to increase the tax base without increasing

the cost of municipal government. The governments

involved often buy into short-term tax gains at massive

long-term costs for eventual infrastructure improvements

to annexed communities. Installing septic tank systems is

common to provide on-site disposal systems, but it is a

temporary solution at best. Because property size must be

sufficient to allow space for septic system replacement, the

cost to the municipality installing a centralized sewer

system will be dramatically increased because of the large

lot size.

Two microbiologic processes occur in all methods that

attempt to decompose domestic wastewater: anaerobic (by

bacteria that do not require oxygen) and aerobic (by

bacteria that require oxygen) decomposition. Aerobic

decomposition is generally preferred because aerobic

bacteria decompose organic matter (sewage) at a rate

much faster than do anaerobic bacteria and odors are less

likely. Centralized wastewater treatment facilities use

aerobic processes, as do most types of lagoons. Septic tank

systems use both processes. 

Septic Tank Systems

Approximately 21% of American homes are served by onsite sewage disposal systems. Of these, 95% are septic

tank field systems. Septic tank systems are used as a

means of on-site wastewater treatment in many homes,

both in rural and urban areas, in the United States. If

maintained and operated within acceptable parameters,

they are capable of properly treating wastewater for a

limited number of years and will need both routine

maintenance and eventually major repairs. Proper

placement and installation is a key to the successful

operation of any on-site wastewater treatment system, but

septic tank systems have a finite life expectancy and all

such systems will eventually fail and need to be replaced.

Figure 10.4 shows a typical rural home with a well and a

septic system. 

Septic tank systems generally are composed of the septic

tank, distribution box, absorption field (also known as the

soil drainfield), and leach field. The septic tank serves three

purposes: sedimentation of solids in the wastewater, storage of

solids, and anaerobic breakdown of organic materials.

To place the septic tank and absorption field in a way that

will not contaminate water wells, groundwater, or streams,

the system should be 10 feet from the house and other

structures, at least 5 feet from property lines, 50 feet from

water wells, and 25 feet from streams. The entire system

area should be easily identifiable. There have been

occasions when owners have paved or built over the area.

The local health code authorities must be consulted on

required distances in their area because of soil and

groundwater issues. 

Aerobic, or aerated, septic systems use a suspended

growth wastewater treatment process, and can remove

suspended solids that are not removed by simple

sedimentation. Under appropriate conditions, aerobic

units may also provide for nitrification of ammonia, as

well as significant pathogen reduction. Some type of

primary treatment usually precedes the aerated tank. The

tanks contain an aeration chamber, with either

mechanical aerators or blowers, or air diffusers, and an

area for final clarification/settling. Aerobic units may be

designed as either continuous flow or batch flow systems.

Healthy Housing Reference Manual 10-3

Figure 10.4. Septic Tank System [7]
The continuous flow type are the most commercially

available units. Effluent from the aerated tank is conveyed

either by gravity flow or pumping to either further

treatment/ pretreatment processes or to final treatment

and disposal in a subsurface soil disposal system. Various

types of pretreatment may be used ahead of the aerobic

units, including septic tanks and trash traps. 

The batch flow system collects and treats wastewater over

a period of time, then discharges the settled effluent at

the end of the cycle [8]. 

Aerobic units may be used by individual or clustered

residences and establishments for treating wastewater

before either further treatment/pretreatment or final onsite subsurface treatment and disposal. These units are

particularly applicable where enhanced pretreatment is

important, and where there is limited availability of land

suitable for final on-site disposal of wastewater effluent.

Because of their need for routine maintenance to ensure

proper operation and performance, aerobic units may be

well-suited for multiple-home or commercial applications,

where economies of scale tend to reduce maintenance

and/or repair costs per user. The lower organic and

suspended solids content of the effluent may allow a

reduction of land area requirements for subsurface

disposal systems. 

A properly functioning septic tank will remove

approximately 75% of the suspended solids, oil, and

grease from effluent. Because the detention time in the

tank is 24 hours or less, there is not a major kill of

pathogenic bacteria. The bacteria will be removed in the

absorption field (drainfield). However, there are soils and

soil conditions that prohibit the ability of a drainfield to

absorb effluent from the septic tank. 

Septic tanks are sized to retain the total volume of sewage

produced by a household in a 24-hour period. Normally a

1,000-gallon tank is the minimum size to use. State or

local codes generally require larger tanks as the potential

occupancy of the home increases (e.g., 1,250 gallons for

four bedrooms) and may require two tanks in succession

when inadequate soils require alternative system

installation. Figure 10.5 shows a typical septic tank.

Distribution boxes are not required by most on-site

plumbing codes or by the U.S. Environmental Protection

Agency. When used, distribution boxes provide a

convenient inspection port. In addition, if a split system

absorption field is installed (two separate absorption

trench systems), the distribution box is a convenient place

to install a diversion valve for annually alternating

absorption fields. 

Absorption Field Site Evaluation

The absorption field has a variety of names, including

leach field, tile field, drainfield, disposal field, and

nitrification field. The effluent from the septic tank is

directed to the absorption field for final treatment. The

suitability of the soil, along with other factors noted

below, determines the best way to properly treat and

dispose of the wastewater.

Most, but unfortunately not all, states require areas not

served by publicly owned sewers to be preapproved for

on-site wastewater disposal before home construction

through a permitting process. This process typically requires a

site evaluation by a local environmental health specialist,

soil scientist, or, in some cases, a private contractor. To

assist in the site evaluation process, soil survey maps from

the local soil conservation service office may be used to

provide general information about soils in the area. 

The form shown in Figure 10.6 is typical of those used in

conducting a soil evaluation.

Sites for on-site wastewater disposal are first evaluated for

use with a conventional septic tank system. Evaluation

factors include site topography, landscape position, soil

texture, soil structure, internal drainage, depth to rock or

other restrictive horizons, and useable area. If the criteria

are met, a permit is issued to allow the installation of a

conventional septic tank system. Areas that do not meet

the criteria for a conventional system may meet lessrestrictive criteria for an alternative type of system. 

10-4 On-site Wastewater Treatment

Figure 10.5. Septic Tank [9]
Many sites are unsuitable for any type of on-site

wastewater disposal system because of severe topographic

limitations, poor soils, or other evaluation criteria. Such

sites should not be used for on-site wastewater disposal

because of the high likelihood of system failure.

Some states and localities may require a percolation test as

part of the site evaluation process. As a primary

evaluation method, percolation tests are a poor indicator

of the ability of a soil to treat and move wastewater

throughout the year. However, information obtained by

percolation tests may be useful when used in conjunction

with a comprehensive soil analysis.

Absorption Field Trench

A conventional absorption field trench (Figure 10.7), also

known as a rock lateral system, is the most common

system used on level land or land with moderate slopes

with adequate soil depth above the water table or other

restrictive horizons. The effluent from the septic tank

flows through solid piping to a distribution box or, in many

cases, straight to an absorption field. With the conventional

system and most alternative systems, the effluent flows

through perforated pipes into gravel-filled trenches and

subsequently seeps through the gravel into the soil.

The local regulatory agency should be consulted about

the acceptable depth of the absorption field trench. Some

states require as much as 4 feet of separation beneath the

bottom of the trench and the groundwater. The depth of

absorption field trenches should be at least 18 inches, and

ideally no deeper than 24 inches. The absorption field

pipe should be laid flat with no slope. There should be a

minimum of 12 to 18 inches of acceptable soil below the

bottom of the trench to any bedrock, water table, or

restrictive horizon. The length of the trench should not

exceed 100 feet for systems using a distribution box.

Serpentine systems may be several hundred feet long and

should be filled with crushed or fragmented clean rock or

gravel in the bottom 6 inches of the trench. Perforated

4-inch-diameter pipe is laid on top of the gravel then

covered with an additional 2 inches of rock and leveled for

a total of 12 inches. A geotextile material or a biodegradable

material such as straw should be placed over the gravel

before backfilling the trench to prevent soil from clogging

the spaces between the rocks. 

One or more monitoring ports should be installed in the

absorption area extending to the bottom of the gravel to

allow measurement of the actual liquid depth in the

Healthy Housing Reference Manual 10-5

Figure 10.6. On-site Sewage Disposal System Site Evaluation Form

Figure 10.7. Cross-section of an Absorption Field [10]
gravel. This is essential for subsequent testing of the

adequacy of the system.

As a general rule, using longer and narrower trenches to

meet square footage requirements produces a better

working and longer lasting ground absorption sewage

disposal system. Studies have shown that as septic systems

age, the majority of effluent absorption by the soil is

provided by lateral movement through the trench

sidewalls. Longer and narrower trenches (such as 400 feet

long by 2 feet wide instead of 200 feet by 4 feet to obtain

800 square feet) greatly increase the sidewall area of the

system for lateral movement of wastewater. 

Alternative Septic Tank Systems 

As the cost of land for home building increases and the

availability of land decreases, land that was once

considered unsuitable is being developed. This land often

has poor soil and drainage properties. Such sites require a

considerable amount of engineering skill to design an

acceptable wastewater disposal system. In many cases,

sites are not acceptable for seepage systems within a

reasonable cost. These systems are primarily regulated by

state and local government and, before use, approval must

be obtained from the appropriate regulatory agencies.

Even if a site is approved in one state or county jurisdiction, a

similar site may not be approved in another.

The primary difficulty with septic tank systems is treating

effluent in slowly permeable or marginal soils. Low-wateruse devices, when installed, may make it possible to use a

small percentage of septic tank systems in marginal soil.

However, low-water-use devices are usually required as

part of a larger effort to develop a usable alternative

sewage disposal system. Alternative sewage disposal

methods that can be used if regular subsurface disposal is

not appropriate are numerous [11]. Some of the more

common alternative systems are described below.

Mound Systems

A mound system (Table 10.1) is elevated above the

natural soil surface to achieve the desired vertical

separation from a water table or impervious material. The

elevation is accomplished by placing sand fill material on

top of the best native soil stratum. At least 1 foot of

naturally occurring soil is necessary for a mound system to

function properly. Minimizing water usage in the home also

is critical to prevent effluent from weeping through the

sides of the mound (Figure 10.8).

When a mound system is constructed, the septic tank

usually receives wastewater from the house by gravity

flow. A lift station is located in the second compartment

or in a separate tank to pump the effluent up to the

distribution piping in the mound. Floats in the lift station

control the size of the pumped effluent dose. An alarm

should be installed to alert the homeowner of pump failure so

that repairs can be made before the pump tank overfills.

Low-Pressure Pipe Systems

Low-pressure pipe (LPP) systems may also be used where

the soil profile is shallow. These systems are similar to

mounds except that they use naturally occurring soil as it

exists on-site instead of elevating the disposal field with

soil fill material. LPP systems are installed with a

trenching machine at depths of 12 to 18 inches. The LPP

system consists of a septic tank, high-water alarm,

pumping tank, supply line, manifold, lateral line, and

submersible effluent pump (Figure 10.9).

10-6 On-site Wastewater Treatment

Table 10.1. Mound System Advantages and Disadvantages
When septic tank effluent rises to the level of the pump

control in the pumping tank, the pump turns on, and

effluent moves through the supply line and distribution

laterals. The laterals contain small holes and are typically

placed 3 to 8 feet apart. From the trenches, the effluent

moves into the soil where it is treated. The pump turns

off when the effluent falls to the lower control. Dosing

takes place one to two times daily, depending on the

amount of effluent generated. Pump malfunctions set off

an alarm to alert the homeowner. The time between doses

allows the effluent to be absorbed into the soil and also

allows oxygen to reenter the soil to break down solids that

may be left behind. If the pump malfunctions, an alarm

notifies the homeowner to contact a qualified septic

system contractor. The pump must be repaired or replaced

quickly to prevent the pump tank from overflowing.

Table 10.2 shows the advantages and disadvantages of LPP

systems.

Plant-rock Filter Systems (Constructed Wetlands)

Considered experimental in some states, plant-rock filter

systems are being used with great success in Kentucky,

Louisiana, and Michigan. Plant-rock filters generally

consist of a septic tank (two-compartment), a rock filter,

and a small overflow lateral (absorption) field. Overflow

from the septic tank is directed into the rock filter. The

rock filter is a long narrow trench (3 to 5 feet wide and

60 to 100 feet long) lined with leak-proof polyvinyl

chloride or butylplastic to which rock is added. A 2- to

4-inch-diameter rock is used below the effluent flow line

and larger rock above (Figure 10.10). 

Plant-rock filter systems are typically sized to allow

1.3 cubic feet of rock area per gallon of total daily waste

flow. A typical size for a three-bedroom house would be

468 square feet of interior area. Various width-to-length

ratios within the parameters listed above could be used to

obtain the necessary square footage. The trenches can

even be designed in an “L” shape to accommodate small

building lots.

Treatment begins in the septic tank. The partially treated

wastewater enters the lined plant-rock filter cell through

solid piping, where it is distributed across the cell. The

plants within the system introduce oxygen into the

Healthy Housing Reference Manual 10-7

Table 10.2. Low-pressure Pipe Systems Advantages and Disadvantages

Figure 10.9. Low Pressure On-site System [12]

Figure 10.8. Mound System Cutaway [3]
wastewater through their roots. As the wastewater

becomes oxygenated, beneficial microorganisms and fungi

thrive on and around the roots, which leads to digestion

of organic matter. In addition, large amounts of water are

lost through evapotranspiration. The kinds of plants most

widely used in these systems include cattails, bulrush,

water lilies, many varieties of iris, and nutgrass. Winter

temperatures have little effect because the roots are doing

the work in these systems, and they stay alive during the

winter months. Discharge from wetlands systems may

require disinfection. The advantages and disadvantages of

the plant-rock filter system are shown in Table 10.3. 

Maintaining On-site Wastewater Treatment

Systems

Dos and don’ts inside the house:

• Do conserve water. Putting too much water into

the septic system can eventually lead to system

failure. (Typical water use is about 60 gallons per

day for each person in the family.) The standard

drain field is designed for a capacity of

120 gallons per bedroom. If near capacity, systems

may not work. Water conservation will extend the

life of the system and reduce the chances of

system failure. 

• Do fix dripping faucets and leaking toilets.

• Do avoid long showers.

• Do use washing machines and dishwashers only

for full loads.

• Do not allow the water to run continually when

brushing teeth or while shaving.

• Do avoid disposing of the following items down

the sink drains or toilets: chemicals, sanitary

napkins, tissues, cigarette butts, grease, cooking

oil, pesticides, kitty litter, coffee grounds,

disposable diapers, stockings, or nylons.

• Do not install garbage disposals. 

• Do not use septic tank additives or cleaners. They

are unnecessary and some of the chemicals can

contaminate the groundwater. 

Dos and don’ts for outside maintenance:

• Do maintain adequate vegetative cover over the

absorption field. 

• Do not allow surface waters to flow over the tank

and drainfield areas. (Diversion ditches or

subsurface tiles may be used to direct water away

from system.) 

• Do not allow heavy equipment, trucks, or

automobiles to drive across any part of the system. 

10-8 On-site Wastewater Treatment

Figure 10.10. Plant-rock Filter System [12]

Table 10.3. Plant-rock Filter System Advantages and Disadvantages 
• Do not dig into the absorption field or build

additions near the septic system or the repair area. 

• Do make sure a concrete riser (or manhole) is

installed over the tank if not within 6 inches of

the surface, providing easy access for measuring

and pumping solids. (Note: All tanks should have

two manholes, one positioned over the inlet

device and one over the outlet device.) 

There is no need to add any commercial substance to

“start” or clean a tank to keep it operating properly. They

may actually hinder the natural bacterial action that takes

place inside a septic tank. The fecal material, cereal grain,

salt, baking soda, vegetable oil, detergents, and vitamin

supplements that routinely make their way from the

house to the tank are far superior to any additive.

Symptoms of Septic System Problems

These symptoms can mean you have a serious septic

system problem: 

• Sewage backup in drains or toilets (often a black

liquid with a disagreeable odor). 

• Slow flushing of toilets. Many of the drains will

drain much slower than usual, despite the use of

plungers or drain-cleaning products. This also can

be the result of a clogged plumbing vent or a

nonvented fixture.

• Surface flow of wastewater. Sometimes liquid

seeps along the surface of the ground near your

septic system. It may or may not have much of an

odor and will range from very clear to black in

color.

• Lush green grass over the absorption field, even

during dry weather. Often, this indicates that an

excessive amount of liquid from the system is

moving up through the soil, instead of down, as it

should. Although some upward movement of

liquid from the absorption field is good, too

much could indicate major problems. 

• The presence of nitrates or bacteria in the

drinking water well indicates that liquid from the

system may be flowing into the well through the

ground or over the surface. Water tests available

from the local health department will indicate

whether this is a problem. 

• Buildup of aquatic weeds or algae in lakes or

ponds adjacent to your home. This may indicate

that nutrient-rich septic system waste is leaching

into the surface water, which may lead to both

inconvenience and possible health problems. 

• Unpleasant odors around the house. Often, an

improperly vented plumbing system or a failing

septic system causes a buildup of disagreeable odors.

Table 10.4 is a guide to troubleshooting septic tank problems.

Septic Tank Inspection

The first priority in the inspection process is the safety of

the homeowner, neighbors, workers, and anyone else for

which the process could create a hazard. 

• Do not enter septic tanks or cesspools.

• Do not work alone on these tanks.

• Do not bend or lean over septic tanks or cesspools.

• Note and take appropriate action regarding unsafe

tank covers.

• Note unsanitary conditions or maintenance needs

(sewage backups, odor, seepage).

• Do not bring sewage-contaminated clothing into

the home.

• Have current tetanus inoculations if working in

septic tank inspection.

Methane and hydrogen sulfide gases are produced in a

septic tank. They are both toxic and explosive. Hydrogen

sulfide gas is quite deceptive. It can have a very strong

odor one moment, but after exposure, the odor may not

be noticed.

Inspection Process

As sewage enters a septic tank, the rate of flow is reduced

and heavy solids settle, forming sludge. Grease and other

light solids rise to the surface, forming a scum. The

sludge and scum (Figure 10.11) are retained and break

down while the clarified effluent (liquid) is discharged to

the absorption field.

Sludge eventually accumulates in the bottom of all septic

tanks. The buildup is slower in warm climates than in

colder climates. The only way to determine the sludge

Healthy Housing Reference Manual 10-9
10-10 On-site Wastewater Treatment

Table 10.4. Septic Tank System Troubleshooting 

Figure 10.11. Sludge and Scum in Multicompartment Septic Tank [13]
depth is to measure the sludge with a probe inserted through

an inspection port in the tank’s lid. Do not put this job off until

the tank fills and the toilet overflows. If this happens, damage

to the absorption field could occur and be expensive to repair. 

Scum Measurement

The floating scum thickness can be measured with a probe.

The scum thickness and the vertical distance from the

bottom of the scum to the bottom of the inlet can also be

measured. If the bottom of the scum gets within 3 inches

of the outlet, scum and grease can enter the absorption

field. If grease gets into the absorption field, percolation is

impaired and the field can fail. If the scum is near the

bottom of the tee, the septic tank needs to be cleaned out.

The scum thickness can best be measured through the

large inspection port. Scum should never be closer than 3

inches to the bottom of the baffle. The scum thickness is

observed by breaking through it with a probe, usually a pole.

Sludge Measurement

To measure sludge, make a sludge-measuring stick using a

long pole with at least 3 feet of white cloth (e.g., an old

towel) on the end. Lower the measuring stick into the

tank, behind the outlet baffle to avoid scum particles,

until it touches the tank bottom. It is best to pump each

tank every 2 to 3 years. Annual checking of sludge level is

recommended. The sludge level must never be allowed to

rise within 6 inches of the bottom of the outlet baffle. In

two-compartment tanks, be sure to check both compartments.

When a septic tank is pumped, there is no need to

deliberately leave any residual solids. Enough will remain

after pumping to restart the biologic processes. 

References

1. University of California Cooperative Extension,

Calaveras County. Septic tanks: the real poop. San

Andreas, CA: University of California Cooperative

Extension, Calaveras County; no date. Available

from URL: http://cecalaveras.ucdavis.edu/realp.htm.

2. University of Nebraska-Lincoln. Residential onsite wastewater treatment: septic system and

drainfield maintenance. Lincoln, NE: University

of Nebraska-Lincoln; 2000. Available from URL:

http://ianrpubs.unl.edu/wastemgt/g1424.htm.

3. Rosenberg CE. The cholera years. Chicago: The

University of Chicago Press; 1962.

4. Salvato J, Nemerow NL, Agardy FJ, editors.

Environmental engineering. 5th ed. New York:

John Wiley and Sons; 2003.

5. Advanced Composting Systems. Phoenix

composting toilet system. Whitefish, MT:

Advanced Composting Systems; no date. Available

from URL: http://www.compostingtoilet.com.

6. BioLet USA, Inc. Composting toilets.

Newcomerstown, OH: BioLet USA, Inc.; no date.

Available from URL: http://www.biolet.com.

7. Mankl K, Slater B. Septic system maintenance.

Columbus, OH: The Ohio State University 

Extension; no date. Available from URL:

http://ohioline.osu.edu/aex-fact/0740.html.

8. Hutzler NJ, Waldorf LE, Fancy J. Aerated tanks

(aerobic units). In: Performance of aerobic

treatment units. Madison, WI: University of

Wisconsin - Madison; no date. Available from

URL: http://www.ci.austin.tx.us/wri/treat5.htm.

9. Center for Disease Control. Basic housing

inspection. Atlanta: US Department of Health

and Human Services; 1976. 

10. Purdue Research Foundation. Environmental

education software series. West Lafayette, IN:

Purdue Research Foundation; 1989. Available

from URL: http://www.inspect-ny.com/

septic/trench.gif.

11. North Carolina Cooperative Extension Service.

On-site wastewater treatment websites.

Jacksonville, NC: North Carolina Cooperative

Extension Service; 2002. Available from URL:

http://www.ces.ncsu.edu/onslow/staff/drashash/

enved/sepsites.html. 

12. Clay Township Regional Waste District. Septic

systems. Indianapolis: Clay Township Regional

Waste District; 2004. Available from URL:

http://www.ctrwd.org/septics.htm. 

13. Jackson Purchase Resource Conservation and

Development Foundation, Inc. Septic systems: an

overview. Cynthiana, KY: Jackson Purchase Resource

Conservation and Development Foundation, Inc.;

no date. Available from URL:

http://www.jpf.org/LRV/septic.htm. 

Healthy Housing Reference Manual 10-11
Additional Sources of Information

Agency for Toxic Substances and Disease Registry. Science

page, Office of the Associate Administrator for Science.

Atlanta: US Department of Health and Human Services;

no date. Available from URL:

http://www.atsdr.cdc.gov/science/. 

American Society of Civil Engineers. Available from

URL: http://www.asce.org. 

Burks BD, Minnis MM. Onsite wastewater treatment

systems. Madison, WI: Hogarth House, Ltd.; 1994. Textbook

and reference manual on all aspects of on-site treatment. 

International Code Council. International private sewage

disposal code, 2000. Falls Church, VA: International

Code Council; 2000. 

National Onsite Wastewater Recycling Association

(NOWRA). Available from URL: http://www.nowra.org

or 1-800-966-2942.

National Small Flows Clearinghouse. Available from

URL: http://www.nesc.wvu.edu/nsfc/nsfc_index.htm or

1-800-624-8301. 

US Army Corps of Engineers. Available from URL:

http://www.usace.army.mil.

10-12 On-site Wastewater Treatment