EPA 832-R-04-001

September 2004

United States

Environmental Protection

Agency

Office of Water

Office of Wastewater Management

Washington DC 20460

Primer for Municipal 

Wastewater Treatment 

Systems
Primer for Municipal Wastewater Treatment Systems

Clean Water Act Requirements for Wastewater Treatment 4

The Need for Wastewater Treatment 5

Effects of Wastewater on Water Quality 5

Some of the Key Challenges Faced by Wastewater Treatment Professionals Today 6

Collecting and Treating Wastewater 6

 Centralized Collection 6

 Combined Sewer Systems 7

 Sanitary Sewer System 9

Pollutants: 8

 Oxygen-Demanding Substances 8

 Pathogens 8

 Nutrients 8

 Synthetic Organic and Inorganic Chemicals 8

 Thermal 8

Wastewater Treatment 9 

 Primary Treatment 9

 Preliminary Treatment 9

 Primary Sedimentation 11

 Basic Wastewater Treatment Processes 10

 Physical 10

 Biological 10

 Chemical 10

 Secondary Treatment 11

 Attached Growth Processes 11

 Suspended Growth Processes 12

 Lagoons 13

 Land Treatment 14

 Slow Rate Infiltration 14

 Rapid Infiltration 15

 Overland Flow 15

 Constructed Wetlands 15

 Disinfection 16

 Chlorine 16

 Ozone 16

 Ultraviolet Radiation 16

Pretreatment 16

Advanced Methods of Wastewater Treatment 17

 Nitrogen Control 17

 Biological Phosphorus Control 18

 Coagulation-Sedimentation 18

 Carbon Adsorption 19

The Use or Disposal of Wastewater Residuals and Biosolids 19

 Land Application 20

 Incineration 20

 Beneficial Use Products from Biosolids 21

Decentralized (Onsite or Cluster) Systems 21

 Treatment 22

 Conventional Septic Tanks 22

 Aerobic Treatment Units 22

 Media Filters 22

 Dispersal Approaches 23

 Absorption Field 23

 Mound System 23

 Drip Dispersal System 24

 Evapotranspiration Beds 24

 Management of Onsite/Decentralized Wastewater Systems 24

Asset Management 24

 Operation 25

 Maintenance 25

Common Wastewater Treatment Terminology 25

4

Clean Water Act Requirements for 

Wastewater Treatment

The 1972 Amendments to the Federal 

Water Pollution Control Act (Public Law 92-

500–, known as the Clean Water Act (CWA), 

established the foundation for wastewater 

discharge control in this country. The CWA’s 

primary objective is to ‘restore and maintain the 

chemical, physical and biological integrity of the 

nation’s waters.’

The CWA established a control program for 

ensuring that communities have clean water 

by regulating the release of contaminants 

into our country’s waterways. Permits that 

limit the amount of pollutants discharged 

are required of all municipal and industrial 

wastewater dischargers under the National 

Pollutant Discharge Elimination System (NPDES) 

permit program. In addition, a construction 

grants program was set up to assist publiclyowned wastewater treatment works build the 

improvements required to meet these new limits. 

The 1987 Amendments to the CWA established 

State Revolving Funds (SRF) to replace grants as 

the current principal federal funding source for 

the construction of wastewater treatment and 

collection systems. 

Over 75 percent of the nation’s population is 

served by centralized wastewater collection 

and treatment systems. The remaining 

population uses septic or other onsite systems. 

Approximately 16,000 municipal wastewater 

treatment facilities are in operation nationwide. 

The CWA requires that municipal wastewater 

treatment plant discharges meet a minimum of 

‘secondary treatment’. Over 30 percent of the 

wastewater treatment facilities today produce 

cleaner discharges by providing even greater 

levels of treatment than secondary.
5

Primer for Municipal 

Wastewater Treatment 

Systems

The Need for Wastewater Treatment

Wastewater treatment is 

needed so that we can 

use our rivers and streams 

for fishing, swimming and 

drinking water. For the first 

half of the 20th century, 

pollution in the Nation’s 

urban waterways resulted in 

frequent occurrences of low 

dissolved oxygen, fish kills, 

algal blooms and bacterial 

contamination. Early efforts 

in water pollution control 

prevented human waste 

from reaching water supplies 

or reduced floating debris 

that obstructed shipping. 

Pollution problems and their 

control were primarily local, 

not national, concerns. 

Since then, population 

and industrial growth have 

increased demands on our 

natural resources, altering 

the situation dramatically. 

Progress in abating pollution 

has barely kept ahead of 

population growth, changes 

in industrial processes, 

technological developments, 

changes in land use, 

business innovations, 

and many other factors. 

Increases in both the 

quantity and variety of goods 

produced can greatly alter 

the amount and complexity 

of industrial wastes and 

challenge traditional 

treatment technology. The 

application of commercial 

fertilizers and pesticides, 

combined with sediment 

from growing development 

activities, continues to be a 

source of significant pollution 

as runoff washes off the 

land.

Water pollution issues now 

dominate public concerns 

about national water quality 

and maintaining healthy 

ecosystems. Although a 

large investment in water 

pollution control has helped 

reduce the problem, many 

miles of streams are still 

impacted by a variety of 

different pollutants. This, 

in turn, affects the ability of 

people to use the water for 

beneficial purposes. Past 

approaches used to control 

water pollution control must 

be modified to accommodate 

current and emerging issues

Effects of Wastewater on 

Water Quality

The basic function of the 

wastewater treatment plant 

is to speed up the natural 

processes by which water 

purifies itself. In earlier 

years, the natural treatment 

process in streams and 

lakes was adequate to 

perform basic wastewater 

treatment. As our population 

and industry grew to their 

present size, increased 

levels of treatment prior 

to discharging domestic 

wastewater became 

necessary. 
6

Collecting and Treating 

Wastewater

The most common form 

of pollution control in the 

United States consists of 

a system of sewers and 

wastewater treatment plants. 

The sewers collect municipal 

wastewater from homes, 

businesses, and industries 

and deliver it to facilities 

for treatment before it is 

discharged to water bodies 

or land, or reused. 

Centralized Collection

During the early days of our 

nation’s history, people living 

in both the cities and the 

countryside used cesspools 

and privies to dispose of 

domestic wastewater. Cities 

began to install wastewater 

collection systems in the late 

nineteenth century because 

of an increasing awareness 

of waterborne disease and 

the popularity of indoor 

plumbing and flush toilets. 

The use of sewage collection 

systems brought dramatic 

improvements to public 

health, further encouraging 

the growth of metropolitan 

areas. In the year 2000 

approximately 208 million 

people in the U.S. were 

served by centralized 

collection systems.

No Discharge 

Greater than

Secondary

Secondary

Less than

Secondary

Raw Discharge

0

20

40

60

80

100

120

140

160

180

200

220

Population Receiving Different Levels of 

Wastewater Treatment

Population Served (millions)

(Data form U.S. Public Health Service multi wastewater inventories: 

2000 USEPA Clean Watershed Needs Survey)

Before the CWA 

After the CWA

1 Raw discharges were eliminated by 1996

2 Data for the "no-discharge" category were unavailable for 1968

2

1

Some of the key challenges faced by wastewater 

treatment professionals today:

 Many of the wastewater treatment and collection facilities 

are now old and worn, and require further improvement, 

repair or replacement to maintain their useful life;

 The character and quantity of contaminants presenting 

problems today are far more complex than those that presented challenges in the past; 

 Population growth is taxing many existing wastewater 

treatment systems and creating a need for new plants; 

 Farm runoff and increasing urbanization provide additional sources of pollution not controlled by wastewater 

treatment; and

 One third of new development is served by decentralized 

systems (e.g., septic systems) as population migrates further 

from metropolitan areas.
7

Combined Sewer Systems

Many of the earliest sewer systems were combined sewers, designed to collect both sanitary 

wastewater and storm water runoff in a single system. These combined sewer systems were 

designed to provide storm drainage from streets and roofs to prevent flooding in cities. 

Later, lines were added to carry domestic wastewater away from homes and businesses. 

Early sanitarians thought that these combined systems provided adequate health protection. 

We now know that the overflows designed to release excess flow during rains also release 

pathogens and other pollutants. 

Simplified Urban Water Cycle
8

Pollutants

Oxygen-Demanding Substances

Dissolved oxygen is a key element in water quality that is necessary to support aquatic life. 

A demand is placed on the natural supply of dissolved oxygen by many pollutants in wastewater. This is called biochemical oxygen demand, or BOD, and is used to measure how well 

a sewage treatment plant is working. If the effluent, the treated wastewater produced by a 

treatment plant, has a high content of organic pollutants or ammonia, it will demand more 

oxygen from the water and leave the water with less oxygen to support fish and other aquatic 

life.

Organic matter and ammonia are “oxygen-demanding” substances. Oxygen-demanding substances are contributed by domestic sewage and agricultural and industrial wastes 

of both plant and animal origin, such as those from food processing, paper mills, tanning, 

and other manufacturing processes. These substances are usually destroyed or converted 

to other compounds by bacteria if there is sufficient oxygen present in the water, but the dissolved oxygen needed to sustain fish life is used up in this break down process.

Pathogens

Disinfection of wastewater and chlorination of drinking water supplies has reduced the occurrence of waterborne diseases such as typhoid fever, cholera, and dysentery, which remain 

problems in underdeveloped countries while they have been virtually eliminated in the U.S.

Infectious micro-organisms, or pathogens, may be carried into surface and groundwater by 

sewage from cities and institutions, by certain kinds of industrial wastes, such as tanning and 

meat packing plants, and by the contamination of storm runoff with animal wastes from pets, 

livestock and wild animals, such as geese or deer. Humans may come in contact with these 

pathogens either by drinking contaminated water or through swimming, fishing, or other 

contact activities. Modern disinfection techniques have greatly reduced the danger of waterborne disease.

Nutrients

Carbon, nitrogen, and phosphorus are essential to living organisms and are the chief nutrients present in natural water. Large amounts of these nutrients are also present in sewage, 

certain industrial wastes, and drainage from fertilized land. Conventional secondary biological treatment processes do not remove the phosphorus and nitrogen to any substantial 

extent -- in fact, they may convert the organic forms of these substances into mineral form, 

making them more usable by plant life. When an excess of these nutrients overstimulates the 

growth of water plants, the result causes unsightly conditions, interferes with drinking water 

treatment processes, and causes unpleasant and disagreeable tastes and odors in drinking 

water. The release of large amounts of nutrients, primarily phosphorus but occasionally nitrogen, causes nutrient enrichment which results in excessive growth of algae. Uncontrolled 

algae growth blocks out sunlight and chokes aquatic plants and animals by depleting dissolved oxygen in the water at night. The release of nutrients in quantities that exceed the 

affected waterbody’s ability to assimilate them results in a condition called eutrophication or 

cultural enrichment.

Inorganic and Synthetic Organic Chemicals

A vast array of chemicals are included in this category. Examples include detergents, household cleaning aids, heavy metals, pharmaceuticals, synthetic organic pesticides and herbicides, industrial chemicals, and the wastes from their manufacture. Many of these substances are toxic to fish and aquatic life and many are harmful to humans. Some are known 

to be highly poisonous at very low concentrations. Others can cause taste and odor problems, and many are not effectively removed by conventional wastewater treatment.

Thermal

Heat reduces the capacity of water to retain oxygen. In some areas, water used for cooling 

is discharged to streams at elevated temperatures from power plants and industries. Even 

discharges from wastewater treatment plants and storm water retention ponds affected by 

summer heat can be released at temperatures above that of the receiving water, and elevate 

the stream temperature. Unchecked discharges of waste heat can seriously alter the ecology 

of a lake, a stream, or estuary. 
9

Sanitary Sewer Systems

Sanitary sewer collection 

systems serve over half the 

people in the United States 

today. EPA estimates that 

there are approximately 

500,000 miles of publiclyowned sanitary sewers 

with a similar expanse of 

privately-owned sewer 

systems. Sanitary sewers 

were designed and built 

to carry wastewater from 

domestic, industrial and 

commercial sources, but 

not to carry storm water. 

Nonetheless, some storm 

water enters sanitary sewers 

through cracks, particularly 

in older lines, and through 

roof and basement drains. 

Due to the much smaller 

volumes of wastewater 

that pass through sanitary 

sewer lines compared to 

combined sewers, sanitary 

sewer systems use smaller 

pipes and lower the cost of 

collecting wastewater. 

Wastewater Treatment

In 1892, only 27 American 

cities provided wastewater 

treatment. Today, more 

than 16,000 publicly-owned 

wastewater treatment plants 

operate in the United States 

and its territories. The 

construction of wastewater 

treatment facilities 

blossomed in the 1920s and 

again after the passage of 

the CWA in 1972 with the 

availability of grant funding 

and new requirements 

calling for minimum levels 

of treatment. Adequate 

treatment of wastewater, 

along with the ability to 

provide a sufficient supply 

of clean water, has become 

a major concern for many 

communities.

Primary Treatment 

The initial stage in the 

treatment of domestic 

wastewater is known as 

primary treatment. Coarse 

solids are removed from 

the wastewater in the 

primary stage of treatment. 

In some treatment plants, 

primary and secondary 

stages may be combined 

into one basic operation. 

At many wastewater 

treatment facilities, influent 

passes through preliminary 

treatment units before 

primary and secondary 

treatment begins.

Preliminary Treatment

As wastewater enters a 

treatment facility, it typically 

flows through a step called 

preliminary treatment. A 

screen removes large floating 

objects, such as rags, cans, 

bottles and sticks that may 

clog pumps, small pipes, and 

down stream processes. The 

screens vary from coarse to 

fine and are constructed with 

parallel steel or iron bars 

with openings of about half 

an inch, while others may 

be made from mesh screens 

with much smaller openings.

Screens are generally placed 

in a chamber or channel and 

inclined towards the flow of 

the wastewater. The inclined 

screen allows debris to be 

caught on the upstream 

surface of the screen, and 

allows access for manual 

or mechanical cleaning. 

Some plants use devices 

known as comminutors or 

barminutors which combine 

the functions of a screen and 

a grinder. These devices 

catch and then cut or shred 

the heavy solid and floating 

material. In the process, the 

pulverized matter remains 

in the wastewater flow to be 

removed later in a primary 

settling tank.

“the ability to 

provide a sufficient 

supply of clean 

water continues to 

be a major national 

concern”

Workers install sewer line
10

After the wastewater has 

been screened, it may flow 

into a grit chamber where 

sand, grit, cinders, and small 

stones settle to the bottom. 

Removing the grit and gravel 

that washes off streets or 

land during storms is very 

important, especially in 

cities with combined sewer 

systems. Large amounts 

of grit and sand entering a 

treatment plant can cause 

serious operating problems, 

such as excessive wear of 

pumps and other equipment, 

clogging of aeration devices, 

or taking up capacity in tanks 

that is needed for treatment. 

In some plants, another 

finer screen is placed after 

the grit chamber to remove 

any additional material that 

might damage equipment or 

interfere with later processes. 

The grit and screenings 

removed by these processes 

must be periodically 

collected and trucked to a 

landfill for disposal or are 

incinerated.

.

Basic Wastewater Treatment Processes

Physical

Physical processes were 

some of the earliest methods 

to remove solids from 

wastewater, usually by 

passing wastewater through 

screens to remove debris 

and solids. In addition, 

solids that are heavier than 

water will settle out from 

wastewater by gravity. 

Particles with entrapped 

air float to the top of water 

and can also be removed. 

These physical processes are 

employed in many modern 

wastewater treatment 

facilities today.

Biological

In nature, bacteria and 

other small organisms in 

water consume organic 

matter in sewage, turning 

it into new bacterial cells, 

carbon dioxide, and other 

by-products. The bacteria 

normally present in water 

must have oxygen to do 

their part in breaking down 

the sewage. In the 1920s, 

scientists observed that these 

natural processes could be 

contained and accelerated 

in systems to remove organic 

material from wastewater. 

With the addition of oxygen 

to wastewater, masses of 

microorganisms grew and 

rapidly metabolized organic 

pollutants. Any excess 

microbiological growth 

could be removed from 

the wastewater by physical 

processes.

Chemical

Chemicals can be used to 

create changes in pollutants 

that increase the removal 

of these new forms by 

physical processes. Simple 

chemicals such as alum, 

lime or iron salts can be 

added to wastewater to 

cause certain pollutants, 

such as phosphorus, to floc 

or bunch together into large, 

heavier masses which can 

be removed faster through 

physical processes. Over the 

past 30 years, the chemical 

industry has developed 

synthetic inert chemicals 

know as polymers to 

further improve the physical 

separation step in wastewater 

treatment. Polymers are 

often used at the later 

stages of treatment to 

improve the settling of excess 

microbiological growth or 

biosolids.
11

Primary Sedimentation

With the screening 

completed and the grit 

removed, wastewater still 

contains dissolved organic 

and inorganic constituents 

along with suspended 

solids. The suspended solids 

consist of minute particles of 

matter that can be removed 

from the wastewater 

with further treatment 

such as sedimentation or 

gravity settling, chemical 

coagulation, or filtration. 

Pollutants that are dissolved 

or are very fine and remain 

suspended in the wastewater 

are not removed effectively 

by gravity settling. 

When the wastewater enters 

a sedimentation tank, it slows 

down and the suspended 

solids gradually sink to the 

bottom. This mass of solids 

is called primary sludge.

Various methods have been 

devised to remove primary 

sludge from the tanks. 

Newer plants have some type 

of mechanical equipment 

to remove the settled solids 

from sedimentation tanks. 

Some plants remove solids 

continuously while others do 

so at intervals.

Secondary Treatment

After the wastewater has 

been through Primary 

Treatment processes, it 

flows into the next stage of 

treatment called secondary. 

Secondary treatment 

processes can remove up to 

90 percent of the organic 

matter in wastewater by 

using biological treatment 

processes. The two most 

common conventional 

methods used to achieve 

secondary treatment are 

attached growth processes 

and suspended growth 

processes..

Attached Growth 

Processes

In attached growth (or fixed 

film) processes, the microbial 

growth occurs on the surface 

of stone or plastic media. 

Wastewater passes over 

the media along with air to Solids removed from 

automated bar screens

Aerated Grit Chamber
12

provide oxygen. Attached 

growth process units include 

trickling filters, biotowers, 

and rotating biological 

contactors. Attached growth 

processes are effective at 

removing biodegradable 

organic material from the 

wastewater.

A trickling filter is simply 

a bed of media (typically 

rocks or plastic) through 

which the wastewater passes. 

The media ranges from 

three to six feet deep and 

allows large numbers of 

microorganisms to attach 

and grow. Older treatment 

facilities typically used 

stones, rocks, or slag as the 

media bed material. New 

facilities may use beds made 

of plastic balls, interlocking 

sheets of corrugated plastic, 

or other types of synthetic 

media. This type of bed 

material often provides 

more surface area and 

a better environment for 

promoting and controlling 

biological treatment than 

rock. Bacteria, algae, fungi 

and other microorganisms 

grow and multiply, forming 

a microbial growth or slime 

layer (biomass) on the 

media. In the treatment 

process, the bacteria use 

oxygen from the air and 

consume most of the organic 

matter in the wastewater as 

food. As the wastewater 

passes down through the 

media, oxygen-demanding 

substances are consumed by 

the biomass and the water 

leaving the media is much 

cleaner. However, portions 

of the biomass also slough 

off the media and must settle 

out in a secondary treatment 

tank.

Suspended Growth 

Processes

Similar to the microbial 

processes in attached growth 

systems, suspended growth 

processes are designed 

to remove biodegradable 

organic material and 

organic nitrogen-containing 

material by converting 

ammonia nitrogen to 

nitrate unless additional 

treatment is provided. In 

suspended growth processes, 

the microbial growth is 

suspended in an aerated 

water mixture where the air 

is pumped in, or the water is 

agitated sufficiently to allow 

oxygen transfer. Suspended 

growth process units include 

variations of activated 

sludge, oxidation ditches and 

sequencing batch reactors. 

The suspended growth 

process speeds up the work 

of aerobic bacteria and 

other microorganisms that 

break down the organic 

matter in the sewage by 

providing a rich aerobic 

environment where the 

microorganisms suspended 

in the wastewater can work 

more efficiently. In the 

aeration tank, wastewater is 

vigorously mixed with air and 

microorganisms acclimated 

to the wastewater in a 

suspension for several hours. 

This allows the bacteria 

Sequencing Batch 

Reactor

Trickling Filters 
13

and other microorganisms 

to break down the organic 

matter in the wastewater. 

The microorganisms grow 

in number and the excess 

biomass is removed by 

settling before the effluent 

is discharged or treated 

further. Now activated 

with millions of additional 

aerobic bacteria, some of 

the biomass can be used 

again by returning it to an 

aeration tank for mixing with 

incoming wastewater.

The activated sludge 

process, like most other 

techniques, has advantages 

and limitations. The units 

necessary for this treatment 

are relatively small, requiring 

less space than attached 

growth processes. In 

addition, when properly 

operated and maintained, 

the process is generally 

free of flies and odors. 

However, most activated 

sludge processes are more 

costly to operate than 

attached growth processes 

due to higher energy use 

to run the aeration system. 

The effectiveness of the 

activated sludge process 

can be impacted by elevated 

levels of toxic compounds in 

wastewater unless complex 

industrial chemicals are 

effectively controlled through 

an industrial pretreatment 

program.

An adequate supply of 

oxygen is necessary for the 

activated sludge process to 

be effective. The oxygen 

is generally supplied by 

mixing air with the sewage 

and biologically active 

solids in the aeration 

tanks by one or more of 

several different methods. 

Mechanical aeration can be 

accomplished by drawing 

the sewage up from the 

bottom of the tank and 

spraying it over the surface, 

thus allowing the sewage 

to absorb large amounts of 

oxygen from the atmosphere. 

Pressurized air can be forced 

out through small openings 

in pipes suspended in the 

wastewater. Combination 

of mechanical aeration and 

forced aeration can also be 

used. Also, relatively pure 

oxygen, produced by several 

different manufacturing 

processes, can be added 

to provide oxygen to the 

aeration tanks.

From the aeration tank, 

the treated wastewater 

flows to a sedimentation 

tank (secondary clarifier), 

where the excess biomass 

is removed. Some of the 

biomass is recycled to the 

head end of the aeration 

tank, while the remainder is 

“wasted” from the system. 

The waste biomass and 

settled solids are treated 

before disposal or reuse as 

biosolids.

Lagoons

A wastewater lagoon 

or treatment pond is a 

scientifically constructed 

pond, three to five feet 

deep, that allows sunlight, 

Brush Aerators in an Oxidation Ditch

Centerfeed well of a clarifier for 

removing excess biomass
14

algae, bacteria, and oxygen 

to interact. Biological and 

physical treatment processes 

occur in the lagoon to 

improve water quality. The 

quality of water leaving the 

lagoon, when constructed 

and operated properly, is 

considered equivalent to the 

effluent from a conventional 

secondary treatment system. 

However, winters in cold 

climates have a significant 

impact on the effectiveness 

of lagoons, and winter 

storage is usually required. 

Lagoons have several 

advantages when used 

correctly. They can be used 

for secondary treatment 

or as a supplement to 

other processes. While 

treatment ponds require 

substantial land area and 

are predominantly used 

by smaller communities, 

they account for more 

than one-fourth of the 

municipal wastewater 

treatment facilities in this 

country. Lagoons remove 

biodegradable organic 

material and some of the 

nitrogen from wastewater.

Land Treatment 

Land treatment is the 

controlled application of 

wastewater to the soil where 

physical, chemical, and 

biological processes treat 

the wastewater as it passes 

across or through the soil. 

The principal types of land 

treatment are slow rate, 

overland flow, and rapid 

infiltration. In the arid 

western states, pretreated 

municipal wastewater has 

been used for many years 

to irrigate crops. In more 

recent years, land treatment 

has spread to all sections of 

the country. Land treatment 

of many types of industrial 

wastewater is also common.

Whatever method is 

used, land treatment can 

be a feasible economic 

alternative, where the land 

area needed is readily 

available, particularly 

when compared to costly 

advanced treatment plants. 

Extensive research has been 

conducted at land treatment 

sites to determine treatment 

performance and study 

the numerous treatment 

processes involved, as 

well as potential impacts 

on the environment, e.g. 

groundwater, surface water, 

and any crop that may be 

grown.

Slow Rate Infiltration

In the case of slow rate 

infiltration, the wastewater 

is applied to the land and 

moves through the soil 

where the natural filtering 

action of the soil along 

with microbial activity and 

plant uptake removes most 

contaminants. Part of the 

water evaporates or is used 

by plants. The remainder is 

either collected via drains or 

wells for surface discharge or 

allowed to percolate into the 

groundwater.

Slow rate infiltration is 

the most commonly used 

land treatment technique. 

The wastewater, which is 

sometimes disinfected before 

application, depending on 

the end use of the crop and 

the irrigation method, can 

be applied to the land by 

spraying, flooding, or ridge 

and furrow irrigation. The 

method selected depends on 

cost considerations, terrain, 

and the type of crops. Much 

of the water and most of the 

nutrients are used by the 

plants, while other pollutants 

are transferred to the soil 

by adsorption, where many 

are mineralized or broken 

down over time by microbial 

action. 

Wastewater Lagoon
15

BOD5 Loading (metric tons/day)

BOD5 Removal Efficiency (%)

Biologically Degradable Wastewater Treated in the U.S. has 

increased since 1940, however, treatment efficiency has 

improved so that pollution has decreased.

Year

Influent BOD5

Effluent BOD5

Removal Efficiency

80,000 

70,000 

60,000 

50,000 

40,000 

30,000 

20,000 

10,000 

0

1940 1950 1960 1970 1980 1990 1996 2016 

1 165 gal/capita-day is based on data in the Clean Water Needs Surveys for 1978 

through 1986 and accounts for residential, commercial, industrial, stormwater, and 

infiltration and inflow components.

Rapid Infiltration

The rapid infiltration 

process is most frequently 

used to polish and recover 

wastewater effluents for 

reuse after pretreatment by 

secondary and advanced 

treatment processes. It is 

also effective in cold or 

wet weather and has been 

successfully used in Florida, 

northeastern and arid 

southwestern states. Large 

amounts of wastewater 

are applied to permeable 

soils in a limited land area 

and allowed to infiltrate 

and percolate downward 

through the soil into the 

water table below. If the 

water is to be reused, it can 

be recovered by wells. The 

cost-effectiveness of this 

process depends on the soil’s 

ability to percolate a large 

volume of water quickly and 

efficiently, so suitable soil 

drainage is important.

Overland Flow

This method has been used 

successfully by the food 

processing industries for 

many years to remove solids, 

bacteria and nutrients from 

wastewater. The wastewater 

is allowed to flow down a 

gently-sloped surface that is 

planted with vegetation to 

control runoff and erosion. 

Heavy clay soils are well 

suited to the overland flow 

process. As the water flows 

down the slope, the soil and 

its microorganisms form a 

gelatinous slime layer similar 

in many ways to a trickling 

filter that effectively removes 

solids, pathogens, and nutrients. Water that is not 

absorbed or evaporated is 

recovered at the bottom of 

the slope for discharge or 

reuse.

Constructed Wetlands

Wetlands are areas where 

the water saturates the 

ground long enough to 

support and maintain 

wetland vegetation such 

as reeds, bulrush, and 

cattails. A “constructed 

wetlands” treatment system is 

designed to treat wastewater 

by passing it through the 

wetland. Natural physical, 

chemical, and biological 

wetland processes have been 

recreated and enhanced 

in constructed wetlands 

designed specifically to treat 

wastewater from industries, 

small communities, storm 

runoff from urban and 

agricultural areas, and acid 

mine drainage. Significant 

water quality improvements, 

including nutrient reduction, 

can be achieved

Constructed Wetlands
16

Disinfection

Untreated domestic 

wastewater contains microorganisms or pathogens that 

produce human diseases. 

Processes used to kill or 

deactivate these harmful 

organisms are called 

disinfection. Chlorine is the 

most widely used disinfectant 

but ozone and ultraviolet 

radiation are also frequently 

used for wastewater effluent 

disinfection.

Chlorine

Chlorine kills microorganisms by destroying 

cellular material. This 

chemical can be applied to 

wastewater as a gas, a liquid 

or in a solid form similar to 

swimming pool disinfection 

chemicals. However, any 

free (uncombined) chlorine 

remaining in the water, 

even at low concentrations, 

is highly toxic to beneficial 

aquatic life. Therefore, 

removal of even trace 

amounts of free chlorine 

by dechlorination is often 

needed to protect fish 

and aquatic life. Due to 

emergency response and 

potential safety concerns, 

chlorine gas is used less 

frequently now than in the 

past.

Ozone

Ozone is produced from 

oxygen exposed to a high 

voltage current. Ozone is 

very effective at destroying 

viruses and bacteria and 

decomposes back to oxygen 

rapidly without leaving 

harmful by products. Ozone 

is not very economical due to 

high energy costs.

Ultraviolet Radiation

Ultra violet (UV) disinfection 

occurs when electromagnetic 

energy in the form of light in 

the UV spectrum produced 

by mercury arc lamps 

penetrates the cell wall of 

exposed microorganisms. 

The UV radiation retards the 

ability of the microorganisms 

to survive by damaging 

their genetic material. UV 

disinfection is a physical 

treatment process that 

leaves no chemical traces. 

Organisms can sometimes 

repair and reverse the 

destructive effects of UV 

when applied at low doses.

Pretreatment

The National Pretreatment 

Program, a cooperative 

effort of Federal, state, 

POTWs and their industrial 

dischargers, requires industry 

to control the amount of 

pollutants discharged into 

municipal sewer systems. 

Pretreatment protects the 

wastewater treatment 

facilities and its workers 

from pollutants that may 

create hazards or interfere 

with the operation and 

performance of the POTW, 

including contamination of 

sewage sludge, and reduces 

the likelihood that untreated 

pollutants are introduced into 

the receiving waters.

Under the Federal 

Pretreatment Program, 

municipal wastewater 

plants receiving significant 

industrial discharges must 

develop local pretreatment 

programs to control 

industrial discharges into 

their sewer system. These 

programs must be approved 

by either EPA or a state 

acting as the Pretreatment 

Approval Authority. More 

than 1,500 municipal 

treatment plants have 

developed and received 

approval for a Pretreatment 

Program.

Land Treatment - Rapid Infiltration
17

Advanced Methods of 

Wastewater Treatment

As our country and the 

demand for clean water 

have grown, it has become 

more important to produce 

cleaner wastewater effluents, 

yet some contaminants are 

more difficult to remove 

than others. The demand 

for cleaner discharges has 

been met through better 

and more complete methods 

of removing pollutants at 

wastewater treatment plants, 

in addition to pretreatment 

and pollution prevention 

which helps limit types of 

wastes discharged to the 

sanitary sewer system. 

Currently, nearly all WWTPs 

provide a minimum of 

secondary treatment. In 

some receiving waters, the 

discharge of secondary 

treatment effluent would 

still degrade water quality 

and inhibit aquatic life. 

Further treatment is needed. 

Treatment levels beyond 

secondary are called 

advanced treatment. 

Advanced treatment 

technologies can be 

extensions of conventional 

secondary biological 

treatment to further 

stabilize oxygen-demanding 

substances in the wastewater, 

or to remove nitrogen and 

phosphorus. Advanced 

treatment may also 

involve physical-chemical 

separation techniques 

such as adsorption, 

flocculation/precipitation, 

membranes for advanced 

filtration, ion exchange, 

and reverse osmosis. In 

various combinations, these 

processes can achieve any 

degree of pollution control 

desired. As wastewater is 

purified to higher and higher 

degrees by such advanced 

treatment processes, the 

treated effluents can be 

reused for urban, landscape, 

and agricultural irrigation, 

industrial cooling and 

processing, recreational uses 

and water recharge, and 

even indirect augmentation 

of drinking water supplies. 

Nitrogen Control

Nitrogen in one form 

or another is present in 

municipal wastewater and 

is usually not removed by 

secondary treatment. If 

discharged into lakes and 

streams or estuary waters, 

nitrogen in the form of 

ammonia can exert a 

direct demand on oxygen 

or stimulate the excessive 

growth of algae. Ammonia 

in wastewater effluent can be 

toxic to aquatic life in certain 

instances.

By providing additional 

biological treatment beyond 

the secondary stage, 

nitrifying bacteria present 

in wastewater treatment can 

biologically convert ammonia 

to the non-toxic nitrate 

through a process known as 

nitrification. The nitrification 

process is normally sufficient 

to remove the toxicity 

associated with ammonia in 

the effluent. Since nitrate 

is also a nutrient, excess 

amounts can contribute to 

the uncontrolled growth of 

algae. In situations where 

nitrogen must be completely 

removed from effluent, an 

Monitoring a discharger as part of a Pretreatment Program
18

additional biological process 

can be added to the system 

to convert the nitrate to 

nitrogen gas. The conversion 

of nitrate to nitrogen gas is 

accomplished by bacteria 

in a process known as 

denitrification. Effluent 

with nitrogen in the form of 

nitrate is placed into a tank 

devoid of oxygen, where 

carbon-containing chemicals, 

such as methanol, are added 

or a small stream of raw 

wastewater is mixed in with 

the nitrified effluent. In this 

oxygen free environment, 

bacteria use the oxygen 

attached to the nitrogen in 

the nitrate form releasing 

nitrogen gas. Because 

nitrogen comprises almost 

80 percent of the air in the 

earth’s atmosphere, the 

release of nitrogen into the 

atmosphere does not cause 

any environmental harm.

Biological Phosphorus 

Control

Like nitrogen, phosphorus 

is also a necessary nutrient 

for the growth of algae. 

Phosphorus reduction is 

often needed to prevent 

excessive algal growth 

before discharging effluent 

into lakes, reservoirs and 

estuaries. Phosphorus 

removal can be achieved 

through chemical addition 

and a coagulationsedimentation process 

discussed in the following 

section. Some biological 

treatment processes 

called biological nutrient 

removal (BNR) can also 

achieve nutrient reduction, 

removing both nitrogen 

and phosphorus. Most of 

the BNR processes involve 

modifications of suspended 

growth treatment systems 

so that the bacteria in these 

systems also convert nitrate 

nitrogen to inert nitrogen gas 

and trap phosphorus in the 

solids that are removed from 

the effluent.

Coagulationsedimentation

A process known as chemical 

coagulation-sedimentation 

is used to increase the 

removal of solids from 

effluent after primary 

and secondary treatment. 

Solids heavier than water 

settle out of wastewater by 

gravity. With the addition of 

specific chemicals, solids can 

become heavier than water 

and will settle.

Alum, lime, or iron salts 

are chemicals added to 

the wastewater to remove 

phosphorus. With these 

chemicals, the smaller 

particles ‘floc’ or clump 

together into large masses. 

The larger masses of 

particles will settle faster 

when the effluent reaches the 

next step--the sedimentation 

tank. This process can 

reduce the concentration of 

phosphate by more than 95 

percent.

CHEMICAL

Nitrification Process Tank
19

Although used for years in 

the treatment of industrial 

wastes and in water 

treatment, coagulationsedimentation is considered 

an advanced process 

because it is not routinely 

applied to the treatment of 

municipal wastewater. In 

some cases, the process 

is used as a necessary 

pretreatment step for other 

advanced techniques. This 

process produces a chemical 

sludge, and the cost of 

disposing this material can 

be significant.

Carbon adsorption

Carbon adsorption 

technology can remove 

organic materials from 

wastewater that resist 

removal by biological 

treatment. These resistant, 

trace organic substances can 

contribute to taste and odor 

problems in water, taint fish 

flesh, and cause foaming 

and fish kills.

Carbon adsorption consists 

of passing the wastewater 

effluent through a bed or 

canister of activated carbon 

granules or powder which 

remove more than 98 

percent of the trace organic 

substances. The substances 

adhere to the carbon surface 

and are removed from the 

water. To help reduce the 

cost of the procedure, the 

carbon granules can be 

cleaned by heating and used 

again.

The Use or Disposal of 

Wastewater Residuals 

and Biosolids

When pollutants are removed 

from water, there is always 

something left over. It may 

be rags and sticks caught on 

the screens at the beginning 

of primary treatment. It may 

be the solids that settle to 

the bottom of sedimentation 

tanks. Whatever it is, there 

are always residuals that 

must be reused, burned, 

buried, or disposed of in 

some manner that does not 

harm the environment.

The utilization and disposal 

of the residual process solids 

is addressed by the CWA, 

Resource Conservation 

and Recovery Act (RCRA), 

and other federal laws. 

These Federal laws reenforce the need to employ 

environmentally sound 

residuals management 

techniques and to 

beneficially use biosolids 

whenever possible. Biosolids 

are processed wastewater 

solids (“sewage sludge”) 

that meet rigorous standards 

allowing safe reuse for 

beneficial purposes. 

Currently, more than half 

of the biosolids produced 

by municipal wastewater 

treatment systems is 

applied to land as a soil 

conditioner or fertilizer and 

the remaining solids are 

incinerated or landfilled. 

Ocean dumping of these 

solids is no longer allowed.

Prior to utilization or 

disposal, biosolids are 

stabilized to control odors 

and reduce the number of 

disease-causing organisms. 

Sewage solids, or sludge, 

when separated from the 

ADSORPTION

Biosolids Digestor
20

wastewater, still contain 

around 98 percent water. 

They are usually thickened 

and may be dewatered 

to reduce the volume to 

be transported for final 

processing, disposal, or 

beneficial use. Dewatering 

processes include drying 

beds, belt filter presses, 

plate and frame presses, 

and centrifuges. To improve 

dewatering effectiveness, 

the solids can be pretreated 

with chemicals such as lime, 

ferric chloride, or polymers 

to produce larger particles 

which are easier to remove.

Digestion is a form of 

stabilization where the 

volatile material in the 

wastewater solids can 

decompose naturally and the 

potential for odor production 

is reduced. Digestion without 

air in an enclosed tank 

(anaerobic solids digestion) 

has the added benefit of 

producing methane gas 

which can be recovered and 

used as a source of energy. 

Stabilization of solids may 

also be accomplished by 

composting, heat treatments, 

drying or the addition 

of lime or other alkaline 

materials. After stabilization, 

the biosolids can be safely 

spread on land.

Land Application

In many areas, biosolids 

are marketed to farmers as 

fertilizer. Federal regulation 

(40 CFR Pert 503) defines 

minimum requirements 

for such land application 

practices, including 

contaminant limits, field 

management practices, 

treatment requirements, 

monitoring, recordkeeping, 

and reporting requirements. 

Properly treated and 

applied biosolids are a 

good source of organic 

matter for improving soil 

structure and help supply 

nitrogen, phosphorus, and 

micronutrients that are 

required by plants.

Biosolids have also been 

used successfully for many 

years as a soil conditioner 

and fertilizer, and for 

restoring and revegetating 

areas with poor soils due to 

construction activities, strip 

mining or other practices. 

Under this biosolids 

management approach, 

treated solids in semiliquid or dewatered form 

are transported to the soil 

treatment areas. The slurry 

or dewatered biosolids, 

containing nutrients and 

stabilized organic matter, is 

spread over the land to give 

nature a hand in returning 

grass, trees, and flowers to 

barren land. Restoration of 

the countryside also helps 

control the flow of acid 

drainage from mines that 

endangers fish and other 

aquatic life and contaminates 

the water with acid, salts, 

and excessive quantities of 

metals.

Incineration

Incineration consists of 

burning the dried solids 

to reduce the organic 

residuals to an ash that 

can be disposed or reused. 

Incinerators often include 

heat recovery features. 

Undigested sludge solids 

have significant fuel value as 

a result of their high organic 

content. However, the water 

content must be greatly 

reduced by dewatering or 

drying to take advantage 

of the fuel potential of 

the biosolids. For this 

reason, pressure filtration 

dewatering equipment is 

used to obtain biosolids 

which are sufficiently dry 

to burn without continual 

reliance on auxiliary fuels. 

In some cities, biosolids are 

Land Application of Biosoilds
21

mixed with refuse or refusederived fuel prior to burning. 

Generally, waste heat is 

recovered to provide the 

greatest amount of energy 

efficiency.

Beneficial Use Products 

from Biosolids

Heat dried biosolids pellets 

have been produced and 

used extensively as a 

fertilizer product for lawn 

care, turf production, citrus 

groves, and vegetable 

production for many years. 

Composting of biosolids 

is also a well established 

approach to solids 

management that has been 

adopted by a number of 

communities. The composted 

peat-like product has shown 

particular promise for use 

in the production of soil 

additives for revegetation of 

topsoil depleted areas, and 

as a potting soil amendment.

Effective pretreatment 

of industrial wastes 

prevents excessive levels 

of unwanted constituents, 

such as heavy metals 

(i.e. cadmium, mercury, 

and lead) and persistent 

organic compounds from 

contaminating the residuals 

of wastewater treatment and 

limiting the potential for 

beneficial use.

Effective stabilization 

of wastewater residuals 

and their conversion to 

biosolid products can be 

costly. Some cities have 

produced fertilizers from 

biosolids which are sold to 

help pay part of the cost 

of treating wastewater. 

Some municipalities use 

composted, heat dried, 

or lime stabilized biosolid 

products on parks and other 

public areas.

Decentralized (Onsite 

and Cluster) Systems

A decentralized wastewater 

system treats sewage from 

homes and businesses 

that are not connected to 

a centralized wastewater 

treatment plant. 

Decentralized treatment 

systems include onsite 

systems and cluster systems. 

An onsite system is a 

wastewater system relying on 

natural processes, although 

sometimes containing 

mechanical components, 

to collect, treat, disperse 

or reclaim wastewater 

from a single dwelling or 

building. A septic tank and 

soil adsorption field is an 

example of an onsite system. 

A wastewater collection and 

treatment system under some 

form of common ownership 

that collects wastewater from 

two or more dwellings or 

buildings and conveys it to 

a treatment and dispersal 

system located on a suitable 

site near the dwellings or 

buildings is a cluster system.

Decentralized systems 

include those using 

alternative treatment 

technologies like media 

filters, constructed wetland 

systems, aerobic treatment 

units, and a variety of soil 

dispersal systems. Soil 

dispersal systems include Egg-shaped Digestors

Composted Biosolids
22

pressure systems such as 

low pressure pipe and drip 

dispersal systems. These 

systems treat and disperse 

relatively small volumes 

of wastewater, and are 

generally are found in 

rural and suburban areas. 

While septic tanks and soil 

absorption systems have 

significant limitations, 

decentralized systems can 

effectively protect water 

quality and public health 

from groundwater and 

surface water contamination 

if managed properly (i.e. 

properly sited, sized, 

designed, installed, 

operated, and maintained). 

Nitrate concentrations in 

groundwater that exceed the 

drinking water standards can 

cause health problems.

Treatment

Onsite wastewater systems 

contain three components: 

a treatment unit which treats 

water prior to dispersal 

into the environment; a 

soil dispersal component 

which assures that treated 

water is released into the 

environment at a rate which 

can be assimilated; and a 

management system which 

assures proper long term 

operation of the complete 

system. Disinfection 

of the treated effluent 

may be provided prior 

to dispersal. A typical 

onsite system consists of a 

septic tank followed by an 

effluent distribution system. 

Alternative treatment systems 

include aerobic treatment 

and sand filtration systems.

Conventional Septic 

Tanks

A septic tank is a tank buried 

in the ground used to treat 

sewage without the presence 

of oxygen (anaerobic). The 

sewage flows from the 

plumbing in a home or small 

business establishment into 

the first of two chambers, 

where solids settle out. The 

liquid then flows into the 

second chamber. Anaerobic 

bacteria in the sewage break 

down the organic matter, 

allowing cleaner water 

to flow out of the second 

chamber. The liquid typically 

discharges through a subsurface distribution system. 

Periodically, the solid matter 

in the bottom of the tank, 

referred to as septage, must 

be removed and disposed of 

properly.

Aerobic Treatment Units

Aerobic treatment units 

are also used to provide 

onsite wastewater treatment. 

They are similar to septic 

tanks, except that air is 

introduced and mixed with 

the wastewater inside the 

tank. Aerobic (requiring 

oxygen) bacteria consume 

the organic matter in the 

sewage. As with the typical 

septic system, the effluent 

discharge from an aerobic 

system is typically released 

through a sub-surface 

distribution system or may be 

disinfected and discharged 

directly to surface water. 

Aerobic treatment units also 

require the removal and 

proper disposal of solids that 

accumulate in the tank. 

Media Filters

Media filters are used to 

provide further treatment 

of septic tank effluent, 

and provide high levels of 

nitrification. They can be 

designed to pass the effluent 

once or multiple times 

through the media bed. 

Media, such as sand, acts as 

a filter. The media is placed 

two to three feet deep above 

a liner of impermeable 

Septic tank and distribution box
23

material such as plastic 

or concrete. Septic tank 

effluent is applied to the filter 

surface in intermittent doses 

and is further treated as it 

slowly trickles through the 

media. In most media filters, 

wastewater is collected in 

an underdrain then either 

pumped back to the filter 

bed or to other types of 

treatment.

Dispersal Approaches

Traditional onsite systems 

include treatment units 

followed by a drainfield or 

absorption field. Wastewater 

from the treatment unit is 

dispersed through a suitable 

soil layer where it receives 

additional treatment by the 

soil microorganisms and 

filtering properties of the 

soil. If the soil is unsuitable 

for the installation of a soil 

absorption field, alternative 

methods can be used to 

further treat or distribute the 

treated effluent. The most 

common alternative dispersal 

systems include low pressure 

pipe, mounds, drip disposal, 

and evapotranspiration beds.

Absorbtion Field

When soil conditions permit, 

the most common method 

to disperse septic tank or 

aerobic system effluent is an 

absorption field consisting 

of a series of perforated 

parallel pipes laid in trenches 

on gravel or crushed stone 

or as a direct discharge to 

the soil through trenches. 

Typically, effluent flows into 

the absorption field from 

a distribution box which 

maintains an even flow of 

effluent to the absorption 

field. From there, the 

effluent drains through the 

stone and into the soil which 

provides further treatment.

Mound System

When the soil is not 

conducive to percolation or 

when the groundwater level 

is high, a mound system is 

commonly used. A mound 

system is a distribution 

system constructed above 

the original ground level 

by using granular material 

such as sand and gravel 

to receive the septic tank 

effluent before it flows to 

the native soil below. The 

effluent flows to a dosing 

tank that is equipped with a 

pump. Here the effluent is 

stored until there is sufficient 

liquid. Once the liquid is 

pumped out, it moves evenly 

throughout the mound before 

reaching less permeable 

soil or ground water. The 

granular material acts as 

a treatment medium and 

improves the removal of Mound system under construction 

(photo courtesy of Ayres Associates)

Onsite aerobic treatment unit
24

pollutants in ways that 

may not be provided by 

substandard native soils.

Drip Dispersal System

Where soils are very thin or 

have reduced permeability, 

drip dispersal systems can 

be utilized. The typical drip 

system operates like drip 

irrigation at a moderately 

high pressure. The 

components of a drip system 

include filters to remove 

solids, a network of drip 

tubes to disperse liquid into 

soil, tanks to hold liquid, and 

controllers to regulate the 

flow to the drip system.

Evapotranspiration Beds

Evapotranspiration (ET) bed 

is an onsite dispersal system 

where pretreated wastewater 

evaporates from the soil 

surface or is transpired by 

plants into the atmosphere. 

Usually, ET beds are used in 

arid climates and there is no 

discharge either to surface 

or ground water. Vegetation 

is planted on the surface of 

the sand bed to improve the 

transpiration process and 

landscaping enhances the 

aesthetics of the bed.

Management of Decentralized Systems

Ensuring performance of 

decentralized wastewater 

treatment systems is an 

issue of national concern 

because these systems are 

a permanent component 

of our nation’s wastewater 

infrastructure. Twentyfive percent of households 

nationwide and one-third 

of the new homes being 

constructed are served by 

onsite systems. Many of 

the existing systems do not 

perform adequately due 

to a lack of management. 

Therefore, EPA promotes 

the sustained management 

of decentralized wastewater 

systems to enhance their 

performance and reliability. 

EPA strongly encourages 

communities to establish 

management programs for 

the maintenance of onsite 

systems in addition to 

improving local requirements 

for onsite system siting and 

system design. Communities 

benefit from effective 

onsite system management 

programs by enjoying 

improved protection of public 

health and local surface 

water and groundwater 

resources, preserving rural 

areas, protecting property 

owners’ investments through 

increased system service 

life, and avoiding the need 

to finance costly central 

wastewater collection and 

treatment systems. 

Asset Management

America’s public waterbased infrastructure – its 

water supply, wastewater, 

and storm water facilities, 

and collection/distribution 

systems – is integral to our 

economic, environmental 

and cultural vitality. 

Much of this country’s 

public wastewater system 

Licensed wastewater treatment plant operator

Sewer line maintenance
25

infrastructure has crossed 

the quarter-century mark, 

dating back to the CWA 

construction grant funding 

of the 1970s. Many of our 

collection systems date from 

the end of World War II and 

the population boom of the 

post war era. The oldest 

portions of the collection 

system pipe network exceed 

100 years of service. 

Significant parts of this 

infrastructure are severely 

stressed from overuse and 

the persistent under-funding 

of repair, rehabilitation, 

and replacement. In an 

increasing number of 

communities, existing 

systems are deteriorating, 

yet the demand for 

new infrastructure to 

accommodate growth 

presses unabated. A 

revitalized approach to 

managing capital wastewater 

assets for cost effective 

performance is emerging 

in this country. This asset 

management approach 

focuses on the cost effective 

sustained performance of the 

wastewater collection and 

treatment system assets over 

their useful life.

Operation 

Wastewater collection and 

treatment systems must 

be operated as designed 

to adequately protect 

water quality and human 

health. Most systems are in 

operation every day of the 

year, rain or shine. Licensed 

and trained operators are 

responsible for the dayto-day performance of the 

wastewater system. Their 

responsibilities include 

budget and business 

administration, public 

relations, analytical testing, 

and mechanical engineering 

as well as overseeing the 

collection system and 

wastewater treatment 

processes.

Maintenance

Wastewater collection and 

treatment systems must 

provide reliable service 

and avoid equipment 

breakdowns. Most 

equipment breakdowns 

can be avoided if system 

operators inspect the 

equipment, including 

sewer lines and manholes, 

regularly. Preventive 

maintenance uses data 

obtained through the 

inspections in a systematic 

way to direct maintenance 

activities before equipment 

failures occur. A good 

program will reduce 

breakdowns, extend 

equipment life, be costeffective, and help the 

system operators better 

perform their jobs.

Common Wastewater 

Treatment Terminology

Activated Sludge is a suspended 

growth process for removing 

organic matter from sewage 

by saturating it with air and 

microorganisms that can 

break down the organic 

matter.

Advanced Treatment involves 

treatment levels beyond 

secondary treatment.

Aeration Tank is a chamber for 

injecting air and oxygen into 

water.

Aerobic refers to a life or a 

process that occurs in the 

presence of oxygen.

Aerobic Treatment Units

provide wastewater 

treatment by injecting 

air into a tank, allowing 

aerobic bacteria to treat the 

wastewater.

Algae are aquatic plants which 

grow in sunlit waters and 

release oxygen into the 

water. Most are a food 

for fish and small aquatic 

animals, but some cause 

water quality problems.

Alternative System A 

wastewater treatment or 

collection system utilized 

in lieu of a conventional 

system.

Anaerobic refers to a life or a 

process that occurs in the 

absence of free oxygen.

Bacteria are small living 

organisms which help 

consume the organic 

constituents of sewage.

Barminutor is a device mounted 

on bar screens in a 

wastewater treatment plant 

to shred material, such 

as rags and debris, that 

accumulates on the bars.

Bar Screen is composed of 

parallel bars that remove 

larger objects from 

wastewater. 
26

Black Water is the term given 

to domestic wastewater that 

carries animal, human, or 

food wastes.

Biological Nutrient Removal

(BNR) is the use of bacteria 

to remove nutrients from 

wastewater. 

Biomass is microbial growth.

Biosolids are treated sewage 

sludge solids that have 

been stabilized to destroy 

pathogens and meet 

rigorous standards allowing 

for safe reuse of this material 

as a soil amendment. 

Biotower is a unit in which the 

waste is allowed to fall 

through a tower packed with 

synthetic media on which 

there is biological growth 

similar to the trickling filter. 

BOD (Biochemical Oxygen 

Demand) is a measure 

of oxygen consumed in 

biological processes that 

break down organic matter 

in water.

Carbon Adsorption is a method 

to treat wastewater in which 

activated carbon removes 

trace organic matter that 

resists degradation.

Chlorination is the process 

of adding chlorine gas or 

chlorine compounds to 

wastewater for disinfection. 

Chlorinator is a device that adds 

chlorine, in gas or liquid 

form, to wastewater to kill 

infectious bacteria.

Clarifier also known as a settling 

tank, removes solids from 

wastewater by gravity settling 

or by coagulation. 

Clean Water Act (Federal 

Water Pollution Control Act) 

originally enacted in 1948 

and amended in 1972, 1981 

and 1987, the Clean Water 

Act has as its objective the 

restoration and maintenance 

of the “chemical, physical, 

and biological integrity of 

the Nation’s waters.”

Coagulation is the clumping 

together of solids to make 

them settle out of the 

sewage faster. Coagulation 

of solids is improved by the 

use of chemicals such as 

lime, alum, iron salts, or 

polymers

Combined Sewers carry both 

sewage and stormwater 

runoff.

Comminutor is a device to catch 

and shred heavy solid matter 

at the headworks of the 

wastewater treatment plant.

Composting is the natural 

biological decomposition 

of organic material in the 

presence of air to form 

a stabilized, humus-like 

material.

Conventional Systems are 

wastewater treatment 

systems that have been 

traditionally used to collect 

municipal wastewater in 

sewers and convey it to a 

central facility for treatment 

prior to discharge to surface 

waters. Either primary or 

secondary treatment may be 

provided in a conventional 

system.

Denitrification is the reduction 

of nitrite to nitrogen gas. 

Denitrification is carried 

out in wastewater treatment 

tanks by bacteria under 

anoxic conditions. The 

bacteria use the nitrate for 

energy, and in the process, 

release nitrogen gas. The 

nitrogen gas, a major 

constituent of air, is released 

to the atmosphere.

Diffused Air is a technique by 

which air under pressure 

is forced into sewage in 

an aeration tank. The air 

is pumped into the tank 

through a perforated pipe 

and moves as bubbles 

through the sewage.

Digestion of solids takes place in 

tanks where volatile organic 

materials are decomposed 

by bacteria, resulting 

in partial gasification, 

liquefaction, and 

mineralization of pollutants.

Disinfection is the killing of 

pathogenic microbes 

including pathogenic 

bacteria, viruses, helminths, 

and protozoans. 

Dispersal/Percolation involves 

a volume of wastewater 

applied to the land, 

penetrating the surface, 

and passing through the 

underlying soil.

Dissolved Oxygen (DO) is 

the amount of free oxygen 

in solution in water, or 

wastewater effluent. 

Adequate concentrations 

of dissolved oxygen are 

necessary for fish and other 

aquatic organisms to live 

and to prevent offensive 

odors.

Eligible Costs are those 

wastewater reduction 

activities that can be funded 

with State Revolving Fund 

(SRF) loans.

Effluent is the treated liquid that 

comes out of a treatment 

plant after completion of the 

treatment process.

Eutrophication is the normally 

slow aging process by 

which a lake evolves 

into a bog or marsh and 

ultimately disappears. 

During eutrophication, the 

lake becomes enriched 

with nutrients, especially 

nitrogen and phosphorus, 

which support the excess 

production of algae and 

other aquatic plant life. 

Eutrophication may be 

accelerated by many human 

activities.

Evapotranspiration is the 

uptake of water from the 

soil by evaporation and by 
27

transpiration from the plants 

growing thereon.

Floc is a clump of solids formed 

in sewage by biological or 

chemical action.

Flocculation is the process 

by which clumps of solids 

in sewage are made to 

increase in size by chemical 

action.

Gray Water refers to domestic 

wastewater composed of 

wash water from sinks, 

shower, washing machines 

(does not include toilet 

wastewater).

Grinder Pump is a mechanical 

device which shreds 

wastewater solids and raises 

the fluid pressure level high 

enough to pass wastewater 

through small diameter 

pressure sewers.

Grit Chamber is a small 

detention basin designed to 

permit the settling of coarse, 

heavy inorganic solids, such 

as sand, while allowing the 

lighter organic solids to pass 

through the chamber.

Groundwater is the zone 

beneath the ground surface 

saturated with water that has 

seeped down through soil 

and rock.

Impervious means resistant to 

penetration by fluids or by 

roots.

Incineration involves combustion 

of the organic matter in 

sewage sludge, producing a 

residual inert ash.

Infiltration is the penetration of 

water through the ground 

into sub-surface soil or the 

passing of water from the 

soil into a pipe, such as a 

sewer.

Influent refers to water, 

wastewater, or other liquid 

flowing into a reservoir, 

basin or treatment plant, or 

any unit thereof.

Inorganic refers to compounds 

that do not contain carbon.

Interceptors are large sewer 

lines that collect the flows 

from smaller main and trunk 

sewers and carry them to the 

treatment plant.

Intermittent sand filter involves 

a bed of sand or other 

fine-grained material to 

which wastewater is applied 

intermittently in flooding 

doses.

Lagoon is a shallow pond in 

which algae, aerobic and 

anaerobic bacterial purify 

wastewater. 

Land Application is the 

controlled application of 

wastewater or biosolids onto 

the ground for treatment 

and/or reuse.

Lateral Sewers are small pipes 

that are placed in the 

ground to receive sewage 

from homes and businesses 

and convey it to main, trunk 

and interceptor sewer lines 

leading to the wastewater 

treatment plant.

Mechanical Aeration uses 

mechanical energy to inject 

air from the atmosphere into 

water to provide oxygen to 

create aerobic conditions.

Media Filters involves a bed of 

sand or other fine-grained 

material to which wastewater 

is applied, generally to 

physically remove suspended 

solids from sewage. 

Bacteria on the media 

decompose additional 

wastes. Treated water drains 

from the bed. Solids that 

accumulate at the surface 

must be removed from the 

bed periodically.

Microbes is shorthand for 

microorganisms.

Million Gallons Per Day (MGD) 

is a measurement of the 

volume of water.

Mound System is an effluent 

disposal system involving a 

mound of soil built up on the 

original ground surface to 

which effluent is distributed. 

National Pollutant Discharge 

Elimination System

(NPDES) is a program 

established by the Clean 

Water Act (CWA) that 

requires all wastewater 

discharges into “waters of 

the United States” to obtain 

a permit issued by the US 

Environmental Protection 

Agency (EPA) or a state 

agency authorized by the 

EPA.

Nitrification is the biochemical 

oxidation of ammonium to 

nitrate. 

Nitrogenous Wastes are wastes 

that contain a significant 

concentration of nitrogen.

Nutrients are elements or 

compounds essential as 

raw materials for plant 

and animal growth and 

development.

Organic Matter is the 

carbonaceous material 

contained in plants or 

animals and wastes.

Overland Flow is land treatment 

which involves the controlled 

application of wastewater 

onto grass-covered gentle 

slopes, with impermeable 

surface soils. As water flows 

over the grass-covered soil 

surface, contaminants are 

removed and the water is 

collected at the bottom of 

the slope for reuse.

Oxidation involves aerobic 

bacteria breaking down 

organic matter and oxygen 

combining with chemicals in 

sewage.

Oxidation Pond is an aerated 

man-made pond used for 

wastewater treatment.

Ozonation is a disinfection 

process where ozone is 
28

generated and added to 

wastewater effluent to kill 

pathogenic organisms. 

Pathogens are disease-causing 

microorganisms, including 

pathogenic bacteria, viruses, 

helminths, and protozoans. 

Percolation is the movement 

of water through subsurface soil layers, usually 

continuing downward to the 

groundwater.

Permeability is a measure of 

the ease with which water 

penetrates or passes through 

soil. 

Phosphorus is a nutrient that 

is essential to life, but in 

excess, contributes to the 

eutrophication of lakes and 

other water bodies.

Pollution results when 

contaminants in human, 

animal, vegetable, mineral, 

chemical or thermal waste 

or discharges reach water, 

making it less desirable 

for domestic, recreation, 

industry, or wildlife uses.

Polymer is a long chain organic 

compound produced by 

the joining of primary units 

called monomers. Polymers 

are used to improve settling 

of suspended solids, remove 

solids from wastewater, 

and improve dewatering of 

biosolids.

Pressure Sewers are a system 

of pipes in which the water, 

wastewater or other liquid is 

transported under pressure 

supplied by pumps.

Pretreatment involves treatment 

of wastes or wastewater by 

industries performed prior 

to the discharge to the sewer 

system.

Primary Treatment is the 

initial stage of wastewater 

treatment that removes 

floating material and 

material that easily settles 

out.

Pump is a mechanical device 

for raising or lifting water or 

other fluid, or for applying 

pressure to fluids in pipes.

Receiving Waters are 

waterbodies (i.e. rivers, 

lakes, oceans, or other 

water courses) that receive 

discharges of treated or 

untreated wastewater.

Rotating Biological Contactor

(RBC) is a wastewater 

treatment process involving 

large, closely-spaced 

plastic discs rotated about a 

horizontal shaft. The discs 

alternately move through 

the wastewater and the air, 

developing a biological 

growth on the surface of the 

discs that removes organic 

material in the wastewater.

Sanitary Sewer is the collection 

system for transporting 

domestic and industrial 

wastewater to municipal 

wastewater treatment 

facilities. Stormwater is not 

directed into this system but 

is handled by a separate 

system.

Secondary Treatment is the 

second stage in most 

wastewater treatment 

systems in which bacteria 

consume the organic matter 

in wastewater. Federal 

regulations define secondary 

treatment as meeting 

minimum removal standards 

for BOD, TSS, and pH in 

the discharged effluents 

from municipal wastewater 

treatment facilities.

Sedimentation Tanks are 

wastewater treatment tanks 

in which floating wastes are 

skimmed off and settled 

solids are removed for 

disposal. 

Seepage is the slow movement 

of water through small 

cracks or pores of the soil, 

or out of a pond, tank or 

pipe.

Septage refers to the residual 

solids in septic tanks or 

other on-site wastewater 

treatment systems that must 

be removed periodically for 

disposal.

Septic Tanks are a type of onsite 

wastewater treatment system 

in which the organic waste 

is decomposed and solids 

settle out. The effluent 

flows out of the tank to a 

soil adsorption field or other 

dispersal system.

Sequencing Batch Reactors

(SBR) are a variation 

of the activated sludge 

process where all treatment 

processes occur in one tank 

that is filled with wastewater 

and drawn down to 

discharge after treatment is 

complete. 

Settleable Solids are solids that 

are heavier than water and 

settle out of water by gravity.

Sewers are a system of pipes 

that collect and deliver 

wastewater and/or 

stormwater to treatment 

plants or receiving waters.

Soil Absorption Field is a 

subsurface area containing 

a trench or bed with a 

minimum depth of 12 

inches of clean stones and 

a system of piping through 

which treated wastewater 

effluent is distributed into the 

surrounding soil for further 

treatment and disposal.

Slow Rate Land Treatment

involves the controlled 

application of wastewater 

to vegetated land at a few 

inches of liquid per week.

Storm Sewers are a separate 

system of pipes that carry 

rain and snow melt from 

buildings, streets and yards 

to surface waters.

Suspended Solids are the small 

particles suspended in water 

or wastewater.
29

For more information see: 

www.epa.gov.owm

Transpiration is the process 

by which water vapor is 

released to the atmosphere 

by living plants. 

Trickling Filter is a fixed film 

process that involves a tank, 

usually filled with a bed of 

rocks, stones or synthetic 

media, to support bacterial 

growth used to treat 

wastewater.

Ultraviolet Radiation (UV) is a 

disinfection process where 

wastewater is exposed to UV 

light for disinfection.

Virus is the smallest form of 

a pathogen which can 

reproduce within host cells.

Wastewater Treatment Plant is 

a facility involving a series 

of tanks, screens, filters, and 

other treatment processes 

by which pollutants are 

removed from water.

Water Table is the elevation of 

groundwater or saturated 

soil level in the ground.