Drinking Water Treatment

Public Water Systems

Public Water Systems (PWSs) come in all shapes 

and sizes, and no two are exactly the same. They 

may be publicly or privately owned and maintained. 

While their design may vary, they all share the same 

goal: providing safe, reliable drinking water to 

the communities they serve. To do this, most water 

systems must treat their water. The types of treatment 

provided by a specific PWS vary depending on the 

size of the system, whether they use ground water or 

surface water, and the quality of the source water.

Tapping a Source of Water

Large-scale water supply systems tend to rely on 

surface water sources, while smaller systems tend 

to rely on ground water. Around 32 percent of the 

population served by community water systems 

(CWSs) drink 

water that 

originates as 

ground water. 

Ground water is 

usually pumped 

from wells 

ranging from 

shallow to deep 

(50 to 1,000 

feet). The remaining 68 percent of the population 

served by CWSs receive water taken primarily from 

surface water sources like rivers, lakes, and reservoirs.

Treating Raw Water

The amount and type of treatment applied by a PWS 

varies with the source type and quality. Many ground 

water systems can satisfy all federal requirements 

without applying any treatment, while others need 

to add chlorine or additional treatment. US EPA 

is developing a ground water rule that will specify 

the appropriate use of disinfection and will address 

other components of ground water systems to assure 

public health protection. Because surface water 

systems are exposed to direct wet weather runoff 

and to the atmosphere and are therefore more easily 

contaminated, federal and state regulations require 

that these systems treat their water. Disinfection 

of drinking water is one of the major public 

health advances of the 20th century. However, the 

disinfectants themselves can react with naturally 

occurring materials in the water to form unintended 

byproducts which may pose health risks. A major 

challenge for water suppliers is balancing the risks 

from microbial pathogens and disinfection byproducts. 

The Stage 1 Disinfectants and Disinfection Byproducts 

Rule and the Interim Enhanced Surface Water 

Treatment Rule together address these risks.

Water suppliers use a variety of treatment processes 

to remove contaminants from drinking water. These 

individual processes may be arranged in a “treatment 

train” (a series of processes applied in sequence). 

All public water systems must have at least 15 

service connections or serve at least 25 people per 

day for 60 days of the year.

Drinking water standards apply to water systems 

differently based on their type and size:

Community Water System (there are approximately 

54,000) - A public water system that serves 

the same people year-round. Most residences 

including homes, apartments, and condominiums 

in cities, small towns, and mobile home parks are 

served by Community Water Systems.

Non-Community Water System - A public water 

system that serves the public but does not serve 

the same people year-round. There are two types 

of non-community systems:

Non-Transient Non-Community Water 

System (there are approximately 20,000) - A 

noncommunity water system that serves the same 

people more than six months per year, but not 

year-round, for example, a school with its own 

water supply is considered a non-transient system.

Transient non-community water system (there are 

approximately 89,000) - A non-community water 

system that serves the public but not the same 

individuals for more than six months, for example, 

a rest area or campground may be considered a 

transient water system.

SAFE DRINKING WATER ACT 1974 - 2004 PROTECT OUR HEALTH FROM SOURCE TO TAP
The most commonly used processes include filtration, 

flocculation and sedimentation, and disinfection for 

surface water. Some treatment trains also include 

ion exchange and adsorption. Water utilities select a 

combination of treatment processes most appropriate 

to treat the contaminants found in the raw water used 

by the system.

Types of Treatment

Flocculation/Sedimentation: Flocculation 

refers to water treatment processes that combine or 

coagulate small 

particles into larger 

particles, which 

settle out of the 

water as sediment. 

Alum and iron 

salts or synthetic 

organic polymers 

(used alone or 

in combination 

with metal salts) 

are generally 

used to promote 

coagulation. Settling 

or sedimentation 

occurs naturally as 

flocculated particles 

settle out of the 

water.

Filtration: Many 

water treatment 

facilities use 

filtration to remove all particles from the water. Those 

particles include clays and silts, natural organic 

matter, precipitates from other treatment processes in 

the facility, iron and manganese, and microorganisms. 

Filtration clarifies water and enhances the 

effectiveness of disinfection.

Ion Exchange: Ion exchange processes are used 

to remove inorganic contaminants if they cannot be 

removed adequately by filtration or sedimentation. Ion 

exchange can be used to treat hard water. It can also 

be used to remove arsenic, chromium, excess fluoride, 

nitrates, radium, and uranium.

Absorption: Organic contaminants, unwanted 

coloring, and taste-and-odor-causing compounds can 

stick to the surface of granular or powder activated 

carbon and are thus removed from the drinking water.

Disinfection (chlorination/ozonation): Water 

is often disinfected before it enters the distribution 

system to ensure that potentially dangerous microbes 

are killed. Chlorine, chloramines, or chlorine dioxide 

are most often used because they are very effective 

disinfectants, not only at the treatment plant but 

also in the pipes that distribute water to our homes 

and businesses. Ozone is a powerful disinfectant, 

and ultraviolet radiation is an effective disinfectant 

and treatment for relatively clean source waters, but 

neither of these are effective in controlling biological 

contaminants in the distribution pipes.

Monitoring Water Quality

Water systems monitor 

for a wide variety of 

contaminants to verify 

that the water they 

provide to the public 

meets all federal 

and state standards. 

Currently, the nation’s 

community water 

systems (CWSs) and 

nontransient noncommunity water 

systems (NTNCWSs) 

must monitor for more 

than 83 contaminants. 

The major classes of 

contaminants include 

volatile organic 

compounds (VOCs), 

synthetic organic 

compounds (SOCs), 

inorganic compounds 

(IOCs), radionuclides, 

and microbial organisms (including bacteria). Testing 

for these contaminants takes place on varying 

schedules and at different locations throughout the 

water system.

Transient non-community water systems may monitor 

less frequently and for fewer contaminants than CWSs. 

Because these types of systems serve an ever-changing 

population, it is most important for them to monitor 

for contaminants such as microbiologicals and nitrate 

that can cause an immediate, acute public health 

effect. Water systems also monitor for a number of 

contaminants that are currently not regulated. These 

monitoring data provides the basis for identifying 

contaminants to be regulated in the future.

Distribution to Customers

An underground network of pipes typically delivers 

drinking water to the homes and businesses served 

by the water system. Small systems serving just a 

The Water Cycle - Drinking water can come from both surface water 

and ground water. The water cycle begins with rainwater and snow melt 

that gathers in lakes and rivers which interact with ground water.
Office of Water (4606) www.epa.gov/safewater EPA 816-F-04-034 

Storage: Water is placed in 

a closed tank or reservoir 

for disinfection to take 

place. The water then flows 

through pipes to homes 

and businesses in the 

community.

Source: AWWA Drinking Water Week Blue Thumb Kit

Filtration: The water passes 

through filters, some made 

of layers of sand, gravel, and 

charcoal that help remove 

even smaller particles.

Sedimentation: 

The heavy particles 

(floc) settle to the 

bottom and the 

clear water moves 

to filtration.

Disinfection: A small amount of chlorine is added or 

some other disinfection method is used to kill any 

bacteria or microorganisms that may be in the water.

Coagulation removes dirt and other particles suspended in 

water. Alum and other chemicals are added to water to form 

tiny sticky particles called “floc” which attract the dirt 

particles. The combined weight of the dirt and the alum 

(floc) become heavy enough to sink to the bottom during 

sedimentation.

Lake or Reservoir

Water Treatment Plant

Follow a drop of water from the source through the treatment process. Water may be treated differently in 

different communities depending on the quality of the water which enters the plant. Groundwater is located 

underground and typically requires less treatment than water from lakes, rivers, and streams.

handful of households may be relatively simple. Large 

metropolitan water systems can be extremely complex 

. sometimes with thousands of miles of piping serving 

millions of people. Although water may be safe when 

leaving the water treatment plant it is important to 

ensure that this water does not become contaminated 

in the distribution system because of such things as 

water main breaks, pressure problems, or growth of 

microorganisms. Much of the existing drinking water 

infrastructure was built many years ago. The US 

EPA Infrastructure Needs Survey, released in 2001, 

estimated that drinking water systems will need to 

invest $150.9 billion over a 20 year period to ensure 

the continued source development, storage, treatment, 

and distribution of safe drinking water. Many agree 

this is a very conservative low estimate.

For More Information

To learn more about drinking water treatment 

and treatment techniques, call the Safe Drinking 

Water Hotline at 1-800-426-4791 or visit the 

safewater web site at www.epa.gov/ safewate r.