United States     EPA 833-F-98-002
        Environmental Protection May 1998
        Agency

        Office of Water (4204)	
&EPA  How
        Waste water
        Treatment
        Works...
        The Basics

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      One of the most common forms of pollution
      control in the United States is wastewater
treatment. The country has a vast system of
collection sewers, pumping stations, and treatment
plants. Sewers collect the wastewater from homes,
businesses, and many industries, and deliver it to
plants for treatment. Most treatment plants were
built to clean wastewater  for discharge into streams
or other receiving waters, or for reuse.
    Years  ago, when sewage was dumped into
waterways, a natural process of purification began.
First, the sheer volume of clean water in the stream
diluted wastes. Bacteria and other small organisms
in the water consumed the sewage and other
organic matter, turning it into new bacterial cells;
carbon dioxide and other products. Today's higher
populations and greater volume of domestic and
industrial  wastewater require that communities
give nature a helping hand.
    The basic function of wastewater treatment is to
speed up the natural processes by which water is
purified. There are two basic stages  in the treat-
ment of wastes, primary and secondary,  which are
outlined here.  In the primary stage, solids are
allowed to settle and removed from wastewater.
The secondary stage uses biological  processes to
further purify  wastewater.  Sometimes, these stages
are combined into one  operation.

Primary Treatment

As sewage enters a plant for treatment, it flows
through a screen, which removes large floating
objects such as rags and sticks that might clog
pipes  or damage equipment. After sewage has
been screened, it passes into a grit chamber, where
cinders, sand, and small stones settle to the bottom.
A grit chamber is particularly important in commu-
nities with combined sewer systems  where sand or
gravel may wash into sewers along with storm
water.
    After screening is completed and grit has been
removed,  sewage still contains organic and
inorganic  matter along with other suspended solids.

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These solids are minute particles that can be
removed from sewage in a sedimentation tank.
When the speed of the flow through one of these
tanks is reduced, the suspended solids will gradu-
ally sink to the bottom, where they form a mass of
solids called raw primary biosolids formerly
sludge). Biosolids are usually removed from tanks

             PRIMARY TREATMENT
Influe
   Screens   Grit chamber

                       Sedimentation tank

by pumping, after which it may be further treated
for use as a fertilizer, or disposed of in a land fill or
incinerated.
   Over the years, primary treatment alone has
been unable to meet many communities' demands
for higher water quality. To meet them, cities and
industries normally treat to a secondary treatment
level,  and in some cases, also use advanced
treatment to remove nutrients and other contami-
nants.

Secondary Treatment

The secondary stage  of treatment removes about
85 percent of the organic matter in sewage by
making use of the bacteria in it. The principal
secondary treatment techniques used in secondary
treatment are the trickling filter and the activated
sludge process.
   After effluent leaves the sedimentation tank in
the primary stage it flows or is pumped to a facility
using one or the other of these processes.  A
trickling filter is simply a bed of stones from three
to six feet deep through which sewage passes.

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          SECONDARY  TREATMENT
                                        Effluent
     \Return Activated biosolids  yWaste Biosolids

       Activated Biosolids Process

More recently, interlocking pieces of corrugated
plastic or other synthetic media have also been
used in trickling beds. Bacteria gather and multi-
ply on these stones until they can consume most of
the organic matter. The cleaner water trickles out
through pipes for further treatment. From a
trickling  filter, the partially treated sewage flows to
another sedimentation tank to remove excess
bacteria.
    The trend today is towards the use of the
activated sludge process instead of trickling filters.
The activated sludge process speeds up the work of
the bacteria by bringing air and sludge heavily
laden with bacteria into close contact with sewage.
After the sewage leaves the settling tank  in the
primary stage, it is pumped into an aeration tank,
where it  is mixed with air and sludge loaded with
bacteria and allowed to remain for several hours.
During this time, the bacteria break down the
organic matter into harmless by-products.
    The sludge, now activated with additional
billions of bacteria and  other tiny organisms, can
be used again by returning it to the aeration tank
for mixing with air and new sewage. From the
aeration tank, the partially treated sewage flows to
another sedimentation tank for removal of excess
bacteria.
    To complete secondary treatment, effluent from
the sedimentation tank is usually disinfected with
chlorine  before being discharged into receiving

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waters. Chlorine is fed into the water to kill
pathogenic bacteria, and to reduce odor. Done
properly, chlorination will kill more than 99
percent of the harmful bacteria in an effluent.
Some municipalities now manufacture chlorine
solution on site to avoid transporting and storing
large amounts of chlorine, sometimes in a gaseous
form.  Many states now require the removal of
excess chlorine before discharge  to surface waters
by a process called dechlorination. Alternatives to
chlorine disinfection, such as ultraviolet light or
ozone, are also being used in situations where
chlorine in treated sewage effluents may be
harmful to fish and other aquatic  life.

Other Treatment Options

New pollution problems have placed additional
burdens on wastewater treatment systems. Today's
pollutants, such as heavy metals, chemical com-
pounds, and toxic substances, are more difficult to
remove from water. Rising demands on the water
supply only aggravate the problem. The increasing
need to reuse water calls for better wastewater
treatment. These challenges are being met through
better methods of removing pollutants at treatment
plants, or through prevention of pollution at the
source.  Pretreatment of industrial waste, for
example, removes many troublesome pollutants at
the beginning, not the end, of the pipeline.
   To return more usable water to receiving lakes
and streams, new methods for removing pollutants
are being developed. Advanced waste treatment
techniques in use or under development range from
biological treatment capable of removing nitrogen
and phosphorus to physical-chemical separation
techniques such filtration,  carbon adsorption,
distillation,  and reverse osmosis.  These wastewa-
ter treatment processes, alone or  in combination,
can achieve almost  any degree of pollution control
desired, Waste effluents purified by such treat-
ment, can be used for industrial,  agricultural, or
recreational purposes, or even drinking water
supplies.

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