Sewage (Wastewater) Treatment*
Sewage, or wastewater, includes all the water
from a household that is used for washing and toilet
wastes. Rainwater flowing into street drains and
some industrial wastes enter the sewage system in
many cities. Sewage is mostly water and contains
little particulate matter, perhaps only 0.03%. Even so,
in large cities the solid portion of sewage can total
more than 1000 tons of solid material per day.
Until environmental awareness intensified, a
surprising number of large American cities had only
a rudimentary sewage treatment system or no system
at all. Raw sewage, untreated or nearly so, was simply discharged into rivers or oceans. A flowing, wellaerated stream is capable of considerable selfpurification. Therefore, until expanding populations
and their wastes exceeded this capability, this casual
treatment of municipal wastes did not cause problems. In the United States, most cases of simple discharge have been improved. But this is not true in
much of the world. Many of the communities bordering the Mediterranean dump their unprocessed sewage into the sea. At one Asiatic tourist resort, a hotel
posted instructions that toilet paper was not to be
flushed in the toilets—presumably because floating
paper would make it clear that the sewage outlets
were near the beach. In areas of Europe and South
Africa where tourism is essential to the economy,
local administrations are attempting to reassure visitors about bathing water quality with the Blue Flag
campaign. The presence of the flag (Figure 1) shows
that the coastal waters meet certain minimal standards of sanitation.
Figure 1. A beach displaying a blue flag.
What sort of bacterial populations would need
to be quantified to set standards for beach-area
Primary Sewage Treatment
The usual first step in sewage treatment is called
primary sewage treatment (Figure 2). In this process, large floating materials in incoming wastewater
are screened out, the sewage is allowed to flow
through settling chambers to remove sand and similar
gritty material, skimmers remove floating oil and
grease, and floating debris is shredded and ground.
After this step, the sewage passes through sedimentation tanks, where more solid matter settles out. Sewage solids collecting on the bottom are called
sludge—at this stage, primary sludge. About 40–
60% of suspended solids are removed from sewage
by this settling treatment, and flocculating chemicals
that increase the removal of solids are sometimes
added at this stage. Biological activity is not particularly important in primary treatment, although some
digestion of sludge and dissolved organic matter can
occur during long holding times. The sludge is removed on either a continuous or an intermittent basis,
and the effluent (the liquid flowing out) then undergoes secondary treatment.
Biochemical Oxygen Demand
An important concept in sewage treatment and in
the general ecology of waste management, biochemical oxygen demand (BOD) is a measure of the
biologically degradable organic matter in water. Primary treatment removes about 25–35% of the BOD
BOD is determined by the amount of oxygen required by bacteria to metabolize the organic matter.
The classic method of measurement is the use of special bottles with airtight stoppers. Each bottle is first
filled with test water or dilutions. The water is initially aerated to provide a relatively high level of dissolved oxygen and is seeded with bacteria if necessary. The filled bottles are incubated in the dark for 5
days at 20°C, and the decrease in dissolved oxygen is
determined by a chemical or electronic testing
method. The more oxygen that is used up as the bacteria degrade the organic matter in the sample, the
greater the BOD, which is usually expressed in milligrams of oxygen per liter of water. The amount of
oxygen that normally can be dissolved in water is
only about 10 mg/liter; typical BOD values of
wastewater may be 20 times this amount. If this
Figure 2. The stages in typical sewage treatment. Microbial activity occurs aerobically in trickling filters or activated
sludge aeration tanks and anaerobically in the anaerobic sludge digester. A particular system would use either activated
sludge aeration tanks or trickling filters, not both, as shown in this figure. Methane produced by sludge digestion is
burned off or used to power heaters or pump motors.
Identify the stages and processes. Which processes require oxygen?.
wastewater enters a lake, for example, bacteria in the
lake begin to consume the organic matter responsible
for the high BOD, rapidly depleting the oxygen in the
Secondary Sewage Treatment
After primary treatment, the greater part of the
BOD remaining in the sewage is in the form of dissolved organic matter. Secondary sewage treatment, which is predominantly biological, is designed
to remove most of this organic matter and reduce the
BOD (Figure 3). In this process, the sewage undergoes strong aeration to encourage the growth of aerobic bacteria and other microorganisms that oxidize
the dissolved organic matter to carbon dioxide and
water. Two commonly used methods of secondary
treatment are activated sludge systems and trickling
In the aeration tanks of an activated sludge system, air or pure oxygen is passed through the effluent
from primary treatment (Figure 4). The name is derived from the practice of adding some of the sludge
from a previous batch to the incoming sewage. This
inoculum is termed activated sludge because it contains large numbers of sewage-metabolizing microbes. The activity of these aerobic microorganisms
oxidizes much of the sewage organic matter into carbon dioxide and water. Especially important members of this microbial community are species of Zoogloea bacteria, which form bacteria containing
masses in the aeration tanks called floc, or sludge
granules (Figure 5).
Soluble organic matter in the sewage is incorporated into the floc and its microorganisms. Aeration is
discontinued after 4 to 8 hours, and the contents of
the tank are transferred to a settling tank, where the
Figure 3. An activated sludge system of secondary sewage
What are the similarities between winemaking and activated sludge sewage treatment?
floc settles out, removing much of the organic matter.
These solids are subsequently treated in an anaerobic
sludge digester, which will be described shortly.
Probably more organic matter is removed by this
settling-out process than by the relatively short-term
aerobic oxidation by microbes.
The clear effluent is disinfected and discharged.
Occasionally, the sludge will float rather than settle
out; this phenomenon is called bulking. When this
happens, the organic matter in the floc flows out with
the discharge effluent, resulting in local pollution.
Bulking is caused by the growth of filamentous bacteria of various types; Sphaerotilus natans and Nocardia species are frequent offenders. Activated
sludge systems are quite efficient: they remove 75–
95% of the BOD from sewage.
Trickling filters are the other commonly used
method of secondary treatment. In this method, the
sewage is sprayed over a bed of rocks or molded
plastic (Figure 6a). The components of the bed must
be large enough so that air penetrates to the bottom
but small enough to maximize the surface area available for microbial activity. A biofilm of aerobic microbes grows on the rock or plastic surfaces (Figure
6b). Because air circulates throughout the rock bed,
these aerobic microorganisms in the slime layer can
oxidize much of the organic matter trickling over the
surfaces into carbon dioxide and water. Trickling
filters remove 80–85% of the BOD, so they are generally less efficient than activated sludge systems.
However, they are usually less troublesome to operate and have fewer problems from overloads or toxic
sewage. Note that sludge is also a product of trickling
Another biofilm-based design for secondary
sewage treatment is the rotating biological contactor system. This is a series of disks several feet in
diameter, mounted on a shaft. The disks rotate
slowly, with their lower 40% submerged in wastewater. Rotation provides aeration and contact between
the biofilm on the disks and the wastewater. The rotation also tends to cause the accumulated biofilm to
slough off when it becomes too thick. This is about
the equivalent of floc accumulation in activated
Disinfection and Release
Treated sewage is disinfected, usually by chlorination, before being discharged (Figure 6c). The discharge is usually into an ocean or into flowing
streams, although spray-irrigation fields are sometimes used to avoid phosphorus and heavy metal contamination of waterways.
Sewage can be treated to a level of purity that allows its use as drinking water. This is the practice
now in some arid-area cities in the United States and
will probably be expanded. In a typical system, the
treated sewage is filtered to remove microscopic suspended particles, then passed through a reverse osmosis purification system to remove microorganisms.
Any remaining microorganisms are killed by exposure to UV light or other disinfectants.
Primary sludge accumulates in primary sedimentation tanks; sludge also accumulates in activated
sludge and in trickling filter secondary treatments.
For further treatment, these sludges are often pumped
to anaerobic sludge digesters (Figure 6d and Figure
7). The process of sludge digestion is carried out in
large tanks from which oxygen is almost completely
excluded. In secondary treatment, emphasis is placed
on the maintenance of aerobic conditions so that organic matter is converted to carbon dioxide, water,
and solids that can settle out. An anaerobic sludge
digester, however, is designed to encourage the
growth of anaerobic bacteria, especially methaneproducing bacteria that decrease these organic solids
by degrading them to soluble substances and gases,
mostly methane (60–70%) and carbon dioxide (20–
30%). Methane and carbon dioxide are relatively
innocuous end-products, comparable to the carbon
dioxide and water from aerobic treatment. The methane is routinely used as a fuel for heating the digester
and is also frequently used to run power equipment in
the plant. There are essentially three stages in the
activity of an anaerobic sludge digester. The first
stage is the production of carbon dioxide and organic
acids from anaerobic fermentation of the sludge by
various anaerobic and facultatively anaerobic microorganisms. In the second stage, the organic acids are
metabolized to form hydrogen and carbon dioxide, as
well as such organic acids such as acetic acid. These
products are the raw materials for a third stage, in
which the methane-producing bacteria produce methane (CH4). Most of the methane is derived from the
energy- yielding reduction of carbon dioxide by hydrogen gas:
Other methane-producing microbes split acetic
acid (CH3COOH) to yield methane and carbon dioxide:
After anaerobic digestion is completed, large
amounts of undigested sludge still remain, although it
is relatively stable and inert. To reduce its volume,
this sludge is pumped to shallow drying beds or water-extracting filters. Following this step, the sludge
can be used for landfill or as a soil conditioner, sometimes under the name biosolids. Sludge is assigned to
two classes: class A sludge contains no detectable
pathogens, and class B sludge is treated only to reduce numbers of pathogens below certain levels.
Most sludge is class B, and public access to applica-
tion sites is limited. Sludge has about one-fifth the
growth-enhancing value of normal commercial lawn
fertilizers but has desirable soil-conditioning qualities, much as do humus and mulch. A potential problem is contamination with heavy metals that are toxic
Homes and businesses in areas of low population
density that are not connected to municipal sewage
systems often use a septic tank, a device whose operation is similar in principle to primary treatment
(Figure 8). Sewage enters a holding tank, and suspended solids settle out. The sludge in the tank must
be pumped out periodically and disposed of. The
effluent flows through a system of perforated piping
into a leaching (soil drainage) field. The effluent entering the soil is decomposed by soil microorganisms.
The microbial action necessary for proper functioning of a septic tank can be impaired by excessive
amounts of products such as antibacterial soaps, drain
cleaners, medications, “every flush” toilet bowl
cleaners, and bleach. These systems work well when
not overloaded and when the drainage system is
properly sized to the load and soil type. Heavy clay
soils require extensive drainage systems because of
the soil’s poor permeability. The high porosity of
sandy soils can result in chemical or bacterial pollution of nearby water supplies.
Many industries and small communities use oxidation ponds, also called lagoons or stabilization
ponds, for water treatment. These are inexpensive to
build and operate but require large areas of land. Designs vary, but most incorporate two stages. The first
stage is analogous to primary treatment; the sewage
pond is deep enough that conditions are almost entirely anaerobic. Sludge settles out in this stage. In
the second stage, which roughly corresponds to secondary treatment, effluent is pumped into an adjoining pond or system of ponds shallow enough to be
aerated by wave action. Because it is difficult to
maintain aerobic conditions for bacterial growth in
ponds with so much organic matter, the growth of
algae is encouraged to produce oxygen. Bacterial
action in decomposing the organic matter in the
wastes generates carbon dioxide. Algae, which use
carbon dioxide in their photosynthetic metabolism,
grow and produce oxygen, which in turn encourages
the activity of aerobic microbes in the sewage. Large
amounts of organic matter in the form of algae accumulate, but this is not a problem because the oxidation pond, unlike a lake, already has a large nutrient
Some small sewage-producing operations, such
as isolated campgrounds and highway rest stop areas,
use an oxidation ditch for sewage treatment. In this
method, a small oval channel in the shape of a racetrack is filled with sewage water. A paddle wheel
similar to that on an old-time Mississippi steamboat,
but in a fixed location, propels the water in a selfcontained flowing stream aerated enough to oxidize
Tertiary Sewage Treatment
As we have seen, primary and secondary treatments of sewage do not remove all the biologically
degradable organic matter. Amounts of organic matter that are not excessive can be released into a flowing stream without causing a serious problem. Eventually, however, the pressures of increased population
might increase wastes beyond a body of water’s carrying capacity, and additional treatments might be
required. Even now, primary and secondary treatments are inadequate in certain situations, such as
when the effluent is discharged into small streams or
recreational lakes. Some communities have therefore
developed tertiary sewage treatment plants. Lake
Tahoe in the Sierra Nevada Mountains, surrounded
by extensive development, is the site of one of the
best-known tertiary sewage treatment systems. Similar systems are used to treat wastes entering the
southern portion of San Francisco Bay.
The effluent from secondary treatment plants
contains some residual BOD. It also contains about
50% of the original nitrogen and 70% of the original
phosphorus, which can greatly affect a lake’s ecosystem. Tertiary treatment is designed to remove essentially all the BOD, nitrogen, and phosphorus. Tertiary
treatment depends less on biological treatment than
on physical and chemical treatments. Phosphorus is
precipitated out by combining with such chemicals as
lime, alum, and ferric chloride. Filters of fine sands
and activated charcoal remove small particulate matter and dissolved chemicals. Nitrogen is converted to
ammonia and discharged into the air in stripping
towers. Some systems encourage denitrifying bacteria to form volatile nitrogen gas. Finally, the purified
water is chlorinated.
Tertiary treatment provides water that is suitable
for drinking, but the process is extremely costly. Secondary treatment is less costly, but water that has
undergone only secondary treatment still contains
many water pollutants. Much work is being done to
design secondary treatment plants in which the effluent can be used for irrigation. This design would
eliminate a source of water pollution, provide nutrients for plant growth, and reduce the demand on already scarce water supplies. The soil to which this
water is applied would act as a trickling filter to remove chemicals and microorganisms before the water reaches groundwater and surface water supplies.
For questions 1–2, answer whether
a. the process takes place under aerobic conditions.
b. the process takes place under anaerobic conditions.
c. the amount of oxygen doesn’t make any difference.
1. Activated sludge system
Which type of sewage treatment is designed to
remove almost all phosphorus from sewage?
What is the relationship between BOD and the
welfare of fish?
In addition to pollution prevention, what other
value does sewage treatment have?
G. Tortora, B. Funke, C. Case. 2010. Microbiology: An Introduction. San Francisco: Benjamin Cummings.