Sewage and its treatment have significant impacts on surface water quality. Sewage contains organic materials, suspended solids, nutrients, pathogens and other pollutants that degrade water quality. Historically, sewage was discharged untreated into waterways, but modern sewage systems route waste to treatment plants. Treatment involves primary, secondary and sometimes tertiary stages to remove pollutants through physical, chemical and biological processes. Despite treatment, poorly treated sewage still poses risks to surface water and human health if pathogens are not fully removed. Proper sewage treatment and disposal are needed to protect water resources.
2. – Contents:
– Sewage
– Seweragesystem
– Sewagetreatment
– Historyof drainage system
– Purpose of cleansurface water
– Howwastewateror sewagegenerated
– Pollutantsof water
– Principlepollutants
– Impactof sewagedischarge on thesurface water
– Developmentof sewagetreatment
– Combinedsystem
– Sanitarysystem
– Clusteredsystem
– Sewage treatmentplant
3. Impact of sewage and sewage
treatment on surface water
The quality of water resources defines the quality of life but on the contrary water crisis has been the
major environmental concern for the past decade.
Water resources are getting contaminated by various factors which include both domestic and
industrial pollutants.
Freshwater availability is one of the major problems facing the world and one third of drinking water
requirement of the world is obtained from surface sources like rivers, dams, lakes, and canals.
These sources of water also serve as best sinks for the discharge of domestic and industrial wastes.
4. Sewage:
Sewage is a type of wastewater that is produced by a community of people. It
is characterized by the volume or rate of flow, physical condition, chemical and
toxic constituents, and its bacteriologic status. It consists mostly of:
grey water (from sinks, bathtubs, showers, dishwashers, and clothes
washers),
black water (the water used to flush toilets, combined with
the human waste that it flushes away); soaps and detergents;
and toilet paper.
5. Sewerage system:
– It is network of pipes, pumps, and force mains for the collection of wastewater,
or sewage, from community. Modern sewerage systems are:
domestic sewers
industrial sewers
storm sewers.
– Sometimes a combined system provides only one network of pipes, mains, and
outfall sewers for all types of sewage and runoff.
6. It is the process of removing contaminants from
municipal wastewater, containing mainly household sewage plus
some industrial wastewater. Physical, chemical, and biological
processes are used to remove contaminants and produce treated
wastewater that is safe enough for release into the environment.
A by-product of sewage treatment is a semi-solid waste or slurry,
called sewage sludge. The sludge has to undergo
further treatment before being suitable for disposal or application
to land
7. History of drainage system:
Many ancient cities had
drainage systems, but they
were primarily intended to
carry rainwater away from
roofs and pavements.
There was little progress in
urban drainage or
sewerage during the
middle Ages.
Before the 20th century,
sewers usually discharged
into a body of water such
as a stream, river, lake, bay,
or ocean. There was no
treatment, so the
breakdown of the human
waste was left to
the ecosystem. Today, the
goal is that sewers route
their contents to
a wastewater treatment
plant rather than directly
to a body of water.
8. Purpose of clean surface water:
Surface water has
been exploited for
several purposes
by humans. It
serves as a source
of:
Potable water
after treatment
as a source of
domestic water
without treatment
particularly in
rural areas in
developing
countries
It has been used
for irrigation
purposes by
farmers,
fishermen get
their occupation
from harvesting
fish in so many
freshwater
sources.
It is used for
swimming and
also serves as
centers for tourist
9. How waste water or sewage generated
Domestic wastewater results from
water use in residences, businesses,
and restaurants.
Industrial wastewater comes from
discharges by manufacturing and
chemical industries.
Rainwater in urban and agricultural
areas picks up debris, grit, nutrients,
and various chemicals, thus
contaminating surface runoff water.
10.
11. Pollutants of water:
Water pollutants may originate from point sources or from dispersed sources.
Point source pollutant: it is one that reaches water from a single pipeline or channel, such
as a sewage discharge or outfall pipe. Surface runoff from farms, for example, is a dispersed
source of pollution, carrying animal wastes, fertilizers, pesticides, and silt into nearby
streams. Point-source pollutants are easier to control than dispersed-source pollutants.
Dispersed sources: are broad, unconfined areas from which pollutants enter a body of
water. Urban storm water drainage, which may carry sand and other gritty materials,
petroleum residues from automobiles, and road deicing chemicals, is also considered a
dispersed source because of the many locations at which it enters local streams or lakes
12. Principal pollutants
Organic material
The amount of organic material in sewage
is indicated by the biochemical oxygen
demand, the more organic material there is
in the sewage, the higher the BOD, which is
the amount of oxygen required by
microorganisms to decompose the organic
substances in sewage.
Dissolved oxygen is an important water
quality factor for lakes and rivers. The
higher the concentration of dissolved
oxygen, the better the water quality. When
sewage enters a lake or stream,
decomposition of the organic materials
begins. Oxygen is consumed as
microorganisms use it in their metabolism.
This can quickly deplete the available
oxygen in the water.
13. Suspended solids
Another important characteristic of sewage
is suspended solids. The volume of sludge
produced in a treatment plant is directly
related to the total suspended solids
present in the sewage.
Industrial and storm sewage may contain
higher concentrations of suspended solids
than domestic sewage. The extent to which
a treatment plant removes suspended
solids, as well as BOD, determines the
efficiency of the treatment process.
14. Plant nutrients
Domestic sewage contains compounds like:
nitrogen
Phosphorus
These two elements that are basic nutrients essential for growth of plants. In lakes, excessive amounts
of nitrates and phosphates can cause rapid growth of algae. Algal blooms, often caused by sewage
discharges, accelerate the natural aging of lakes in a process called eutrophication.
15. – Bacteria……….... (Salmonella, Shigella, Campylobacter, Vibrio cholerae).
– Viruses…………... (hepatitis A, rotavirus, enteroviruses).
– Protozoa………… (Entamoeba histolytica, Giardia lamblia, Cryptosporidium
parvum).
– Parasites …………such as helminthes and their eggs roundworm, hookworm and
whipworm
16. – Environmental impact of sewage:
– Poorly treated wastewater can have a profound influence on the receiving
watershed. The toxic impacts may be acute or cumulative.
– Acute impacts from wastewater effluents are generally due to high levels of
ammonia and chlorine, high loads of oxygen‐demanding materials, or toxic
concentrations of heavy metals and organic contaminants.
– Cumulative impacts are due to the gradual buildup of pollutants receiving
surface water, which only become apparent when certain threshold is
exceeded. All aquatic organisms have a temperature range for their optimum
function and survival.
17. – Contamination of surface water with pathogenic organisms in wastewater could
result in the transmission of waterborne diseases for people who use the water
resource for domestic and other purposes downstream. About 25% of all deaths
worldwide are the result of infectious diseases caused by pathogenic
microorganisms.
– Bacteria :
– Campylobacter jejune ………….. Gastroenteritis
– Escherichia coli ………………Gastroenteritis
– E. coli O157:H7 ………………Bloody diarrhea, hemolytic uremic syndrome
19. Developments in sewage
treatment
– Combined systems
– Systems that carry a mixture of both domestic sewage and storm sewage are
called combined sewers.
– Combined sewers typically consist of large-diameter pipes or tunnels, because of
the large volumes of storm water that must be carried during wet-weather
periods.
– They are very common in older cities but are no longer designed and built as part
of new sewerage facilities.
– Because wastewater treatment plants cannot handle large volumes of storm
water, sewage must bypass the treatment plants during wet weather and be
discharged directly into the receiving water.
20. Another method for controlling combined sewage
involves the use of swirl concentrators.
These direct sewage through cylindrically shaped
devices that create a vortex, or whirlpool, effect. The
vortex helps concentrate impurities in a much smaller
volume of water for treatment.
21. Sanitary systems
– New wastewater collection facilities are designed as separate systems,
carrying either domestic sewage or storm sewage but not both. Storm sewers
usually carry surface runoff to a point of disposal in a stream or river.
– Sanitary sewers, on the other hand, carry domestic wastewater to a sewage
treatment plant. Pretreated industrial wastewater may be allowed into
municipal sanitary sewerage systems, but storm water is excluded
22. Clustered wastewater treatment
systems
In certain instances when it is not feasible to connect
residences or units to public sewer systems, communities
may opt for a clustered wastewater treatment system.
Such facilities are smaller versions of centralized
treatment plants and serve only a limited number of
connections
23. – There are three levels of wastewater treatment:
– Primary level
– Secondary level
– Tertiary (or advanced) level
24. Primary treatment removes about
60 percent of total suspended
solids and about 35 percent of
BOD; dissolved impurities are not
removed.
Secondary treatment removes
more than 85 percent of both
suspended solids and BOD.
When more than 85 percent of
total solids and BOD must be
removed, or when dissolved nitrate
and phosphate levels must be
reduced, tertiary treatment
methods are used. Tertiary
processes can remove more than
99 percent of all the impurities
from sewage, producing an effluent
of almost drinking-water quality
25. Primary treatment
– Primary treatment removes material that will either float or readily settle out
by gravity. It includes the physical processes of screening, comminution, grit
removal, and sedimentation.
– Screens cause block floating debris such as wood, rags, and other bulky objects
that could clog pipes or pumps.
– Comminutor may be used to grind and shred debris that passes through the
screens.
– The shredded material is removed later by sedimentation or flotation
processes.
26. Primary treatment
– Grit chamber are designed to slow down the flow so that solids such as sand,
coffee grounds, and eggshells will settle out of the water.
– Suspended solids that pass through screens and grit chambers are removed
from the sewage in sedimentation tanks. These tanks, also called primary
clarifiers, provide about two hours of detention time .
– As the sewage flows through them slowly, the solids gradually sink to the
bottom. The settled solids known as raw or primary sludge are moved along
the tank bottom by mechanical scrapers
27.
28. – Secondary treatment removes the soluble organic matter that
escapes primary treatment. It also removes more of the
suspended solids.
– Removal is usually accomplished by biological processes in which
microbes consume the organic impurities as food, converting
them into carbon dioxide, water, and energy for their own
growth and reproduction.
29. There are three basic biological treatment methods:
– the trickling filter,
– the activated sludge process,
– The oxidation pond.
– Rotating biological contacter.
30. Trickling filter
– A trickling filter is simply a tank filled with a deep bed of stones.
– Settled sewage is sprayed continuously over the top of the stones and trickles
to the bottom, where it is collected for further treatment.
– As the wastewater trickles down, bacteria gather and multiply on the stones.
– The steady flow of sewage over these growths allows the microbes to absorb
the dissolved organics, thus lowering the biochemical oxygen demand (BOD)
of the sewage.
31. – Settling tanks, called secondary clarifiers, follow the
trickling filters. These clarifiers remove microbes that
are washed off the rocks by the flow of wastewater.
32. Activated sludge
The activated sludge treatment system consists of an aeration tank followed by a secondary
clarifier. Settled sewage, mixed with fresh sludge that is recirculated from the secondary clarifier, is
introduced into the aeration tank.
Compressed air is then injected into the mixture through porous diffusers located at the bottom of
the tank.
As it bubbles to the surface, the diffused air provides oxygen and a rapid mixing action. Air can also
be added by the churning action of mechanical propeller-like mixers located at the tank surface.
Under such oxygenated conditions, microorganisms thrive, forming an active, healthy suspension
of biological solids mostly bacteria called activated sludge
33. – Oxidation ponds, also called lagoons or stabilization ponds,
are large, shallow ponds designed to treat wastewater
through the interaction of sunlight, bacteria, and algae.
– Algae grow using energy from the sun and carbon dioxide
and inorganic compounds released by bacteria in water.
During the process of photosynthesis, the algae release
oxygen needed by aerobic bacteria.
34. Rotating biological contactor
– In this treatment system a series of large plastic disks mounted
on a horizontal shaft are partially submerged in primary effluent.
– As the shaft rotates, the disks are exposed alternately to air and
wastewater, allowing a layer of bacteria to grow on the disks and
to metabolize the organics in the wastewater.
35.
36. secondary effluent may be treated further by several
tertiary processes.
– For the removal of additional suspended solids and BOD from
secondary effluent, effluent polishing is an effective treatment. It is
most often accomplished using granular media filters, much like the
filters used to purify drinking water
37. – Removal of plant nutrients
– When treatment standards require the removal of plant nutrients from the
sewage, it is often done as a tertiary step. Phosphorus in wastewater is usually
present in the form of organic compounds and phosphates that can easily be
removed by chemical precipitation.
– A method called nitrification-denitrification can be used to remove the
nitrates. It is a two-step biological process in which ammonia nitrogen is first
converted into nitrates by microorganisms. The nitrates are further
metabolized by another species of bacteria, forming nitrogen gas that escapes
into the air.
38. Land treatment
– In some locations, secondary effluent can be applied directly to
the ground and a polished effluent obtained by natural processes
as the wastewater flows over vegetation and percolates through
the soil. There are three types of land treatment: slow-rate, rapid
infiltration, and overland flow.
– In the slow-rate, or irrigation, method, effluent is applied onto
the land by ridge-and-furrow spreading (in ditches) or by
sprinkler systems. Most of the water and nutrients are absorbed
by the roots of growing vegetation.
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