TABLE OF CONTENT
TABLE OF CONTENT.............................................................................................................1
LIST OF FIGURE......................................................................................................................1
1 INTRODUCTION...........................................................................................................2
2 WASTE-WATER TREATMENT PROCESS................................................................2
2.1 Physical Treatment Process...............................................................................................2
2.1.1 Screening...................................................................................................................3
2.1.2 Flow Equalization......................................................................................................3
2.1.3 Mixing and Flocculation............................................................................................3
2.1.4 Gravity Separation Theory.........................................................................................3
2.1.5 Grit Removal..............................................................................................................4
2.1.6 High-Rate Clarification (HRC)..................................................................................4
2.1.7 Flotation.....................................................................................................................4
2.2 Chemical Treatment Process..............................................................................................4
2.2.1 Chemical coagulation.............................................................................................4
2.2.2 Chemical precipitation...........................................................................................5
2.2.3 Chemical disinfection.............................................................................................5
2.2.4 Chemical oxidation................................................................................................7
2.3 Biological Treatment.........................................................................................................7
3 CONCLUSION...............................................................................................................9
4 REFERENCE..................................................................................................................9
LIST OF FIGURE
Figure 1: Location of physical unit operation in wastewater treatment plant flow diagram.....2
Figure 2: Screening process.......................................................................................................3
Figure 3: Grit removal chamber.................................................................................................4
Figure 4: Biological Treatment Process.....................................................................................9
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1 INTRODUCTION
Wastewater is water whose physical, chemical or biological properties have been changed as
a result of the introduction of certain substances which render it unsafe for some purposes such as
drinking.
Wastewater treatment, also called sewage treatment, the removal of impurities from wastewater, or
sewage, before it reaches aquifers or natural bodies of water such as rivers, lakes, estuaries, and
oceans. Since pure water is not found in nature (i.e., outside chemical laboratories), any distinction
between clean water and polluted water depends on the type and concentration of impurities found in
the water as well as on its intended use (Nathanson, 2022).
Water and wastewater treatment plants are designed on a unit operations concept in which one
operation is optimized to accomplish one task, although more than one problem substance may be
remedied in the operation. Each operation generally has ramifications on other downstream treatment
processes, and tradeoffs between increasing the efficiency of one process or another depend on water
characteristics and costs of each operation. A brief description of the processes used for water and
wastewater treatment is given here with an explanation of where the processes may occur in the
treatment stream (Droste & Gehr, 2019).
2 WASTE-WATER TREATMENT PROCESS
2.1Physical Treatment Process
Operations used for the treatment of wastewater in which change is brought about
by means of the application of physical forces are known as Physical unit operations/ Physical
treatment.
Figure 1: Location of physical unit operation in wastewater treatment plant flow diagram
Most commonly used physical unit operations are the following (Steer, 1940).
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2.1.1 Screening
Screening is the first unit activity that is typically seen in wastewater treatment plants. The solids
detected in the wastewater that needs to be treated are retained by screens, which are devices having
apertures that are typically all the same size.
Figure 2: Screening process
2.1.2 Flow Equalization
The operational issues brought on by fluctuations in flow rate are solved by flow equalization. Based
on the features of the collecting system, it can be utilized in a variety of different conditions and is
essentially the dampening of flow rate variations to achieve a constant or nearly constant flow rate.
2.1.3 Mixing and Flocculation
Mixing is an important unit operation in many phases of wastewater treatment including
Mixing of one substances completely with another
Blending of miscible liquids
Flocculation of wastewater particles
Continuous mixing of liquid suspensions
Heat transfer
Flocculation is a transport step that brings about the collisions between the destabilized particles
(suspended solid and BOD) needed to form larger particles that can be removed readily by settling or
filtration.
2.1.4 Gravity Separation Theory
One of the most often employed unit procedures in wastewater treatment is the removal of suspended
and colloidal elements from wastewater by gravity separation. The term "sedimentation" refers to the
gravity settling process used to separate suspended, heavier-than-water particles.
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2.1.5 Grit Removal
Grit can be removed from wastewater using centrifugal separation of solids or grit chambers. Sand,
gravel, cinders, or other heavy solid materials with subsidence velocities or specific gravities
significantly higher than those of the organic putrescible solids in wastewater can be removed using
grit chambers.
Figure 3: Grit removal chamber
2.1.6 High-Rate Clarification (HRC)
High-rate clarification employs physical/chemical treatment and utilizes flocculation and
sedimentation systems to achieve rapid settling.
2.1.7 Flotation
One procedure used to separate solid or liquid particles from the liquid phase is flotation. By
introducing tiny gas (air) bubbles into the liquid phase, separation is induced.
2.2Chemical Treatment Process
The principal chemical unit processes used for wastewater treatment include:
1. Chemical coagulation
2. Chemical precipitation
3. Chemical disinfection
4. Chemical oxidation
2.2.1 Chemical coagulation
Chemical coagulation refers to the processes and mechanisms that lead to the chemical instability of
small particles and the production of larger particles through perikinetic flocculation (aggregation of
particles with sizes between 0.01 and 1 m).
Role of chemical coagulants
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Understanding the properties of the colloidal particles present in wastewater is crucial to
understanding the function that chemical coagulants and flocculants play in removing those particles.
2.2.2 Chemical precipitation
The precipitation reactions involved with
Alum
When alum is added to wastewater containing calcium and magnesium bicarbonate
alkalinity a precipitate of aluminum hydroxide will form. The overall reaction are as follows:
3Ca (HCO3)2 + Al2(SO4)3.18H2O ⇌ 2Al(OH)3+3 CaSO4+ 6CO2+18H2O
Lime
When lime alone added as a precipitant, the principles of clarification are explained by the
following reactions-
For the carbonic acid,
H2CO3+Ca (OH)2 ⇌ CaCO3 +2H2O
For the alkalinity
Ca (HCO 3)2+ Ca (OH)2 ⇌ 2CaCO3+2H2O
Ferric chloride
Ferric chloride is the Iron salt used most commonly in precipitation applications. When
this salt is added to wastewater the following reactions take place,
2FeCl3+ 3Ca (HCO3)2 ⇌ 2Fe(OH)3+ 3CaCl2+6CO2
Ferric chloride and Lime
If lime is added to supplement the natural alkalinity of the waste water, the following
reaction can be assumed to occur,
2FeCl3 + 3Ca(OH)2 ⇌ 2Fe(OH)3+3CaCl2
2.2.3 Chemical disinfection
Before the effluent is returned to the water system, the removal of organisms from the treated water is
often the last stage. Disinfection stops the spread of waterborne infections by bringing bacteria and
microbial populations down to a controlled level.
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Disinfection methods
Disinfection is most accomplished by the use of
1. Chemical agents
Chlorine and its compounds
Bromine
Iodine
Ozone
Phenol and phenolic compounds
Alcohols
2. Physical agents
Physical disinfectants that can be used are heat, light, and sound wave. Heating water to the boiling
point, for example, will destroy the major disease producing non-spore-forming bacteria.
Sunlight is also a good disinfectant, due to primarily to the UV radiation portion of the
electromagnetic (EM) spectrum. The decay of microbes observed in oxidation ponds is due to their
exposure to the UV component of the sunlight. Special lamps developed to emit UV rays have been
used successfully to disinfect water and wastewater.
3. Mechanical means
The typical removal efficiencies for various treatment operations and processes are reported in the
following table
Process Percent removal
Coarse screens 0-5
Fine screens 10-20
Grit chamber 10-25
Plain sedimentation 25-75
Chemical precipitation 40-80
Trickling filters 90-95
Activated sludge 90-98
Chlorination of treated wastewater 98-99.999
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4. Radiation
The major types of radiation are electromagnetic, acoustic, and particle. Gamma rays are emitted from
radioisotopes, such as cobalt-60. because of their penetration power, gamma rays have been used to
disinfect (sterilize) both water and wastewater.
2.2.4 Chemical oxidation
BOD and COD
The overall reaction for the oxidation of organic molecules comprising BOD with Ozone and
Hydrogen peroxide can be represented as;
Multiple arrows in the direction signify the number of steps are involved in the overall reaction. The
overall reaction rates are too slow to be applicable for waste water treatment.
Ammonia
The chemical process in which chlorine is used to oxidize the ammonia nitrogen in solution to
nitrogen gas and other stable compounds is known as breakpoint chlorination.
NH4++ HOCl → NH2Cl+H2O + H+
NH2Cl + HOCl → NHCl2+ H2O
0.5 NHCl 2+ 0.5H2O → 0.5NOH + H+ + Cl-
0.5 NHCl 2 +0.5NOH → 0.5N2 +0.5HOCl +0.5H+ +0.5Cl-
The overall reaction is given as:
NH4+ +1.5HOCl → 0.5N2 +1.5 H2O +2.5H+ +1.5Cl-
The most important advantages of this process is that, with proper control, all(98%) the ammonia
nitrogen in the wastewater can be oxidized. The disadvantages are build up of HCl, TDS and chloro-
organic compounds are recognizable.
2.3Biological Treatment
The goal of biological wastewater treatment is to reduce the amount of biochemical oxygen demand
(BOD) in wastewater by establishing contact between the water to be treated and bacteria that feed on
the organic components in the wastewater. In other words, BOD reduction is the goal of biological
treatment.
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Some important terms in Biological treatment
Aerobic process: Biological treatment process that occur in the presence of oxygen
Anaerobic process: Biological treatment process that occur in the absence of oxygen
Anoxic process: The process by which nitrate nitrogen converted biologically to nitrogen gas
in the absence of oxygen. This process is also known as Denitrification.
Facultative process: In this process organism can function in the presence or absence of
molecular oxygen.
Suspended growth process(SGP): In which the microbes responsible for degradation are
maintained in suspension within the liquid.
Attached growth process(AGP): In which the microbes responsible for degradation are
attached to some inert medium, such as rocks, plastic materials etc.
Lagoon process: A generic term applied to treatment processes that take place in ponds or
lagoons.
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Figure 4: Biological Treatment Process
3 CONCLUSION
The pollution of rivers and streams with chemical contaminants is one of the most crucial
environmental problems. Waterbome chemical pollution entering rivers and streams causes
tremendous destruction. Thus it is essential to treat the wastewater before it is discharged into the
environment. The principal objective of wastewater treatment is generally to allow human and
industrial effluents to be disposed of without danger to human health or unacceptable damage to the
natural environment.
Advanced wastewater treatment can be used to achieve any level of treatment desired. Advanced
treatment is necessary in some treatment systems to remove nutrients from wastewater. Advanced
wastewater treatment plants utilize sophisticated processes and equipment. They are relatively
expensive to run and operating costs as well as effluent quality are sensitive to the quality of
operation. The ultimate goal of wastewater treatment should be managing wastewater effectively,
economically, and ecologically.
4 REFERENCE
Droste, R., & Gehr, R. (2019). Theory and practice of water and wastewater treatment ( PDFDrive
).
Nathanson, J. A. (2022). wastewater treatment.
Steer, F. W. (1940). Wastewater Engineering Treatment and Reuse. In Notes and Queries (Vol.
179, Issue 18, p. 317). https://doi.org/10.1093/nq/179.18.317-a
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