9. 9
1.0 INTRODUCTION
• Nuisance of Waste Water
• Health & Environmental Concerns
• Industrial Wastewater
• Typical Composition of Untreated Domestic Wastewater
• Characteristics of Wastewater
10. 10
Nuisance of Wastewater
• It is desirable and becoming necessary to remove
immediately wastewater from its sources of generation,
followed by treatment and disposal because:
– Untreated wastewater usually contains many pathogenic or
disease-causing micro-organisms that dwell in the human
intestinal tract of that may be present in certain industrial waste.
– Also, nutrients which can stimulate the growth of aquatic plants
are found in wastewater.
– Wastewater may contain toxic compounds.
• Wastewater collected from municipalities and
communities must ultimately be returned to receiving
waters or to the land.
11. 11
• Nuisance and health conditions have brought about an
increasing demand for more effective means of
wastewater management.
– The impracticability of procuring sufficient areas for the disposal
of untreated wastewater on land, particularly for larger cities, led
to the adoption of more intensive methods of treatment.
12. 12
• Primary treatment is essentially dealing with physical operations in
which screening and sedimentation are used to remove the floating
and settleable solids found in wastewater.
• Secondary treatment uses biological and chemical processes to
remove most of the organic matter.
• Advanced treatment uses additional combinations of unit
operations and processes to remove other constituents such as
nitrogen and phosphates.
• Land treatment processes or the natural systems combine
physical, chemical and biological treatment mechanisms.
– These systems produce water with quality similar to or better than that
from advanced wastewater treatment.
13. 13
Health and Environmental Concerns
• Odours are one of the most serious environmental
concerns to the public.
• The control of odours, particularly, the control of
hydrogen sulphide generation is of great concern in
collection systems and at treatment plants
– The sulphide produced in sewers is released as hydrogen
sulphide
– The release of excess hydrogen sulphide will cause accelerated
corrosion of concrete sewers and headwork structures and to the
release of odours.
14. 14
• Special efforts should be made to control and contain the
development of odours in the design, installation and the
proper sitting of wastewater collection and treatment
facilities.
15. 15
INDUSTRIAL WASTEWATERS
• Industrial wastewaters can be classified as:
– Domestic wastewaters
– Process wastewaters, and
– Cooling wastewaters
• Plant workers, shower facilities and cafeterias produce
domestic wastewaters.
• Process wastewaters are produced by product washing,
spills and leaks.
16. 16
• Various cooling processes produce cooling wastewaters
– They can be once-pass systems or multiple-recycle cooling
systems.
• In the once-pass cooling systems, large volumes of
cooling waters are used and returned to the
environment.
• In the multiple-recycle cooling system, using cooling
towers, wastewaters are the result of blow-down which is
required to prevent excess buildup of salts.
• Domestic wastewaters pose the potential for pathogenic
micro-organisms
– Normal sanitary-sewage system is normally used to handle the
domestic wastewaters to prevent the spread of pathogenic
micro-organisms.
17. 17
• There is normally no potential hazard of pathogenic
micro-organisms in the process wastewaters
– However, they are potential hazard to the environment through
chemical reactions, directly or indirectly
– Some process waste are toxic and pose a direct health hazard to
biological life in the environment
– Other process wastes are readily bio-degraded and create an
immediate oxygen demand.
• Cooling wastewaters are the least hazardous
– However, process wastewaters may be present in the cooling
wastewaters resulting from the leaks in the cooling systems.
18. 18
Physical Characteristics
• Wastewater characteristics can be classified as
– Physical
– Chemical, and
– Biological
• The physical properties are
– Solids
– Odour
– Colour
– Temperature and
– Density
19. 19
• The most important physical characteristics of
wastewater is its total solids content.
• The total solids content is composed of the following
items:
– Floating matter
– Settleable matter
– Colloidal matter, and
– Matter in solution
20. 20
• Odours in wastewater are nuisance to the environment
– Offensive odours can cause poor appetite for food, impaired
respiration, nausea and vomiting
– Thus, odours in wastewater have been considered as the first
concern of the public in the implementation of wastewater
facilities.
• Generally, odour in fresh wastewater is less
objectionable than the odour of wastewater that has
undergone anaerobic (devoid of oxygen) decompositon.
– Hydrogen sulphide is produced by anaerobic micro-organisms
that reduce sulphate to sulphide
– Hydrogen sulphide is responsible for the most characteristic
odour of stale or septic wastewater.
Odour
21. 21
• In domestic wastewater, odours are due to gases
produced by the decomposition of organic matter
– Or by substances added to the wastewater.
• In industrial wastewaters, odours are caused by the
presence of odorous compounds or compounds that
generate odours during the wastewater treatment
process.
– The control of odours is a major consideration in the design and
operation of wastewater facilities covering collection, treatment
and disposal.
22. 22
Colour
• Condition relating the age of the wastewater is
qualitatively determined by the colour and odour of the
wastewater.
– Fresh wastewater is usually a light brownish-grey colour
– The colour changes sequentially from grey to dark grey and
finally to black as the travel time to wastewater in the collection
system increases and more anaerobic conditions develops.
• Black wastewater is often described as septic
23. 23
Temperature
• Wastewater temperature tends to be higher than
the temperature of the water supply
– Due to the addition of warm water from households and
industrial activities.
• It is important to consider the temperatuare of
wastewater because it affects:
– Chemical reaction and reaction rates
– Aquatic life, and
– The suitability of the water for beneficial uses.
24. 24
• Also, oxygen is less soluble in warm water than in cold
water.
– Abnormally high temperatures can cause the undesirable growth of
wastewater fungus and water plants.
– For bacterial activity, the optimum temperatures are from about 25 to
35°C.
• For the most part, temperature is not a critical issue
below 37°C if wastewaters are to receive biological
treatment.
– The effects of temperature on the performance of biological
treatment processes are discussed later in biological treatment
topic
• An increase in wastewater temperature causes an
increase in the rate of biochemical reactions.
– This is also accompanied by the decrease in the quantity of
oxygen present in surface waters.
25. 25
– This combined effect results in serious depletion in dissolved
oxygen concentration in the summer months.
• A sudden change in temperature can cause a high rate
of mortality of aquatic life.
• Oxygen is a critical environmental resource in receiving
streams and lakes.
– Aquatic life requires reasonable dissolved-oxygen (DO) levels
– The minimum stream DO levels is set by EPA at 5mg/L during
summer operations.
26. 26
Density
• Density or specific gravity of wastewater is an important
parameter because
– Of the tendency of density currents formation in sedimentation
tanks and in other treatment units.
• The density of domestic wastewater is essentially the
same as that of water at the same temperatures,
provided that is does not contain significant amounts of
industrial waste.
27. 27
Turbidity
• Insoluble particulates impede the passage of light
through water by
– Scattering and absorbing the rays.
• This interference of light passage is referred to as
turbidity.
• The standard is a suspension of silica of specified
particle size selected so that
– A 1.0 mg/L suspension measures as 1.0 NTU.
28. 28
Chemical Characteristics
• The chemical characteristics of wastewater are classified
into
– Organic matters or organics
– Inorganic matters or inorganics
– Gases
• Organic compounds are composition of:
– Carbon
– Hydrogen and
– Oxygen
– Together with nitrogen in some cases
29. 29
• The organic composition of industrial wastes varies
widely
– Because different raw materials are used by each specific industry.
30. 30
Volaltile Organic Compounds (VOCs)
• VOCs are organic compounds that have
– A boiling point equal to or less than 100°C, and/or
– A vapour pressure greater than 1 mm Hg at 25°C.
• The release of VOCs in sewers and at treatment plants
is of great concern because
– They pose a significant public health risk
– They contribute to a general increase in reactive hydrocarbons in
the atmosphere, leading to the formation of photochemical
oxidants.
31. 31
Agricultural Pesticides
• Pesticides, herbicides and other agricultural chemicals
are trace organic compounds
– They are toxic to most life forms
– They can be significant contaminants of surface waters.
• These chemicals are mainly from surface runoff from
agricultural and park lands
– They are not common constituents of domestic wastewater.
• Concentration of agricultural chemicals in wastewater
can cause
– Fish kills
– Contamination of the flesh of fish
– Impairment of water supplies.
32. 32
Priority Pollutants
• Priority pollutants are both organic and inorganic
– They are identified by the Environment Protection Agency
• They are selected on the basis of their known or
suspected
– Carcinogenicity (carcinogen means cancer-producing
substance)
– Multagenicity (mutation, genetic change which when transmitted
to offspring gives rise to heritable variation)
– Teratogenity, or (Teratogeny, production of monstrosities)
– High acute toxicity
• Many of the organic priority pollutants are volatile
organic compounds (VOCs)
33. 33
• Typical examples of priority pollutants are:
– Non-metals
• Arsenic, As
• Selenium, Se
– Metals
• Barium, Ba
• Cadmium, Cd
• Chromium, Cr
• Lead, Pb
• Nercury, Hg
• Silver, Ag
– Organic Compounds
• Benzene, C6H6
• Ethylbenze, C6H5C2H5
• Toluence, C5HC5H3
34. 34
– Halogenated Compounds
• Chlorobenzene, C6H5C1
• Chloroethene, CH2CHC1
• Dichloromethane, CH2C12
• Tetrachloroethane, CC12CC12
– Pesticides, Herbicides, Insecticides
• They are listed by trade names. They are also halogenated
compounds
• Examples: Endrin (C12H8OC16) Lindane (C6H6C16) etc.
35. 35
Inorganics
• The natural water and wastewater contain
several inorganic matters
– The inorganic components of wastewater and natural waters
have to be considered in establishing and controlling water
quality.
• Some of the rocks and minerals are dissolved in water
when they come in contact with the natural water.
• Concentration of inorganic constituents are increased by
the natural process.
36. 36
• Wastewaters are seldom treated for removal of the
inorganic constituents that are added in the use cycle.
• It is important to examine the nature of some of the
constituents in terms of:
– pH value
– Chlorides
– Alkalinity and
– Nitrogen concentration.
37. 37
Chlorides Concentration
• The chloride concentration is an important parameter for
the determination of water quality.
• Chlorides are found in natural water
– They result from the leaching of chloride-containing rocks and
soils
– From salt water intrusion
• Other sources of chlorides are
– Agricultural, industrial and domestic wastewaters
– Human excreta contain about 6g of chlorides per person per
day.
38. 38
Alkalinity
• Wastewater is normally alkaline
– The alkalinity in wastewater is due to the presence of
• Hydroxides
• Carbonates, and
• Bicarbonates of calcium, magnesium, sodium, potassium or
ammonia
• Other compounds such as silicates, phosphate
• The concentration of alkalinity in wastewater is important
in
– Chemical treatment
– Biological nutrient removal etc.
39. 39
Nitrogen
• Nitrogen and phosphorus are known as nutrients or
biostimulants
– They are essential to the growth of protista and plants.
• Total nitrogen is comprised of
– Organic nitrogen, ammonia, nitrite and nitrate.
• Ammonia nitrogen exists in aqueous solution as either
– The ammonium ion or ammonia, depending on the pH of the
solution:
NH3 + H2O < > NH4+ + OH‾
40. 40
– At pH above 7, the equilibrium is displaced to the left
– At pH below 7, the ammonium ion is predominant
• Nitrite nitrogen is relatively unstable and is easily
oxidised to the nitrate form.
• Nitrites present in wastewater effluents are oxidised by
chlorine
– Thus it increases the chlorine dosage requirements and the cost
of disinfection.
– The most highly oxidised form of nitrogen found in wastewaters
is the nitrate nitrogen.
• The most highly oxidised form of nitrogen found in
wastewaters is the nitrate nitrogen.
41. 41
Phosphorus
• The orthophosphate, polyphosphate and organic
phosphate are the usual forms of phosphorus found in
aqueous solutions.
• The orthophosphates are
PO4⁻³, HPO4⁻², H2PO4‾, H3PO4
– They are available for biological metabolism without further
breakdown
• The polyphosphates are
– Molecules with two or more phosphorus atoms, oxygen atoms,
and in some cases, hydrogen atoms combined in a complex
molecule.
42. 42
SULPHUR
• Sulphate ions are present in most water supplies and
wastewater.
• Under anaerobic conditions, sulphate is reduced
biologically to sulphide
– The sulphide can then combine with hydrogen to form hydrogen
sulphide, H2S.
• The generalised reaction are:
Organic matter + SO4⁻² →S⁻² + H2O + CO2
bacteria
S⁻² + 2H+ → H2S.
43. 43
• H2S will accumulate at the crown of the pipe
– The accumulated H2S in turn can be oxidised to sulphuric acid
– Sulphuric acid is corrosive to sewer pipes.
44. 44
Toxic Inorganic Compounds
• Many of the toxic inorganic compounds are classified as
priority pollutants.
– The toxic inorganic compounds are
• Copper
• Lead
• Silver
• Chromium
• Arsenic acid
• Boron
• They are toxic in varying degrees to micro-organisms
and must be considered in the design of a biological
treatment plant.
45. 45
– The micro-organisms may be killed and the biological treatment
ceased by the introduction of these ions in treatment plant.
– In sludge digesters, copper is toxic at a concentration of 100
mg/L.
– Chromium and nickel are toxic at concentrations of 500 mg/L
etc.
46. 46
Gases
• The following gases are found in untreated wastewater:
– Nitrogen ) found in all waters
– Oxygen ) exposed to air
– Carbon dioxide )
– Ammonia ) Produced by decompositon
– Hydrogen Sulphide, and ) of the organic matter in
– Methane ) wastewaters.
• Chlorine, Ozone and the oxides of sulphur and nitrogen
are not present in untreated wastewater.
47. 47
Dissolved Oxygen
• The respiration of aerobic micro-organisms requires
dissolved oxygen.
• Oxygen is only slightly dissolved in water.
• The solubility of gas depends on
– The partial pressure of the gas in the atmosphere
– The temperature, and
– The purity of the water.
• It is desirable to have dissolved oxygen in wastewater
because
– It prevents the formation of noxious odour.
48. 48
Methane
• Mathane gas is the main by-product of the anaerobic
decomposition of the organic matter in wastewater.
– It is colourless and odourless
– It is combustible hyddrocarbon and it has high fuel value.
• Occasionally, mathane is produced because of
anaerobic decay in accumulated bottom deposits.
– Since even small amounts of oxygen is toxic to the organisms
responsible for methane production, mehtane gas will not be
found in large quantities in untreated wastewater.
49. 49
• Since methane is highly combustible, the explosion
hazard is high
– It is important that manholes and sewer junctions or chambers
where methane gas may be present (or collected) should be
ventilated with a portable blower during and before the workers
are working in them.
• Safety measures should be taken and
– Notices should be posted about the plant warning of explosion
hazards in treatment plant where methane is produced.
50. 50
Hydrogen Sulphide
• Hydrogen sulphide is produced by
– The anaerobic decomposition of organic matter containing
sulphur, or
– The reduction of mineral sulphites and sulphates.
• Hydrogen sulphide is
– Colourless and
– Inflammable
– It has a characteristic odour of rotten eggs.
51. 51
• Hydrogen sulphide will combine with iron present in
waste-water to form ferrous sulphide (FeS).
– Resulting in the blackening of wastewater and sludge.
• Other metallic sulphides are also produced.
52. 52
Micro-organisms
• The micro-organisms found in surface water and
wastewater are classified as
– Eucaryotes
– Eubacteria, and
– Archaebacteria
• The eucaryotes group includes
– Algae
– Fungi
– Protozoa
– Mosses
– Ferns, etc.
53. 53
• Eubacteria group includes most bacteria.
• Archaebacteria group includes methanogens, halophiles
and thermacidophiles.
54. 54
2.0 COLIFORMS, BOD, COD AND TKN
• Coliforms
• Biochemical Oxygen Demand (BOD)
• Theoretical Oxygen Demand (ThOD)
• Chemical Oxygen Demand (COD)
• BOD5, CBOD & NBOD
• BOD Rate Equation
• BOD Calculations
• Total kjeldahl Nitrogen (TkN)
55. 55
Coliforms
• The intestinal tract of man contains countless rod-
shaped bacteria known as coliform organisms.
– Each person discharges from 100 to 400 billion coliform
organisms per day, in addition to other kinds of bacteria
– Thus, the presence of coliform organisms is taken as an
indication that pathogenic organisms may be present, and
– The absence of coliform organisms is taken as an indication that
the water is free from disease-producing organisms
• Because the numbers of pathogenic organisms present
in wastes and polluted water are few and difficult to
isolate and identify,
– The coliform organisms, which is more numerous and more
easily tested for, is commonly used as an indicator organism.
56. 56
Th OD
• The amount of oxygen required to oxidize a substance to
carbon dioxide and water may be calculated by
stoichiometryif the chemical compositon of the
substance is known.
– This amount of oxygen is known as the Theoretical Oxygen
Demand (Th OD)
57. 57
Chemical Oxygen Demand
• In contrast to the Th OD, the Chemical Oxygen Demand,
COD, is a measured quantity that does not depend on
knowledge of the chemical compositon of the substance
in the water.
– In the COD test, a strong chemical oxidizing agent (chromic
acid) is mixed with a water sample and then boiled.
– The difference between the amount of oxidizing agent at the
beginning of the test and that reamining at the end of the test is
used to calculate the COD.
58. 58
Biochemical Oxygen Demand
• If the oxidation of an organic compound is carried out by
microorganisms using the organic matter as a food
source, the oxygen consumed is known as Biochemical
Oxygen Demand, or BOD.
– The actual BOD is less than the Th OD due to the incorporation
of some of the carbon into new bacterial cells.
• The test is a bioassay that utilizes microorganisms in
conditions similar to those in natural water to
– Measure indirectly the amount of biodegradable organic matter
present.
• Bioassay means to measure by biological means.
59. 59
BOD Test
• A water sample is inoculated with bacteria that consume
the biodegradable organic matter to obtain energy for
their life processes.
– Because the organisms also utilize oxygen in the process of
consuming the waste, the process is called aerobic
decompositon.
– This oxygen consumption is easily measured.
– The greater the amount of organic matter present, the greater
the amount of oxygen utilized.
• The BOD test is an indirect measurement of organic
matter because
– We actually measure only the change in dissolved oxygen
concentration caused by the microorganisms as they degrade
the organic matter.
60. 60
• Although not all organic matter is biodegradable and the
actual test procedures lack precision,
• The BOD test is still the most widely used method of
measuring organic matter because of
• The direct conceptual relationship between BOD and oxygen
depletion in receiving waters.
• Only under rare circumstance will the Th OD, COD, and
BOD be equal.
61. 61
BOD5
• The five-day BOD5 was chosen as the standard value for
most purpose because
– The test was devised by sanitary engineers in England, where
rivers have travel times to the sea of less than five days, so there
was no need to consider oxygen demand at longer times.
• Since there is no other time which is any more rational
than five-days, this value has become firmly established.
62. 62
CBOD and NBOD
• Not only the carbon in organic matter is oxidized
– Many other organic compounds, such as proteins, also contain
nitrogen that can be oxidized with the consumption of molecular
oxygen.
• The two processes must be considered separately
– Because the mechanisms and rates of nitrogen oxidation are
distinctly different from those of carbon oxidation.
• Oxygen consumption due to oxidation of carbon is called
carbonaceous BOD (CBOD)
– And oxygen consumption due to nitrogen oxidation is called
nitrogenous BOD (NBOD)
63. 63
Total Kjeldahl Nitrogen (TKN)
• TKN is a measure of the total organic and ammonia
nitrogen in the wastewater.
– J. Kjeldahl (Pronounced ‘kell dall’) developed the test in 1883.
• It gives a measure of the availability of nitrogen for
building cell,
– As well as the potential nitrogen oxygen demand that will have to
be satisfied
• In this method, the aqueous sample is first boiled to drive
off the ammonia, and then
– It is digested
– During the digestion, organic nitrogen is converted to ammonia.
64. 64
Example
• Given: The BOD tests for the raw wastewater were set
up by pipetting 5.0 ml into each 300 ml bottle.
For one pair of bottles, the test results were:
The initial dissolved oxygen (DO) = 8 mg/L
The final DO = 3.6 mg/L (after 5 days of incubation at 20°C)
Determine the BOD5.
• Solution
– Since the sample is unseeded, the relationship equation:
BOD = D1-D2
P
65. 65
– Where D1 = 8mg/L
D2 = 3.6 mg/L
P = Decimal fraction of wastewater
sample used
= 5/300
BOD5 = 8-3.6 = mg/L
5/300
67. 67
Screening
• Wastewater treatment is directed towards removal of
pollutants (contaminants) with least effort.
• Suspended solids are removed by either
– Physical separation or
– Chemical separation
• Screening is the first physical unit operation encountered
in a wastewater treatment plant.
• A screen is a device with opening of any shape, circular
or rectangular slots are common.
68. 68
• It consists of parallel bars or rods, called a bar rack or
bar screen
– It may consist of wires, gratings, wire mesh or perforated plate,
and is called a screen.
69. 69
Bar Racks
• Bar racks are used to protect pumps, valves, pipelines
etc. from damage or clogging by rags and large objects.
• Steels or stainless steels bar of screening surface size
ranging 0.6 to 1.5 in. are used,
– Generally used in pre-treatment operation
– The size is classified as coarse.
70. 70
Screenings
• Screenings are the material retained or bar racks and
screens.
• Coarse screenings consist of materials or debris such as
– Plastics
– Rocks
– Rags
– Branches
– Pieces of lumber
– Leaves
– Papers
– Tree roots, etc.
71. 71
• Coarse screenings are collected on racks or bars of 5/8
in. or greater spacing.
• Fine screenings are retained on screens with openings
less than 5/8 in. (15mm).
72. 72
Disposal of Screenings
• Screenings may be disposed or removed by
– Hauling to disposal areas such as landfill, (the most commonly
used)
– Burial on the plant site, for small installation only
– Incineration
– As municipal solid wastes, or
– Discharged to grinders or mascerators where they are ground
and returned to the wastewater.
73. 73
Communication
• To comminute means to cut up into small fragments.
• Communication are used to cut up coarse solids into a
smaller, more uniform size so as
– To improve the downstream operation and processes and
– Grit may include egg shells, bone chips, seeds and large organic
particles such as food wastes.
• However, the comminuted solids may present
downstream problem.
– It is particularly bad with rags because the rags tend to
recombine after cutting up into ropelike strands, if agitated (in grit
chambers and aerated channels).
– Thus, clogging pump impellers, sludge pipelines, etc.
74. 74
Grit Chambers
• Grit chambers are used to remove grit
– The grit consists of sand, gravel, cinders or other solid materials
– Grit may include egg shells, bone chips, seeds and large organic
particles such as food wastes.
• Grit chambers are designed to
– Provide protection against abrasion and wear in moving
mechanical equipment
– Reduce the risk of forming heavy deposits in pipelines and
channels.
75. 75
• It is essential to remove grit ahead of centrifuges, heat
exchangers and high-pressure diaphragm pumps
– Grit chambers are usually installed after the bar racks and before
the primary sedimentation tanks.
76. 76
Types of Grit Chambers
• Grit chambers may be classified into
– Horizontal-flow with square or rectangular section
– Aerated, and
– Vortex-type
• The horizontal-flow grit chamber has the flow passing
through the chamber in a horizontal direction
– It has a series of influent distribution vanes or gates and a weir
section at the effluent end.
– The vanes or gates distribute the influent over the cross section
of the tank.
– The distributed wastewater flows in straight lines across the tank
– The effluent overflows the weir in a free discharge.
77. 77
• The aerated grit chamber has a spiral-flow aeration tank
– The spiral velocity is induced and controlled by the tank
dimensions as well as the amount of air supplied to the unit.
• The vortex-type grit chamber uses a cylindrical tank
– The flow enter the tank tangentially to create a vortex-flow
pattern
– The grits are separated by centrifugal and gravitational forces
• The square horizontal-flow grit chambers are designed
to remove 95% of the 100-mesh particles at peak flow.
• Aerated grit chambers are designed to remove 65-mesh
particales (0.2 mm) or larger at peak flow.
78. 78
• The vortex-type grit chambers are designed to remove
95% of the 50-mesh (0.33 mm) at peak flow, (85% of 70-
mesh, 65% of 100-mesh)
79. 79
Flow Equalisation
• There are variations in the flowrate of influent-
wastewater and strength (concentration) of wastewater
in all wastewater treatment facilities.
• The purposes of flow equalisation are as follows:-
– To overcome the operational problems caused by flowrate
variations,
– To improve the performance of the downstream processes,
– To reduce the size and cost of downstream treatment facilities.
80. 80
• Flow equalisation is the damping of flowrate variation so
that
– A constant or nearly constant flowrate is achieved.
• Flow equalisation may have
– In-line arrangement, or
– Off-line arrangement.
81. 81
In-Line Equalisation
• Figure below shows the in-line equalization incorporated
in a wastewater treatment plant.
Bar rack and/or
Comminutor
Grit Removal
00
Equalization Basin
Pumping
To Primary
Treatment
Mixing
Untreated
Wastewater
82. 82
• In in-Line arrangement, all of the flow pass through the
equalisation basin.
– A considerable amount of constituent and flowrate damping can
be achieved by in-line equalisation.
83. Off-line Equalisation
• The off-line equalisation arrangement is shown below.
83
Bar Rack
And/or
Comminutor
OO
Equalisation
Basin
Untreated
Wastewater
Grit Removal Overflow structure
To Primary
Treatment
Pumping
Station
Mixing
84. 84
• In the off-line arrangement, only slight damping is
achieved
– Only the flowrate above some predetermined flowrate is
deverted into the equalisation basin.
– The pumping requirements are minimised.
85. 85
Equalisation Basin
• For flow equalisation, the following design factors must
be considered:
– Basin construction
– Mixing and air requirement
– Pump and pump control systems
• The basin may be of
– Concrete
– Earthen, or
– Steel constructin
• The earthen basins are the least expensive
– The side slopes of basin may very between 3:1 and 2:1.
87. 87
Gravity Sedimentation
• Sedimentation is the separation from water of
suspended particles.
– The particles are heavier than water
– Sedimentation is by gravitational settling.
• The terms sedimentation and settling are used
interchangeably
– A sedimentation basin may also be called as
• Sedimentation tank
• Settling basin, or
• Settling tank
88. 88
• Example of sedimentation applicaton are:
– Grit removal and particulate matter removal in the primary
settling basin.
– Biological-floc removal in the activated-sludge settling basin
– Chemical-floc removal when the chemical coagulation process is
used.
89. 89
Gravity Sedimentation Tanks
• Gravity sedimentation tanks are used to remove slowly
settling particles.
• The sedimentation tanks can be
– Rectangular or
– Circular
• The design of sedimentation tanks are based on:
– Retention time
– Surface overflow rate, and
– Minimum depth
90. 90
• The removal efficiency is affected by
– The hydraulic flow pattern through the tank.
• For maximum settling efficiency, the wastewater flow
must be distributed properly through the sedimentation
valume.
• It is important to note that
– The energy contained in the incoming wastewater flow must be
dissipated before the solids can settle.
• After the solids have settled, the settled effluent should
be collected without creating serious hydraulic currents
– Sedimentation process could be adversely affected by hydraulic
currents.
91. 91
• Effluent weirs are placed at the end of rectangular
sedimentation tanks and
– Around the periphery of circular sedimentation tanks.
• Effluent weirs are placed to ensure uniform flow out of
tanks.
• The settled solids are removed from the sedimentation
tank floor by
– Scrapping and hydraulic flow.
• Sludge hoppers are used in conventional sedimentation
tanks
– To collect the concentrated sludge and
– To prevent removal of excess volume of water with the settled
solids.
92. 92
• Fig CS and Fig RS are the cross-section diagrams of
conventional sedimentation tanks.
94. 94
Retention Time
• The gravity sedimentation tanks are normally designed
to provide for 2-hr retention based on average flow.
– Longer retention period are allowed for light solids or inert solids
that do not change during their retention in the tank.
• Sedimentation time should not be too long
– Because the solids will become too densely compact, affecting
solid collection and removal.
• Organic solids generally will not compact to more than 5
to 10%.
– Inorganic solids will compact up to 20% to 30%.
95. 95
• Centrifugal sludge pumps can handle solids up to 5 or
6%
– Positive-displacement sludge pumps can pump solids up to
10%.
• There is a tendency for sludge to lose fluid propertise
when solids are above 10%
– And the sludge with solids above 10% must be handled as semi-
solid rather than a fluid.
96. 96
Minimum Dept
• The minimum depth of sedimentation tanks is generally
3.0 m or 10 ft.
– The minimum diameter of a circular sedimentation tank is 6.0 m
or 20 ft.
– The length-to width ratio of rectangular sedimentation tanks is
5:1.
97. 97
Flotation
• Flotation process may be used in place of primary
sedimentation for removal of suspended and floating
solids.
• Flotation is a unit operation that will separate solids or
liquid particles from a liquid phase.
– Separation is achieved by the introduction of air bubbles into the
liquid phase.
• The bubbles will attach to the particulate matter.
– The combined air bubbles and particle will create buoyancy
forces that are high enough to cause the particle to rise to the
surface (floating).
98. 98
– In this way, particles or solids that have a higher density than the
liquid can then be made to rise.
• Flotation can also be used to float particles with lower
density than the liquid, such as oil suspension in water.
99. 99
Flotation Agents
• Air is used as the flotation agent for municipal
wastewater treatment.
• The following methods are used:
– Dissolved-Air Flotation
• Air is injected while the liquid is under pressure.
• This is followed by release of the pressure.
– Air Flotation
• Aeration at atmospheric pressure
• Air bubbles are formed by introducing air directly into the liquid
phase through a revolving impeller or through diffusers.
100. 100
– Vacuum Flotation
• Saturation with air at atmospheric pressure, followed by application
of a vacuum to the liquid.
• Vacuum flotation consists of saturating the wastewater with air
either
– Directly in an aeration tank, or
– By permitting air to enter on the suction side of a wastewater pump.
• The application of partial vacuum causes the dissolved air to come
out of solution as minute bubbles.
• The bubbles and attached solid particles rise to the surface, forming
a scum blanket which is removed by skimming operation.
– For flotation application, design air-solids ratios have not been
well defined.
• However, air quantities of 2 to 3% by volumne of wastewater
flowrate yield satisfactory rasults.
101. 101
Chemical Additives
• In the flotation operation, various chemical additives are
commonly used to enhance the degree of removal.
• These chemical additives will create a surface or a
structure that can easily absorb or entrap air bubbles.
• Inorganic chemicals such as
– Aluminum and ferric salts and activated silica are used
– They bind the particles together, creating a structure that can
easily entrap air bubbles.
102. 102
• Organic polymers are used to change the nature of
either the air-liquid interface or the solid-liquid interface
or both.
– These compounds will collect on the interface to bring about the
desired change.
103. 103
Advantages of Flotation
• Flotation are used for
– Untreated wastewater, and
– Settled wastewater.
• Flotation has the advantage of
– High surface-loading rates and
– High removal of grease and floatable material.
• The main advantage of flotation over sedimentation is
that
– It can remove more completely and in shorter time the very small
or light particles that settled slowly.
• The floated particles can be collected by a skimming
operation.
106. 106
Purpose of Secondary Treatment
• The secondary treatment is used mainly.
– To remove the soluble BOD that has not been removed by the
primary treatment and
– To carry out further removal of suspended solids.
• The basic requirements for conventional aerobic
secondary biological treatment are
– The availability of many microorganisms
– Good contact between these organisms and the organic
material,
– The availability of oxygen, and
– Having favorable environmental conditions such as
• Favourable temperature and
• Sufficient time for the organisms to work.
107. 107
• Many methods have been used in the past to meet these
basic requirements.
• The common approaches include:
– The activated sludge
– The trickling filters
– Oxidation lagoons or ponds.
108. 108
Objectives of Biological Treatment
• The main objectives of the biological treatment of
wastewater are:
– Coagulation and removal of the nonsettleable colloidal solids
and
– Stablization of the organic matter.
• For domestic wastewater, the principal objective is:
– Reduction of the organic content and the nutrients such as
nitrogen and phosphorus.
• For agricultural return wastewater, the main objective is:
– Removal of the nutrients, particularly nitrogen and phosphorus,
that are responsible for stimulating the growth of aquatic plants.
109. 109
• For industrial wastewater, the main objective is to
remove or lower the concentration of organic and
inorganic compounds.
110. 110
Use of Microorganisms
• A variety of microorganisms, primarily bacteria, are used
for:
– The coagulating of nonsettleable colloidal solids.
– The removal of carbonaceous BOD, and
– The stabilization of organic matter.
• The colloidal and dissolved carbonaceous organic matter
are converted by microorganisms into various gases and
into cell tissue.
• The specific gravity of the cell tissue is slightly greater
than that of water
– Thus, the removal of resulting cells from the treated liquid can be
achieved by gravity settling.
111. 111
Carbon for Microorganisms
• For an organism to reproduce and function properly, it
must have
– An energy source
– Carbon for the synthesis of new cellular material
– Nutrients such as nitrogen, phosphorus, sulphur, calcium, etc.
• Carbon and energy sources are usually referred to as
substrates.
• Organic matter and carbon dioxide are two of the most
common sources of cell carbon for microorganisms.
112. 112
• The energy needed for cell synthesis may be supplied by
– Light or
– Chemical oxidation reaction.
113. 113
Bacterial Growth
• Secondary treatment uses biological processes to
stabilize waste components
– Most biological treatment processes are comprised of complex,
interrelated, mixed biological populations.
– The mixture of microorganisms is usually referred to as
biomass.
• A portion of the waste is oxidized, releasing energy,
– The remainder is used as building blocks of protoplasm.
• The energy released by biomass metabolism is used to
produce the new units of protoplasm.
114. 114
• Thus, the advantage of using the biomass to stabilize
waste is that
– It provides the energy and basic chemical components required
for reproduction.
115. 115
Biological Waste Conversion
• The process of biological waste
conversion may be expressed in terms of
the following equation:
Waste + Biomass + Electron → More + End Products
(electron acceptor ↑ Biomass
donor) Proper (Oxidized
Environmental electron donor,
Conditions Reduced electron
acceptor)
116. 116
• The waste generally serves as an electron donor
– And it needs an electron acceptor.
• The electron acceptors include:
– Molecular oxygen
– Carbon dioxide
– Oxidized forms of nitrogen
– Sulphur and organic substances.
117. 117
Electron Acceptors & End Products
• The end products of the reaction are determined by the
electron acceptor.
• A list of typical end products produced by various
electron acceptors is given as follows:
Electron Acceptors End Products
• Molecular oxygen Water, CO2, oxidized nitrogen
• Oxidized nitrogen N2, N2O, NO, CO2, H20
• Oxidized sulphur H2S, S, CO2, H2O
• CO2, acetic acid, formic acid CH4, CO2, H2
• Complex organics H2, simple organics, CO2, H20
118. 118
• In general, the energy level of end products are much
lower than that of waste components,
– As a result, there is a release of energy.
119. 119
Environmental Control
• It is indicated in the earlier equation that proper
environmental conditions are required for the reaction to
take place.
• The environmental conditions are required by the
biomass, not the electron donor or acceptor.
• The environmental conditions include.
– pH
– Temperature
– Nutrients
– Ionic balance, etc.
120. 120
• biomass can function over a wide pH range generally
from 5 to 9.
– However, some microbes requires a much narrower pH range.
– It is also important to maintain a relatively constant pH in the
process.
– Continual changes in pH are detrimental.
• Most organisms can function well over a broad range of
temperature
– But do not adjust well to frequent fluctuation of even a few
degrees.
• Thus it is necessary to have a controlled environment
and biological community (biomass) in the design of
biological waste-treatment units.
121. 121
Need for Sludge Disposal
• More biomass is produced after the process of biological
waste conversion, see equation given earlier.
– This is desirable because it provides a continual production of
the organisms required to stabilize the waste.
• However, an excess level will build up and the process
could cause choking on organisms.
– It is necessary that some organisms are wasted from the system.
• The wasted organisms are called sludge.
– The ultimate disposal of sludge is a major cost companent of all
biologically based processes.
122. 122
Terms used for Biological Treatment
Processes
• The following terms are commonly used to define
various biological processes:
– Aerobic processes
• Biological treatment processes that occur in the presence of oxygen.
– Anaerobic processes
• Biological treatment processes that occur in the absence of oxygen.
– Nitrification
• The biological process by which ammonia is converted first to nitrite and
then to nitrate.
– Denitrification
• The biological process by which nitrate is converted to nitrogen and other
gaseous end products.
123. 123
– Anoxic denitrification
• The process by which nitrate nitrogen is converted biologically to
nitrogen gas in the absence of oxygen. This process is also known
as anaerobic denitrification.
– Carbonaceous BOD removal
• The biological conversion of the carbonaceous organic matter in
wastewater to cell tissue and various gaseous end products. In the
conversion, it is assumed that the nitrogen present in the vaious
compounds is converted to ammonia.
– Biological nutrient removal
• The removal of nitrogen and phosphorus in biological treatment
processes.
– Substrate
• The organic matter or nutrients that are converted during biological
treatment or that may be limiting in biological treatment.
124. 124
– Suspended – growth processes
• The biological treatment processes in which the
microorganisms responsible for the conversion of the organic
matter or other constituents in the wastewater to gases and
cell tissue are maintained in suspension within the liquid.
– Attached – growth processes
• The biological treatment processes in which the micro –
organisms responsible for the conversion of the organic
matter or other constituents in the wastewater to gas and cell
tissue are attached to some inert medium such as rock, slag,
or specially designed ceramic or plastic materials. These
processes are also known as fixed – film processes.
125. 125
Various Biological Treatment Processes
• There are five major groups of biological treatment,
namely:
– Aerobic processes
– Anoxic processes
– Anaerobic processes
– Combined aerobic, anoxic and anaerobic processes, and
– Pond processes
• These processes are further subdivided, depending on
whether treatment is accomplished in:
– Suspended – growth systems,
– Attached – growth systems, or
– Combinations thereof (of the two).
126. 126
Microbiology in Wastewater Treatment
• In nature, the key role of the bacteria is to decompose
organic matter produced by other living organisms.
• All biological processes for wastewater treatment are in
fact derived from processes occurring in nature.
– They speed up the decomposition of waste by controlling the
environment required for optimum growth of the microorganism
involved.
127. 127
• The principal applications of these processes are for:
– The removal of the carbonaceous organic matter in wastewater,
usually measured as BOD, total organic carbon (TOC), or
chemical oxygen demand (COD).
– Nitrification
– Denitrification
– Phosphorus removal, and
– Waste stabilization
128. 128
Aerobic Suspended – Growth Treatment
Processes
• The various suspended – growth biological treatment
processes for the removal of carbonaceous organic
matter are as follows:
– The activated – sludge process
– Aerated lagoons
– A sequencing batch reactor, and
– The aerobic digestion process.
• The activated – sludge process is most commonly used
for the secondary treatment of domestic wastewater.
– This process is presented in the following pages.
129. 129
Activated – Sludge Process
• The activated – sludge process was developed in
England in 1914.
• The activated – sludge process derives its name from
the biological mass (activated sludge) produced when air
is continuously injected into the wastewater.
• Many versions of the original process are in use today
– Fundamentally they are all similar.
130. 130
• Basically, in this process:
– A mixture of wastewater and biological sludge (micro-organisms)
is agitated and aerated.
• The sludge (biological solids) are then separated from
the treated wastewater and returned to the aeration
process as needed.
131. 131
Activated Sludge
• In the activated – sludge process, organic waste is
introduced into a reactor (aeration tank) where an
aerobic bacterial culture is maintained in suspension.
– In this process, microorganisms are mixed thoroughly with the
organics under conditions that stimulate their growth through the
use of organics as foods.
– As the microorganisms grow and are mixed by agitation of the
air, the individual organisms clump together (flocculate) to form
an active biological called activated sludge.
• The mixture of activated sludge and wastewater in the reactor
(aeration tank) is called mixed liquor.
132. 132
Return Sludge
• The mixed liquor then flows from the reactor to a
secondary clarifier.
– The activated sludge will settle out in the secondary clarifier.
• Since high population of microbes is required to permit
rapid breakdown of the organics in wastewater, most of
the settled sludge is returned to the reactor, hence it is
called return sludge.
• Some of the return sludge has to be diverted or wasted
to the sludge handling system for treatment and
disposal.
– Because more activated sludge is produced than in desirable in
the process.
133. 133
Chemical Equation
• In the reactor, the bacterial culture performs the
conversion as described by the following equations:
Oxidation and Synthesis:
COHNS + O2 + nutrients → CO2 + NH3 + C5H7NO2 + other end products
(organic (new bacterial cells)
matters)
Endogenous respiration:
↓
C5H7NO2 + 5O2 → SCO2 + 2H2O + NH3 + energy
(cells)
bacteria
134. 134
• In these equation, the organic matter in wastewater is
represented by COHNS.
• Although the endogenous respiration reaction results in
relatively simple end products and energy, stable organic
end products are also formed.
• The aerobic environment in the reactor is achieved by
the use of diffusers or mechanical aeration, which also
serves to maintain the mixed liquor in a completely
mixed regime.
• In the activated – sludge process, the bacteria are the
most important microorganisms because they are
responsible for the decomposition of the organic material
in the influent.
135. 135
• In the reactor or aeration tank, a portion of the organic
waste is used by aerobic and facultative bacteria to
obtain energy for the synthesis of the remainder of the
organic material into new cells.
– Only a portion of the original waste is actually oxidized to low
energy compounds such as NO3⁻, SO4⁻², and CO2.
– The remainder is synthesized into a cellular material.
– Also, many intermediate products are formed before the end
products are produced.
• In general, the bacteria in the activated-sludge process
include member of:
– The genera such as
Pseudomonas
137. 137
Conventional Activated Sludge Systems
• In conventional activated sludge systems, it is typically
required that
– The wastewater be aerated for six to eight hours in long,
rectangular reactors.
– About 8m³ of air is required to treat each m³ of wastewater
– Sufficient air is used to keep the sludge in suspension.
• The injection of air is at near the bottom by the aeration
tank (reactor) through a system of diffusers.
• The volume of sludge returned to the aeration tank is
about 20 to 30 percent of the wastewater flow.
138. 138
Wasted Sludge
• The activated sludge process is controlled by wasting a
portion of the microorganisms each day. This is carried
out
– To maintain the proper amount of the microorganisms to
efficiently degrade the BOD.
– Wasting means that a portion of the microorganisms is
discarded from the process.
– The discarded microorganisms are called waste activated sludge
(WAS).
• A balance is the then achieved between growth of new
organisms and their removal by wasting.
139. 139
– If too much sludge is wasted, the concentration of
microorganisms in the mixed liquor will becomes too
low for effective treatment.
– If too little sludge is wasted, a large concentration of
microorganisms will accumulate and, ultimately,
overflow the secondary tank and flow into the
receiving stream.
141. 141
Practical Application
• The practical application of the activated – sludge
process is considered briefly as follows:
– In the design of the activated – sludge process, consideration
must be given to
• Selection of the reactor type
• Loading criteria
• Sludge production
• Oxygen required and transfer
• Nutrient requirements
• Control of Filamentous organisms, and
• Effluent characteristics.
142. 142
Trickling Filters
• Trickling filters have been a popular biological treatment
process for nearly 100 years.
• Trickling filters have a bed of coarse material (media)
over which wastewater is continuously distributed.
• The coarse materials include:
– Stones or rocks
– plastic
143. 143
• Ideally, the filter medium should possess the following:
– Providing a high surface area per unit of volume
– High durability
– Low cost
– Does n clog easily
• Rock media such as high-quality granite or blast furnace
slag were commonly used until the mid – 1960s.
– Rock media have been replaced by plastic, redwood, or
pressure – treated wood because of problems such as minimal
void areas and the potential for biomass clogging.
145. 145
Distributing Systems
• The wastewater is typically distributed over the surface
of the rocks by rotating arm
– The rotary distributor has become a standard trickling filter
process because it is reliable and easy to maintain.
• The distributor has two or more arms mounted on a pivot
in the centre of the filter, revolving in a horizontal plane
– The arms are hollow and contain nozzles
– The wastewater is discharged through these nozzles over the
filter bed.
146. 146
• The distributor unit is driven by
– An electric motor, or
– By the dynamic reaction of the wastewater discharging from the
nozzles.
147. 147
Secondary Clarifier
• As the wastewater trickles through the bed, a microbial
growth establishes itself on the surface of the stone or
packing in a fixed film.
– The microorganisms cling and grow in a slime on the rocks as
they feed on the organic matters.
• Excess growth of microorganisms, if not removed, would
cause undesirably high levels of suspended solids in the
plant effluent.
– Thus a sedimentation tank is needed to allow these solids to
settle out in it.
– The sedimentation tank is termed as secondary clarifier or final
clarifier.
148. 148
• Thus, the function of the secondary settling tank is to
produce a clarified effluent
– All the sludge from trickling – filter settling tanks is removed to
sludge – processing facilities (digester).
149. 149
Recirculation
• In trickling filter design, recirculation is provided for
return of portion of the effluent to flow through the filter.
– The ratio of the returned flow to the incoming flow is called the
recirculation ratio.
• Recirculation practised in stone filters has the following
advantages:
– It increases contract efficiency by bringing the waste into contact
more than once with active biological material.
– It dampens variations in loading over a 24 hour period. The
strength of the recirculated flow lags behind that of the incoming
wastewater. Thus, recirculation dilutes strong influent and
supplements weak influents.
150. 150
– It improves distribution over the surface, thus reducing the
tendency to clog and also reduce filter flies.
– It prevents the biological slimes from dying out and dying during
night time periods when flows may be too low to keep the filter
wet continuously,
• Recirculation practised for plastic media will provide the
desired wetting rate to keep the microorganisms alive.
151. 151
Underdrains
• The wastewater collection system in a trickling filter has
underdrains.
– The underdrains catch the filtered wastewater and solids
discharged from the filter medium and convey them to the final
sedimentation tank.
152. 152
Stabilization Ponds
• Stabilization ponds have been used to treat wastewater
– Particularly as wastewater treatment systems for small
communities.
• Domestic wastewater can be effectively stabilized by the
natural biological process that occurs in shallow ponds.
• Waste stabilization pond has been used as an all –
inclusive term that refers to a pond of lagoon used to
treat organic waste by biological and physical processes.
153. 153
– In fact, many terms have been used to describe different types of
systems employed in wastewater treatment.
– In recent years, oxidation pond has been widely used as a
collective term of all types of ponds.
• In general, stabilization ponds can be classified
according to the presence of oxygen, as:-
– Aerobic
– Facultative
– Anaerobic
– Maturation or tertiary, and
– Aerated
154. 154
Aerobic Ponds
• Aerobic ponds have the following features:
– Large, shallow earthen basins or ponds, less than 1.5 m in depth
– Used for the treatment of wastewater by natural processes
involving the use of both
• Algae and
• Bacteria
– Dissolved oxygen is maintained in ponds throughout the entire
depth, mainly by the action of photosynthesis.
– The pond is shallow to allow light to penetrate to the bottom,
thereby maintaining active algae photosynthesis throughout the
entire system.
– Stabilization of the organic material in the aerobic pond is
achieved by the action of aerobic bacteria.
155. 155
• In aerobic photosynthesis ponds, the oxygen is supplied
by
– Natural surface reaeration, and
– Algae photosynthesis
• Algae photosynthesis will release oxygen
– The oxygen is used by the bacteria in the aerobic degradation of
organic matter.
– Nutrients and carbon dioxide produced by the aerobic
degradation are then used by the algae.
156. 156
Facultative Ponds
• Facultative ponds are the most common type used as
wastewater treatment systems for small communities.
• Facultative ponds have the following features:
– The stabilization of waste is brought about by a combination of
• Aerobic
• Anaerobic, and
• Facultative (aerobic-anaerobic) bacteria
– The ponds are 1 to 2.5m deep, having three zones:
• An aerobic upper zone where aerobic bacteria and algae exist in
symbolic relationship, maintained by photosynthesis and surface
reaeration.
157. 157
• A facultative middle zone that is partly aerobic and partly
anaerobic, in which the decomposition of organic waste is
carried out by facultative bacteria.
• An anaerobic bottom zone where decomposition of
accumulated solids are carried out by anaerobic bacteria.
158. 158
Facultative Stabilization Processes
• The following processes are encountered in the
facultative stabilization ponds.
– Large solids will settle out at the bottom to form an anaerobic
sludge.
– The solids in the sludge are broken down by anaerobic bacteria,
producing dissolved organics and gases such as CO2, H2S, and
CH4 which are either oxidized by the aerobic bacteria or vented
to the atmosphere.
– Soluble and colloidal organic materials will be oxidized by
aerobic and facultative bacteria.
– Oxygen for oxidation is released by algae which grow
abundantly near the surface.
– Organic oxidation produces carbon dioxide, serving as a carbon
source for the algae.
– Oxygen is maintained in the upper layer of facultative ponds by
the presence of algae and by surface reaeration.
159. 159
Advantages of Facultative Ponds
• Facultative ponds are popular because of the following
reasons:
– Cost Factor
• Capital, operating and maintenance costs are less than those of
other biological systems.
– Management Factor
• Long retention times facilitate the management of large fluctuation
in wastewater flow and strength with no significant effect on effluent
quality.
160. 160
Anaerobic Ponds
• Anaerobic ponds are used mainly as a pretreatment
process to treat high temperature, high-strength organic
wastewater containing high concentration of solids.
– However, they have been used to treat municipal wastewater as
well.
• Anaerobic ponds have the following features:
– Deep earthen ponds with appropriate inlet and outlet pipings
– Depths up to 30ft (9.1m) have been built to conserve heat
energy and to maintain anaerobic conditions.
– Typically, anaerobic conditions prevail throughout the entire
depth, except for extremely surface zone.
161. 161
• The wastes that are added to the pond will settle to the
bottom.
• The partially clarified effluent is usually discharged to
another treatment process for further treatment.
162. 162
Anaerobic Stabilization Process
• In anaerobic ponds, stabilization is obtained by
– A combination of precipitation and the anaerobic
conversion of organic wastes to
• CO2
• CH4
• Other gaseous end products
• Organic acids, and
• Cell tissues
• There are two distinct stages in anaerobic treatment of
complex waste:
163. 163
– The first stage is known as acid fermentation, involving the
breakdown of complex organic materials to (mainly) short-chain
acids and alcohols.
– The second stage is known as methane fermentation, involving
the conversion of these materials to gases, mainly CO2 and CH4.
164. 164
Anaerobic Conditions
• The factor for determining whether the biological activity
– The magnitude of the organic loading and
– The availability of dissolved oxygen
• The anaerobic condition of a pond is maintained by
applying a BOD5 load that exceeds oxygen production
from photo-synthesis.
– The decrease in surface area and the increase in depth will
reduce photosynthesis.
165. 165
• Anaerobic ponds become turbid (muddy and thick) from
the presence of reduced metal sulphides
– Thus, the penetration of light is restricted and algae growth
becomes negligible.
166. 166
Advantages of Stabilization Ponds
• Stabilization ponds are popular in small towns.
Particularly in sites
– Where extensive industrial expansion is not anticipated, and
– Where the topography and soil condition of land is suitable for
siting.
• The advantage are:
– Lower capital or initial cost, compared to that of a mechanical
plant
– Lower operating costs
– Possible regulation of effluent discharge, this enables control of
pollution during critical times of year.
– Treatment system is not significantly affected by a leaky sewer
system that collects storm water.
167. 167
Disadvantages of Stabilization Ponds
• Disadvantages are:
– Extensive land area involved
– Poor assimilative capacity for certain industrial wastes
– Potential odour problems
– The town may expand and new development will intrude the
lagoon site
– Difficult to meet the effluent quality standard for suspended
solids of 30 mg/L
168. 168
5.0 CHEMICAL TREATMENT SYSTEMS
• Chemical Precipitation
• Adsorption
• Disinfection
• Disinfection with Chlorine Compounds
• Other Means of Disinfection
169. 169
Chemical Unit Processes
• Chemical unit processes used for the treatment of
wastewater are brought about by means of or through
chemical reaction.
– They are usually used together with the physical unit operations
and the biological unit processes.
• Chemical unit processes are additive processes (with the
exception of activated-carbon adsorption)
– Chemicals are added to the wastewater to achieve or enhance
the removal of suspended solids
– The physical unit operation and the biological unit processes are
subtractive in removing the suspended solids from the
wastewater.
170. 170
• Chemical unit processes, together with various physical
operations are developed for
– The complete secondary treatment of untreated wastewater,
including the nitrogen removal and phosphorus removal.
• Chemical unit processes are also used together with
biological treatment
– To remove phosphorus by chemical precipitation.
171. 171
Chemical Precipitation
• Chemical precipitation in wastewater treatment involves
– The addition of chemicals to change the physical state of
dissolved and suspended solids, and
– To facilitate the removal of the suspended solids by
sedimentation.
• Thus, chemical precipitation is used to
– Improve the performance of primary settling facilities
– Remove phosphorus.
• Phosphorus removal is done in advanced wastewater
treatment.
172. 172
Chemical Precipitation & Gravity
Sedimentation
• Light weight suspended solids and colloidal solids can
be removed by chemical precipitation and gravity
sedimentation.
– The tiny particles are agglomerated into large particles by the
chemical precipitation.
– The large particles can then settle rapidly in normal
sedimentation tanks.
• The precipitation reaction results in removal of
suspended solids.
– However, it also increases the amount of sludge to be handled.
– The chemical sludge must be taken into account together with
the characteristics of the original suspended solids in the
evaluation of sludge – processing systems.
173. 173
Coagulants
• Chemicals used as coagulants in wastewater treatment
are:
– Aluminium sulphate (Alum), A12 (SO4)3.18H2O
A12 (SO4)3.14H2O
– Ferric chloride, FeC13
– Ferrous Sulphate, FeSO4, 7H2O
– Ferric Sulphate, Fe(SO4)3, Fe2 (SO4)3, 3H2O
– Lime, Ca (OH)2
(XH2O) indicates the number of water molecules)
• The choice of coagulant depends upon:
– The chemical characteristics of the particles being removed
– The pH of the wastewater, and
– The cost and availability of the precipitation.
174. 174
Rapid Mixer
• A rapid mixing system is normally required in chemical
precipitation
• The rapid mixing system and the flocculation system are
installed ahead of the rectangular sedimentation tank.
Influents Effluent
Flocculation Sedimentation
(Rect. Tank)
Sludge
Rapid Mixing
OO
175. 175
• In the case of circular sedimentation tank, the rapid-
mixer and flocculation units are built into the tank.
• Rapid mixers are designed to give 30s retention at
average flow
– Sufficient trubulence is required to mix the chemicals with the
influents.
• The flocculation units are designed for slow mixing at 20-
min retention
– The particles are caused to collide by flocculation, and increase
in size, without excessive shearing.
176. 176
Improvement in Plant Performance
• It is possible to obtain a clear effluent by chemical
precipitation
– Substantially free from matter in suspension or in the colloidal
state.
• Removal of soluble organics is a function of the
coagulant chemical,
– Iron salt produces best results, and lime the poorest.
• Metal removal is a function of pH and the ionic state of
the metal.
177. 177
• Chemical precipitation can remove
– 95 percent of the suspended solids
– Up to 50 percent of the soluble organics and the bulk of the
heavy metals in a wastewater.
• In comparison, a sedimentation without chemical
precipitation can remove
– Only 50 to 70 percent of the total suspended matter, and
– 30 to 40 percent of the organic matter.
178. 178
Chemical Precipitation for Phosphate
Removal
• The removal pf phosphorus from wastewater can be
carried out by
– Making phosphate into suspended solids and
– Subsequently removing these solids.
• Phosphorus can be incorporated or formed into either
– Biological solids, or
– Chemical precipitates
• This topic will be discussed in the advanced wastewater
treatment.
179. 179
Adsorption Process
• Adsorption process is involved in collecting soluble
substances that are in solution on a suitable interface.
– The interface can be between the liquid and a gas, a solid or
another liquid.
• Adsorption process on an activated carbon is employed
to improve the quality of treated wastewater effluent
(after the normal biological treatment).
– The carbon is used to remove a portion of the remaining
dissolved organic matter.
• Activated – carbon absorbers are commonly used for
odour control.
180. 180
Activated Carbon
• The preparation of activated carbon is briefly described
as follows:-
– First, char is made from materials such as:
• Almond
• Coconut
• Woods,
• Coal, etc.
– Char is produced by
• Heating the materials (in a retort) to a red heat to drive off
hydrocarbon
– Activation is then carried out by exposing the char to
an oxidised gas at a high temperature.
• The gas develops a process structure in the char, creating a
large internal surface area.
181. 181
• After activation, the carbon is then separated into
different sizes with different capacities.
• The two size characteristics are:
– Powdered activated carbon (PAC), it has a diameter of less than
200 mesh.
– Granular activated carbon (GAC), it has a diameter greater than
0.1 mm.
• Both GAC and PAC are used for wastewater treatment.
• Activated carbon has different rates of adsorption for
different substances.
182. 182
• Activated carbon may be effective in removing
– Hydrogen sulphide and will work on reducing organic odour.
• The removal of odours depend on the concentration of
the hydrocarbon in the odorous gas
– The hydrocarbon are absorbed first before compounds such as
hydrogen sulphide are removed.
183. 183
Carbon Regeneration
• For economical application, it is essential to use an
efficient means of regenerating the carbon after its
adsorption capacity has been reached.
• Regeneration of granular carbon can be easily done in a
furnace by oxidizing the organic matter and thus
removing it from the carbon surface.
– However, about 5 to 10 percent of the carbon is also destroyed
in the process of carbon regeneration and must be replaced with
new carbon.
184. 184
• The methodology for regenerating powdered activated
carbon is not well-defined.
– This is a major problem with the application of PAC.
• The use of PAC produced from solid wastes may obviate
the need to regenerate the spent carbon.
185. 185
Disinfection
• Disinfection is a process
– To render water safe from pathogenic bacteria
• Disinfection can be accomplished by the use of:
– Chemical agents
– Physical agent
– Mechanical means, and
– Radiation.
186. 186
Chemical Agents
• The chemical agents include:
– Chlorine and chlorine compounds
– Bromine
– Iodine
– Ozone
– Alcohols
– Soaps and synthetic detergents
– Various alkalies and acids
• The most common disinfectants are the oxidizing
chemicals, and
– Chlorine is the most commonly used.
187. 187
• Bromine and iodine have also been used for wastewater
disinfection.
• Ozone is a highly effective disinfectant
– Its use is increasing
• Highly acidic or alkaline water can also be used to
destroy pathogenic bacteria
– Water with a pH greater than 11 or less than 3 is relatively toxic
to most bacteria.
188. 188
Physical Agents
• Physical disinfectants are
– Heat and
– Light
• Heating water to the boiling point will destroy the major
disease – producing bacteria.
– But, it is not economically feasible to disinfect large quantities of
wastewater by heating because of the high cost involved.
• Sunlight is a good disinfectant
– Particularly, the ultraviolet radiation.
189. 189
• Ultraviolet rays emitting from special lamps have been
used to sterilize small quantities of water
– The efficiency of the process depends on the rays penetration
into water.
– It is difficult to use ultraviolet radiation in aqueous systems.
190. 190
Factors Influencing Disinfections
• The following factors will affect the disinfection
performance:
– Contact time
– Concentration and type of chemical agent
– Temperature
– Number and types of organisms
– Nature of suspending liquid
• In general:
– The longer the contact time, the greater the kill, for a given
concentration of disinfectant
– Increasing the temperature gives more rapid kill
191. 191
– The larger the organism concentration, the longer the
time required for a given kill,
• Although in a dilute system such as wastewater, the
concentration of organisms is not a major consideration.
• The effectiveness of disinfectants will also depend on the
types of organisms
– Viable growing bacteria cells are easy to kill
– But, bacterial spores are extremely resistant; many of the
chemical disinfectants normally used will not be effective.
192. 192
Chlorination
• Chlorination is used because:
– It is readily available as gas, liquid or powder
– Cheap
– Easy to use, high solubility (7000 mg/L)
– It leaves a residual in solution which is not harmful and helps
protect distribution system
– It is very toxic to most microorganisms.
• Chlorine gas is normally used as a bioxide and
disinfectant in water.
193. 193
Chlorine Compounds
• The most common chlorine compounds used in
wastewater treatment plants include:
– Chlorine gas, C12
– Calcium hypochlorite, Ca(OC1)2
– Sodium hypochlorite, NaOC1
– Chlorine dioxide, C1O2
• The use of calcium and sodium hypochlorite is mostly
found in very small treatment plant such as package
plants
– Because simplicity and safety are for more important than cost in
this application.
194. 194
• Sodium hypochloride is used at large wastewater
treatment plants for safety reason.
• Chlorine gas is the most commonly used form.
195. 195
Superchlorination / Dechlorination
• Superchlorination / dechlorination
– It is used where pollution is high, lowland river waters
– Heavy initial dose of chlorine is added, killing everything
– Any objectional excess is then removed by dechlorination using
sulphur dioxide
– Contact time is 20-30 minutes
– It leaves a small amount of residual chlorine.
196. 196
Dechlorination
• Chlorination is commonly used to destroy pathogenic
and other harmful organisms that may cause danger to
human health.
– However, some organic compounds in wastewater may react
with the chlorine to produce toxic compounds that can cause
long-term adverse effect on the use of water.
• It is therefore necessary to dechlorinate wastewater
treated with chlorine in certain applications
– So that the effects of the toxic chlorine residual on the
environment will be minimized.
197. 197
Sulphur Dioxide for Dechlorination
• Dechlorination is applied after the breakpoint chlorination
process for the removal of ammonia nitrogen.
• Sulphur dioxide is used most commonly for
dechlorination
– Activated carbon has also been used.
• Sulphur dioxide gas added in water will successively
remove:
– Free chlorine
– Monochloramine
– Dichloramine
– Nitrogen trichloride and
– Poly-n-chlor compounds
199. 199
Ozonization
• Ozonization process or ozonation is used to:
– Remove taste and odour as well as dissolved / colloidal organic
matter
– Good colour removal
– Contact time is 5 minutes
– The process is very effective but expensive
• Ozone is a highly unstable toxic blue gas; it should be
produced on site as close to the point of use as possible.
• Ozone is also a very effective virucide
– It is generally believed to be more effective than clorine.
200. 200
• Ozonation does not produce dissolve solids and is not
affected by the ammonium ion or pH influent to the
process.
• For these reasons, ozonation is considered a viable
alternative to either chlorination or hypochlorination,
especially where dechlorination may be required.
201. 201
Notes on Ozone
• The application of Ozone to disinfect water supplies was
first carried out in France in the early 1900s.
– Eventually its use spread into several Western European
countries (primarily in Europe).
• The common application for ozone at the European
installation is to control taste -, odour -, and colour –
producing agents.
• Being chemically unstable, ozone decomposes to
oxygen very rapidly after generation,
– Thus it must be generated on-site.
202. 202
• The most efficient method of producing ozone is by
electrical discharge
– Ozone is generated either form air or pure oxygen when a high
voltage is applied across the gap of narrowly spaced electrodes.
203. 203
6.0 ADVANCED WASTEWATER
TREATMENT
• Additional Treatment for Water Reuse
• Various Advanced Wastewater Treatment Methods
• Suspended – Solids Removal by Filtration
• Filter Applications
• Refractory Adsorption
• Carbon Adsorption
• Phosphorus Removal
• Nitrogen Sources and Control
• Biological Nitrification
• Biological Denitrification
• Ammonia Stripping
• Removal of Toxic Compounds
204. 204
Additional Treatment for Water Reuse
• Advanced wastewater treatment refers to the additional
treatment needed to
– Remove contaminants (both suspended and dissolved
substances) remaining after conventional secondary treatment
• The term tertiary treatment is also used.
• Contaminants of municipal water results from:
– Human excreta
– Food preparation wastes, and
– A wide variety of organic and inorganic industrial wastes.
205. 205
• Conventional treatment uses physical-biological
processes, and possibly chlorination, to
– Reduce biochemical oxygen demands, suspended solids and
pathogen.
• The maximum acceptable level of organic matter in a
wastewater effluent after biological treatment is defined
in terms of BOD and suspended-solids concentrations.
• Secondary treatment processes when coupled with
disinfection (mainly chlorination) may remove:
– Over 85 percent of the BOD and suspended solids and
– Nearly all pathogen.
206. 206
• However, secondary treatment processes can achieve
only minor removal of some pollutents such as
– Nitrogen
– Phosphorus
– Soluble COD and
– Heavy metals
• These pollutents may be of major concern in some
circumstances
– Thus, it may be necessary to remove these pollutents by
advanced wastewater treatment.
• Advanced wastewater treatment processes improve
effluent quality to the point that it is adequate for many
reuse purposes.
207. 207
Various Advanced Wastewater Treatment
Methods
• The popular advanced treatment methods are given as
follows:
– Suspended-solids Removal
• Filtration through granular media
• Chemical coagulation and clarification
– Organic Matter Removal
• Adsorption on granular activated carbon
• Extended biological oxidation
– Phosphorus Removal
• Biological-chemical precipitation and clarification
• Chemical coagulation and clarification.
208. 208
– Nitrogen Removal
• Biological nitrification / Denitrification
• Ammonia reduction by air stripping
– Heavy Metal Removal
• Lime precipitation
– Dissolved-Solids Removal
• Reverse osmosis
209. 209
Suspended – Solids Removal by
Filtration
• Secondary treatment processes, such as the activated-
sludge process, are highly efficient for removal of
biodegradable colloidal and soluble organs.
– However, the typical effluent still contains a BOD of 20 to 50
mg/L.
• The secondary clarifiers are not perfectly efficient to
settle out the microorganisms from the biological
treatment processes.
– These organisms will contribute both to the suspended solids
and to the BOD5 because the process of biological decay of
dead cells exerts an oxygen demand.
210. • Filtration process can be used to remove the residual
suspended solids
– Including the unsettled microorganisms
– The residual BOD5 is also reduced by removing the
microorganisms
210
211. Purpose of Filtering
• Removal of suspended solids from the effluent of a
conventional treatment plant may serve to
– Reduce the organic content, or
– To pretreat the wastewater for subsequent processing.
• Examples are:
– For effective disinfection, it is necessary to remove suspended
solids that can harbour and protect pathogenic bacteria and virus
from the oxidizing action by chlorine or ozone.
– To prevent fouling, carbon adsorption columns are preceded by
filtration.
211
212. Filter Applications
• Conventional sand filters, similar to those used in water
treatment, can be used.
– These filters often clog quickly, thus frequent back washing is
required.
• It is desirable to have
– The larger filter grain sizes at the top of the filters
– This will lengthen filter runs and reduce backwashing
– Also, it will permit the trapping of some of the larger particles of
biological floc at the surface without plugging the filter.
212
213. • Multimedia filters use
– Low-density coal for the large grain sizes
– Medium – density sand for intermediate sizes, and
– High – density garnet for the smallest size filter grains.
• This is so arranged that
– During backwashing, the greater density offsets the smaller
diameter
– And the coal will remain on top, the sand in the middle while the
garnet remains on the bottom.
• Typically, plain filter can reduce the suspended solids of
activated – sludge effluent from 25 to 10 mg/L.
213
214. Refractory Organics
• Soluble organic materials that are resistent to biological
breakdown will persist in the effluent
– Even after the processes of secondary treatment, coagulation,
sedimentation and filtration.
• These persistent materials are called refractory organics.
• Refrectory organics can be detected in the effluent as
soluble COD
– The typical values of secondary effluent COD are 30 to 60 mg/L.
214
215. Carbon Adsorption
• Refractory organics can be effectively removed by
– Adsorbing them on activated carbon.
• Carbon is activated by heating in the absence of oxygen
– The activation process results in the formation of many pores
within each carbon particle.
– The greater the surface area of the carbon (with many pores),
the greater its capacity to hold organic material.
• The current practice is installing the granular-carbon
columns as tertiary conditioning after the chemical
precipitation and granular-media filtration.
215
216. • After the adsorption capacity of the carbon has been
exhausted, the spent carbon can be regenerated for
reuse.
• Powdered carbon is commonly used in water treatment
– But it has not widely used in wastewater processing because of
the difficulty of regeneration.
216
217. Phosphorus Removal
• Phosphorus is typically found as mono-hydrogen
phosphate (HPO4²‾) in wastewater.
• Chemical precipitation, using alum and iron coagulants or
lime, is effective in phosphate removal.
– Alum, double sulphate of aluminium and potassium.
• The precipitation reaction between alum and phosphate is:
A12(SO4)3 + 2HPO4²‾ ↔ 2A1PO4 ↓+ 2H ++ 2SO4²‾
• The precipitation reaction between ferric chloride and
phosphate is:
FeC13 + hpo4²‾ ↔ FePO4 ↓ +H+ + 3C1‾
217
218. • The chemical reaction using lime is
5Ca(OH)2 + 3HPO4²‾ ↔ Ca5(PO4)3 OH↓ + 3H2O + 6OH‾
• Alum and Ferric chloride reduce pH
– Whereas lime increases pH.
• The effective range of pH for alum and ferric chloride is
between 5.5 and 7.0.
– Lime is added when there is not enough alkalinity so as to buffer
the system to this range of pH.
218
219. Phosphorus Removal by Chemical
Addition
• The addition of certain chemicals, (such as alum, ferric
chloride or sulphate, and lime) to wastewater produces
insoluble or low-solubility salts.
• In the precipitation of phosphorus, a reaction basin and a
settling tank are required for the removal of precipitate.
• Since ferric chloride and alum may be added directly in
the aeration tank in the activated sludge system,
– The aeration tank can serve as a reaction tank, and
– The precipitate can be removed in the secondary clarifier.
219
220. • But, this arrangement is not possible with lime
– When lime is used, the high pH required with lime to produce the
precipitate is harmful to the activated sludge organism.
• The use of lime for phosphorus removal is declining due
to
– The operation and maintenance problems associated with the
handling, storage, and feeding of lime.
220
221. Nitrogen Sources
• Most nitrogen in surface waters is from:
– Land drainage and
– Dilution of wastewater effluents
• In domestic waste, the primary sources of nitrogen are:
– Faeces
– Urine, and
– Food-processing discharges
• Bacteria decomposition produces ammonia by
deamination of nitrogenuous organic compounds
221
222. • Continued aerobic oxidation results in nitrification.
• The nitrogen forms of interest are
– Organic
– Inorganic and
– Gaseous nitrogen
222
223. Nitrogen Control
• Nitrogen in any soluble form CNH3, NH4+, NO2‾ and
NO3‾ but not N2 gas is a nutrient and
– The removal of nitrogen from wastewater may be necessary to
help control algal growth in the receiving body.
• Nitrogen in the form of ammonia exerts an oxygen
demand and
– This can be toxic to fish
• Removal of nitrogen can be carried out either by
– Biological process, or
– Chemical process.
223
224. • The biological process is called
– Nitrification/denitrification
• The chemical process is called
– Ammonia stripping.
224
225. Biological Nitrification
• The removal of nitrogen can be effectively carried out by
– Biological nitrification – denitrification
• It is a two-step process:
– The first step is the conversion of ammonia aerobically to nitrate
(NO3‾) which is termed nitrification.
– The second step is the conversion of nitrates to nitrogen gas,
this is termed denitrification.
• The nitrification step is expressed in chemical term as
follows:
NH4++ 2O2 ↔ NO3‾ + H2O + 2H+
– Bacteria must be present to cause the reaction to occur.
225
226. • The rate of nitrification in wastewater is essentially linear
– It is a function of time and independent of ammonia – nitrogen
concentration
– Temperature, pH and dissolved oxygen are important
parameters
– Nitrification rate decreases with temperature drop
– The optimum pH for nitrification is 8.2 – 8.6.
226
227. Biological Denitrification
• Nitrie and nitrate are bacterially reduced to gaseous
nitrogen by biological denitrification.
• Denitrification is an anoxic process because it occurs in
the absence of dissolved oxygen.
• The process can be expressed chemically as follows:
2NO3‾ + Organic matter → N2 + CO2 + H2O
• As indicated above, organic matter (carbon) is needed
for denitrification
– The wastewater to be denitrified must contain sufficient carbon
to provide the energy source for the bacteria.
227
228. • The carbon requirement may be provided by wastewater
and cell materials, or
– By an external source such as methanol (CH3OH)
• An organic carbon source acts as a hydrogen donor
(oxygen acceptor) and
– To supply carbon for biological synthesis.
228
229. Ammonia Stripping
• Nitrogen in the form of ammonia can be removed form
wastewater by the physical-chemical process of
ammonia stripping.
• Ammonia stripping process involves the following steps:
– Raising the wastewater pH to convert the ammonium ion into
ammonia
– The ammonia can then be stripped from the water by passing
large quantities of air through the water.
• The rate of ammonia transfer is enhanced by converting
most of the ammonia to a gaseous form at a high pH.
229
230. – Usually in the range of 10.5 to 11.5
• The ammonia stripping reaction is expressed as:
NH4 + OH‾ ↔ NH3 + H2O
• The hydroxide (high pH) is usually provided by adding
lime.
• The lime will also react with CO2 in the air and water
– Thus calcium carbonate scale is formed and this must be
removed periodically.
230
231. Disadvantages of Ammonia Stripping
• Ammonia stripping is simple in concept
– But is has disadvantages that make it expensive to operate and
maintain.
• The disadvantages are:
– It is temperature sensitive
• Ammonia solubility increases with lower temperature
• Fogging and icing occur in cold climate
– Pollution sensitive
• Ammonia reaction with sulphur dioxide may cause air pollution
problems.
– Maintenance & Operation Problems
• It usually requires lime for pH control, this will increase treatment
cost and cause lime-related operating and maintenance problems.
231
232. Removal of Toxic Compounds
• Various methods are used for the treatment of toxic
compounds.
• The nature of toxicity is complex
– The specific characteristics of the wastewater and the nature of
compound must be considered in the application of treatment
methods.
• The various treatment processes for toxic compounds
removal are reviewed as follows:
– Activated-carbon adsorption process
• To remove natural and synthetic organic compounds including
VOCs pesticides, heavy metals.
232
233. – Conventional biological treatment process (activated – sludge,
trickling filter).
• To remove phenols and selected hydrogenated hydrocarbons
– Air stripping
• To remove ammonia and volatile organic compounds (VOCs)
– Chemical coagulation, sedimentation, and filtration
• To remove heavy metals
– Activated – sludge – powdered activated carbon
• To remove heavy metals, ammonia and selected refractory priority
pollutants.
233
234. 7.0 SLUDGE TREATMENT & DISPOSAL
• Sludge Sources & Characteristics
– Sludge Problem
– Primary of Raw Sludge and Scum
– Secondary Sludge and Scum
– Sludge from Chemical Precipitation
– Activated Sludge
– Tertiary Sludge
• Sludge Treatment Processes
– Thickening (Concentration)
– Stabilization
– Conditioning
– Dewatering
– Reduction
• Sludge Disposal 234
235. Sludge Problem
• In the process of treating wastewater to remove
impurities, another problem is created : sludge.
– In fact, the higher the degree of wastewater treatment, the larger
is the residue of sludge that must be handled.
– The exceptions of this rule are where land applications or
polishing lagoons are used.
• The objective of processing sludge is to
– Extract water from the solids and
– Dispose of the dewatered residue.
• Satisfactory treatment and disposal of the sludge is a
very complex and costly operation in municipal
wastewater treatment system.
235
236. Primary or Raw Sludge and Scum
• Quantities of sludge and scum depend upon the nature
of the collection system.
• Sludge from the bottom of the primary clarifiers contains
from 3 to 8 percent solids which is about 70 percent
organic.
– 1 percent solid is about 1 g solid/100 mL sludge volume.
• The sludge rapidly becomes anaerobic and is highly
odoriferous.
236
237. • Sludge from primary settling tank is usually gray and
slimy and
– In most cases, it has an extremely offensive odour.
• Primary sludge can be readily digested under suitable
conditions of operation.
237
238. Secondary Sludge and Scum
• The secondary sludge consists of
– Microorganisms and inert materials that have been wasted from
the secondary treatment processes.
• The solids are about 90 percent organic.
• This sludge becomes anaerobic when the supply of air is
removed.
– They create noxious conditions if not treated before disposal.
• The solids content depends on the source.
– Wasted activated sludge is typically 0.5 to 2 percent solids.
– Trickling filter sludge contains 2 to 5 percent solids.
238
239. Sludge from Chemical Precipitation
• Sludge form chemical precipitation with metal salts is
usually dark in colour.
– The surface may be red if it contains much iron.
• Lime sludge is grayish brown.
• The ordour of chemical sludge may be objectionable
– It is not as bad as primary sludge.
• If the sludge is left in the tank, it undergoes
decomposition similar to primary sludge.
– But at a slower rate.
239
240. • Substantial quantities of gas may be given off and
– The sludge density is increased by long residence times in
storage.
240
241. Activated Sludge
• Activated sludge generally has a brownish, flocculent
appearance.
– Dark colour means that the sludge may be approaching a septic
conditions.
• A lighter colour than usual, indicates that underaeration
may have been occurred with a tendency for the solids
to settle slowly.
• The sludge tends to become septic rapidly and
– Giving a disagreeable odour putrefaction.
• Activated sludge will digest readily aline or when mixed
with primary sludge.
241
242. Tertiary Sludges
• The characteristics of sludges form the tertiary treatment
processes depend on the nature of the process.
• For example:
– Phosphorus removal produces a chemical sludge that is difficult
to handle and treat.
• When phosphorus removal occurs in the activated
sludge process, the chemical sludge is combined with
the biological sludge,
– Making the biological sludge more difficult to treat.
242
243. • Nitrogen removal by denitrification produces a biological
sludge with properties very similar to those of waste
activated sludge.
243
244. Sludge Treatment Processes
• The basic processes for sludge treatment are as follows:
– Thickening (Concentration)
• To separate as much as possible by gravity or flotation.
– Stabilization
• To convert the organic solids to more refractory (inert) forms.
• The inert forms can be handled or used as oil conditioners without
causing a nuisance or health hazard through processes referred to
as “digestion”.
• These are biochemical oxidation processes.
– Conditioning
• The sludge is treated with chemicals or heat so that that water can
be readily separated.
244
245. – Dewatering
• Water is separated from sludge by vacuum, pressure or drying.
– Reduction
• Solids are converted to a stable form by
– Wet oxidation or
– Incineration
• These are chemical oxidation processes, the volume of sludge is
decreased, hence the term reduction.
245
