Handout

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Environmental Engineering
CSE 29364
Prof. W. Chu, CEE
About this subject
 Assessment weight : Examination 70% &
Coursework 30%
 Laboratory work 30% (Lab. Group Report, 4 -
6 students in one group, should be submitted
within two weeks after the laboratory session
during the lecture)
 Water/Waste Laboratory ZS1102
 lab 1: Basic Water Quality parameters.
 lab 2: Solids, Alkalinity and Hardness.
 lab 3: BOD & COD.
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Suggested References
 Environmental Engineering, Peavy et. al.,
McGraw-Hill.
 Wastewater Engineering Treatment and Reuse,
Metcalf and Eddy, McGraw-Hill.
 Water Supply and Sewerage, McGhee,
McGraw-Hill.
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Introduction of WATER
Pollution
Prof. W. Chu

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WATER POLLUTANTS
 Oxygen-Demanding Wastes
 Pathogens
 Nutrients
 Salts
 Thermal Pollution
 Heavy Metals
 Pesticides
 Volatile Organic Compounds
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Oxygen-Demanding Wastes
 Dissolved oxygen (DO). The saturated value
of dissolved oxygen in water is on the order
of 8 to 15 mg/L, depending on temperature
and salinity. Minimum amounts required for a
healthy fish population may be as high as 5-8
mg/L for active species.
 Oxygen-demanding wastes are substances
that oxidize in the receiving body of water,
reducing the amount of DO available.
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Oxygen-Demanding Wastes
 The chemical oxygen demand, or COD, is the
amount of oxygen needed to chemically
oxidize the wastes
 The biochemical oxygen demand, or BOD, is
the amount of oxygen required by
microorganisms to biologically degrade the
wastes. BOD has traditionally been the most
important measure of the strength of organic
pollution. BOD reduction in a wastewater
treatment plant is a key indicator of process
performance. 8

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Pathogens
 Pathogens are disease-producing organisms
that grow and multiply within the host.
 bacteria responsible for cholera, bacillary
dysentery, typhoid, and paratyphoid fever;
 viruses responsible for infectious hepatitis and
poliomyelitis;
 protozoa, which cause amebic dysentery and
giardiasis; and helminths, or parasitic worms,
which cause diseases such as
schistosomiasis and dracontiasis (guinea
worm).
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Pathogens
 The intestinal discharges of an infected
individual, a carrier, may contain billions of
these pathogens, which, if allowed to enter
the water supply, can cause epidemics of
immense proportions. Carriers may not even
necessarily exhibit symptoms of their disease,
which makes it even more important to
carefully protect all water supplies from any
human waste contamination.
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Pathogens
 Even developed countries such as the United
States, typhoid, for example, was killing
approximately 28,000 Americans each year.
 In 1885, almost 90,000 people in Chicago
died of typhoid or cholera when untreated
sewage was drawn directly into the drinking
water supply during a severe storm.
 Chlorination, which began in the United
States in 1908, that outbreaks of waterborne
diseases became rare.
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Pathogens
 The World Health Organization estimates that
approximately 80 percent of all sickness in
the world is attributable to inadequate water
or sanitation.
 Contaminated water caused by poor
sanitation can lead to both waterborne and
water-contact diseases.
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Pathogens
 Waterborne diseases are those acquired by
ingestion of pathogens not only in drinking
water, but also from water that makes it into a
person's mouth from washing food and
hands. E.g. open wells or streams that are
easily polluted.
 Giardiasis (賈第蟲病) caused by the Giardia
lamblia protozoa, which passed through the
feces of carriers pose an unusual threat to
surface water and municipal supply systems.
They are not easily destroyed by chlorination.13
Pathogens
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Pathogens
 Water-contact diseases do not even require
that individuals ingest the water.
Schistosomiasis (血吸蟲病) affecting 200
million people and spread by free swimming
larva in the water, called cercaria. Thay
attach themselves to human skin, penetrate
it, and enter the bloodstream. Cercaria
mature in the liver into worms that lay masses
of eggs on the walls of the intestine.
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Pathogens
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Nutrients
 Nutrients are chemicals, such as nitrogen,
phosphorus, carbon, sulfur, calcium,
potassium, iron, manganese, boron, and
cobalt, that are essential to the growth of
living things.
 Excess nutrients stimulate the growth of
algae (algae bloom), the use of water for
drinking water supply, and as a viable habitat
for other living things can be adversely
affected.
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Eutrophication accelerated results
with human input of nutrients to a lake
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Nutrients
 Bloom of algae which eventually die and
decompose, which removes oxygen from the
water and make DO insufficient to sustain
normal life forms.
 Algae and decaying organic matter add color,
turbidity, odors, and objectionable tastes to
water. The process of nutrient enrichment,
called eutrophication, is especially important
in lakes.
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Nutrients
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Nutrients
 The nutrient that is least available relative to
the plant's needs is called the limiting
nutrient. This suggests that algal growth can
be controlled by identifying and reducing the
supply of that particular nutrient.
 Carbon is usually available from a number of
natural sources including alkalinity, dissolved
carbon dioxide from the atmosphere, and
decaying organic matter, so it is not often the
limiting nutrient.
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Nutrients
 Usually either nitrogen or phosphorus
controls algal growth rates. In general,
seawater is most often limited by nitrogen,
while freshwater lakes are most often limited
by phosphorus.
 Major sources of both nitrogen and
phosphorus include municipal wastewater
discharges, runoff from animal feedlots, and
chemical fertilizers.
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Nutrients
 Some bacteria and blue-green algae can
obtain nitrogen directly from the atmosphere.
These life forms are usually abundant in lakes
that have high rates of biological productivity,
making the control of nitrogen in such lakes
extremely difficult.
 Acid rain can also contribute nitrogen to lakes.
 The only unusual source of phosphorus is from
detergents. To limit the nearby use of
phosphate in detergents is critical.
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Nutrients
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Nutrients
 Nitrogen in water (mostly nitrate, NO3
-) can
be converted into nitrites (NO2
-) by intestinal
bacteria of infants. Nitrites have a greater
affinity for hemoglobin in the bloodstream
than does oxygen, and results in “blue baby”
syndrome. In extreme cases the victim may
die from suffocation.
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Salts
 Dissolved solids, or salts, typically include
cations as sodium, calcium, magnesium, and
potassium, and anions such as chloride,
sulfate, and bicarbonate.
 Total dissolved solids (TDS)
 Fresh water < 1500 mg/L
 Brackish water up to 5000 mgIL
 Saline water > 5000 mg/L
 Seawater contains 30,000-34,000 mg/L.
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Salts
 Drinking water has a recommended
maximum contaminant level for TDS of 500
mg/L.
 Livestock can tolerate higher concentrations.
(U.S. Geological Survey)
 poultry at 2860 mg/L
 pigs at 4290 mg/L
 beef cattle at 10,100 mg/L.
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Salts
 Of greater importance is the salt tolerance of
crops.
 TDS > 500 mg/L, the need for careful water
management to maintain crop yields
 TDS up to 1500 mg/L, can be tolerated by
most crops with little loss of yield
 TDS > 2100 mg/L, unsuitable for irrigation
except for the most salt tolerant crops.
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Thermal Pollution
 A large steam-electric power plant requires
an enormous amount of cooling water. A
typical nuclear plant, for example, warms
about 40 m3/s of cooling water by 10C as it
passes through the plant's condenser. If that
heat is released into a local river or lake, the
resulting rise in temperature can dramatically
affect life in the vicinity of the thermal plume.
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Thermal Pollution
 As water temperature increases, more
oxygen is needed for aquatic life.
 Metabolic rates increased by about a factor of
2 for each 10C rise in temperature. (More
oxygen consumption by organisms)
 The available supplies of DO reduced (Waste
assimilation is quicker, and the amount of DO
that the water can hold decreases)
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Heavy Metals
 Heavy metal is metal with specific gravity
greater than about 4 or 5, or the term is
simply used to denote metals that are toxic.
 Toxic metals includes aluminum, arsenic,
beryllium, bismuth, cadmium, chromium,
cobalt, copper, iron, lead, manganese,
mercury, nickel, selenium, strontium, thallium,
tin, titanium, and zinc. Some of these metals,
such as chromium and iron, are essential
nutrients in our diets, but in higher doses are
extremely toxic. 31
Heavy Metals
 Metals may be inhaled, as is often the case
with lead, for example, and they may be
ingested (food and water).
 Metals have a range of adverse impacts on
the body, including nervous system and
kidney damage, creation of mutations, and
induction of tumors.
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Pesticides
 Pesticide is used to cover a range of
chemicals that kill organisms that humans
consider undesirable.
 Three groups: organochlorines (chlorinated
hydrocarbons), organophosphates, and
carbamates. Many of them are EDC
(Endocrine disrupting chemicals).
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Pesticides
 Endocrine disrupting chemicals that
interfere with endocrine (or hormone system)
in animals, including humans. These
disruptions can cause cancerous tumors,
birth defects, and other developmental
disorders like feminizing of males or
masculine effects on females, etc. Any
system in the body controlled by hormones,
can be derailed by EDCs.
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Volatile Organic Compounds
 Volatile organic compounds (VOCs) are
among the most commonly found con-
taminants in groundwater. They are often
used as solvents in industrial processes and
a number of them are either known or
suspected carcinogens or mutagens.
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Volatile Organic Compounds
 Five toxic VOCs presence in water:
 Vinyl chloride (chloroethylene): a carcinogen
used in the production of PVC resins.
 Tetrachloroethylene: a solvent and a heat
transfer medium, used in the manufacture of
chlorofluorocarbons. It causes tumors in
animals.
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Volatile Organic Compounds
 Trichloroethylene (TCE): a solvent commonly
used to clean everything, a suspected
carcinogen and the most frequently found
contaminants in groundwater.
 1,2-Dichloroethane: a metal degreaser cause
injury to the central nervous system, liver, and
kidneys.
 Carbon tetrachloride: a common household
cleaning agent, very toxic if ingested
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Population and Water Demand
 Hong Kong has 6.98M people including
transients such as visitors and the armed forces.
 The average daily consumption of fresh water
was 2.6M m3/d and average daily use of
seawater for flushing was 0.64M m3/d.

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Types of Water Consumption
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Water supply resources
 The statistical data for water resources for the last 10 years indicate that nowadays
about more than 70% of raw fresh water is supplied from the East River (Dongjiang)
in Guangdong province
 and only less than 30% of raw fresh water is supplied by local catchment sources.

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Dongjiang water
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Water supply system in Hong Kong
Plover
Cove
High
Island

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Water quality requirements
 Water contains a variety of chemical, physical, and biological
substances which are either dissolved or suspended in it.
 Water also contains living organisms which react with its
physical and chemical elements. For these reasons, water must
often be treated before it is suitable for use.
 Water quality requirements are established in accordance with
the intended use of the water.
 Quality is usually judged as the degree to which water conforms
to physical, chemical, and biological standards set by the user.
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Physical characteristics
 Tastes, odour, colour, and turbidity are controlled in public water
supplied partly because they make drinking water inedible, but also
prohibit the use of water in beverages, food processing, and textile.
 Tastes and odours are caused by the presence of volatile chemicals
and decomposing organic matter.
 Colour in water is caused by minerals such as iron and manganese,
organic material, and coloured wastes from industries. Colour in
domestic water may stain fixtures and dull clothes.
 Turbidity, as well as being aesthetically objectionable, is a health
concern because the particles involved could harbor pathogens. Water
with enough suspended clay particles (10 turbidity units) will be visually
turbid. Surface water sources may range in turbidity from 10 - 1,000
units; however, it is possible for very turbid rivers to have 10,000 units
of turbidity.

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Chemical characteristics
 The many chemical compounds dissolved in water may be of natural or
industrial origin and may be beneficial or harmful depending on their
composition and concentration.
 For example, small amounts of iron and manganese may not just
cause colour; they can also be oxidized to form deposits of ferric
hydroxide and manganese oxide in water mains and industrial
equipment. These deposits reduce the capacity of pipes and are
expensive to remove.
 Hard waters are generally considered to be those waters that require
considerable amounts of soap to produce foam and they also produce
scale in hot water pipes, heaters, and boilers.
 Sulfates, chlorides, and nitrates of calcium and magnesium are not
removed by boiling. These salts cause noncarbonate hardness,
sometimes called “permanent” hardness.
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Biological characteristics
 From the perspective of human use and consumption, the most
important biological organisms in water are pathogens, those
organisms capable of infecting, or of transmitting diseases to,
human.
 These organisms are not native to aquatic systems and usually
require an animal host for growth and reproduction. They can,
however, be transported by natural water systems, thus
becoming a temporary member of the aquatic community.
 Many species of pathogens are able to survive in water and
maintain their infectious capabilities for significant periods of
time. These water borne pathogens include species of bacteria,
viruses, protozoa, and parasitic worms.

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Drinking water quality standard
 HK mainly adopts WHO guidelines for
drinking water quality standard
 Part A: Microbiological quality
 Part B: Chemicals of health significance
 Part C: Other parameters
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Part A: Microbiological quality

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Part B: Chemicals of health
significance
Organic
chlorine
Heavy
metals
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Part B: Chemicals of health
significance

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Part B: Chemicals of health
significance
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Part C: Other parameters

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Water conservation
 Conservation remains integral to any water
management policy and in this context we have
been implementing a range of measures
including greater use of sea water for toilet
flushing across a range of buildings, the
replacement of aging water pipes to reduce
leakage and then continuous monitoring and
management of water pressure which also
helps us detect and reduce leakage.
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Nature and Characteristics of
Wastewater
 Definition of Wastewater
 Every community produces both liquid & solid wastes.
The liquid portion-wastewater is essentially the water
supply of the community after it has been consumed
by variety of users.
 Classification
 Municipal wastewater,
 Industrial wastewater,
 Stormwater

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Municipal Wastewater
 The excreted waste from human is called sanitary sewage.
 Wastewater from residential area is referred to as domestic sewage and
includes kitchen, bath, laundry, and floor drain wastes. These, together with the
liquid wastes from commercial and industrial establishments, are termed
municipal wastewater.
 This wastewater is normally connected in a public sewer system and directed to
treatment facilities for safe disposal.
 Quantities of municipal wastewater are commonly determined from water use.
Because water is consumed by humans, utilized in industrial products, used for
cooling, and required for activities such as lawn watering and street washing,
around 70 to 90 percent of the water supplied reaches the sewers.
 However, the above assumption may not be always correct due to infiltration
(groundwater leakage into the sewer system through poor joints) or storm water,
which enters the sanitary sewer system through illicit connections (roof
downspouts and road catch basins) and inflow (through manhole openings).
Composition of municipal wastewater
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Industrial Wastewater
 Wastewater from industries include employees’ sanitary wastes, process wastes
from manufacturing, wash waters, and relatively uncontaminated water from
heating and cooling operations. The waters from processing are the major
concern. They vary widely with the type of industry. In some cases,
pretreatment to remove certain contaminants or equalization to reduce
hydraulic/organic shock-loads may be necessary before the wastewater can be
discharged into the public sewer system.
 Wastes are specific for each industry and can range from strong (high BOD5)
biodegradable wastes like those from meat packing, through wastes such as
those from plating shops and textile mills, which may be inorganic and toxic and
require on-site physical-chemical treatment before discharge to the public sewer
system.
 The volume or strength of industrial wastewater is often compared to that of
domestic sewage in terms of a population equivalent (PE) based on typical per
capita contribution. (assuming one unit equals to 54 g BOD per day)
Population equivalents of
wastewater from industries
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Stormwater
 The runoff from rainfall, snowmelt, and street washing is less contaminated than
municipal wastewater. It therefore receives little or no treatment before being
discharged into storm sewers (for direct disposal into receiving waters).
 The quantity of stormwater which runs off from a municipality varies widely with the
time of year, the type of terrain, and the intensity and duration of the storms which
occur.
 Runoff Coefficient: A fraction varies from about 0.2 (parks and lawns) to 1 (roofs and
paved areas). An overall average value for a municipality might range between 0.3 to
0.5 during fairly intense storms.
 Stormwater runoff, particularly in cities, contains dust and other particulate from roads,
leaves from trees, grass cuttings from lawns and parks, and fallout from air pollution.
 The concentration of these contaminants is highest when they are first flushed into
the sewer system during the early stages of runoff and then decreases as the rain
continues.
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Effects of Pollutants
 Water pollution occurs when the discharge of
wastes impairs water quality or disturbs the
natural ecological balance.
 The contaminants which cause problems
include pathogens (disease causing
organisms), organic matter, solids, nutrients,
toxic substances, colour, foam, heat, and
radioactive materials.

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Organic matters
 Biological decomposition of waste organic matter is a relatively slow
reaction, and gradually depleting the dissolved oxygen in a river as the
water flows downstream.
 Oxygen is replaced by reaeration at the surface and photosynthetic activity
of green plants. The maximum oxygen deficit depends on the
interrelationship of biological oxygen utilization and reaeration.
 Fishes and most aquatic life are stiffed by a lack of oxygen, and unpleasant
tastes and odours are produced if the content is sufficiently reduced.
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Organic matters
 Settleable organic solids can create sludge deposits that
decompose, causing high oxygen demand and intensified odour.
 Floating solids are unsightly and obstruct passage of light vital
to plant growth. Thin films of oil can also reduce the rate of
reaeration.

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Water pollution in Hong Kong
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Inorganic Solids
 Inert suspended solids, such as silt and mine
slurries, produce turbidity that reduces light
penetration and, therefore, interferes with
photosynthesis.
 Solids that settle out of solution blanket the
bottom organisms in a river and hinder the
reproduction cycle of fishes.

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Toxic substances
 Acids, alkalis, and toxic chemicals adversely affect aquatic life
and impair recreational uses of water.
 Sharp change of pH at the discharging point of a river or lake
eliminates less tolerant animal and plant species. It may also induce
considerable toxicity in water. For example, ammonia is much more
toxic in alkaline water (NH3) than acidic one (NH4
+).
 Heavy metals such as mercury are serious pollutants, since they
form stable compounds that persist in nature and are concentrated
in the food chain. The fishing industry has sustained economic
losses in recent years because unacceptable levels of mercury or
other heavy metals were discovered in fishes from contaminated
waters, resulting in government condemnation of the affected
catches.
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Non-toxic salts
 Buildup of salts from domestic wastes and waste brines can interfere with water
reuse by municipal, industries (manufacturing textiles, paper, and food
products), and agriculture for irrigation water. Salts like sodium chloride and
potassium sulfate pass through conventional water and waste-water treatment
plants unaffected.
 Inorganic phosphorus and nitrogen salts induce the growth of algae and
aquatic weeds in surface waters. (Eutrophication)
 The majority phosphates originate from fertilizer washed from agricultural land
and phosphate builders used in synthetic detergents. The latter source
contributes approximately 60 percent of the phosphorus in domestic waste, and
often the majority found in industrial wastes.
 Ammonia nitrogen is extremely soluble and is readily transported by surface
runoff from cultivated farmland. In waste-water treatment, the nitrogen in
organic compounds is released as soluble inorganic nitrogen.
 Removal of nitrogen and phosphorus in conventional biological waste
treatment is generally only 30 to 50 percent.

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Non-aesthetic wastes
 Foam-producing matter and colour, although often not harmful,
lead to an undesirable appearance to receiving water; they are
considered indicator of contamination.
 Taste- and odour-producing compounds interfere with the
palatability of the water for drinking purposes. Their source may
be industrial origin, or may results from blooms of algae
encouraged by nutrient enrichment from waste disposal.
 An increase in water temperature often magnifies the
offensiveness of polluted water. Discharging heated water. such
as cooling water from power plants, accelerates dissolved oxygen
depleting, promotes the growth of blue-green algae, intensifies
tastes and odours, and may stress fish and other aquatic life.
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Typical characteristics of sewage
Constituent Concentration (mg/L) related to wastewater strength
Strong Medium Weak
BOD 400 220 110
COD 1000 500 250
SS 350 220 100
Nitrogen
- Total 85 40 20
- Organic 35 15 8
- Ammonia 50 25 12
- Nitrite 0 0 0
- Nitrate 0 0 0
Phosphorus
- Total 15 8 4
- Organic 5 3 1
- Inorganic 10 5 3
Alkalinity as CaCO3 150 100 50

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Sewage Disposal in Hong Kong
 Everyday, the people of Hong Kong produce some 2.2
million cubic metres of sewage, enough to fill up
1,200 Olympic-size swimming pools.
 About 95% of the population are now served by the
public sewer system with over 98% of the sewage
produced being collected and treated from
preliminary treatment (screening) plants to
secondary treatment (biological) plants treating
sewage from residential, commercial and industrial
sources in the territory prior to disposal to the sea for
dilution and dispersion through submarine outfalls.
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How much of Hong Kong's sewage is
treated, and to what levels?
Preliminary
Treatment
Primary
Treatment
Chemically
Enhanced
Primary
Treatment
Secondary
Treatment
Total
28.7% 0.4% 54.5% 16.4% 100%

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Stonecutter Island (昂 船 洲) - Chemically
Enhance Primary Sedimentation
Stonecutters Island Sewage
Treatment Works Site
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Final Disposal of Treated Effluent
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WATER &
WASTEWATER
TREATMENT
Prof. W. Chu

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Aims of water treatment
 Free from chemicals, microorganisms in amount in
order to prevent hazards in health
 To make it acceptable such as odour and taste
 To lower the content of Fe, Mn to prevent colouring of
cloth and damaging the pumping facilities
 reasonably soft (Ca2+ and Mg2+)
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A typical water treatment process
Coagulation
Flocculation
Sedimentation
Filtration
Disinfection
Raw water
Water Distribution System

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WATER TREATMENT -
Coagulation and
Flocculation
Prof. W. Chu
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Terms of coagulation and
flocculation
 Coagulation refers to the process of destabilization
of the colloid particles by the addition of some
material to the water.
 Flocculation refers to the collision and aggregation
of the destabilized particles into large flocs.
Flocculation describes only the transport step
involving the collision frequency and hydrodynamics
of floc formation after the particles have become
destabilized.

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Coagulation and Flocculation
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80
Settling velocity of particles

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Particle size
1 Å = 0.0001 m
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Colloidal stability
 In most surface water, colloidal surfaces
are negatively charged. The negatively
charged colloid attracts a cloud of positive
ions around it due to electrostatic forces.
 an inner layer including adsorbed ions
and a diffuse layer where the ions are
distributed due to electrical forces and
fluid motions.
 The inner layer is called the Stern layer
and is about the thickness of a hydrated
ion from the surface. Within second
diffuse layer there is a shear plane which
represents the limit to which the counter
ions can be swept from the surface by
fluid motion.
 The ions within the shear plane move with
the particle; those outside of it move
independently of the particle and are
subject to fluid and thermal motions.

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Coagulation theory
 Double-layer compression
 Charge neutralization
 Entrapment (Sweep coagulation)
 Bridging
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Double-layer Compression
• Increase the ion strength to compress the thickness of the double layer
(e.g. 100Å for 0.001 molar but 10Å at 0.1 molar solutions). Rapid
coagulation occurs as Zeta potential is reduced to 20 mV. An example
of ionic layer compression occurs in nature when a turbid stream flows
into the ocean.
• Since the ionic strength depends upon the square of the ionic charge, so
Na+ < Ca2+ < Al3+. In water treatment plants, chemical coagulation is
usually accomplished by the addition of trivalent metallic salts such as
Al2(SO4)3 or FeCl3.

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Charge neutralization
Adsorption of specific ions on the surface of the particulate.
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Entrapment (Sweep coagulation)
 Al3+ + 3H2O  Al(OH)3↓ + 3H+
 The last product formed in the hydrolysis, if alum is used, is
aluminum hydroxide, Al(OH)3 forms in amorphous, gelatinous
flocs that are heavier than water and settle by gravity.
 Colloids may become entrapped in a floc. The solids remain
trapped within the settling floc and appear to be swept from the
water.
 This procedure generates a large amount of wet aluminum or
iron sludges which must be de-watered and disposed.

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Bridging
Synthetic polymers also may
be used instead of, or in
addition to, metallic salts.
These polymers may be linear
or branched and are highly
surface reactive.
Thus, several colloids may
become attached to one
polymer and several of the
polymer-colloid groups may
become enmeshed, resulting
in a settleable mass.
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Coagulants
 (1) Alum coagulants
 - Al2(SO4)3.14H2O
 - Sodium aluminate Na2Al2O4
 (2) Iron coagulants
 - Copperas (FeSO4
.7H2O)
 - Chlorinated copperas
 - FeCl3
 - Fe2(SO4)3
 (3) Polymers
 Polyaluminum chloride (PAC)

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pH and alkalinity
 Al2(SO4)3 + 6H2O  2Al(OH)3 + 6H+ + 3SO4
2-
 The formation of Al(OH)3 will produce acidity, which
should be neutralized by addition of alkalinity (e.g.
CaCO3)
 1 mg/L of Al2(SO4)3 requires 0.5 mg/L of alkalinity.
 For optimum operation, pH is required in the range of
5.0 - 6.0
90
Dosages
 If a water contains insufficient alkalinity, the addition of a metallic coagulant may depress the
pH below the range in which the particular salt is effective. In such circumstances, an
alkaline salt must be added to increase the buffer capacity of the solution. The adequacy of
the alkalinity can be estimated from the following simplified equations for the commonly
used coagulants:
Alum (aluminum sulfate, the most commonly used)
Al2(SO4)318H2O + 3Ca(HCO3)2  2Al(OH)3 + 3CaSO4 + 18H2O + 6CO2
Ferric chloride
2FeCl3 + 3Ca(HCO3)2  2Fe(OH)3 + 3CaCl2 + 6CO2
Ferric sulfate
Fe2(SO4)3 + 3Ca(HCO3)2  2Fe(OH)3 + 3CaSO4 + 6CO2
Ferrous sulfate and lime
FeSO47H2O + Ca(OH)2  Fe(OH)2 + CaSO4 + 7H2O
Fe(OH)2 + O2 + 2H2O  4Fe(OH)3 (in the presence of O2)

46. 46
91
Steps of coagulation-flocculation
process:
1. Add coagulating chemicals
2. A rapid agitative mixing to achieve uniform distribution of chemical for reaction.
(if mixing is not complete, only part of it reacted)
3. Chemical and physico-chemical charge occur leading to coagulation and
formation of microscopic particles.
4. A gentle agitation of the water to cause agglomeration of the microscopic matter
to form settleable floc. (if rapid mixing, the flocs will be broken down again)
92
Rapid mixing

47. 47
93
Velocity gradient (G)
 Design parameters for rapid-mix units are mixing time
t and velocity gradient G (of two fluid particles).
G =
P
V






1 2
/ • G = velocity gradient, s-1
• P = power input, W (N.m/s)
• V = volume of mixing basin, m3
• µ = viscosity, N.s/m2
1
-
s
10
m
0.1
m/s
0
.
1
distance
speed
relative



G
94
Flocculation
• Design parameter for flocculation is Gt, a dimensionless number.
Values of Gt form 104 to 105 are commonly used, with t ranging from
10 to 30 min.
• Large G values with short times tend to produce small, dense flocs,
while low G values and long times produce larger, lighter flocs.
• Since large, dense flocs are more easily removed in the settling basin,
it may be advantageous to vary the G values over the length of the
flocculation basin.
• The small, dense flocs produced at high G values subsequently
combine into larger flocs at the lower G values. Reduction in G values
by a factor of 2 from the influent end to the effluent end of the
flocculator has been shown to be effective.

48. 48
95
Typical design of a
Coagulation/Flocculation Process
 Rapid-mixing tanks operate best at G values from 700 to 1000,
with detention times of approximately 2 min.
• Flocullator has the values of Gt from 104 to 105 are commonly
used, with t ranging from 10 to 30 min.
96
Flocculation tank

49. 49
97
WATER/WASTEWATER
TREATMENT -
Sedimentation
Prof. W. Chu
Sedimentation for Water and
Wastewater Treatment
 Water Treatment: Sedimentation is used to
remove the chemical floc generated in
coagulation-flocculation process by gravity.
It’s critical to lower the loading of the following
filtration process.
 Wastewater Treatment: To remove large
objects and grit to protect from damage the
equipment; To remove 50-70% organic
suspend solids (SS) and 25-40% BOD from
the wastewater in order to reduce the load on
the secondary treatment
98

50. 50
99
Types of Settling
 Type 1. - Discrete settling (Unhindered settling)
 Particles being settled keep their individuality, i.e., they do not
coalesce with other particles. Thus the physical properties of the
particles (size, shape, specific gravity) are unchanged during the
process. The settling of sand particles in grit chambers is a
typical example of discrete settling (e.g. sand grains).
 Type 2. - Flocculent settling
 Agglomeration of the settling particles is accompanied by
changes in density and settling velocity. The sedimentation
occurring in primary clarifiers is an example (e.g. most organic
materials and biological solids)
 Type 3 - Zone settling (Hindered settling)
 Particles from a lattice (or blanket) which settles as a mass
exhibiting a distinct interface with the liquid phase. Examples
include sedimentation of activated sludge in secondary clarifiers
and that of alum flocs in water treatment processes
 Type 4 - Compression settling (Compaction)
 Compaction occurs at bottom of the sedimentation tanks.
100
Ideal Settling Behaviour
(Type 1 - Discrete Settling)
Particle Settling Velocity

51. 51
101
Settling equations
Driving force (Submerged
weight of the particle) can be
expressed as
FG = gravity driving force
p = particle density
 = fluid density
Vp = particle volume
Drag force on a particle is given
by:
FD = drag force
CD = dimensionless drag
coefficient
Ap = projected area of particle
vs = settling velocity of particle
p
p
G gV
F )
( 
 
 F C A
v
D D p
S
 
2
2
102
If FG = FD
 Equating the above expressions after substituting for Ap = ¼ d2
and Vp = 1/6d3 and re-arranging, results in the following
expression for vs.
 In practice, it is found that drag coefficient CD is a function of the
Reynolds Number for spherical particles can be represented by
the following expressions (𝑅𝑒 =
𝜌𝐷𝑣
𝜇
)
 For Re < 1, (Laminar region)
 For 1 < Re < 104, (Transition region)
 For 104 < Re , (Turbulent region)
v
gd
C
S
p
D


4
3
( )
 

Re
24

D
C
CD   
24 3
0 34
1
2
Re Re
.
CD  0 4
.

52. 52
103
Drag coefficient CD
104
Two special cases
Re < 1 (Laminar flow) Re > 104 (Turbulent flow)
Stokes’ Law
CD 
24
Re
CD 0 4
.
v
g
d
S
p


18
2
 

v gd
S
p


3 33
.
( )
 

v
gd
C
S
p
D


4
3
( )
 


53. 53
105
Ideal sedimentation tank
Long-rectangular Basin (Horizontal Flow)
106
Overall removal efficiency
A
Q
v 
0 t
H
v
L
u
o  
0
u
Q
WH

F
h
H
v t
v t
v
v
v
Q A
x
s s s
   
0
0 0 0 /
“Overflow Rate” or
“Surface loading rate”




0
0
0
0
1
)
1
(
f
sdf
v
v
f
F

0
0
0
1
f
sdf
v
v
t0 is the time of particle travel
Fx is fraction removed for particle size with vs
1-f0 is fraction of particles with v > v0
F is the total mass fraction removed
v0
f0
For particle with vs ≧ v0 →100% removed
For particle with vs < v0 → removed with a
fraction Fx

54. 54
107
Example: Settling column analysis of
type 1 suspension
 A settling analysis is run on a type - 1 suspension. The column
is 2 m deep, and data are shown below:
 What will be the theoretical removal efficiency in a settling basin
with a loading rate of 25 m3/m2-d (25 m/d)?
 Solution
 1. Calculate mass fraction remaining and corresponding settling
rates.
Time min 0 60 80 100 130 200 240 420
Conc. mg/L 300 189 180 168 156 111 78 27
Time min 0 60 80 100 130 200 240 420
Conc. mg/L 300 189 180 168 156 111 78 27
Mass fraction
remaining
0.63 0.60 0.56 0.52 0.37 0.26 0.09
v0 x 102
, m/min 3.3 2.5 2.0 1.55 1.0 0.83 0.48
189/300 = 0.63
2/60 x 102 = 3.3
108
Solution
3. Determine v0 = 25 m/d = 1.74 x 10-2 m/min
4. Determine f0 = 54 %
5. Determine Δxvs, by graphical integration.
6. Determine overall removal efficiency:
F = (1- f0) + (∑Δx vs)/v0
= (1 - 0.54) + 0.46/1.74 = 72 %
x (or f) vs xvs
0.06 1.50 0.09
0.06 1.22 0.07
0.1 1.00 0.10
0.1 0.85 0.09
0.1 0.70 0.07
0.06 0.48 0.03
0.06 0.16 0.01
xvs= 0.46

55. 55
109
Settling Column Test for Flocculent
Sedimentation (Type 2)
Draw isoremoval lines
(like contour map)
110
Hindered Settling & Zone Settling-
Type 3

56. 56
111
Batch analysis
112
Compression Settling (Type 4)
 At very high particle concentrations, compression
settling occurs as the settled solids are compressed
under the weight of overlying solids, the void spaces
are gradually diminished, and water is squeezed out
of the matrix.
 Compression settling is important in gravity
thickening processes. It is also particularly important
in activated sludge final settling tanks.

57. 57
113
Sedimentation tanks
 Long-Rectangular Basin
( Horizontal Flow)
 Long-rectangular basins are
commonly used in treatment
plants processing large flows.
This type of basin is
hydraulically more stable, and
flow control through large
volumes is easier with this
configuration.
114

58. 58
115
Design parameters for rectangular
horizontal flow tank
 For Q in m3/d and OFR in m3/m2d,
 Tank Surface Area (m2), A = Q/OFR
 Tank Length (m), L = where  = L/W
 Detention time (hour),
 Forward velocity,
A

A
Q
H
t
/
24

WH
Q
u 
116
Weir overflow rate
Weir loading rate = Q/length of weir
Weir loading rate range from 6 m3/m.h for
light flocs to about 14 m3/m.h for heavier
discrete-particle suspensions are commonly
used.

59. 59
117
Design Criteria of Sedimentation
Tank
 Overflow rates recommended:
 For primary settling followed by secondary treatment--32 to 48
m3/m2·d at average flow, 80 to120 m3/m2·d at peak flow.
 For primary settling with waste activated sludge--24 to 32
m3/m2·d at average flow, 48 to 70 m3/m2·d at peak flow.
 For coagulation/flocculation--20 to 33 m3/m2·d at average flow
 Recommended side water depth:
 3 to 5 m for rectangular clarifiers, 3.6 m typical.
 3 to 5 m for circular clarifiers, 4.5 m typical.
118
Design Criteria of Sedimentation
Tank
 Hydraulic detention times recommended:
 Primary settling followed by secondary treatment: range 1.5 to
2.5 hours, 2.0 hours typical.
 Primary settling receiving waste activated sludge: range 1.5 to
2.5 hours, 2.0 hours typical.
 For coagulation/flocculation : range 2.0 to 8.0 hours
 Weir loading rates recommended:
 Primary settling followed by activated sludge:125 to 500 m3/m·d
at average flow, 250 m3/m·d typical.
 Primary settling receiving waste activated sludge:125 to 500
m3/m·d at average flow, 250 m3/m·d typical.
 For coagulation/flocculation floc:144 to 336 m3/m·d

60. 60
119
Design Criteria of Sedimentation
Tank
 Linear Flow-through Velocity: In practice, the linear flow-through
velocity (scour velocity) has been limited to 1.2 to 1.5 m/min to
avoid re-suspension of settled solids.
 Surface Geometry: To minimize scouring of settled solids,
surface geometry is another design variable that has been used
in attempting to control scouring of solids from high linear flow-
through velocities or wind. Although the length to width ratio of
rectangular tanks has historically been used as such a design
tool, it is not considered to be reliable. Common length-to-width
ratios employed for design range from 3:1 to 5:1.
120
Long-Rectangular Basin

61. 61
121
Removal efficiency vs. surface loading
rate (Q/A) for primary settling tank
122
Example: Designing a long-
rectangular settling basin
 A city must treat about 15,000 m3/d of water. Flocculating
particles are produced by coagulation, and a column analysis
indicates that an overflow rate of 30 m/d will produce
satisfactory removal at a depth of 3.0 m. Determine the size of
the required settling tank.

62. 62
123
Solution:
 1. Compute surface area (provide two tanks at 7500
m3/d each)
 Q = OFR x A
 7500 m3/d = A x 30 m/h
 A = 7500/30 = 250 m2
 2. Selecting a length-to-width ratio of 3/1, calculate
surface dimensions.
 w x 3w = 250 m2
 Width = 9.13, say 9 m
 Length = 27.39, say 27 m
124
Solution:
 3. Check retention time

 t =
 4. Check horizontal velocity
 u = Q/WH =
 5. Check weir overflow rat.
h
3
.
2
h
24
d
1
x
m/d
7500
m
3.0
x
m
27
x
m
9
rate
flow
volume


m/h
6
.
11
m
3.0
x
m
9
24h
d
x
/
m
7500 3

d
h
m
m
m
h
m
m
x
h
d
x
d
m
.
/
6
9
.
30
9
1
24
1
7500 3
3
3



Weir overflow rate range from 6
m3/m.h for light flocs to about 14
m3/m.h for heavier discrete-
particle suspensions are
commonly used.
Total length of weir = 9 m x 5 = 45 m

63. 63
125
Circular Basin ( Radial Flow)
126
A Circular Basin in STW

64. 64
127
Features of circular tanks
 Circular tanks have certain advantages
 Sludge removal mechanisms are simpler and require less maintenance.
 Excessive weir overflow should never be a problem because the entire
circumference is used for overflow.
 Problems
 It is essential that the weir plates be precisely level, since a very slight difference
in elevation will result in considerable short circuiting (direct channelling from
influent to effluent).
 Uneven distribution and wind currents can also cause short circuiting. These
factor make flow control more difficult in circular basins than in long-rectangular
ones.
 Because flow-control problem become more difficult to control as tanks size
increases, it is usually advisable to limit circular tank diameters to 30 m or less.
128
WATER TREATMENT
- Filtration
Prof. W. Chu

65. 65
129
FILTRATION
 Definition
Water filtration can be defined as a physical-chemical process for
separating suspended and colloidal impurities from water by
passage through a bed of granular material.
 Use of filtration
In water treatment plants, filtration is most often used as:
 - A polishing step to remove small flocs or precipitant particles not
removed in the settling of coagulated or softened waters.
 - Protect granular activated carbon against fouling, and increase
carbon adsorption efficiencies by reducing the load of applied
organics.
 -Be combined with biological denitrification by using the fine grains
of the filter bed as an attached growth medium.
130
Function of filtration in water treatment plants
1. Remove particulate and colloidal matter not settleable after
either biological or chemical flocculation or both.
2. Increase removal of suspended solids, turbidity, phosphorus,
BOD, COD, heavy metals, asbestos, bacteria, virus, and other
substances.
3. Improve the efficiency and reduce the cost of disinfection
through removal of suspended matter and other interfering
substances.
4. Assure continuous plant operation and consistent effluent
quality. Increase overall plant reliability by overcoming
common irregularities in biological and chemical treatment.

66. 66
131
Slow sand filter
132
Particle size of filter media
 Effective size: Size of the opening through
which 10% sand by weight will pass
 Uniformity coefficient: Size of the opening
through which 60% of the sand by weight will
pass divided by the effective size D60/D10.
The smaller the coefficient, the more uniform
the grade of sand.

67. 67
133
Typical data in a slow sand filter
 effective size: 0.2 - 0.4 mm (0.3 mm)
 uniformity coefficient:1.7 - 3.0
 Filtration rate: 2 - 5 m/d
134
Disadvantages of slow sand filters
 Low sand filters have large space requirement and are capital-
intensive; additionally, they do not function well with highly turbid
water since the surface clogs quickly, requiring frequent
cleaning.
 Cleaning was accomplished periodically (usually no more
frequently than once a month): draining the filters and
mechanically removing the top few centimeters of sand, along
with the accumulated solids and the biological mat.
 Use of slow sand filters has declined because of their high
construction cost, large filter area needed, and unsuitability for
treating highly turbid and polluted waters requiring chemical
coagulation.

68. 68
135
Rapid sand filter
136
Operation of a rapid sand filter
 The most common type of device for treating municipal water supplies
is the rapid sand filter, which removes nonsettleable flocs and
impurities remaining after chemical coagulation and sedimentation of
the raw water.
 The rapid sand filter utilizes a bed of silica sand ranging from 0.6 to
0.75 m in depth. Sizes may range from 0.35 to 1.0 mm or even larger,
but normally with sizes from 0.45 to 0.55 mm.
 Common filtration rates in rapid sand filters range from 2.5 to 5.0 m/h.
 An important feature of the rapid sand filter is that it is cleaned by
hydraulic backwashing which resulting in stratification of the medium.
 By careful selection of media with regard to size and density, it is
possible, to approximate this reverse gradation. Dual-media filters do
this to some extent, and mixed-media filters essentially approximate
reverse gradation.

69. 69
137
Filters with different arrangement of
media
A B C D
138
Filtration
mechanism
(a) Straining
(b) Sedimentation
(c) Interception
(d) Adhesion
(e) Flocculation

70. 70
139
Filtration efficiency affected by filter
media
 Traditionally, silica sand has been the medium most commonly used in
granular-medium filters. Modern filter applications often make use of
anthracite coal and garnet sand in place of, or in combination with,
silica sand. the important properties of these materials are size, size
distribution, and density.
 In general, filter efficiency increases with smaller grain size, lower
porosity, and greater bed depth.
 Coarse to fine (down-flow) filters contain much more storage space for
materials removed from the water and permit the practical use of much
finer materials in the bottom of the bed.
 Dual media, which should be considered merely an intermediate step in
the development of mixed media, is less resistant to breakthrough than
a rapid sand filter, while mixed media is more resistant than either.
140
Filtration efficiency affected by
water characteristics
 The turbidity of the effluent from a properly operating filter
should be less than 0.5 NTU (Nephelometric Turbidity Units).
With proper pretreatment, filtered water should be essentially
free of color, iron, and manganese.
 Large microorganisms, including algae, diatoms, and amoebic
system are readily removed from properly pretreated water by
filtration.
 Filtration is employed for the removal of finely divided
suspended material carrying over from secondary clarification
or chemical precipitation units. Since PO4, COD, and BOD may
also present in suspended form, removal part of these
constituents by filtration is possible.
Turbidity standards of 5,
50, and 500 NTU

71. 71
141
Filtration efficiency affected by
water characteristics
 Temperature
 Cold water is notably more difficult to the filter than warm water,
but usually there is no control over water temperature.
 Filterability
 Filterability, related to the nature, size and adhesive qualities of
the suspended and colloidal impurities in the water, is the most
important property.
 By recording filter effluent turbidity, appropriated adjustments
can be made in chemical treatment of filter influent to obtain
optimum filterability in the plant filters.
 Maximum filterability is generally more important than maximum
turbidity reduction to generate a reasonable water .
142
Filterability
 Filterability index (F) =
t
VC
HC
0
H = head loss through sand filter
C = average filtrate quality
C0 = input suspension quality
V = flow velocity approaching the top of the bed
t = duration of filter run
The filterability index is dimensionless and the value of C/Co is simply a ratio
relating input and filtrate water quality in some way (e.g. turbidity or suspended
solids level).
A lower value of F indicates a better filterability.

72. 72
143
Filter media arrangement
Before backwash After backwash
144
Dual-media filters
Dual-media filters are usually constructed of silica sand and anthracite coal.
Dual-media filters thus have the advantage of more efficiency in utilizing pore space
for storage. This results in longer filter runs and greater filtration rates because of
lower head losses.
A disadvantage of dual-media filters is that the filtered material (dirt) is held rather
loosely in the anthracite layer. Any sudden increase in hydraulic loading dislodges the
dirt and transports it to the surface of the sand layer, resulting in rapid blinding at this
level.

73. 73
145
Mixed-media filters
146
A typical installation
Media Fraction Specific
gravity
Effective
size
Anthracite 60% 1.6 1.0 mm
Silica sand 30% 2.6 0.4 mm
Garnet sand 10% 4.2 0.15 mm

74. 74
147
Dual-media filter
148
Filter operation
 The two basic modes of operating granular-
medium filters are:
 (1) constant head-variable flow and
 (2) constant flow-variable head.
 More recent design of larger filter plants
usually makes use of a combination of the
above modes of operation.

75. 75
149
Filter operation
 A constant flow is delivered to a bank of several filters through a common
header and is allowed to distribute itself according to the operating rate of each
individual filter.
 The height of the water column is the same above all the filter units, with the
cleanest filter accepting the greatest flow.
 When the flow rate though any one unit decreases to a predetermined level, that
filter is taken off-line and backwashed.
 Removal of one filter results in an increase in flow to the remaining filters, with a
subsequent increase in head and flow rate through each filter.
 When backwashing is completed, the newly cleaned filter is returned to service
and will accommodate a larger flow rate.
 Water level will therefore drop slightly in all the filters, resulting in a decrease
flow through each filter.
150
WATER TREATMENT
- Disinfection
Prof. W. Chu

76. 76
151
DISINFECTION
 Disinfection is used in water treatment to reduce
pathogens (disease-producing microorganisms) to an
acceptable level.
 Disinfection is not the same as sterilization.
Sterilization implies the destruction of all living
organisms. Drinking water need not be germ-free.
 Three categories of human enteric pathogens are
normally of consequence: bacteria, viruses, and
amoebic cysts. Purposeful disinfection must be
capable of destroying all three.
152
Water disinfectants
 They must destroy the kinds and numbers of pathogens that may be introduced
into water within a practicable period of time over an expected range in water
temperature.
 They must meet possible fluctuations in composition, concentration, and
condition of the waters or wastewaters to be treated.
 They must be neither toxic to humans and domestic animals nor unpalatable or
otherwise objectionable in required concentrations.
 They must be dispensable at reasonable cost and safe and easy to store,
transport, handle, and apply.
 Their strength or concentration in the treated water must be determined easily,
quickly, and (preferably) automatically.
 They must persist within disinfected water in a sufficient concentration to provide
reasonable residual protection against its possible recontamination before used ,
or because this is not a normally attainable property, the disappearance of
residuals must be a warning that recontamination may have taken place.

77. 77
153
Disinfection with Chlorine –
Chlorination
 The most common chlorine compounds used in wastewater
treatment plants are:
 chlorine gas (Cl2) - Gas
 calcium hypochlorite (Ca[OCl]2) - Powder
 sodium hypochlorite (NaOCl) - Liquid
 Calcium and sodium hypochlorite are most often used in very
small treatment plants, such as package plants, where simplicity
and safety are far more important than cost.
154
Chemistry of chlorination
 When chlorine in the form of Cl2 gas is added to water, two
reactions take place: hydrolysis and ionization.
 Hydrolysis may be defined as:
 Cl2 (g) + H2O  H+ + HOCl +Cl- (Hypochlorous acid)
 Ionization may be defined as:
 HOCl  H+ + OCl- (Ka = 3 x 10-8) (Hypochlorite ion)
 The sum of HOCl and OCl- is called the free available chlorine
and is the primary disinfectant employed.

78. 78
155
Hypochlorite salts
 Reactions are as follows:
 Ca(OCl)2 + 2H2O  2HOCl + Ca(OH)2
 NaOCl + H2O  HOCl + NaOH
(where HOCl  OCl- + H+)
𝐾𝑎 =
𝑂𝐶𝑙− [𝐻+]
[𝐻𝑂𝐶𝑙]
and 𝑝𝐾𝑎 = −𝑙𝑜𝑔𝐾𝑎
156
Dissociation of chlorine is pH
dependent
The relative distribution of these two
species is very important because the
killing efficiency of HOCl is about 40 to
80 times that of OCl-.
Thus, chlorine exists predominantly as
HOCl at pH levels between 4.0 and 6.0.
Below pH 1.0, depending on the chloride
concentration, the HOCl reverts back to
Cl2.
Above pH 7.52, hypochlorite ions (OCl-)
predominate. Hypochlorite ions exist
almost exclusively at levels of pH around
9 and above.
pKa = 7.52

79. 79
157
Mechanism of chlorination
 At low concentration, chlorine probably kills microorganisms by
penetrating the cell and reacting with the enzymes and
protoplasm.
 At higher concentration, oxidation of the cell wall will destroy the
organism.
 Hypochlorous acid (HOCl) is the more effective than the
hypochlorite ion (OCl-) by approximately two orders of
magnitude.
 Microorganism kill by disinfectants is assumed to follow the CT
concept, that is, the product of disinfectant concentration (C)
and time (T) yields a constant. CT is widely used as a criteria for
disinfection.
158
Factors that affect disinfection
efficiency of chlorine
1. Form of chlorine
2. pH
3. Concentration
4. Contact time
5. Type of organism
6. Temperature

80. 80
159
Chloramines
 The reaction of chlorine with ammonia are of great significance
in water processes as follows:
 NH3 + HOCl  NH2Cl (monochloramine) + H2O
 NH2Cl + HOCl  NHCl2 (dichloramine) + H2O
 NHCl + HOCl  NCl3 (trichloramine) + H2O
 Chlorine that exists in water in chemical combination with
ammonia or organic nitrogen compounds is defined as
combined available chlorine.
 The proportion of monochloramine, dichloramine and
trichloramine formed depends on the molar ratio of chlorine to
ammonia and the pH of the water.
160
Combined residual chlorination
 Combined available chlorine forms have lower oxidation potentials than
free available chlorine forms and, therefore, are less effective as
oxidants. Moreover, they are also less effective disinfectants. In facts,
about 25 times dosage is necessary to obtain equivalent bacterial kills
under the same conditions of pH, temperature and contact time.
 The use of combined chlorine as disinfectant has been encouraged by
the evidence that free chlorine contributes to the production of THM
and that chloramines, being less reactive, are less likely to create these
compounds
 Although combined chlorine residual is not a good disinfectant, it has
an advantage over free chlorine residual in that it is reduced more
slowly and, there, persist for a longer time in the distribution system.

81. 81
161
Application of combined residual
chlorination
1. If the water contains sufficient ammonia to produce with added
chlorine a combined available chlorine residual of the desired
magnitude, the application of chlorine alone suffices.
2. If the water contains too little or no ammonia, the addition of
both chlorine and ammonia is required.
3. If the water has existing free available chlorine residual, the
addition of ammonia will convert the residual into combined
available residual chlorine. A combined available chlorine
residual should contain little or no free available chlorine.
162
Free residual chlorination
 If the water contains no ammonia (or other
nitrogenous materials), the application of
chlorine will yield free residual.
 If the water does contain ammonia that
results in the formation of a combined
available chlorine residual, it must be
overcome by applying an excess of chlorine.

82. 82
163
Breakpoint chlorination
164
Application of breakpoint chlorination
 The main reason for adding enough chlorine to obtain
a free chlorine residual is that an effective disinfection
can be ensured.
 The amount of chlorine that must be added to reach
a desired level of residual is called the chlorine
demand.
 The point at which the concentration begins to
increase again is called the breakpoint, and the
dosage required to reach that point is called the
breakpoint dosage

83. 83
165
Chlorine contact tank and chlorine
storage
Chlorine gas cylinder
166
Example: Estimation of required
chlorine residuals
 Estimate the chlorine residual that must be
maintained to achieve a coliform count equal to or
less than 200/100 mL in an effluent from an
activated-sludge treatment facility, assuming that the
effluent contains a coliform count of 107/100 mL. The
specified contact time is 30 min.

 What will be the required residual to meet the
specified effluent coliform count for a peak hourly
flowrate with a factor of 2.75?

84. 84
167
Solution:
 Determine the chlorine residual need to meet the effluent
discharge requirement:
 Nt/N0 = (1 + 0.23 CT)-3 (Collins, 1970)
 CT = 155.8 mg/L.min
 For a value of equal to 30 min, C = 155.8/30 = 5.2 mg/L
 Determine the residual for the peak hourly flowrate
 Cp = 5.2 x 2.75 = 14.3 mg/L
168
Toxicity of chlorine residuals
 Chlorination is one of the most commonly used methods for the
destruction of pathogenic and other harmful organisms that may
endanger human health.
 Many organic compounds in wastewater may react with the
chlorine to form toxic compounds that can have long-term
adverse effects on the beneficial uses of the waters to which
they are discharged.
 To minimize the effects of these potentially toxic chlorine
residuals on the environment, it has been found necessary to
dechlorinate wastewater treated with chlorine.

85. 85
169
Dechlorination with sulfur dioxide
 Sulfur dioxide gas successively removes free chlorine,
monochloramine, dichloramine, nitrogen trichloride, and poly-n-
chlor compounds.
 Reactions with chlorine:
 SO2 + H2O  HSO3
- + H+
 HOCl + HSO3
-  Cl- + SO4
2- +2H+ (HSO3
-: hydrogen sulfide)
 SO2 + HOCl + H2O  Cl- + SO4
2- + 3H+ (SO4
2- : sulfate)
 Reactions with chloramines:
 SO2 + H2O  HSO3
- + H+
 NH2Cl + HSO3
- + H2O  Cl- + SO4
2- + NH4
+ + H+
 SO2 + NH2Cl + 2H2O  Cl- + SO4
2- + NH4
+ + 2H+
170
Practice of using sulfur dioxide
 1.0 mg/L of sulfur dioxide will be required for the dechlorination
of 1.0 mg/L of chlorine residue (expressed as Cl2).
 Because the reactions of sulfur dioxide with chlorine and
chloramines are nearly instantaneous, contact time is not
usually a factor and contact chambers are not used, however,
rapid and positive mixing at the point of application is an
absolute requirement.
 Excess sulfur dioxide dosages should be avoided not only
because of the chemical wastage but also because of the
oxygen demand exerted by the excess sulfur dioxide.
 HSO3
- + 0.5O2  SO4
2- + H+

86. 86
171
Dechlorination with activated carbon
 Carbon adsorption for dechlorination provides complete removal
of both combined and free residual chlorine.
 Reactions with chlorine:
 C + 2Cl2 + 2H2O  4HCl + CO2 (or 4H+ + 4Cl- + CO2)
 Reactions with chloramines:
 C + 2NH2Cl + 2H2O  CO2 + 2NH4
+ + 2Cl-
 C + 4NHCl2 + 2H2O  CO2 + 2N2 + 8H+ + 8Cl-
172
Practice of using activated carbon
 Because granular carbon in column applications has
proved to be very effective and reliable, activated
carbon can be considered where dechlorination is
required.
 However, this method is quite expensive. It is
expected that the primary application of activated
carbon for dechlorination will be in situations where
high levels of organic removal are also required.

87. 87
173
Ozonation
 Ozone is a pungent-smelling, unstable gas. It is a form of
oxygen in which three atoms of oxygen are combined to form
the molecule O3. Because of its instability, it is generated at the
point of use.
 O3 + H2O  HO3
+ + OH۰ (Hydroxyl Radical at high pH)

 Ozone is an extremely reactive oxidant, and it is generally
believed that bacterial kill through ozonation occurs directly
because of cell wall disintegration (cell-lysis).
 Ozone is widely used in drinking water treatment in Europe and
is continuing to gain popularity in the US. It is a powerful oxidant,
more powerful even than hypochlorous acid. It has been
reported to be more effective than chlorine in destroying viruses
and cysts.
174
CT values for 99.99% giardia cyst inactivation
Temperature (C) 0.5 5 10 15 20 25
Chlorine Dioxide 81 54 40 27 21 14
Ozone 4.5 3 2.5 2 1.5 1
Chloramines 3800 2200 1850 1500 1100 750
In addition to being a strong oxidant, ozone has the advantage of not
forming THMs or any of the chlorinated DBPs.
Ozone residuals can be acutely toxic to aquatic life forms.
However, because ozone dissipates rapidly, ozone residuals will normally
not be found by the time the effluent is discharged into the receiving water.

88. 88
175
Application of ozone for disinfection
176

89. 89
177
Ultraviolet Radiation
 Ultraviolet (UV) light is in the range 200 to 390 nm.
 For example: A mercury vapor arc lamp that emits UV at 254 nm

 Ultraviolet light is a physical rather than a chemical disinfecting
agent.
 Radiation with a wavelength of around 254 nm penetrates the
cell wall of the microorganism and is absorbed by cellular
materials including DNA and RNA, which either prevents
replication or causes death of the cell to occur.
 Water must be relatively free from turbidity
178
Ultraviolet Radiation
 For practical purposes, the inactivation of bacteria by UV
radiation can be described using first-order kinetics. (dC/dt = -kC)
 Because ultraviolet light is not a chemical agent, no toxic
residuals are produced. At present, disinfection with ultraviolet
light is considered to have no adverse or beneficial
environmental impacts.
 The depth of light penetration still limits the liquid film thickness
around each lamp to about 50 to 80 mm. Multiple lamps are
used to provide greater coverage.
 Its major disadvantages are that it leaves no residual protection
for the distribution system and it is more expensive than
chlorination.

90. 90
179
UV disinfection at Shek Wu Hui STW
180
Typical Water Treatment Plant in HK

91. 91
181
 Exercise 1. The chlorine residuals measured when various dosages of chlorine
were added to a wastewater are given below. Determine (a) the breakpoint dosage
and (b) the design dosage to obtain a residual of 0.75 mg/L free available chlorine.
Dosage, mg/L 0.1 0.5 1.0 1.5 2.0 2.5 3.0
Residual, mg/L 0.0 0.4 0.8 0.4 0.4 0.9 1.4
Solution: Breakpoint dosage = 1.75 mg/L;
Design dosage = 2.6 mg/L
182
 Exercise 2. Determine the amount of activated carbon that
would be required per year to dechlorinate treated effluent
containing a chlorine residual of 5 mg/L (as Cl2) from a plant
with an average flowrate of 2500 L/d. What dosage of sulfur
dioxide would be required?
 Solution: C + 2Cl2 + 2H2O  4HCl + CO2
Carbon required per year = 5 mg/L /(2x71) x 12 x 2500 L/d x 365 x 1/106 mg/kg
= 0.385 kg/y
Cl2 + H2O  HOCl + HCl
SO2 + HOCl + H2O  Cl- + SO4
2- + 3H+
Cl2 + SO2 + 2H2O  2Cl- + SO4
2- + 4H+
SO2 dosage = 5 mg/L x (64/71) = 4.5 mg/L

92. 92
183
BIOLOGICAL TREATMENT
(Secondary Treatment - mainly for WW)
Prof. W. Chu
184
Secondary Treatment Processes
Secondary treatment
(Biological treatment)
To remove 40 - 50% of the original
suspend solids and most of the
original dissolved organics and
inorganics in order to meet the
minimum standards for
discharge.

93. 93
185
 Various types of microorganisms are active in the
breakdown of organic matter and resulting a
stabilization of organic waste.
 Aerobic organisms require oxygen for their
metabolic processes.
 Anaerobic organisms function in the absence of
oxygen and obtain their energy from organic
compounds.
 Facultative organisms can function aerobically in
the presence of oxygen or anaerobically in the
absence of oxygen.
 The main types of microorganisms encountered in
wastewater treatment are bacteria, protozoa and
algae.
Microorganism in wastewater treatment
186
Mechanism of biological reaction
 In aerobic biological treatment system the
reactions occurring are:
 Organics (BOD) + O2 + N + P cells + CO2 + H2O
 Cells + O2 CO2 + H2O + N + P
microorganism
 





 
 ism
microorgan
Microorganism ↑
BOD ↓
DO ↓
pH 6-8

94. 94
187
Food-to-microorganism ratio
 F/M = daily
per
total mass of substrate applied
unit biomass
The food-to-microorganism ratio is expressed as
the daily total mass of substrate applied per unit
biomass and is widely used as a measure of the
average process loading for biological treatment
systems.
188
Cell growth
 rg = rate of bacterial growth (mass/volume.time)
 µ = specific growth rate (time-1) – 1st order
 X = concentration of microorganism, (mass/volume)
 S = substrate (mass/volume), e.g. BOD
 Ks = half rate constant (mass/volume)
dX
dt
r X
g
   



maxS
K S
s
dX
dt
SX
K S
s


max

95. 95
189
Cell yield
 where = the rate of food utilization (mg/l)
 Y = decimal fraction of food mass converted to
biomass ( )
dX
dt
Y
dS
dt
 
dS
dt
mg L
/ biomass
mg / L food utilized
190
Food utilization
 The factor Y varies depending on the metabolic
pathway used in the conversion process.
 Typical values of Y for aerobic reactions are about
0.4 to 0.8 kg biomass per kg of BOD5, while
anaerobic reactions range from 0.08 to 0.2 kg
biomass per kg of BOD5.
)
(
1 max
S
k
Y
SX
dt
dX
Y
dt
dS
s 






96. 96
191
Net sludge growth rate
 In practice, the above equation is incomplete without considering
the reduction of biomass through endogenous respiration.
Endogenous decay is also taken to be first order in biomass
concentration.
 kd = endogenous decay rate (time-1) – 1st order
dX
dt
SX
K S
k X
s
d



max
dX
dt
SX
K S
s


max
192
Sludge age
 Sludge Age (SA)
 (or Solids Retention Time (SRT) or Mean Cell
Residence Time (MCRT) )
 i.e. The time required to refresh the whole
volume of sludge in the reactor
dt
dX
X
C 



1

97. 97
193
Classification of treatment processes
 Suspended-growth processes are the biological
treatment processes in which the
microorganisms responsible for the conversion of
the organic matter or other constituents in the
wastewater gases and cell tissue are suspended
within the liquid.
 Attached-growth processes are 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 attached
to some inert medium such as rock, slag, or
specially designed ceramic or plastic materials.
Attached growth treatment processes are also
known as fixed-film processes.
194
Activated Sludge Process
Components:
a. Aeration tank (bio-reactor)
b. Aeration system
c. Final sedimentation tank (Solid-liquid separation)
e. Return activated sludge system
f. Excess activated sludge withdrawal system (to sludge
treatment/disposal)

98. 98
195
Operational principle
 In practice, wastewater flows continuously into an aeration tank where air is injected to mix the activated
sludge with the wastewater and to supply the oxygen needed for the organisms to break down the organics.
 The mixture of activated sludge and wastewater in the aeration tank is called mixed liquor.
 The mixed liquor flow from the aeration tank to a secondary classifier where the activated sludge is settled out.
 Most of the settled sludge is returned to the aeration tank (and hence is called return sludge) to maintain a
proper population of microbes (F/M ratio) to permit rapid breakdown of the organics.
 Because more activated sludge is produced than is desirable in the process, some of the return sludge is
wasted to the sludge handling system for treatment and disposal.
Mixed liquor suspended solids
(MLSS)
196
Operational principle
 In conventional activated sludge systems, the wastewater is typically aerated for 6 to 8
hours in long, rectangular aeration basins.
 About 8 m3 of air is provided for each m3 of wastewater treated. Sufficient air is provided
to keep the sludge in suspension. The air is injected near the bottom of the aeration tank
through diffusers or by surface aerators).
 The volume of sludge returned to the aeration basin is typically 20 to 30 percent of the
wastewater flow.
 The activated sludge process is controlled by wasting a portion of the microorganisms
each day in order to maintain the proper amount of bacteria to efficiently degrade the
BOD5.
 A balance is then achieved between growth of new organisms and their removal by
wasting. If too much sludge is wasted, the concentration of bacteria in the mixed liquor
will become too low for effective treatment. If too little sludge is wasted, a large
concentration of bacteria will accumulate and, ultimately, overflow the secondary tank
and flow into the receiving water body.

99. 99
197
Plug Flow
BOD (Oxygen demand)
Oxygen supply
Influent Effluent
Under
supplied
Over
supplied
198
Complete Mixed
BOD (Oxygen demand)
Oxygen supply
Influent Effluent
BODi BODe

100. 100
199
Main design parameters
 Hydraulic Retention Time (HRT) and Aeration period
 Aeration period is calculated in the same manner as detention time as below:
𝜃 𝑡 =
𝑉
𝑄
 where  (or t) = aeration period or detention time (time)
 V = volume of aeration tank (volume)
 Q = flow rate (volume/time)
 BOD loading
 BOD loading is usually expressed in terms of grams BOD applied per day per
cubic meter of liquid volume in the aeration tank

 BOD loading =
settled wastewater BOD per day
of aeration tank
volume
(g/m3.d)
200
Food-to-microorganism ratio (F/M)
 The food-to-microorganism ratio is a way of expressing BOD
loading (or BOD removed in some text books) with regard to the
microbial mass in the system.

𝐹
𝑀
=
𝑄×𝐵𝑂𝐷
𝑉×𝑀𝐿𝑆𝑆
(or
𝐹
𝑀
=
𝑄×(𝑆0−𝑆)
𝑉×𝑀𝐿𝑆𝑆
)
 Where So = BOD in influent
 S = BOD in effluent
 F/M = food-to-microorganism ratio, grams of BOD
 per day per gram of MLSS
 Q = wastewater flow, cubic meters per day
 BOD = applied BOD, grams per cubic meter
 V = liquid volume of aeration tank, cubic meters
 MLSS = mixed liquor suspended solids
(d-1)

101. 101
201
Example:
 The designed average daily flow of an extended aeration
package sewage treatment plant is 500 m3/day with an average
influent BOD5 of 300 mg/L. The design F/M ratio is 0.1/d and the
operating MLSS is 3,000 mg/L. Determine the volume of
aeration tank and BOD loading of aeration tank.
 Solution:
 V =
 BOD loading = =
V
QBOD 500
500
300
m x 300 g / m
m
g / d / m
3 3
3
3
/ d

3
3
3
3
500
g/m
3000
x
/d
1
.
0
g/m
300
x
/
m
500
)
/
(
m
d
MLSS
M
F
QBOD


202
Sludge age or mean cell residence time
 Sludge age (c) =
 where c = sludge ate or mean cell residence time, days
 SSe = suspended solids in wastewater effluent, mg/L
 SSw = suspended solids in waste sludge, mg/L
 Qe = quantity of wastewater effluent, m3/d
 Qw = quantity of waste sludge, m3/d
MLSS
SSe
x V
x Q + SS x Q
e w w

102. 102
203
Typical data of operational parameters
for activated sludge process
BOD
loading
(g/m3.d)
F/M
(1/d)
c
(d)
Aeration
period
(h)
Return sludge
rates
(%)
BOD
removal
(%)
500-600 0.2-0.5 5-15 6.0-7.5 20-40 80-90
204
Kinetics in completely mixed with
recycle system

103. 103
205
Assumptions;
1. The influent and effluent biomass concentrations
are negligible compared to biomass at other
points in the system.
2. The influent food concentration S0 is immediately
diluted to the reactor concentration S
because of the complete-mix regime.
3. All reactions occur in the reactor; i.e., neither
biomass production nor food utilization occurs
in the clarifier.
Mass Balance
206
𝑋 =
𝑐𝑌(𝑆0 − 𝑆)
(1 + 𝐾𝑑𝑐)

104. 104
207
Aeration of activated sludge
 An oxygen concentration of 1.5 to 2.5 mg/l is necessary to
maintain effective treatment, and reduction below this range can
cause problems.
 A suitable design target value when using air at atmospheric
pressure as an oxygen source is 2 mg/l.
208
Mixing requirements
 Although the obvious primary function of an aeration
device is to induce an oxygen transfer into the liquid,
other important functions in activated sludge systems
or aerated lagoon systems is to keep the waste solids
and biological flocs in suspension, and to provide for
mixing of the contents of the aeration tank, thus
blending the feed with the aeration mixture.
 These other functions sometimes influent the layout
of the device, and sometimes are the critical feature
of the design.

105. 105
209
Diffused air aeration
210
Air diffuser
 Porous diffusers produce many bubbles of approximately 2.0 to 2.5 mm in
diameter and is more efficient with respect to oxygen transfer, because of
the large surface area per volume of air. However, head loss through the
small pores necessities greater compression of the air and thus greater
energy requirements.
 non-porous diffuser (coarse bubble diffuser) inject fewer bubbles of a
larger (up to 25 mm diameter) size, which offer less maintenance and lower
head loss, but poorer oxygen transfer efficiencies.

106. 106
211
Mechanical surface aerators
Floating surface aerator
212
Operating requirement of activated
sludge processes
 Organic loading rate, as expressed by F/M ratio, is affected by
both the rates at which organic matter as measured by BOD is
added to the aeration tank, and the mass of MLVSS brought into
contact with that organic matter.
 Both average and peak rates of loading are important to
consistent plant performance, so that care should be taken to
keep flow rates as uniform as possible and to ensure that
primary sedimentation tanks perform efficiently.

107. 107
213
Some operational problems with
activated sludge processes
 Sludge bulking usually results form the growth of filamentous forms of sludge
microorganisms and may be the consequence of either a deficiency in nutrient
concentration, especially nitrogen (high carbohydrate wastes), or a low DO
concentration in the aeration tanks. Poor effluent clarification and loss of sludge
solids in the effluent may result.
Normal Activated Sludge
with Low SVI
Filamentous Bulking
Activated Sludge with High SVI
214
Variation of activated sludge
processes
 Extended Aeration
 Short-term Aeration or High-rate Activated Sludge
 Contact Stabilization Process
 High-purity Oxygen Activated Sludge Systems
 Sequencing Batch Reactor (SBR)
 Intermittent Decanting Extended Aeration (IDEA)
 Oxidation or Stabilization Ponds

108. 108
215
Conventional activated sludge process
BOD
loading
(g/m3.d)
F/M
(1/d)
c
(d)
Aeration
period
(h)
Return sludge
rates
(%)
BOD
removal
(%)
500-600 0.2-0.5 5-15 6.0-7.5 20-40 80-90
216
Modified activated sludge processes

109. 109
217
Extended Aeration
Conventional Extended aeration
Large flows Small flows
HRT 6 – 8 h HRT 18 – 36 h
F/M 0.2 – 0.4 F/M 0.04 – 0.15
Sludge age 5 – 15 d Sludge age > 15 d
MLSS 1500 – 3000 mg/L MLSS 3000 – 6000 mg/L
BOD removed 80 –90% BOD removed 85 – 95%
Extended Aeration is a
completely mixed
process operated at a
long hydraulic
detention time  and
high sludge age c.
218
Application of Extended Aeration
 The process is limited in application to small flow
where its inefficiency is outweighed by its stability
and simplicity of operation.
 Many extended aeration plants are prefabricated
units (“package plants”) which require little more than
foundations and electrical and hydraulic connections.
 In selecting or specifying package plants the
engineer should give careful consideration to the
quality and capacity of pumps, motors, and blowers
(compressors) as well as the capacity of the system.

110. 110
219
• Low rate, suspended growth system
• Can be operated intermittently or continuously.
Continuous operation requires secondary
clarifiers.
• Loading rates = 0.16~0.24 kg BOD/m3·day
• Useful for small communities; but large space
required.
Oxidation Ditch
220
Short-term Aeration or High-rate
Activated Sludge
 Short-term Aeration or High-rate Activated Sludge is
a pre-treatment process similar in application to a
roughing filter. (HRT = 1-2 h)
 Retention times and sludge ages are low, which
leads to a poor quality effluent and relatively high
solids production.
 This process has potential application as the first
stage of a two-stage process designed for biological
nitrification.

111. 111
221
Comparison of 3 activated processes
Parameter High rate Conventional Extended
aeration
F/M High Medium Low
HRT Short Medium Long
SRT Short Medium Long
Reaction rate High Medium Low
Effluent quality Poor Good Better
222
Trickling Filter (TF)
 Rotary distributor & underdrain system
 - Influent wastewater is pumped up a vertical riser to a rotary distributor for spreading
uniformly over the filter surface.
 - Rotary arms are driven by the wastewater flow out of the distributor nozzles.
 - Underdrains carry away the effluent and permit circulation of air.
 Stone-media trickling filter
 - The most common media are crushed rock, slag or field stone (durable, insoluble, and
resistant to spalling)
 - The size range for stone media is 75 - 125 mm diameter
 - Bed depth range is 1.5 to 2 m
 Plastic media
 Plastic media have considerable advantages over traditional stone media
 - The voidage is >90%
 - The surface area per unit volume is 3 - 6 times higher

112. 112
223
Operational principle
 - The wastewater is sprayed over a bed of crushed rock
 - Microbial films on the fixed media are produced.
 - As the wastewater flows over the slime layer, organic
matter and dissolved oxygen are extracted, and
metabolic end products such as carbon dioxide are
released.
 - Dissolved oxygen in the liquid is supplied by absorption
from the air in the voids surrounding the filter media.
 - An important element in trickling filter operation is the
provision for return of a portion of the effluent to flow
through the filter. This practice is called recirculation.
Settled sewage + O2 CO2 + New bacterial cells
Bacteria
 

224
Recirculation
 Recirculation is practiced in trickling filter for the following reasons:
 - To increase contact efficiency by bringing the waste into contact more than once with active
biological material.
 - To dampen variations in loading over a 24-hour period. The recirculated flow neutralize the
strength of the incoming wastewater. Thus, recirculation dilutes strong influent and
supplements weak influent.
 - To raise the DO of the influent
 - To improve distribution over the surface (due to higher flow), thus reducing the tendency to
clog and also reduce filter files.
 - To prevent the biological slimes from drying out and drying during night time periods when
flows may be too low to keep the filter wet continuously.

113. 113
225
Operational parameters
 BOD loading = Settled wastewater BOD/Volume of filter media
 BOD loading =
 Where BOD loading = g/m3.d
 Settled BOD = raw wastewater BOD remaining after primary
 Volume of media = volume of stone in the filters, m3
 The ratio of the returned flow to the incoming flow is called the
recirculation ratio.
 R =
 where R = recirculation ratio
media
filter
of
Volume
BOD
wastewater
Settled
Q
QR
226
Typical Loading for Trickling Filters
High Rate Two Stage
BOD loading
(g/m3.d)
500 - 1500 700 - 1100
Hydraulic loading (m3/m2.d) 10 - 30 10 - 30
Recirculation ratio (R) 0.5 - 3.0 0.5 - 4.0

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227
Trickling filters in STW
228
Bio-tower (i.e. Deep TF)

115. 115
229
Nature of bio-towers
Bio-towers have several advantages over classical trickling filters:
 The porosity and nature of the packing allow greater loading
rates and virtually eliminate plugging problems.
 Increased ventilation minimizes odour problems under most
operating conditions.
 The compact nature of the reactor allows for economical
housing.
Disadvantages include a relatively high pumping cost necessitated
by the large recycle requirement and the head loss through the
deep bed.
230
Design of bio-towers
 The most commonly used formula was proposed by Eckenfelder
and is of the form:

 where Se = effluent substrate concentration, BOD5, mg/L
 So = influent substrate concentration, BOD5, mg/L
 D = depth of the medium, m
 Q/A = hydraulic loading rate, m3/m2.min
 k = treatability constant relating to the wastewater and
 the medium characteristics, min-1
 n = coefficient relating to the medium characteristics
 The values of the treatability constant k range from 0.01 to 0.1.
 Average values for municipal wastewater on modular plastic media are
around 0.06 at 20C. (design based on critical condition: e.g. Winter)
 kT = k20(1.035)T-20
 n
A
Q
kD
e
e
S
S 

0

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231
Biotower/biofilter with recirculation
 Most systems apply recirculation, the equation for a
recirculation system must be modified as follows:

𝑆𝑒
𝑆𝑎
=
𝒆
−
𝒌𝑫
(
𝑸+𝑸𝒓
𝑨
)𝒏
𝟏+𝑹 −𝑹𝒆
−
𝒌𝑫
(
𝑸+𝑸𝒓
𝑨
)𝒏
 Sa = the BOD5 of the mixture of raw and recycled mixture
applied to the medium
 R = ratio of the recycled flow to the influent flow.
(Q+Qr)/A = hydraulic loading rate, m3/m2.min
232
Bio-tower applications

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233
Rotating Biological Contactors (RBC)
Structure and operation of RBC
- A rotating biological contactor (RBC) is constructed of bundles of plastic packing
attached radially to a shaft, forming a cylinder of media.
- The shaft is placed over a contour-bottomed tank
- The media are submerged approximately 40 %
- During submergence, wastewater can enter the voids in the packing
- When rotated out of the tank, the liquid trickles out of the voids between the surfaces
and is replaced by air.
- Altering exposure to organics in the wastewater and oxygen in the air.
- Excess biofilm drops from the media is carried out in the effluent for sedimentation
234
Typical design data for RBC
 The spacing between sheets in the media used is 19 mm for BOD removal and
12 mm for nitrification
 The disks are 3.7 m in diameter and operate at 40 percent submergence
 The operating speed is 1.5 rpm
 The peripheral velocity is 17.4 m/min for a 3.66-m diameter cylinder
 Typical recommendations for domestic wastewater treatment to produce an
effluent of <30 mg/L of BOD and <30 mg/L of SS are:
 Average organic loading is 7.5 g/m2.d of soluble BOD or 15 g/m2.d of total BOD
 Maximum loading on the first stage is 30 g/m2.d of soluble BOD or 60 g/m2.d of
total BOD
 A temperature correction for additional RBC surface area of 15 % for each 2.8C
below a design wastewater temperature of 13C

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235
A rotating biological contact unit
236
Advantage and disadvantage of RBC
 The process appears to be suitable for the treatment of municipal
wastewater; it has a large surface area for biofilm growth. This large
amount of biomass permits shorter contact time, maintains a secondary
treatment standards.
 Recirculating effluent through the reactor is not necessary.
 The sloughed biomass is relatively dense and settles well in the
secondary clarifier. Other advantages include low power requirement
and simple operating procedures.
 Disadvantage of the system include a lack of documented operating
experience, high capital cost, high shaft loading, and sensitivity to
temperature.
 Covers must be provided to protect the media form damage by the
elements and from excessive algal growths. Adequate housing also
helps to minimize temperature problem in colder climates.

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237
SLUDGE TREATMENT
Prof. W. Chu
238
Where are sludges generated?

120. 120
239
Screenings
 Screenings include all
types of organic and
inorganic materials
large enough to be
removed on bar racks.
 The organic content
varies, depending on
the nature of the system
and the season of the
year.
240
Grit
 Grit is usually made up of the heavier
inorganic solids that settle with
relatively high velocities.
 The sand, broken glass, nuts, bolts,
and other dense material that is
collected in the grit chamber is not
true sludge in the sense that it is not
fluid. However, it still requires
disposal.
 Because grit can be drained of water
easily and is relatively stable to
biological activity, it is generally
trucked directly to a landfill without
further treatment.

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241
Scum/grease
 Scum consists of the floatable materials skimmed
from the surface of primary and secondary settling
tanks and from grit chambers and chlorine contact
tanks, if so equipped.
 Scum may contain grease, vegetable and mineral
oils, animal fats, wax, soaps, food waste, vegetable
and fruit skin, hair, paper and cotton, cigarette tips,
plastic materials, condom, grit particles, and similar
material.
 The specific gravity of scum is less than 1.0 and
usually around 0.95.
242
Primary sludge
 Sludge from the bottom of the primary clarifiers
is usually gray, containing from 3 to 8 percent
solids (1 percent solids = 10,000 mg/L) and is
approximately 70% organic.
 This sludge rapidly becomes anaerobic and is
highly odorous.

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243
Secondary sludge
 Activated sludge generally has brown flocculent appearance. If
the colour is dark, the sludge may be approaching a septic
condition. If the colour is lighter than usual, there may have been
under aeration with a tendency for the solids to settle slowly.
 Sludge in good condition has an inoffensive “earthy” odour. The
sludge tends to become septic rapidly and then has a
disagreeable odour of decomposition. Activated sludge will
digest readily alone or when mixed with primary sludge.
244
Secondary sludge
 Humus sludge from trickling filters is brownish, flocculent, and
relatively inoffensive than fresh. It generally undergoes
decomposition more slowly than other undigested sludges.
When trickling-filter sludge contains many worms, if may
become inoffensive quickly and digests readily.
 The secondary sludges are about 90% organic and the solids
content depends on the source.
 Wasted activated sludge is typically 0.5 to 2% solids, while
trickling filter sludge contains 2 to 5% solids.
 In some cases, secondary sludges contain large quantities of
chemical precipitates because the aeration tank is used as the
reaction basin for the addition of chemicals to remove
phosphorus.

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245
Concentrations of different sludges
Source Typicalconcentration,%
Primarysludge, withoutthickening 2-7
Wasteactivatedsludge 0.5-1.5
Wastetricklingfiltersludge 1-5
Digestedsludge 4-10
Dewateredsludge 12-50
Solid content of 1% = 10,000 mg/L
246
Sludge Treatment
 Thickening
 Separation of as much water as possible by gravity or floatation
 Stabilization
 Conversion of organic solids to more inert forms by “digestion”, (so
that they do not create odor or cause oxygen demand)
 Conditioning
 Treatment with heat or chemicals so that water can be more easily
removed
 De-watering
 Separation of as much water as possible by vacuum, pressure or
drying
•Raw sludges are in liquid form and water content needs to be removed
as much as possible.
•If sludges contain high fraction of organic contents, it is not biologically
stable.

124. 124
247
Gravity thickening
 Gravity thickening is a simple and inexpensive process that has been
used widely on primary sludges for many years. It is essentially a
sedimentation process similar to that which occurs in all settling
tanks.
 Purely primary sludge can be thickened from 1-3% to 10% solids.
248
Thickening – Flotation
 Flotation involves separation of
solids from the water phase by
attaching the solids to fine air
bubbles to decrease the density
of the particles so that they float
instead of sinking. The rising
solids are called the "float". The
float is skimmed off the surface
and further processed in the
sludge train.

125. 125
249
Dissolved-air flotation (DAF) system
 In dissolved-air flotation (DAF) systems, air is dissolved in the wastewater under a
pressure of several atmospheres, followed by release of the pressure to the
atmospheric level.
 In small pressure systems, the entire flow may be pressurized by means of a pump to
275 to 350 kPa with compressed air added at the pump suction. The entire flow is
held in a retention tank under pressure for several minutes to allow time for the air to
dissolve.
 In the larger units, a portion of the DAF effluent (15 to 120%) is recycled.
250
Comparison of Flotation with Gravity
Sedimentation
 Capital costs are lower for flotation units than for gravity sedimentation.
For the case of activated sludge, allowable overflow rates for flotation
are about double the values for gravity sedimentation, these resulting in
lower capital costs, because the rose velocity of activated sludge by
flotation exceeds the settling velocity by sedimentation.
 On the other hand, operating costs are usually higher mainly owing to
the cost of power for air compression.
 Higher effluent quality is obtained from flotation units, where solids
removal of the order of 95% or higher are common. Flotation yield can
be considerably improved by addition of coagulants. The most common
coagulants utilized are alum, ferric chloride, and polyelectrolytes.
 The current trend is toward using gravity thickening for the primary
sludges and flotation thickening for activated sludges, and then
blending the thickened sludges for further processing.

126. 126
251
Sludge Treatment: Thickening
 Flotation
 Especially effective on
activated sludge
 Increases solids
content from 0.5 - 1%
to 3-6%
 Gravity thickening
 Best with primary
sludge
 Increases solids
content from 1-3% to
10%
Primary
Sludge
Gravity Thickening
Secondary
Sludge
Flotation
Further processing
252
Sludge Stabilization (Digestion)
Aims:
 inhibit, reduce, or eliminate the potential for
putrefaction
 reduce pathogens, and
 eliminate offensive odours,
Principal methods:
 aerobic digestion;
 anaerobic digestion, and
 composting.

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253
Aerobic Digestion
 Aerobic digestion may be used to treat
 (1) waste-activated sludge only,
 (2) mixtures of waste-activated sludge or trickling-filter sludge and
primary sludge, or
 (3) waste sludge from extended aeration plant.
 Aerobic digestion has been used primarily in plants of a size less than
0.2 m3/s, but in recent years the process has been employed in larger
wastewater-treatment plants with capacities up to 2 m3/s.
254
Aerobic reaction
 Aerobic digestion is similar to the
activated-sludge process. As the supply of
available substrate (food) is depleted, the
microorganisms begin to consume their
own protoplasm to obtain energy for cell
maintenance reactions.
 Cell tissue is oxidized aerobically to carbon
dioxide, water, and ammonia. In actuality,
only about 75 to 80 percent of the cell
tissue can be oxidized; the remaining 20-
25 percent is composed of components
and organic compounds that are not
biodegradable.
 The ammonia is subsequently oxidized to
nitrate as digestion proceeds.
Nonbiodegradable volatile suspended
solids will remain in final product from
aerobic digestion.

128. 128
255
Anaerobic Digestion
 Anaerobic digestion involves the decomposition of organic and
inorganic matter in the absence of molecular oxygen.
 In the anaerobic digestion process, the organic material in
mixture of primary settled and biological sludges is converted
biologically, under anaerobic conditions, to a variety of products
including methane (CH4) and carbondioxide (CO2).
 The process is carried out in an airtight reactor. Sludge,
introduced continuously or intermittently, is retained in the
reactor for varying periods of time.
 The stabilized sludge, withdrawn from the reactor, is reduced in
organic and pathogen content.
256
Reaction Mechanisms
 (1) Hydrolysis Process –
conversion of insoluble high
molecular compounds (lignin,
carbohydrates, fats) to lower
molecular compounds;
 (2) Acidogenesis Process –
conversion of soluble lower
molecular components of fatty
acids, amino acids and sugars
(monosaccharides) to lower
molecular intermediate products
(volatile acids, alcohol,
ammonia, H2 and CO2), and
 (3) Methanogenesis Process –
conversion of volatile acids &
intermediate products to final
product of methane and CO2.

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257
Solids and Hydraulic Retention Times
 Anaerobic digester sizing is based on providing sufficient
residence time in well-mixed reactors to allow significant
desctruction of volatile suspended solids (VSS) to occur.
 Sizing criteria that have been used are (1) solids retention time
(SRT), the average time, the solids are held in the digestion
process, and (2) the hydraulic retention time (HRT), the average
time, the liquid is held in the digestion process.
For digestion systems without recycle SRT = HRT.
 There is a minimum SRT for each reaction. If the SRT is less
than the minimum SRT, bacteria cannot grow rapidly enough
and the degestions process will fail eventually.
258
Temperature
 Most anaerobic digestion systems are designed to operate in
the mesophilic temerature range between 30 and 38°C.
 Other systems are designed for operation in the thermophilic
temperature range of 50 and 57°C.
 While selection of the design operating temperature is
important, maintaining a stable operating temperature is more
important because the bacteria, especially the methane formers,
are sensitive to temperature changes.
 Generally, temperature changes greater than 1°C/d affect
process performance, and thus changes less than 0.5°C are
recommended.

130. 130
259
Single-stage digestion (Standard Rate)
 Standard-rate process does not employ sludge
mixing, but rather the digester contents are allowed
to stratify into zones.
 Sludge feeding and withdrawal are intermittent
rather than continuous.
 Retention time ranges between 30 and 60 days for
heated digesters.
 Organic loading rate is between 0.48 and 1.6 kg
total volatile solids per m3 of digester volume per
day.
 Major disadvantage of the standard-rate process is
the large tank volume required because of long
retention times, low loading rates, and thick scum
layer formation.
 Systems of this type are generally used only at
treatment plants having a capacity of 0.04 m3/s or
less.
260
Two-stage digestion
The high-rate system evolved as a result of continuing efforts to improve the
standard-rate unit, In this process, two digesters operating in series separate
the functions of fermentation and solids/liquid separation.
Retention time: 10-15 d;
Organic loading rate: 1.6-2.2 kg/m3.d

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261
Two-stage digesters
262
Gas production
 Total gas production is usually estimated from the percentage of
volatile solids reduction.
 Typical values vary from 0.75 to 1.12 m3/kg of volatile solids
destroyed.
 Gas production can fluctuate over a wide range, depending on
the volatile solids content of the sludge feed and the biological
activity in the digester.
 Excessive gas production rates sometime occur during startup
and may cause foaming and escape of foam and gas from
around the edges of floating digester covers.

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263
Gas use
 Methane gas at standard temperature and pressure (20°C and 1 atm) has a
low heating value of 35,800 kJ/m3. Because digester gas is only 65 percent
methane, the lower heating value of digester gas is approximately 22,400
kJ/m3.
 In larger plants, digester gas may be used as fuel for boiler and internal-
combustion engines which are, in turn, used for pumping wastewater,
operating blowers, and generating electricity.
 Hot water from heating boilers or from engine jackets and exhaust heat boilers
may be used for sludge heating and for building heating, or gas-fired sludge-
heating boilers may be used.
 Because digester gas contains hydrogen sulfide, nitrogen, particulates, and
water vapor, the gas frequently has to be cleaned in dry or wet scrubbers
before it is used in internal-combustion engines.
264
Sludge Conditioning (Pretreatment of dewatering)
 Chemical conditioning
One of the most commonly used is the addition of coagulants such as
ferric chloride, lime, or organic polymers. Ash from incinerated
sludge has also found used as a conditioning agent.
 In recent years, organic polymers have become increasingly popular for
sludge conditioning. Polymers are easy to handle, require little storage
space, and are very effective. The conditioning chemicals are injected
into the sludge just before the dewatering process and are mixed with
the sludge.

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265
Heat treatment
 Another conditioning approach is to heat the sludge at high
temperatures (175 to 230C) and pressures (1,000 to 2,000
kPa).
 Under these conditions, much like those of a pressure cooker,
water that is bound up in the solids is released, improving the
dewatering characteristics of the sludge.
 Heat treatment has the advantage of producing a sludge that
dewaters better than chemically conditioned sludge.
 The process has the disadvantages of relatively complex
operation and maintenance and the creation of highly polluted
cooking liquors that when recycled to the treatment plant impose
a significant added treatment burden.
266
Sludge dewatering
 Sludge drying beds
 Vacuum filtration
 Filter press
 Centrifugation

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267
Sludge drying bed
 1). Pump 0.20 to 0.30 m of stabilized liquid sludge onto the drying bed surface.
 2). Add chemical conditioners continuously, if conditioners are used, by
injection into the sludge as it is pumped onto the bed.
 3). When the bed is filled to the desired level. allow the sludge to dry to the
desired final solids concentration. (2-3 months)
 4). Remove the dewatered sludge either mechanically or manually.
 5). Repeat the cycle.
268
Sludge drying beds