WATER SUPPLY & WASTEWATER
ENGINEERING
TOPIC 6 :
SEWAGE TREATMENT
Prepared By :
Tuan Noor Laily Tuan Mat
Civil Engineering @ pkb
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Why do we need to
treat wastewater ?
◦ •To prevent groundwater pollution
◦ •To prevent sea shore
◦ •To prevent soil
◦ •To prevent marine life
◦ •Protection of public health
◦ •To reuse the treated effluent For agriculture For groundwater
recharge for industrial recycle
◦ •Solving social problems caused by the accumulation of
wastewater
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6.1.1
CHARACTERISTICS
OF SEWAGE
◦ Sewage treatment is the
process of removing
contaminants from
wastewater and household
sewage, both runoff
(effluents), domestic,
commercial and institutional.
It includes physical,
chemical, and biological
processes to remove
physical, chemical and
biological contaminants. Its
objective is to produce an
environmentally safe fluid
waste stream (or treated
effluent) and a solid waste
(or treated sludge) suitable
for disposal or reuse (usually
as farm fertilizer).
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Physical, chemicals and biological properties of wastewater
Characteristic Sources
Physical properties:
◦ Color Domestic and industrial wastes, natural decay of organic
materials
◦ Odor Decomposing wastewater, industrial wastes.
◦ Solids Domestic water supply, domestic and industrial wastes,
soil erosion, inflow infiltration
◦ Temperature Domestic and industrial wastes
Chemical constituents:
◦ Organic:
◦ Carbohydrates Domestic, commercial, and industrial wastes
◦ Fats, oils, and grease Domestic, commercial, and industrial wastes
◦ Pesticides Agricultural wastes
◦ Phenols Industrial wastes
◦ Proteins Domestic, commercial, and industrial wastes
◦ Priority pollutants Domestic, commercial, and industrial wastes
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Physical, chemicals and biological properties of wastewater
◦ Surfactants Domestic, commercial, and industrial wastes
◦ Volatile organic compounds Domestic, commercial, and industrial wastes
◦ Other Natural decay of organic materials
◦ Inorganic:
◦ Alkalinity Domestic wastes, domestic water supply, groundwater
infiltration
◦ Chlorides Domestic wastes, domestic water supply, groundwater
infiltration
◦ Heavy metals Industrial wastes
◦ Nitrogen Domestic and agricultural wastes
◦ pH Domestic, commercial, and industrial wastes
◦ Phosphorus Domestic, commercial, and industrial wastes natural runoff
◦ Priority polluter Sulfur Domestic water supply; domestic, commercial and industrial
wastes
◦ Gases:
◦ Hydrogen sulfide Decomposition of domestic wastes
◦ Methane Decomposition of domestic wastes
◦ Oxygen Domestic water supply , surface-water infiltration
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Physical, chemicals and biological
properties of wastewater
Biological constituents:
◦ Animals Open watercourses and treatment plants
◦ Plants Open watercourses and treatment plants
◦ Eubacteria Domestic wastes, surface water infiltration, treatment plants .
◦ Archaebacteria Domestic wastes, surface-water infiltration, treatment plant
◦ Viruses Domestic wastes
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Biological
◦ Main groups of Microorganisms:-
◦ The main microorganisms of concern in wastewater treatment are Bacteria, Fungi, Algae,
Protozoa, Viruses, and pathogenic microorganisms groups.
◦ Bacteria:-Types: Spheroid, rod curved rod, spiral, filamentous.
Some important bacteria:-
◦ Pseudomonas:-reduce NO3 to N2, So it is very important in biological nitrate removal in
treatment works.
◦ Zoogloea:-helps through its slime production in the formation of flocs in the aeration tanks.
◦ Sphaerotilus natuns:Causes sludge bulking in the aeration tanks.
◦ Bdellovibrio:destroy pathogens in biological treatment.
◦ Acinetobacter:Store large amounts of phosphate under aerobic conditions and release it
under an –anaerobic condition so, they are useful in phosphate removal.
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Biological
◦ Nitrosomonas: transform NH4into NO2-
◦ Nitrobacter: transform NO2-to NO3-
◦ Coliform bacteria:-The most common type is E-Coli or Echerichia Coli, (indicator for the
presence of pathogens).
◦ E-Coli is measured in (No/100mL)
Fungi:
◦ •Important in decomposing organic matter to simple forms.
Algae:
◦ • Cause eutrophication phenomena. (negative effect)
◦ • Useful in oxidation ponds. (positive effect)
◦ • Cause taste and problems when decayed. (negative effect)
Protozoa:
◦ •Feed on bacteria so they help in the purification of treated waste water.
◦ •Some of them are pathogenic
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Biological
◦ Viruses:
◦ Viruses are a major
hazard to public health.
Some viruses can live as
long as
◦ 41 days in water and
wastewater at 20 oC.
They cause lots of
dangerous diseases.
◦ Pathogenic organisms:
◦ The main categories of
pathogens are:-
◦ Bacteria, Viruses,
protozoa, helminthes
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6.1.2 WASTEWATER TREATMENT
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PRELIMINARY
AND PRIMARY
TREATMENT
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PRELIMINARY TREATMENT
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Secondary Treatment
•Trickling Filter
Introduction
In most wastewater treatment systems, the trickling filter follows primary treatment and includes
secondary settling tank or clarifier as shown below. The process is a fixed film biological treatment
method designed to remove BOD and suspended solids.
The trickling filter consists of several major components including distribution system, media,
underdrains, effluent channel, secondary settling tank, and recirculation pumps and piping. Each of
these components has one or more purposes.
In operation, wastewater is distributed evenly over the surface of the trickling filter media. As the
wastewater flows over the surface of the media the organisms in the slime remove the organic
matter from the flow.
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b. Oxidation Pond
Introduction
Wastewater treatment using ponds can be an economical way of
treatment which produces effluent that is highly purified. The number
and the type of ponds used are the determining factors as to the
degree of treatment that is provided.
Another name for wastewater treatment ponds is waste stabilization
ponds . For this lesson we use waste stabilization ponds because these
ponds help to stabilize the wastewater before it is passed on to
receiving water. They can also be referred to as oxidation
ponds or sewage lagoons .
The waste stabilization pond is a biological treatment process , where
bacteria use organic matter in the wastewater as food. The three
types of bacteria at work in most ponds are the aerobic, anaerobic,
and the facultative bacteria.
Because of unpleasant conditions associated with the anaerobic
decomposition, plant operators must make sure that there is
enough dissolved oxygen (D.O.) in the pond to make sure that it will
be the aerobic and facultative bacteria that will be predominant,
rather than having anaerobic decomposition take place.
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Oxidation Ponds *Aerobic Ponds- Aerobic ponds are generally shallow, less
that 1 m deep, allowing sunlight to pass through to the
• The wastewater stays in treatment longer bottom. The presence of sunlight in the entire depth
ensures that algal photosynthesis is active throughout the
than other systems entire system. During the day, the alga uses the sunlight,
carbon dioxide, and inorganic material provided from
Anaerobic Ponds - Anaerobic ponds function in the
absence of oxygen, sunlight, and nitrogen and rely on
bacteria to grow. In this process, the alga releases large organic loading rather than photosynthesis. The
• A large amount of space is required amounts of oxygen into the ponds. Aeration with wind or
mechanical aerators also can increase dissolved oxygen
anaerobic process can be broken down into three steps.
levels in the water. Aerobic bacteria break down organic
• Potential for foul odors suspended solid waste, and their metabolism uses the
oxygen that the algae provides.
Several different types of oxidation ponds are
commonly used:
Acetogenesis- The component subunits are subject to
Hydrolysis- Anaerobic bacteria hydrolize complex waste to
fermentation and metabolic processes to form volatile
form their component subunits
acids and alcohols.
Facultative Ponds- Facultative ponds (Figure 2) have an
aerobic top portion, a facultative middle zone, and an
Methane Fermentation- Anaerobic bacteria convert the anaerobic bottom. The facultative zone usually is aerobic
end products to methane and other gases. during the daylight, and anaerobic during the night.
Facultative bacteria can use oxygen to oxidize waste, but
can also use nitrogen in the absence of oxygen.
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Activated Sludge
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Rotating biological contactors (RBC)
Rotating biological contactors (RBC) are a conventional aerobic biological wastewater
treatment unit. Conventional biological treatment means activated sludge systems and fixed film
systems such as trickling filters, or RBC (NOLDE 1996). The advantage of all these systems is that they
are compact (i.e. in densely populated urban settings) and that they efficiently reduce organic
matter (JENSSEN 2004). However, they are high-tech and generally require skilled staff for construction
as well as for operation.
Rotating biological contactors (RBC), also called rotating biological filters, are fixed-bed reactors consisting
of stacks of rotating disks mounted on a horizontal shaft. They are partially submerged and rotated
as wastewater flows through. They are used in conventional wastewater treatment plants assecondary
treatment after primary sedimentation of domestic grey- or blackwater, or any
other biodegradable effluent. The microbial community is alternately exposed to the atmosphere and
the wastewater, allowing both aeration and assimilation of dissolved organic pollutants and nutrients for
their degradation.
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Sludge treatment and
disposal In all biological waste treatment processes some surplus
sludge is produced. The objective of residual
The sludges accumulated in a wastewater treatment management is:
process must be treated and disposed of in a safe and • Reduction of water content.
effective manner. The purpose of digestion is to reduce the
amount of organic matter and the number of disease- • Stabilization of sludge solids.
causing microorganisms present in the solids. The most
• Reduction in sludge solids volume.
common treatment options include anaerobic
digestion, aerobic digestion, In facultative type aerated lagoons and algal waste
and composting. Incineration is also used, albeit to a much stabilization ponds, the surplus sludge settles out in the unit
lesser degree.
itself and is removed only once in a few years after emptying
Sludge treatment depends on the amount of solids
the unit, exposing the wet sludge to natural drying, and
generated and other site-specific conditions. Composting carting away the dried sludge for agricultural use or land
is most often applied to small-scale plants with aerobic filling.
digestion for mid-sized operations, and anaerobic
In extended aeration process where aerobic digestion of
digestion for the larger-scale operations.
surplus sludge is done, the sludge can be taken directly for
The sludge is sometimes passed through a so-called pre- dewatering and disposal.
thickener which de-waters the sludge. Types of pre- In case of activated sludge and trickling filter plants, the
thickeners include centrifugal sludge thickeners[ rotary
sludge is taken (along with the primary sludge) to a sludge
drum sludge thickeners and belt filter presses. Dewatered
sludge may be incinerated or transported offsite for digester for further demineralization and thereafter it is
disposal in a landfill or use as an agricultural soil dewatered.
amendment. 28
Sludge Treatment
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Disposal of Sludge
Final disposal of sludge is to land and sometimes to the sea, in one of
the following ways:
• Agricultural use of dried or wet sludge.
• Use of dried sludge as landfill in absence of agricultural demand.
• Spreading wet sludge on eroded or waste land, contouring the field,
so as to gradually build up a top soil of agricultural value.
• Disposing off wet sludge along with solid wastes for (i) composting,
or (ii) sanitary landfill.
• Transporting and dumping into the sea.
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Treated Effluent Disposal
The proper disposal of treatment plant effluent or reuse requirements is an essential part of planning and designing
wastewater treatment facilities. Different methods of ultimate disposal of secondary effluents are discussed as follows.
Natural Evaporation
The process involves large impoundments with no discharge. Depending on the climatic conditions large impoundments
may be necessary if precipitation exceeds evaporation. Therefore, considerations must be given to net evaporation,
storage requirements, and possible percolation and groundwater pollution. This method is particularly beneficial where
recovery of residues is desirable such as for disposal of brines.
Groundwater Recharge
Methods for groundwater recharge include rapid infiltration by effluent application or impoundment, intermittent
percolation, and direct injection. In all cases risks for groundwater pollution exists. Furthermore, direct injection implies
high costs of treating effluent and injection facilities.
Irrigation
Irrigation has been practiced primarily as a substitute for scarce natural waters or sparse rainfall in arid areas. In most
cases food chain crops (i.e. crops consumed by humans and those animals whose products are consumed by humans)
may not be irrigated by effluent. However, field crops such as cotton, sugar beets, and crops for seed production are
grown with wastewater effluent.
Wastewater effluent has been used for watering parks, golf courses and highway medians.
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Recreational Lakes
The effluent from the secondary treatment facility is stored in a lagoon for approximately 30 days. The effluent from the
lagoon is chlorinated and then percolated through an area of sand and gravel, through which it travels for approximately
0.5 km and is collected in an interceptor trench. It is discharged into a series of lakes used for swimming, boating and
fishing.
Aquaculture
Aquaculture, or the production of aquatic organisms (both flora and fauna), has been practiced for centuries primarily for
production of food, fiber and fertilizer. Lagoons are used for aquaculture, although artificial and natural wetlands are also
being considered. However, the uncontrolled spread of water hyacinths is itself a great concern because the flora can clog
waterways and ruin water bodies.
Municipal Uses
Technology is now available to treat wastewater to the extent that it will meet drinking water quality standards. However,
direct reuse of treated wastewater is practicable only on an emergency basis. Many natural bodies of water that are used
for municipal water supply are also used for effluent disposal which is done to supplement the natural water resources by
reusing the effluent many times before it finally flows to the sea.
Industrial Uses
Effluent has been successfully used as a cooling water or boiler feed water. Deciding factors for effluent reuse by the
industry include (1) availability of natural water, (2) quality and quantity of effluent, and cost of processing, (3) pumping
and transport cost of effluent, and (4) industrial process water that does not involve public health considerations.
Discharge into Natural Waters
Discharge into natural waters is the most common disposal practice. The self-purification or assimilative
capacity of natural waters is thus utilized to provide the remaining treatment.
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How a Septic Tank Works
•The heavier, solid particles in the sewage settle to the bottom of the
Your household liquid waste flows from the house into the septic tank tank forming a layer of sludge. Lighter materials, including fat and
where the sewage undergoes a settlement process and is partially grease, float to the surface forming the scum layer.
treated by bacteria. Bacteria living in the septic tank break down some of the organic solids
This partially treated waste then travels from the tank to a disposal or into liquid components, helping to reduce the build-up of sludge in the
absorption field, where it is further broken down by natural processes. tank.
The type of disposal field depends on soil type, the slope of the section, The scum helps prevent odours escaping and stops air entering. The
vegetation and underlying rock. treated effluent flows out of the tank through an outlet pipe as new
waste-water enters.
Your septic system is a living ecosystem where bacteria do the work of
digesting and treating waste. At this stage the effluent still contains large amounts of dissolved
The septic tank retains fats and solids, and releases the liquid portion pollutants including salts and nutrients such as compounds of nitrogen
of the waste. This liquid effluent flows into the disposal field and is and phosphorus.
further treated by the soil. It also contains disease causing pathogens such as viruses, bacteria and
worms.
A healthy septic tank is a well balanced ecosystem where good bacteria
thrive in the right amounts to digest waste and treat the effluent water.
◦ The contents of a healthy septic tank form 3 layers
• A layer of fats called scum, which floats on the surface of the liquid
waste.
• A layer of clear liquid waste called effluent.
• A layer of solids called sludge, which sinks to the bottom
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Imhoff Tank
The Imhoff tank, named for German engineer Karl Imhoff(1876–1965), is a chamber suitable for the reception and
processing of sewage. It may be used for the clarification of sewage by simple settling and sedimentation, along
withanaerobic digestion of the extracted sludge. It consists of an upper chamber in which sedimentation takes place, from
which collected solids slide down inclined bottom slopes to an entrance into a lower chamber in which the sludge is collected
and digested. The two chambers are otherwise unconnected, with sewage flowing only through the upper sedimentation
chamber and no flow of sewage in the lower digestion chamber. The lower chamber requires separate biogas vents and pipes
for the removal of digested sludge, typically after 6-9 months of digestion. The Imhoff tank is in effect a two-story septic
tank and retains the septic tank's simplicity while eliminating many of its drawbacks, which largely result from the mixing of
fresh sewage and septic sludge in the same chamber.
Imhoff tanks are being superseded in sewage treatment by plain sedimentation tanks using mechanical methods for
continuously collecting the sludge, which is moved to separate digestion tanks. This arrangement permits both improved
sedimentation results and better temperature control in the digestion process, leading to a more rapid and complete digestion
of the sludge
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Super Sept System Step 3: Circulation
Inner chamber contents circulate through
Step 1: Influent access points into the top of an outer,
Chamber contains plastic media settlement chamber. The contents slowly
designed to provide a large surface sink and re-enter the inner chamber at the
area to which naturally occurring system’s bottom where bacteria continue
micro-organisms in the influent organic matter breakdown. Process repeats
become attached. effectively breaking down 100% of all organic
waste.
Step 4: Discharge
Step 2: Aeration After 100% of the waste has been
An air blower mounted on a plinth at broken down by the micro-organisms,
ground level pumps air into a the polluting strength of the wastewater
diffuser located in the inner and the total solids that remain in
chamber. The extra oxygen suspension will have been reduced 40-
accelerates the growth of naturally fold, and the effluent is a clear, odor-free
occurring micro-organisms which liquid.
break down all of the organic waste.
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