Describing sewage treatment plant, it's process with various examples. Link to download: https://dimpstrail.gumroad.com/l/vjicar

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SewageWaterTreatment
DIMPLE PODDAR
Sem – 09 | Date: 03 Sep 2020
Smt. K. L. Tiwari college of architecture

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SEWAGE WATER TREATMENT
 Sewage treatment is the process of removing contaminants from
municipal wastewater, containing mainly household sewage plus
some industrial wastewater. Also referred to as wastewater treatment.
 Physical, chemical, and biological processes are used to remove
contaminants and produce treated wastewater (or treated effluent) that is safe
enough for release into the environment.
 A by-product of sewage treatment is a semi-solid waste or slurry,
called sewage sludge. The sludge has to undergo further treatment before
being suitable for disposal or application to land.
Figure 1: Different sources of sewage water

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Municipal wastewater treatment systems
Pre – treatment of Industrial wastewater
 Industrial wastewaters must be pretreated prior to being discharged to
municipal sewer system
 Pretreatment requirements set by regulatory agencies
 Why: remove materials that will not be treated by municipal system, remove
materials that inhibit the biological processes in secondary treatment
 For example: silver ions are toxic to bacteria which might affect biological
process.
 Thus silver ions are removed at pre-treatment before biological process so
that bacterial performance do not get affected.

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Figure 2: Basic Sewage water treatment system

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Preliminary Treatment
 Upon arrival via the sewer system, the wastewater is sent through a bar screen,
which removes large solid objects such as sticks and rags.
 Leaving the bar screen, the wastewater flow is slowed down entering the grit tank,
to allow sand, gravel and other heavy material that was small enough not to be
caught by the bar screen to settle to the bottom.
 All the collected debris from the grit tank and bar screen is disposed of at a sanitary
landfill.
Primary treatment (screening)
Figure 3: Bar racks
Bar racks
 Purpose: remove larger
objects
 Solid material stored in
hopper and sent to landfill
 Mechanically or manually
cleaned.

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Figure 4: Grit chambers
Primary treatment (settling)
 Primary treatment separates suspended solids and greases from wastewater.
 Wastewater is held in a tank for several hours allowing the particles to settle to the
bottom and the greases to float to the top.
 The solids drawn off the bottom and skimmed off the top receive further treatment as
sludge.
 The clarified wastewater flows on to the next stage of wastewater treatment.
Settling/ Sedimentation
 Solid liquid separation process in which a suspension is separated into two phases –
1. Clarified supernatant leaving the top of the sedimentation tank (overflow).
2. Concentrated sludge leaving the bottom of the sedimentation tank (underflow).
Purpose of settling
 To remove coarse dispersed phase.
Grit chambers
 Purpose: remove inert dense
material, such as sand,
broken glass, silt and pebbles
 Avoid abrasion of pumps and
other mechanical devices.
 Material is called “grit”

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 To remove coagulated and flocculated impurities.
 To remove precipitated impurities after chemical treatment.
 To settle the sludge (biomass) after activated sludge process / tricking filters.
Figure 5: Primary settling basin

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Equalization tank
 The sewage from the bar screen chamber and oil, grease and grit trap comes to the
equalization tank.
 The equalization tank is the first collection tank in an STP.
 Its main function is to act as buffer: To collect the incoming raw sewage that comes at widely
fluctuating rates, and pass it on to the rest of the STP at a steady (average) flow rate.
 During the peak hours, sewage comes at a high rate. The equalization tank stores this
sewage, and lets it out during the non-peak time when there is no/little incoming sewage.

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1. Inlet pipe
2. Sewage
3. Raw
sewage lift
pumps
4. Delivery
pipe
5. Coarse
bubble
diffusers
6. Air supply
pipeline
Secondary Treatment (activated sludge)
 Secondary treatment is a biological treatment process that removes dissolved
organic matter from wastewater.
 Sewage microorganisms are cultivated and added to the wastewater.
 The microorganisms use organic matter from sewage as their food supply.
 This process leads to decomposition or biodegradation of organic wastes.
 Basic approach is to use aerobic biological degradation:
 organic carbon + O2 microorganisms
CO2 + new cells
 Objective is to allow the BOD to be exerted in the treatment plant rather than in the
stream

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Basic ingredients
 High density of microorganisms (keep organisms in system)
 Good contact between organisms and wastes (provide mixing)
 Provide high levels of oxygen (aeration)
 Favorable temperature, pH, nutrients (design and operation)
 No toxic chemicals present (control industrial inputs)
Activated Sludge
 Process in which a mixture of wastewater and microorganisms is agitated and aerated
 Leads to oxidation of dissolved organics
 After oxidation, separate sludge (mostly microbial cells, water, and other contaminants)
from wastewater
 Induce microbial growth
– Need food, oxygen

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– Want Mixed Liquor Suspended Solids (MLSS) of 3,000 to 6,000 mg/L

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Secondary Clarifier/ Settling Tank
 The purpose and function of the secondary clarifier is threefold:
1. Allow settling of biomass solids in the Mixed Liquor (biomass slurry) coming out of
the aeration tank, to the bottom of the clarifier
2. To thicken the settled biomass, in order to produce a thick underflow
3. To produce clear supernatant water, in the overflow from the clarifier
 There are three popular design variations in the unmechanized clarifier tank.
 They differ in the manner in which the sludge is collected and returned to the Aeration
tank.

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Settling tank with air-lift pump
Settling tank with directsuction electric pump
1. Sewage inlet pipe
2. Center-feed well
3. Steeply sloped walls of the
tank
4. Bacterial flocs
5. Compressed air pipe
6. Inverted funnel
7. Delivery pipe
8. Clear water
1. Sewage inlet pipe
2. Center-feed well
3. Steeply sloped walls of the
tank
4. Bacterial flocs
5. Sludge delivery pipe
6. Pumps
7. Header assemnbly
8. Clear water

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Settling tank with buffer sump
Mechanized Clarifier Tank
 The three types of clarifier tanks described so far were not mechanized:
 In those tanks, the sludge settles and moves to the deepest part of the tank due to
gravity, from where a pump takes it to the aeration tank.
 In a mechanized clarifier tank, the sludge settles at the bottom over a wide area, and
a few rubber wiper blades (called “squeegees”) sweep it to a pit at the center of the
tank, from where a pump takes it to the aeration tank.
1. Sewage inlet pipe
2. Center-feed well
3. Steeply sloped walls of the tank
4. Bacterial flocs
5. Sludge delivery pipe
6. Buffer sump
7. Compressed air pipeline
8. Coarse air bubble diffusers
9. Pumps
1. Sewage inlet pipe
2. Center-feed well
3. Bottom
4. Weir
5. Launder
6. Outlet pipe
9. A motor
10. Speed-reduction gear box
11. Shaft
12. Frame
13. Platfaorm

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Details of the sludge-collection pit
Figure 6: Sludge movement in the tank
Sludge Recirculation
 The indivisible combination of the aeration tank, settling tank and sludge
recirculation constitutes an “activated sludge biological treatment system”.
 All three must be fine-tuned to act in unison to produce the desired high level of
treatment. The optimum desired age of the microbes is between 25 to 30 days.
1. Tank floor
2. Sludge-collection pit
3. Outlet port
4. Outlet pipe
5. Rcc pillar
6. Bush housing

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 At the same time, an STP needs to maintain a high level of microbes in the aeration tank.
Both these objectives are achieved by recirculating the sludge from the settling tank, and
also bleeding out of excess microbes from the system at regular intervals.
Figure 7: Movement of sludge in tanks
1. Two identical
pumps
2. Suction pipes
3. Delivery pipe
4. Bypass pipeline
5. Valves

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Filter Feed Pumps (FFP)
 Filter feed pumps are used to take the water from the clarified water sump and pass it
through the pressure sand filter and activated carbon filter installed in series.
Pressure Sand Filter (PSF)
 The pressure sand filter (PSF) is used as a tertiary treatment unit to trap the trace
amounts of solids which escape the clarifier, and can typically handle up to 50 mg/l of
solids in an economical manner.
 This unit is essentially a pressure vessel that is filled with graded media (sand and
gravel). The water filtered with PSF is passed on to the next stage in the STP chain: the
Activated Carbon Filter.
 A good average design filtration rate is 12 m3 / m2 /hr of filter cross-sectional area, and
most filters used in STP applications are designed on this basis.

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Activated Carbon Filter (ACF)
 An activated carbon filter, like the Pressure Sand Filter, is a tertiary treatment unit.
 It receives the water that is already filtered by the Pressure Sand Filter and improves
multiple quality parameters of the water: BOD, COD, clarity (turbidity), color and
odor.
 Just as the PSF, the ACF also needs to be backwashed, albeit at a lesser frequency to
dislodge any solid particles trapped by simple filtration action.
 When the carbon gets exhausted (indicated by no improvement in water quality
across the ACF), fresh carbon needs to be filled into the filter.

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Final treatment
 Treated water is disinfected and then it is sent out for wastewater reuse activities or for
discharging in river/streams. Mostly chlorination and/or ultra violet irradiation is used
for disinfection purposes.
Disinfection Of Treated Water
 The treated water is disinfected to destroy and render harmless disease-causing
organisms, such as bacteria, viruses, etc.
 The most common methods of disinfection include Chlorination, Ozonation and UV
radiation. Of these, Chlorine finds widespread application.

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 The primary action of the chemical involves damaging the cell wall, resulting in cell
lysis and death. In most STPs, the common form of Chlorine used is Sodium
Hypochlorite (Hypo) available commercially at 10-12 % strength, being safe, easy to
handle and having a reasonable shelf life
Excess Sludge Handling
 Biological treatment of wastewater perforce produces excess biological solids due to
the growth and multiplication of bacteria and other microorganisms in the system.
 The excess biomass thus produced needs to be bled out of the system, and disposed
off efficiently. This is a five-step process: sludge removal, storage, conditioning,
dewatering and disposal.
 Sludge is removed (“bled”) from the system from the sludge recirculation pipeline
(through a branch). The sludge is in the form of a thick slurry. It is taken into a
sludge-holding tank, and kept under aeration (to prevent the living organisms from
putrefying) until dewatering operations can be carried out.
1. Filter plates
2. End-plate
3. Rails
4. Hydraulic jack
5. Plunger
6. Inlet pipe
7. Filtrate discharge

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 Before dewatering, polymer or other chemicals may be added for conditioning the
sludge, to facilitate the process. Sludge is then dewatered in a filter press/ Sludge
bag/ centrifuge.
Figure 8: Sludge collection tank
Figure 9: Typical measurements points