Transcript Wastewater Treatment - WEC CIVILIANS

Wastewater Treatment
Wastewater Treatment Methods
• Wastewater treatment system consists of a combination of different
treatment units, which either reduce the quantity of impurities or
qualify wastewater to undergo action in next treatment unit. Each
treatment unit employ one or combination of physical, chemical or
biological measures. Based upon the type of measure used,
treatment method can be broadly classified into Unit Operations and
Unit Processes.
Unit Operations
• Treatment measures in which the application of physical forces
predominates are known as unit operations. Major treatment measures
falling under this category are as follows:
i. Screening
ii. Mixing
iii. Flocculation
iv. Sedimentation
v. Floatation
vi. Elutriation
vii. Vacuum filtration
viii. Heat Transfer and drying
Screening
• Screening is the first operation at any wastewater treatment works. This process
essentially involves the removal of large non-biodegradable and floating solids
that frequently enter a wastewater works, such as rags, papers, plastics, tins,
containers and wood.
• Efficient removal of these constituents will protect the downstream plant and
equipment from any possible damage, unnecessary wear & tear, pipe blockages
and the accumulation of unwanted material that will interfere with the required
wastewater treatment processes.
• Wastewater screening is generally classified into either coarse screening or fine
screening. Screens may be manually or mechanically cleaned, with only the older
and smaller treatment facilities using manually cleaned screens as their primary
or only screening device. Coarse screens are typically used as primary protection
devices, and usually have openings of 10mm or larger.
• Fine screens are used to remove material that may cause operation &
maintenance problems in downstream processes, particularly in
systems that lack primary treatment. Typical opening sizes for fine
screens are 3 to 10mm.
Mixing
• Mixing is required in various units of a treatment plant, and it can be
achieved by rotating mixers, static mixers or creation of a hydraulic
jump.
Flocculation
•
• Flocculation is "a process of contact and adhesion whereby the
particles of a dispersion form larger-size clusters."
Sedimentation
• Sedimentation is the process in which suspended particles settle out
of the fluid in which they are entrained, and come to rest against a
barrier. This is due to their motion through the fluid in response to
the forces acting on them.
Floatation
• The process of floating a solid or liquid impurities in wastewater due
to their low densities is called floatation.
Elutriation
•
• Elutriation is a method for separating insoluble solids into different
particle sets based on their size, shape and density, using different
methods or forces. Normally the forces used for elutriation includes,
gravity, buoyancy, electrochemical forces or centrifugal forces.
Vacuum filtration
• Filtration is a technique used to separate a solid from a liquid. The
solid is separated from the liquid phase by passing the mixture over a
filtering media. Vacuum filtration is process in which speed of
filtration process is increased by sucking liquid through membrane.
This process is normally used for sludge dewatering.
Heat Transfer and drying
• Heat transfer and drying are simultaneous processes normally used in
sludge processing and dewatering. In first step, heat is transferred to
refuse due to which water in refuse start evaporating which is called
drying.
Unit Processes
• Type of treatment in which removal of contaminants is brought about
by addition of chemicals or use of biological mass are called unit
processes. Therefore, unit processes are further categorized into
chemical and biological unit processes.
Chemical Unit Processes
• Reduction or removal is brought about by means of chemical reactions by
adding chemicals and providing suitable environment which facilitates the
desired reactions. Major treatment methods falling under this category re
as follows
• Chemical neutralization
To control or adjust system pH.
• Chemical coagulation
To remove colloidal particles by chemical destabilization and flocculation
• Chemical precipitation
To enhance the removal of suspended solids, phosphorous, heavy metals and
BOD in specific system conditions
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• Chemical oxidation
• To remove grease, ammonia, BOD, COD and for odor control in
particular requirement.
• Chemical disinfection
To kill pathogens in influent and treated effluents
Biological Unit Processes
• The wastewater treatment methods in which treatment purpose is
achieved by microorganisms. Major treatment methods falling under this
category are classified as follows.
• Suspended growth process
• Microorganisms, performing biological activity are in suspended and evenly
mixed in wastewater. Some examples of this category are
• Activated Sludge Process
• Aerated Lagoons
• Oxidation Ponds
• Aerobic and Anaerobic digesters
Attached growth process
Microorganisms, performing biological activity are attached and make
colonies to some medium. Wastewater pass through the medium and
microorganisms attached to medium extract solids/contaminant and
fulfil treatment purpose. Some examples of this category are
• Trickling Filter
• Rotating Biological Contractors (Bio Rotors)
• Bio Towers
• Up-flow filters
Wastewater Treatment Systems
• Set of combination used from available unit operations and processes
for wastewater treatment can be categorized in four treatment steps
i. Preliminary Treatment
ii. Primary Treatment
iii. Secondary Treatment
iv. Tertiary Treatment / Advanced Treatment
Preliminary Treatment
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Objective: Preliminary treatment fulfil following purposes
Receive influent
Flow Measurement
Screening to protect next treatment units from large size solids
Pumping, as required to lift wastewater above ground level so that
wastewater flow should be derived from gravity in following treatment
steps
• Grit Removal to protect mechanical equipment from abrasive wear.
• A schematic process flow diagram of preliminary treatment is as follows
Primary Treatment
• Objective: To reduce organic load on secondary treatment processes.
• Primary treatment removes heavier solids from wastewater by
sedimentation. Primary clarifier reduces 60 – 70% of suspended
inorganic solids and 30 – 32% of organic solids
• A schematic process flow diagram of preliminary treatment is as
follows
Secondary Treatment
• Objective: To convert organic material (BOD) into solid biomass for
removal in secondary clarifier (SST).
• Several Treatment alternatives exist including
Activated Sludge (Suspended Growth)
Trickling Filter (Attached Growth)
Tertiary Treatment / Advanced Treatment
• Objective: To further reduce the amount of residual impurities in
effluent of secondary treatment
• Advanced treatment is normally provided in following cases.
i. The quality of conventionally treated wastewater (secondary effluent)
is unsuitable for final disposal requirements into receiving water body.
ii. The concentrations of residual organic materials or suspended solids
require further reduction or complete removal for specific reuse or
recycling of wastewater after secondary treatment
iii. The concentrations of nutrients like nitrogen & phosphorous is high
for final disposal or reuse / recycling of wastewater.
Functions of Treatment Plant Units
• Major functions of treatment units shown in schematic diagram of wastewater
treatment plant are as under
i. Sump and Pump Unit
To collect community wastewater in sump unit and pump it to next units at raised
level so that flow is maintained by gravity.
ii. Screen
To remove large floating material in wastewater to protect next treatment units
from damaging and clogging
iii. Grit Chamber
To remove high density suspended particles by settlement
iv. Skimming Tank
To collect floating solids, oil and grease from water surface
v. PST (Primary Sedimentation Tank)
To remove fine suspended inorganic and settle able organic solids
(BOD) and floating scrum.
vi. Aeration Chamber
Acts as biological treatment unit in which microorganisms in is
suspended form contribute toward treatment. Much of COD and BOD
is removed in this process.
vii. SST (Secondary Sedimentation /Settling Tank)
To thicken and segregate biological sludge from wastewater
viii. Sludge Digester
To treat sludge before final disposal
Selection of Treatment Plant System
• Selection of a particular treatment system and choosing a suitable
combination of treatment units depends upon following factors
1. Degree of treatment to bring quality of final effluent. This means that type
and amount of impurities in influent that need to be reduced to a certain
level.
2. Regulatory /legal requirement of amount of impurities in effluent.
3. To ensure that final effluent is safe for end disposal or acceptable for
specific reuse.
4. Topography and space available for treatment plant site.
5. Availability or non-availability mechanical and skilled personal to handle
treatment plant.
• It is also important to consider following points while selecting a
specific treatment system
• Reduction of inorganic material component of wastewater is much
easier and cheaper than the removal of organic contents of
wastewater
• Removal of suspended solids require from wastewater requires lesser
time and effort than colloidal and dissolved solids.