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7/21/2019 LESSON 5 Wastewater Treatment and Disposal
http://slidepdf.com/reader/full/lesson-5-wastewater-treatment-and-disposal 1/24
pegslides ©2014
Wastewater Treatment and
Disposal
Lesson 5
7/21/2019 LESSON 5 Wastewater Treatment and Disposal
http://slidepdf.com/reader/full/lesson-5-wastewater-treatment-and-disposal 2/24
Primary Treatment
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Grit removal
The removal of gritty materialsuspended in raw sewage such as sand,
coffee grounds, eggshells, etc.
Flow velocity should not be less thanthe self-cleansing velocity ( 0.6 m/s)
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Grit
Inert dense material such as sand, broken glass, silt and pebbles
When not removed from wastewater,
they abrade pumps and devices
Settle in corners and bends reducing
flow capacity
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Grit chambers
Long, narrow tanks wherein productionin velocity and collection of grit are
made
Rising air bubbles help to keep theorganic solids in suspension while grit
settles in the bottom
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Water Treatment Process
1.Primary treatment
a. Screening
b. Comminuting
c. Grit removal
d. Sedimentation
2. Secondary treatment
a. Sloughing
b. Recirculation
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Sedimentation
When flow velocities and turbulenceare minimal, particles that are
denser than water settle to the
bottom of the tank.
Sludge – layer of accumulated solids
at the bottom of the tank
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Primary clarifiers
Settling tanks that receive sewageafter grit removal
Coagulants
chemicals that allow very small suspended particles to collide, stick together and
form settleable flocs
increase removal efficiency of primary
settling tanks
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Secondary Treatment
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Secondary Treatment
To achieve BOD and TSS removalefficiencies of at least 85% as
required by the Clean Water Act, this
process must follow
Purpose: to remove the suspended
solids that did not settle out in the
primary tanks
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Microorganisms
Biological treatment of sewageinvolves the use of microbes to
consume the organic pollutants as
food.
Examples:
Bacteria
Fungi
Algae
Protozoa
Rotifers and Crustaceans
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Why should we keep microbes happy?
Microbes metabolize the biodegradable
organics. Thus converting them into
CO2.
They stabilize the organic pollutantsin a controlled, artificial
environment (the tank)
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Most Common Biological Treatments
1. Trickling Filter
2. Activated Sludge Process
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Trickling Filters
Consists of a bed of crushed rock
(~2m deep)
Usually in circular with a diameter
as large as 60m
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Trickling Filter Process
Primary effluent
is wastewater that flows out from a
treatment plant or individual treatment
process
is sprayed over the surface of the
crushed stone bed and trickles downward
to an underdrain system
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Trickling Filter
Is not a filter at all, in the true
sense of the word.
The stones in a trickling filter only
serve to provide a large amount ofsurface area for the biological
growth
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Underdrain System
Serves to collect and carry away the
wastewater from the bottom of the bed
and to permit air circulation upward
through the stones
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Sloughing
Pronounced as “sluffing”
When slime layer gets thicker and
gets washed off from surfaces of
stones by flowing water
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Secondary Clarifier
Also known as final clarifier
A sedimentation tank where the
collected trickling filter effluent
in the underdrain system is conveyed
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Recirculation
Can serve to improve the pollutant
removal efficiency
Direct Recirculation – one common
pattern of recirculation
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Activated Sludge
Is a suspended-growth system because
microbes are thoroughly mixed and
suspended in the wastewater flows
rather than attached to a particular
surface
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