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 Wastewater Treatment and

Disposal

Lesson 5

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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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