Raw Water Intake, Screening,

Aeration and Grit RemovalAeration and Grit Removal

Content

• Introduction

• Raw water intakestructures– Types of intakestructures

– Intake site selection

– Intake-designconsideration– Intake-designconsideration

• Screening– Types of screening

– Aeration

– Purpose of aeration

– Types of aerators

Introduction

Raw Water Intake

• Raw water intakes withdraw water from a river, lake, or reservoir over a predetermined range of pool levels.

• Screens remove large floating objects • Screens remove large floating objects from the water – to protectpumpingequipment.

• Aeration removes gases and volatile compounds and also to oxidize certain dissolved metals.

Raw Water Intake Structures

• Control withdrawal of raw water from a surface water source.

• Selectively withdraw the best quality water while excluding fish, floating water while excluding fish, floating debris, coarse sediment, and other objectionable suspended matter.

• Intake contains gates, screens, control valves, pumps, chemical feeders, flow meters, offices and machine shop

Types of Intake Structures

1. Floating intakes

2. Submerged intakes

Types of intake structures

Intake Site Selection

• Water quality

• Water depth

• Stream or current velocities

• Foundation stability

• Access• Access

• Power availability

• Proximity to water treatment plant

• Environmental impact

• Hazard to navigate

Intake Design Consideration

Intake velocities

• High velocities – head loss, entrain suspended matter ,trap fish, and other aquatic animals.

Intake-port location

• Water quality in each stratum may vary.

• To achieve, multiple intake ports set at other aquatic animals.

• Velocity below 8 cm/s allows aquatic animals to escape, and minimize the suspended matter.

intake ports set at various levels are generally provided.

• Top intake – less than2 m below normal level.

• Bottom intake – least1 m above the bottom

Screening

• It is a unit operation that removes suspended matter from water.

• Screens may be classified as coarse, fine, or micro strainer, depending on fine, or micro strainer, depending on the size of material removed.

• Located at intake structure, raw water pump station, or water treatment plant.

Screening

1. Coarse screen or trash rack

• To prevent large object from entering the conveyance system.

• Consists of vertical flat bars, or, in some • Consists of vertical flat bars, or, in some cases, round pipes spaced with 5 to 8 cm of clear opening.

• Installed outside of any sluice gate.

• The velocity through the coarse screen is generally less than 8 cm/s.

Coarse Screens

Fixed coarse screen

Rotary coarse screen

Screening

2. Fine screen

• To remove smaller objects that may damage pumps or other equipment.

• Screens consists of heavy wire mesh with 0.5 cm square opening.cm square opening.

• The typical design velocity through the effective area is in the range of 0.4 to 0.8 m/s.

• There are two types: Traveling screens and passive screen installation.

Fine Screens

Fine Screens

Aeration

Aeration involves bringing air or other gases in contact with water.

The purpose of aeration are

1. Reduce the concentrations of taste and odor causing substance by volatilizationcausing substance by volatilization

2. To oxidize iron and manganese, rendering them insoluble.

3. To dissolve oxygen in water to make it taste better

4. To remove compounds for better water treatment ( H2S removal before chlorination and CO2 removal before softening)

Types of Aerators

Cascade aerator

Water is sent down gravitically and

oxygenated as it come into contact

with the air bubbles caused by

turbulent flow.

Types of AeratorsDiffused air aerators

Water is enriched with oxygen as it come into

contact with the air bubbles.

Grit Removal System

• Grit chambers are designed to remove grit, consisting of sand, gravel, sanders, or other heavy solid materials that have specific gravities or setting velocities substantially greater than those of organic particles in wastewater.

• Grit chambers are most commonly located after the bar screens and before the primary sedimentation.

• These are just like sedimentation tanks, design mainly to • These are just like sedimentation tanks, design mainly to remove heavier particles or coarse inert and relatively dry suspended solids from the wastewater. – There are two main types of grit chambers like rectangular

horizontal flow types and aerated grit chambers.

– In the aerated grit chamber the organic solids are kept in suspension by rising aerted system provided at the bottom of the tank.

Grit Chambers: Velocity Controlled

Grit Removal System

Grit chambers are provided to:

• Protect moving mechanical equipment

from abrasion and accompanying

abnormal wear.abnormal wear.

• Reduce formation of heavy deposits in

pipelines, channels and conduits.

Grit Removal System

• Horizontal flow (Rectangular or square) (configuration type)

Designing a Rectangular horizontal flow type grit chamber:– Cross-sectional area, A = (Q / V ) for each unit – Cross-sectional area, Ax = (Qdesign / Vh) for each unit

(Vh ≈ 0.3 m/sec), depth ≈ 1-1.5 m

– Assuming (tD = 1-2 minutes), determine the length L = Vh * tD (Add 10% additional)

– Check the SLR (1200-1700 m3/m2-day) and Vs (≥ 0.01 m/sec). Grit produced is about 1.5 ft3/ML of wastewater flow. Add to depth {0.3 m free board + grit}

Type I Settling -- Stokes’ Law

µρρ

18

)( 2dgv ss

−=

where

ν = settling velocityνs = settling velocity

ρs = density of particle (kg/m3)

ρ = density of fluid (kg/m3)

g = gravitational constant (m/s2)

d = particle diameter (m)

µ = dynamic viscosity (Pa�s)

Example: Grit Chamber Design

• Design a grit chamber to remove sand

particles (ρp = 2650 kg/m3) with a mean

diameter of 0.21 mm. Assume the sand

is spherical and the temperature of the

wastewater is 20 oC. The wastewater wastewater is 20 oC. The wastewater

flow is 10,000 m3/d. A velocity of 0.3

m/s will be automatically maintained,

and the depth must be 1.5 times the

width at maximum flow.

Example

• Calculate settling velocity

µρρ

18

)( 2dgv ss

−=

( )×

− m

kgkgm 24101.299826508.9 - ( )

⋅

×

×

−=

−

sm

kg

m m

kg

m

kg

s

m332

3

24

1000.118

101.299826508.9 -

s

v

s

m039.0=sv

Example

• Calculate the cross-sectional area

v

QAx =

=

s

min

min

d

m

s

d

m3

6014403.0000,10xA

2m 39.0=xA

Example

• Calculate the width and depth

25.15.1 WWWAx =×=

m 51.05.1

39.0

5.1

5.05.0

=

=

= xAW

m 76.05.151.05.1 =×=×=WD

Example

• Determine the detention time required for a

particle to fall the entire tank depth

s m/s

m 4.19

039.0

76.0===d

v

Dt

• Determine the length to achieve this

detention time

s m/s

4.19039.0

===s

dv

t

m m/s s 85304.19 ..vtL d =×=×=

Example

• Thus, the tank must have dimensions

W = 0.51 m

D = 0.76 m

L = 5.8 mL = 5.8 m

Grit Removal System

Assignment

Will a grit particle with radius of 0.1 mm and

SG of 2.65 settled in a horizontal grit

chamber that is 13.5 m length if the average chamber that is 13.5 m length if the average

flow, Q is 0.15 m3/s with the width of

chamber 0.56 m and horizontal velocity

0.25 m/s? Water temperature is 22oC,