Lecture 2 . Design of Preliminary and Primary Treatment

The Islamic University of Gaza- Civil Engineering Department

Advance wastewater treatment and design (WTEC 9320)

By

Husam Al-Najar

General Wastewater Treatment Processes

Preliminary

Treatment

Primary

Treatment

Tertiary (Advanced)

Treatment Disinfection

Sedimentation and Flotation

Secondary

Treatment

Biological Treatment

Sedimentation

Chemical Phosphorous

Removal

Biological Nutrient Removal

Multimedia Filtration

Screening

Grit removal

Pre-Aeration

Flow Metering and Sampling

Solids

Treatment

Digestion

Disposal

Preliminary and Primary Treatment

Preliminary Primary

Lift Station Influent weir for

flow

measurements

Bar Screen Grit

Chamber Primary

Clarifier

Primary clarifiers should

remove 50-70% of

influent solids

Mechanical treatment

– Flow Measurement

– Removal of large objects

– Removal of sand and grit

– Primary Sedimentation

• Remove large objects

• Ex: sticks, rags, toilet

paper, tampons

• Clog equipment in

sewage treatment plant

• fats, oils, and greases

• larger settleable solids including human waste, and

• floating materials

Secondary Treatment: Fixed Film Biological Treatment Process

Trickling filters (biological tower ).

Rotating biological contactors (RBC).

Packed bed reactors

Fluidized bed biofilm reactors.

Secondary Treatment: Suspended Growth Process Schematic Conventional activated sludge system

Oxidation ditches

Sequential batch reactor (SBR)

Aerated lagoons

Waste stabilization ponds

Up flow anaerobic sludge blanket (UASB)

Disinfection: Remove disease-causing organisms from wastewater

Chlorination

Most common

Advantages: low cost & effective

Disadvantages: chlorine residue

could be harmful to environment

UV light radiation

Damage the genetic structure of bacteria,

viruses and other pathogens.

Advantages: no chemicals are used water taste

more natural

Disadvantages: high maintenance of the UV-

lamp

Ozonation

Oxidized most pathogenic microorganisms

Advantages: safer than chlorination fewer disinfection by-product

Disadvantage: high cost

Treatment Process Summary

Design of Preliminary and Primary Treatment

Bar screens: Screens are used in wastewater treatment for the removal of coarse solids.

Screens are either manually or mechanical cleaned.

Manual bar screen

• Bar spacing is in range of 2-5 cm

• The screen is mounted at an angle of 30-45

• Bars are usually 1 cm thick, 2.5 wide

• Minimum approach velocity in the bar screen channel is

0.45 m/s to prevent grit deposition.

• Maximum velocity between the bars is 0.9m/s to

prevent washout of solids through the bars.

Mechanical bar screen

• Bar spacing is in range of 1.5-4 cm

• The screen is mounted at an angle of 30-

75

• Bars are usually 1 cm thick, 2.5 wide

• Minimum approach velocity in the bar

screen channel is 0.45 m/s to prevent grit

deposition.

• Maximum velocity between the bars is

0.9 m/s to prevent washout of solids

through the bars.

Approach Channel

Design of the bar screen channel (Approach Channel)

The cross section of the bar screen channel is determined from the continuity equation:

Qd = AcVa

Ac = Qd/ Va

Qd = design flow, m3/s

Ac = bar screen cross section, m2

Va = Velocity in the approach channel, m/s

Usually, rectangular channels are used, and the ratio between depth and width is taken

as 1.5 to give the most efficient section.

7.0

1

2

)( 22

xg

VVH

ab

l

The head loss through the bar screen

Hl = head loss

Va = approach velocity, m/s

Vb = Velocity through the openings, m/s

g = acceleration due to gravity, m/s2

Bar Screen Design Example

A manual bar screen is to be used in an approach channel with a maximum velocity of 0.64 m/s,

and a design flow of 300 L/s. The bars are 10 mm thick and openings are 3 cm wide. Determine

1. The cross section of the channel, and the dimension needed

2. The velocity between bars

3. The head loss in meters

4. The number of bars in the screen

Solution

1. Ac= Qd/Va= 0.3/0.64 = 0.47 m2

Ac= W x1.5W =1.5 W x W

W = 0.56 m, Depth (d) = 1.5 W = 0.84 m

2.

= 0.84 x 0.56 (3/3+1) = 0.35 m2

From continuity equation: Va Ac= Vb Anet Vb= 0.64 x 0.56 x 0.84/0.35 = 0.86 m/s < 0.9 m/s ok

barc

c

cnet

tS

SAA

Continue Solution

3. Head loss:

7.0

1

2

)( 22

xg

VVH

ab

l

7.0

1

81.92

)64.086.0( 22

xx

Hl

= 0.024 m

4. Number of Bars n tbar + (n-1)Sc = W

n x 1 + (n-1) x 3= 56 n= 14.75 = 15

Grit Chamber

Removal of inorganic matter which has high density and particle size 0.1 to

0.2 mm in order to protect pumps from abrasion and to protect digesters from

getting clogged.

Sand, gravel, broken glass, egg shells, and other material having a settling

velocity substantially greater than the organic material in wastewater

Grit Chamber Function

• To protect mechanical equipment from abrasion and wear; reduce the

formation of deposits in pipelines and channels; and reduce the frequency

of digester cleaning that is required because of accumulated grit.

• To separate the grit from the organic material in the wastewater.

• This separation allows the organic material to be treated in subsequent

processes.

Types of Grit Chamber

Horizontal flow Aerated Grit Chamber Vortex Grit Chamber

• Extremely simple mechanical design

• No moving parts below the water surface

• Can use the blower air for air lift pumping as well

• Possible septic condition of the plant influent may

be alleviated through pre-preparation in the grit chamber

• Efficient grit removal over variable flow rate

• Rugged precision bearing drive

• Compact design reduce civil work expensed)

• Low energy consumption Simple

and inexpensive maintenance requirement

Aerated Grit Chamber Vortex Grit Chamber

Horizontal flow Grit Chamber

D

s

s

C

dgV

3

)(4

s

Design of Horizontal Flow Grit Chamber

Settling Theory

Vs = settling velocity of particles

= density of particles

= liquid density

d = particle diameter

CD = drag coefficient

f

gdV

s

h

)8 (

Vh = scour velocity

= Friction factor of particles

= Darcy-weisbach friction factor

f Vs

Vh

Particle

Density (kg/m3)

Settling Velocity (Vs)

m/h

0.1 mm 0.2 mm

Sand 2650 25 74

Organic matter 1200 3.0 12

Organic matter 1020 0.3 1.2

Design Example of Settling Channel

A suspension contains particles of grit with a diameter of 0.2 mm and specific gravity of 2.65.

For particles of this size CD= 10, f= 0.03, and = 0.06. Find the length and the dimension of

the cross section of rectangular grit chamber for treatment plant having a daily flow of 11000

m3.

Solution

1103

02.0)165.2(9804

xx

xxVs

03.0

02.0980)165.2(06.08 xxxVh

= 2.1 cm/s

= 23 cm/s

Scour velocity of particles

Scour velocity of organic solids

D

s

s

C

dgV

3

)(4

f

gdV

s

h

)8 (

Vh

VS A

W

H

L

Continue Solution

Cross section area of the grit channel A = W * H = Q/Vh

While the length of the channel could be find from Vs / Vh = H / L

A= W*H= Q/Vh = 0.13 (m3/s)/0.23 (m/s)= 0.55 m2 Assume W= 1 m , then H = 0.55 m

Vs/ Vh= H/L 2.1/23 = o.55/L L= 6.04 m

Design Example of Settling Channel and Control Device: Design a set of

rectangular grit basins with proportional flow weir for a plant which has a peak flow

of 80,000 m3/day, max flow of 65,000, an average flow of 50,000 m3/day and a

minimum flow of 20,000 m3/day. Use three basins. Make the peak depth equal to the

width. The design velocity (Vh) is 0.25 m/s.

Solution

The peak flow per channel will be 80,000/3= 26,666 m3/day = 0.31 m3/s.

The max flow per channel is:

65,000/3= 21666 m3/day = 0.25 m3/s

The average flow per channel is:

50,000/3= 16,666 m3/day = 0.19 m3/s.

The minimum flow per channel is: 20,000/3= 6,666 m3/day = 0.077 m3/s.

A = Q/V A max = 0.31/0.25 = 1.24 m2.

The depth H = W = 1.11 m .

The length of the channel = H (Vh/Vs)

L =1.11 (0.25/0.021) = 13.2 m

Continue Solution

The equation used to calculate y in the table is:

Y= Q/(Vh*W ), for example

Q= 0.077m3/s

Y= 0.077/(0.25*1.11)= 0.28 m= 280 mm

y = (2/3.1) *Y (similar to >> dc = (2/3.1)* H)

= (2/3.1) *280 = 181 mm

The weir must be shaped so that:

Q = 8.18 * 10-6 wy1.5 (where m3/min)

w= width of the proportional weir at depth (y).

for example :

y= 181 mm w= 231 mm

Flow (Q)

Channel Dimension

Control device Dimension

A = Q/V

W=Y at peak

flow

Y= Q/(Vh*W )

y = (2/3.1) *Y

Q = 8.18 * 10-6 wy1.5

(where m3/min)

m3/s

m3/min

W

(mm)

Y

(mm)

y

(mm)

w

(mm)

0.0167 1 1100 60 39 502

0.077 4.62 1100 280 181 231

0.19 11.4 1100 690 445 148

0.25 15.0 1100 909 586 129

0.31 16.6 1100 1100 710 107

Continue Solution

Proportional flow weir for use with rectangular grit chamber

W

Y w

y

Channel Wall

Rejected Area

Opening

Primary Sedimentation Tanks- Circular

The depth of the clarification zone is commonly referred to as a clear water zone (CWZ) depth, while the

depth of the zone of sludge accumulation is named sludge blanket depth (SBD). The sum of the CWZ

depth and the SBD is typically defined as a side water depth (SWD)

Primary Sedimentation Tanks- Rectangular

Design Guideline Source Surface Overflow Rate

(m3/m2.day)

Hydraulic Detention Time (hrs.)

Metcalf & Eddy

(Primary Settling Followed by

Secondary Treatment)

32 – 48 (at average flow)

80 – 120 (at peak hourly flow)

1.5 – 2.5

Randall, Barnard & Stensel

For SWD of 1.83 – 3.05 m:

≤ 2.184 x SWD2 (at average flow)

≤ 4.368 x SWD2 (at peak hourly flow).

For SWD of 3.05 – 4.57 m:

≤ 6.672 x SWD (at average flow)

≤ 13.344 x SWD (at peak hourly flow)

NA

Ten State Standards

≤ 40 (at average flow)

≤ 60 (at peak hourly flow)

Tank surface area is determined based on the

larger of the two SORs.

Minimum SWD = 2.1 m

NA

Qasim 30 – 50 (at average flow)

40 (typical at average flow)

70 – 130 (at peak hourly flow)

100 (typical at peak hourly flow)

1.0 – 2.0

Key Design Criteria For Primary Sedimentation Tanks

20 40 60 80 100

20

40

60

80

100

Surface overflow rate (m3/m2/d)

Rem

oval

%

Surface Overflow Rate (m3/m2.day) and Suspended solids and BOD removals

Max. Flow

Min. Flow

Time (hours)

Average Flow

m3/hr.

Design Example: Primary Sedimentation Tanks: Find the surface area and the depth of the circular

primary sedimentation tanks for the following flow in the figure using Metcalf & Eddy, Randall,

Barnard & Stensel, Ten State Standards and Qasim guidelines.