1 Sludge Treatment and Disposal-1

8/3/2019 1 Sludge Treatment and Disposal-1

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Sludge Treatment and Disposal - 1

Mohamed Hasnain Isa

Civil Engineering Department

Universiti Teknologi PETRONAS31750 Tronoh, Perak, Malaysia


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Able to manage biosolids resulting fromtreatment of water and wastewater.

Able to apply engineering principles to design

water and wastewater treatment processes.

Able to identify/solve water and wastewater

problems (domestic and industrial)..

Learning Outcomes 2, 3, 5, 6


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Objectives

To identify the sources and characteristics of

water and wastewater sludges.

To explain methods for sludge thickening.

To analyse and design gravity sludge thickener.


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Sources and Characteristics of Solids and Sludgesdepend on the type of plant and method of operation


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Sludge Thickeningphysical process

increases the concentration of solids of sludge

reduces sludge volume to facilitate ease in handling,

treatment and disposal


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Table Recommended hydraulic overflow rates for

gravity thickeners

Type of sludge Max. overflow rate, m3/m2.d

Primary 15.5-31

Waste activated sludge 4-8

Combined primary and

waste activated sludge

6-12


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Sludge discharge

Scraper blades

Sludge influentSlats/pickets

Inlet baffle

Supernatant

overflow

Sludge discharge

Scraper blades

Sludge influentSlats/pickets

Inlet baffle

Supernatant

overflow

Figure Gravity sludge thickener


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Figure Gravity sludgethickener (partially filled)

Figure Gravity sludgethickener (empty)


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Table Typical sludge concentrations and solid loading for

gravity thickeners

Type of sludge Solids concentration, % Solids loading

Unthickened Thickened kg/m2.d

Separate:

Primary sludge 2-6 5-10 100-150

Trickling filter 1-4 3-6 40-50RBC 1-3.5 2-5 35-50

Air-ASP 0.5-1.5 2-3 20-40

High purity oxygen-ASP 0.5-1.5 2-3 20-40

Extended aeration-ASP 0.2

-1.02

-32

5-4

0Anaerobically digested primary

sludge from primary digester

8 12 120


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Type of sludge Solids concentration, % Solids loading

Unthickened Thickened kg/m2.d

Combined:

Primary and trickling filter 2-6 5-9 60-100

Primary and RBC 2-6 5-8 50-90

Primary and waste activatedsludge

0.5-1.5 4-6 25-70

2.5-4.0 4-7 40-80

Trickling filter and waste

activated sludge

0.5-2.5 2-4 20-40

Chemical (tertiary):

High lime 3-4.5 12-15 120-300

Low lime 3-4.5 10-12 50-150

Iron 0.5-1.5 3-4 10-50

contd.


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Figure Suspended solids and BOD removal as a function

of surface overflow rate


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1515Figure Excessive sludge production v. F/M ratio


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Dissolved Air Floatation

air is forced into solution under pressure and then mixed

with sludge in the influent

the mixture enters a floatation tank and the rising air

bubbles carry the sludge flocs to the liquid surface

the layer of thickened sludge formed on the liquid surfaceis skimmed off

the skimmed effluent exits as underflow

sludge can be thickened to about 6 % solids by floatation


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thickening of solids by floatation is primarily influenced by

air-to-solids ratio, sludge characteristics (in particular the

SVI), solids loading rate and polymer application

the air-to-solid ratio at which the float solids are maximised

ranges from 2 to 4 %

good thickening has been observed with SVI less than

200

higher loadings can be used with dissolved air floatation

than gravity thickeners due to the rapid separation of solids

from the wastewater


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Influent

Sludge

Pressurized

air

Effluent

Sludge skimming

Rising air bubbles

with sludge

Influent

Sludge

Pressurized

air

Effluent

Sludge skimming

Rising air bubbles

with sludge

Figure Sludge thickening by dissolved air floatation


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Table Typical solids loadings for sludge thickeners using

dissolved air floatation

Type of sludge Solids loading, kg/m2.h

Without chemical

addition

With chemicals

addition

Air-ASP:

Mixed liquor 1.2-3.0 Up to 10Settled 2.4-4.0 Up to 10

High purity oxygen-ASP 3.0-4.0 Up to 10

Trickling filter 3.0-4.0 Up to 10

Primary and air ASP 3.0-6.0 Up to 10

Primary and trickling filter 4.0-6.0 Up to 10

Primary sludge only 4.0-6.0 Up to 12.5


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a

a

SfPs

SA )1(3.1

!

where

A/S = air to solids ratio, mL (air)/mg (solids)sa = air solubility, mL/L

f = fraction of air dissolved at pressure P, usually 0.5

P = pressure, atm =

Sa = influent suspended solids, mg/L

35.101

35.101p

If entire flow is pressurised,


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QSRfPs

SA

a

a )1(3.1

!

where

R = pressurised recycle, m3

/dQ = mixed liquor flow, m3/d

If only recycle is pressurised,

Temp., C 0 10 20 30

sa, mL/L 29.2 22.8 18.7 15.7


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Q.1 The flow diagram for a wastewater treatment plant is as

shown below. The following information is also given:

Treatment plant:Diameter of PST = 20 m

Volume of aeration tank = 2 500 m3

MLSS in aeration tank = 3 000 mg/l

Wastewater:

Influent suspended solids = 220 mg/l

Influent BOD = 250 mg/l

Effluent BOD = 20 mg/l

Flow = 20 000 m3

/d

Sludge solids:

Primary sedimentation tank = 3.5 %

Secondary sedimentation tank = 0.8 %

Gravity thickener = 5 %


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Determine:

(i) the mass and volumetric loading rates to the thickener.

(ii) the percent volume reduction by the gravity thickener.

Primary

Sedimentation

Tank

Secondary

Sedimentation

TankAeration Tank

EffluentInfluent

Gravity

Thickener

Return liquid

Thickened sludge

disposal

Return sludge

Primary

Sedimentation

Tank

Primary

Sedimentation

Tank

Secondary

Sedimentation

Tank

Secondary

Sedimentation

TankAeration TankAeration Tank

EffluentInfluent

Gravity

Thickener

Gravity

Thickener

Return liquid

Thickened sludge

disposal

Return sludge

Primary

Sedimentation

Tank

Secondary

Sedimentation

TankAeration Tank

EffluentInfluent

Gravity

Thickener

Return liquid

Thickened sludge

disposal

Return sludge

Primary

Sedimentation

Tank

Primary

Sedimentation

Tank

Secondary

Sedimentation

Tank

Secondary

Sedimentation

TankAeration TankAeration Tank

EffluentInfluent

Gravity

Thickener

Gravity

Thickener

Return liquid

Thickened sludge

disposal

Return sludge


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Q.2 Design a gravity thickener for a wastewater treatment

plant with the following sludge characteristics:

Type of sludge Specific gravity Solids

%

Flow rate

m3/d

Average design conditions:

Primary sludge 1.03 3.3 400

Waste activated 1.005 0.2 2250

Peak design conditions:

Primary sludge 1.03 3.4 420

Waste activated 1.005 0.23 2500


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Q.3 Design a floatation thickener without and with pressurised

recycle to thicken the solids in an activated sludge mixed

liquor from 0.2

to4%. The following conditions apply:

Optimum A/S ratio = 0.012 mL/mg

Temperature =2

0 CAir solubility = 18.7 mL/L

Recycle system pressure = 280 kPa

Fraction of saturation = 0.5

Surface loading rate = 9 L/m2.min

Sludge flow rate = 500 m3/d


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Q4.


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Thank you

1 Sludge Treatment and Disposal-1

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