ESC3204+5

8/13/2019 ESC3204+5

http://slidepdf.com/reader/full/esc32045 1/14

1

Faculty of Environmental Studies Universiti Putra Malaysia 2013

Sedimentation

1

Sedimentation: separation of suspended particles thatare heavier than water by gravitational settling

Sedimentation and settling are used interchangeably

Sedimentation basin = sedimentation tank, clarifier,settling basin, settling tank

Accelerated gravity settling: removal of particles insuspension by gravity settling in an accelerated flow field


Most WW contain solids and many treatment

processes generate solids

- Phosphate precipitation, coagulation,

activated sludge bioxidation

Particle in WW that will settle by gravity within a

reasonable period of time can be removed by

“sedimentation” in sedimentation basins (also

known as clarifiers)

2


“Settleable” doesn’t necessarily mean that theseparticles will settle easily by gravity

Some need to be coaxed out of suspension orsolution by the addition of chemicals

(coagulants) or increased gravity (centrifugationor filtration)

Because of high volumetric flow rates in WWTP,gravity sedimentation is the only practical,economical method

3Faculty of Environmental Studies Universiti Putra Malaysia 2013

Gravity separation can obviously be applied only

to those particles which have density greater

than water

Goals of gravity sedimentation

• Produce a clarified (free of suspended solids)

effluent

• Produce a highly concentrated solid sludge

stream

4

8/13/2019 ESC3204+5


2


Types of Sedimentation Basins

5

Rectangular tanks Circular tank


They are sized in order to have an optimal

sedimentation speed

If sedimentation speed is too high, most

particles will not have sufficient time to settle,

and will be carried with the treated water

If the speed is too low, the tanks will be of an

excessive size

6


Theory of Gravitational Settling

To separate solids from liquid using the force ofgravity. In sedimentation, only suspended solids

(SS) are removed.


1. Type I settling (free settling):

• discrete, non-flocculent particles in a dilute

suspension

• settle as separate units, and there is no

apparent flocculation or interaction between

the particles

• settling of sand particles in grit chambers

• Particle settling according to Stoke’s Law

• Design parameter is surface overflow rate

(Q/AS)

8

8/13/2019 ESC3204+5


3


2. Type II settling (settling of flocculated

particles):

• settling of flocculent particles in a dilute

suspension

• flocculate during settling and increase in size

and settle at a faster velocity

• settling in primary clarifiers

• Design parameters: surface overflow rate,

depth of tank or hydraulic retention time


3. Type III settling (zone or hindered settling):

• settling in an intermediate concentration

• the particles are so close together that

interparticle forces hinder the settling of

neighboring particles

• settle at a constant velocity

• settling in secondary clarifiers (upper part)

10


4. Type IV settling (compression settling):

• settling in a high concentration

• the particles touch each other and settling can

occur only by compression of the compacting

mass

• settling in secondary clarifiers (lower part)


Zone Settling & Compression

Zone settling occurs when a flocculent suspensions with highconcentration (on the order of 500 mg/L) settles by gravity.

Flocculant forces between particles causes settling as a matrix

(particles remain in a fixed position relative to each other asthey settle)

When matrix sedimentation is constrained from the bottom, itbegins to compress

Such a situation occurs when the matrix encounters thebottom of the tank in which it is settling – this is called Type IVsettling

12

8/13/2019 ESC3204+5


4



The height of the interface (between the

clarified zone and the zone settling zone) versus

time is plotted

Velocity of this interface is steady after some

induction period but changes with time as

compression begins

The slope of the steady interface subsidence

rate represents zone settling velocity (ZSV)

14


Factors affecting zone settling velocity

• Suspended solids concentration

• Depth of settling column

• Stirring (0.5 – 2 rpm to prevent “arching”)

• Temperature

• Polymer addition (affects matrix structure)


Used to separate solid or liquid particles from a liquid

phase

How? Introduce fine gas (air) bubbles into the liquid

phase• Bubbles attach to the particulate matter - particle to

rise to the surface

• Particles can be collected by skimming operation

Used to remove suspended matter and to concentrate

biosolids

Flotation

16

8/13/2019 ESC3204+5


5


Flotation vs. sedimentation

Advantages of flotation over sedimentation

• Very small/light particles that settle slowly can beremoved more completely and in a shorter time

• High rise velocity permits small tankage

• Ability to handle variable solids loading (can adjust airflow)

• Can provide high float concentration (good thickening)

• Can remove low density particles which would requirelong settling period

Disadvantages

• Capital, energy, operating costs


Types of Flotation Process

18

1. Dissolved Air Flotation

The removal is achieved by dissolving air in the

wastewater under pressure and then releasing the air at

atmospheric pressure in a flotation tank or basin

The released air forms tiny bubbles which adhere to the

suspended matter causing the suspended matter to float

to the surface of the water where it may then be

removed by a skimming device


Sometimes polymer is added at the feed to enhance sticking of the bubble


Recycle

Often recycle is employed because the

flocculent suspended solids are too fragile to be

directly aerated

Larger quantities of air can be introduced since

recycle flow can be greater than feed flow

20

8/13/2019 ESC3204+5


6


2. Dispersed (induced) Air Flotation

Also known as induced-air flotation

Seldom used in municipal wastewater treatment, but is

used in industrial apllications for the removal of emulsifiedoil and suspended solids form high-volume waste or

process waters

Air bubbles are formed by introducing the gas phase

directly into the liquid phase through a revolving impeller

(acts as a pump)

Air is induced and dispersed into the liquid by pumping

action21



3. Vacuum Flotation

Waste is saturated with air at 1 atm then a

vacuum is applied to create relative

supersaturation

This results in bubble formation. Because there

is a max of 1 atm pressure difference, there is a

severe limitation on the amount of air available

for flotation


24

Chemical Unit

OperationsTreatment methods in which the

removal or conversion of pollutants

by the addition of chemicals or by

chemical reactions

8/13/2019 ESC3204+5



Process Application

Coagulatio n The chemical destabilization of particles in WW to bring

about their aggregation during perikinetic and orthokinetic

flocculation

Disinfection With chlorine, chlorine compounds, bromine, ozone

Control of slime growths in sewers

Control of odors

Neutralizatio n Control of pH

Oxidation Removal of BOD, ammonia

Destruction of microorganisms

Control of odors in sewers, pump stations

Removal of resistant organic compounds

Precipitation Enhancement removal of TSS and BOD in primary

sedimentation facilities

Removal of phosphorus, heavy metals

Corrosion control in sewers due to H2S


Consideration

• A net increase in the dissolved constituents inthe WW

- Chlorine is added, TDS of effluent is increased

• Handling, treatment and disposal of largevolumes of sludge that is produced fromprecipitation

• Cost of the chemicals

26


Coagulation

All of the reactions and mechanisms involved in thechemical destabilization of particles and in theformation of larger particles through perikineticflocculation (aggregation of particles in the sizerange from 0.01 to 1 mm)

Coagulant – chemical that is added to destabilizethe colloidal particles in WW so that floc formationcan result

Flocculent – a chemical (organic) added to enhancethe flocculation process


Flocculation – the process whereby the size ofparticles as a result of particle collisions

Microflocculation = perikinetic flocculation

particle aggregation is brought about by the

random motion of fluid molecules (Brownianmotion)

Macroflocculation = orthokinetic flocculation

particle aggregation is brought about byinducing velocity gradients and mixing in thefluid and differential settling

28

8/13/2019 ESC3204+5



In the treatment…

Water Treatment

• coagulation and flocculation are essential components o f thetreatment processes

Wastewater Treatment

• chemical phosphorus removal

• In overloaded WWTP, coagulant is added to enhance 1o treatment – reducing SS and organic loads from 1o clarifiers

Can be used to remove:

• Natural organic matter – humic substances, woody substances

• Pathogens – Cryptosporidium,Giardia

• Inorganics – arsenic, flouride

• phosphorus


Colloidal particles

Particles which are just big enough to have a

surface which is microscopically observable or

which is capable of adsorption of another phase

30


50 – 70% of the organic matter in domestic WW is

composed of colloidal matters

In water treatment, color, turbidity, viruses, bacteria,

algae and organic matter are primarily either in

colloidal form or behave as colloids

Surface area ~ 1 sq. yd. to 1 acre/gram

1-micron colloid – 1 year to settle (by gravity) of 1 foot

Too small to be filtered by standard filtration devices


Measurement of colloid concentration

Surface area might be an excellent measure of

colloid concentration – difficult measurement

Standard suspended solids measurement won’t

work because colloids will pass through filters

The best method is _________________ or the

measurement of light scattered by the colloids

32

8/13/2019 ESC3204+5



Removal of Hydrophobic Colloids

• Destabilization (or Coagulation)

Reduce the forces acting to keep the particles apart after

they contact each other (i.e. lower repulsion forces)

• Flocculation

Process of bringing destabilized colloidal particles together toallow them to aggregate to a size where they will settle bygravity

After coagulation/flocculation, gravity sedimentation orfiltration are employed to remove flocculated colloids


Colloidal stability

Colloidal stability can be influenced by the

degree of affinity for the solvent in which colloid

is suspended

Regardless of how the surface charge develops –

which promotes stability – must be overcome if

these particles are to be aggregated

(flocculated) into larger particles with enough

mass to settle easily

34


Hydrophobic – colloids are (-) charged and

dispersion is stabilized by electrostatic repulsion

e.g. clays, metal oxides, sulfides (chemical

origin)

Hydrophilic – colloids have a great affinity for

the solvent (e.g. water), colloids are slightly

charged, dispersion is stabilized by hydration

e.g. gelatin, starch, proteins (biological origin)


Coagulants

Al3+ and Fe3+ accomplish destabilization of

colloids through

- Adsorption and charge neutralization

- Enmeshment in sweep floc

- Adsorption and interparticle bridging

36

8/13/2019 ESC3204+5



Hydrolysis

The hydrolysis products of Al3+ and Fe3+ are responsible

for particle aggregation

Coordination compounds:

Amphoteric: can exist both in strong acids and bases:

6 +

↔ +

6

− ↔ −

37

Proposed hydrolysis of trivalent metal

salts

With addition of enough base




Aluminum Sulfate (Alum)

When alum is added to WW containing Ca, Mg

bicarbonate alkalinity

3 ∙ 18 ↔ 2 3 6 18

40

3 x 100 as CaCO3 666.5

2 x 78 3 x 136 6 x 44 18 x 18

8/13/2019 ESC3204+5



Lime

↔ 2

↔ 2 2

A sufficient lime must be added to combine with

all the free carbonic acid and with the carbonic

acid of the bicarbonates to produce CaCO3


Ferrous Sulfate and Lime

Ferrous sulfate must be added with lime to form precipitate:

∙ 7

↔

7

↔

If sufficient alkalinity is not available lime is added in excess:

2 ↔ 2 2

Ferrous hydroxide can be oxidized to ferric hydroxide:

1

4

1

2 ↔

42


Ferric Chloride

Ferric chloride is used due to problems with ferrous

sulfate

2 3

↔ 2 3 6

If lime is added:

2 3 ↔ 2 3


Ferric Sulfate and Lime

2 3

↔ 2 3

44

8/13/2019 ESC3204+5



Factors affecting coagulation

Temperature

• Affect low turbidity water, by shifting theoptimum pH

• Pre-polymerized coagulants (PACl), polyferricsulfate can be less sensitive to changes

Sequence of chemical addition

• pH correction>metal coagulant>flocculant aid

• The best sequence can be determined by Jar Test


Jar Test

• Simulate the coagulation/flocculation process

in a batch mode• A series of batch test are run to get optimal

dosage = lowest residual turbidity

– pH

– coagulant type and dosage

– coagulant aid

46


Water samples Various dosages rapid

mixing (coagulation)

gently stirred (flocculation) stop stirring


Phosphorus Removal

Excess calcium ions will react with phosphate to

precipitate hydroxylapatite

10+ 6+ 2− ↔

Phosphate with aluminum/iron:

+ −

↔ +

+ −

↔ +

48

8/13/2019 ESC3204+5





Factors affecting the choice of chemical for

phosphorus removal:

• Influent phosphorus level

• WW suspended solids

• Alkalinity

• Chemical cost (transportation)

• Reliability of chemical supply

• Sludge handling facilities

• Ultimate disposal methods

• Compatibility with other treatment processes


Strategies for Phosphorus Removal

Precipitation – chemicals added to raw WW and PO4 isremoved with 1o sludge

Co-precipitation – chemicals added to form precipitatesthat are removed along with waste biological sludge

- effluent from 1o sedimentation, mixed liquor, effluentfrom biological treatment process before 2o

Post-precipitation – chemicals added to effluent 2o sedimentation and removed in separate sedimentationfacilities or effluent filters

52

8/13/2019 ESC3204+5



Metal salt addition

- 1o sedimentation tanks

- 2o treatment

- 2o clarifiers (+ polymer)

Lime

- 1o sedimentation tanks

- Following 2o treatment

- Effluent filtration


Removal of NOM

• Humic substances from the aqueous

extraction of living woody substances, soilorganic matter

• Removed due to:

– Color

– Transport for toxic substances/micro-pollutants

– React with chlorine

54


• Removal is effective with aluminum sulfate,

ferric chloride, ferric sulfate, polyaluminum

chloride, acid alum (aluminum sulfate with 1-

15% H2SO4)

• TOC or NOM reductions depend on:

– Type and dosage of coagulant – pH, temperature, water quality

– NOM characteristics

• Optimum pH for:

– Ferric salts: 3.7 – 4.2

– Aluminum sulfate: 5.0 – 5.5


Removal of Pathogens

• USEPA requires 99.9% (3-log) Giardia removal

and 99% (2-log) removal of Cryptosporidium

• WTP with coagulation, flocculation,

sedimentation, filtration – 2.5-log Giardia

56

Date post:	04-Jun-2018
Category:	Documents
Upload:	zhelresh-zeon
View:	221 times
Download:	0 times

ESC3204+5

Documents