TERTIARY PROCESS

PRIMARY PROCESS

SECONDARY PROCESS

Wastewater

Bar Rack/Comminutor

Grit Chamber

Equalization Basin

Primary Settling

Biological Treatment

Secondary Settling

Advance Wastewater Treatment

PRETREATMENT PROCESS

e.g: Depth Filter, Membrane filter, adsorption, Ion exchange, gas stripping etc

Refer Table 5-1: Typical physical unit operations

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Retain solid found in influent WW to the treatment plant

Principle role

Screening

Coarse screen Remove coarse materials (e.g. sticks, rags, etc) from the flow stream that could :

damage subsequent process equipment

Reduce overall treatment process reliability & effectiveness

Contaminate waterway

Fine screen (used in place of/following coarse screen) Where greater removals of solids are required to remove coarse materials (e.g. sticks, rags , etc) from the flow stream that could

Protect process equipment

Eliminate materials that may inhibit the beneficial reuse of biosolid

The bar screen may be manually cleaned or mechanically cleaned (performed frequently enough to prevent solids buildup and reduce flow into the plant)

lightThe flow passes through the screen and the large solids are trapped on the bars for removal.

Bar Screen/ Bar Rack

May consists of parallel bars, rods or wires (coarse screen) perforated plate (fine screen)

The bar screen may be coarse (6 – 150 mm/ 0.25 – 6 inch openings) or fine (< 6 mm/0.25 inch openings).

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Remove grit (sand, egg shells or other heavy solid materials) that tends to settle in corners, bends, reducing flow capacity and ultimately clogging pipes and channels.

Grit chamber are provided to

Grit ChamberProtect moving mechanical equipment from abrasion

Reduce formation of heavy deposits in pipelines, channels & conduits

Reduce the frequency of digester cleaning caused by exessive accumulation of grit

Grit removal processes use gravity/velocity, aeration or centrifugal force to separate the solids from the wastewater.

The most common method of grit disposal is transport to a landfill. In some large plants, grit is incinerated with solids

Once screened, the wastewater passes into two aerated grit chambers. Low-pressure air entering the grit chamber creates a rolling motion that causes grit and dense solids to settle to the tank bottom.

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used to intercept coarse solid & shred them in the screen channel

The use of comminutor and macerator is adventageous in a pumping station to:

Comminutors & Macerators Protect pump againts clogging by rags & large objects

Eliminate the need to handle & dispose of screenings

However, shredded solid (plastic bags, rags) tends to form ropelike strands & can clog pump impellers, sludge pipelines & heat exchangers).

The solids are cut up into a smaller, more uniform size of for return to the flow stream for subsequent removal.

Design consideration:•may be preceded by grit chambers to prolong life•Constructed with bypass arrangement

Macerators

• To minimize fluctuations in WW characteristics in order to provide optimum conditions for subsequence process

• To provide adequate dampening of organic fluctuations in order to prevent shock loading to biological system

• Provide adequate pH control • Provide cont. feed to biological system• Provide capacity for controlled discharge• To prevent high conc. of toxic materials from entering the biological

treatment plants.

Bar Screen / communi

tor

Grit Removal

Equalization Basin

Primary Treatmen

t

Effluent for further treatment

Q

t

Q

t

In-line arrangement: •all of the flow passes through the equalization basin•Can be used to achieve considerable amount of constituent conc. and flowrate damping.

Off-line arrangement:•Only flow above predetermined flow limit is diverted to equalization basin•Used to capture ‘first flush’ from combined collection system

Volume requirements for equalization tank

• Obtain from ‘Cumulative volume vs Time of day’ graph

(refer textbook page 336)

• Volume = vertical distance from point of tangency to the straight line representing the average flowrate

Objectives:Prepare WW for biological treatment (stabilize organic)Remove + 60% SS and 35% BOD5 in sewageImportant because the reduction of the suspended solids and BOD5

1)lowers the O2 demand, 2)decreases the rate of energy consumption and 3)Reduces operational problem with downstream biological treatment 4)remove scum (grease, oil, plastics, and other floatable materials) and inert particulate matter which are not removed in grit chamber

( primary & secondary process handle MOST of the NON-TOXIC wastewater)

Principle form of primary treatment: SEDIMENTATION

Note: sedimentation tank = sedimentation basin, clarifier, settling basin, settling tank

Objectives:Speed up natural process of breaking down biodegradable organicsRemove up to 85 % SS and BOD5

Devices/structures:• Activated sludge, extended aeration, rotating biological contacting

(RBC), trickling filter, aerated lagoons, sequencing batch reactor etc

• Biological degradation of soluble organics.• Mostly aerobically in an open aerated vessels @ lagoon• Speed up natural processes of breaking down biodegradable

organics• Cannot remove N, P, heavy metals, pathogens, bacteria and

viruses.• After treatment, microorganism and other carried over solids are

allowed to settle.• A fraction of sludge is recycle• Excess sludge along with sediment solids has to be disposed off.

Objectives:Nutrients removal, chlorination and dechlorinationProcess added after biological treatment in order to remove specific group/ types of residualCan remove + 95% BOD5, P, SS, bacteria and N

Devices/structures:• Filtration –removes SS• Granular Activated Carbon – removes organics• Chemical oxidation – removes oxidizable organics

• Expensive to process LARGE VOLUME of WW

of organic matter or by substance added to the WW.

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Depends on the degree of treatment required to bring the quality of raw wastewater to a permissible level of treated wastewater (eg. Effluent from the treatment plant)

This ensures that the final effluent is either safe for disposal or acceptable for specific reuse or recycling.

Other significant factor that will influence the selection of a treatment system

Selection of treatment process

Ref: (Karia and Christian,2006)

Availability of funds and land at the treatment site

Non-availability of suitable mechanical equipment and skilled personnel for running and maintaining the plant.

The topography of land at the treatment site

of organic matter or by substance added to the WW.

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Reduction of inorganic material component of wastewater is much easier and cheaper than removal of organics contents of wastewater

Removal of suspended solids from wastewater requires lesser time and efforts than of colloidal and dissolved solids

In many countries, the Environmental Protection Act requires at least the secondary treatment system for all publicly owned treatment works such as municipal wastewater treatment plant, so that effluent requirements of 30mg/L for BOD and 100mg/L of SS are achieved.

The points to keep in mind while selecting the treatment process

Ref: (Karia and Christian,2006)

Essential consideration

Strength & characteristics

of WW

Flow rate and their fluctuations Mass

loading

Design criteria

of organic matter or by substance added to the WW.

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The strength of wastewater is normally expressed in terms of pollution load, which is determined from the concentrations of significant physical, chemical and biological content of wastewater.

Characteristics of WW depend on the quality of water used by the community, culture of population, type of industries present & treatment given by industries to their WW.

The strength of WW measured as mass per unit volume of WW (Units: mg/L )

If characteristics of raw WW show the concentration of specific constituents like BOD & SS within the standard permissible limits, there is no need to treat the WW.

Strength & characteristics

of WW

of organic matter or by substance added to the WW.

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Is the quantity or volume of wastewater in terms of rates

It is the total quantity of wastewater generated daily and to be treated every day.

The flow rate units:m3/day or m3/s or MLD (million Litres per Day)

The volume of WW depend on the water consumption by the population for its various activities

FLOW RATE &THEIR

FLUCTUATIONS

Normally a treatment plant is designed on the daily average flow basis which is known as plant capacity.

Example: 1 MLD (Million Litres per Day) plant means = the plant designed for 1 – ML daily average flow of WW

of organic matter or by substance added to the WW.

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The mass pollution load is defined as flow rate & strength of WW & is expressed as load per unit time

Example:WW having 1000 m3/d flow & 200 mg/L (g/m3) BOD has the mass pollution load of BOD equal to 200 kg/d (1000 m3/d X 200 g/m3 X 10-3 g/kg)

In the case of treatment plant that receives flow of combined sewerage system, the seasonal variation in the rainy season will lower down the BOD & SS concentration due to the dilution because of the added amount of storm water. On the other hand, a higher concentration of BOD & SS may be observed during the dry weather period.

MASS LOADING

Therefore, in almost all cases, a flow-weighted average should be used because it is more accurate method of analysis

Where ,

XW = flow –weighted average concentration on the constituentXi = average concentration of the constituent during the “i” time periodQi = average flow rate during “i” time period

i

iiW Q

QXX

of organic matter or by substance added to the WW.

The data determined through the research and laboratory scale model studies as well as those obtained from the operational experience of field and pilot scale WW treatment facility. The values of such guideline parameters are called design criteria and available in the literature.

The most frequently assumed criteria for designing a conventional WW treatment plant (WWTP):

• Detention period or time• Flow through velocity• Settling velocity• Surface loading rate @ over flow rate• Weir loading rate • Organic loading (BOD @ COD @ VSS loading)• Food to Microorganism ratio, F/M• Mean cell Residence Time• Hydraulic Loading• Volumetric Loading• Basin geometry (L:B:D) length, breadth and depth ratio.

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Process of heavier solid particles in suspension, settle to the body of tank by gravity

Removal of SS from WW

Sedimentation

Depends on Velocity of flow

Size and shape of particles

Viscosity of water

Types of particles

Discrete /non-flocculant particles

Flocculant particles

Size & velocity constant during the settling

Size & velocity fluctuates during the settling

LESS COSTLY than many other treatment processes

Common operation & found almost in WWT plant

The settling of discrete particles can be analysed by means of the classic laws of sedimentation by Newton & Stokes.

pp gVFg )(

2

2pwpd

d

ACF

Gravitational force,

Frictional drag force,

Refer page 363 in text book

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In the design of sedimentation basin, the settling velocities of the particles MUST be KNOWN.

sedimentation

The knowledge of settling velocity of particle is used to determining the depth of a treatment unit to separate the suspended solids (particulate matter) by gravity settling and for checking the adequacy of length or diameter of a tank to remove particles before the effluent flows out of the basin.

A

Qvc

Where,vc = particle settling velocityQ = flowrate of WWA = surface of sedimentation tank

Idealized discrete particles settling in 3 different type of basins

Inle

t zo

ne

Outl

et

zone

Sludge zone

RECTANGULAR BASIN CIRCULAR BASIN

UPFLOW BASIN

Sludge zone

Inle

t zo

ne

Inle

t zo

ne

Outl

et

zone

Outl

et

zone

Inle

t zo

ne

Outl

et

zone

Settlingzone

Settlingzone

Settlingzone

Rectangular Basin

Circular Basin

Classification of particles settling

Type 1Discrete

Type 2Flocculant

Type 3Zone

Particles DOES NOT change in size, shape & density during the settling process

Particles settle discretely at a constant velocity

Settle as individual particles & do not flocculate

Occurs during: Presedimentation for sand removal

Grit Chamber

Flocculate during sedimentation

Particles size constantly changing

Settling velocity is changing increase with depth & extent of flocculation

Occurs during: Alum or iron coagulation

Primary sedimentation basins

The floc particles adhere together & the mass settle as a blanket (layer)

Distinct clear zone & sludge zone present

Concentration HIGH (greater than 500 mg/L)

Occurs during: Activated sludge sedimentation

Sludge thickeners

Solid settle

water

Initially, all the sludge is at uniform concentration A

A settling proceeds, the collapsed solid on the bottom of the settling unit (D) build up at constant rate.

C is zone of transition through which the settling velocity decrease as a result of high conc. of solid.

Through the transition zone C, the settling velocity will decrease due to the increasing density & viscosity of the suspension surrounding the particles.

When the rising layer of settle solid reaches the interface, a compression zone occur.

Transition zone

Dense solid

A

B

A

D

C CA

D

BB

D

High of sludge liq interface

Settling properties of flocculated sludge

Sett

ling

Zone

Tra

nsi

tion

Zone

Com

pre

ssi

on

Zone

The following design criteria are generally assumed to design a Primary Settling Tank / Sedimentation

A) GENERAL

No. of Tanks 2 or more (usually)

Types of tanks Circular or rectangular

Removal of Sludge and Scum Mechanical (usually)

Tank bottom slope 60-150 mm/m

Speed of sludge scraper 0.02 – 0.05 rpm

Refer Table 5-20 in textbook

B) DIMENSIONS Range Typical

Rectangular Tank

Length (m) 15-100 30

Width (m) 3-30 10

Depth 2.5-5 4

Circular Tank Diameter (m) 3-60 30

Depth (m) 3-5 4

Bottom slope, (mm/mm)

0.02 – 0.05 0.03

Range Typical

Detention Time, t (hr) 1.0 – 4.0 2.0

Flow Through velocity (m/min) 0.6 – 3.6 0.9

SLR (m3/m2/hr) at average flow 1.2 – 2.5 1.6

Peak Hourly Flow 2.0 – 5.0 4.2

WLR (m3/m/d) 125 - 500 250

C) TECHNICAL

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Clarifiers

Types

Overflow rate (surface loading rate)

Primarily used in WWT to separate solids from liquids in effluent streams.

Criteria for sizing clarifier (settling tank)

Tank depth at the side wall

Detention time Scour velocity

Clarifiers

Definition: The average daily flow rate divided by the surface area of the tank.

A

QOR

overflow rate @surface settling rate

(m3/m2d)

Average daily flowrate (m3/day)

Total surface area of the tank (m2)

Depth of tank

The water depth at the side wall measuring from the tank bottom to the top of the overflow weir.

Exclude the additional depth resulting from slightly sloping bottom that is provided in both circular and rectangular clarifiers.

Influent

Occupied with sludge

Influent

Effluent weir

H

Effluent weir loading (typical= 250 m3/m.d) is equal to quantity of WW flowing divided by the total weir length, Lw

dm

m

L

Q

W .loadingeir Effluent w

3

Average daily flowrate (m3/day)

Total weir length(m)

Q

Vd t

Tank volume (m3)

Detention time =(day)

Average daily flowrate

(m3/day)

Detention timelength of time a particle or a unit volume of WW remains in a reactor

2/1

H)1(8

V

f

gdsk

Scour Velocityhorizontal velocity through the tank to avoid resuspension of settled particles

Where:VH = horizontal velocity that will just produce scour (m/s)k = cohesion constant that depends on type of material being scoured (unitless)s=specific gravity of particlesg=acceleration due to gravity (9.81 m/s2)d=diameter of particlesf=Darcy-Weisbach friction factor (unitless)

of organic matter or by substance added to the WW.

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Used for the removal of lighter SS, oil & grease.

Also used to concentrate biological sludge and to separate both the fine solid and a liquid particles from the liquid phase

Introducing fine gas (air) bubbles into the liquid phase.Bubbles will attach to the particulate matter , thus increase the

buoyant force, cause the particle to rise to the WW surface .Floated particles are collected by skimming operation .

Thus, the operation is just the opposite of that of gravity sedimentation where particles get removed at the bottom of the tank.

Flotation

Advantage: Removal of smaller particles in a shorter time and more complete.

Degree of particle removal can be enhanced by addition of chemical additives

Example of Flotation System

Frequently used in industrial WW

treatment

Frequently used in Municipal WW

treatment

air bubbles are formed by introducing the air in the form of gas phase directly into the liquid phase either by a revolving impeller or through air diffusers at the atm pressure.

WW is first saturated with air either directly in the aeration tank or by introducing air at the pump side (at Patm) Then partial vacuum is applied. This results in generation of small air bubbles which attached themselves to the particles and make them rise, forming a scum blanket. Typically a cylindrical tank maintain under vacuum is applied and continuously fed with WW

Flotation is achieved first by dissolving the air in

the WW or in a portion of treated effluent (liquid) under high pressure in

the pressurizing or retention tank and then

reducing the pressure of the WW through a

pressure-reducing valve to atmospheric level

during feeding it to the flotation tank to form the

rising air bubbles.

A predetermine fraction of effluent from the flotation unit is taken to the

pressurized tank where it is pressurized, and the air is dissolved below the

saturation level.

The flow is then mixed with the influent entering the flotation unit through a pressure-reducing valve so that air

bubbles come out from the recycled flow and get attached with the particles of

incoming raw wastewater that are to be removed by flotation.

Systems based on recirculation of effluent

In large treatment plants, normally 15-20% of the effluent is recycled Small treatment plants operate without recycling the effluent.

Example: Dissolved-air Flotation (DAF)

Effluent is recirculated Effluent is not recycled

Wastewater influent is first retained for some time in the pressure tank where pressure of wastewater is increased to as high as 275-

350kPa and air is dissolved in it.

Then the flow is fed to the flotation unit through a pipeline having a pressure-

reducing valve.

As the pressure is released from wastewater, the dissolved air comes out of

the solution as fine bubbles which are used for particle separation by flotation.

Dispersed-Air Flotation

Dissolved-Air Flotation (no recycle)

Please refer diagram in your textbook

Dissolved-Air Flotation (with recycle)

Please refer diagram in your textbook

Design consideration

Concentration of particles to be

removed

Particle rise velocity or

buoyant force

Air/solid ratio

Solids loading rate

Quantity of air required for

formation of air bubbles

Dissolved air flotation units are usually designed on the basis of the air to solid ratio, A/S, using the following equations:

For system with recycle For system without recycle

QS

RfPs

S

A

a

a )1(3.1

a

a

S

fPs

S

A )1(3.1

Where, A = volume of air (ml) S = mass of solids (mg) 1.3 = weight of 1ml of air (mg) sa = solubility of air in (ml/L) (temp depended funct)

Refer page 422-423

f = fraction of air dissolved at pressure P (atm)P = operating pressure (atm)Sa = influent suspended solids or sludge solids

(mg/L)R = pressurized recycled flow (m3/d)Q = mixed liquor flow (m3/d)

Typical Design Criteria

Hydraulic retention time, HRT

20-30min

(for efficient primary clarification)

Rising rate or surface loading rate

0.06 – 1.63 m3/min-m2

Rising velocity of air-solid mix

when no flocculants are used

2.56 – 12.7 cm/min

when flocculants are used

20 – 60 cm/min

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Employ for the removal of SS, following coagulation in physical-chemical treatment or as tertiary treatment

following the biological WW treatment process

SS are removed using granular filter medium (depth filter) principally by straining mechanism, consists of

surface removal and depth removalFiltration

The efficiency of filtration process is depends on•character of media•character of SS•temperature•flow rate•bed depth•time (throughput volume)

Primary design/operating parameters

Quality (SS conc.) of effluent

Headloss through the filter & accesories

Note: Headloss is the reduction of total head or pressure drop of a liquid as it moves through a system.

The filter run terminates when:

Filtration

the total head loss reaches maximum point (high enough)

excess SS or turbidity appears in the effluent.

Filtration rate will effect :

The build up of head loss

the effluent quality attainable

The head loss through the filter can be described by D’Archy’s Law

L

hKV fp

Where,V = superficial approach velocity (ft/min).Kp = coefficient of permeability (ft/min). This will change with time

hf = frictional head loss (ft)

L = depth of filter (ft).

Material Shape Relative Density

Porosity (%)

Effective Size (mm)

Silica sand Rounded 2.65 42 0.4-1.0

Silica sand Angular 2.65 53 0.4-1.0

Ottawa sand

Spherical 2.65 40 0.4-1.0

Silica gravel

Rounded 2.65 40 1.0-5.0

anthracite Angular 1.5-1.7 55 0.4-1.4

garnet angular 3.1-4.3 46 0.2-0.4

The selection of medium filters is depends on the type of filter and operation.See example in Table 11-6, 11-8, 11-9

Use to separate dissolved and colloidal constituents from WW

Membrane Process

Components in water are driven through a membrane under the driving force of:

pressure

electrical potential

The size of the opening in the membrane are a major determinant of species that can pass because the opening present a physical barrier to any substances that are larger

than openings.

concentration gradient

A semi permeable membrane is SELECTIVE to the species it passes.

Semi permeable membrane

Permeate RetentateThe liquid passing through

the membraneThe fraction not passing through the membrane.

separation mechanism

nature of driving force

Membrane process classification(Refer Table 11-17 in textbook)

membrane’s material

size of separation

•Thickness=0.2-0.25μm,supported by porous substrate•Flat sheets,fine hollow fiber,or tubular form•For WW treatment,typically organic membrane.•E.g:polypropylene, cellulose acetate, aromatic polyamides and TFC

•Hyrostatic pressure difference (MF, UF, NF, RO)• concentration difference (dialysis)

•MF, UF = straining•NF = straining & diffusion•RO= diffusion

•Macropores= >50 nm•Mesopores=2-50 nm•Micropores= <2 nm•For RO = very fine pores, known as dense

Membrane Operation in MF and UF:

Cross flow•2 types: a) without reservoir b) with reservoir

•Feed water is pumped with cross flow tangential to the membrane.•Water that does not pass through the membrane is recirculated after blending with additional feed water•Cross flow with resorvoir-water that does not pass through membrane is recirculated to storage tank.

Direct feed or dead-end

•No cross flow•All water applied to membrane passes through the membrane

Refer page 1112 , Figure 11-37.

REVERSE OSMOSIS (RO) –if a pressure gradient opposite in direction and greater than osmotic pressure, flow from the more concentrated to the less concentrated region will occur

OSMOTIC PRESSURE – balancing pressure difference between pressure and chemical potential , occur when two solutions having different solute conc. are separated by semipermeable membrane.

Where,Fw = flux of water (mass/area.time)

A= area of the membranekw = water mass transfer coefficient ∆Pa = average imposed pressure gradient∆π=osmotic pressure gradientQp = permeate stream flow

A

QPkF paww )(

The flux of water through the membrane (Eq 11-43);

The flux of solute depends on the concentration gradient and resistance parameter.

Membrane Process

Measurement of the ability of membrane to

reject the passage of a species i,

Rate of Rejection =

Where,Fi= flux of solute, kg/m2s

ki = solute mass transfer coefficient, m/sΔCi = solute concentration gradient, kg/m3

Cp = conc of solute in the permeate,kg/m3

Qp = permeate stream flow, m3/s A = membrane area, m2

A

CQCkF

ppiii

Where,Ri = rejection rateCif = conc of species i in the feedCp = conc of species i in permeate

100x ,%

if

ipifi C

CCR

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Fouling of a membrane increase resistance to flow and reduces the flux of water through a membrane

Backwashing or chemical treatment may be applied to remove foulants.

Membrane Process

Irreversible fouling of membranes is the most serious problems.

Oxidizing agents such as chlorine or ozone attack membranes and change their structures.