Chapter 4b - Wastewater Treatment 2

8/12/2019 Chapter 4b - Wastewater Treatment 2

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Chapter 4B

Wastewater Treatment:Primary and Secondary

1

Treatment


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Learning Outcomes

At the end of this learning activities the

students are expected be able to;

1. Recognize the function of primary

2

2. Apply knowledge on secondary

treatment on wastewater

3. Apply different treatment methods onwastewater


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3


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Flow equilization is done after the wastewater

passed through screens and grit chambers The objectives of flow equilization are:

To equalize the flows to minimize flow surge

Flow Equalization

4

To equalize the organic loads to dampen fluctuations To neutralize the pH variations to bring it to the range

6.5-8.5

To provide a continuous wastewater flow to the plant To control of high toxicity loads.


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Example of Flow Equilization

5


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Flow Equalization Flow equalization is needed to overcome the

variation in flow rates into a w/w treatment plant The purpose of flow equalization is to dampenthis variations so that the wastewater can betreated at near constant rate

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Flow equalization can significantly improve theperformance of existing plant and increase itsuseful capacity

Flow equalization is achieved by constructinglarge basin that collect and store the wastewaterflow and from which the wastewater is pumpedto treatment plant.


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Flow equalization is achieved by reducing the variations

in flow rate and/or concentrations of the wastewaterbeing fed to the treatment facility by using equalizationbasins.

Flow equalization

Flow Equalization

7

Common in industries that operate a 5 day week. The flow is balanced or spread out equally over 7 days so that

the flow arriving into the plant is the same for each of the 7 days.

Organic equalization

Industries may at different times during the week have a highCOD effluent, lasting only a few hours.

If this sent directly through the treatment plant it may cause ashock load with consequent problem.

Then balancing the high load should be done by retaining thepollutant load in a balance or equalization tank prior to treatment.


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Nutrient Balancing

Where nutrient will be added to the influent wastewater if thewastewater be deficient in nutrient.

pH Balancing Should be re uired for influent of wastewater treatment lant

Flow Equalization

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which too high or low in pH for secondary treatment. It is desirable that the pH be in range 6.5 -8.5 for activated

sludge treatment system.

Many industrial wastewater not within the range therefore needto be balance.


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Primary Treatment

(Physical Treatment)

Primary treatment is to provide protection to

WWTP equipments Primary treatment remove large objects by bar

racks and screens

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Grits (sand, broken pebbles, broken glass andsilt) are also removed in grit chambers to protect

expensive equipments such as pumps

After going through screens and grit chambers,wastewater is then kept in the primary settling

tanks for a suitable period of time (retention

time) to remove other settleable solids


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Primary Settling Basins

(Rectangular)

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Rectangular Primary SettlingTank

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Circular Primary Settling Tank

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Circular Primary Settling Tank

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Primary Settling Tank Design(typical values)

Size rectangular: 3-24 m wide x 15-100 m long

circular: 3-90 m diameter

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Detention time: 1.5-2.5 hours Overflow rate: 25-60 m3/m2.day

Typical removal efficiencies

solids: 50-60% BOD5: 30-35%


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Using an overflow rate of 26 m/d and detention

time of 2 hrs, find the size of primary

Example of PrimaryTank Design

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. .

What would be the overflow rate? Assume 15

sedimentation tanks with length to width ratio of

4.7.


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Solution

17


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Unit Processes of

Secondary Treatment

Conventional aerobic secondary biologic treatmentare the availability of many microorganisms,organic material, oxygen and favorableenvironment (temperature and sufficient time)

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The stabilization of organic material (pollutant) isaccomplished by microbes which convert colloidaland dissolved organic matter into gases andprotoplasm

ExampleC5H7O2N + 7O2 4CO2 + 3H2O + HNO3


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Aerobic Decomposition (AD)

of Wastewater

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Occurs in the presence of oxygen Organic material oxidized aerobically by microbes resulting in large

production of new cells generating sludge (dead and living cells)

AD is only suitable for low strength wastewater (ie < 500 mg/l BOD).

For high strength w/w (>1000 mg/l BOD), AD not suitable because ofdifficulty in supplying of enough oxygen and also because of the amountof sludge produced

Organic Matter + O2new cells + energy + CO2 + H2O(CHONSP)


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22Aeration Tanks in Aerobic Treatment


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Anaerobic Decomposition (AnD)

Occurs in the absence of oxygen. This

process is also called fermentation

It is a two step process;

First, complex organic compoundsare fermented to low-molecular

weight volatile fatty acids (VFA)

Secondly, the organic acids are

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convened to methane (CH4)

AnD yields CO2, CH4 and H2O as

major end products

Because of small amount of energy

released, the amount of cell production

is low, thus sludge production is alsolow.

Organic Matter (CHONSP) + Combined O2 (from organics)

new cells + energy + CH4

+ CO2

+ H2O + other end products


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Gaseous oxygen is excluded from the reactions by physical

containment.

Anaerobes utilize electron acceptors from sources other thanoxygen gas. These acceptors can be the organic material itself or

may be supplied by inorganic oxides from within the input material.

When the oxygen source in an anaerobic system is derived from the

Anaerobic Digestion

organ c mater a tse , t e nterme ate en pro ucts are

primarily alcohols, aldehydes, and organic acids, plus carbon

dioxide.

In the presence of specialised methanogens, the intermediates are

converted to the 'final' end products of methane, carbon dioxide, and

trace levels of hydrogen sulfide.

In an anaerobic system, the majority of the chemical energy

contained within the starting material is released by methanogenic

bacteria as methane.

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There are two conventional operational temperaturelevels for anaerobic digesters, which are determined

by the species of methanogens in the digesters: Mesophilicwhich takes place optimally around 30-

38C or at ambient temperatures between 20- 45Cwith mesophiles

Anaerobic Digestion

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mesophilic archaea are the primarymicroorganism present

Thermophilicwhich takes place optimally around49- 57 at elevated temperatures up to 70C withthermophiles

thermophilic archaea - are the primarymicroorganisms present


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Anaerobic WW Treatment Facility

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Comparing AD and AnD

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Comparing AD and AnD

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AEROBIC: More sludge to manage (60%)

often causing problems

Main product CO2 and H2O

No odor

ANAEROBIC: Less sludge to manage(10%)

Main product CH4, CO2 and H2O and

Bad odor due to H2S and NH3 gases


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Biological Treatment Main agent of biological treatment is the

microorganisms (microbes) that thrives in the

municipal wastewater

These microbes consumed the organic

po u an s n e was ewa er as e r oo

The degradation of organics (pollutant) is done

aerobically or anaerobically

The microbes reproduce and multiply in thewastewater resulting in more numbers to

continue degrading pollutants until the

wastewater rendered clean29


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Bacterial Growth Requirements1. Terminal electron acceptor

2. Macronutrients

a. carbonb. nitrogen

c. phosphorus

3. Micronutrientsa. trace metals

b. vitamins

4. Environmenta. moistureb. temperature

c. pH


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Bacteria Growth in Pure

Cultures


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Phases of Growth (Wikipedia) During lag phase, bacteria adapt themselves to growth conditions.

It is the period where the individual bacteria are maturing and notyet able to divide. During the lag phase of the bacterial growthcycle, synthesis of RNA, enzymes and other molecules occurs.So in this phase the microorganisms are not dormant.

Exponential phase (sometimes called the log phase or thelogarithmic phase) is a period characterized by cell doubling. The

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num er o new ac er a appear ng per un me s propor ona o

the present population. Exponential growth cannot continueindefinitely, however, because the medium is soon depleted ofnutrients and enriched with wastes.

During stationary phase, the growth rate slows as a result ofnutrient depletion and accumulation of toxic products. This phase

is reached as the bacteria begin to exhaust the resources that areavailable to them. This phase is a constant value as the rate ofbacterial growth is equal to the rate of bacterial death.

At death phase, bacteria run out of nutrients and die.


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Mathematics of Growth log growth phase

....222222 0000 ++++= PPPPP

nPP )2(0=

2logloglog 0 nPP +=


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Example Bread yeast cells divide and form 2 cells every

5 minutes. If you place 105

cells in a suitableenvironment, how many cells will you have in

30 minutes?

sgeneration6nerationminutes/ge5

minutes30==n

( ) cells10x4.6210 665 ==P


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How is this accomplished? Create a very rich

environment for

growth of a diverse

microbial community

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MostAbundant Microbes in W/W

Aerobic treatment

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Amoeba Rotifer

Filamentous

Ciliated Protozoa

Flagellated Protozoa

Vorticella


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Secondary Treatment Provide BOD removal beyond what is achievedin primary treatment

removal of soluble BOD additional removal of suspended solids

Basic approach is to use aerobicbiological

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degradation:Organic Matter + O2 New Cells + CO2 +

(CHONSP) H2O + NO2 + PO4

Objective is to allow the BOD to be exerted in

the treatment plant rather than in the stream


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Monod Equation

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max = maximum growth rate, t-1

S = concentration of limiting food in solution, mg/L

Ks = half saturation constant, mg/L

= concentration of limiting food when, = max


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In the log growth phase;

Monod Equation;

XdtdX =

SK

S

s

m

+=

dX/dt = growth rate of biomass = growth rate constant

X = concentration of biomass

m = maximum growth rate, t-1

S = concentration of limiting food in

solution, mg/L

Ks = half saturation constant, mg/L

= concentration of limiting food when,

=

..1

.2

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Taking into account naturaldie-off;

. m

Xk

SK

SX

dt

dXd

s

m

+

=

kd= endogenous decay

3


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Rate of food utilization dS/dt would equal rate of biomass production

dt

dX

Ydt

dS 1=

Combining equation 1, 2 and 3

Y = decimal fraction of food mass converted

to biomass

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SKYdt s

m

+=


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Basic Ingredients

High density of microorganisms (keep

organisms in system)

Good contact between organisms and

wastes (provide mixing)

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Provide high levels of oxygen (aeration) Favorable temperature, pH, nutrients

(design and operation)

No toxic chemicals present (control

industrial inputs)


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Dispersed growth vs

Fixed Growth

Dispersed Growth suspended organisms Activated sludge

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Aerated lagoons, stabilization ponds

Fixed Growth attached organisms

Trickling filters

Rotating Biological Contactors (RBCs)


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Activated Sludge Process in which a mixture of wastewater and

microorganisms (biological sludge) is agitatedand aerated (disperse growth)

Leads to oxidation of dissolved or anics

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After oxidation, separate sludge fromwastewater

Induce microbial growth

Need food, oxygen

Want Mixed Liquor Suspended Solids

(MLSS) of 3,000 to 6,000 mg/L


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Activated Sludge

Mixed LiquorAeration Tank Air

w/winfluent

edSludge

)Air

Q,So

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, ,

SecondaryClarifier

Waste ActivatedSludge (WAS)

ReturnActiva

(R

AS

Treated

w/w Discharged toRiver or LandApplication

(Q+Qr) X,S

(Q-Qw),

S,Xe

Qr

,Xr

,S

Qw,Xr,S


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Activated Sludge Processr

Xw

Qe

Xw

QQXd

kS

sK

SXmV

oQX +=

++ )()(

Q = wastewater flowrate, m3/d

Xo = microbes concentration (VSS) entering aeration tank, mg/L

V = volume o aeration tank m3

Eq. 8.10

(pg 491)

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m

= maximum growth rate, d-1

S = soluble BOD5 in aeration tank end effluent, mg/L

X = microbes concentration (MLVSS) in aeration tank, mg/L

Ks = half velocity constant, mg/L

= soluble BOD5 concentration at one half the maximum growth rate

kd= decay rate of microbes, d-1Qw = flow rate of liquid containing microbes to be wasted, mg/L

Xe = microbes concentration in effluent from settling tank, mg/L

Xr= microbes concentration (VSS) in sludge being wasted, mg/L


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At steady state

( )( )

( ) SQSQQSKY

SXVQS wws

mo +=

+

Food in

influent

Food

consumed

Food in

efffluent

Food in

WAS++ =

Eq. 8.13

(pg 493)

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Combining Eq. 6.10 and 6.13

( ) dorw

kSSX

Y

V

Q

VX

XQ= Eq. 8.17

(pg 493)


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The inverse of left side of Eq. 8.17 defines

the mean cell residence time

c

rwXQ

VX=

The concentration of BOD5 in the effluent (S) is fixed,

Eq. 8.19

(pg 493)

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( )( ) 1

1

+=

dmc

cds

k

kKS

Eq. 8.21(pg 494)

From Eq. 6.17, the concentration of microbes in the tank,

( )( )( )cd

c

k

SSYX

+

=

1

0Eq. 8.23

(pg 494)


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Activated sludge

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Activated Sludge Vid


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Secondary Clarifier

49East Lansing WWTP


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Example 1Ex. A wastewater treatment plant to treat wastewater to meet

effluent standard of 25 mg/L BOD and 30 mg/L suspended

solids. The treatment plant flow rate is 0.029 m3/s. The effluentfrom the primary tank has BOD of 240 mg/L. Using the

following assumptions, estimate the required volume of the

aeration tank

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1. BOD of effluent suspended solids is 70% of the allowable

suspended solids concentration

2. Growth constant values are estimated to be;

Ks = 100 mg/L BOD, kd = 0.025 /d, m = 10/d,

Y = 0.8 mg VSS/mg BOD removed

3. Design MLVSS is 3000 mg/L


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( )( ) 1

1

+=

dmc

cds

k

kKS


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( )cdc

k +

=

1

F d t Mi i R ti


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Food to Micro-organism Ratio

(F/M Ratio) The F/M ratio is one of the major design parameter

0=VX

QSF

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(MLVSS)solidssuspended

atileliquor volmixed

volume

BODsolubleinitialrateflowwhere

50

=

=

==

X

V

SQ


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F/M Ratio Low F/M (low rate of wasting)

starved organisms

more complete degradation

larger, more costly aeration tanks

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2

higher power costs (to supply O2) less sludge to handle

High F/M (high rate of wasting)

organisms are saturated with food low treatment efficiency


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F/M Ratio F/M Ratio is controlled by

wasting part of the biomassthereby reducing MLVSS

High rate of wasting causeshigh F/M ratio (meaning morefood than organism) thuscausing poor treatment

Parameters Tank A Tank B

F/M

c

Sludge

Low

Long

Little

High

Short

Much

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Low F/M Ratio causes microbeto starve thus resulting in morecomplete degradation of waste(pollutant)

Long cell mean residence time

(c) is not always usedbecause this would result inbigger tank and longeraeration time (thus wouldincrease power consumption)

Power High Low

F/M values typically range

from 0.1 to 1.0 mg/mg


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F/M Ratio ExampleFlow = 0.15 m3/s, BOD5 = 84.0 mg/L

Volume of reactor = 970m3, MLVSS = 2000 mg/L

Calculate F/M Ratio in mg/mg.day

Total mass of subsrate (Food) = Q x BOD concentration

= 3

56

. ,

Mass of MLVSS = Volume of tank x Concentration of MLVSS

= 970 m3 x 2000 mg/L

Therefore F/M ratio = 0.15 m3/s x 84 mg/L x 86,400 s/d = 0.56 mg/mg.d

970 m3 x 2000 mg/L

Typical F/M Ratio values are 0.1 to 1.0 mg/mg


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S

57

VXM

=


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Sludge Volume Index (SVI) SVI is used to control the rate of sludge return to the reactor

basin in activated sludge process

SVI can be used as an indication of the settlingcharacteristics of the sludge, thereby impacting on the returnrate and MLSS

SVI is defined as the volume in ml occupied by 1 g of

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ac va e s u ge a er e aera e quor as se e

minutes as calculated below;

g

mgx

MLSS

SVSVI 1000=

SVI = Sludge Volume Index, mL/g

SV = Volume of settled solids in one liter graduated

cylinder after 30 minutes settling, mL/L

MLSS = Mixed liquor suspended solids, mg/L


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SVIOne sample of wastewater contain 4000 mg/L of MLSS. After settled

for one hour the volume of sludge in the 1L cylinder after 30 minutes

is 400 mL. Calculate SVI

SVI = SV x 1000 mL

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SVI = 400 mL x 1000 mL = 100 mL/g

4000 mg/L

Typical values of SVI is 80 150 mL/g for activated sludge

operating with concentration 2000 3500 mg/l of MLSS


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Activated Sludge Design td = approximately 6 - 8 hr

Long rectangular aeration basins Air is injected near bottom of aeration

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Aeration system used to providemixing

MLVSS and F/M controlled by wastinga portion of microorganisms


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Tutorials Refer to text book page 495, Example 4

Please do this example as well as questionsfrom page 561 564

Nos. 8.8, 8.10, 8.11, 8.12, 8.18, 8.22, 8.24

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A Trickling Filter is a fixed bed, biological filter that operates under(mostly) aerobic conditions. Pre-settled wastewater is trickled orsprayed over the filter. As the water migrates through the pores of

the filter, organics are degraded by the biomass covering the filtermaterial.

Advantages

Small land area re uired com ared to Constructed Wetlands.

Trickling Filters

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Can be operated at a range of organic and hydraulic loading

rates. Disadvantages/limitations

- High capital costs and moderate operating costs- Requires expert design and construction.- Requires constant source of electricity and constant w/w flow.

- Flies and odours are often problematic.- Not all parts and materials may be available locally.- Pre-treatment is required to prevent clogging.- Dosing system requires more complex engineering.


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Trickling Filter Plant Layout

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Trickling Filters


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Trickling Filters

Rotating distribution arm sprays primary

effluent over circular bed of rock or other

coarse media

Air circulates in pores between rocks

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Biofilm develops on rocks and micro-

organisms degrade waste materials as

they flow past

Organisms slough off in clumps when filmgets too thick


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Trickling Filters

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Slime growth on rocks in trickling filter

Latest Technology


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Latest Technology

Trickling filter medium

68High Density Polyethelene

(HDPE) Polypropelyne (PP)

(HDPE)

Trickling


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Trickling

Filters

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Trickling Filters Not a true filtering or sieving process

Material only provides surface on whichbacteria to grow

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lighter - can get deeper beds (up to 12 m) reduced space requirement

larger surface area for growth

greater void ratios (better air flow) less prone to plugging by accumulating slime


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Trickling Filter Example

A trickling filter has diameter of 15 m anddepth of 5 m, if flow is 3000 m3/day, calculate

the surface overflow rate SAR

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Answer : SOR = Q/A= 3000/(D2/4)

= .? m

3

/day.m

2


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Rotating Biological Contactors Called RBCs

Consists of series of closely spaceddiscs mounted on a horizontal shaft

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an ro a e w e ~ o eac sc s

submerged in wastewater Discs: light-weight plastic

Slime is 1-3 mm in thickness on disc


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Rotating Biological Contactors

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RBC

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RBC On Site

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Rotating Biological Contactors

PrimarySettling

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Sludge

Treatment

SecondarySettling

Sludge Treatment


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Low-tech solutions Aerobic ponds

Facultative ponds Anaerobic ponds

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On-site Treatment

(Septic Tanks)

WW Stabilization Ponds


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Waste or Wastewater Stabilisation Ponds (WSPs) are artificial

man-made lagoons in which blachwater, greywater or faecal

sludge are treated by natural occurring processes and theinfluence of solar light, wind, microorganisms and algae. The

ponds can be used individually or in series of an anaerobic,

S ab a o o ds

acu a ve an aero c ma ura on pon . s are ow-cos

for O & M and BOD and pathogen removal is high. However,large surface areas and expert design are required.

The effluent still contains nutrients (e.g. N and P) and is

therefore appropriate for the reuse in agriculture (irrigation)

or aquaculture (e.g. fish- or macrophyte ponds) but not fordirect recharge in surface waters.

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Anaerobic Ponds


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Primarily used as a pretreatment process

for high strength, high temperature wastes Can handle much high loadings

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s age:

Acid fermentation: Organics Org. acids

Methane fermentation Org. Acids CH4 and

CO2

Facultative ponds


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p

Ponds 1 - 2.5 m deep

HRT = 30 - 180 d Not easily subject to

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fluctuations in Q,loading

Low capital, O&M

costs

Facultative

Anaerobic

Facultative Ponds


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Facultative zone


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Septic Tanks


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In locations where sewers and a centralized

wastewater treatment system are not available,

on site disposal must be used

Septic systems most common for individual

87

residences

Engineered systems used for unfavorable site

conditions

Larger systems required for housing clusters,rest areas, commercial and industrial facilities

Septic Systems


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Septic Tank settling, flotation and anaerobic degradation


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Septic

Systems

89


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Example


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Design a septic tank and tile field systemfor highway rest area. Use the followingassumptions; Avrg daily traffic = 6000 vehicle/d

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Percent turn in = 10%

Use rate = 20 liters/turn in Maximum use rate = 2.5 x average

GWT = 4.2 below GL

Soil percolation = 5min/cm


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Oxidation Ditches


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Oxidation Ditch


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Wetlands

(From: http://www.city.pg.bc.ca/finished.htm)


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Wetlands

Use of natural or artificial wetlands

98

oa ng p an s ac as ers an suppor

for bacteria

(From: Environmental Science, 4th ed., B.J. Nebeland R.T. Wright, Prentice-Hall, N.J., 1981)

Constructed Wetland


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Constructed Wetland


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Reed bed filtration system


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101

Facility Options


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Facility Options Considerations for wastewater treatment

facility options

costs

ca ital

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operation and maintenance (including energy)

availability of space

degree of treatment required by DOE

permit municipal or municipal plus industrial

flow rate

Facility Options


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Facility Options

Considerations for wastewater

treatment facility options distance from residential properties

103

problems with: odors, flies, other nuisances

agricultural usage or land application

options

presence of pathogens

experience of design engineers

The inverse of left side of Eq. 8.17 defines

the mean cell residence time


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c

rwXQ

VX=

The concentration of BOD5 in the effluent (S) is fixed,

Eq. 8.19

104

( )( ) 1

1

+

=dmc

cdsk

kKS

Eq. 8.21

From Eq. 6.17, the concentration of microbes in the tank,

( )( )( )cd

c

k

SSYX

+=

1

0Eq. 8.23


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END OF

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Documents

Chapter 4b - Wastewater Treatment 2