Lecture8- Ch8

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    Activated Sludge Principles

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    Activated Sludge Principles• astewater is aerated in a tan!

    • Bacteria are encouraged to grow "# providing

    − $%#gen

    − &ood 'B$()

    − Suita"le temperature

    − *ime

    • As "acteria consume B$(+ the# grow and multipl#

    • *reated wastewater ,lows into secondar# clari,ier

    • Bacterial cells settle+ removed ,rom clari,ier as sludge

    • Part o, sludge is rec#cled "ac! to activated sludge tan!+ to maintain

      "acteria population

    • -emainder o, sludge is wasted

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    Applications o, activated sludge

     processes Process Application

    Conventional  .ow/strength domestic waste+ suscepti"le to shoc! loads

    Complete-mix General application+ resistant to shoc! loads+ sur,ace aerators

    Step-aeration General application to wide range o, wastes

     Modified-aeration Intermediate degree o, treatment where cell tissue in the e,,luent is not o"0ectiona"le

    Contact-stabilization E%pansion o, e%isting s#stems+ pac!age plants+ ,le%i"le

     Extended-aeration Small communities+ pac!age plants+ ,le%i"le+ sur,ace aerators

     Kraus process .ow/nitrogen+ high strength wastes

     High-rate aeration 1se with tur"ine aerators to trans,er o%#gen and control the ,loc si2e+ generals application

     Pure-oxygen General application+ use where limited space is availa"le+ re3uires e%pensive o%#gen source+

    tur"ine or sur,ace aerators

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    Conventional Activated Sludge

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    Completel#/mi%ed Activated

    Sludge

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    Step/aeration Activated Sludge

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    Contact Sta"ili2ation

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    $%idation (itchraus Process

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    (esign parameters ,or activated

    sludge processes Process   θ

    χ

      d) θ (d) F/M Qr/Q X (g/!)

    "on#entional  5/65 4/8 79/74 795/5 6+577/:+777

    "oplete-ix  5/65 :/5 79/7; 795/6 :+777/;+777

     $tep-aeration 5/65 :/5 79/74 795/7

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    $perational characteristics o,

    activated sludge processes Process Flo odel    Aeration syste   *+, reo#al efficiency

    () 

    "on#entional   Plug/,low (i,,used air+ mechanical aerators 85/>5

    "oplete-ix   Complete/mi% (i,,used air+ mechanical aerators 85/>5

     $tep-aeration  Plug/,low (i,,used air 85/>5

     Modified-aeration  Plug/,low (i,,used air ;7/

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    astewater Characteri2ation

    • AS design re3uires determining@ 6) aeration "asin volume 9) sludge production :) o%#gen needed and 4) the e,,luent concentration o,

    important parameters

    • *o design AS process+ characteri2ation o, wastewater is re3uired

    • astewater characteristics *8/6+ p;;; can "e grouped into the

    ,ollowing categories@

     =  car"onaceous su"strates+

     =  nitrogen compounds+

     =  phosphorus compounds+

     =  total and volatile suspended solids+

     =   and al!alinit#

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    astewater Characteri2ation

    • Car"onaceous Constituents ?easured "# B$( or C$(• 1nli!e B$(+ some portion o, C$( is non"iodegrada"le C$( is

    ,ractionali2ed in &8/4+ p;;8

    • $, interest is whether the C$( is dissolved or solu"le and how much

    is particulate+ comprised o, colloidal and suspended solids

    • *he non"iodegrada"le solu"le C$(+ n"sC$(+ will "e ,ound in the

    AS e,,luent and the non"iodegrada"le particulates will contri"ute to

    the sludge

    • Because the non"iodegrada"le particulate C$(+ n"pC$(+ is organic+

    it will contri"ute to the SS concentration o, the wastewater andmi%ed li3uor in the AS and is re,erred to as the non"iodegrada"le

    volatile suspended solids+ n"SS

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    astewater Characteri2ation

    • *he in,luent wastewater will also contain nonvolatile in,luentsuspended inert solids+ i*SS+ that add to the ?.SS

    • &or "iodegrada"le C$(+ understanding the ,ractions that are

    measured as solu"le+ solu"le readil# "iodegrada"le 'r"C$()+ and

     particulate is important ,or AS process design

    • *he r"C$( is 3uic!l# assimilated "# the "iomass+ while the

     particulate+ must ,irst "e dissolved "# e%tracellular en2#mes and are

    thus assimilated at much slower rates

    • *he r"C$( is o, particular interest+ *8/:+ p;;>+ and has a direct

    e,,ect on the AS "iological !inetics and process per,ormance

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    • A@ $%#gen used ,or r"C$(• B@ $%#gen used ,or nitri,ication• C@ $%#gen used ,or particular C$(• (@ $%#gen used ,or endogenous deca#

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    astewater Characteri2ation

    • C$( and B$( ma# "e correlated as the ,ollowing@

     "C$( consumed in the B$( test is e3ual to the o%#gen consumed

    '1B$() plus the o%#gen e3uivalent o, the remaining cell de"ris@

     "C$( 1B$( 649 , d 'D) "C$(

     "C$(B$( ratio varies "etween 6;/6

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    astewater Characteri2ation

    •  Fitrogenous Compounds &8/5+ p;

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    astewater Characteri2ation

    • Summar# *a"ulation P ;

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    &undamentals o, Process Anal#sis

    and Control• Process design considerations@

     =  -eactor t#pe

     =  inetics

     =  S-*

     =  Sludge production

     =  $%#gen re3uirements

     =  $thers

    • -eactor t#pe selection considerations *8/4+ p ;

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    &undamentals o, Process Anal#sis

    and Control• Sludge production@ E%cess solids are produced in the AS process and

    must "e properl# disposed o, or the# will accumulate and e%it in the

    e,,luent

    • P+SS  Do"sH'S7/S)'6!g67:g) e3 8/64+ p;86

    *he Do"s term is illustrated in &8/

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    &undamentals o, Process Anal#sis

    and Control•  Futrient re3uirements@ Based on cell mass+ 694 "# weight o,

    nitrogen is re3uired and phosphorus is usuall# assumed to "e a"out

    65 o, the nitrogen As a general rule+ ,or S-* values J

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    &undamentals o, Process Anal#sis

    and Control• E%ample@• Given@ A ?. has a *SS o, :577mgl and settles to a volume o, 9

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    &undamentals o, Process Anal#sis

    and Control• Secondar# Clari,ication@ *he design is t#picall# "ased on the sur,ace

    over,low rate and solids loading rate+ *8/

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    &undamentals o, Process Anal#sis

    and Control

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    &undamentals o, Process Anal#sis

    and Control• E,,luent Characteristics@ *he ma0or parameters o, interest are

     =  organic compounds+ sB$( usuall# less than : mgl

     =  suspended solids+ 5/65 mgl

     =  and nutrients

    • Process Control =  ?aintaining ($ in the aeration tan!s

     =  -egulating -AS

     =  Controlling AS

    • *he most commonl# used parameter ,or controlling the AS process isS-* *he waste AS ,low ,rom the rec#cle line is usuall# used to

    maintain the desired S-* *he ?.SS is also used as a control

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    &undamentals o, Process Anal#sis

    and Control• *he ($ should "e 65/9 mgl in all areas o, the aeration tan! alues

    a"ove 9 mgl ma# improve nitri,ication 'when B$( is high) alues

    a"ove 4 mgl do not improve operations "ut signi,icantl# increase

    aeration costs

    • -AS Control@

     =  *he -AS is returned ,rom the ,inal clari,ier to the inlet o, the

    aeration tan!

     =  *he solids ,orm a sludge "lan!et in the "ottom o, the clari,ier

     =  -eturn sludge pumping rates o, 57/

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    &undamentals o, Process Anal#sis

    and Control• Settlea"ilit#@ *o calculate return/sludge ,lowrate+ several techni3ues are used@

    Settlea"ilit# test@

    In a 6777 ml graduated c#linder the volume o, settlea"le solids a,ter :7 minutes is divided

     "# the volume o, clari,ied li3uid 'supernatant)

    SI 'Sludge olume Inde%) test@

     

    677I   ×=

     L

     

      ratio

    677××= L

     

     Qflowrate

    ( )[ ]

    )ercenta(ease*)ressed+&%%

    where;

    =

    −=

    !

    ! R

     " 

    S#  " QQ 6467746774

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    &undamentals o, Process Anal#sis

    and Control• Sludge asting@ *o maintain a given S-*+ the e%cess AS produced

    each da# must "e wasted+ AS

     =  *he sludge can "e wasted ,rom the -AS line or the aeration tan!

     =  *he -AS is more concentrated there"# re3uiring smaller pumps

     =  *he AS is discharged to the primar# sedimentation tan!s ,or co/thic!ening or to sludge thic!ening ,acilities prior to digestion

     =  I, wasting is ,rom the -AS line@

     =  I, wasting is done ,rom the aeration tan!@

    ( ),4)-eq(8 

    SR$  X 

    X Q

     R

    !   =

    ,)-eq(8 SR$ 

     Q! =

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    &undamentals o, Process Anal#sis

    and Control• $perational Pro"lems@

     = Bul!ing sludge@ *he ?.SS ,loc does not compact or settle well and ,loc

    is discharged in the clari,ier e,,luent *he principal cause is ,ilamentous

     "acteria which are ver# competitive at low su"strate+ nutrient or ($

    conditions

     = -ising sludge@

    K *he sludge has good settling characteristics "ut rises to the sur,ace

    K *he most common cause is denitri,ication in which nitrites and

    nitrates are converted to nitrogen gas+ F9 which ma!es the mass

     "uo#antK -ising sludge is di,,erentiated ,rom "ul!ing sludge "# the presence o,

    small gas "u""les and ,loating sludge in the secondar# clari,iers

    K -ising sludge pro"lems ma# "e overcome "# reducing the detention

    time in the clari,ier "# increasing the -AS rate

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    &undamentals o, Process Anal#sis

    and Control• $perational Pro"lems@

     = &oaming@

    K %ocardia can "e responsi"le ,or e%cessive ,oaming

    K *he "acteria have h#dropho"ic cell sur,aces and attach to air

     "u""les where the# sta"ili2e the "u""les to cause ,oamK 1suall# ,ound a"ove the ?.

    K %ocardia can "# controlled "# avoiding trapping ,oam in the

    secondar# treatment process and using chlorine spra#

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    Processes ,or B$( -emoval and

     Fitri,ication

    • *hree Activated/Sludge process design e%amples are provided in thissection '8/4) to demonstrate the application o, the ,undamental

     principles to B$( removal and nitri,ication

    • *he e%amples are@

    6 A single sludge complete/mi% activated/sludge process without

    and with nitri,ication E%ample 8/9

    9 A se3uencing "atch reactor 'SB-) with nitri,ication E%ample 8/:

    : A staged nitri,ication process E%ample 8/4

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    Processes ,or B$( -emoval and

     Fitri,ication

    Se3uencing Batch -eactor • 'SB-) is a ,ill/and/draw activated/sludge treatment s#stem In SB-

    aeration and sedimentation are carried out se3uentiall# in the same

    tan! *he process ta!es place in ,ive steps@

    6 ,ill @

     =  addition o, wastewater to reactor 

     =  li3uid level rises ,rom 95 to 677

     =  normall# lasts 95 o, ,ull c#cle time

    9 react@

     =  complete the reaction

     =   .asts :5 o, c#cle time

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    Processes ,or B$( -emoval and

     Fitri,ication

    Se3uencing Batch -eactor : settle@

     =  to allow solid separation to occur

     =  more e,,icient than continuous ,low s#stems

     =   .asts 974draw@

     =  to remove clari,ied treated waste lasts ,rom

     =  5 / :7 o, c#cle time+ t#picall# 45 minutes

    5 idle@ =  to provide time ,or one reactor to complete

    its ,ill c#cle "e,ore switching to another unit

     =  Sometimes omitted

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    , l d

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    Processes ,or B$( -emoval and

     Fitri,ication

    Se3uencing Batch -eactor • sludge wasting usuall# occurs during settle or idle phases

    • no need ,or rec#clingL "oth aeration and settling occur in the same

    cham"er 

    • Process !inetics@Accumulation in,low = out,low reaction

     r QS QS  dt 

    ds suo   +−=

    ( )S  K Y  XS r 

    dt ds

     s

    m su

    +

    −==  µ 

    ( )  t 

    Y  X 

    S S S 

    S  K    m

    t ot 

    o s    

      

      =

    −+ µ 

    ln

    P , B$( - l d

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    Processes ,or B$( -emoval and

     Fitri,ication

    Staged activated/sludge process• Consists o, a series o, complete/mi% reactors

    • &or the same reactor volume+ rectors in series can provide greater

    treatment e,,icienc# than a single complete/mi% reactor+ or provide a

    greater treatment capacit#

    • *he o%#gen upta!e is higher in the ,irst stage and decreases graduall#

    P , B$( - l d

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    Processes ,or B$( -emoval and

     Fitri,ication

    $verview o, "iological nitrogen removal processes• All "iological nitrogen removal processes include aero"ic 2one

    'nitri,ication) and ano%ic 2one 'denitri,ication)

    • Categories o, suspended growth "iological nitrogen removal

     processes include '6) single/sludge or '9) two/sludge

    • Single/stage processes@ 'three t#pes)

     =   preano%ic@ initial contact o, in,luent and return activated sludge

    is in the ano%ic 2one 'commonl# used)

     =  Postano%ic@ ano%ic 2one ,ollows the aero"ic 2one

     =  Simultaneous nitri,ication/denitri,ication 'SFdF)@ "oth 2ones

    e%isis in a single reactor -e3uires ($ control

    • *wo/sludge processes@ consists o, two separate stages ,or nitri,ication

    ,ollowed "# denitri,ication 'not commonl# used)

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    Preano%ic

    Postano%ic

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    Simultaneous

    *wo/sludge

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    Processes ,or Phosphorous -emoval

    Process ,or "iological phosphorous removal• *hree "iological phosphorous removal 'BP-) con,iguration are

    commonl# used@

     =   Phoredo% 'AM$)@ represent an# process with an anaero"icaero"ic

    se3uence to promote BP- Fitri,ication does not ta!e occur

     =  A9$@ process se3uence+ anaero"icano%icaero"ic Fitri,icationta!es place

     =  1C* '1niversit# o, Cape *own)@ used ,or wea! wastewater

    where the addition o, nitrate would have signi,icant e,,ect on the

    BP- per,ormance• *he PhoStrip process@ com"ines "iological and chemical processes

    ,or phosphorous removal

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    (esign o, Ph#sical &acilities ,or AS Process

    (esign o, Aeration *an!sA,ter selecting the activated sludge process and the aeration s#stem+

    the ne%t step is to design the aeration tan!s and support ,acilities

    Aeration Tanks:

    •constructed o, rein,orced concrete

    •capacit# is determined ,rom process design

    •,or plants in a capacit# range o,

    75 = 67 ?gald minimum two tan!s

    67 = 65 ?gald 4 tan!s

    J57 ?gald J ; tan!s

    Some large plans have :7 to 47 tan!s

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    (esign o, Ph#sical &acilities ,or AS Process

    Aeration Tanks:•wastewater depth in the tan! should "e 65 = 95 ,t ,or di,,users to wor!

    e,,icientl#

    •,ree "oard ,rom 6 = 9 ,t a"ove waterline should "e provided

    •width to depth ratio 6@6 = 99@6 '65@6 is common)

    •,or large plants channel length can e%ceed 577 ,t per tan! 

    •tan!s ma# consist o, one to ,our channels

    •length/to/width ratio o, each channel should "e at least 5@6

    •,or mechanical aeration s#stem+ one aerator per tan! is commonl#

    used with a ,ree "oard :5 = 5 ,t

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    Suspended Growth Aerated .agoons

    Consists o, shallow earthen "asins var#ing in depth ,rom 9/5m provided with mechanical aerators

     mechanical aerators provide o%#gen and mi%ing

    Suspended growth aerated lagoons are operated on a ,low/through

     "asis or with rec#cle

    .agoons with solid rec#cle are essentiall# the same as the activated

    sludge process

    *#pes o, Suspended growth aerated lagoons@

    &acultative partiall# mi%ed

    Aero"ic ,low/through with partial mi%ing

    Aero"ic with solids rec#cle and nominal complete mi%ing

    *he general characteristics o, these lagoon s#stems are summari2ed in

    *a"le 8/9>

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    Suspended Growth Aerated .agoons

    &acultative partiall# mi%ed *he energ# input is su,,icient to meet o%#gen re3uirement "ut not su,,icient to

    maintain all o, the solids in suspension

    A portion o, incoming solids will settle a long with a portion o, the "iological

    solids 'AS)

    Settled solids will undergo anaero"ic decomposition

    *he term ,acultative is derived ,rom the aero"ic and anaero"ic processes that

    occur in the lagoon

    &acultative lagoons must "e dewatered and the accumulated soilds removed

     Fot commonl# used

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    Suspended Growth Aerated .agoons

    Process design ,or ,low/through lagoons@BOD removal:  the "asis o, design is S-* + t#pical values o, S-* range

    ,rom : = ; da#s $nce S-* is selected S can "e calculated using

    e3uations ,rom Ch

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    Suspended Growth Aerated .agoons

    Process design ,or ,low/through lagoons@&or lagoons in series+ the ,ollowing e3uation can "e used@

    Oxygen requirements:

    Can "e computed in the same wa# as ,or activated sludge process

    $%#gen re3uirements have "een ,ound to var# ,rom 7< = 64 the amount o, B$(5 removed

    ( )[ ]   ( )[ ]

    seriesinlagoonso, num"erwhereL

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    Suspended Growth Aerated .agoons

    Process design ,or ,low/through lagoons@Temperature:

    *emperature e,,ect include@

    reduced "iological activit# and treatment e,,icienc#

    ,ormation o, ice

    *emperature can "e estimated using@

    the proportionalit# ,actor incorporates@

    • heat trans,er coe,,icients

    • e,,ect o, sur,ace area increase due to aeration

    • e,,ect o, wind and e,,ect o, humidit#

    ( )

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    Biological *reatment with ?em"rane

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    Biological *reatment with ?em"rane

    Separation

    ?B- Process (escription ?em"rane "ioreactors '?B-s) com"ine the use o, "iological processes

    and mem"rane technolog# to treat wastewater

    As shown in ,igure 6+ within one process unit+ a high standard o, treatment

    is achieved+ replacing the conventional arrangement o, aeration tan!+

    settling tan! and ,iltration that generall# produces what is termed as atertiar# standard e,,luent

    *he dependence on disin,ection is also reduced+ since the mem"ranes with

     pore openings+ generall# in the 776/75 Om range+ trap a signi,icant

     proportion o, pathogenic organisms '&igure 9)

    $perating at a mi%ed li3uor suspended solids '?.SS) concentration o, upto 97+777 mg. and a sludge age o, :7/;7 da#s+ ?B-s o,,er additional

    advantages over conventional activated sludge plants+ including a smaller

    ,ootprint

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    Biological *reatment with ?em"rane

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    Biological *reatment with ?em"rane

    Separation ?B- Process Advantages

    *he a"ilit# to eliminate secondar# clari,ier and operate at higher ?.SS

    concentrations provide the ,ollowing advantages@

    igher volumetric loading rate resulting in shorted h#d detention time

    .onger S-* resulting in less sludge production

    $perate at lower ($ concentration

    igh/3ualit# e,,luent '*SS+ B$(+ "acteria+ tur"idit#+ etc) *a"le 8/:7

    .ess space re3uired ,or wastewater treatment

    ?B- Process disadvantages@

     igh capital cost and energ# cost

    .imited data on mem"rane li,e+ 'high cost ,or mem"rane replacement)

    ?em"rane ,ouling

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