Post on 06-Jul-2018
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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$( is high) alues
a"ove 4 mgl do not improve operations "ut signi,icantl# increase
aeration costs
• -AS Control@
= *he -AS is returned ,rom the ,inal clari,ier to the inlet o, the
aeration tan!
= *he solids ,orm a sludge "lan!et in the "ottom o, the clari,ier
= -eturn sludge pumping rates o, 57/
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&undamentals o, Process Anal#sis
and Control• Settlea"ilit#@ *o calculate return/sludge ,lowrate+ several techni3ues are used@
Settlea"ilit# test@
In a 6777 ml graduated c#linder the volume o, settlea"le solids a,ter :7 minutes is divided
"# the volume o, clari,ied li3uid 'supernatant)
SI 'Sludge olume Inde%) test@
677I ×=
L
S
ratio
677××= L
S
Qflowrate
( )[ ]
)ercenta(ease*)ressed+&%%
where;
=
−=
!
! R
"
S# " QQ 6467746774
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&undamentals o, Process Anal#sis
and Control• Sludge asting@ *o maintain a given S-*+ the e%cess AS produced
each da# must "e wasted+ AS
= *he sludge can "e wasted ,rom the -AS line or the aeration tan!
= *he -AS is more concentrated there"# re3uiring smaller pumps
= *he AS is discharged to the primar# sedimentation tan!s ,or co/thic!ening or to sludge thic!ening ,acilities prior to digestion
= I, wasting is ,rom the -AS line@
= I, wasting is done ,rom the aeration tan!@
( ),4)-eq(8
SR$ 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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