dynamics of wastewater treatment systems

Dynamics of Wastewater Treatment

SystemsGustaf Olsson

Lund University, Sweden

[email protected]

Control of Biological WWT 2002

Gustaf Olsson, IEA, Lund University

Models

Models for

• understanding mechanisms• design• control



Mass Balances

Conservation of mass:

(rate of change of vessel contents) =

(rate of inflows) – (rate of outflows) +(rate generated) – (rate consumed)



Simple respirometer

The DO (SO) concentration:

Respiration rate r :

oo

o

SK

Srr

+−= max

rdt

dSo =



Dissolved Oxygen Dynamics



DO responses

Air flow rate

Oxygen concentration



Dissolved oxygen dynamics

Oxygen transfer rate:

DO mass balance:

VSSaKrate OsatOL ⋅−⋅= )( ,

VrVSSaKSqSqdt

VSdOsatOLOoutinOin

O ⋅+−+−= )()(

,,



Controlled DO response

Change in inlet DO(disturbance)

Change in DOsetpoint

DO conc

InletDO conc

Air flow rate



Controlling the DO response

Influencing the Kl a

airL uconstaK •≈

)( , OrefOairOair SSKuu −+=



Organic carbon removal



Carbon removal activated sludge

Bio reactor

Influent

Sludge outtake

Effluent

Sludge recirculation

Aeration



Simple nutrient control



Simple biological kinetics

ProcessComponents

Nutrient N Biomass BKinetics

Aerobicheterotrophicgrowth BY

1− BNN

N XsK

s

+

µ̂1



Simple bioreactor response

Influent substrate decrease

Substrate

Biomass



Soluble carbon removal



Carbon removal kinetics

ProcessComponents

Nutrient BiomassKinetics

Aerobichetero-trophicgrowth

HY

1− HOO

O

SS

SH X

sK

s

sK

s

+

+

µ̂1

Oxygen

H

H

Y

Y 1−

Hetero-trophicdecay

Pf−1 -1 HH Xb



Carbon removal response

Increase in air flow

Decrease in influent substrate

Biomass



Carbon removal



Nitrogen removal



Pre-denitrification plant

Aerobic reactor

Sludge outtakeSludge recirculation

Influent

Internal recirculation

Effluent

Anoxic reactor



Nitrogen Removal Process

Nitrification

Denitrification

Ammonium

Nitrate

Free gaseous nitrogen

Dissolved oxygen

Easily degradable organic matter



N removal – basic mechanisms



Nitrogen removal kinetics

ProcessesComponents

CarbonBiomass

heterotrophic

Kinetics

Aerobic hetero-trophic growthAnoxic hetero-trophic growtnAerobic auto-trophic growthHeterotrophicdecayAutotrophic

decay

Oxygen NH4 NO3Biomassautotrophic



Nitrification (batch)



Denitrification (batch)



Phosphorous removal



Plant design for bio-P removal

Aerobic reactor

Sludge outtakeSludge recirculation

Influent

Anoxic reactor

Internal recirculation

EffluentBio-P

reactor



P removal

Processes

• Fermentation• P release

• P uptake• PAO growth

• PHA breakdown• PP breakdown

• PAO breakdown

Components• SF – fermentable COD• SA – volatile fatty acids• Dissolved oxygen• Phosphate• PHA – polyhydroxyl-

alkanoates• PP – polyphosphate• PAO



P removal – basic mechanisms



P release basic mechanisms

• Fermentation of fermentable COD to VFA. VFA used by the organisms to store carbon as poly-hydroxyl-alkanoates (PHA)

• P release from poly-phosphate into solution while VFA is converted to PHA



P uptake basic mechanisms

• P uptake from solution to PP using the PHA and DO

• Growth of PAO biomass, utilizing PHA and DO



Easy degradable organic matter, VFA Phosphate

Carbon dioxide

Phosphate

PHA storage

Accumulated poly phosphate storage

Anaerobic condition:no dissolved oxygen nor nitrate present

Aerobic or anoxic conditions:nitrate and/or dissolved oxygen presentcondition

Phosphate Accumulating Organism (PAO)

Net P uptake

PO

4-P

, ppm

Accumulated poly phosphate storage

PHA storage

Concnetration in reactor

P removal mechanisms



0

5

10

15

20

27-sep 28-sep 29-sep 30-sep 01-okt 02-okt 03-okt 04-okt 05-okt 06-okt

pp

mNO3

NH4

PO4

Typical nutrient variationsNH4

PO4

NO3



P release



P uptake



Hydraulicmodels



Simple tank hydraulics

Volume VArea A

qin qout

outin qqdt

dhA

dt

dV −==

αhbNconstqout ⋅⋅⋅=



Settling



Multilayer model

Layer 1

Layer m

Layer n

Overflow

Underflow

Feed Layer m-1

Layer m+1

....

....

....

....



Multilayer model

( ) ( )nmi

ffAxxqdt

dxAh iiiiiu

iii

,...,1

11

+=

−+−= −−

Underflow

Solids fluxixC

iiii eCxvxf 21

−==



Settler – Feed Flowrate Increase



Settler – Underflow Decrease



Settler Profile – 5 layers



Generaldynamic models



State model

( )( )ptdtutxgty

ptdtutxfdt

dx

),(),(),()(

),(),(),(

=

=

x(t) = state variablesu(t) = manipulated input variablesd(t) = disturbances input variablesy(t) = output variables



Input-output models

( )ptdtutyhdt

dy),(),(),(=

)()(...)2()(

)(...)2()()(

21

21

tvtntubttubttub

tntyattyattyaty

n

n

+∆⋅−++∆⋅−+∆−++∆⋅−++∆⋅−+∆−=

Time discrete form:



Summary

• Mass balances of substrates, organisms and dissolved oxygen

• Processes for C, N and P removal• Settler dynamics is crucial• Many different time scales

dynamics of wastewater treatment systems

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