The Activated Sludge Model No 1ASM1
Bengt Carlsson
The Peterson process matrixGeneral expression for a component z in a completely mixed reactor
zin
zin
rDzDzz
VrQzQzzV
+−=
+−=.
.
where V=volume of the reactor, Q=inflow rate=outflowrate, D=Q/V=dilution rate, and rz is the process rate Note that for simple biomass growth.
In the process matrix discussed below, only process rates are described.
XSrZ )(µ=
Biomass with Monod growth rate and decay:
bXXXK
SrS
X −+
= maxµ
If we assume that all decayed biomass becomes substrate the process rate for substrate is:
bXXXK
SY
rS
S ++
−= max1 µ
Example 1
X S r
Growth 1
Decay -1 1 bX
Y1
− XXK
S
S +maxµ
Peterson matrix for Example 1
↓
→
ProcessComponents
The bisubstrate model
Slowly biodeg substrateXs
Easily biodeg substrateSs
BiomassX
Inert matterXP
Hydrolysis
Growth of biomass
Decay of biomass1-ff
Example 2. A Bisubstrate model
Assumptions:
•Biomass X has growth rate r and a decay bX.A typical example of r is
• The fraction (1-f) of the decayed biomass becomes slowly biodeg matter Xs, and fraction f becomes inert matter Xp.
•The yield is Y and hydrolysis of Xs has growth rate kXS
XSK
SSK
SrOO
O
S ++= maxµ
SS kXr
YdtdS
+−=1
SS kXbXf
dtdX
−−= )1(
XbrdtdX 11 −=
Xbfdt
dX P =
Substrate dynamics:
Change of slowly biodeg substrate:
Accumulation of inert matter:
Growth and decay of biomass
SS XS XP X rate
Growth -1/Y 1 r
Decay (1-f) f -1 b X
Hydro-lysis
1 -1 k XS
• In ASM1, a bisubstrate model is used for the carbon oxidation process.
• If the substrate and biomass is measured inCOD [g COD/m3] the oxygen consumption is easy to calculate by adding the factors in the growth process:
rY
YrrY
)1(11 −−=+−
ASM1
• ASM1 proposed 1987, probably the most used modelfor the ASP. Previously called IAWQ Model No 1
• ASM2: 1995, model for biological P removal
• ASM3 1999, improvements of ASM1. Substrate goesthrough a storage process…
• ADM1 2001, Anaerobic Digester Model No 1
• ASM1 describes an activated sludge system withcarbon oxidation, nitrification, denitrification.
• In total 8 processes are modelled:Growth of biomass (3), decay (2), ammonificationof organic N (1), hydrolysis (2)
•Heterotrophic biomass XB,H:- Oxidise carbon under aerobic conditions- Denitrify (SNO =>N2) under anoxic
conditions (if substrate available).
•Autotropic biomass XB,A:- Nitrify (SNH =>SNO) under aerobic
conditions.
Divisions of carbonaceous material
Total COD
Biodeg. COD:
Soluble SSParticulate XS
NonbiodegCOD:
Soluble SIParticulate XIand XP
Active biomass
HeterotrophsXB,H
AutothropsXB,A
Nitrogeneous componentsMost important components:
•Ammonia SNH•Nitrate (and nitrite) SNO
•Biodeg N: SND and XND
In summary: 12 components (+ alkalinity)
Growth rates and processes:
growth of heterotrophs XB,H- Aerobic growth- Anoxic growth- Decay
HBH
HBNOS
NO
OHO
HO
SS
SgH
HBOO
O
SS
SH
HB
Xb
XSK
SSK
KSK
S
XSK
SSK
Sdt
dX
,
,,
,
,,
−
++++
++=
∧
ηµ
µ
Growth of autotrophs XB,A- Aerobic growth- Decay
ABA
ABOAO
O
NHNH
NHA
AB
Xb
XSK
SSK
Sdt
dX
,
,,
,
−
++=
∧
µ
Model parameters
ASM1 has 19 model parameters, where some (typically max growth rates) are temperature dependent.
Default values exist, but several parameters may needto be tuned to mimic a specific plant.
Some set of parameters may lead to (approximately) the same model behaviour!
Also the influent water needs to be characterized.
See: Calibrating, identifiability and optimal experimental design of activated sludge models,B Peterson, PhD 2000. Thesis available for download
Limitations of ASM1
Examples:
• The pH is assumed to be near neutrality
• Many of the relations are empirical or based on hypothesis
• Cells need nutrients to grow which are not modelled
• Constant temperature. In order to allow for temperature variations, Arrerenius relations may be used
Model useExample of use:
• Testing and evaluation new control strategies
• Education and process understanding
• Evaluating new processes and/or operating modes
• Process optimization
• Prediction
The goal of the modelling should determine how careful the model should be calibrated.