ADM1 - Anaerobic Digestion Model No. 1

IWA standard model for comprehensive anaerobic digestion simulation including biochemical and physicochemical processes

Overview

ADM1 (Anaerobic Digestion Model No. 1) is the internationally recognized biokinetic model developed by the IWA Task Group for Mathematical Modelling of Anaerobic Digestion Processes, published in 2002. It provides a comprehensive framework for simulating the complex biochemical and physicochemical processes occurring in anaerobic digesters, including hydrolysis, acidogenesis, acetogenesis, methanogenesis, and gas-liquid transfer.

When to Use

Recommended For

• Anaerobic digesters (mesophilic and thermophilic)
• Biogas production optimization
• Sludge digestion and stabilization
• Co-digestion studies
• Process control and optimization
• Volatile fatty acid (VFA) accumulation analysis
• pH and alkalinity management

Not Recommended For

• Aerobic processes (use ASM1, ASM2d, ASM3)
• Short SRT systems (<10 days)
• Highly simplified design calculations
• Systems without adequate characterization data

State Variables

ADM1 uses 35 state variables in three categories: soluble components, particulate components, and cations/anions.

Soluble Components (12)

Index	Symbol	Description	Unit
0	S_su	Monosaccharides (sugars)	kg COD/m3
1	S_aa	Amino acids	kg COD/m3
2	S_fa	Long chain fatty acids (LCFA)	kg COD/m3
3	S_va	Total valerate	kg COD/m3
4	S_bu	Total butyrate	kg COD/m3
5	S_pro	Total propionate	kg COD/m3
6	S_ac	Total acetate	kg COD/m3
7	S_h2	Hydrogen gas	kg COD/m3
8	S_ch4	Methane gas	kg COD/m3
9	S_IC	Inorganic carbon	kmol C/m3
10	S_IN	Inorganic nitrogen	kmol N/m3
11	S_I	Soluble inerts	kg COD/m3

Particulate Components (12)

Index	Symbol	Description	Unit
12	X_c	Composites	kg COD/m3
13	X_ch	Carbohydrates	kg COD/m3
14	X_pr	Proteins	kg COD/m3
15	X_li	Lipids	kg COD/m3
16	X_su	Sugar degraders	kg COD/m3
17	X_aa	Amino acid degraders	kg COD/m3
18	X_fa	LCFA degraders	kg COD/m3
19	X_c4	Valerate and butyrate degraders	kg COD/m3
20	X_pro	Propionate degraders	kg COD/m3
21	X_ac	Acetoclastic methanogens	kg COD/m3
22	X_h2	Hydrogenotrophic methanogens	kg COD/m3
23	X_I	Particulate inerts	kg COD/m3

Cations and Anions (3)

Index	Symbol	Description	Unit
24	S_cat	Cations (Na+, K+, etc.)	kmol/m3
25	S_an	Anions (Cl-, etc.)	kmol/m3
26	S_hco3	Bicarbonate	kmol/m3

Gas Phase Components (3)

Index	Symbol	Description	Unit
27	S_gas,h2	Hydrogen in gas phase	kg COD/m3
28	S_gas,ch4	Methane in gas phase	kg COD/m3
29	S_gas,co2	Carbon dioxide in gas phase	kmol/m3

Biochemical Processes (19)

ADM1 includes 19 biochemical processes organized into five main categories.

Process Summary

#	Process	Category
1	Disintegration	Extracellular
2	Hydrolysis of carbohydrates	Extracellular
3	Hydrolysis of proteins	Extracellular
4	Hydrolysis of lipids	Extracellular
5	Uptake of sugars	Acidogenesis
6	Uptake of amino acids	Acidogenesis
7	Uptake of LCFA	Acetogenesis
8	Uptake of valerate	Acetogenesis
9	Uptake of butyrate	Acetogenesis
10	Uptake of propionate	Acetogenesis
11	Uptake of acetate	Methanogenesis
12	Uptake of hydrogen	Methanogenesis
13	Decay of X_su	Decay
14	Decay of X_aa	Decay
15	Decay of X_fa	Decay
16	Decay of X_c4	Decay
17	Decay of X_pro	Decay
18	Decay of X_ac	Decay
19	Decay of X_h2	Decay

Process Hierarchy

Disintegration
    |
    +-- X_c -> X_ch + X_pr + X_li + X_I + S_I
    |
Hydrolysis
    |
    +-- X_ch -> S_su
    +-- X_pr -> S_aa
    +-- X_li -> S_fa + S_su
        |
Acidogenesis
    |
    +-- S_su -> S_bu + S_pro + S_ac + S_h2
    +-- S_aa -> S_va + S_bu + S_pro + S_ac + S_h2
        |
Acetogenesis
    |
    +-- S_fa -> S_ac + S_h2
    +-- S_va -> S_pro + S_ac + S_h2
    +-- S_bu -> S_ac + S_h2
    +-- S_pro -> S_ac + S_h2
        |
Methanogenesis
    |
    +-- S_ac -> S_ch4 + S_IC (acetoclastic)
    +-- S_h2 + S_IC -> S_ch4 (hydrogenotrophic)

Disintegration

Composite particulate matter (X_c) breaks down into carbohydrates, proteins, lipids, and inert fractions:

$$X_c \xrightarrow{k_{dis}} f_{ch,xc} \cdot X_{ch} + f_{pr,xc} \cdot X_{pr} + f_{li,xc} \cdot X_{li} + f_{xI,xc} \cdot X_I + f_{sI,xc} \cdot S_I$$

Rate: $$\rho_1 = k_{dis} \cdot X_c$$

Hydrolysis

First-order kinetics for conversion of polymers to monomers:

Carbohydrates: $$\rho_2 = k_{hyd,ch} \cdot X_{ch}$$

Proteins: $$\rho_3 = k_{hyd,pr} \cdot X_{pr}$$

Lipids: $$\rho_4 = k_{hyd,li} \cdot X_{li}$$

Acidogenesis

Uptake of monomers by acidogenic bacteria with Monod kinetics:

Sugar uptake: $$\rho_5 = k_{m,su} \cdot \frac{S_{su}}{K_{S,su} + S_{su}} \cdot X_{su} \cdot I_5$$

Amino acid uptake: $$\rho_6 = k_{m,aa} \cdot \frac{S_{aa}}{K_{S,aa} + S_{aa}} \cdot X_{aa} \cdot I_6$$

Acetogenesis

Uptake of LCFA and VFAs by syntrophic bacteria:

LCFA uptake: $$\rho_7 = k_{m,fa} \cdot \frac{S_{fa}}{K_{S,fa} + S_{fa}} \cdot X_{fa} \cdot I_7$$

Valerate uptake: $$\rho_8 = k_{m,c4} \cdot \frac{S_{va}}{K_{S,c4} + S_{va}} \cdot X_{c4} \cdot \frac{S_{va}}{S_{bu} + S_{va} + \epsilon} \cdot I_8$$

Butyrate uptake: $$\rho_9 = k_{m,c4} \cdot \frac{S_{bu}}{K_{S,c4} + S_{bu}} \cdot X_{c4} \cdot \frac{S_{bu}}{S_{bu} + S_{va} + \epsilon} \cdot I_9$$

Propionate uptake: $$\rho_{10} = k_{m,pro} \cdot \frac{S_{pro}}{K_{S,pro} + S_{pro}} \cdot X_{pro} \cdot I_{10}$$

Methanogenesis

Final methane production via two pathways:

Acetoclastic methanogenesis: $$\rho_{11} = k_{m,ac} \cdot \frac{S_{ac}}{K_{S,ac} + S_{ac}} \cdot X_{ac} \cdot I_{11}$$

$$CH_3COOH \rightarrow CH_4 + CO_2$$

Hydrogenotrophic methanogenesis: $$\rho_{12} = k_{m,h2} \cdot \frac{S_{h2}}{K_{S,h2} + S_{h2}} \cdot X_{h2} \cdot I_{12}$$

$$4H_2 + CO_2 \rightarrow CH_4 + 2H_2O$$

Biomass Decay

First-order decay of all seven biomass groups: $$\rho_{13-19} = k_{dec,i} \cdot X_i$$

Where i = su, aa, fa, c4, pro, ac, h2

Inhibition Functions

ADM1 incorporates multiple inhibition mechanisms to capture process limitations.

pH Inhibition

ErgoPlanner uses the IWA lower-inhibition exponential form in every tier (C# reference, browser preview, Rust engine); it is pinned by the parity benchmark:

$$I_{pH} = \begin{cases} 1 & pH \ge pH_{UL} \ \exp\left(-3 \cdot \left(\frac{pH - pH_{UL}}{pH_{UL} - pH_{LL}}\right)^2\right) & pH < pH_{UL} \end{cases}$$

There is no hard cut-off below pH_LL: the factor keeps decaying smoothly. Three organism groups carry their own limits: acidogens and acetogens (aa), acetoclastic methanogens (ac) and hydrogenotrophic methanogens (h2).

Where:

• pH_UL = upper pH limit (no inhibition above it)
• pH_LL = lower pH limit (sets the width of the transition band)

Hydrogen Inhibition

Non-competitive inhibition on LCFA and VFA degraders:

$$I_{h2} = \frac{K_{I,h2}}{K_{I,h2} + S_{h2}}$$

Applied to processes 7-10 (acetogenesis).

Free Ammonia Inhibition

Non-competitive inhibition on acetoclastic methanogens:

$$I_{NH3} = \frac{K_{I,NH3}}{K_{I,NH3} + S_{NH3}}$$

Free ammonia concentration: $$S_{NH3} = \frac{S_{IN} \cdot K_{a,NH4}}{K_{a,NH4} + [H^+]}$$

Combined Inhibition Terms

For sugar and amino-acid uptake (processes 5-6): $$I_{5,6} = I_{pH,aa} \cdot I_{IN,lim}$$

For LCFA, valerate/butyrate and propionate uptake (processes 7-10), each with its own hydrogen constant: $$I_{7-10} = I_{pH,aa} \cdot I_{IN,lim} \cdot I_{h2}$$

For acetoclastic methanogens (process 11): $$I_{11} = I_{pH,ac} \cdot I_{IN,lim} \cdot I_{NH3}$$

For hydrogenotrophic methanogens (process 12): $$I_{12} = I_{pH,h2} \cdot I_{IN,lim}$$

Where I_IN,lim represents inorganic-nitrogen limitation: $$I_{IN,lim} = \frac{S_{IN}}{K_{S,IN} + S_{IN}}$$

Disintegration, hydrolysis and decay are not inhibited.

Physicochemical Processes

Acid-Base Equilibria

ADM1 includes six acid-base pairs in equilibrium:

Carbon dioxide system: $$CO_2 + H_2O \leftrightarrow HCO_3^- + H^+$$ $$K_{a,CO2} = \frac{[HCO_3^-][H^+]}{[CO_2]} = 10^{-6.35}$$

Ammonia system: $$NH_4^+ \leftrightarrow NH_3 + H^+$$ $$K_{a,NH4} = \frac{[NH_3][H^+]}{[NH_4^+]} = 10^{-9.25}$$

VFA dissociation (acetate example): $$HAc \leftrightarrow Ac^- + H^+$$ $$K_{a,ac} = 10^{-4.76}$$

Charge balance: $$S_{cat} + [NH_4^+] + [H^+] = S_{an} + [HCO_3^-] + [Ac^-] + [Pro^-] + [Bu^-] + [Va^-] + [OH^-]$$

Gas-Liquid Transfer

Dissolved H2, CH4 and CO2 move between the liquid and the headspace by two-film transfer with Henry's law at the operating temperature (Batstone et al. 2002; BSM2 form, Rosen & Jeppsson 2006). A positive rate means gas leaves the liquid:

$$\rho_{T,h2} = k_La \cdot (S_{h2} - 16 \cdot K_{H,h2} \cdot p_{h2})$$ $$\rho_{T,ch4} = k_La \cdot (S_{ch4} - 64 \cdot K_{H,ch4} \cdot p_{ch4})$$ $$\rho_{T,co2} = k_La \cdot (S_{CO2} - K_{H,co2} \cdot p_{co2})$$

Where:

• k_La = volumetric mass transfer coefficient (1/d, 200 by default)
• S_CO2 = dissolved CO2(aq), taken from the carbonate speciation of S_IC at the current pH
• K_H,i = Henry's law coefficient (M/bar), corrected to the operating temperature
• 16 and 64 = kg COD per kmol of H2 and CH4
• p_i = headspace partial pressure (bar): p_h2 = S_gas,h2 · RT/16, p_ch4 = S_gas,ch4 · RT/64, p_co2 = S_gas,co2 · RT, with R = 0.083145 bar·m3/kmol/K

Henry's law coefficients at 35C:

Gas	K_H (M/bar)
H2	7.38 x 10^-4
CH4	1.16 x 10^-3
CO2	2.71 x 10^-2

Headspace and Biogas Flow

The headspace holds the three gas-phase states. Gas arrives by transfer and leaves through the outlet, which is held at a fixed pressure (atmospheric):

$$\frac{dS_{gas,i}}{dt} = -S_{gas,i} \cdot \frac{q_{gas}}{V_{gas}} + \frac{V_{liq}}{V_{gas}} \cdot \rho_{T,i}$$

$$q_{gas} = \max\left(0,; k_p \cdot (P_{gas} - P_{atm})\right), \quad P_{gas} = p_{h2} + p_{ch4} + p_{co2} + p_{H2O}$$

The water vapour pressure follows the BSM2 expression, 0.0557 bar at 35C. The outlet coefficient k_p defaults to the BSM2 value (5.0 x 10^4 m3/d/bar for a 3400 m3 digester); the browser preview scales it with the liquid volume. Setting k_p to 0 closes the headspace: no biogas leaves, which is useful for mass-balance checks.

Time Integration

The reactor equations (liquid plus headspace) are advanced with the classic four-stage Runge-Kutta (RK4) method at a fixed step:

• Browser preview: RK4 with your step size. The headspace makes the equations stiff (relaxation times under a minute at full scale), so each step is split automatically into substeps inside the explicit stability limit.
• Rust engine: a semi-implicit mode is the default (the fast gas and hydrogen states are solved implicitly, the slow states with RK4). Classic explicit RK4 on all 27 states is available as an option for comparison against the reference.

Parameters

Kinetic Parameters (Mesophilic, 35C)

Parameter	Symbol	Default	Unit	Description
Disintegration rate	k_dis	0.5	1/d	Composite breakdown
Hydrolysis carbohydrates	k_hyd,ch	10.0	1/d	Carbohydrate hydrolysis
Hydrolysis proteins	k_hyd,pr	10.0	1/d	Protein hydrolysis
Hydrolysis lipids	k_hyd,li	10.0	1/d	Lipid hydrolysis
Max uptake sugars	k_m,su	30.0	1/d	Sugar uptake
Max uptake amino acids	k_m,aa	50.0	1/d	Amino acid uptake
Max uptake LCFA	k_m,fa	6.0	1/d	LCFA uptake
Max uptake valerate/butyrate	k_m,c4	20.0	1/d	C4 VFA uptake
Max uptake propionate	k_m,pro	13.0	1/d	Propionate uptake
Max uptake acetate	k_m,ac	8.0	1/d	Acetate uptake
Max uptake hydrogen	k_m,h2	35.0	1/d	H2 uptake

Half-Saturation Coefficients

Parameter	Symbol	Default	Unit
K_S,su	K_S_su	0.5	kg COD/m3
K_S,aa	K_S_aa	0.3	kg COD/m3
K_S,fa	K_S_fa	0.4	kg COD/m3
K_S,c4	K_S_c4	0.2	kg COD/m3
K_S,pro	K_S_pro	0.1	kg COD/m3
K_S,ac	K_S_ac	0.15	kg COD/m3
K_S,h2	K_S_h2	7 x 10^-6	kg COD/m3
K_S,IN	K_S_IN	1 x 10^-4	kmol N/m3

Decay Rates

Parameter	Symbol	Default	Unit
Decay sugar degraders	k_dec,su	0.02	1/d
Decay AA degraders	k_dec,aa	0.02	1/d
Decay LCFA degraders	k_dec,fa	0.02	1/d
Decay C4 degraders	k_dec,c4	0.02	1/d
Decay propionate degraders	k_dec,pro	0.02	1/d
Decay acetoclastic methanogens	k_dec,ac	0.02	1/d
Decay H2 methanogens	k_dec,h2	0.02	1/d

Inhibition Parameters

Parameter	Symbol	Default	Unit
pH upper limit (acetogens)	pH_UL_aa	5.5	-
pH lower limit (acetogens)	pH_LL_aa	4.0	-
pH upper limit (acetoclastic)	pH_UL_ac	7.0	-
pH lower limit (acetoclastic)	pH_LL_ac	6.0	-
H2 inhibition constant	K_I,h2	3.5 x 10^-6	kg COD/m3
Free ammonia inhibition	K_I,NH3	1.8 x 10^-3	kmol N/m3

Stoichiometric Parameters

Parameter	Symbol	Default	Unit
Yield sugar degraders	Y_su	0.10	kg COD/kg COD
Yield AA degraders	Y_aa	0.08	kg COD/kg COD
Yield LCFA degraders	Y_fa	0.06	kg COD/kg COD
Yield C4 degraders	Y_c4	0.06	kg COD/kg COD
Yield propionate degraders	Y_pro	0.04	kg COD/kg COD
Yield acetoclastic	Y_ac	0.05	kg COD/kg COD
Yield hydrogenotrophic	Y_h2	0.06	kg COD/kg COD

Disintegration/Hydrolysis Fractions

Parameter	Symbol	Default
f_sI,xc	f_sI_xc	0.10
f_xI,xc	f_xI_xc	0.20
f_ch,xc	f_ch_xc	0.20
f_pr,xc	f_pr_xc	0.20
f_li,xc	f_li_xc	0.30
f_fa,li	f_fa_li	0.95
f_h2,su	f_h2_su	0.19
f_bu,su	f_bu_su	0.13
f_pro,su	f_pro_su	0.27
f_ac,su	f_ac_su	0.41

Temperature Effects

Arrhenius Temperature Correction

$$k(T) = k(T_{ref}) \cdot \exp\left[\frac{E_a}{R}\left(\frac{1}{T_{ref}} - \frac{1}{T}\right)\right]$$

Where:

• E_a = activation energy (J/mol)
• R = gas constant (8.314 J/mol/K)
• T = absolute temperature (K)
• T_ref = reference temperature (typically 308 K for mesophilic)

Typical Activation Energies

Process Group	E_a (kJ/mol)
Hydrolysis	20-40
Acidogenesis	40-60
Acetogenesis	50-70
Methanogenesis	60-80

Temperature Ranges

Regime	Temperature Range	Optimal
Psychrophilic	10-20C	15C
Mesophilic	30-40C	35C
Thermophilic	50-60C	55C

Temperature-Dependent Parameters

At thermophilic conditions (55C), rates typically increase 2-3x compared to mesophilic, but:

• Free ammonia inhibition increases
• Propionate accumulation more common
• Process more sensitive to disturbances

ASM-ADM1 Interface

When coupling aerobic and anaerobic systems, state variables must be transformed.

ASM to ADM1 Transformation

ASM Variable	ADM1 Variable(s)
X_S	X_c (composites)
X_BH, X_BA	X_c (composites)
X_P	X_c (composites)
S_S	S_su, S_aa (split by f_aa)
S_I	S_I
X_I	X_I
S_NH	S_IN
S_NO	Not used (denitrification separate)

COD-Based Mapping

Primary sludge characteristics: $$f_{ch,PS} = 0.30, \quad f_{pr,PS} = 0.25, \quad f_{li,PS} = 0.25, \quad f_{I,PS} = 0.20$$

Waste activated sludge characteristics: $$f_{ch,WAS} = 0.20, \quad f_{pr,WAS} = 0.50, \quad f_{li,WAS} = 0.10, \quad f_{I,WAS} = 0.20$$

Example Interface Configuration

{
  "primarySludge": {
    "COD": 50000,
    "fractions": {
      "carbohydrates": 0.30,
      "proteins": 0.25,
      "lipids": 0.25,
      "inerts": 0.20
    }
  },
  "wasActivatedSludge": {
    "COD": 30000,
    "fractions": {
      "carbohydrates": 0.20,
      "proteins": 0.50,
      "lipids": 0.10,
      "inerts": 0.20
    }
  }
}

Configuration in ErgoPlanner

Enabling ADM1

1. Select anaerobic digester unit
2. Set Calculation Mode to Advanced
3. Select Biokinetic Model: ADM1
4. Configure influent characterization
5. Set temperature (mesophilic/thermophilic)
6. Configure kinetic parameters (or use defaults)
7. Run simulation

Parameter Input

{
  "model": "adm1",
  "temperature": 35,
  "volume": 5000,
  "parameters": {
    "disintegration": {
      "k_dis": 0.5,
      "f_sI_xc": 0.10,
      "f_xI_xc": 0.20,
      "f_ch_xc": 0.20,
      "f_pr_xc": 0.20,
      "f_li_xc": 0.30
    },
    "hydrolysis": {
      "k_hyd_ch": 10.0,
      "k_hyd_pr": 10.0,
      "k_hyd_li": 10.0
    },
    "uptake": {
      "k_m_su": 30.0,
      "k_m_aa": 50.0,
      "k_m_fa": 6.0,
      "k_m_c4": 20.0,
      "k_m_pro": 13.0,
      "k_m_ac": 8.0,
      "k_m_h2": 35.0
    },
    "halfSaturation": {
      "K_S_su": 0.5,
      "K_S_aa": 0.3,
      "K_S_fa": 0.4,
      "K_S_c4": 0.2,
      "K_S_pro": 0.1,
      "K_S_ac": 0.15,
      "K_S_h2": 7e-6
    },
    "inhibition": {
      "K_I_h2_fa": 5e-6,
      "K_I_h2_c4": 1e-5,
      "K_I_h2_pro": 3.5e-6,
      "K_I_NH3": 1.8e-3,
      "pH_UL_aa": 5.5,
      "pH_LL_aa": 4.0,
      "pH_UL_ac": 7.0,
      "pH_LL_ac": 6.0
    },
    "yields": {
      "Y_su": 0.10,
      "Y_aa": 0.08,
      "Y_fa": 0.06,
      "Y_c4": 0.06,
      "Y_pro": 0.04,
      "Y_ac": 0.05,
      "Y_h2": 0.06
    },
    "decay": {
      "k_dec_all": 0.02
    },
    "gasTransfer": {
      "k_La": 200.0,
      "K_H_h2": 7.38e-4,
      "K_H_ch4": 1.16e-3,
      "K_H_co2": 2.71e-2
    }
  }
}

Outputs

Biogas Production

Key outputs include:

Output	Unit	Description
Q_gas	m3/d	Total biogas flow
Q_ch4	m3/d	Methane flow
Q_co2	m3/d	CO2 flow
CH4 content	%	Methane percentage
Specific gas production	m3/kg VS	Per VS destroyed

Theoretical methane yield: $$Y_{CH4,theoretical} = \frac{350 \cdot (1-Y)}{1000} \text{ Nm}^3 \text{ CH}4/\text{kg COD}$$

Where 350 L CH4/kg COD is the theoretical maximum at STP.

Digestate Quality

Parameter	Typical Range	Unit
VS destruction	45-55%	%
COD removal	50-60%	%
pH	7.0-7.5	-
Alkalinity	2500-5000	mg CaCO3/L
VFA (as acetate)	<500	mg/L
Ammonia	500-1500	mg N/L

Process Diagnostics

Indicator	Healthy Range	Warning
VFA/Alkalinity ratio	<0.3	>0.4
pH	6.8-7.4	<6.5 or >7.8
Propionate/Acetate	<1.4	>2.0
H2 in biogas	<100 ppm	>500 ppm
CH4 content	55-70%	<50%

Example: Municipal Sludge Digester

Configuration

Parameter	Value
Volume	5,000 m3
Temperature	35C
HRT/SRT	20 days
Mixing	Complete

Influent (Mixed Primary + WAS)

Parameter	Value
Flow	250 m3/d
Total COD	60 kg COD/m3
VS	40 kg/m3
VS/TS	0.75
TKN	2.5 kg N/m3

Expected Results

Parameter	Value
Biogas production	3,000-3,500 m3/d
CH4 content	60-65%
VS destruction	50-55%
Effluent VFA	<300 mg/L
pH	7.1-7.3
NH4-N	800-1,200 mg/L

Calibration Guidelines

Key Parameters to Calibrate

1. k_dis - Disintegration rate (most important for sludge)
2. k_hyd,ch/pr/li - Hydrolysis rates
3. f fractions - Substrate composition
4. k_m,ac - Acetoclastic methanogen uptake
5. K_I,NH3 - Ammonia inhibition constant

Calibration Procedure

1. Determine influent characterization (COD fractionation)
2. Match steady-state biogas production with k_dis, k_hyd
3. Match VFA profiles with uptake parameters
4. Validate pH predictions
5. Check ammonia inhibition effects
6. Dynamic validation with step changes

Common Issues

Issue	Likely Cause	Parameter Adjustment
Low CH4	Inhibition active	Check pH, NH3, H2
VFA accumulation	Slow methanogenesis	Increase k_m,ac
pH drop	Overloading	Reduce OLR, check alkalinity
Poor VS destruction	Slow hydrolysis	Increase k_hyd values

Troubleshooting

Issue	ADM1 Indicator	Solution
Low gas production	Low X_ac, X_h2	Reduce OLR, check temperature
Souring	High S_ac, S_pro	Add alkalinity, reduce feed
Foaming	High S_fa	Reduce lipids, add antifoam
Ammonia inhibition	High S_IN	Dilute feed, co-digest with low-N substrate
Unstable pH	Low S_IC	Add alkalinity buffer
H2 accumulation	Low X_h2 activity	Check trace elements, reduce OLR

Limitations

1. No sulfate reduction - SRB competition not modeled
2. Simplified disintegration - Single composite class
3. Homogeneous mixing - No spatial gradients
4. No trace nutrients - Micronutrient limitations ignored
5. Fixed stoichiometry - No adaptation to substrate changes
6. Two methanogenic pathways - Syntrophic acetate oxidation not included
7. Many parameters - Requires extensive calibration data
8. pH calculation - Can be numerically challenging
9. Dissolved hydrogen is stiff - In the browser preview, a very large hydrogenotroph population can drive dissolved H2 to its floor within one step; use a smaller time step if H2 oscillates. The Rust engine's default semi-implicit mode does not have this limitation.

Extensions

ADM1-S (Sulfate Reduction)

Adds sulfate reduction pathway:

• S_SO4 -> S_H2S
• H2S inhibition of methanogens
• Competition between SRB and methanogens

ADM1-Nit

Adds nitritation in digester effluent:

• For partial nitritation treatment
• Integration with anammox

ADM1-P

Adds phosphorus dynamics:

• Struvite precipitation
• Phosphorus release and uptake

Advanced Topics

Co-digestion

For co-digestion, characterize each substrate:

{
  "substrates": [
    {
      "name": "Primary Sludge",
      "flowFraction": 0.6,
      "COD": 60000,
      "fractions": {"ch": 0.30, "pr": 0.25, "li": 0.25}
    },
    {
      "name": "Food Waste",
      "flowFraction": 0.4,
      "COD": 150000,
      "fractions": {"ch": 0.50, "pr": 0.20, "li": 0.20}
    }
  ]
}

Trace Element Limitation

Optional extension for trace element effects:

$$I_{TE} = \frac{S_{TE}}{K_{S,TE} + S_{TE}}$$

Critical trace elements:

• Cobalt (Co) - B12 synthesis
• Nickel (Ni) - F430 cofactor
• Selenium (Se) - Selenocysteine
• Tungsten (W) - Formylmethanofuran

Validation

IWA STR21 Benchmark Cases

ADM1 should reproduce:

1. Steady-state biogas production within 10%
2. VFA profiles during overload
3. Recovery after inhibition events
4. Response to temperature changes

Model Verification Checklist

• Mass balance closure (COD, N, C)
• pH within expected range
• Biogas composition matches measurements
• VFA profiles during transients
• Response to organic load changes

Related Models

• ASM1 - Activated sludge model for aerobic processes
• ASM2d - Extended model with biological P removal
• ASM3 - Storage-based activated sludge model
• Sumo2N - Two-step nitrification model

References

1. Batstone, D.J. et al. (2002). "The IWA Anaerobic Digestion Model No. 1 (ADM1)." Water Science and Technology, 45(10), 65-73.
2. IWA Task Group (2002). "Anaerobic Digestion Model No. 1 (ADM1)." IWA Scientific and Technical Report No. 13.
3. Rosen, C. & Jeppsson, U. (2006). "Aspects on ADM1 Implementation within the BSM2 Framework." Technical Report, Lund University.
4. Batstone, D.J. et al. (2015). "Mathematical Modelling of Anaerobic Digestion Processes: Applications and Future Needs." Reviews in Environmental Science and Bio/Technology.
5. Donoso-Bravo, A. et al. (2011). "Model selection, identification and validation in anaerobic digestion." Water Research, 45(17), 5347-5364.
6. Siegrist, H. et al. (2002). "Mathematical model for meso- and thermophilic anaerobic sewage sludge digestion." Environmental Science & Technology.