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Biogas Production ​

Comprehensive biogas yield prediction, composition modeling, and energy recovery potential for anaerobic digestion

Overview ​

Biogas production from anaerobic digestion represents a major energy recovery opportunity in wastewater treatment. ErgoPlanner models biogas yield, composition, and energy content using substrate-specific calculations aligned with ADM1 biochemistry.

Biogas Fundamentals ​

Composition ​

ComponentTypical RangeSource
CH455-75%Methanogenesis
CO225-45%All AD processes
H2S100-10,000 ppmProtein, sulfate
NH350-200 ppmProtein degradation
H2OSaturatedVapor at temp
Siloxanes0-50 mg/m3Synthetic products

Energy Content ​

$$LHV = %CH_4 \times 35.8 \text{ MJ/Nm}^3$$

CH4 %LHV (MJ/Nm3)kWh/Nm3
55%19.75.5
60%21.56.0
65%23.36.5
70%25.17.0

Yield Prediction ​

Basic Yield Model ​

$$Q_{biogas} = VS_{destroyed} \times Y_{specific}$$

Where:

  • VS_destroyed = kg VS/d
  • Y_specific = 0.8-1.0 m3/kg VS

Substrate-Specific Yields ​

Substratem3 CH4/kg VSm3 CO2/kg VSTotal m3/kg VS
Carbohydrates0.420.420.83
Proteins0.500.501.00
Lipids1.010.431.44
Mixed sludge0.500.350.85

Buswell Equation ​

Theoretical methane yield from organic composition:

$$C_nH_aO_bN_c + \left(n - \frac{a}{4} - \frac{b}{2} + \frac{3c}{4}\right)H_2O \rightarrow$$

$$\left(\frac{n}{2} + \frac{a}{8} - \frac{b}{4} - \frac{3c}{8}\right)CH_4 + \left(\frac{n}{2} - \frac{a}{8} + \frac{b}{4} + \frac{3c}{8}\right)CO_2 + cNH_3$$

Example: Glucose (C6H12O6)

n=6, a=12, b=6, c=0

CH4 yield = (6/2 + 12/8 - 6/4) = 3 mol CH4/mol glucose

At STP: 3 × 22.4 L = 67.2 L CH4/mol = 373 mL CH4/g glucose

VS Destruction Efficiency ​

$$\eta_{VS} = \frac{VS_{in} - VS_{out}}{VS_{in}} \times 100%$$

Digestion TypeTemperatureSRT (d)VS Destruction
Mesophilic35°C2045-55%
Mesophilic35°C3050-60%
Thermophilic55°C1550-60%
Thermophilic55°C2055-65%

Temperature Effects ​

$$k(T) = k(35) \times \theta^{(T-35)}$$

Temperature Rangetheta (θ)
Mesophilic (30-40°C)1.04
Thermophilic (50-60°C)1.08

Composition Model ​

CH4/CO2 Ratio ​

Depends on substrate oxidation state:

$$%CH_4 = 50 + \frac{a}{2n} - \frac{b}{n}$$

For typical wastewater sludge: 60-65% CH4

Trace Components ​

H2S Production ​

$$S_{H2S} = S_{sulfate} \times \eta_{reduction} + S_{protein} \times f_{S}$$

Sulfate (mg/L)H2S (ppm)Concerns
<100<500Low
100-300500-2000Moderate
>300>2000High - treatment needed

Siloxanes ​

SourceConcentration
Municipal sludge1-15 mg/m3
Industrial sludge15-50 mg/m3

Removal threshold for CHP: <5 mg/m3

Saturated Water Vapor ​

$$P_{H2O,sat} = e^{(17.27 \times T)/(237.3 + T)} \times 0.611 \text{ kPa}$$

At 35°C: ~6% of biogas is water vapor

Biogas Calculation ​

Mass Balance Approach ​

Inputs:

  • Primary sludge: VS load, composition
  • WAS: VS load, composition
  • Co-substrates: FOG, food waste

Process:

  1. Calculate biodegradable fraction
  2. Apply VS destruction efficiency
  3. Apply substrate-specific yields
  4. Sum component contributions

Example Calculation ​

Given:

  • Primary sludge: 2,000 kg VS/d (60% carbs, 25% proteins, 15% lipids)
  • WAS: 1,000 kg VS/d (50% proteins, 40% carbs, 10% lipids)
  • VS destruction: 55%

Step 1: VS destroyed

VS_destroyed = (2000 + 1000) × 0.55 = 1,650 kg/d

Step 2: Biogas by substrate

Primary sludge:

Carbs: 2000 × 0.60 × 0.55 × 0.83 = 548 m³/d
Proteins: 2000 × 0.25 × 0.55 × 1.00 = 275 m³/d
Lipids: 2000 × 0.15 × 0.55 × 1.44 = 238 m³/d
Subtotal: 1,061 m³/d

WAS:

Proteins: 1000 × 0.50 × 0.55 × 1.00 = 275 m³/d
Carbs: 1000 × 0.40 × 0.55 × 0.83 = 183 m³/d
Lipids: 1000 × 0.10 × 0.55 × 1.44 = 79 m³/d
Subtotal: 537 m³/d

Total: 1,598 m³/d biogas

Step 3: Composition CH4 content: ~62% → 991 m³ CH4/d

Process Configuration ​

Digester Types ​

TypeSRTVS DestYield
CSTR (mesophilic)20 d50%0.8-1.0 m³/kg VS
CSTR (thermophilic)15 d55%0.9-1.1 m³/kg VS
Two-stage25 d60%1.0-1.2 m³/kg VS
TPAD18 d55%0.9-1.1 m³/kg VS

Co-Digestion ​

Co-substrateMethane YieldMix Ratio
Fats/Oils/Grease0.9-1.0 m³/kg VS5-20%
Food waste0.5-0.8 m³/kg VS10-30%
Glycerol0.7-0.9 m³/kg VS1-5%
Crop residues0.3-0.5 m³/kg VS10-30%

Benefits:

  • Increased biogas yield
  • Better C:N ratio
  • Improved VS destruction
  • Dilution of inhibitors

Inhibition Factors ​

Ammonia Toxicity ​

$$I_{NH3} = \frac{K_I}{K_I + S_{NH3}}$$

Free ammonia (mg/L)Effect
<200No inhibition
200-500Slight inhibition
500-1500Moderate inhibition
>1500Severe inhibition

Free ammonia calculation:

$$S_{NH3} = S_{TAN} \times \frac{10^{pH}}{K_a + 10^{pH}}$$

VFA Accumulation ​

Total VFA (mg/L)pHEffect
<10007.0+Normal
1000-30006.5-7.0Stress
>3000<6.5Upset

Sulfide Inhibition ​

Free H2S (mg/L)Effect
<50No inhibition
50-100Slight inhibition
100-200Moderate inhibition
>200Severe inhibition

Biogas Treatment ​

Treatment Requirements by Use ​

End UseH2SCO2SiloxanesH2O
CHP engine<500 ppmN/A<5 mg/m³<80% RH
Boiler<1000 ppmN/AN/A<90% RH
Microturbine<200 ppmN/A<0.1 mg/m³<60% RH
Fuel cell<1 ppm<2%<0.01 mg/m³<5% RH
Grid injection<4 ppm❤️%<0.1 mg/m³Dewpoint
Vehicle fuel<16 ppm❤️%<0.1 mg/m³Dewpoint

Treatment Technologies ​

TechnologyRemovesEfficiency
Iron spongeH2S95-99%
Biological scrubberH2S99%+
Activated carbonSiloxanes, H2S95%+
Water scrubbingCO2, H2S95-99%
PSA/membraneCO295-99%
Chiller/desiccantH2ODewpoint

Configuration ​

Example Configuration ​

json
{
  "biogasProduction": {
    "calculationMode": "advanced",
    "digester": {
      "type": "CSTR",
      "temperature": 35,
      "srt": 20,
      "volume": 5000
    },
    "feedstock": {
      "primarySludge": {
        "vsLoad": 2000,
        "composition": {
          "carbohydrates": 0.60,
          "proteins": 0.25,
          "lipids": 0.15
        }
      },
      "was": {
        "vsLoad": 1000,
        "composition": {
          "carbohydrates": 0.40,
          "proteins": 0.50,
          "lipids": 0.10
        }
      }
    },
    "inhibition": {
      "ammoniaCheck": true,
      "vfaCheck": true,
      "sulfideCheck": true
    },
    "outputs": {
      "biogasYield": true,
      "composition": true,
      "energyContent": true
    }
  }
}

Monitoring Parameters ​

Stability Indicators ​

ParameterHealthy RangeWarningAlarm
pH6.8-7.46.5-6.8<6.5
VFA/Alk<0.30.3-0.5>0.5
Ammonia<1500 mg/L1500-2500>2500
CH4 %>55%50-55%<50%
H2<100 ppm100-500>500 ppm

Production Metrics ​

MetricUnitTypical
Specific yieldm³/kg VS dest0.8-1.0
Volumetric ratem³/m³/d0.5-1.5
CH4 productivitym³ CH4/m³/d0.3-0.9

Troubleshooting ​

IssueIndicatorCauseSolution
Low yield<0.7 m³/kgLow VS destructionIncrease SRT, check T
High H2S>3000 ppmHigh sulfate/proteinAdd Fe, reduce sulfate
Low CH4%<55%CO2 accumulationCheck alkalinity
FoamingVisualOrganic overloadReduce loading
VFA buildup>1000 mg/LAcidogenesis > methanogenesisReduce load, add lime

References ​

  1. Metcalf & Eddy (2014). Wastewater Engineering, 5th Ed.
  2. Batstone, D.J. et al. (2002). ADM1 Scientific and Technical Report. IWA Publishing.
  3. Tchobanoglous, G. et al. (2003). Solid Waste Engineering
  4. US EPA (2014). AgSTAR Biogas Handbook
  5. Weiland, P. (2010). "Biogas production" Applied Microbiology and Biotechnology