Energy Recovery

Biogas utilization, CHP systems, thermal recovery, and energy self-sufficiency for wastewater treatment

Overview

Energy recovery from biogas represents the primary pathway to achieving energy-positive wastewater treatment. ErgoPlanner models complete energy balances including CHP electrical and thermal efficiency, parasitic loads, and grid export potential.

Energy Balance Framework

Plant Energy Flow

                    INPUTS                              OUTPUTS
                    ──────                              ───────
              ┌─────────────────┐
              │    BIOGAS       │
              │  Energy Content │──────────────┐
              └─────────────────┘              │
                                               ▼
              ┌─────────────────┐      ┌──────────────────┐
              │      GRID       │      │       CHP        │
              │   Electricity   │──────│   Generator      │
              └─────────────────┘      └────────┬─────────┘
                      │                         │
                      │                    ┌────┴────┐
                      ▼                    ▼         ▼
              ┌─────────────────┐   ┌──────────┐  ┌──────────┐
              │   TREATMENT     │   │ ELECTRIC │  │ THERMAL  │
              │    PROCESS      │◄──│  OUTPUT  │  │  OUTPUT  │
              └─────────────────┘   └──────────┘  └──────────┘

Energy Metrics

Metric	Unit	Calculation
Energy consumption	kWh/ML	Total electrical demand
Energy production	kWh/ML	CHP generation
Net energy	kWh/ML	Production - consumption
Self-sufficiency	%	Production / consumption × 100
Carbon credit	kg CO2e	Exported × grid factor

CHP Systems

Combined Heat and Power

CHP converts biogas to both electricity and useful heat:

$$E_{total} = Q_{biogas} \times %CH_4 \times LHV_{CH4}$$

$$E_{electrical} = E_{total} \times \eta_{electrical}$$

$$E_{thermal} = E_{total} \times \eta_{thermal}$$

Where:

• LHV_CH4 = 35.8 MJ/Nm³ = 9.94 kWh/Nm³
• eta_electrical = Electrical efficiency (30-42%)
• eta_thermal = Thermal efficiency (40-50%)

CHP Technology Comparison

Technology	Electrical η	Thermal η	Total η	Size Range
IC engine	35-42%	40-50%	80-90%	50 kW - 5 MW
Gas turbine	25-32%	50-55%	80-85%	500 kW - 50 MW
Microturbine	25-30%	50-55%	80-85%	30-250 kW
Fuel cell (MCFC)	45-55%	25-35%	75-85%	250 kW - 3 MW
Stirling engine	25-30%	50-55%	80%	1-100 kW

IC Engine Performance

Electrical Efficiency Curve

$$\eta_e(load) = \eta_{max} \times \left(1 - k \times (1 - load)^2\right)$$

Where k ≈ 0.15

Load Factor	Relative η
100%	100%
75%	97%
50%	88%
25%	71%

Typical IC Engine Specifications

Parameter	Value
Electrical efficiency	38-42%
Thermal efficiency	43-47%
Maintenance interval	2,000 hours
Major overhaul	60,000 hours
Oil consumption	0.3 g/kWh
Availability	90-95%

Biogas Fuel Requirements

Parameter	IC Engine	Microturbine	Fuel Cell
CH4 minimum	40%	35%	40%
H2S maximum	500 ppm	200 ppm	1 ppm
Siloxanes max	10 mg/m³	0.1 mg/m³	0.01 mg/m³
Moisture	<80% RH	<60% RH	<5% RH
Pressure	50-200 mbar	200-500 mbar	5-50 mbar

Thermal Recovery

Heat Sources

Source	Temperature	Recovery Potential
Engine jacket	85-95°C	25-30% of fuel
Exhaust gas	400-500°C	15-20% of fuel
Oil cooler	70-80°C	3-5% of fuel
Charge air	40-60°C	2-3% of fuel

Heat Utilization

Application	Temperature Required	Priority
Digester heating	35-55°C	High
Building heating	40-70°C	Medium
Sludge drying	80-120°C	Medium
Hot water	60-80°C	Low
Absorption chiller	85-100°C	Low

Digester Heating Demand

$$Q_{heating} = Q_{feed} \times \rho \times C_p \times (T_{dig} - T_{feed}) + Q_{losses}$$

Where:

• Q_feed = Feed flow (m³/d)
• rho = Density ≈ 1020 kg/m³
• Cp = Specific heat ≈ 4.18 kJ/kg·K
• T_dig = Digester temperature (35 or 55°C)
• Q_losses = Heat losses through walls

Typical demand: 40-60 kWh/m³ sludge (mesophilic)

Heat Balance Example

Given:

• Biogas production: 1,000 Nm³/d @ 62% CH4
• CHP: 500 kWe, η_e = 40%, η_th = 45%
• Digester heating: 800 kWh/d

Calculation:

Energy input:

E_input = 1000 × 0.62 × 9.94 = 6,163 kWh/d

CHP outputs:

E_electrical = 6,163 × 0.40 = 2,465 kWh/d
E_thermal = 6,163 × 0.45 = 2,773 kWh/d

Thermal balance:

Available: 2,773 kWh/d
Required for digester: 800 kWh/d
Surplus heat: 1,973 kWh/d

Energy Self-Sufficiency

Calculation

$$Self\text{-}Sufficiency = \frac{E_{generated}}{E_{consumed}} \times 100%$$

Benchmark by Plant Type

Plant Configuration	Typical Self-Sufficiency
Conventional AS (no AD)	0%
AS + Primary sludge digestion	30-50%
AS + Full sludge digestion	50-70%
BNR + Digestion	40-60%
A-stage + B-stage + Digestion	70-100%
Co-digestion	80-150%
Advanced recovery	100-200%

Energy-Positive Examples

Feature	Impact on Balance
Co-digestion (FOG, food waste)	+30-100% biogas
Thermal hydrolysis	+15-25% biogas
A-stage carbon capture	+20-40% net
Membrane aeration	-30% consumption
Anammox (sidestream)	-20% aeration

Grid Export

Carbon Credit Calculation

$$Credit_{CO2} = E_{export} \times EF_{grid}$$

Where:

• E_export = Exported electricity (kWh)
• EF_grid = Grid carbon intensity (kg CO2/kWh)

Export Economics

Factor	Value
Feed-in tariff	£50-100/MWh
ROCs/REGO	£5-20/MWh
Avoided import	Grid price
Network charges	-£5-15/MWh

Grid Connection Options

Type	Capacity	Voltage
LV connection	<100 kW	400V
HV connection	100 kW - 1 MW	11 kV
Grid connection	>1 MW	33 kV

Alternative Energy Uses

Biomethane Upgrade

Convert biogas to grid-quality biomethane:

Parameter	Raw Biogas	Biomethane
CH4	55-70%	>97%
CO2	30-45%	❤️%
H2S	100-10,000 ppm	<4 ppm
Energy density	6 kWh/m³	10 kWh/m³

Upgrade technologies:

• Water scrubbing (95-98%)
• Chemical scrubbing (99%)
• PSA (97-99%)
• Membrane separation (95-99%)
• Cryogenic (99%+)

Vehicle Fuel

Application	Fuel Quality	Infrastructure
Municipal fleet	Biomethane	Fueling station
Collection vehicles	CNG/biomethane	Slow fill overnight
Public transport	Biomethane	Fast fill

Thermal-Only Use

Where CHP not viable:

Option	Efficiency	Application
Boiler	85-95%	Direct heat
Absorption chiller	COP 0.7	Cooling
ORC	10-15% electrical	Low-grade heat

Configuration

Example Configuration

{
  "energyRecovery": {
    "chp": {
      "technology": "ic_engine",
      "capacity": 500,
      "electricalEfficiency": 0.40,
      "thermalEfficiency": 0.45,
      "availability": 0.92,
      "minimumLoad": 0.50
    },
    "thermal": {
      "digesterHeating": true,
      "buildingHeating": true,
      "heatExport": false,
      "absorptionChiller": false
    },
    "grid": {
      "exportEnabled": true,
      "importTariff": 150,
      "exportTariff": 80,
      "gridCarbonFactor": 0.21
    },
    "alternativeUse": {
      "biomethaneUpgrade": false,
      "vehicleFuel": false,
      "boilerBackup": true
    }
  }
}

Performance Indicators

Key Metrics

Metric	Unit	Target
Specific energy consumption	kWh/m³	<0.4
Self-sufficiency	%	>70%
CHP availability	%	>90%
Heat utilization	%	>80%
Carbon intensity	kg CO2e/m³	<0.3

Monitoring Dashboard

Parameter	Frequency	Purpose
Biogas flow	Continuous	Yield tracking
CH4 content	Daily	Energy content
CHP output	Continuous	Performance
Heat demand	Continuous	Balance
Grid import/export	15-min	Cost/credit

Optimization Strategies

Maximize Generation

Strategy	Benefit
Co-digestion	+30-100% biogas
THP pretreatment	+15-25% yield
Two-stage digestion	+10-15% VS destruction
Temperature optimization	+5-10% kinetics
Mixing optimization	+5% gas release

Minimize Consumption

Strategy	Savings
ABAC aeration control	20-30% aeration
VFD on all motors	15-25% pumping
LED lighting	50-80% lighting
Heat recovery	10-20% heating
Process optimization	10-15% overall

Troubleshooting

Issue	Cause	Solution
Low CHP output	Low biogas quality	Check H2S, moisture
Poor heat utilization	Mismatched supply/demand	Heat storage, scheduling
High import	Low biogas yield	Review AD performance
Export curtailment	Grid constraints	Storage, load shifting
Derating	High ambient temp	Cooling, capacity planning

Related Topics

• Biogas Production
• GHG Emissions
• Scope 1/2/3
• ADM1
• Reporting

References

1. UKWIR (2019). Energy Optimization in Water Utilities
2. WEF (2017). Energy Production and Efficiency Research Foundation
3. IEA Bioenergy (2020). Biogas Country Report Summary
4. US EPA (2021). Combined Heat and Power Partnership
5. CIBSE (2012). CHP for Buildings