Anaerobic Digester (Mesophilic)

Heated biological reactor operating at 35-38°C to stabilize organic sludge through anaerobic decomposition, reducing volatile solids by 40-55% while producing biogas (60-70% CH₄) for energy recovery.

Overview

Anaerobic digestion is the primary stabilization process for municipal wastewater sludge. Microorganisms break down organic matter in the absence of oxygen, producing methane-rich biogas and stabilized biosolids. This is the most energy-efficient sludge treatment process, often producing net positive energy when optimized.

When to Use

Medium to large WWTPs (>10,000 PE typically)
Primary + secondary sludge stabilization
When biogas/energy recovery is valued
Odor control requirement (enclosed process)
Class B biosolids production acceptable
Long-term economic optimization (20+ year payback acceptable)

Key Characteristics

High VS destruction (40-55% for conventional, up to 65% with THP)
Biogas production: 0.8-1.0 m³/kg VS destroyed
Long SRT: 15-20 days minimum (mesophilic)
Temperature control critical (35-38°C)
Significant capital cost (tanks, heating, mixing, gas handling)
Positive energy balance possible with CHP

Operating Modes

Basic Mode

Fixed VS destruction efficiency (default 50%)
Simple first-order kinetics
Default biogas yield (0.9 m³/kg VS)
Constant temperature and SRT
Basic heat loss model

Advanced Mode

ADM1 (Anaerobic Digestion Model No. 1) integration
Dynamic biogas composition (CH₄, CO₂, H₂S, NH₃)
Temperature-dependent kinetics
VFA accumulation and pH prediction
Inhibition modeling (ammonia, sulfide, LCFA)
Detailed heat balance (mixing, walls, gas, feed)
Stratification and mixing optimization

Mathematical Model

Governing Equations

VS Destruction:

$$VS_{destroyed} = VS_{in} \times \eta_{VS}$$

Where:

$\eta_{VS}$ = VS destruction efficiency (0.40-0.55 conventional, up to 0.65 with THP)
Dependent on: SRT, temperature, VS composition, mixing

First-Order Kinetics (Basic Mode):

$$\eta_{VS} = \frac{k_d \times SRT}{1 + k_d \times SRT}$$

Where:

$k_d$ = degradation rate constant (d⁻¹)
At 35°C: $k_d$ = 0.04-0.06 d⁻¹ for WAS, 0.06-0.10 d⁻¹ for primary
$SRT$ = solids retention time (days), typically 15-25d

Temperature Correction (Arrhenius):

$$k_d(T) = k_d(35°C) \times \theta^{(T-35)}$$

Where:

$\theta$ = 1.08-1.12 (temperature coefficient)
$T$ = operating temperature (°C)

Biogas Production:

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

Where:

$Y_{gas}$ = biogas yield (m³/kg VS destroyed)
Primary sludge: 0.9-1.1 m³/kg VS
WAS: 0.7-0.9 m³/kg VS
Mixed (50/50): 0.8-1.0 m³/kg VS

Methane Content:

$$CH_4 = 60-70% \text{ (v/v typical)}$$ $$CO_2 = 30-40%$$ $$H_2S = 500-3000 \text{ ppm (without treatment)}$$

Biogas Energy Content:

$$E_{biogas} = Q_{biogas} \times CH_4% \times 9.97 \text{ kWh/m}^3$$

At 65% CH₄: 6.5 kWh/m³ biogas

SRT:

$$SRT = \frac{V \times X}{Q_{waste} \times X_{waste}}$$

Where:

$V$ = digester volume (m³)
$X$ = VS concentration in digester (kg/m³)
$Q_{waste}$ = waste sludge flow (m³/d)
$X_{waste}$ = waste VS concentration (kg/m³)

Volatile Solids Loading Rate:

$$OLR = \frac{Q \times VS_{in}}{V}$$

Where:

$OLR$ = organic loading rate (kg VS/m³·d)
Typical range: 1.5-4.0 kg VS/m³·d
Conservative design: 1.6-2.4 kg VS/m³·d

Heat Balance

Heat Required:

$$Q_h = Q_{sensible} + Q_{losses} - Q_{input}$$

Sensible heat (bring feed to temperature):

$$Q_{sensible} = Q_{feed} \times \rho \times C_p \times (T_{dig} - T_{feed})$$

Where:

$\rho$ = density ≈ 1000 kg/m³
$C_p$ = specific heat ≈ 4.18 kJ/kg·°C
Typical: 20-40 kWh/m³ feed (temperate climate, feed @ 15°C → 35°C)

Heat losses:

$$Q_{losses} = U \times A \times (T_{dig} - T_{amb})$$

Where:

$U$ = overall heat transfer coefficient (W/m²·K)
- Insulated: 0.3-0.6 W/m²·K
- Uninsulated: 2-4 W/m²·K
$A$ = surface area (m²)
Typical: 3-6% of digester heat content per day (well insulated)

Heat from mixing:

$$Q_{mixing} = P_{mix} \times \eta_{heat}$$

Typically 5-20% of total heat requirement

Design Parameters

Input Parameters

Parameter	Symbol	Unit	Range	Default	Description
Feed VS concentration	$VS_{in}$	%	2-6	4.0	Volatile solids in feed
Feed flow rate	$Q$	m³/d	-	-	Daily sludge feed
Target SRT	-	days	15-25	18	Solids retention time
Operating temperature	$T$	°C	30-40	35	Mesophilic range
VS destruction target	$\eta_{VS}$	%	40-55	50	Design efficiency
Mixing intensity	$P/V$	W/m³	5-12	8	Power per unit volume
Feed temperature	$T_f$	°C	10-20	15	Incoming sludge temp

Output Parameters

Parameter	Unit	Description
Digester volume	m³	Required reactor volume
VS destroyed	kg/d	Daily VS reduction
Biogas production	m³/d	Daily biogas generation
CH₄ production	m³/d	Methane volume
Energy potential	kWh/d	Gross energy available
Heating demand	kWh/d	Heat required
Digestate flow	m³/d	Stabilized sludge output
Digestate VS	%	Remaining volatile solids

Configuration Guide

Step-by-Step Setup

Characterize Feed Sludge
- Determine VS content (% of TS)
- Composition: % primary, % WAS
- Flow rate (average, peak)
- Temperature
- Typical: Primary @ 70-75% VS, WAS @ 65-75% VS
Select Operating Parameters
- Temperature: 35°C (mesophilic standard)
- SRT: 15-20 days (standard), 12-15 days (high-rate with THP)
- OLR: 1.6-2.4 kg VS/m³·d (conservative)
- VS destruction: 50% (design target)

Calculate Digester Volume

Method 1: SRT-based

For Q = 100 m³/d feed @ 4% VS (40 kg/m³)
VS load = 100 × 40 = 4,000 kg VS/d

Assume 50% destruction → Digester VS = 20 kg/m³ (2% VS)
SRT = 18 days
V = (4,000 × 18)/(20 × X_balance)

Simplified: V = Q × SRT = 100 × 18 = 1,800 m³
Add 20% for gas storage/freeboard → 2,160 m³
Use 2,200 m³ (2 × 1,100 m³ digesters)

Method 2: OLR-based

OLR_design = 2.0 kg VS/m³·d
VS_load = 4,000 kg/d
V = 4,000/2.0 = 2,000 m³
Add 20% → 2,400 m³

Size Mixing System
- Gas recirculation: 8-12 W/m³ (common in UK)
- Mechanical mixers: 5-8 W/m³
- For 2,200 m³: 17-26 kW mixing power

Design Heating System

Sensible heat:
Qs = 100 m³/d × 1000 kg/m³ × 4.18 kJ/kg·°C × (35-15)°C
Qs = 8,360 MJ/d = 2,322 kWh/d

Heat losses (insulated, D=15m, H=12m):
A_surface = πDH + 2×π(D/2)² = π×15×12 + 2×π×7.5² = 919 m²
U = 0.4 W/m²·K, ΔT = 35-10 = 25°C (winter)
Q_loss = 0.4 × 919 × 25 × 24 × 3600 / 1,000,000 = 796 MJ/d = 221 kWh/d

Total heat required = 2,322 + 221 = 2,543 kWh/d
Boiler capacity (peak): 2,543/20 = 127 kW + 30% safety = 165 kW

Estimate Biogas Production

VS destroyed = 4,000 × 0.50 = 2,000 kg/d
Biogas yield = 0.9 m³/kg VS (mixed sludge)
Q_biogas = 2,000 × 0.9 = 1,800 m³/d @ 65% CH₄
Energy = 1,800 × 6.5 = 11,700 kWh/d

CHP efficiency: 35% electric, 45% heat
Electricity: 11,700 × 0.35 = 4,095 kWh/d
Heat recovered: 11,700 × 0.45 = 5,265 kWh/d

Net heat balance: 5,265 - 2,543 = +2,722 kWh/d surplus!
(Available for plant heating or export)

Typical Design Values

Sludge Type	OLR (kg VS/m³·d)	SRT (d)	VS Dest (%)	Biogas (m³/kg VS)	Temp (°C)
Primary only	2.0-3.5	15-20	50-60	1.0-1.2	35-38
WAS only	1.2-2.0	18-25	35-45	0.7-0.9	35-38
Mixed (50/50)	1.6-2.8	15-20	45-55	0.85-1.0	35-38
THP + Mixed	3.0-6.0	12-15	55-65	1.0-1.2	35-38

Performance Criteria

VS Destruction Targets

Configuration	SRT (d)	VS Destruction (%)	Notes
Conventional	15-18	40-50	Standard design
Conventional (long SRT)	20-25	50-55	Improved stabilization
High-rate (THP)	12-15	55-65	Requires pretreatment
Two-stage	2-3 + 15-18	50-60	Acid + gas phase

Biogas Quality

Parameter	Typical Range	Notes
CH₄	60-70%	Primary-rich: higher; WAS-rich: lower
CO₂	30-40%	Balance to CH₄
H₂S	500-3000 ppm	Depends on sulfate in feed
H₂O	Saturated	Condense before use
Temperature	35°C	At production

Energy Balance (Typical)

For 50,000 PE WWTP (≈2,500 kg VS/d):

Biogas: 2,200 m³/d @ 65% CH₄
Gross energy: 14,300 kWh/d
CHP output: 5,000 kWh electric + 6,400 kWh heat
Heat demand: 1,600 kWh/d (digester) + 800 kWh/d (buildings)
Net: 5,000 kWh/d electric + 4,000 kWh/d heat surplus
Plant consumption: ~8,000 kWh/d total
Biogas covers 60-70% of plant power needs

Troubleshooting

Issue	Cause	Solution
Low biogas production	Low VS destruction	Check SRT, temperature, mixing
	Toxic/inhibitory compounds	Reduce loading, identify source
	Poor mixing	Increase mixing intensity, check dead zones
	Temperature drop	Check heating system, insulation
VFA accumulation	Organic overload	Reduce feed rate, check OLR
	pH drop (<6.8)	Add alkalinity (lime, caustic)
	Ammonia inhibition	Dilute, reduce protein-rich feeds
	Temperature shock	Gradual temp changes only
Foaming	High F/M, surfactants	Anti-foam spray, reduce loading
	Sudden loading increase	Gradual feed ramping
	Grease/fat accumulation	Skim, reduce FOG input
High H₂S	Sulfate in feed	Reduce industrial contribution
	Low pH	Increase alkalinity
	Use for desulfurization: Biogas scrubber, iron dosing
Poor VS destruction	Low SRT	Increase SRT to 18-20d
	Cold temperature	Increase to 35-37°C
	Poor biodegradability	THP pretreatment, longer SRT
Stratification	Poor mixing	Increase mixing power, add mixers
	Temperature gradient	Improve heat distribution
	Grit/sand accumulation	Desludge, improve grit removal
Odor from digestate	Incomplete digestion	Check SRT, VS destruction
	Septicity developing	Minimize storage time

Operation & Maintenance

Daily Checks

Gas production rate (flow meter)
Digester temperature (multiple points)
Feed flow rate
Gas pressure (maintain slight positive)
Mixing system operation
Visual inspection for foam
Review VFA if online analyzer available

Weekly Maintenance

Sample digestate for TS, VS, pH, alkalinity
Check heating system operation
Inspect gas piping for condensate
Verify flame arrestors clear
Review biogas composition (CH₄, CO₂, H₂S)
Check pressure relief valves

Monthly Tasks

Full digestate characterization (TS, VS, TKN, TP, metals)
Inspect mixing equipment
Calibrate gas flow meters
Check heat exchanger efficiency
Inspect foam breakers/spray system
Review VS destruction calculation

Annual Tasks

Internal inspection (drain required, or ROV)
Grit/sand desludging if needed
Heating coil descaling
Gas storage membrane inspection
Full instrumentation calibration
Energy audit (heat balance verification)
Mixing system overhaul

Advanced Features (ADM1 Mode)

Anaerobic Digestion Model No. 1

ADM1 provides mechanistic modeling of digestion:

Process Steps:

Disintegration (particulate → soluble)
Hydrolysis (polymers → monomers)
Acidogenesis (monomers → VFA + H₂)
Acetogenesis (VFA → acetate + H₂)
Methanogenesis (acetate/H₂ → CH₄)

Key Benefits:

Predicts biogas composition (CH₄/CO₂ ratio)
VFA accumulation and pH dynamics
Inhibition (ammonia, H₂S, pH)
Temperature effects on all kinetic steps
Startup optimization
Troubleshooting (diagnose VFA accumulation cause)

Inputs Required:

Feed characterization: Carbohydrates, proteins, lipids, inerts
Cation/anion content (for pH buffering)
Temperature profile
Mixing regime

Outputs:

Dynamic biogas flow and composition
pH, VFA species (acetate, propionate, butyrate)
Ammonia, sulfide concentrations
Optimal OLR for stable operation

Co-Digestion

Adding external organics:

Co-substrate	VS (%)	Biogas (m³/kg VS)	Benefit	Limitation
Food waste	20-30	0.8-1.2	+50-100% biogas	Permits, quality control
Fats, oils, grease (FOG)	80-95	1.2-1.8	+200-300% biogas	Inhibition risk, mixing
Agricultural waste	15-30	0.6-0.9	+30-60% biogas	Fiber content, C/N ratio
Brewery waste	5-10	0.7-1.0	+20-40% biogas	Volume, BOD spike

Design considerations:

Permits for waste acceptance
Quality control (no contaminants)
Separate receiving/storage
Pasteurization may be required (ABP regulations)
Monitor ammonia (high-protein wastes)

Worked Example

Given

WWTP: 50,000 PE
Primary sludge: 1,500 kg VS/d @ 5% VS
WAS: 1,000 kg VS/d @ 3% VS
Combined: 2,500 kg VS/d
Feed temperature: 15°C (winter)
Target: 35°C, 18d SRT

Solution

Step 1: Calculate volume

Use OLR = 2.0 kg VS/m³·d
V = 2,500/2.0 = 1,250 m³ (active)
Add 20% for gas/freeboard: 1,500 m³ total
Use 2 × 800 m³ digesters (1,600 m³ total)

Verify SRT:
Assuming 2% VS in digester (after 50% destruction):
Mass in digester = 1,600 × 20 = 32,000 kg VS
SRT = 32,000/2,500 = 12.8 days ✗ (too low)

Revise to 3 × 800 m³ = 2,400 m³:
Mass = 2,400 × 20 = 48,000 kg VS
SRT = 48,000/2,500 = 19.2 days ✓

Step 2: Biogas production

VS destruction: 2,500 × 0.50 = 1,250 kg/d
Biogas yield: 0.9 m³/kg VS
Q_biogas = 1,250 × 0.9 = 1,125 m³/d @ 65% CH₄
Q_CH4 = 1,125 × 0.65 = 731 m³/d
Energy = 1,125 × 6.5 = 7,313 kWh/d

Step 3: Heat requirement

Feed: (1,500/50 + 1,000/30) = 63 m³/d @ 15°C
Sensible: 63 × 1000 × 4.18 × (35-15) / (3600 × 1000) = 1,464 kWh/d

Surface area (3 × 10m D × 10m H):
A = 3 × (π×10×10 + 2×π×5²) = 1,099 m²
Heat loss (U=0.4, ΔT=25°C):
Q_loss = 0.4 × 1,099 × 25 × 24 / 1000 = 263 kWh/d

Total heat: 1,464 + 263 = 1,727 kWh/d

Step 4: CHP sizing

CHP input: 7,313 kWh/d biogas
Electric output (35%): 2,560 kWh/d (107 kW avg)
Heat output (45%): 3,291 kWh/d
Heat surplus: 3,291 - 1,727 = 1,564 kWh/d ✓

Use 150 kWe CHP (allows for peaks)

Step 5: Economic benefit

Electricity: 2,560 kWh/d × £0.15/kWh × 365 = £140,000/year
Heat (displaced gas): 1,564 kWh/d × £0.08/kWh × 365 = £45,600/year (export or plant use)
Total annual benefit: £185,600/year

Capital cost: £2-3M for digestion + CHP
Simple payback: 11-16 years
With RHI/incentives: 7-10 years

THP - Pretreatment to enhance digestion
Thermophilic-Digester - Higher temperature alternative
Acid-Phase-Digester - Two-stage system
Gas-Phase-Digester - High-solids alternative
Belt-Press / Centrifuge - Downstream dewatering

References

WEF (2018). Design of Municipal Wastewater Treatment Plants, MOP 8, 6th Ed., Ch. 18.
Metcalf & Eddy (2014). Wastewater Engineering, 5th Ed., pp. 1635-1695.
Batstone, D.J. et al. (2002). "Anaerobic Digestion Model No. 1 (ADM1)." Water Science & Technology, 45(10), 65-73.
Appels, L. et al. (2008). "Principles and potential of the anaerobic digestion of waste-activated sludge." Progress in Energy and Combustion Science, 34(6), 755-781.
UK Water Industry Research (2013). Optimizing Biogas Production from Anaerobic Digestion.
CIWEM (2013). Sewage Sludge: Anaerobic Digestion, Manual of Practice.
EU (2001). Working Document on Sludge, 3rd Draft.

Clarification

Disinfection

Filtration

Membranes

Anaerobic Digester (Mesophilic) ​

Overview ​

When to Use ​

Key Characteristics ​

Operating Modes ​

Basic Mode ​

Advanced Mode ​

Mathematical Model ​

Governing Equations ​

Heat Balance ​

Design Parameters ​

Input Parameters ​

Output Parameters ​

Configuration Guide ​

Step-by-Step Setup ​

Typical Design Values ​

Performance Criteria ​

VS Destruction Targets ​

Biogas Quality ​

Energy Balance (Typical) ​

Troubleshooting ​

Operation & Maintenance ​

Daily Checks ​

Weekly Maintenance ​

Monthly Tasks ​

Annual Tasks ​

Advanced Features (ADM1 Mode) ​

Anaerobic Digestion Model No. 1 ​

Co-Digestion ​

Worked Example ​

Given ​

Solution ​

Related Units ​

References ​

Anaerobic Digester (Mesophilic)

Overview

When to Use

Key Characteristics

Operating Modes

Basic Mode

Advanced Mode

Mathematical Model

Governing Equations

Heat Balance

Design Parameters

Input Parameters

Output Parameters

Configuration Guide

Step-by-Step Setup

Typical Design Values

Performance Criteria

VS Destruction Targets

Biogas Quality

Energy Balance (Typical)

Troubleshooting

Operation & Maintenance

Daily Checks

Weekly Maintenance

Monthly Tasks

Annual Tasks

Advanced Features (ADM1 Mode)

Anaerobic Digestion Model No. 1

Co-Digestion

Worked Example

Given

Solution

Related Units

References