Anaerobic digestion simultaneously reduces sludge volume, stabilises pathogenic material, and generates biogas — an energy source that typically meets 50–100% of a WWTP’s energy demand when coupled with combined heat and power (CHP).
Anaerobic digestion proceeds through hydrolysis (breakdown of complex organics), acidogenesis (conversion to volatile fatty acids), acetogenesis (conversion to acetic acid, H&sub2;, CO&sub2;), and methanogenesis (conversion to CH&sub4;). The methanogenesis stage is rate-limiting and most sensitive to operational upsets. Temperature, pH (6.8–7.4), alkalinity (>2,000 mg/L CaCO&sub3;), and freedom from inhibitors are the key control parameters.
Mesophilic digestion operates at 35–38°C and is the global standard for municipal sludge. Thermophilic digestion (52–58°C) achieves 10–15% higher volatile solids destruction and full pathogen destruction (equivalent to BS EN 15002 Category A), but is more sensitive to process upsets and has higher heat demand. Thermophilic is favoured where land application of biosolids requires Category A hygienisation.
Typical biogas yield: 0.25–0.45 m³ CH&sub4; per kg VS destroyed. Biogas composition: 60–70% CH&sub4;; 30–40% CO&sub2;; traces of H&sub2;S (50–2,000 ppm) requiring removal before CHP. A 50,000 PE WWTP with mesophilic digestion typically generates 500–800 m³/day biogas, meeting 50–70% of site electricity demand via CHP. Use our biogas yield calculator.
Biogas-fired combined heat and power units generate 0.5–1.0 kWh per m³ of biogas. Waste heat from exhaust gas and engine cooling is recovered to maintain digester temperature at 35–55°C.
| Parameter | Mesophilic (35–38°C) | Thermophilic (52–58°C) |
|---|---|---|
| HRT (hydraulic retention time) | 20–30 days | 12–20 days |
| VS destruction | 40–55% | 50–65% |
| Pathogen reduction | 2–3 log (Category B) | 4–6 log (Category A) |
| Specific biogas yield | 0.25–0.35 m³/kg VS | 0.30–0.45 m³/kg VS |
| Heat demand | Lower (easier to maintain temperature) | Higher (greater heat input required) |
| Stability / process robustness | High | Lower — more sensitive to upset |
| Digestate odour | Moderate | Lower (better stabilisation) |
| UK regulatory classification (PAS 110) | Category B (restricted use) | Category A (unrestricted) |
Egg-shaped digesters offer the best mixing efficiency and grit-free operation but have high capital cost. Cylindrical flat-floor digesters with internal mixing are the most common UK municipal design. Floor slope minimum 1:6 to allow grit removal. Minimum freeboard 300 mm above operating level for gas accumulation and foam management.
Effective mixing prevents stratification, temperature gradients, and scum layer formation. Options: gas recirculation (bubbling compressed biogas through bottom lances); mechanical mixing (slow-speed top-entry or side-entry mixers); pump recirculation through external heat exchanger. Mixing intensity: 5–8 W/m³ for effective homogenisation.
Common inhibitors: ammonia (toxic above 3,000 mg/L NH&sub4;-N for mesophilic); hydrogen sulphide (>500 mg/L dissolved); heavy metals (Cu, Zn, Ni — check industrial feed quality); volatile fatty acid accumulation (>2,000 mg/L signals process stress). Monitor VFA:alkalinity ratio daily — target <0.3.
Stabilised digestate is a valuable biofertiliser with 3–5% nitrogen, 1–2% phosphorus, and 1–2% potassium. Dewatered cake can be pelletised for agricultural distribution, creating a revenue stream.
Co-digestion for biogas uplift: Adding high-energy co-substrates (food waste, FOG, glycerol) to sewage sludge digesters can increase biogas yield by 50–200%. The UK’s Environment Act 2021 mandates separate food waste collection from 2025, creating a significant co-digestion feedstock supply. Our engineers can assess co-digestion feasibility and process integration for your existing digesters.
Pre-digestion thickening to maximise digester VS loading and biogas yield.
Sludge ThickeningEstimate biogas production and CHP energy generation from your sludge characteristics.
Biogas CalculatorHigh-rate anaerobic digestion (EGSB, UASB) for food and brewery wastewater.
Food Industry ADOur engineers can assess your sludge characteristics and recommend the most efficient dewatering and disposal route.
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