Biogas in WWTP is the most underused operational opportunity in Spain’s water sector. Most plants with anaerobic digestion operate with VS removal of 35-50% digesting only primary and secondary sludge, when they could reach 45-65% by co-digesting with the organic fraction of municipal solid waste (OFMSW). Well-managed co-digestion raises biogas production, reduces the dewatered sludge to be managed by 20-30%, and improves the traceability of the digestate as a valorisable by-product. The operational question is not whether co-digestion works, but how it is designed without compromising effluent quality or the sludge line.
Biogas in WWTP production is one of the most mature use cases of industrial anaerobic digestion: it has been operated for decades and is a standard part of the design of medium and large plants. But the sector’s learning curve has stalled on the first step: digesting only the internal sludge and accepting the biogas generated as a by-product of sludge treatment. The second step, where the real value lies, is operating anaerobic digestion as a business unit: maximising biogas production through co-digestion, reducing the final management cost of dewatered sludge and monetising the digestate in line with regulations. This article describes what changes between primary and secondary sludge, why co-digestion with OFMSW is the highest-return step, what to watch in digestate quality, and when thermal pre-treatment starts to pay off economically.
How biogas in WWTP differs from agro-industrial biogas
Anaerobic digestion in a WWTP shares its biochemistry with agro-industrial digestion but operates under very different contractual and regulatory constraints. Three structural differences explain why WWTPs operate under different criteria:
- The primary objective is not biogas: it is compliance with the effluent discharge to the receiving watercourse. Biogas is a by-product of sludge treatment. Any decision about the digester must ensure that the sludge line is not destabilised, because the digester return flows (supernatant, dewatering liquor from the digested sludge) go back to the WWTP head and affect the discharge.
- The substrate is endogenous and very stable: primary and secondary sludge have a predictable composition and a constant flow (linked to the treated water flow). This facilitates operational control but limits the production ceiling: the methane potential (BMP) of sludge is relatively low (150-350 NmL CH4/g VS) compared with energy substrates such as silage (290-340) or FOG (800-1,000).
- Co-digestion is optional and highly profitable: a WWTP with oversized digestion capacity (very common in plants designed before primary optimisation) can accept external substrates on a gate-fee basis, generating two simultaneous revenues: the treatment fee for the incoming waste and the additional biogas produced.
WWTP + OFMSW co-digestion: the highest-return case
Co-digestion of WWTP sludge with OFMSW (Organic Fraction of Municipal Solid Waste) is, in economic terms, the biogas use case with the best TCO in the sector. Three effects accumulate: the average BMP of the diet rises (OFMSW has a BMP of 350-500 NmL CH4/g VS versus the 150-350 of sludge), the plant’s productivity absorbs more volume by using the digester’s idle capacity, and VS removal rises from 35-50% to 45-65% because OFMSW is more biodegradable than sludge, proportionally reducing the dewatered sludge to be managed.
The critical condition for WWTP + OFMSW co-digestion to work is the stability of the input. The OFMSW received by the WWTP must be previously pre-treated (removal of impurities, shredding, homogenisation) and characterised by batch. If the WWTP receives OFMSW with unseparated plastics, glass or metals, the mechanical problems in pumps, heat exchangers and mixers cancel out the economic return. The operational rule: the WWTP must not become a waste pre-treatment plant, but receive an already-conditioned substrate.
Typical WWTP + OFMSW co-digestion ratio
The OFMSW fraction in the mixture is usually between 10 and 25% of the total VS fed to the digester. Below 10% the economic effect is marginal. Above 25%, the risk of destabilisation increases (propionic acid accumulation, ammonia peaks if the OFMSW contains a high protein fraction) and the management of return flows to the WWTP head becomes more complicated. The optimal operational and economic point is 15-20% OFMSW on a VS basis in most medium and large plants.
VS removal and final sludge disposal
The final management cost of dewatered sludge is one of the highest OPEX items in a medium WWTP, with typical values between 40 and 90 €/t of wet sludge depending on the destination (agricultural valorisation, external composting, landfill, incineration). Each additional percentage point of VS removal in the digester translates into a proportional reduction in the dewatered sludge produced.
In figures: a 200,000 PE WWTP with typical VS removal of 42% digesting only sludge, which moves to 55% by implementing co-digestion with 18% OFMSW, reduces its annual dewatered sludge production by approximately 22%. For a plant managing 8,000 t/year of dewatered sludge at 65 €/t, that is equivalent to an annual saving of around 115,000 € in final disposal alone, before counting the gate fee charged for accepting the OFMSW and the value of the additional biogas.
Digestate quality and agronomic limitations
The digestate (the liquid phase of digested sludge after dewatering) can be valorised agronomically as an organic amendment, but Spanish regulations (Royal Decree 1051/2022 on sustainable nutrition of agricultural soils) and European regulations (EU Regulation 2019/1009 on fertilising products) impose strict limits on heavy metals, pathogens and impurities. The agronomic quality of the digestate from a co-digesting WWTP depends critically on the quality of the external substrates accepted.
| Parameter | Typical threshold (agronomic digestate) | Main risk |
|---|---|---|
| Cd (cadmium) | < 3 mg/kg DM | Uncharacterised industrial substrates |
| Pb (lead) | < 150 mg/kg DM | OFMSW with metallic impurities |
| Hg (mercury) | < 1.5 mg/kg DM | Poor screening |
| Pathogens (E. coli) | < 1,000 CFU/g DM | Incomplete digestion or absent thermal pre-treatment |
| Salmonella | Absent in 25 g | Mesophilic digestion without hygienisation |
| Impurities (plastics) | < 0.5% by weight | OFMSW poorly screened at source |
Thermal pre-treatments in biogas in WWTP: when they pay off
Thermal pre-treatments (THP, Thermal Hydrolysis Process, with commercial technologies such as CAMBI, Exelys, BIOTHELYS) subject the sludge to thermal hydrolysis at 150-170 °C and high pressure before it enters the digester. The biochemical effect is twofold: solubilisation of hard-to-biodegrade organic matter (especially secondary sludge, rich in biomass with resistant cell walls), and hygienisation of the sludge (class A under the regulations). The operational result is a 20-40% increase in the BMP of secondary sludge and the possibility of agronomic valorisation of the digestate without sanitary limitations.
The economic viability threshold of THP is generally found in plants of > 250,000-300,000 PE for two reasons: the CAPEX is high (several million euros), and the thermal OPEX requires economies of scale for the extra biogas produced to offset the energy consumed. For medium plants (50,000-200,000 PE), the ROI is usually marginal unless there are additional drivers (demanding agronomic destination, high sludge disposal costs). In small plants, THP rarely pays off.
Operational case: 150,000 PE WWTP with OFMSW co-digestion
150,000 PE WWTP with two mesophilic anaerobic digesters (38 °C) operating at an HRT of 22 days. Previous configuration: exclusive digestion of mixed sludge (60% primary, 40% thickened secondary). Average production: 750 m³ biogas/day. VS removal: 41%. Dewatered sludge produced: 6,200 t/year at 65 €/t.
Intervention: incorporation of pre-treated OFMSW
Agreement with a nearby mechanical-biological treatment (MBT) plant to receive pre-treated OFMSW (removal of impurities and prior shredding). Gradual input from 5% to 18% of total VS over 16 weeks, with reinforced monitoring of FOS/TAC, individual VFAs (especially propionic acid), total ammonia and supernatant quality.
Result after 12 months of stable operation
Key results after 12 months. Biogas production: 750 → 1,180 m³/day (+57%). VS removal: 41% → 56% (+15 points). Annual dewatered sludge: 6,200 → 4,850 t (−22%).
Revenue from OFMSW gate fees: +145,000 €/year. Saving on final sludge management: +88,000 €/year. Revenue from additional biogas (valorised in cogeneration): +52,000 €/year. Total direct return: ≈ 285,000 €/year. Additional CAPEX required (OFMSW reception tank + dosing pump): 320,000 €. Payback: ≈ 14 months.
Frequently asked questions about biogas in WWTP
How much biogas does WWTP sludge produce?
The typical BMP of primary WWTP sludge is between 250 and 350 NmL CH4/g VS; that of thickened secondary sludge between 150 and 250 NmL CH4/g VS (lower due to the greater resistance of the biomass cell walls). In industrial operation, real specific productivity is typically 60-75% of the theoretical BMP, conditioned by the digester’s HRT and the composition of the mixture. Co-digestion with OFMSW can raise the diet’s average productivity by 30 to 50%.
What type of OFMSW can be co-digested with WWTP sludge?
The OFMSW suitable for co-digestion with WWTP sludge must be previously pre-treated in an MBT or sorting plant: removal of impurities (plastics, glass, metals), shredding to a size < 12 mm and homogenisation. Receiving untreated OFMSW at the WWTP does not pay off: the mechanical problems in pumps, heat exchangers and mixers cancel out the economic return. The minimum characterisation must include VS, C/N ratio, impurity content and the absence of critical pathogens.
How does co-digestion affect the digestate and its agronomic use?
Co-digestion with OFMSW can improve the nutrient content (N, P, K) of the digestate but can also worsen its quality if the OFMSW contributes heavy metals or non-separable impurities. Spanish (RD 1051/2022) and European (EU 2019/1009) regulations impose strict limits on Cd, Pb, Hg, pathogens and impurities for agronomic use. The analytical traceability of the digestate is critical: batch-by-batch characterisation, not just quarterly.
When does a thermal pre-treatment such as CAMBI or Exelys pay off?
Thermal pre-treatment (THP) generally pays off in plants of more than 250,000-300,000 PE due to the high CAPEX required and the need for economies of scale to amortise the thermal OPEX. In medium plants the ROI is usually marginal, unless there are additional drivers such as a mandatory class A agronomic destination or high sludge disposal costs. In small plants it rarely pays off.
Does your WWTP digest only sludge?
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References and regulations
Mehariya, S. et al. (2018). Co-digestion of food waste and sewage sludge. Chemosphere, 211, 670-681. doi.org/10.1016/j.chemosphere.2018.07.027
Bolzonella, D. et al. (2018). Anaerobic codigestion in wastewater treatment plants. Renewable and Sustainable Energy Reviews, 92, 1-13. doi.org/10.1016/j.rser.2018.04.062
Royal Decree 1051/2022, of 27 December, establishing rules for sustainable nutrition in agricultural soils. boe.es/eli/es/rd/2022/12/27/1051
Regulation (EU) 2019/1009 on EU fertilising products. eur-lex.europa.eu/eli/reg/2019/1009/oj