Sustainable biogas purification by integration of a nature-based solution for digestate centrate treatment with biological H2S removal

The wastewater treatment sector has the potential to significantly reduce its own energy consumption (estimated at between 1% and 3% of global energy use) and to produce renewable energy, for example, by producing biogas from sludge.

Anaerobic digestion (AD) has acquired a particularly important role in the technical and economic management of wastewater treatment plants (WWTPs) since, in addition to removing pathogens and stabilising biosolids, it enables the recovery of volatile solids in biogas and co-digestion with other organic wastes to increase the biogas production.

The use of AD can reduce greenhouse gas (GHG) emissions by an estimated 3.29 to 4.36 billion tonnes of CO2 equivalent, representing approximately 10-13% of global GHG emissions from selected agro-bioenergy activities. Currently, only around 10% of WWTPs in Europe produce biogas through sludge digestion. This limited adoption is mainly due to the decentralised nature and varying sizes of WWTPs, which limits the economic viability of AD.

Sustainable and low-cost purification solutions for biogas are crucial, including the future deployment of technologies that valorise biogas (including its CO2 content) beyond its current valorisation in boilers, combined heat and power (CHP), and enrichment to biomethane for injection in the natural gas grid. To accelerate the availability and cost-effectiveness of technologies using/producing biofuels in the treatment of complex waste streams such as biogas, it is essential to remove undesirable impurities. A critical impurity in biogas is H2S, as a corrosive and toxic gas.


Project objective:

LIFE GreenCouple project aims to enhance the sustainability of wastewater treatment plants (WWTPs) and anaerobic digestion (AD) facilities which produce digestate and biogas by boosting the valorisation of waste streams.

This will be achieved through a solution consisting of improved pre-treatment of waste streams —including digestate, centrate and biogas)—using nature-based approaches. LIFE GreenCouple will scale-up and validate the integration of two sustainable biotechnologies, aiming to reduce the environmental impact of digestate centrate by treating it with a nature-based solution and subsequent valorisation in biogas purification processes. This solution will decrease operational costs (OPEX) of current methods for digestate centrate treatment and biogas purification, aiming to accelerate the roll-out of both AD and the downstream technologies used for biogas valorisation.

Key specific objectives:

- Scale-up and build two sustainable biogas purification prototypes based on the integration of two biotechnologies: i) a Nature-Based Solution (NbS), consisting in specifically innovative Constructed Wetlands (CW) to nitrify digestate centrate, remove phosphorous and contaminants of emerging concern; ii) a Suspended Biomass Bioreactor (SBB) operated under anoxic conditions using the nitrified centrate as electron and nutrient source to remove Hydrogen Sulphide (H2S) from biogas.
- Validate the use of CW as a cost-effective alternative to activated sludge centrate treatment processes to obtain a biogenic nitrate + nitrite rich stream to be used in the SBB, in addition to carry out phosphorous and contaminants of emerging concern removal.
- Validate the biological removal of H2S using a SBB operated under anoxic conditions to decrease OPEX in the current strategies.
- Validate the proposed solution at three demo sites with different centrate and biogas characteristics: Gandia and Ontinyent WWTPs with AD plants (Valencia, Spain) and Huntstown agro-industrial AD plant (Dublin, Ireland). Prototype A (Demonstration plant) will be installed in Gandia WWTP and Prototype B (Replication plant) will be designed as a transportable system to be installed at Ontinyent WWTP and Huntstown AD plant.

Expected results:

- Amount of centrate treated: 2,920 m3/year
- Amount of biogas treated: 893,520 Nm3/year
- H2S removed: 711 kgS-H2S/year-
- N valorised in SBBs: 0.647 t/year
- P removed: 1,827 kg/year
- S removed from biogas: 711 kg/year
- Fe salts consumption reduction: 51 t/year
- Activated carbon consumption reduction: 375 t/year
- DWTP Sludge usage in CW: 22 t/year
- Biological anaerobic sludge produced reduction: 112 t/year
- Reduction in primary energy use: 0.008 GWh/year
- GHG emissions reduction: 1.492 CO2eq/year
- S-SOx emissions reduction: 169 kg/year

🚩 𝗧𝗵𝗲 𝗽𝗿𝗼𝗯𝗹𝗲𝗺:
The EU aims at energy neutrality for urban wastewater treatment by 2040. But still, there are many challenges in Wastewater Treatment Plants (WWTPs), like removing pathogens and contaminants, stabilizing biosolids, recovering volatile solids, and increasing biogas production.

Converting waste into biogas and energy is key to the circular economy and green transition, and a relevant solution for this is through anaerobic digestion (AD) of biomass waste, which includes sludge from urban wastewater treatment.

Current systems are energy-intensive, and treating sludge and byproducts, like AD centrate, can be expensive and polluting. These byproducts often contain high concentrations of nutrients or harmful gases, which can reduce the quality of biogas, harm ecosystems, and increase operational costs.

🟢 𝗧𝗵𝗲 𝗽𝗿𝗼𝗷𝗲𝗰𝘁:
The LIFE GreenCouple project is developing a nature-based solution to improve wastewater and biogas treatment. By combining constructed wetlands (CW) with an anoxic suspended biomass bioreactor (SBB), the project tackles two significant challenges:

→ Removing nutrients from AD centrate
→ Cleaning biogas by removing hydrogen sulphide (H₂S)

The aim is to reduce environmental impact, boost energy efficiency, and cut costs at wastewater and biogas plants — all while relying on low-energy, low-maintenance technologies.
The researchers behind the project are Professor Thomas Højlund Christensen, Associate Professor Ricardo Gabbay de Souza, and Senior Researcher Anders Fredenslund.

🌍 𝗧𝗵𝗲 𝘀𝘂𝘀𝘁𝗮𝗶𝗻𝗮𝗯𝗶𝗹𝗶𝘁𝘆 𝗰𝗵𝗮𝗹𝗹𝗲𝗻𝗴𝗲:
This project supports the green transition by valorising waste streams, increasing resource recovery, and reducing greenhouse gas emissions.
The technologies will be evaluated through life cycle assessment (LCA) and microbiome dynamics modelling to identify critical hotspots and confirm their environmental benefits.

The project also promotes biodiversity and circular use of resources, including reusing aluminium sludge from water treatment in the wetland system.

🚀 𝗧𝗵𝗲 𝗶𝗻𝗻𝗼𝘃𝗮𝘁𝗶𝗼𝗻:
What makes LIFE GreenCouple unique is the combination of two nature-based technologies in one integrated system.
Constructed wetlands recover nutrients like ammonium and phosphorus from centrate, while the SBB enables biological removal of hydrogen sulphide from biogas – a significant innovation compared to conventional systems.
Using waste materials as reactive substrates adds another circular layer to the concept.

The result:
🔹 Cleaner water
🔹 Cleaner energy
🔹 Lower costs
🔹 Higher environmental performance