PROJECT DESCRIPTION
BACKGROUND
One of the most efficient technologies for reducing greenhouse gas (GHG) emissions in the waste sector is anaerobic digestion (AD). AD is a process which offers opportunities not only to minimise waste volumes but also to produce renewable energy in the form of biogas and biomethane. AD technologies have traditionally been considered as an adequate treatment for the sewage sludge generated in wastewater treatment plants (WWTPs), as they enhance the production of alternative energy sources to sustain the wastewater treatment process. This can cut operational costs and limit the dependence on other energy sources.
However, the process has limitations. The hydrolysis of complex organic waste is commonly ineffective, leading to both long retention times, which result in large bioreactor sizes, and reduced biogas yields. To improve hydrolysis and accelerate methane production, several studies have recommended chemical, thermal or biological pre-treatment methods, or a combination of these techniques. Such practices could break down the complex sludge structure, reducing the solid mass, disintegrating complex extracellular substances and boosting methane production and kinetics during AD. Furthermore, sonication – which is a process of applying sound energy to agitate particles or discontinuous fibres in a liquid – is widely used to manage sewage in wastewater plants. This technique facilitates the breakdown of organic matter by microorganisms. This enables shorter biomass retention times, which reduces the size of the reactors and, subsequently, the capital investment.
Despite being a mature technology, AD operated in mono-digestion mode – one sole feedstock – faces several challenges. The simultaneous digestion of two or more feedstocks, called anaerobic co-digestion, is an alternative that can help stabilise the digestion process and increase the production of biogas, which in turn improves the economic viability of the process. Co-digestion has been deployed at scale in several WWTPs worldwide, but tools to help it reach its full potential have yet to be developed.
OBJECTIVES
LIFE MERLIN aims to boost the biogas production in WWTPs by an innovative combination of urban wastewater sludge pre-treatment technologies and a smart co-digestion process. The biogas will feed the Combined Heat and Power (CHP) engines of the WWTP, converting it into energy for the plant. This will bolster the production of renewables.
The specific project objectives are to:
- Maximise combined heat and power engines’ electricity production on-site by increasing the amount of biogas produced, thus minimising the dependency of the WWTP sites on the grid;
- Minimise GHG emissions and increase sustainability through a circular approach for WWTPs, sludge management and disposal, and biogas production using sludge AD;
- Establish a localised waste disposal ecosystem tailored to each demonstration site;
- Ensure the secure and reliable operation of the co-digestion process within the LIFE MERLIN project to maximise biogas production and prevent any process inhibition.
RESULTS
The project’s expected results are as follows:
- The MERLIN technological solution will pretreat 50% of the total sludge produced in Monte Orgegia and 33% of the sludge produced at the Murcia Este site. The co-digestion process will be implemented at Monte Orgegia and the amount of co-substrate added to the AD in Murcia Este WWTP will increase by up to 15%. During the following 5 years, 100% of the sludge generated at both demo sites will be treated by the LIFE MERLIN technology;
- An increase in biogas production of up to 575,888 normal cubic metres (Nm3) per year, which will be burnt on-site to produce electricity. This will result in a decrease in the import of electricity equal to that generated by the CHP unit and an increase in primary renewable energy production, resulting in 2.36 gigawatt hours (GWh) per year for the Monte Orgegia site and 8.15 GWh per year for Murcia Este WWTP;
- The MERLIN method will be replicated at 6 sites in Spain including in Alicante, Rincón de León, Barcelona, Galicia, Palencia and León. Resulting carbon savings will amount to 9,387 tonnes of carbon dioxide equivalent (CO2eq) per year;
- Once the LIFE MERLIN technology is implemented, GHG emissions will decrease from 3,846 tonnes of CO2eq per year to 3,597 tonnes per year. In 5 years, it is expected that the pre-treatment will be applied to the sludge generated at both sites, achieving further carbon savings in both locations;
- At the end of the project, it is expected that 4 full-time equivalents (FTE) will be necessary to operate the demonstration sites and scale up the replication of the technology in other WWTPs. Once the solution is replicated in the other six WTTPs, 35 FTP will be necessary.
