Can we create a perfect loop of urban bio-waste processing and recovery?

With a European funding package of 7.7 million euros, DECISIVE is one of the projects to have received funding through the H2020 programme¹. Its goal is to come up with innovative ways to solve the problem of urban waste management.

What is new and different about the DECISIVE concept?

Anne Trémier: Currently, urban waste travels some distance after collection to be treated and converted into a resource. The idea behind the DECISIVE project is to process waste as close to its source as possible. Previous studies led by INRAE on the local treatment of household waste (composting in particular) have shown that, once people are aware of what happens to their waste and how it is processed and reused, they produce less of it. This inspired us to begin work on a circular waste management system: I feed myself, I therefore produce waste, I collect it and enable it to be processed into a resource in a form that can be used there and then. Our system provides a way to scale up this principle to neighbourhood levels of waste processing and valorisation and embeds it in a circular economy. We aim to unlock a reduction in waste production by building on community participation and partnerships with energy producers and the transport sector, ultimately increasing the sustainability of urban development.

In concrete terms, what will the micro-digester processing flow look like?

Anne Trémier: Just like any standard anaerobic digestion process, this ‘miniature’ processing flow will allow production of both a biogaz that can be directly converted to energy and a digestate (the residue of the process) that yields high added value – a green pesticide in this instance. To make sure that this type of processing can be carried out appropriately in a dense urban setting, the processing plants will treat a maximum of 200 tonnes of bio-waste a year², equivalent to the waste generated by 800 to 1000 households. This is where the technological challenge comes in: we need to design an anaerobic digestion process that is capable of operating at this scale, that will meet its energy targets by keeping consumption low, that is both simple and compact and yet has a high enough specification to meet the security and regulatory requirements for operation within the community.

DECISIVE was launched in 2016, what are its achievements so far?

Anne Trémier: The basic concept has now been developed for this neighbourhood waste processing flow and we have created two particular tools to help in its implementation. The first is a decision-making tool that allows local communities to work out the added value of this sort of local and decentralised system by comparing it with their existing waste processing strategies with the help of a suite of technical, environmental, economic and social indicators. The second tool devised by the INRAE team is a spatial planning tool. Using GIS mapping, it incorporates data on a given area’s bio-waste resources and existing constraints (regulation, logistics etc) in order to map the most appropriate locations on the ground for waste management units. Intended for use by local authorities, but also by consultants and other operators involved in bio-waste management, these tools will very shortly be made available on a platform that can be accessed through the Decisive project website.

A further remarkable achievement has been the installation of a fully-functioning micro-digester in situ (using technologies already available on the market), hosted by a horticultural college in Ecully, near Lyon. Supplied by a neighbouring school canteen, a hotel complex and a number of high-quality restaurants in Lyon, this demonstration unit, which came on line in October 2019, should enable us to test out the feasibility of an entirely local process, in other words, to close the loop, converting organic waste into energy and fertiliser that can be directly reused on site for vegetable production.

Beyond getting the organisational framework in place, what progress have you made in developing the process itself?

Anne Trémier: To make the system as economical as possible in its water and energy use, we have used INRAE’s expertise in the field of anaerobic digestion to develop an innovative continuous solid state fermentation process or, in less technical terms, a self-sustaining system with a sufficient and regular supply of food waste and no need for extra water. We have come up with a laboratory prototype digester that has now been in operation for a year and that uses a unique process (patent pending) to treat 20 litres of waste at a time. We are currently building a further prototype that will treat up to 100 litres of waste and will enable us to test the process in real conditions using the waste from a public sector canteen. Meanwhile, our Spanish partners at the Universidad Autònoma de Barcelona, who have been tasked with investigating the possible manufacture of a green pesticide from the digestate, have validated their proposed method³ and established the usefulness of the resultant molecule. The process will next be tested at an existing industrial anaerobic digester site.

What is the project’s greatest challenge?

Anne Trémier: If we continue to make decisions based only on economic and financial indicators, we already know that the task of finding a high-performing business model will be a difficult one. Our greatest challenge is to develop models where equal weight is given to economic, social and environmental indicators, making it possible for us to determine the conditions that would allow the process to become not just viable but perhaps even indispensable. It’s a very major change to have to make …

Find out more

• Article. The Conversation (par A. Tremier): Micro-méthaniseurs : la solution pour mieux valoriser les déchets en ville ?

^{1 This project was funded by the European Union’s Horizon 2020 programme (n°689229).}

^{2 For comparison, existing small agricultural anaerobic digesters treat 1000 – 2000 tonnes of organic waste per annum.}

^{3 The chosen method is to grow cultures of the Bacillus Thuringiensis microorganism, known for its insecticidal properties, on the anaerobic digestate.}