Regional energy transition: the codigestion of wastewater treatment sludge and fermentable waste offers solutions
Consistent with the aims of COP21, codigestion of fermentable waste with wastewater treatment sludge offers practical solutions for individual regions. The challenge is to generate renewable energy (electricity, heat and biomethane) locally and optimise the process profitably by pooling waste from different sources.
More than a million tonnes of wastewater treatment sludge are produced every year by wastewater treatment plants. Like every type of fermentable waste (food waste and plant waste), this sludge can, under the right conditions, generate methane-rich biogas that can be recovered in the form of energy. This energy resource is currently under exploited (Recovering the biogas released by wastewater treatment plant (STEP) waste, Ministry for Sustainable Development). Why? Because the upfront investment cost presents a very real barrier to the profitable operation of methanation units (The fundamentals of methanation, ADEME), which limits their development to large-scale wastewater treatment plants with a capacity of more than 100,000 PE.
A solution well suited to rural and suburban local authorities
For rural and suburban local authorities with wastewater treatment plants of modest size (10,000-30,000 PE), co-treatment of sludge and local biowaste offers a solution that could lead by example in terms of regional ecology.
It offers both:
– an alternative treatment channel for organic waste (from catering, retail, food processing, landscape maintenance, etc.) in the context of a circular economy (wasting nothing and converting everything within the same region) – and a local source of renewable energy generation that is optimised and made profitable by pooling waste from different sources. It is fully consistent with the targets set out in the energy transition law introduced by the Ministry for Ecology and Sustainable Development (Ministère de l’Ecologie et du Développement Durable) published on 18 August 2015, which commits France to achieving a 32% contribution from renewables by 2030 and reducing national greenhouse gas emissions by 40% over the same period.
Easy to implement?
Although this solution has the potential to put regions on the road to the energy transition and generate synergies between local stakeholders, putting it into practice requires the prior provision of information systems to locate sources of waste, identify collection logistics, and model the volume of methane production relative to the type of waste available. Another challenge is encouraging the involvement of a broad range of local stakeholders, which requires a high level of administrative commitment on the part of the local authority.
The dining hall at La Chesnaye preschool and primary school
In 2014, Saur worked with the local authority of Mordelles in the Ille-et-Vilaine region of France to:
. Assess the potential biowaste resource (plant, catering and food industry waste) . Model the biomethane production process using the Codilog package (Design support software for a wastewater treatment plant sludge codigestion project using suburban organic waste, Astee conference 2015), developed in partnership with IRSTEA (the French National Science and Technology Research Institute for the Environment and Agriculture) . Define the optimum mixtures required to generate maximum energy (more than 10,000 MWh per year).
The project is now being researched by the Rennes Metropolitan authority and AILE (The Association for Local Energy and Environment Initiatives), which has been appointed to experiment with local renewable energy development projects within the European project framework.