Anaerobic Digestion: A central feature of the Circular Economy

Anaerobic Digestion: A central feature of the Circular Economy

Author : Ildefonso Rocamora Miguel, EngD student. His host university is Cranfield University and his project, 'Batch-dry digestion: optimising the science behind it' is sponsored by Amey.

After some time out of the university I never thought that I would be back at Cranfield University and back to science again. But with the support of Amey Plc, Cranfield University and the STREAM program here I am again, ready to tackle an EngD program with the goal of improving the current anaerobic digestion processes that Amey currently runs in their two energy from waste plants in England. The goal being to improve biogas production for the production of renewable energy and at the same time minimize gas emissions and contaminants.

The company is currently using anaerobic digestion as part of the United Kingdom’s agenda to solve the problem of the high amount of organic matter (food waste, garden residues, etc..) still present in the waste bags of the people that do not separate them properly at their homes and is never reused. The organic matter in our black bin bags currently finishes its life in landfills, contributing to increasing the footprint of our way of life and increasing pollution and greenhouse gas emissions to the atmosphere, aggravating the greenhouse effect that worryingly raises global temperature.

With the aim of reducing the disposal to landfill to the bare minimum, Amey’s process plants perform a mechanical treatment, where recyclables still present in the plant are recovered, the organic matter is recovered from the general waste and the rest is sent to produce electricity by incineration or gasification. This reduces the disposal to landfill to less than three percent, which reduces dramatically the use of landfills, although further studies are being carried to minimize this amount to zero.

The organic matter obtained from the general waste is then sent to an anaerobic digestion process, storing the biodegradable organic matter in the absence of oxygen, where microorganism will break down the organic matter, producing biogas and a solid residue. The biogas produced in the anaerobic digestion is rich in methane, a gas with high calorific value, that can be burned in engines to produce electricity to be supplied to the grid, and all of this using a residue that before was being disposed to landfill. On top of that, the solid residue obtained from the anaerobic digestion is a product rich in nutrients, and will be used as a fertilizer for agriculture and landscaping, reducing dramatically the amount of product sent to landfill after the process and providing a natural and high quality fertilizer for local farmers at cheap price.

This process is one example of the types of solution that we need to adopt to extricate ourselves from the current resource use model of take, make and dispose, where any used materials end up in landfill without any reuse or recycling. Recovering reusable fractions of solid waste supports a circular economy philosophy, where minimizing the use of naturals resources, reuse and recycle are the main priorities. Anaerobic digestion promises an ideal long-term option to support multiple recirculation of resources, transforming a waste stream to produce renewable energy and fertilizer without the need of using any fossil fuels related technology.