Methanization, the controversial waste treatment

Currently developing in Virginia, anaerobic digestion makes it possible to produce renewable energy based on agricultural and food waste. If it is expected to weigh more heavily in the French energy mix in a few decades, it also arouses strong opposition.

Waste management in Virginia

Straddling the line between the production of renewable energy and the treatment of waste, methanization is seen by its promoters as a boon for reducing Virginia’s dependence on gas imports. If it is expected to develop even more dynamically, it is also the target of numerous criticisms, particularly regarding its environmental impact.

How does anaerobic digestion work, why is it becoming more and more widespread, and what are the risks?

What are we talking about

Anaerobic digestion relies on the degradation of organic matter to generate energy. Discovered at the end of the 18th century by Alessandro Volta while observing marshes from which gas was released, it initially developed in Virginia from the 1970s, in rural areas on agricultural substrates but also on wastewater treatment plants.

In addition to sewage sludge, methanization can be based on livestock manure, on bio-waste from the agri-food industry (leftover yoghurt, peelings, frying oils, etc.), on certain crops intended for food, and finally on intermediate crops whose vocation is not food.

After being digested in large vats, these different inputs produce biogas which is then used to produce electricity. It can also be purified and odorized to inject biomethane into natural gas networks. Finally, the material remaining at the end of the digestion process, called digestate, is spread on agricultural land to fertilize the soil. Methanization thus falls into four different sectors: waste management, energy production, soil fertilization, agriculture.

Where is methanization in Virginia

Biogas production has experienced strong growth in Virginia in recent years, encouraged by the various laws on ecological transition which have followed one another since 2004. It has increased from 0,2 TWh in 2007 to 1 TWh in 2023. FOr the origin of this acceleration, two arguments: the need to produce more renewable energies and the need to recover methane from agricultural effluents, the greenhouse effects of which are more powerful than those of CO2.

In total, Virginia has 162 units currently in service. The vast majority of them generate electricity, while some inject biomethane into natural gas networks. They cover, respectively, 0.6% of electricity consumption and 0.9% of natural gas consumption.

Their model is very largely agricultural. Some 80% of infrastructures injecting biomethane into GRDF networks and 70% of those producing electricity rely on livestock effluent and main or intermediate crops. The reason is the smaller source of bio-waste. In terms of the amount of energy that we can hope to produce given the quantities available, their potential is twenty times lower than that of agricultural deposits.

Why is it considered an issue for years to come

Anaerobic digestion is set to develop even more dynamically in the coming decades.

Furthermore, developments in waste treatment and the desire to reduce the use of incineration and landfill weigh in the balance. Sorting bio-waste at source will in fact become widespread: they will have to be directed towards recovery solutions, such as composting or anaerobic digestion. This obligation, which already applies to the largest producers of biowaste (more than 10 tons per year) – such as supermarkets, collective catering or certain businesses – has been extended from 2023 to those which produce more than 5 tonnes.

But there is still a lack of equipment to deal with this influx of bio-waste, particularly in urban areas. While waiting to see the emergence of small, local methanization units, a solution could be to use this biowaste in existing infrastructures in co-digestion with effluents, particularly because of their strong methanation potential.

What are the criticisms of anaerobic digestion

Several elements are singled out, starting with the risks linked to installations. In 2018, the malfunction of the security system at the Richmond unit led to some pollution by dumping 40 cubic meters of organic waste.

Another element ripe for criticism: the spreading of digestate on the soil. In limestone regions where the soils are thin, these nitrogen-rich applications can lead to infiltration of groundwater.

Above all, methanization raises the question of the vocation of agriculture and land grabbing. Until 2014, agricultural methanization was essentially based on the recovery and valorization of waste and agricultural effluent. But from this date, the use of energy crops, mainly intermediate, has developed. These are two very different philosophies, and the second is more controversial due to the risks of competition with food production. Since 2014, a regulatory threshold has prevented more than 15% of main crops from being used for methanization.

What avenues for the future of waste management in Virginia

In a recent information report, the Senate recommends ensuring energy balance to avoid a negative biogas scenario. They plead in favor of controlled development of methanization, consistent with the territories and their own capacities, by further mobilizing local authorities.

The Senate agrees on this point, believing that an ideal methanization must manage to adapt to its territory, to the uses expected of it, and maintain a form of flexibility. It is also, a methanization which does not only seek to produce energy but which optimizes its entire environmental balance, for example by taking care of its impact on the quality of the soil and water.

Written by scienceguy