Blog Post

Introduction

Picture a giant stomach. To make it less off-putting, let’s imagine it’s a sleek, steel tank. When we feed this “stomach,” it digests the organic material, breaking it down to extract energy and nutrients, just like our own digestive system. This is essentially what happens in Anaerobic Digestion (AD).

In this process, microorganisms break down organic waste such as agricultural waste, manure, and food scraps in an oxygen-free environment inside the tank. While the biogas produced is used to generate renewable energy, there’s another valuable byproduct that’s often overlooked: digestate. Rich in nitrogen, phosphorus, and other essential minerals, digestate is much more than waste, it’s a potent resource for agriculture and environmental sustainability.

Let’s take a step closer at digestates.

What are digestates?

Having introduced digestates as a valuable byproduct of AD, a process designed to bring value to organic waste and produce renewable energy, let’s examine its characteristics more in detail. Digestate is the nutrient-rich material remaining after AD has transformed organic waste into biogas—a mixture of methane and carbon dioxide—for energy. This residual substance is loaded with essential nutrients.

Upon leaving the anaerobic digester, digestates are comprised of both liquid and solid fractions. A mechanical process is used to separate these two phases:

• The liquid fraction, often referred to as “nitrogenous water", typically contains less than 15% dry matter. [1] This liquid can be utilized for irrigation, supplying essential nutrients to crops while helping to reduce landfill waste. However, it is important to monitor its application to prevent excess nitrate accumulation in the soil.
• The solid digestates contain between 20% and 40% dry matter and a variable organic carbon content. These anaerobic digestates still retain a portion of biodegradable organic matter, making them valuable by-products for further processing. Composting these digestates is particularly effective because it serves multiple purposes: (1) it breaks down the remaining organic matter, ensuring the stability of the digestates when applied to soil; (2) the aerobic conditions during composting generate heat, which promotes the drying of the material and eliminates potential pathogens; (3) through the humification process, the transformation of organic matter into humus, the quality of the organic matter is enhanced, improving its effectiveness as a soil conditioner. [2]

By undergoing this composting stage, anaerobic digestates can achieve maturity and become safe for soil application, contributing to sustainable land use and soil health.

Benefits of digestates

The use of digestates in agriculture offers numerous benefits, particularly in the context of declining global phosphate reserves, which are diminishing at an alarming rate. Digestates offer a valuable method for recycling nutrients, thereby helping to preserve essential resources and enhancing circular economy. By enriching the soil with essential nutrients and enhancing its structure, water retention, and drainage, we support sustainable agricultural practices. This not only improves root growth but also promotes better aeration and boosts microbial activity.

Moreover, incorporating digestates into farming practices effectively valorises organic waste, significantly reducing landfill usage and greenhouse gas (GHG) emissions. When organic materials are disposed of in landfills, they can release potent GHG such as methane and nitrous oxide. These gases are significantly more harmful than carbon dioxide; methane is 21 times more harmful, while nitrous oxide is 310 times more damaging. By digesting organic materials, these dangerous emissions can be avoided.

Using digestates as a natural source of nutrients also diminishes the reliance on chemical fertilizers and pesticides, thereby reducing the environmental impact of agriculture. In Europe, the production of each ton of mineral fertilizer through the Haber-Bosch process—which is responsible for fixing most nitrogen for agricultural use—results in an average emission of 9.7 tons of CO₂ equivalent.[3] This not only harms the environment but also perpetuates dependence on imported natural gas for energy.

Challenges

Despite these significant benefits, several challenges need to be addressed to realize their full potential.

Regulatory hurdles: A major obstacle is the lack of a cohesive legal framework. Many countries in Europe do not have any or appropriate legislation concerning digestates use, resulting in barriers to revalorize organic residues and hindering the potential of AD. In addition, some countries do not recognise AD as a recycling process [4] what holds back even more its wide implementation across Europe. This lack of recognition and regulatory support also leads to economic viability issues for AD plants, as they must treat digestates as waste rather than a valuable product. Consequently, this results in additional expenses related to management costs, undermining the overall sustainability and profitability of AD facilities.

Environmental concern: If applied improperly, digestates can harm plant growth and soil health, and even cause environmental problem due to leaching or contamination. Depending on the , Different types of digestate result from varying organic waste used during the anaerobic digestion, each with its own advantages and disadvantages. It’s important to evaluate them to find the most sustainable way to use them.

Lack of awareness: Many  farmers are hesitant to spread digestates on their land due to a lack of information about their benefits The lack of awareness and clear guidance on proper application, environmental impact, and long-term benefits has led to uncertainty among farmers, making them reluctant to adopt digestates as a soil amendment.

FENIX Project

The FENIX project is focused on maximizing the potential of an existing AD plant located in Marmagne, France. This facility processes organic waste from supermarkets to produce biomethane, which is injected into the natural gas grid. As we have learnt above on this article, the process generates digestates as a by-product. The project aims to improve their management and value by increasing their potential mixing them with biochar (discussed in a previous blog post) to create a formulated soil amendment product that enhances soil fertility, water retention, nutrient uptake, and microbial activity, while also providing economic benefits to the agricultural sector.

To achieve this, the FENIX project will collect digestates from the Marmagne AD plant and conduct tests on various biochar combinations while analysing their characteristics. By leveraging the AD process, the project addresses organic waste management challenges, generates renewable energy, and creates valuable soil amendments.

FENIX aims to explore the combination of two types of organic matter derived from renewable gas production—biochar and digestates—to increase recycling of nutrients coming from waste and create a regulated soil amendment product specifically designed for poor soils and modern farming.

FENIX plays a vital role in promoting a more sustainable and circular economy while supporting eco-friendly farming practices. The implementation of the developed soil amendment formula can significantly enhance agricultural productivity, leading to healthier soils.

References:

[1] Anaya-Reza, O., Altamirano-Corona, M. F., Basurto-García, G., Patricio-Fabián, H., García-González, S. A., Martinez-Hernandez, E., & Durán-Moreno, A. (2024). Wet anaerobic digestion of organic fraction of municipal solid waste: experience with long-term pilot plant operation and industrial scale-up. Bioprocess and Biosystems Engineering, 47(2), 235-247.

[2] Teglia, C., Tremier, A., & Martel, J. L. (2011). Characterization of solid digestates: Part 2, assessment of the quality and suitability for composting of six digested products. Waste and Biomass Valorization, 2, 113-126.

[3] Ahlgren, S., Baky, A., Bernesson, S., Nordberg, Å., Norén, O., & Hansson, P. A. (2012). Consequential life cycle assessment of nitrogen fertilisers based on biomass–a Swedish perspective. Insciences J., 2(4), 80-101.

[4] Lamolinara, B., Pérez-Martínez, A., Guardado-Yordi, E., Fiallos, C. G., Diéguez-Santana, K., & Ruiz-Mercado, G. J. (2022). Anaerobic digestate management, environmental impacts, and techno-economic challenges. Waste Management, 140, 14-30.

By: Z. Cardell, J. Gómez
Editorial: L. Salinas
November 2024