What happens when liquid digestate is injected with air?

When liquid digestate is injected with air, it undergoes a process often referred to as aerobic polishing or aeration, which introduces oxygen into the material. This process can alter the chemical and biological properties of the liquid digestate, making it more suitable for specific applications such as hydroponic fertilization or reducing environmental risks associated with its disposal. Below is a detailed explanation of what happens during this process, based on available research.

Effects of Aeration on Liquid Digestate

• Nutrient Transformation and Stabilization: Aeration facilitates the oxidation of organic compounds and nitrogen forms in the liquid digestate. For instance, ammoniacal nitrogen (NH4+-N), which is abundant in liquid digestate (often 70–90% of total nitrogen), can be partially converted to nitrate (NO3-) through nitrification by aerobic bacteria. Nitrate is a more readily available form of nitrogen for plants, particularly in hydroponic systems, enhancing the digestate's value as a fertilizer 45 :

• "Nowadays, the adoption of sustainable agricultural practices, including the reduction of synthetic fertilizers, has become a challenge for the agriculture sector. In this experimental work, the effect of the liquid fraction of digestate (by-product of the anaerobic digestion process) as a fertilizer was evaluated. The aim of the research was to verify to which extent digestate can affect growth and quality parameters of orange fruits, comparing the results to those obtained for fruits grown on soil treated with conventional mineral fertilizers. To assess the effectiveness of the treatments, different qualitative and quantitative parameters of Citrus fruits were measured. In particular, the results showed slight differences between the two treatments, suggesting that digestate may be used for the production of high-quality fruits. Moreover, in some orchards, the Citrus fruits of the plants treated with digestate showed a higher concentration of health-promoting compounds, such as vitamin C, flavonoids, phenolic content, when compared to the control group. Thus, digestate can be considered an optimal source of plant nutrients and can be used as a crop growth promoter, since it represents an effective strategy for reducing the mineral fertilizers input.:

• Reduction of Organic Matter and Odors: The introduction of air promotes aerobic microbial activity, which further breaks down residual organic matter in the digestate. This process can reduce the biochemical oxygen demand (BOD) and minimize unpleasant odors, making the digestate less likely to cause air pollution when applied to fields or stored 67.

• pH Adjustment and System Stability: Aeration can influence the pH of liquid digestate. Studies have shown that adjusting pH during aerobic polishing can optimize nutrient recycling capabilities, ensuring the digestate is more compatible with specific agricultural applications like hydroponics. This process helps stabilize the material, reducing the risk of nutrient imbalances when used as a fertilizer 4.

• Pathogen and Weed Seed Reduction: While anaerobic digestion already reduces pathogens and weed seeds, additional aeration can further enhance this effect by creating conditions less favorable for anaerobic pathogens and by supporting thermal or oxidative processes that neutralize remaining contaminants 6.

Applications and Considerations

• Hydroponic Fertilizer Preparation: Aerobic polishing of liquid digestate has been explored as a method to prepare it for use in hydroponic systems. The oxygenation process helps tailor the nutrient profile (e.g., increasing nitrate content) to meet the needs of hydroponic crops, while also reducing the risk of clogging irrigation systems due to organic residues 4.

• Environmental Benefits: Aeration reduces the risk of water and soil pollution by lowering the potential for nutrient runoff. The slow-release nature of nutrients in aerated digestate, compared to raw digestate or synthetic fertilizers, minimizes environmental impacts such as nitrogen leaching into water bodies 67.

• Challenges: While aeration offers benefits, it must be carefully managed. Excessive aeration or improper pH control can lead to nutrient loss (e.g., volatilization of ammonia) or microbial imbalances. Additionally, the process may require energy input, which could offset some sustainability gains if not optimized 3.

In summary, injecting air into liquid digestate triggers aerobic processes that transform its nutrient content, reduce odors and organic load, and enhance its suitability for agricultural applications like hydroponic fertilization. This treatment also mitigates environmental risks associated with raw digestate disposal, though it requires careful management to maximize benefits.

1. https://www.sciencedirect.com/science/article/pii/S096085242201375X
2. https://www.sciencedirect.com/science/article/pii/S0929139323002640
3. https://iwaponline.com/wst/article/82/1/144/75620/Effect-of-liquid-digestate-recirculation-on-biogas
4. https://www.mdpi.com/2071-1050/16/10/4077
5. https://www.frontiersin.org/journals/sustainable-food-systems/articles/10.3389/fsufs.2023.1128103/full
6. https://europeanbiogas.eu/wp-content/uploads/2015/07/Digestate-paper-final-08072015.pdf
7. https://www.stgreenpower.co.uk/insight/the-benefits-of-using-digestate-instead-of-fertiliser-a-sustainable-solution-for-agriculture
8. https://www.mdpi.com/2073-4395/13/3/782