This installation aims to demonstrate that nutrient-rich by-products such as fish sludge and poultry manure can be transformed into valuable fertilizers for plant production.
The objective is to develop a small-scale but manageable production system capable of converting organic residues into a concentrated liquid fertilizer adapted to hydroponic cultivation.
How manure is transformed into hydroponic fertiliser
Animal residues such as poultry manure and fish sludge contain high levels of nutrients, including nitrogen, phosphorus and potassium. However, these nutrients are initially present in organic forms that are not directly suitable for hydroponic cultivation. The challenge is therefore to transform them into a clean and plant-available nutrient solution while ensuring compatibility with sensitive growing systems.
To achieve this, the pilot unit relies on a three-step treatment process:
- Aerobic digestion and settling
- Filtration, oxidation and sanitation
- Nitrification
1. Aerobic digestion and settling
Together, these steps make it possible to progressively convert raw organic residues into a fertilizer solution that is safer, more stable and richer in plant-available nutrients.
The first step of the process is aerobic digestion. During this phase, heterotrophic microorganismsdegrade the organic matter present in the manure and fish sludge. As mineralization progresses, nutrientslocked in the organic material are released into mineral forms. For example, organic nitrogen is converted into ammonium, which can later be transformed into nitrate during nitrification.
Aeration is a key parameter in this phase. Oxygen supply must be sufficient to maintain aerobic conditions and prevent the formation of undesirable gases, while excessive turbulence must be avoided so as not to disrupt floc formation and settling.
After digestion, the digestate is transferred to a settling tank, where floc formation and sedimentation allow the solid fraction to separate from the clarified liquid phase.
2. Filtration, oxidation and sanitation
After settling, the clarified digestate enters a second treatment phase aimed at improving its physical, chemical and sanitary quality before nitrification.
Because the starting materials are animal residues, several issues must be addressed at this stage:
- remaining suspended solids, which can create anaerobic zones and cause problems in downstream processes or around plant roots;
- pathogens, which must be reduced before agricultural reuse;
- residual organic matter, which can stimulate heterotrophic bacteria and interfere with nitrification.
To remove particles, the system uses bag filters, which offer flexibility in mesh size depending on treatment needs. To further improve liquid quality, ozone is injected before UV treatment. This helps reduce residual organic matter and contributes to sanitization before the final biological step.
This treatment stage is particularly important because it creates more favorable conditions for the final nitrification process and helps produce a fertilizer that is more suitable for hydroponic use.
3. Nitrification
The final stage of the process is nitrification, during which specialized microorganisms convert ammonium into nitrate, the main nitrogen form preferred in hydroponic systems.
This step is particularly sensitive, as nitrifying bacteria can be affected by residual organic matter, remaining solids and high free ammonia concentrations. In digestates with high ammonium concentrations, the free ammonia (NH3) threshold that inhibits nitrifying bacteria can be reached very quickly. This makes nitrification control especially challenging when working with nutrient-rich animal residues.
To overcome this issue, the Gembloux pilot unit has been designed around a two-stage nitrification strategy:
- a retention tank, where digestate is stored.
- a nitrification tank, where ammonium is progressively oxidized.
The digestate is transferred from the retention tank to the nitrification tank via a regulated dripping system, which allows precise control of the ammonium load and helps prevent inhibitory peaks of free ammonia.
This stage requires close control of pH, temperature, oxygen concentration and the use of biocarriers to support bacterial growth. Since nitrification is slower than aerobic digestion, the system is designed with batch production upstream and continuous nitrification downstream.
Soon operational in ULiège bioponic pilot system
The unit will soon be installed and commissioned in Gembloux. The next steps will focus on start-up, parameter optimization and fertilizer production trials, followed by plant tests on lettuce and basil.
The current objective is to produce around 10 liters per day of concentrated digestate-based fertilizer.
With this new pilot installation, ULiège Gembloux and the BIOBOOST-Pro project are taking an important step towards the development of innovative, bio-based fertilizers for hydroponic production systems.