3.1.3. The Soiling®-Process Represents an Efficient Approach to Convert Digestate to Fertilizer

Digestates contain high amounts of NH4-N, but not NO3-N required for plant growth in soilless cultivations. NH4 conversion to NO3 can be achieved by composting, by the soil microbiome and by nitrification fermenters. One efficient nitrification fermenter is the Soiling®-module from Jassen Kunststoffzentrum GmbH (EP 3684909A1 [12]). Verification of Jassen GmbH shows that before the soiling®-treatment, the NH4 amount was 200 mg/L. After 9 days of aeration with 25 m3/d oxygen, the NH3 amount decreased to 15 mg/L. This means 92.5% of the NH4-N is nitrified into NO3-N (calculation in Supplementary File 9). This is superior to other nitrification systems, like the nitrification system of Wang et al., 2017, which has an efficiency of about 87.2%. Therefore, the soiling®-system was used for all further calculations. In addition, the project partner Jassen Kunststoffzentrum GmbH could provide exact numbers for the conversion of the digestate taken from ADP to the fertilizer. Here, a digestate yield of 2.63 L fertilizer per day containing 0.00594 kg/NO3-N per liter. In total, this requires 535.7 kJ energy for each kg N. For comparison: the production of fertilizer using artificial nitrogen fixation process (Haber–Bosch process) requires already 10.800 kJ per kg N for the production of NH4, which has still to be converted to nitrate. A precondition for the nitrification step is a separation of the liquid and solid components of the digestate. This separation could be achieved by a screw press that is easy to handle and

has a low energy consumption (about 0.5 kJ). The remaining solid fraction could also be used to upgraded by composting, but this was not further considered here. To estimate how much N can be produced as fertilizer from a hydroponic system, the N flow according to the Simba model was considered (Figure 7).

**Figure 7.** N flow according to the Simba model. Ntot = total nitrogen, NH4 = ammonium, NH3 = ammonia, NO3 = nitrate. Quantities relate to one year.
