In the anaerobic treatment of sludge or organic waste, where biogas is produced from the carbonaceous compounds, the nutrients such as nitrogen and phosphate remain for the most part in the aqueous phase. They are then mostly present in the low-carbon fermentation residue as ammonium and phosphate.
Wastewater streams from biogas plants for sewage sludge fermentation with high loading rates, so-called high-load digestion, are characterized, for example, by high ammonium and phosphate concentrations of up to 1300 mg NH4 per liter and 200 mg phosphate per liter respectively. In these wastewater streams, ammonium is currently converted to nitrogen or precipitated together with phosphate in energy-intensive process steps. Reuse is therefore not possible.
The nutrients nitrogen and phosphate can be removed and recovered from these process waters: By using the process water as a nutrient solution for the cultivation and propagation of microalgae, the nitrogen present in the form of ammonium and phosphate as a nutrient can be utilized by the algae.