Fraunhofer Institute for Interfacial Engineering and Biotechnology IGBIn 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.
Biological nutrient removal and reuse
Use of filtrate water from fermentation for the cultivation of algae
Within the project "More biogas from waste and micro-algae residues poor in lignocellulose by combined bio-/hydrothermal gasification – EtaMax", the algae Phaeodactylum tricornutum, which contains the omega-3 fatty acid EPA (eicosapentaenoic acid), was successfully cultivated using filtrate water from two different municipal biogas plants as culture medium. For the economic efficiency of algae cultivation, this means – in addition to the use of waste gas CO2 – a further contribution to cost reduction.
Use of nutrient-rich treated wastewater in agriculture
In global terms, agriculture is one of the largest consumers of water. Especially where water is scarce, new concepts and processes for water reuse are therefore in demand, especially in agriculture.
Combining irrigation with fertilization
As mentioned above, anaerobically treated wastewater still contains a large amount of inorganic phosphate and ammonium salts - nutrients that are urgently needed in agriculture. So why not use wastewater for irrigation and as a source of nutrients at the same time?
In the "HypoWave" research project, an interdisciplinary team is investigating how the efficiency of hydroponic plant production can be further increased by using municipal wastewater for irrigation. To this end, a pilot plant for the reuse of specifically treated municipal wastewater for hydroponic production will first be built near Wolfsburg. There, the technical processes, plant production, the economic efficiency of the plant and the quality of the products produced will be investigated.