Municipal wastewater treatment plants generally use air in the aeration process to introduce O2 for nitrification. O2 is sometimes used in industrial wastewater treatment plants, particularly in the case of high organic loads, substances that are difficult to degrade or insufficient aeration basin size. Compared to air injection, however, this results in significantly higher running costs.
Increasingly problematic are so-called trace substances, i.e. pharmaceutical residues, hormone-like substances, pesticides, etc. For this reason, the EU draft of the municipal wastewater directive requires a 4th treatment stage for larger wastewater treatment plants. Ozonation, in which the trace substances are oxidatively cracked, has proven its worth. However, for wastewater treatment plants of size class 4 (GK4), the supply of LOx (liquid oxygen - cryogenic liquid O2) by cASU (cryogenic Air separation Unit) alone incurs costs of over €150,000 per year. The alternative on-site O2 production with PSA generators (pressure swing adsorption) saves costs, but still leads to considerable costs of €30,000 (own electricity: €0.08/kWhel.) to €90,000 (external purchase: €0.25/kWhel.) due to the electricity requirement of > 0.9 kWh/m3 O2. The required gas drying has not yet been taken into account.
Fraunhofer IKTS has developed a high-temperature membrane separation process based on highly selective MIEC membranes (MIEC - Mixed Ionic Electronic Conductor). Their permeability for O2 is based on the simultaneous transport of oxide ions and electronic charge carriers. Thanks to optimized operating conditions and highly efficient heat exchangers, a specific energy requirement similar to that of the optimized cASU was achieved, but already at very low O2 throughputs. This means that the process can be used for on-site O2 production, with energy costs being more than halved compared to PSA. This is the basis for a spin-off project (BMWK, FKZ: 03EFSTH033), which is driving forward the construction and sale of O2 generators under the name POXOS® (Pure Oxygen on Site).
