(German title: Denitrifikation von Molke und Molkeprodukten - Kombination von Chromatographie und biologischen Nitratabbau)

During the production of cheese, up to 0.25 g of nitrate per liter is added to the cheese milk to prevent late bloating. During this fermentation error, also known as butyric acid fermentation, lactic acid is broken down by Clostridium tyrobutyricum into butyric acid, acetic acid, hydrogen and carbon dioxide. This results in increased nitrate levels in the whey produced as a by-product, which can be around 60 mg l^-1 depending on the production process. In order to enable the use of this whey for the production of dietetics, confectionery, nutraceuticals and functional foods, it is very important to reduce the nitrate content to below 10 mg l^-1.

To this end, an innovative process development for reducing the nitrate content to below 10 mg⋅l-1 was established at the Department of Bioprocess Engineering. By using a two-stage chromatography process, it was possible to reduce the wastewater load from reprocessing by over 95%. This is achieved by a microbial reprocessing stage of the salt eluent. The eluent containing sodium chloride from the anion exchange chromatography is continuously fed into a membrane bioreactor with Paracoccus denitrificans. The microorganism can reduce the nitrate contained in the eluent to gaseous nitrogen under anaerobic conditions. This allows the eluent to be used repeatedly after cross-flow ultrafiltration

Once the individual denitrification stages had been successfully established, a continuously operating laboratory system was put into operation. This was followed by a transfer to a mini pilot mafler with which the continuous denitrification of 250 ml h^-1 whey concentrate could be carried out. The system could be operated over a period of >260 h and 40 cycles without changing the eluent. After coordinating all basic operations, a fully automated and remotely controllable pilot plant with a denitrification capacity of 450 l d^-1 was developed. The plant has a 25 l bioreactor, the contents of which are purified by a 0.8 m² ultrafiltration membrane. The chromatographic stage was established with two 2.3 l anion exchange columns, which were continuously loaded and regenerated alternately. The measurement and control software was developed specifically for this system at the department. The chromatographic stage was able to reduce the nitrate content of the whey concentrate supplied to below 3 mg l^-1. The biological stage was able to completely remove the retained nitrate from the eluent.