Construction of a purification plant in the Postojna Caves

The new concession holder of Postojna Cave has decided to renovate the concert hall in the cave and build a post office and souvenir shop next to it. In order to achieve this, the Environment Agency ordered them to install a biological treatment plant in addition to the toilets.

Since the cave has a specific atmosphere and the temperature inside is constantly around 10°C, nobody wanted to take the risk of building a biological treatment plant. At that time, Prof. Hallvard Ødegaard of the University of Trondheim was lecturing in Slovenia. Among other things, he spoke about the pilot plant at the SINTEF/NTNU institute with MBBR/MBR technology for biological treatment and the separation of solid particles from treated water. The interesting aspect of this pilot plant is that the tests were carried out at low temperatures and the cleaning effect was extrapolated to 0°C using mathematical modelling. The results showed that biological treatment processes (MBBR) take place even at such low temperatures.

After consultations with Professor Ødegaard, who supported the idea and promised us his help, we decided to build a treatment plant in the Postojna Cave using MBBR/MBR technology. The treatment plant consists of a mechanical pre-treatment (fine screen – 1 mm), accumulation, three stages of MBBR (den. + ox. + nitr.) and solid/liquid membrane separation (KUBOTA).

The execution was very complex, as the work took place 6 km inside the cave, which can only be reached by electric train. The space for the purification plant, which is located directly under the toilets, had to be excavated in the rock. The construction company built too small a space and we had to further modify the installation of the equipment.

It took some time after the initial start-up for the treatment plant to start operating stably. We took over the management of the treatment plant immediately after its commissioning. The peak of the species was reached in 2016 when, for the first time in history, the olm (Proteus anguinus) hatched eggs and gave birth to young in captivity. At the time, the cave received up to 11,000 visitors per day. The treatment plant still meets the required discharge parameters and is operated by the company CID.