By Wojciech Mikołajczak (IMN)
As a textbook example of the use of new and constantly developing technologies for both environmental protection and remediation, we will take a look at a water treatment facility located in Dolní Rožínka, Czech Republic. The history of the plant goes back to 1957 when mining of uranium – one of the most strategic elements in the nowadays energy sector – was commenced.
11 years later, an additional mill, a chemical treatment plant and two tailing ponds were added. The technology was based on carbonate leaching and the uranium separation by anionic exchange resin. The original project design envisioned a closed water circuit with a small water deficiency.
This target was never fulfilled and an annual water surplus of about 300,000 m³ remained. The water accumulated in tailing ponds was partially recycled for the mill. The balance water, due to high salinity (about 33 kg/m³, predominantly sodium sulphate) required purification before discharging. The need to solve this problem increased yearly. [1]
The first step undertaken toward environmental protection was the addition of an evaporator (EV) unit in 1977. The system consisted of eight evaporators connected in series and has operated until now. The last two evaporators work as both evaporators and crystallizers. The capacity of the evaporation unit is about 40 m³/h of treated water. The crystals of Na₂SO₄ are separated in a centrifuge, dried and sold for use in detergent production – annually between 6000 and 8000 tons.
Table 1 Comparison of major operational costs; with and without membrane technology [2]
In 1996, an additional electrodialysis (ED) unit began to operate and was scaled up in 2007 achieving a capacity of 65 m³/h of pre-treated water (softening, filtration and ion exchange for heavy metal removal). The concentrate containing about 100 kg/m³ became feed for EV, while partially desalted dilute (12 kg/m³) undergoes further desalination by reverse osmosis (RO) unit.
The development of this unit was commenced in 2000 and since 2007 full-scale installation is capable to discharge up to 35 m³/h of water into the local river while meeting the effluent restrictions (TDS decreased to 0.18 kg/m³). [3]
In 2008, the period of permitted activity was conditionally extended and finally terminated in December 2016. Up to now, the total yield of deposits was 20,220 tonnes of uranium. Currently, the facility remains under the management of GEAM, a branch of DIAMO state enterprise, and continues the remediation process, releasing to rivers about 6.5 mils. m³ of purified water a year.
The combination of membrane and thermal desalination technologies proved to be the most competitive compared to using each technology separately. Thanks to it, investment and operation costs of wastewater treatment decreased, bringing us closer to zero liquid discharge.
[1] V. Benes; J. Mitas; I. Rihak DIAMO, State Owned Enterprise, “Experience with water treatment and restoration technologies during and after uranium mining,” International Atomic Energy Agency (IAEA), Straz pod Ralskem (Czech Republic), 2000.
[2] F. Toman, “Membrane processes in a water treatment at the uranium chemical mill,” in Book of abstracts PERMEA, Prague, 7 – 11 June, 2009.
[3] J. Kinčl, T. Jiříček, D. Neděla, F. Toman and J. Cakl, “Electromembrane Processes in Mine Water Treatment.,” in IMWA Mine Water and Circular Economy, Lappeenranta, Finland , 2017.