REMOVAL OF RADIONUCLIDES FROM ESTONIAN GROUNDWATER USING AERATION, OXIDATION AND FILTRATION

Groundwater in Estonia was analyzed to determine iron, manganese, sulfides, ammonia, radionuclide (Ra²²⁶, Ra²²⁸) content and TID. It has been previously established that for 70% of wells of Cambrian-Vendian water layer the TID would exceed the EU requirement for drinking water (0.1mSv/yr).

The purpose of this work was to determine technology for groundwater simultaneous purification from iron, manganese, sulfides, ammonia and some radionuclides (Ra226, Ra228). A pilot plant consisting of aeration unit GDT (Gas-Degas Technology, Mazzei Corp.,USA), oxidation tank and two-stage filtration columns was constructed. Different MnO₂ containing catalytic filter materials were tested with zeolite. Along the flow sheet pH, dissolved oxygen and carbon dioxide, turbidity, iron, manganese and ammonia content were monitored mainly by using spectrophotometry (HACH DR/2000).

Radioactivity of water samples was determined by the Estonian Radiation Centre using liquid scintillation counting method and γ-spectrometry. It was established that by intensive aeration of groundwater followed by oxidation with certain contact time, and appropriate selection of filter materials, the pilot plant was able to remove dissolved gases, iron, manganese and radium isotopes, without the need to add any chemicals. During the purification process 90% of gross-alfa and 70% of gross-beta activity was removed. TID of effluent was calculated by Ra²²⁶ and Ra²²⁸, and was about 0.067mSv/yr.

Radium cations are removed by two main mechanisms: first, co-precipitation with iron hydroxide flocks, and second, due to the adsorption onto hydrous manganese oxide particles or onto MnO₂ containing catalytic filter materials.

Backwash waters from two filtration steps were collected and analyzed separately. Measurements of Ra²²⁶ and Ra²²⁸ in the backwash water confirm that channeling of backwash water is in compliance with EU directive. Regarding to the solid wastes, preliminary tests indicate that no radium is accumulated in the filter material. Still, further investigation is needed. On the basis of the results of the pilot plant studies, a full scale plant (2500 m3/day) of Viimsi Water Ltd. was designed by the SWECO Project. The plant will be put into operation in January 2012.