Introduction. The main problem is discussed that is related to application of reverse osmosis membranes method for quality drinking water production from underground water sources that contain dissolved contaminants dangerous for health such as: fluoride, ammonia, lithium, strontium, arsenic, boron etc. It is also mentioned that reverse osmosis is currently efficiently used for drinking water production. The main goal of the present work was evaluation of the new developed method efficiency and calculation of the operational costs to compare with conventional approach to use reverse osmosis to remove lithium from the ground water. Present article demonstrates results of research aimed at developing a new approach to change the ratio of monovalent and bivalent ions in the permeate of nanofiltration membranes. An example of water with high lithium content that exceeds normative value by 24 times is discussed. The developed technique of ion separation is applied and experimental results are demonstrated, thus providing low lithium concentration in product water with increased calcium and TDS by 5 times as compared to the conventional use of reverse osmosis membranes. Operational costs are evaluated and compared with conventionally used approach to produce drinking quality water from ground water.

Materials and methods. A series of experiments were conducted to remove lithium from ground water and to demonstrate the efficiency of the new developed method of ion separation. Experimental results of permeate and concentrate separation are presented compared to reverse osmosis that provided the increase of calcium and TDS values in the product water by 4–5 times compared to permeate produced by reverse osmosis membranes. The economical evaluation of the main technical parameters of the developed method involved calculation of the required membrane area and the number of membrane elements at each stage, calcium carbonate scaling rates and reagent consumption to prevent scaling as well as the amounts of concentrate discharges into the sewer.

Results. Experimental dependencies of the efficiency of different dissolved contaminants removal from the ground water using different types of membranes depending on the recovery values were obtained. Experimentally obtained results confirmed the possibility to increase the calcium concentration and TDS values of the product water by 4–5 times leaving the lithium concentration at the same level. A flow diagram of the developed process is demonstrated based on experimentally obtained results. The increase of product water TDS facilitates the further reduction of concentrate flow rate and operational costs. Design characteristics to calculate operational costs for two options (conventional and proposed) are presented. Economical comparison was performed using results of calculations of membrane surface area on each stage of membrane treatment, scaling rates, reagent consumption, concentrate discharges.

Conclusions. Application of reverse osmosis for removal of monovalent contaminants (such as: lithium, ammonia, boron) from ground water results, in reduction of TDS values that requires additional costs to provide permeate conditioning. Operation of reverse osmosis facilities also is distinguished by scaling problems that also requires additional operational costs to prevent and remove scale deposits. In this paper, a new approach is proposed to apply nanofiltration membranes using double stage treatment and separate monovalent and bivalent ions and increase calcium and TDS content in product water leaving the lithium content at the extreme level. The use of developed method enables to reduce operational costs due to lower membrane replacement costs, reagent consumption for scale prevention and concentrate discharges.