Control over the Ratio between Mono- and Divalent Ions in Drinking Water Treatment by Nanofiltration

The main problems related to the application of the membrane method of reverse osmosis for the preparation of high-quality drinking water from groundwater sources containing a number of contaminants hazardous for health, such as strontium, ammonium, nitrates, lithium, fluorides, arsenic, and boron, are described. A new method of controlling the ion composition of purified water on the basis of nanofiltration membranes with low rejection selectivity to salts is proposed to separate mono- and divalent ions. The operational costs and economic benefit are determined as compared to the traditional approach to the treatment of groundwater with a high lithium concentration. A series of experimental studies on the treatment of groundwater with a high lithium concentration are carried out to substantiate the efficiency of the developed ion-separation method. The results of separating permeate solutions and a treated water concentrate are given to demonstrate that the concentration of hardness ions and total dissolved salts (TDS) in the purified water can be increased by 4–5 times as compared to the permeate resulting from the treatment of groundwater on reverse osmosis membranes. The possibility of controlling the ratio between mono- and divalent ions in the permeate to increase the concentration of hardness ions and TDS in purified water by 4–5 times at a consistently low lithium content is experimentally confirmed. Economic comparison is carried out by using the surface areas of the membranes at each treatment stage, the rates of scaling on the membranes, the required consumption of service reagents, and the flow rates of discharged concentrates, which are calculated from experimental data. The developed method results in decreased operational costs of a membrane drinking water treatment setup due to decreases in the membrane replacement costs, in the consumption of reagents preventing the formation of calcium carbonate scales on the membranes, and in the flow rate of concentrate discharges.