Fluoride removal with extra low energy reverse osmosis membranes: three years of large scale field experience in Finland

Infiltration of surface water into the ground and a retention time of 50 days is a natural method to remove color, iron and manganese — which are typical for Nordic lake waters — and to produce a good quality drinking water. However, due to the geological situation in the Southern parts of Finland, the fluoride concentration increases after the soil passage from a typical value of 0.7 mg/L to values up to 1.8 mg/L. According to the drinking water directives of the European Union, the fluoride concentration shall be limited to 1.5 mg/L. In order to meet this standard, KLV, a public company which provides water to a number of municipalities, is operating a 6000 m3/d reverse osmosis plant since September 2003. The RO permeate water with a fluoride concentration of <0.03 mg/L is blended with the artificial ground water to ensure a fluoride level below 1.3 mg/L in the drinking water. By treating only 30% of the water, the treatment goal could be achieved with the lowest capital costs. FILMTEC™ XLE reverse osmosis membranes are used to minimize the energy consumption and thus the operating costs. The system operated at a pressure of 6–11 bar at a temperature of 5–11°C, which was until recently possible with nanofiltration membranes only. The RO system operated at 80% recovery without addition of any scaling inhibitor. The concentrate was four times oversaturated with calcium fluoride. Regular flushing was employed to prevent the deposition of precipitates. Sacrificial elements were operated on the concentrate to detect the onset of scaling early enough to initiate cleaning. A first preventive cleaning was carried out after half a year. Substantial fouling with natural organic matter occurred after one and a half year. This could be controlled with harsh alkaline cleanings at pH > 12 at a temperature of >35°C. After more than three years of operation, the salt and fluoride rejection of the membrane was as high as in the beginning.

The RO plant has supplied a high water quality with less than 0.03 mg/L fluoride constantly since it started in October 2003. The salt rejection did not show any signs of deterioration, although there was a period of frequent cleanings with high pH at high temperature. The low operating pressure of average 8 bars at a temperature of average 8°C resulted in a low energy consumption. The operation cost of the produced permeate was calculated 0.09 h/m3. This includes electricity, cleaning chemicals, labor and membrane replacement after five years. The plant could be operated at 80% recovery without a scaling inhibitor, although the solubility limit of calcium fluoride was exceeded. Sacrificial control elements have been operated with concentrate from the plant as an early indicator of scaling. The natural organic matter in the feed water might have some scaling inhibiting effects, and the regular flushing of the membranes have certainly helped to prevent the buildup of a scaling layer. The fluoride level of the artificial groundwater has shown an increasing trend over the years. It is expected that the percentage of RO permeate water will have to be increased in future. Therefore three additional lines are currently being built by HyXo Oy to expand the capacity to 9600 m3/day.