EXPERIENCE WITH FULL-SCALE ELECTRODIALYSIS FOR NITRATE AND HARDNESS REMOVAL

In regions with intensive agriculture, nitrate concentrations in groun d water frequently exceed the EC-standard for drinking water of 50 mg/ l NO3. As a result, many water works are confronted with the necessity of nitrate removal. In principle, there are biological and chemo-phys ical technologies available. The separation process by means of electr odialysis shows several advantages such as highly selective desalinati on, high water recovery, practically no addition of chemicals and the possibility of stop and go operation (covering of peak demands). An ad ditional desired side effect of this process is a reduction in water h ardness. The principle of electrodialysis involves the removal of ioni c components from aqueous solutions through ion exchange membranes und er the driving force of an electric held. The degree of desalination, and hence nitrate and hardness removal, is adjusted by the voltage app lied to the membrane cells. The quality of the concentrate is controll ed by the amount of dilution water in the concentrate cycle. fn 1990, Austrian Energy started pilot tests for nitrate removal by means of el ectrodialysis at the Kleylehof well in Eastern Austria. These tests we re carried out for two years. The average nitrate concentration in the raw water was 80 to 100 mg/l NO3. The hydraulic capacity of the pilot plant amounted to 1 m(3)/h. The aims of the research program were to asses the efficiency of desalination in view of nitrate removal produc t quality, water recovery with regard to brine quality, the selection of membranes and long-term experience. Due to the specific requirement s, which were best met by the electrodialysis process, Austrian Energy obtained an order for the design and erection of a full-scale plant i n 1996. The plant should be suitable for seasonal operation in order t o cover peak demands in summer. Two possibilities were planned for the disposal of the produced concentrate. The first involved was discharg e into the local sewage treatment plant, the second - reuse for irriga tion Therefore, a highly selective nitrate removal process was request ed. Because of an increase in the raw water nitrate concentration, the plant had to be designed for a maximum of 160 mg/l NO3. The nitrate s electivity was enhanced by the use of monovalent selective anion excha nge membranes. As opposed to the pilot plant, a new electrodialysis-st ack design with a membrane length of 1 m was employed. The ED-unit con sists of three parallel membrane stacks with a hydraulic capacity of 4 8 m(3)/h each. Thus the plant can be operated with three hydraulic sta ges (48, 96 and 144 m(3)/h). The plant is fully automated and the diff erent hydraulic stages are started and stopped by means of remote cont rol. Plant start-up took place in August 1997. The nitrate concentrati on in the raw water at that time was 120 mg/l NO3. The nitrate removal was adjusted to a product concentration of 40 mg/l NO3. At maximum de salination performance, a minimum nitrate level of 20 mg/l could be ac hieved. With a nitrate removal of 66% at a total desalination rate of only 25%, the obtained nitrate selectivity is relatively high. After a pproximately three months of operation and initial investigations with concentrate degradation in the local savage treatment plant, the nitr ate removal plant was shut down and mothballed during the winter for a restart in May 1998.