Experience with full-scale electrodialysis for nitrate and hardness removal

Abstract

In regions with intensive agriculture, nitrate concentrations in ground water frequently exceed the EC-standard for drinking water of 50 mg/l NO 3. As a result, many water works are confronted with the necessity of nitrate removal. In principle, there are biological and chemo-physical technologies available. The separation process by means of electrodialysis shows several advantages such as highly selective desalination, high water recovery, practically no addition of chemicals and the possibility of stop and go operation (covering of peak demands). An additional desired side effect of this process is a reduction in water hardness. The principle of electrodialysis involves the removal of ionic components from aqueous solutions through ion exchange membranes under the driving force of an electric field. The degree of desalination, and hence nitrate and hardness removal, is adjusted by the voltage applied to the membrane cells. The quality of the concentrate is controlled by the amount of dilution water in the concentrate cycle. In 1990, Austrian Energy started pilot tests for nitrate removal by means of electrodialysis at the Kleylehof well in Eastern Austria. These tests were carried out for two years. The average nitrate concentration in the raw water was 80 to 100 mg/l NO 3. 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, product quality, water recovery with regard to brine quality, the selection of membranes and long-term experience. Due to the specific requirements, which were best met by the electrodialysis process, Austrian Energy obtained an order for the design and erection of a full-scale plant in 1996. The plant should be suitable for seasonal operation in order to cover peak demands in summer. Two possibilities were planned for the disposal of the produced concentrate. The first involved was discharge into the cocal sewage treatment plant, the second—reuse for irrigation. Therefore, a highly selective nitrate removal process was requested. Because of an increase in the raw water nitrate concentration, the plant had to be designed for a maximum of 160 mg/l NO 3. The nitrate selectivity was enhanced by the use of monovalent selective anion exchange membranes. As opposed to the pilot plant, a new electrodialysis-stack design with a membrane length of 1 m was employed. The ED-unit consists of three parallel membrane stacks with a hydraulic capacity of 48 m 3/h each. Thus the plant can be operated with three hydraulic stages (48, 96 and 144 m 3/h). The plant is fully automated and the different hydraulic stages are started and stopped by means of remote control. Plant start-up took place in August 1997. The nitrate concentration in the raw water at that time was 120 mg/l NO 3. The nitrate removal was adjusted to a product concentration of 40 mg/l NO 3. At maximum desalination performance, a minimum nitrate level of 20 mg/l could be achieved. With a nitrate removal of 66% at a total desalination rate of only 25%, the obtained nitrate selectivity is relatively high. After approximately three months of operation and initial investigations with concentrate degradation in the local sewage treatment plant, the nitrate removal plant was shut down and mothballed during the winter for a restart in May 1998.