Experimental determination of nanofiltration models: application to nitrate removal

Abstract

Nowadays, nitrate contamination of groundwater is a growing problem both in developed and developing countries. Such contamination comes mainly from the intensive use of nitrogen fertilizers in agriculture. Other possible sources of water contamination by nitrates are improper discharges of industrial effluents, the inadequate management of leachate from landfill of municipal solid waste and effluents from intensive farming. The maximum permissible concentration of nitrates in the water for public consumption in the EU countries is 50 mg/L, although the World Health Organization established a value of 25 mg/L to be considered a quality water. This problem of nitrate contamination, widespread in countries of the European Union, is very important in Spain, where there are some regions with a high presence of aquifers with nitrate concentrations significantly higher than the established limits. Among the possible treatments for nitrate removal from contaminated waters, nanofiltration (NF) is a technically proven process that eliminates nitrates without causing a significant imbalance in the rest of dissolved salts. In this work, the removal of nitrates from water using a commercial NF membrane is studied. Synthetic feeds with levels of nitrate concentration higher than the ones usually found in contaminated aquifers have been considered. On the other hand, the experimental results have been fitted to the solution-diffusion model, having found mathematical expressions that can be used to predict membrane performance for feed concentrations higher than 100 mg/L. The model considers the concentration on the surface of the membrane, estimated by the film theory model. It has been proved that concentration polarization cannot be neglected for this application. The tested membrane allows nitrate decontamination of water (concentration below the legal limit) in one pass for values of feed concentration up to 100 mg/L, with a transmembrane pressure of 5 bar.