Application of membrane techniques in a water softening process

The paper presents the results of tests carried out on selected membranes, SEPA (Osmonics Inc., USA). The efficiency of membrane water softening was evaluated on the basis of permeate flux and the physicochemical analysis of raw water and permeate. The transport-separation properties of the membranes were determined using (a) the model solutions of NaCl and MgSO4 and (b) the natural waters of high degree of hardness. The performed tests indicate that the DS-5-DK and DS-3-SE membranes may be applied to the softening of water for drinking and household purposes. The model based on the extended Nernst–Planck and the Donnan equations has been quantitatively fitted to the experimental data. The obtained values of model parameters indicate that in the case of RO membranes a mechanism other than the Donnan exclusion is responsible for the high retention of ions. The membrane filtration modelling will be performed in the next stage of project.

1. Using DS-3-SE and DS-5-DK membranes, a considerable reduction of the hardness of extremely hard well waters has been achieved. The total hardness of permeate did not exceed the permissible value for drinking water. At the same time, the substances that make up the taste and nutritional quality of water have been removed to only a small extent. 2. It has been observed that DS-5-DK is a typical NF membrane. It sufficiently reduces the hardness of the tested waters and shows relatively higher productivity than the DS-3-SE membrane. Thus, the DS-5-DK membrane can be recommended for the production of potable and household water. 3. It was proven that the RO process is not favourable for the production of drinking water because of too high retention of micro and macro elements. These elements are responsible for the water health virtues. The water softened on RO membranes can be mixed with water softened by other methods. 4. The model based on the extended Nernst– Planck and Donnan equations can quantitatively be fitted to the experimental data; however, the obtained values of model parameters indicate that in the case of RO membranes, a mechanism other than the Donnan exclusion is responsible for the high retention of ions.