Rheological and permeability characteristics of alginate fouling layers developing on reverse osmosis membranes during desalination

Rheological property data (practically and theoretically significant) of typical desalination-membrane fouling layers are reported for the first time in this study, involving two stages. First, desalination of brackish water (with 30 mg/L sodium alginate) was carried out under constant flux; Ca2+-ion concentration 1 mM–8 mM was used. Linear temporal trans-membrane pressure increase, associated with constant specific fouling resistance α, indicated uniformity of growing alginate layers. Fouling resistance α increased with calcium concentration. Subsequently, fouled membrane segments were tested in a parallel-plate rheometer, operating in oscillatory and steady shearing modes. Key rheological parameters of fouling layers, including storage (G′) and loss (G″) moduli, complex and shear viscosities, were assessed in relation to their permeability and other characteristics. Alginate fouling layers possess viscoelastic properties characteristic of rather stiff gels; moreover, they belong in the class of thixotropic materials with yield stress. Layers from feed-water of small Ca+2-ion concentration exhibit strong elastic character, high viscosities and yield stresses; the elastic character and rheological parameter values are reduced with increasing Ca2+-ion concentration. The high Na-ion concentration in the fouling layers appears to significantly influence their structure; however, considering the complicated dynamics of developing layers, the relative significance of the abundant Na-ions compared to Ca2+-ions is unclear.

► Rheological properties of membrane fouling layers reported for the first time. ► Alginate fouling layers possess viscoelastic properties typical of stiff gels. ► They exhibit characteristics of thixotropic materials with yield stress. ► Fouling layer permeability is reduced with increasing Ca+2-ion concentration. ► Elastic character and yield stress are also reduced with increasing Ca-content.