Bacterial Growth Potential of Antiscalants used in Reverse Osmosis Systems using Seawater Autochthonous Microbial Communities

Abstract

Antiscalants are chemicals used in membrane-based water desalination processes to prevent the scaling of salts on the membrane. Previous studies suggested that antiscalants could lead or contribute biological fouling by providing growth-promoting factors such as source of biodegradable carbon or phosphorus. However, the test in previous studies were conducted using drinking water and pure cultures of bacteria isolated from freshwater. These conditions do not reflect those of desalination systems.

In this study, we determined the microbial growth potential of eight antiscalants under conditions relevant to desalination plants. To this end, autochthonous microbial communities from the Red Sea were used, and we exposed them to chemically diverse antiscalants.

The chemical characterization of the antiscalants showed that their carbon content ranged from 0.02 to 0.15 mg C/ mg antiscalant. Fourier transform infrared and nuclear magnetic resonance spectroscopy allowed us to classify the eight tested antiscalants into three types based on their chemical structure. These types were phosphonate-based, natural inulin-based, and polyacrylate-based antiscalants.

The growth potential of the antiscalants was determined by incubating seawater with antiscalants (50 mg/L) and an initial bacterial cell content of 20,000 cells/ml at 30$^o$C. A reference without antiscalant addition was used for comparisons. The microbial growth was followed by measuring the cell concentrations over time by flow cytometry. Four phosphonate-based antiscalants promoted microbial growth to different degrees, while distinctively one phosphonate did not increased growth significantly under all tested conditions. At the same time, inulin and polyacrylate-based antiscalants did not increase bacterial growth compared to the reference. The growth potential of 8 antiscalants used presently in practice desalination plants ranges between 1,479,793 and 2,074,106 cells/ml based on the results of the growth potential test, strongly suggesting that antiscalant selection and optimization contributes to biofouling prevention and control.