Optimization of a two-stage reverse osmosis pilot system for Caspian seawater desalination using response surface methodology: SEC reduction through recovery increase, Fe fouling, and scaling control

The scarcity of freshwater has become one of the most critical global problems, and seawater desalination is considered an effective approach to tackling this problem. Caspian seawater with low total dissolved solids (TDS) (< 16,000 mg/L) is a potential source for desalination, but it contains high levels of calcium, magnesium, and bicarbonate alkalinity, also more than the permissible amount of iron at the connection points to the rivers. In this study, a two-stage reverse osmosis (RO) pilot was designed for Caspian seawater desalination with the aim of enhancing the recovery rate and reducing specific energy consumption (SEC) as much as controlling the carbonate scaling and Fe fouling. The effects of main input parameters, including operational pressure, initial TDS, and pH, besides their interaction with each other on output variables, namely system desalination removal (SDR), permeate flux, recovery rate, Fe rejection, and Fe fouling probability, were assessed and optimized using response surface methodology (RSM). An adequate correlation between the predicted and experimental data was gained through the analysis of variance (ANOVA). In the first stage, the scaling controlling was performed by pH adjusting in acidic ranges. The maximum SDR, permeate flux, and recovery rate (96.86%, 41.47 Lmh, and 48.17%, respectively), and the minimum SEC and Fe fouling probability (1.809 kWh/m³ and 0.25, respectively) were obtained at average initial TDS of 12,000 mg/L, operational pressure of 21.9 bar, and pH of 5.235; furthermore, the Fe concentration was completely rejected in all tests. The concentrated water was then introduced to the second stage after removing 91% of the inorganic carbon which was done by acidification. Adding the second stage led to increases in overall recovery and permeate flux of 18.7% and 17.56 Lmh, respectively, and also decreased total SEC by 0.56 kWh/m³, which confirmed the efficiency of the optimized two-stage system.