Document Type : Research Paper
Authors
1
Assist. Prof., Dept. of Water Engineering and Environment, the Caspian Sea Basin Research Center, University of Guilan, Rasht, Iran
2
Chief Executive Officer, Sorena Water Treatment Company, Tehran, Iran
Abstract
Reverse osmosis technology currently accounts for over 70% of global water desalination capacity. However, no well-defined criterion currently exists for determining the number of membrane elements within pressure vessels in RO configuration design. Since this design variable directly affects system performance and associated costs, selecting an optimal PV length remains a challenging task. This study investigated the effect of PV length on the techno-economic performance of brackish water reverse osmosis systems designed for the reuse of pretreated wastewater with a total dissolved solids concentration equivalent to 2000 mg/L NaCl. Two design scenarios, with permeate capacities of 18 and 155 m³/h, were evaluated using configurations containing 4 to 8 membrane elements per PV. Hydraulic and mass-transfer performance was simulated using DuPont WAVE software. The systems were evaluated using seven key indicators: hydraulic behavior, maximum recovery, permeate quality, specific energy consumption, capital expenditure, operating expenditure, and levelized cost of water. Increasing PV length enhanced the maximum achievable system recovery. In both scenarios, salt rejection exceeded 98%, while recovery was the primary factor determining permeate quality. Feed pressure and SEC exhibited nonlinear relationships with recovery and PV length. At high recovery levels, hydraulic balance and permeate backpressure had a greater influence on energy consumption than pressure drop. Accordingly, the 8-element configuration at the small scale and the 6-element configuration at the large scale provided the best balanceamong feed pressure, pressure drop, and energy consumption. From an economic perspective, increasing the PV length increased CAPEX by 7% and 3.5% in the small and large-scale scenarios, respectively. Nevertheless, increasing the PV length and recovery reduced OPEX. The minimum LCOW values were 0.1309 $/m3 for the 8-element configuration at 80% recovery in the small-scale scenario and 0.1100 $/m3 for the 6-element configuration at 75% recovery in the large-scale scenario. These results suggest that in large-scale projects, selecting more balanced PV lengths, such as the 6-element configuration, offers a more favorable techno-economic trade-off compared to longer alternatives, whereas in small-scale systems, the 8-element configuration serves as a justifiable option to achieve the minimum water production cost.
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