Advancing freshwater conservation: techno-economic study of conventional and closed-circuit reverse osmosis systems in full-scale units with challenging feed water conditions

Abstract

This techno-economic study evaluated two approaches for conserving freshwater in full-scale brackish RO units, i.e., conventional secondary RO and closed-circuit RO (CCRO) systems. Three industrial cases with silica-rich, sulfate-rich, and high-salinity waters were analyzed through numerical modeling. Results demonstrated that both secondary RO and CCRO effectively conserved freshwater, with CCRO achieving 5–8% higher recovery rates and being less prone to scaling. However, CCRO recovery was limited by silica and sulfate salts supersaturation and constrained by the maximum operating pressure (41.3 bar) for high-salinity water. CCRO systems required fewer membrane elements, eliminated acid use, and consumed less energy but demanded more pressure vessels, higher antiscalant use, and produced slightly lower permeate quality. Statistical analysis revealed critical operational thresholds for both systems: 83% recovery for multi-stage RO and 89% for CCRO, beyond which performance declines significantly, especially at higher feed salinities. Economically, multi-stage RO consistently exhibited lower capital and total production costs across all case studies, while CCRO’s advantages in recovery and energy efficiency were offset by higher capital and chemical expenses. Nevertheless, CCRO remains a potentially competitive option in moderate-salinity conditions if antiscalant consumption is optimized.