Membrane aging and boron standards govern recovery in reverse osmosis of seawater–treated wastewater blends

Blending treated wastewater (TWW) with seawater upstream of reverse osmosis (RO) can simultaneously increase water reuse and reduce feed salinity and energy demand, but its operational value depends on whether water-quality constraints remain satisfied as membranes age and feed conditions vary. This study quantifies that lifetime operating envelope for a co-located desalination–reuse system on the Saudi Arabian coast of the Arabian Gulf. Pretreatment screening of local membrane-bioreactor effluent established a 35% seawater design boundary under the assumed phosphate-control conditions, highlighting the dependence of the blending limit on site-specific pretreatment performance. A sequential AquaGRID simulation campaign supported development of an automatic relevance determination Gaussian process regression (ARD-GPR) surrogate, which reproduced 32 independent simulator cases withheld from fitting with a permeate-boron RMSE of 0.039 mg/L and correct compliance classification in all cases. Constrained optimization was then performed at three membrane ages across 500 annual feedwater-variability scenarios. Energy-optimal recovery remained at the 60% upper bound at beginning and middle of life, but declined to 55.8% at end of life when the stringent 0.5 mg/L boron criterion became active, increasing specific energy consumption from 1.80 to 1.98 kWh/m³ (+9.9%). Annual feasibility at the 35% seawater boundary declined from 96.8% to 80.8% over membrane life. No tested seawater fraction achieved the 95% end-of-life reliability target under the stringent 0.5 mg/L criterion (maximum 81.0%), whereas the 2.4 mg/L GSO/WHO drinking-water criterion was satisfied throughout. Raising feed pH to 9.0 increased modeled end-of-life compliance to 98.8%. The central finding is an aging-induced shift in the active process constraint: recovery optimization governs while boron remains non-binding, whereas boron control becomes decisive as membrane rejection declines under the more stringent criterion.