Sustainable Water Reclamation from Pretreated Cattle Wastewater Using a Two-Stage Reverse Osmosis System: Experimental Investigation and Response Surface Optimisation

Sustainable wastewater reclamation is important for conserving freshwater resources, improving water-use efficiency, and supporting sustainable agricultural water management. This study experimentally investigated and optimised water recovery from pretreated cattle wastewater using a two-stage reverse osmosis (RO) system, in which the concentrate from the first stage was treated in the second stage to recover additional water. Response surface methodology (RSM) based on a three-factor, three-level Box–Behnken design was used to evaluate the effects of feed pH (7.0–8.0), pressure (100–150 psi), and temperature (25–45 °C) on overall water recovery. The 15 experimental runs achieved water recovery rates of 64.79–74.13%. The quadratic model demonstrated a strong fit to the experimental data (R2 = 99.66%; predicted R2 = 98.20%), although the limited experimental dataset warrants further validation. Numerical optimisation predicted a maximum recovery of 75.0% at pH 7.803, 150 psi, and 45 °C. Validation experiments achieved an average recovery of 74.37%, with an average relative error of −0.84%. Under the optimised conditions, the system produced reclaimed water with electrical conductivity of 58 µS/cm, total dissolved solids of 33 mg/L, chemical oxygen demand of 5 mg/L, and total nitrogen of 0.7 mg/L. By recovering additional water from pretreated cattle wastewater, the two-stage RO configuration offers potential to reduce freshwater demand and improve resource efficiency in agricultural wastewater management.