Directly Coupled Forward–Reverse Osmosis with Ammonium Bicarbonate for Digestate Treatment and Carbon Capture Integration

Abstract This study investigates a directly coupled forward–reverse osmosis (FO–RO) system using ammonium bicarbonate (ABC) as a biocompatible draw solute, chosen for its potential regeneration from carbon dioxide and ammonia, for treating high-fouling liquid digestate from biogas production. Standalone FO and RO tests with ABC demonstrated predictable water and solute transport behavior, enabling the coupled system’s performance to be anticipated from the individual process equilibria. Subsequent hybrid FO–RO experiments examined the influence of feed cross-flow velocity, draw solution molarity and pH, and RO pressure on overall recovery and solute flux. A 6 h continuous run was performed to evaluate fouling behavior and operational robustness. Standalone FO and RO performance followed expected trends with increasing ABC concentration, while direct hydraulic coupling imposed steady-state constraints that limited driving forces and reduced system efficiency. The integrated process remained stable without observable fouling or scaling. This work presents the first demonstration of a directly coupled FO–RO configuration using ABC for real digestate treatment and highlights its potential for scalable circular recovery of water, nutrients, and carbon dioxide.