Parameters Influencing Semi-Batch Reverse Osmosis - Experimental Analysis

Closed loop reverse osmosis (CLRO) is a promising approach for improving recovery and reducing energy consumption compared with conventional reverse osmosis (RO). However, systematic experimental investigations combining projection-based analysis with validation of parameter interactions remain limited. This study evaluates the influence of key operating parameters, including feed ratio (FR), plug flow (PF) membrane recovery (RrPF), closed loop (CL) membrane recovery (RrCL), and concentrate recirculation flow rate (Q̇C_RCL), on system performance using a pilot-scale CLRO system with a feed concentration of 25 g/L NaCl. A CLRO process analysis tool is used to compare projected and measured performance. Four evaluation cases distinguish between idealised tool predictions, idealised first-pass experimental behaviour, PF-CL transition behaviour, and extended multi-pass operation. Parameters associated with CL operation (RrCL and Q̇C_RCL) dominate system performance, significantly affecting permeate flux, pass duration, specific energy consumption, and permeate quality, whereas PF parameters (FR, RrPF) exhibit limited influence. This is attributed to the dominant contribution of CL phase, where higher permeate flux and longer duration govern overall performance. Trends predicted by the projection tool are confirmed experimentally, with deviations within ±5 % for ideal first-pass conditions. When non-ideal effects are considered, performance deviates due to transition durations, incomplete displacement in PF mode, mixing, and salt carry-over, while the same parameter trends are preserved. Moderate CL recovery and recirculation favour high pass recovery with low energy consumption, whereas improved permeate quality requires higher recirculation and CL recovery. The results provide a basis for interpreting and designing CLRO systems under practical conditions.