Effect of solution chemistry on assimilable organic carbon removal by nanofiltration: full and bench scale evaluation

The effectiveness of nanofiltration (NF) to control assimilable organic carbon (AOC) and biodegradable dissolved organic carbon (BDOC), the main indicators of biological stability of finished potable water, was systematically investigated at the 30,000 m³/day NF membrane plant located in Southern Florida. One year of full-scale operation showed that nanofiltration effectively reduced BDOC, but was not able to reject AOC. The insignificant AOC rejection observed was probably due to the low pH, high hardness, and high ionic strength (TDS) of the processing water. In order to verify this hypothesis, a series of well-controlled bench-scale experiments were conducted at simulated solution chemistries. The bench-scale study clearly demonstrated that AOC removal by NF membranes decreased markedly with decreasing pH (a 25% decrease in pH led to a 20% decrease in AOC removal), and increasing hardness (10-fold increase led to a 90% decrease in AOC removal) and ionic strength (a 25-fold increase led to a 50% decrease in AOC removal). These solution environments repress the electrostatic repulsion between charged AOC compounds and membranes, resulting in low AOC rejection. Lastly, an empirical model was statistically developed based on bench-scale data and utilized to estimate full-scale performance. AOC removal predicted by the model showed good agreement with values observed in full-scale operation (R²=0.98).