Natural organic matter (NOM), a ubiquitous component of natural waters, is of concern because it poses water treatment challenges and possible health risks. Health risks associated with NOM include formation of halogenated organic disinfection byproducts; promotion of biological growth in distribution systems; and facilitated transport of heavy metals and hydrophobic organic chemicals through the treatment regimen. For many water utilities, removal of NOM is an integral component of the production of water that is chemically and biologically safe. The most common treatment approach to removing NOM is the conventional water treatment process sequence of coagulation, flocculation, clarification, and filtration. Anion exchange has the potential to be an effective treatment process for NOM removal, but an improved understanding of the interactions among NOM, anion exchange resin, and reactor engineering is required.

Accordingly, the objective of this research is to formulate a mathematical model to describe NOM removal by anion exchange in a completely mixed flow reactor (CMFR). The overall model consists of a macrotransport model coupled with a microtransport model. The macroscale model describes the transport of both NOM and anion exchange resin in a CMFR with resin recycle. The microscale model describes NOM uptake via intraparticle pore diffusion in the anion exchange resin. The transport equations were solved numerically using a finite element scheme. Equilibrium and kinetic parameters were obtained from laboratory experiments using model waters formulated from NOM extracts. The dual macrotransport-microtransport model was validated by comparing predictions from the model to results obtained from a continuous-flow pilot plant study conducted by the authors. The validated model is used to simulate anion exchange treatment in a CMFR with resin recycle for different raw water characteristics, NOM properties, resin properties, and operating conditions.