Applicability of Sherwood correlations for natural organic matter (NOM) transport in nanofiltration (NF) membranes

Abstract

A Sherwood correlation (mass transfer correlation) pertinent to natural organic matter–nanofiltration (NOM–NF) systems was established through the determination of the mass transfer coefficient (k) based on a combined film/thermodynamic approach, under laminar flow conditions. In this study, the NOM transport characteristics inside a NF membrane were quantitatively demonstrated through the pore Peclet number (Pe) calculated from experimental data. Transport experiments were performed, using a plate-and-frame crossflow test unit, at various feed flow rates, with a thin-channel type test cell that could be adjusted to different channel heights. The transport experimental results, with Suwannee River NOM (SRNOM), show that the transport of the SRNOM through the NF membrane (denoted as ESNA, MWCO of 200–250) was dominated by diffusion, which was further confirmed by the determined pore Pe (Pe < 1.0). The Sherwood correlation established for the SRNOM and NF membrane under ambient conditions (neither pH nor ionic strength adjustment) exhibited the standard form: Sh = 0.853Re^0.550Sc^0.363. The k values calculated by the correlations (k_Sh) were in good agreement with those determined from the experiments (k_exp). However, significant discrepancy between the k_Sh and k_exp was observed with alterations in the feed water chemistry. The k_Sh value increased with either decreasing pH or increasing ionic strength, as the experimentally determined diffusion coefficient of the SRNOM increased under these condition, whereas, the k_exp exhibited opposite trends, due to the decreased water permeation (or suction) rate, increased NOM transmission and decreased electrostatic repulsion. The k values for other source water NOM (but with the same membrane) were also estimated by the Sherwood correlation coupled with diffusion cell tests. The discrepancy between the k_Sh and k_exp values was less than 20%, and the discrepancy decreased with increased feed flow rate.

Introduction

Natural organic matter (NOM) occurs ubiquitously in drinking water supplies, and exerts a problematic disinfectant demand, serving as a precursor to the formation of disinfection by-products (DBPs), which are related to the taste and odor, and enhancing bacterial regrowth and biofilm formation in the distribution system. These adverse aspects of NOM in the treatment of drinking water, along with the limited supply of fresh water in many regions of the world, have driven utilities to consider advanced treatment alternatives. Of particular interest are the uses of nanofiltration (NF) and tight-ultrafiltration (tight-UF) as treatment alternatives for the removal of NOM. NF membranes are usually made of polymeric (mostly polyamide) films, with molecular weight cutoffs (MWCO) between 200 and 1000 [1]. They have a looser structure than reverse osmosis (RO) membranes, and are usually negatively charged [2]. This combination of NF membrane properties has prompted their use in the removal of NOM to minimize their adverse aspects in the treatment of drinking water.

Recent research has shown that NOM can be effectively removed by NF membranes through a combination of size exclusion and electrostatic repulsion [3], [4], and the flux decline of NF membranes, due to NOM fouling, can be reduced by controlling various physical and chemical parameters, such as pH, ionic strength, calcium concentration and hydrodynamic conditions (permeation rate, crossflow velocity and channel configurations) [3], [4], [5], [6], [7]. Successful application of NF technology, however, requires a deeper understanding of NOM transport characteristics in NF membranes. The NOM transport characteristics in the concentration polarization (CP) layer, and through the semi-porous (macro-void) structure, of the membrane are influential in the process performance, with respect to both the quality (permeate NOM concentration) and quantity (permeate flux) of the product water.

A quantitative understanding of solute transport characteristics in the CP layer and through the amorphous structure of NF membranes can be achieved by determining the mass transfer coefficient and membrane transport parameters, such as the reflection coefficient and solute diffusive permeability coefficient. The film model [8], [9], thermodynamic approach [10], [11] and Sherwood correlations [12], [13] have been the most widely used models (separately or combined with each other) for determining these parameters as they do not require information on the amorphous structure of the RO and NF membranes, which are hardly described by one simple quantitative parameter [14]. There have been studies that have determined these parameters for various solute-membrane systems, although most have focused on the elucidation of the transport characteristics of ionic species in RO membranes [15], [16], [17], [18], [19] and nonionic macromolecules in UF membranes [20], [21]. However, there have been a few studies investigating the NOM transport characteristics, in the CP layer and through NF membrane pores, and investigating the influence of the water chemistry and hydrodynamic conditions. Moreover, most existing Sherwood correlations may not be relevant for the determination of the mass transfer coefficient of a NOM–NF system as the system is non-ideal compared to the salt–RO systems (i.e., heterogeneous and charged macromolecular characteristics of NOM, NOM transmission through the NF membrane and interaction between NOM and membrane (hydrophobic and electrostatic)).

In this study, the NOM transport characteristics in the CP layer and through the semi-porous structure of an NF membrane were quantitatively investigated by analyzing the experimental data obtained with the combined film/thermodynamic model. Diffusive and convective transmissions of NOM through the membrane pores were quantitatively demonstrated through the pore Peclet number (Pe), which has been defined in this study. Emphasis has been placed on establishing a Sherwood correlation, pertinent to a NOM–NF system, through actual crossflow filtration experiments involving natural water NOM and a NF membrane. The influence of the water chemistry, such as pH and ionic strength, on the NOM transport characteristics in a NF membrane was also investigated by comparing the mass transfer coefficients calculated by the Sherwood correlation (k_Sh) with those determined experimentally (k_exp). To establish the Sherwood correlation, the diffusion coefficient of the NOM was experimentally determined, through diffusion cell tests conducted at various pHs and ionic strengths. Finally, the applicability of the Sherwood correlation was observed with different source water NOM.