All the possible mechanisms and factors affecting flux decline, natural organic matter (NOM) rejection, and NOM transport were studied in this paper. Electrostatic exclusion (charge repulsion) was determined to be an influential factor in minimizing flux decline and membrane fouling, and in maximizing NOM rejection; the effective MWCO (membrane molecular weight cutoff) concept was used to support the notion of electrostatic interactions and to describe an apparent MWCO by a charged membrane. This allowed for a negatively-charged tight ultrafiltration (UF) membrane to be comparatively compared to a nanofiltration (NF) membrane in terms of NOM rejection. The f/k ratio, relating relative back-diffusional transport to convective transport, was identified as a major influential factor in terms of flux decline and NOM rejection. The effect of hydrophobicity on flux decline was not significant when the same f/k ratio was used for different NOM-membrane pairs. The convectional and diffusional transportation of NOM was also evaluated for NF and UF membranes using existing thermodynamic models.

Secondary effluent organic matter (EfOM) from a conventional activated sludge process was filtered through constant-pressure dead-end filtration tests with a sequential ultrafiltration (UF, molecular weight cut-off (MWCO) of 10k Dalton) and nanofiltration (NF, MWCO of 200 Dalton) array to investigate its membrane fouling potential. Advanced analytical methods including liquid chromatography with online carbon detection (LC-OCD) and fluorescent excitation-emission matrix (F-EEM) were employed for EfOM characterization. EfOM consisted of humic substances and building blocks, low molecular weight (LMW) neutrals, biopolymers (mainly proteins) and hydrophobic organics according to the sequence of their organic carbon fractions. The UF rejected only biopolymers and the NF rejected most humics and building blocks and a significant part of LMW neutrals. Simultaneous occurrence of cake layer and standard blocking during the filtration process of both UF and NF was identified according to constant-pressure filtration equations, which was possibly caused by the heterogeneous nature of EfOM with a wide MW distribution (several ten to several million Dalton). Thus the corresponding two fouling indices (kc for cake layer and ks for standard blocking) from UF and NF could characterize the fouling potential of macromolecular biopolymers and low to intermediate MW organics (including humics, building blocks, LMW neutrals), respectively. Compared with macromolecular biopolymers, low to intermediate MW organics exhibited a much higher fouling potential due to their lower molecular weight and higher concentration.

The traditional application of size exclusion chromatography (SEC) with single wavelength UV detection (at 254 nm) for molecular weight (size) estimation can be expanded to include multiple detectors that provide insight into the size, structure, and functionality of natural organic matter (NOM). Other detectors include, foremost, on-line dissolved organic carbon (DOC) as well as fluorescence and variable wavelength UV. SEC coupled with this range of detectors elucidates unique signatures for different types and sources of NOM including allochthonous versus autochthonous (i.e., algal organic matter (AOM)) versus wastewater effluent (i.e., effluent organic matter (EfOM)). Also revealed by SEC are preferential NOM removals and/or transformations by various water and wastewater treatment processes as well as identification of problematical NOM components (e.g., membrane foulants).

Constructed wetlands (CWs) and managed aquifer recharge (MAR) represent commonly used natural treatment systems for reclamation and reuse of wastewater. However, each of these technologies have some limitations with respect to removal of different contaminants. Combining these two technologies into a hybrid CW-MAR system will lead to synergy in terms of both water quality and costs. This promising technology will help in the reduction of bacteria and viruses, trace and heavy metals, organic micropollutants, and nutrients. Use of subsurface flow CWs as pre-treatment for MAR has multiple benefits: (i) it creates a barrier for different microbial and chemical pollutants, (ii) it reduces the residence time for water recovery, and (iii) it avoids clogging during MAR as CWs can remove suspended solids and enhance the reclaimed water quality. This paper analyzes the removal of different contaminants by CW and MAR systems based on a literature review. It is expected that a combination of these natural treatment systems (CWs and MAR) could become an attractive, efficient and cost-effective technology for water reclamation and reuse.