Structure of Combined Colloidal/Organic Fouling Layer on a Nanofiltration Membrane
EMSL Project ID
7998
Abstract
Fouling is the biggest obstacle for efficient application of membrane technology in water and wastewater treatment. Membrane fouling caused by colloidal material and/or dissolved organic matter in natural and waste water results in deterioration of membrane performance (i.e., permeate water flux and quality) and ultimately shortens membrane life. Due to the complex nature of natural water and wastewater, the mechanisms of membrane fouling are not well understood yet, especially when both colloidal materials and macromolecular organic matter are present. The objective of this research is to elucidate the mechanisms of combined membrane fouling by colloidal materials and dissolved organic matter. It has been found that the extent and rate of membrane fouling is controlled by the structure of the fouling layer, a layer of fouling materials accumulated on the membrane surface during filtration. The structure of the fouling layer is mainly affected by the solution chemistry and the operating condition of the membrane system. Therefore, visualization of the surface as well as the inner structure of fouling layers formed under various solution chemistries and operating conditions is imperative to understanding the interactions between foulants and the membrane surface as well as those among fouling materials, which are key to elucidate membrane fouling mechanisms.
In this study, membrane fouling experiments using feed waters with different colloid/natural organic matter (NOM) compositions are conducted under systematically varied solution chemistry to determine the roles of colloids and NOM in combined fouling of nanofiltration membranes. Compared to the case when only one type of foulant is present, the results demonstrate significant synergetic effects when the feed water contains both foulants. The fouling rate increases drastically during combined colloidal/organic fouling. The synergetic effect and the overall fouling rate are found to strongly depend on the colloidal particle size. Moreover, divalent cations, such as calcium and magnesium ions, which were found to play an important role in organic fouling, do not show any difference from monovalent cations. It is hypothesized that the enhanced fouling is attributed to hindered back diffusion of colloids caused by NOM. It is also hypothesized that the inter-molecular bridging among NOM molecules formed by calcium ions is disrupted by the colloids, resulting in a less stable fouling layer. These hypotheses can be tested by imaging the surface and cross-section of the fouling layers formed in the membrane fouling experiments using microscopic technologies.
The fouling layer formed during filtration is a mixture of inorganic colloids and macromolecular organic matter. It is a very thin, loose and hydrated layer. The sample preparation for a normal field emission SEM would definitely destroy the structure of the fouling layer. Therefore, it is necessary to use an environmental SEM so that the damaging sample preparation procedure can be avoided.
Project Details
Project type
Exploratory Research
Start Date
2004-04-05
End Date
2005-04-12
Status
Closed
Released Data Link
Team
Principal Investigator
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