Phytoplankton (Red Tide) Fouling of Pretreatment and Reverse Osmosis Membranes in Seawater Desalination
Microfiltration and ultrafiltration membranes are valuable pretreatment tools for seawater reverse osmosis desalination. When phytoplankton (algal) blooms occur, however, these pretreatment membranes can be severely fouled and require intense cleaning. Further, organic matter permeating the pretreatment membranes can foul the reverse osmosis elements either through direct adsorption or by providing substrate for bacterial biofilms. The aim of this project is to study the fouling effects of marine algae and algogenic organic matter on integrated membrane desalination systems.
Preliminary bench-scale membrane fouling experiments have been performed with a red tide field sample and with laboratory-cultured phytoplankton. Based on these results, a series of experiments is underway to investigate five basic questions. 1) What are the effects of shear forces in releasing organic foulant material from phytoplankton? 2) How do materials, pore sizes, and configurations affect fouling? 3) How does algogenic organic matter change over the life cycle of an algal bloom, especially as bacterial populations fluctuate? How does this variability affect membrane fouling behavior? 4) How do alternatives like coagulation/flocculation compare with membrane pretreatment in phytoplankton removal? 5) What is the quantity and nature of organic matter that permeates the prefiltration membranes and leads to fouling in the reverse osmosis elements?
The main body of experiments is performed with cultured phytoplankton. This allows well-controlled tests that avoid the complications of seasonal variations associated with using natural seawater samples. We will compare the results with field samples of natural algal blooms. Two participating utilities (Long Beach Water District and Marin Municipal Water District) have been identified. The consulting firm MWH will also help locate other utilities where phytoplankton bloom events may occur. The information obtained in these laboratory studies will aid utilities and membrane manufacturers in understanding the fundamental principles involved in phytoplankton fouling so that appropriate technologies can be piloted and implemented.
This project is managed by the American Water Works Association Research Foundation (AWWARF) with funds provided by the Department of Energy (DOE). The program is entitled "Advancing Desalination to Produce Drinking Water." Graduate student David Ladner is the Co-Principal Investigator with Mark Clark. Undergraduate student Derek Vardon is working side-by-side with David on the experiments.
Novel Desalination Membranes using Aquaporin Channels
Our research goal is to develop materials inspired by biological systems for environmental engineering applications. The specific project we are working on at present aims to develop a sustainable low energy membrane technology for treatment of impaired and contaminated water sources. Our hypothesis is that such a technology can be developed using practical membrane processes inspired by biological membranes. Biological systems conduct extremely efficient water filtration. The human kidney for example filters an astounding 180 L of water per day without the need for large positive pressures. Nature overcomes the large diffusion limitation present in most solute rejecting membranes by the use of membrane channel proteins called aquaporins (AQPs2). AQPs are water-conducting channels found in biological membranes and have a unique hourglass architecture with a "pore opening" of 2.8 angstroms; the narrow pore prevents the passage of large molecules. The openings are lined with hydrophobic amino acids. This results in single-file movement of water molecules. Synthetic nanotubes with hydrophobic walls use a similar transport mechanism. By contrast, transport in RO membranes is much slower because small networks of water molecules must diffuse through a high-density polymer matrix.
We propose to synthesize membranes that incorporate AQPs into synthetic block copolymers. These polymers have a sandwich arrangement of hydrophilic (A) and hydrophobic (B) groups similar to biological membranes. Hydrophilic groups lie on the outside and hydrophobic groups are on the inside (ABA arrangement). In a recently published paper we have shown that orders of magnitude improvements in productivity over existing RO membranes can be obtained by using the proposed approach.
This project was envisioned and initiated by graduate student Manish Kumar. He has received several awards to provide intial funding for the work. We have recently received funding from the National Science Foundation - Chemical and Biological Separations Division (Engineering Directorate) to support this project. Dr. Mark M. Clark and Dr. Julie Zilles are Principal Investigators for this project.
Past Projects
Characterization of US Seawaters and Development of Standardized Protocols for Evaluation of Foulants in Seawater Reverse Osmosis Desalination
This project addressed the emerging issue of seawater desalination for water-scarce coastal regions by providing a novel approach to understanding flux decline during seawater reverse osmosis (SWRO). While seawater desalination using RO is not a new concept, the understanding of membrane flux decline in SWRO treatment is limited. This follows from the fact that seawater has not been well characterized in terms of membrane applications. Seawater chemistry is fundamentally different from surface and groundwater. The extrapolation of fresh-water fouling results to SWRO may not be appropriate. An original methodology for evaluation of fouling potential of seawaters needed to be developed. This is particularly true as the ionic strength and the nature and origin of organic material in seawater is different. SWRO flux decline increases power consumption during RO operation which is one of the largest operational cost items for desalination plants (as much as a third). Therefore, understanding flux decline and implementing strategies to combat this issue will have a large beneficial impact on the economics of the SWRO process.
The overall approach of this project was to analyze seawater from different locations within the United States with special emphasis on potential membrane foulants and then perform bench-scale SWRO fouling experiments on these waters. These tests were performed in two different laboratories with the goal of standardizing the RO testing protocol that could be utilized by other workers.
Source waters and fouled membranes were analyzed to characterize the colloidal and dissolved material responsible for fouling. A fundamental understanding of fouling was sought by coupling the experimental fouling data with the seawater characterization.
This project was a collaboration with MWH consulting engineers in Pasadena, California. Samer Adham was the principal investigator, with Mark Clark as the Co-PI. Arun Subramani at MWH, Manish Kumar, and David Ladner performed the experiments. A paper detailing some of the results has been submitted for publication. The final project report is under review at the funding agency, the WateReuse Foundation.
Characterization of Membrane Foulants in Seawater Reverse Osmosis Desalination
This project was the prelude to the Characterization of Seawaters project mentioned just above. It was performed in three phases. First, the bench-scale SWRO unit was constructed and optimized. Several different testing strategies were evaluated to determine the advantages and disadvantages of the various configurations. These tests were performed with sodium chloride solutions, artificial seawater (containing a natural spectrum of salts) and a seawater source obtained from the northern coast of San Diego Bay, California. Experiments performed in Phase One considered such questions as:
The second phase of the project involved testing the SWRO system with different source waters and foulant surrogates to determine if fouling could be effectively evaluated in more extreme or varied circumstances. Foulant surrogates-high concentrations of pure proteins and polysaccharides-were tested. Protein and polysaccharide samples resulted in drastic flux decline, as expected for such extreme conditions (265 mg/l concentration). New source waters were also run, including a sample from Long Beach, California, and a phytoplankton-laden seawater from Sarasota, Florida. The Long Beach sample did not appear to foul the RO membrane. The phytoplankton in the Sarasota seawater did make a significant difference, decreasing flux by a measurable level, but the loss of productivity was less than 20%. Further, longer-term tests spanning four days were completed. When San Diego water was run at high flux for four days, a general flux decline trend began to be noted. By the end of Phase Two, it became clear that roughly 20% flux decline was the most that could be expected in these bench-scale experiments, even for waters laden with high organic-matter concentrations. Since most natural waters have much lower concentrations, bench-scale tests must be carried out in a controlled manner to obtain high resolution and discern small values of flux decline.
After finding that phytoplankton caused significant flux decline in the Sarasota experiment, it became interesting to investigate further the effects of phytoplankton. During Phase Three, phytoplankton were cultured in the laboratory and a series of experiments was run to evaluate the RO performance. Cultures were spiked directly into seawater and RO performance was evaluated, resulting in significant flux decline (~20%) in 24 hours. A similar run with microfiltered phytoplankton also resulted in a fairly significant flux decline (~12%). With ultrafiltered phytoplankton, flux decline was much lower (~3%). These results show that algogenic organic matter (AOM) can reduce flux significantly in RO. Also, there was a dramatic difference between ultrafiltration and microfiltration for pretreatment of phytoplankton-laden water sources.
With Phase-Three experiments resulting in measurable and distinct fouling patterns, the groundwork was laid for an analysis of the fouled membranes in an attempt to correlate flux decline with other parameters. Images of the fouled membranes were evaluated with image processing techniques to measure the relative light absorbance of different fouling fractions. Foulant material was desorbed from the membranes and polysaccharides were measured. These results were compared with polysaccharide and total organic carbon (TOC) measurements for the tested waters. In a late development, it was discovered that the organic material in the foulant layer had fluorescent properties similar to the parent phytoplankton. A laser-scanning fluorometry (LSF) technique was utilized to make measurements of the fluorescence. To the research team's knowledge, this is the first time that such a technique has been utilized for evaluation of membrane fouling.
The major contribution of this research project to the field of SWRO desalination is the optimization of bench-scale testing protocols for use with natural seawater sources. Bench-scale testing proves to be difficult because most natural waters do not exhibit dramatic flux decline trends during short-term tests. However, even when little flux decline is apparent, the amount of material deposited on the surface can be significant. Thus, flux decline alone is not a sufficient indicator of fouling. The membranes should be evaluated after the flux experiment to determine how much material has been deposited. Optical techniques like image analysis and LSF can be used, as well as wet-chemical techniques like polysaccharide measurement. In this way, several different parameters can be compared for each source water to get a broad understanding of the fouling potential.
This project was funded by the U.S. Bureau of Reclamation under the Desalination Research and Development Program. David Ladner was the primary author of the proposal and did the bulk of the experimental work. Former students Nathan Lester and Paul St. Aubyn assisted.
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