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The Water Desalination Research & Development Program Newsletter - No. 13 - Spring 1999

PRIMARY GOAL

The primary goal of the Water Desalination Research & Development (DesalR&D) Program is to develop more cost-effective, technologically efficient, and implementable means to desalinate water. This goal is accomplished by a combination of partnerships with the private sector and academia for basic and applied research projects; piloting research projects with the private sector, academia, municipalities, and communities; and in-house applied research and demonstration projects. For more information about the DesalR&D program, contact the program manager, Kevin Price, at 303-445-2260, or visit the DesalR&D web site at: http://www.usbr.gov/pmts/water/research/DWPR/.

Some 20 cost-shared financial assistance agreements with the private sector and academia were awarded this fiscal year (see Newsletter No. 12). In addition, smaller in-house research projects are being funded. Following are reviews of two cost-shared projects and two in-house projects, all of which are resulting in significant new processes, procedures, or methodologies for the water treatment, membrane, or desalting industries.

MEMBRANE BIOREACTORS

The City of San Diego, CA is conducting a major pilot study, cost-shared with the DesalR&D program, to demonstrate the feasibility and reliability of using membrane bioreactors (MBRs) for the pretreatment of domestic wastewater prior to reverse osmosis (RO) desalting for water purification. MBRs use bioreactors and membrane filters (such as microfiltration [MF] or ultrafiltration [UF] membranes) in lieu of sedimentation and filtration in conventional suspended-growth biological treatment. The investigation, just underway at the City's Aqua 2000 Research Center in Escondido, will compare two different membrane configurations: "in-series" in which low-pressure membrane modules replace a conventional clarifier downstream of the bioreactor; and "submerged" in which the low-pressure membranes are contained within the bioreactor. Three companies are providing pilot-scale MBR systems for testing: ZENON Municipal Systems/C.G.E. (Canada/France), Mitsubishi Rayon Corporation (Japan), and Lyonnaise-des-Eaux/Degremont (France/USA).

The MBR process offers several benefits over the conventional activated sludge process, including smaller space and reactor requirements, better solids removal, improved disinfection, increased volumetric loading, and less sludge production. In addition, the effluent water quality from the MBR exceeds the quality of a conventional activated sludge system. Therefore, MBR effluent should provide superior pretreatment for RO water treatment and desalting. Pilot testing at the Aqua 2000 Research Center will focus on technical- and cost-effectiveness, reliability (to include continuous monitoring of membrane integrity), and the development of data for a full scale plant. MBR performance will be compared with that of a conventional pilot treatment system run in parallel at the site.

INDIRECT POTABLE REUSE

Similar to San Diego, the City of McAllen, TX is conducting an extensive pilot study, cost-shared with the DesalR&D program, to demonstrate the feasibility, reliability, and cost of indirect potable reuse (IPR) technology for the purpose of expanding the City's water supply by an additional 20 years. IPR is the recovery of water from wastewater for its purposeful introduction into a surface or groundwater that ultimately serves as a drinking water supply.

The results of the McAllen phase I IPR pilot study, performed from April to August 1997, demonstrated that treating the City's wastewater with ZENON's ZenoGem MBR process (followed by ultraviolet [UV]/RO) may provide a simpler, potentially less costly, treatment process than with MF followed by UV/RO. Further, this process may produce a better quality reclaimed water than is currently achieved by the City's activated sludge/MF wastewater treatment plant (WWTP).

The phase II study goal is to demonstrate the concept of IPR through treatment and reclamation of the City's wastewater using ZenoGem and RO. Through long-term performance testing of the ZenoGem process and RO treatment of ZenoGem filtrate, it is hoped to demonstrate that ZenoGem has significant operational, cost, and water quality benefits compared to the conventional WWTP/MF process. A comparison will be made between the conventional WWTP MF/RO process and the ZenoGem/RO process, including initial capital expenditures as well as operation and maintenance costs. If successful, this project will also provide the City with the required criteria to design a full-scale ZenoGem facility, as well as the information needed to define costs and design criteria for a ZenoGem/RO process, which would provide the City with a supplemental raw water supply.

In addition to the above objectives, the MBR/RO pilot study is being used to test several integrity monitoring methods. When membranes provide the ultimate barrier between the consumer and pathogenic microbes in wastewater, it is vitally important to be sure that the membrane barrier is intact. Methods being tested in McAllen are on-line particle counting and particle monitoring and monthly pressure hold and dye challenge tests. The American Society for Testing & Materials (ASTM) will be proposing a standard for RO and UF membrane integrity testing for drinking water treatment in the next year. Data produced in this study will provide input to that consenus process.

ELECTROKINETIC ANALYSIS

One on going in-house research project is entitled "Electrokinetic Analysis (EKA) of Membrane Surface Energy." EKA is a method of measuring the zeta potential (ZP) at surface-liquid interfaces. ZP is calculated from the change in electrical potential with flow velocities as indicated by pressure. The ZP can be used as an estimate of the energy available at a membrane surface in contact with a saline solution. Previous work in this area (report #23) indicated that membranes with ZPs of lower absolute value may be less prone to fouling. On further analysis though, it became apparent that the ZP measurement itself was highly variable. Last year an ambitious experiment was performed to identify sources of variance in ZP measurements. Various factors related to instrument set up and operation and membrane handling were compared for their influence on the standard deviation of three measurements. A total of 48 different sets of conditions with three replicates were tested. The results showed that the standard deviation of the measurements could be decreased threefold by soaking the membrane until it was fully hydrated (3 days) and letting the system come to equilibrium at a given flow velocity, solution pH, and conductivity before measuring the electrical potential. The results of the experiment were presented at the North American Membrane Society Meeting last June and have been submitted for publication in the Journal of Membrane Science.

Now that a more accurate measurement technique has been established, the ZP of various membrane types is being analyzed over ranges of pH and conductivity that can be expected under normal usage. The objective is to find out if ZP can be useful in estimating fouling potential.

RESISTANCE MODEL FOR MEMBRANES

This recently completed in-house research project entitled "A Resistance Model for Evaluating Interactions between Natural Organic Matter (NOM) and Membranes at Different Scales of Operation" evaluated and compared NOM fouling of membranes at different scales of operation. This research interfaced with an American Water Works Association Research Foundation (AWWARF) project entitled "Natural Organic Matter Rejection by, and Fouling of, Nanofiltration and Ultrafiltration Membranes" investigated by Dr. Gary Amy at the University of Colorado at Boulder. The relevance of NOM to membrane treatment of drinking water is twofold: (1) NOM is the precursor to disinfection by-products which are regulated under USEPA primary drinking water standards and, (2) NOM is ubiquitous in drinking water supplies and, therefore, frequently a source of fouling during membrane treatment. Bench- and pilot-scale membrane studies are now required under the USEPA Information Collection Rule for some water treatment plants whose source water contains high levels of NOM. The purpose of this mandate is to obtain information on the cost and feasibility of advanced water treatment to remove NOM from the source water. The objectives of this research were to improve our understanding of NOM fouling mechanisms and to develop a NOM fouling model (based on these fouling mechanisms) to predict membrane performance for bench- and pilot-scale studies. The approach of this research was to interpret NOM fouling in terms of NOM-membrane interactions. Two different sources of surface water and two different membranes were tested to provide variation in the intrinsic properties which affect NOM-membrane interactions. Each source water-membrane combination was tested at three different scales of operation. Permeate flux declined through time and was attributed to the development of a NOM gel layer on the membrane surface.

A mathematical gel-resistance model was developed to analyze the NOM-membrane interactions and compare the test results at different scales of operation. The model includes parameters related to properties of the NOM, membrane, and feedwater. NOM was characterized in terms of molecular weight distribution and aromatic structure. Measured feedwater properties included pH, conductivity, and concentration of dissolved organic carbon. Membranes were characterized in terms of molecular weight cutoff of the pores, surface charge, and hydrophobicity.

Application of the gel-resistance model to the membrane test results indicated that properties of the NOM, membrane, and feedwater can be quantitatively related to NOM fouling and the resulting permeate flux decline at each scale of testing. These quantitative relationships were observed at different scales of testing; however, each scale of membrane operation imposes unique operating conditions that also influence the test results. The gel-resistance model also provided a means of interpreting the differences in test results due to the operational differences between each scale of testing.


CONTACT US

Water from Water is published by Reclamation's Water Treatment Engineering and Research Group - Susan Martella, Editor. For more information about the DesalR&D program, contact Kevin Price at: Bureau of Reclamation, 86-69000, PO Box 25007, Denver CO 80225; phone (303) 445-2260; or e-mail a message to MPrice@usbr.gov.