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The Water Treatment Technology Program Newsletter - No. 2 - Summer 1995

WATER TREATMENT TECHNOLOGY PROGRAM (WTTP)

Reclamation established the WTTP as a research effort to work in partnership with academia, private industry, and local communities to address a broad range of research related desalting and water treatment needs. Water from Water is published by Reclamation's Technical Service Center (TSC) in Denver as part of the technology transfer function of the WTTP to keep you informed on the latest activities and developments under the various program Tasks.

Management of the WTTP is assigned to the TSC's Water Treatment Engineering & Research Team. This Group provides water treatment research and engineering design services, and places an emphasis on cost sharing our expertise and services not only with other Reclamation organizations, but also with other departments, agencies, governments, academia, and private industry.

For more information about the Water Treatment Engineering & Research Team, contact Chris Holdren at (303) 445-2178. For more information about the WTTP, contact Kevin Price at (303) 445-2260. Mail may be directed to the address listed below, and to the attention of the Water Treatment Engineering & Research Team, D-8230.

TASK 3 - DESALTING MEMBRANE PROCESS DEVELOPMENT

Reverse osmosis (RO) membranes were originally developed for the purpose of desalting sea and brackish water. Today, RO and related membrane processes have expanded to address almost every facet of water treatment, including wastewater and process water reclamation, and drinking water treatment. These new technologies may offer solutions to a diverse array of specific drinking water problems.

The major advantages of membrane processes are: (1) superior quality of the product water which is attained with fewer chemical additives than conventional treatment, (2) membrane plants can be much smaller than conventional plants because of the modular configuration of membranes and the possible elimination of other processes, and (3) problems and costs associated with sludge dewatering, handling, and disposal are eliminated.

The major disadvantage of membrane processes is their cost. Although capital and operating costs for sea and brackish water membrane desalination have been reduced to competitive levels in many locations over the past 20 years, fouling of the membranes is one factor that keeps the cost from becoming more competitive with conventional treatment. One method of fouling can occur when colloids and dissolved organics interact with the membrane and cause a decrease in the water flux across the membrane. Fouling can also cause an increase in salt passage through the membrane. Although colloids are often considered to be the principal cause of membrane fouling, the mechanisms of colloidal fouling are complex and poorly understood.

UCLA CONTRACT

In order to better understand the effects of membrane colloidal fouling, in 1994, through a competitive bid process, Reclamation contracted with UCLA's Department of Civil and Environmental Engineering to cost share the study "Zeta Potential of Reverse Osmosis Membranes: Implications for Membrane Performance and Feed Water Treatment." The study objectives were to: (1) develop a methodology to measure zeta potentials (charge) of RO membranes using a streaming potential analyzer, (2) investigate the zeta potential of different commercial RO membranes at various solution chemistries, (3) delineate the mechanisms of surface charge acquisition by RO membranes in aqueous solutions, and (4) evaluate the implications of the results for minimizing colloidal fouling and for optimizing pretreatment of feed waters.

According to UCLA, the membrane surface charge has to be characterized to better understand the interaction of colloids and dissolved organics with the membrane. This characterization can be accomplished by evaluating the membrane zeta potential from streaming potential measurements. Determination of zeta potential of RO and nanofiltration (NF) membrane surfaces is critical to membrane fouling research.

The study has concluded, and the following recommendations and conclusions were made: (1) the pH of zero charge of commercial cellulose acetate and thin film composite RO and NF membranes is between 3 and 5, (2) commercial RO and NF membranes are negatively charged at typical operating pH conditions, (3) the surface charge of RO and NF membranes is markedly influenced by the solution chemistry of feed waters, and (4) colloidal fouling can be reduced when colloids and membranes are highly negatively charged. Also it was concluded that the relationship between membrane charge and membrane performance (i.e., water flux, salt rejection, and fouling potential) should be further investigated.

ZENON CONTRACT

In order to reduce the impacts of fouling on NF membranes, in 1994, through a competitive bid process, Reclamation contracted with ZENON Environmental, Inc., to cost share the study "Development of an Advanced Transverse Flow Nanofiltration Membrane Process for High Performance Desalination." NF incorporates a membrane with pore sizes which are classified between those of ultrafiltration and RO membranes. NF has been shown to remove most naturally occurring dissolved organics, however the conventional module is not always well suited for application to surface waters because of build-up of fouling material.

ZENON was contracted to further develop their novel MousticTM transverse flow hollow fiber nanofiltration module. ] The major advantage of this design is that the feedwater flows outside and perpendicular to the hollow fibers, rather than inside, so the membrane is considerably less likely to foul and is easier to clean. The study involved: (1) development of a high tensile strength fiber suitable for brackish water application, (2) production of suitable membranes, and (3) modifications to the existing transverse flow module design for high pressure applications. The high pressure nanofiltration module was tested on synthetic brackish water and performed well with minimal pretreatment. Flux, however, was low in the configured module compared to individual fibers.

The study has concluded, and the following improvements were made to the transverse flow module design: (1) a transverse flow module was constructed which could be operated at the pressures necessary for desalting brackish water, (2) suitable high tensile base fibers were developed, (3) appropriate membrane chemistries were evaluated, and a chlorine-resistant nanofiltration with suitable solute rejection characteristics was produced, and (4) methods for application evaluation of the membrane were investigated and optimized. Also, it was concluded that further membrane development is necessary to improve flow distribution and decrease channeling in the high pressure design.


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.