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The Desalination and Water Purification Research & Development Program Newsletter - No. 17 - Summer 2000

PROGRAM REPORTS

The Desalination and Water Purification Research & Development (DWPR) Program publishes a final report for each project funded by the program. A complete list of the available reports can be found at the program's reports web site: www.usbr.gov/water/publications/reports.html. To obtain single copies of the reports, contact Susan Martella at 303-445-2257.

The projects described below have resulted in final reports. The integrity monitoring work is available as reports #55 and #55a (#55a contains a complete set of data). The membrane bioreactor (MBR) part 1 work is available as report #34. The McAllen, TX, and the MBR part 2 reports are in final draft and will be available at the end year.

INTEGRITY MONITORING

Vacuum hold tests and particle counters are accepted methods for monitoring integrity in microfiltration (MF) and loose ultrafiltration (UF) membrane systems, but such methods are not applicable to tighter UF, nanofiltration (NF), and reverse osmosis (RO) membrane systems. One potential benefit of the tighter systems is they are capable of replacing or reducing the need for disinfection. However, before this can occur, an acceptable method for monitoring these system's integrity is needed. Salt passage, permeate flow, and pressure drop are the usual performance parameters for RO and NF, but it is not known how these parameters relate to the passage of pathogens. Therefore, criteria are needed for determining when an RO or NF system must be taken out of service for repair or membrane replacement. The attributes of a "good" integrity monitoring method are: it is easy to use; it is reliable; it has a correlation to a verifiable breach of integrity; and it must be acceptable to regulatory and industrial organizations.

The DWPR program, in partnership with ASTM, has been involved in research related to these integrity monitoring issues for several years. This year, an integrity monitoring study was performed in conjunction with a separately funded water treatment study at McAllen using a combination of MF/MBR/RO, treating screened and de-gritted sewage. On-line methods, such as total organic carbon (TOC) analysis, particle counts, and particle index, were compared with periodic measurements of pressure hold, food grade dye challenges, and ultraviolet spectral analysis at 254, 455, and 530 nm. The feed and product waters were tested for Pseudomonas aeruginosa twice a week. Pressure hold tests and dye challenges were performed three times over the six-month trial. After a six-month performance test, the system was systematically damaged: brine seals were twisted, o-rings were cut, and membranes were pierced. Following each breach in integrity, 24 hours of performance data was collected and then the periodic tests were performed. The final analysis of baseline and challenge data showed that the only on-line measurement that consistently revealed damage was the TOC analyzer. The periodic dye test, spectral analysis at 254 nm, and pressure hold tests were also consistent in detecting damage.

Data collected during this study was presented to the ASTM D19.08.02 committee for Standards for Reverse Osmosis, Nanofiltration, and Ultrafiltration Membranes at their January 2000 meeting. At the meeting it was decided that the Integrity Monitoring Standard should be written as a Practice. The integrity tests to be incorporated in this new standard will be Diffusive Flow, Pressure Hold, Vacuum Hold, Soluble Dye, Pressure Monitoring, and TOC Monitoring. Gary Ganzi (978-934-9349 x2242) of U.S. Filter is the Method Advocate for the first two methods; Dave Paulson (612-988-6524) of Osmonics for the middle three; and Michelle Chapman Wilbert (303-445-2264) of Reclamation for the last. All six of these methods have been reviewed and accepted for scientific validity and practicality by the ASTM Task Group. A draft of the Standard Practice will be available for review in September. Those interested in reviewing the draft should contact one of the above advocates.

MEMBRANE BIOREACTORS

The increased need for reclaimed water in arid environments has resulted in the emergence of new wastewater reclamation technologies. The MBR is one of these new technologies that combines activated sludge treatment with a membrane separation process. The reactor is operated in a similar way to conventional activated sludge (CAS), however a clarifier is not needed. Instead, a low-pressure membrane, either MF or UF, is used to perform the sludge separation. The combination of an activated sludge and membrane process produces water that has undergone secondary, tertiary, and low-pressure membrane treatment using only one unit operation.

Because a membrane is performing the separation instead of a clarifier, the MBR can be operated at higher mixed liquor suspended solids concentrations, and longer solids retention times. The removal of the clarifier further eliminates such problems as sludge bulking, pin floc, and various other settling problems associated with clarifier operation and the overall footprint of the system is much smaller than a CAS facility.

The DWPR program co-funded research with the City of San Diego and Montgomery Watson to investigate the feasibility of using MBRs for water reclamation. Based on the findings of the first phase of the project (Part 1), the project team concluded that a parallel comparison of commercially available MBR systems needed to be evaluated at pilot-scale. Two submerged MBR systems (Mitsubishi Rayon Corporation from Japan and Zenon Environmental Systems, Inc. from Canada) were evaluated at the Aqua 2000 Research Center in Escondido, CA from April 1998 to April 1999. The project was designed to evaluate the MBR performance treating municipal wastewater and the feasibility of using the MBR permeate as a feed source for thin film composite RO membranes. The first part of the project was dedicated to operating both MBRs in a nitrification and denitrification mode. After completion of Part 1 of the project, both MBR systems were retrofitted and operated in a nitrification only mode by removing the anoxic tank. Throughout both parts of the study, the effluent from each MBR was fed to two separate, single stage RO pilot systems. Both MBR systems were capable of producing a good quality effluent water suitable for use by an RO system with minimal fouling. Both MBRs were operated at reasonable cleaning intervals ranging between one and three months. Both units showed high biochemical oxygen demand removal with values below the detection limit, and significant TOC and chemical oxygen demand reduction. The effluent turbidities were typically less than 0.1 NTU. In addition, both systems showed high removal of total and fecal coliforms and total coliphage.

A cost comparison was also performed analyzing several technologies capable of producing water suitable for use by an RO system. The analysis revealed that the MBR is a cost competitive alternative for producing water suitable for use by an RO system at a 1- and 5-MGD capacity.

PROGRAM STATUS

The two principal thrusts of the DWPR program are: (1) perform research and studies on desalination technologies and related issues to push the state of the art forward so costs can be reduced; and (2) conduct development and demonstration activities to test technological advancements, confirm economics, and gain public acceptance. Due to reduced funding, the award of research and studies projects, as well as demonstration projects, both scheduled to occur in October 2000, is indefinitely postponed. We will know our final fiscal year 2001 budget in October, 2000. For more information about these and other DWPR program activities, contact Kevin Price, Program Manager, at 303-445-2246, or visit the DWPR web site at: http://www.usbr.gov/pmts/water/research/DWPR/.


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.