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Membrane Processes

Membrane processes are divided into two classes:

  1. Micro- and Ultra- filtration (MF and UF) membrane separates through sieving particles (pores)

  2. Reverse Osmosis, Nanofiltration, and Forward Osmosis (RO, NF, and FO) membrane separate through solution and diffusion of molecules and ions.

The difference between the two types in each category is the degree of separation. Click here for a link to a good industrial filtration spectrum. For a detailed description of the membrane processes please click to download DWPR Report #29, "The Desalting and Water Treatment Membrane Manual: A Guide to Membranes for Municipal Water Treatment (2nd Edition)," which includes specifications for membranes available at the time of publication (1998). Table 1 summarizes the various levels of membrane treatment processes and conventional processes that they can replace.


Table 1. Membrane Process Levels of Separation


Membrane Process

Contaminants

Conventional Process

Microfiltration

Turbidity
Suspended solids
Biological contamination

Settling,
coagulation/flocculation,
media filtration, and
disinfection

Ultrafiltration

Above + Virus, color and odor causing organics

Above + powdered activated carbon filtration (difficult)

Nanofiltration

Hardness, sulfate, iron, manganese

Lime softening, coagulation/ floccultation, media filtration

Reverse Osmosis and Forward Osmosis

Dissolved salts

Distillation

Microfiltration. Microfiltration filters water through mediums with tiny pore sizes (from slightly below 0.1 micron to slightly above 1 micron). The pore size refers to the medium--not anything that might acculate on the medium. This is smaller than the size range for bacteria, algae and cysts. However, the pore size is larger than viruses, so microfiltration does not remove viruses well.

Ultrafiltration.Ultrafiltration filters waterthrough mediums with poresizes smaller than microfiltration and larger than nanofiltration. UF is a relatively new process for municipal water treatment. It has been used more extensively in industrial processes for concentrating milk, protein, and cells; recovering process materials such as paint, colloidal metals, and dyes; and recovering oily waste water. In the last 15 years, the application of UF in drinking water treatment has expanded profoundly due to changes in drinking water quality regulations.

Nanofiltration and Reverse Osmosis require hydraulic pressure to overcome the osmotic pressure of the feed solution. Osmosis is a natural process whereby water is transported through a semi-permeable membrane from a solution of low concentration to one of high concentration. Plants use this phenomena to draw in water from soil. Reverse osmosis occurs when pressure in excess of the osmotic pressure is exerted on the high concentration side of the membrane, inducing pure water to diffuse through the membrane to the low concentration side. Nanofiltration membranes have more open structuresthan reverse osmosismembranes, and they reject large ions like calcium and sulfates at higher rate than small ions like sodiumand chloride.

Forward Osmosis uses a draw solution such as sugar and nutrient solutions as a driving force. The diving solution on one side of the membrane draws fresh water from water on the other side that is less saline, but may be turbid and contaminated with pathogenic bacteria and virus. The draw solution in concentrated form is inside a bag formed from semi-permeable membrane. The bag is left in the water source until it is full of diluted nutrient/sugar solution. These are sometimes provided for baby formula in developing countries and for backpackers and military use. Other draw solutions under investigation areammonia-carbon dioxide, nano-magnetic ferritin particles, and seawater.Forward osmosis has not been used for large applications in the past. In recent years, forward osmosis has gained more attention as a method for producing power from the energy released as fresh water rivers enter the ocean. The energy of mixing can be harvested through drawing the fresh water through a membrane into the seawater and using the increase in volume to drive a turbine to generate electricity. Statkraft, an energy supply company based in Oslo, Norway, built an osmotic energy plant near Oslo.

Reclamation has carried out and funded the research of others into many areas of membrane processes including:


Report Title

Performing Agency

Date

Report #

General

The Desalting and Water Treatment Membrane Manual: A Guide to Membranes for Municipal Water Treatment

Bureau of Reclamation

Sep-93

1

Desalting as an Environmentally Friendly Water Treatment Process

Summary Report of an ADA Seminar

Sep-94

13

The Desalting and Water Treatment Membrane Manual, A Guide to Membranes for Municipal Water Treatment, 2nd Edition

Bureau of Reclamation

May-98

29

Pretreatment and Design Considerations for Large-Scale Seawater Facilities(abstracts)

Reiss Environmental, Inc

Feb-08

137

Pilot Studies

Nanofiltration of a High Salinity Groundwater on the Hopi Reservation

Northern Arizona University

May-95

3

Lake Havasu City Water Treatment Study

Bureau of Reclamation

Apr-95

8

Maricopa Ground Water Treatment

Bureau of Reclamation

Feb-96

15

Reverse Osmosis Treatment of Central Arizona Project Water for the City of Tucson

Tucson Water Department

Jan-04

36

Salinity and TOC Removal Using Nanofiltration

University of Texas at El Paso

Aug-02

46

Evaluation of the Port Hueneme Demonstration Plant

Bureau of Reclamation

May-01

65

Pilot Investigation of Slow Sand Filtration and Reverse Osmosis Treatment of Central Arizona Project Water Report Appendices

Bureau of Reclamation

Aug-02

90

Riverton City Desalination of Mineralized Ground Water Pilot Project

Epic Engineering

Apr-09

131

Halfway Wash Water Treatment Pilot Study

Bureau of Reclamation

Aug-06

148

Membrane & Module Development

Development of an Advanced Transverse Flow Nanofiltration Membrane Process for High Performance Desalination

ZENON Environmental, Inc.

Jun-95

9

Brackish Groundwater Treatment and Concentrate Disposal for the Homestead Colonia El Paso, Texas

University of Texas at El Paso

Apr-99

32

Development of an Advanced Transverse Flow Nanofiltration and Membrane Process for High Performance Desalination-Phase II

ZENON Environmental, Inc.

Nov-98

37

Molecular Sieving Hollow Fiber Ceramic Membranes for Reverse Osmosis/Nanofiltration Membranes

Media and Process Technology, Inc.

Feb-97

40

New Design Channel Feed Spacer in Spiral Wound Elements for Pretreatment Cost Reduction

Desalination Systems, Inc.

Sep-98

45

Polyamide Reverse Osmosis Membrane Fouling and Its Prevention: Oxidation-Resistant Membrane Development, Membrane Surface Smoothing, and Enhanced Membrane Hydrophilicity

Separations Systems Technology, Inc.

Nov-00

61

Industry Consortium Analysis of Large Diameter Reverse Osmosis/Nanofiltration Element Diameters

Dow Chemical Company

Sept-04

114

Use of Dendrimers to Enhance Selective Separation of Nanofiltration and Reverse Osmosis Membranes(abstract)

Hydranautics

Aug-09

140

Membrane Characterization

Zeta Potential for Reverse Osmosis Membranes: Implications for Membrane Performance and Feed Water Treatment

University of California at Los Angeles

Dec-96

10

Membrane Element Autopsy Manual

Bureau of Reclamation

Dec-96

17

Evaluation of Methods for Monitoring the Integrity of Reverse Osmosis Membrane Systems

Bureau of Reclamation

Apr-00

55

Boron Rejection by Reverse Osmosis Membranes: National Reconnaissance and Mechanism Study

Georgia Institute of Technology

July 09

127

Evaluation of High
Productivity Brackish Desalination Membrane
Dr. Winston Ho 2008

Fouling & Cleaning

Novel Membrane Process with Rapid Backpulsing for Water Treatment

University of Colorado

Nov-96

18

Enhancement of Membrane Fouling Resistance through Surface Modification

Bureau of Reclamation

Mar-97

22

A Resistance Model for Evaluating Interactions Between Natural Organic Matter (NOM) and Membranes at Different Scales of Operation

Bureau of Reclamation

Sep-99

44

Membrane Fouling: Influence of Natural Organic Matter

Rensselaer Polytechnical Institute

Sept-02

83

Evaluation of Precipitative Fouling for Colorado River Water Desalination using RO

Metropolitan Water District of Southern California

Dec-02

85

Evaluation of Reverse Osmosis Scaling Prevention Devices at High Recovery

Bureau of Reclamation

Mar-03

91

Evaluation of Cleaning Spiral Wound Membrane Elements with the Two-Phase Flow Process (see also Report 121)

Novaflux Technologies, Inc

Jun'03

101

Predicting Membrane Flux Decline Using Parameters Derived from Field-Flow Fractionation Measurements

Colorado School of Mines

Jun-06

102

Impact of Magnetic Fields on RO Separation

University of South Florida

Oct-05

110

Evaluation of Desalination on Waters Under the Influence of Surface Water Runoff for Pretreatment, Water Quality, and Pathogen Removal Performance

Reiss Environmental.

Sept-07

113

Pilot Scale Testing of Membrane Desalination System Utilizing Novel Two-Phase Cleaning Technology

Novaflux Technologies

May-08

121

Novel Fouling Resistant Membranes for Water Purification

University of Texas at Austin

Sept-08

129

Optimization of Chemical Cleaning of Organic-Fouled Reverse Osmosis Membranes

Yale University

Aug-09

141

Reduced Membrane Fouling Potential by Tailored Fluid/Structure Interaction (abstract)

Heat Transfer Research, Inc.

May-08

143

Efficient and effective operation of filtration membrane requires knowledge of a water preparation and membrane sustainment. Specific areas of importance for membrane filtration operation include:

Research in these categories include a variety of pretreatment techniques, techniques for removing specific compounds, mechanisms causing membrane fouling and techniques to remove fouling agents during cleaning.

Research Priorities


Tampa Bay side ported vessel configuration

New flow distributor design on left, the old design on the right.