1. Chapter 6 Reverse Osmosis and Nanofiltration1

2. Membrane filtration Types Theory Practice Reverse Osmosis
Nanofiltration
Ultrafiltration
Microfiltration
Theory
Practice
Problems 2

3. Overview of Membrane Processes
Very effective
Slow rate of transfer across the membrane
High pressure drops
Large membrane areas needed
Can be used to separate colloidal and dissolved solids that are much smaller than those removed by other filtration processes 3

4. MEMBRANE DESIGNATION 1 bar =100 kPa PRESSURE GRADIENT PORE DIAMETER
REMOVAL EFFICIENCY
MEMBRANE DESIGNATION
This figure shows the membrane categories descending from the least porous to the most, and the corresponding performance. As expected, more contaminants can be removed at the expense of higher pressure drop and/or lower flow rate.
(Read off the material removed, pressure drop, and pore diameter off the diagram as you describe the different types. This may be a good page to print and hand out.)
The most open membrane is Microfiltration (MF). Despite the designation of least rigorous, MF removes some of the most common and dangerous pathogens such as Giardia, E-coli, and Cryptosporidium. For this reason, it is widely used for animal drinking water.
Ultrafiltration (UF) is the preferred treatment for potable water. It allows better disinfection of pathogens, because the latter can cover quite a range. It also demonstrates excellent removal of most common viruses.
Other extreme…Reverse Osmosis (RO) is needed for removal of ions and salts and other common organics like Humic and Fulvic acids. Nanofiltration (NF), which is the middle ground, can eliminate the organics nearly as well, but not the ions (salts). In 1984, a study concluded that membrane treatment with a pore diameter between NF and RO would be needed to reduce a lake in Florida THM (Tri-Halo-Methane) level to less than the MCL (Maximum Control Limit) of 0.1 mg/L
The ability of RO to remove ions or salts qualifies it for the most well known and promising application of all...
1 bar =100 kPa 4

5. Organics (e.g., Color , NOM, SOCs)
Membrane Separation
Macro
Micro
Ionic Range
Molecular Range
Molecular Range
Particle Range
Macro Particle Range
Size, Microns
0.001
(nanometer)
0.01
0.1
1.0 10
100
1000
Molecular Weight
(approx..)
100
1,000
100,000
500,000
Dissolved Salts
(ions)
Viruses
Bacteria
Relative
Sizes
Algae
Organics (e.g., Color , NOM, SOCs)
Cysts
Sand
Clays
Silt
Asbestos Fibers
Reverse
Osmosis
Ultrafiltration
Separation
Microfiltration
Conventional Filtration (granular media)
Process
Nano
filtration 5

6. Osmosis vs. Reverse Osmosis
Driving Force ( ∆C, ∆P)
Feed
Permeate
Concentrate
Osmosis is the net movement (diffusion) of a solvent from a region of higher water concentration to a region of lower concentration
Reverse osmosis is the net movement (diffusion) of a solvent (water) from a region of higher salt concentration to a region of lower water concentration 6

7. Reverse osmosis theory
Water flux:
Solute flux:
Where:
Jw = volumetric flux of water
P = transmembrane pressure
 = difference in osmotic pressure between the feed and the permeate
Js = mass flux of water
C = concentration gradient across membrane
Cp = concentration of solute in permeate 7

8. Reverse osmosis theory
Recovery
Rejection 8

9. Membrane Processes: Operation
We will now discuss the driving forces that force water through membranes.
Nearly all water treatment applications use pressure drop and separate the components by size.
Not surprisingly, there is a increased pressure drop with tighter membrane, high flux, and/or a higher concentration of contaminants. This repeats the “trade-off” or “balancing” concept discussed earlier.
One good example of these interrelationships is the dairy industry. With the same membrane and pressure drop, flow rate with a dairy process is only 10% that of pure water. 9

10. RO and NF Membrane Process Arrangement
Spiral wound membrane
Courtesy of Mintrinc 10

11. The layers of a spiral wound membrane
The layers. To provide more detail the following slides are provided without titles.

12. Overview of RO pressure vessels and associated pumps and controls.
Overview of RO pressure vessels, associated pumps and controls for a boiler plant

13. Four pressure vessels arranged in an array

14. Third stage Second stage First stage
The membrane array consists of 3 stages. The first stage has two pressure vessels. The second and third stages have only one pressure vessel.
The membrane array consists of 3 stages. The first stage consists of two pressure vessels. The second and third stages have only one pressure vessel.

15. Operational challenges
Scaling and fouling of membranes
Lower than desired rejection
Flow
(85-90%)
Product Water
Fouling Layer
Permselective Barrier
Flux (10-15%)
Adsorption
Fouling of the membrane occurs by accumulation of suspended or dissolved solids on the external membrane surface, on the membrane pores, or within the pores. 15

16. Biofouling
Biofouling is referred as the undesired development of microbial layers on the surface.
Biofilm organisms are embedded in a matrix of microbial origin, consisting of extracellular polymeric substances (EPS).
Membrane Surface
Microbial Attachment
Microbial Colonization
EPS & Microbial Growth
Montana State University 16

17. Pretreatment Pretreatment to prevent scaling
Removal of ion that cause scaling before processing
Inhibition of crystal growth
Filtration of particulates
To prevent deposition on membranes (organic foulants)
Addition of chemicals:
Antiscalant for scale control
Sulfuric acid to adjust pH
Caustic soda to adjust pH 17

18. Cleaning
Cleaning the membranes is a logical progression from fouling. The most common cleaning methods are
Hydrochloric acid
Caustic
Citric acid, pH adjusted to 4 with ammonium hydroxide Combination of commercial wetting agent and citric acid at pH 4
Disinfection to prevent biological fouling 18

19. Post-treatment RO/NF Concentrate
Permeate has a low pH as alkalinity is removed and acid is often added to prevent scaling
Water is corrosive
Need to strip CO2 and add base and corrosion inhibitor (alkalinity in some cases)
Concentrate
High TDS
Disposal: municipal sewer, ocean discharge, deep well injection 19

20. Outcomes Based on this lecture and Chapter 6, you should be able to
Describe the physical and chemical phenomena that underlie the design and operation of RO and NF membrane filters
Based on the characteristics of the raw water and the desired characteristics of the product water, select the appropriate membrane
Design an array system given appropriate recovery rates and flow rates 20