Chapter 6 Reverse Osmosis and Nanofiltration 1

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

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

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

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

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

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

Reverse osmosis theory Recovery Rejection 8

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

RO and NF Membrane Process Arrangement Spiral wound membrane Courtesy of Mintrinc http://www.mtrinc.com/images/faq/spiral.gif 10

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

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

Four pressure vessels arranged in an array

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.

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

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

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

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

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

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