Chapter 6 Reverse Osmosis and Nanofiltration Membrane filtration

Chapter 6 Reverse Osmosis and Nanofiltration

Membrane filtration ¡ Types l l Reverse Osmosis Nanofiltration Ultrafiltration Microfiltration Theory ¡ Practice ¡ l Problems

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 ¡

PRESSURE GRADIENT MEMBRANE DESIGNATION REMOVAL EFFICIENCY PORE DIAMETER 1 bar =100 k. Pa

Membrane Separation Size, Microns Molecular Weight (approx. . ) Relative Sizes Ionic Range Macro Micro Molecular Range Particle Range 0. 001 (nanometer) 100 0. 01 1, 000 Dissolved Salts (ions) 0. 1 100, 000 1. 0 Macro Particle Range 100 10 1000 500, 000 Viruses Bacteria Algae Organics (e. g. , Color , NOM, SOCs) Cysts Clays Sand Silt Asbestos Fibers Separation Process Reverse Osmosis Nano filtration Ultrafiltration Microfiltration Conventional Filtration (granular media)

Osmosis vs. Reverse Osmosis Permeate Feed Concentrate Driving Force ( ∆C, ∆P) 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

Reverse osmosis theory ¡ ¡ Water flux: Solute flux: l l l l 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

Reverse osmosis theory ¡ Recovery ¡ Rejection

Membrane Processes: Operation

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

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

Four pressure vessels arranged in an array Four pressure vessels.

Third stage Second stage First stage The membrane array consists of 3 stages. The first The membrane array consists 3 stages. The first stage consists of two pressure stage has 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 Adsorption (85 -90%) Flux (10 -15%) Fouling Layer Permselective Barrier Product Water

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

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

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

Post-treatment ¡ RO/NF l l l ¡ Permeate has a low p. H as alkalinity is removed and acid is often added to prevent scaling Water is corrosive Need to strip CO 2 and add base and corrosion inhibitor (alkalinity in some cases) Concentrate l l High TDS Disposal: municipal sewer, ocean discharge, deep well injection

Outcomes Based on this lecture and Chapter 6, you should be able to l l l 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