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Membrane Processes Chapter 15
Resources and Materials: Students should review and utilize the following on-line resources: (pages 95 and )
Membrane Filtration: Why are water systems implementing membrane filtration? What contaminants does membrane filtration remove? How does membrane filtration work?
Why are water systems implementing membrane filtration? Increasing regulatory requirements Economic factors: Requires a smaller amount of treatment chemicals than conventional treatment Can treat lower quality source water Requires less space than conventional treatment
Increasing regulatory requirements Contaminants such as cryptosporidium, arsenic, and disinfection by-products have become the focus of increasingly stringent regulations. These contaminants are readily removed by membrane filtration
What kinds of contaminants can membranes remove? INORGANICS: Metals such as arsenic, iron and manganese Salt and other dissolved solids Hardness ORGANICS: Bacteria, viruses, and protozoa Dissolved organic matter (TOC, NOM) Cyst-forming organisms (giardia and cryptosporidium)
Study the chart : Which process will remove some, but not all, viruses? Which is the only listed process that will remove metals? Which process will remove particles as small as salts, but will not remove metals?
ANSWERS Membrane Processes Chapter 15 Which process will remove some, but not all, viruses? MICROFILTRATION Which is the only listed process that will remove metals? REVERSE OSMOSIS Which process will remove particles as small as salts, but will not remove metals? NANOFILTRATION
How does membrane filtration work? Manufactured membranes operate like sieves. They have pores of a uniform size which permit small particles to pass through and reject larger particles. Hit the Enter key ONCE to Begin the animation
Membrane Processes Chapter 15 In order to operate at efficient rates, a driving force is applied to ‘push’ particles across the membrane Some membranes are “pressure-driven”. Pressure is applied to the raw-water side of the membrane.
Membranes are classified as “pressure-driven” or “electric-driven” according to the type of force applied PRESSURE-DRIVEN Microfiltration Ultrafiltration Nanofiltration Reverse osmosis ELECTRIC-DRIVEN Electrodialysis Electrodialysis reversal
Wastewater (reject water) Pressure-Driven Membrane Processes: Inside to Out configuration Influent water with contaminants Pushed into membrane by pressure Clean Water Forced out through membrane by pressure Membrane Hollow core collectors for treated water Hit Enter to see a cut-away view of the inside of this membrane assembly
Wastewater (reject water) Pressure-Driven Membrane Processes: Outside to In configuration Influent water with contaminants pushed in on top of the membrane by pressure Clean water exits through the central core
Understanding Reverse Osmosis What is Osmosis? The natural tendency of dissimilar solutions to from an equilibrium with equal concentrations throughout the blended solution. For example, if you pour sweet tea and unsweetened tea into two sides of a container separated by a porous screen (see below) the eventual result will be a container full of slightly sweet tea. The movement of molecules across the screen to reach equilibrium is an example of osmosis. 2 hours later
Reverse Osmosis The natural progression of osmosis makes both solutions equally saturated with a contaminant (in the case of the tea, the “contaminant” was sugar). Osmosis is not a helpful process in water treatment because the purpose of treatment is to create a finished water stream with a very low concentration of contaminants, and a wastewater stream containing the major proportion of the contaminants present in the raw water. Therefore, treatment professionals must “reverse” osmosis to force contaminants out of the finished water stream and into the wastewater stream. The force applied to a reverse osmosis membrane is in the form of pressure (from 50 – 300 psi)
Membrane Processes Chapter 15 Reverse Osmosis Reverse Osmosis membranes are the most selective of all of the pressure membranes, and can remove contaminants as small as individual ions such as calcium and magnesium –Higher pressures are required to force water through the very small pores of RO membranes –Clogging and fouling occur with water containing excessive iron, hardness, turbidity and other contaminants – pre-treatment is necessary to remove these contaminants –RO produces a greater percentage of ‘reject’ water: a waste stream requiring analysis and disposal
Membrane Processes Chapter 15 Membrane Cleaning Some pleated membranes are designed to be disposable; these are generally used for individual residential or commercial services or very small water treatment plants. The majority of membranes require some type of cleaning operation. Cleaning is accomplished by reverse flow (Backwashing) and/or chemical cleaning.
Membrane Processes Chapter 15 Triggers for cleaning a pressure-driven membrane filter Time – based: for example, backwash every 4 hours Elapsed time: for example, backwash every 4 hours Pressure required to push water through membrane (similar to head loss on a gravity filter). Scaling or fouling of the membrane Special triggers for Reverse Osmosis membranes: Reject water percentage increases/treated water decreases by more than 5% Effluent concentration of contaminant (for example, salt in a desalination facility) increases by 15% or more
Electrically-Driven Processes Electrodialysis: Electrical current is applied to the solution to force contaminant ions across the membrane into the waste stream. Electrodialysis reversal: The operating principle is the same as electrodialysis, with the added step of periodic current reversal (think of an ‘electric current backwash’) to clean the membrane. The most common application for these processes is desalination of brackish water.
Cleaner influent water = better membrane performance As the purity of the incoming water decreases, the rate of membrane fouling and scaling increases. Increases the frequency of cleaning, time out of service, and cost of cleaning chemicals Decreases the service life of the membrane through increase exposure to oxidants and long-term plugging of pores with build-up of fouling agents Pre-treatment is recommended to lower turbidity, iron, and other contaminants before water is fed to the membrane
Membrane Processes Chapter 15 Process control monitoring for membrane operation % Rate of recovery: Amount of finished water divided by amount of influent raw water X 100 % Reject water: 100% - % rate of recovery Feed water flow rate and pressure (must not exceed rated capacity of membrane to prevent membrane damage) Feed water quality (conductivity, pH, turbidity, etc) to prolong membrane life and prevent fouling Finished water quality (conductivity, ph, turbidity, etc) to detect breakthrough or membrane damage
Mechanical issues in Membrane Operation Routine visual inspection to detect leaks in membrane assemblies Periodic inspection of operating units (unusual bubbling, etc may indicate broken fibers, leaks, etc) Periodic removal from service for integrity testing (apply air pressure and check for bubbling) Routine cleaning with appropriate cleaning agents compatible with specific membrane in use