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Although conventional filtration typically produces a high quality of finished water, some particulate material will not be captured in the filter. Membrane filtration is a mechanical barrier that uses a straining mechanism only to remove material from the water. If the barrier is intact, no particles larger than the membrane's pore size can pass through the filter.
Membrane filtration employs a thin, permeable layer, or sheet of material, which the water passes through to remove impurities and targeted pollutans. The primary removal mechanism in membrane filtration is the physical blockage of particles by the membrane material. Secondary removal mechanisms include the removal of substances that are attached to the particles captured during the primary removal process. The membrane filter is based on pore size and durability. The smaller the pore size in the membrane, the smaller the paticle that will be blocked from passing through the membrane filter.
When a membrane filter is new, particles that are smaller than the pore size will pass through the filter. As the membrane begins to capture particles larger that the pore size, it begins to "ripen". As a membrane ripens, the pore size available for filtering decreases, and smaller particles are captured. A membrane filter will fail when the water no longer passes through it, or when the rate of water passing through it (flux rate) is less than the filter design flow rate.
Principles of Membrane Filtration
A membrane is a semi-permeable thin layer of material capable of separating contaminants as a function of their physical/chemical characteristics. A more common way to express this is: A membrane is a thin layer of material that will only allow certain compounds to pass through it. Which material will pass through the membrane is determined by the size and the chemical characteristics of the membrane and the material being filtered. In order to understand the concept of membrane treatment, the concept of osmosis must be discussed.
Osmosis is a naturally occurring phenomenon that describes the tendency of clean water to dilute dirty water when they are placed across a permeable membrane from each other. Eventually, the concentration of the constituents in the water on the "dirty" side of the membrane will equal the concentration of the constituents on the clean side of the membrane.
Osmotic pressure is the pressure created by the difference in concentration of the constituents on either side of the membrane, and this pressure drives the osmosis process. Osmotic pressure drives the flow of fresh water to the dirty side. As the concentration of the constituents on each side of the membrane reach equilibrium (where the concentration is the same on both sides of the membrane), the osmotic pressure becomes zero and the flow stops.
Osmosis is not desirable from a water treatment standpoint since the goal of treatment is to produce fresh water and not to dilute dirty water with fresh water.
Reverse osmosis (RO) is the process of forcing water from the dirty side through the membrane into the clean water side, while leaving the undesirable constituents behind on the membrane itself. By operating the system opposite of its "normal" direction, fresh water can be produced from raw water. Undesirable constituents will be deposited on the membrane's surface and will eventually clog it. If a membrane system is to be useful, there must be a way to remove this material from the membrane itself as well as from the entire system.
Membranes are used to remove undesirable constituents from the water. If these constituents are dissolved in the water, very tight membranes are required; if the constituents are particulate, then a looser membrane is appropriate. Membrane filters are used to remove microbiological contaminants. Even the loosest membrane will remove Giardia cysts and Cryptosporidium oocysts, but if virus removal is desired in addition, a slightly tighter membrane would be used. There are four levels of membrane filtration. These levels are (from largest to smallest pore size): microfiltration, ultrafiltration, nanofiltration, and reverse osmosis. Each level has a pore size range associated with it and is used to remove certain sized contaminants.
|Filtration Level||Pore size range||Target Contaminants|
|Microfiltration||> 0.10 µm||Particulate material like algae, Giardia, Cryptosporidium, bacteria, and clays|
|Ultrafiltration||0.01 - 0.10 µm||All substances removed by microfilters plus humic acids and some viruses|
|Nanofiltration||0.01 - 0.001 µm||
All substances removed by microfilters and ultrafilters plus dissolved metals and salts
|Reverse Osmosis||< 0.001 µm||All substances removed by microfilters, ultrafilters, nanofilters plus smaller dissolved metals and salts|
µm is a micron, which is one millionth of a meter, and is also known as a micrometer.
The image below illustrates which process, or processes, could be used for a particular contaminant.
The nanofiltration typically has the highest operating pressures of the other membrane processes shown.
Membrane Filtration Categories
There are two general classifications of membrane processes:
Pressure-driven membrane processes use differential pressure as the mode of force to move relatively small water molecules through a membrane while larger molecules are rejected.
Based on the pore size and mechanism of functioning, membranes can be divided into four groups:
Microfiltration membranes function like a sieve, and operate in the range between conventional sand filtration and ultrafiltration. The pore size is > 0.10 µm; therefore, they remove all particles bigger than 0.10 µm, including Cryptosporidium oocysts, Giardia cysts and all bacteria. They are successfully used for water treatment plants with less than 12 MGD capacity and low raw water turbidity.
The membranes used in hollow-fiber microfiltration are made out of polyvinylidene fluroride (PVDF), which are resistant to acids, bases, chlorine, ozone, and permanganate with a life expectancy of 5 to 10 years. This allows them to be cleaned with stron chemicals without causing damage. Membranes are cleaned every 4-6 weeks to remove material that has fouled the membrane. The cleaning process is different for each type of membrane, but generally, acids and basis are used to remove particulates: acids remove cationic material while bases remove anionic material.
Hollow-fiber microfiltration membranes are less than a millimeter in diameter and are contained within a module. The fibers are sealed at the bottom end of the filter module, which directs flow to the outside of the fiber shell. Water passes through the fibers from the outside in, leaving contaminants behind. The filtered water is collected and pumped away.
The flow rate through these fibers is called the flux. Hollow-fiber microfiltration modules are usually installed in the vertical position to allow for good water and air separation during the "flux recovery" process.
These membranes are operated in either a cross-flow or direct-flow mode with the flux ranging between 35-50 gallons/ft2/day (abbreviated gfd) for surface water plants. The process recovery is typically > 95% when treating this type of water source. Recovery is the percent of raw water that becomes treated water that can be distributed to the public.
A certain percentage of the treated water is used for backwashing through reverse flow. To backwash this type of filter, flow is reversed on a regular basis to dislodge particulates taht have foulded the membrane. Air is usually added to increase the velocity at the membrane to help dislodge fouling material. Strong oxidizers can also be added during this backwashing process to aid in contaminant removal. This type of filter has a low percentage of reject water due to the pore size; smaller pores sizes means more reject water.
A microfiltration membrane filter is often used as a pre-filter for ultrafiltration.
Ultrafiltration membranes are similar to the microfiltration membranes except the pore size is 0.01 - 0.10 µm to remove very small particles. They remove all particles bigger than this pore size, including viruses and THM formation precursors with the aid of a pressure difference, pressing the contaminated water through the membrane. This provides an effective barrier for turbidity particles, bacteria and protozoan cysts. The operating pressure ranges between 5 and 30 psi and normally operates at ambient temperature. They are more expensive due to the smaller pore size. The finer the pore size, the more effective the membrane, and the more expensive it is.
Ultrafiltration has the ability to reduce the bacteria to Log-6 and viruses to Log-4 from drinking water without the risk of re-growth.
Nanofiltration membranes have 0.01 - 0.001 µm pore size and is capable of removing dissolved organic compounds that pass through micro and ultrafiltration processes. Since nanofiltration removes most multivalent cations and anions, it is used to soften water and remove disinfection by-product precursors, such as humic acid. Nanofiltration will also filter calcium and magnesium, making it useful for water softening applications. This type of filtration are primarily used in industry (chemicals, pharmaceuticals, etc) and rarely used for municipal water treatment.
The reverse osmosis pressure removes virtually all organic and inorganic contaminants from the water being treated. This process is very similar to distillation, in which only gases and low-molecular substances can pass through the membrane filter of reverse osmosis. This type of system operates at higher pressures than other membrane processes and therefore requires more energy, which drives up the cost of using this type of technology.
Reverse osmosis (RO) membranes are made of two basic materials:
Over time, the RO membranes will reduce efficiency due to fouling and compaction. Fouling is when the particulates block the flow through the membrane while compaction is the result of process pressure. The high pressures used for the reverse osmosis process can cause the pores of the membrane to collapse closed over time. The compaction rate is higher when operating temperatures and pressures are higher. It is possible to clean these fouled membranes with acid, but often times they must be replaced once they are fouled.
There are certain flags that may arise indicating the reverse osmosis membrane may need to be cleaned, including:
Sometimes we are beyond cleaning and may require a shut down. Certain conditions will indicate if a shutdown is necessary, including:
Problems that may occur during reverse osmosis operation include:
There are four different membrane configurations used in reverse osmosis, including:
Only spiral-would and hollow-fiber membranes are used in treating drinking water, so we will only discuss those two configurations here.
Most reverse osmosis membranes are of this configuration. Spiral-wound membranes are made with two flat sheets of membrane material separated by porous sheets. These membrane layers are sealed on three sides, with product water leaving the cartridge through the open end after being forced through the membrane due to the pressure differential.
Pleated membranes were specifically designed to meet the requirements of the Interim Enhanced Surface Water Treatment Rule (IESWTR) and provide a barrier against Cryptosporidium. These membranes are used on waters that are of good quality and have little fouling potential, including surface water plants and groundwater plants under the influence of surface water.
The pore size of the pleated membrane is 1 µm, which is smaller than Cryptosporidium oocysts. Each membrane module has 150 ft2 of surface area and is designed to operate at a flux of 430 gfd, which works out to about 45 gpm per module. Ten modules are combined in parallel to create a system capable of delivering 450 gpm of treated water.
This type of module is designed as a Cryptosporidium barrier and is not able to be backwashed. Instead, they are replaced when the differential presssure becomes excessive across the membrane. This type of filter is only used when the total suspended solids is < 0.3 mg/L.
The hollow-fiber membranes consist of a cartridge containing thousands of fibers that surround the feed water distribution core. The fibers are laid in the shape of a "U" within the bundle and the ends are enclosed in the end sheet. Water flows from the outside of the fiber into the middle where it then flows out as product water.
Feedwater for Reverse Osmosis
Since RO filters cannot be backwashed, it is important to remove any particulates that may cause membrane fouling before the feed water enters the RO membrane assembly. Pretreatment also plays a role in extending the life of RO membranes.
Surface water usually requires more pretreatment and monitoring than groundwater since it is exposed to outside elements. Pretreatment for reverse osmosis filtration can include:
To help prevent calcium and magnesium carbonate scaling on the membrane, sulfuric acid is usually added to the feed water. Cartridge type filters are often installed prior to the reverse osmosis unit to remove any small turbidity particles that were not removed by other pretreatment processes.
The recovery rate, or amount of treated water, is directly related to the pressure applied to the unit. Higher recovery rates require higher operating pressures. Although these systems can operate at pressures as low as 50 psi, most utilities operate between 300-600 psi to maximize water recovery.
One of the biggest downfalls to RO membranes is the amount of reject water produced and how to dispose of it. Reverse osmosis systems can reject as much as 50% of the water treated. This reject water has a higher solids concentration than the original water and may not be suitable for sanitary sewer disposal because it may cause problems at the wastewater treatment plant. If the reject water can't be sent to the sanitary sewer, other options include:
Reverse Osmosis Post-treatment
After water has been treated through reverse osmosis, there are additional treatment processes that may be necessary, including:
Electric-driven processes use electrical current to move ions across a membrane. There are two variations of the same proces:
Electrodialysis membranes use direct electric current to separate dissolved electrolytes from the solution. The current transports the ion contaminants through a membrane from the lower concentration to the higher concentration.
This process simply reverses the current applied through electrodialysis. The current is reversed to cause the ion flow to be reversed, which provides automatic flushing of scale-forming material from the membrane surface.
Although these two technologies are electric driven and innovative, they have not been widely used within the water treatment industry.
After membrane filtration comes adsorption involving activated carbon, which we will cover in the next lesson.
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Answer the questions in the Lesson quiz. You will need to log into Canvas to take the quiz. You may take the quiz 3 times, if needed, and an average will be taken from your attempts for final grade calculation.