Ultrafiltration

By Dr. Gil Dhawan

---

Ultrafiltration is a separation process using membranes with pore sizes in the range of 0.1 to 0.001 micron. Typically, ultrafiltration will remove high molecular-weight substances, colloidal materials, and organic and inorganic polymeric molecules. Low molecular-weight organics and ions such as sodium, calcium, magnesium chloride, and sulfate are not removed. Because only high-molecular weight species are removed, the osmotic pressure differential across the membrane surface is negligible. Low applied pressures are therefore sufficient to achieve high flux rates from an ultrafiltration membrane. Flux of a membrane is defined as the amount of permeate produced per unit area of membrane surface per unit time. Generally flux is expressed as gallons per square foot per day (GFD) or as cubic meters per square meters per day.

Ultrafiltration membranes can have extremely high fluxes but in most practical applications the flux varies between 50 and 200 GFD at an operating pressure of about 50 psig in contrast, reverse osmosis membranes only produce between 10 to 30 GFD at 200 to 400 psig.

---

Ultrafilter vs. Conventional Filter

Ultrafiltration, like reverse osmosis, is a cross-flow separation process. Here liquid stream to be treated (feed) flows tangentially along the membrane surface, thereby producing two streams. The stream of liquid that comes through the membrane is called permeate. The type and amount of species left in the permeate will depend on the characteristics of the membrane, the operating conditions, and the quality of feed. The other liquid stream is called concentrate and gets progressively concentrated in those species removed by the membrane. In cross-flow separation, therefore, the membrane itself does not act as a collector of ions, molecules, or colloids but merely as a barrier to these species.

Conventional filters such as media filters or cartridge filters, on the other hand, only remove suspended solids by trapping these in the pores of the filter-media. These filters therefore act as depositories of suspended solids and have to be cleaned or replaced frequently. Conventional filters are used upstream from the membrane system to remove relatively large suspended solids and to let the membrane do the job of removing fine particles and dissolved solids. In ultrafiltration, for many applications, no prefilters are used and ultrafiltration modules concentrate all of the suspended and emulsified materials.

---

Concentration Polarization

When a membrane is used for a separation, the concentration of any species being removed is higher near the membrane surface than it is in the bulk of the stream. This condition is known as concentration polarization and exists in all ultrafiltration and reverse osmosis separations. The result of concentration polarization is the formation of a boundary layer of substantially high concentration of substances being removed by the membrane. The thickness of the layer and its concentration depend on the mass of transfer conditions that exist in the membrane system. Membrane flux and feed flow velocity are both important in controlling the thickness and the concentration in the boundary layer. The boundary layer impedes the flow of water through the membrane and the high concentration of species in the boundary layer produces a permeate of inferior quality in ultrafiltration applications relatively high fluid velocities are maintained along the membrane surface to reduce the concentration polarization effect.

---

Recovery

Recovery of an ultrafiltration system is defined as the percentage of the feed water that is converted into the permeate, or:

	Where:	R =	Recovery
		P =	Volume of permeate
		F =	Volume of Feed

---

Ultrafiltration Membranes

Ultrafiltration Membrane modules come in plate-and-frame, spiral-wound, and tubular configurations. All configurations have been used successfully in different process applications. Each configuration is specially suited for some specific applications and there are many applications where more than one configuration is appropriate. For high purity water, spiral-wound and capillary configurations are generally used. The configuration selected depends on the type and concentration of colloidal material or emulsion. For more concentrated solutions, more open configurations like plate-and-frame and tubular are used. In all configurations the optimum system design must take into consideration the flow velocity, pressure drop, power consumption, membrane fouling and module cost.

---

Membrane Materials

A variety of materials have been used for commercial ultrafiltration membranes, but polysulfone and cellulose acetate are the most common. Recently thin-film composite ultrafiltration membranes have been marketed. For high purity water applications the membrane module materials must be compatible with chemicals such as hydrogen peroxide used in sanitizing the membranes on a periodic basis.

---

Molecular-Weight Cutoff

Pore sizes for ultrafiltration membranes range between 0.001 and 0.1 micron. However, it is more customary to categorize membranes by molecular-weight cutoff. For instance, a membrane that removes dissolved solids with molecular weights of 10,000 and higher has a molecular weight cutoff of 10,000. Obviously, different membranes even with the same molecular-weight cutoff, will have different pore size distribution. In other words, different membranes may remove species of different molecular weights to different degrees. Nevertheless, molecular-weight cutoff serves as a useful guide when selecting a membrane for a particular application.

---

Factors Affecting the Performance of Ultrafiltration

There are several factors that can affect the performance of an ultrafiltration system. A brief discussion of these is given here.

Flow Across the Membrane Surface. The permeate rate increases with the flow velocity of the liquid across the membrane surface. Flow velocity if especially critical for liquids containing emulsions or suspensions. Higher flow also means higher energy consumption and larger pumps. Increasing the flow velocity also reduces the fouling of the membrane surface. Generally, an optimum flow velocity is arrived at by a compromise between the pump horsepower and increase in permeate rate.

Operating Pressure. Permeate rate is directly proportional to the applied pressure across the membrane surface. However, due to increased fouling and compaction, the operating pressures rarely exceed 100 psig and are generally around 50 psig. In some of the capillary-type ultrafiltration membrane modules the operating pressures are even lower due to the physical strength limitation imposed by the membrane module.

Operating Temperature. Permeate rates increase with increasing temperature. However, temperature generally is not a controlled variable. It is important to know the effect of temperature on membrane flux in order to distinguish between a drop in permeate due to a drop in temperature and the effect of other parameters.

---

Performance of Ultrafiltration Systems

In high purity water systems, ultrafiltration is slowly replacing the traditional 0.2-micron cartridge filters. In Japan, practically all of the semiconductor industry follows this practice. An ultrafiltration membrane with a molecular-weight cutoff of 10,000 has a nominal pore size of 0.003 micron. When an ultrafiltration membrane is used instead of a 0.2-micron cartridge filter, particle removal efficiency is greatly improved. In addition, ultrafiltration membranes are not susceptible to the problem of bacteria growing through them, as is the case with 0.2-micron filters.

In a recent study (1), the performance of an ultrafilter was compared with that of a 0.2-micron cartridge filter. Some of these results are given in Table A.

The Ultrafilter used in the study had a molecular-weight cutoff of 100,000- (pore size 0.006 micron). As the requirements for the quality of high purity water become more stringent, we can expect to see an increasing use of ultrafiltration as a final filter.

Table A
Effectiveness of Ultrafiltration Particle Counts on Waters
Test Location	0.2 Micron Filtered DI Rinse Water	Unfiltered DI Rinse Water
1	200-300	20-30*
2	175-200	0-25
3	120	5
4	275	125*
*Plumbing after UF not upgraded

---

Operation and Maintenance

Ultrafiltration system operation and maintenance is similar to that of reverse osmosis systems. Daily records of feed and permeate flow, feed pressure and temperature, and pressure drop across the system should be kept. Membranes should be cleaned when the system permeate rate drops by 10% or more. Feed flow is critical to the operation of ultrafiltration systems. A drop in feed flow may be due to a problem in the prefilter (if any), with the flow control valve, or with the pump itself. When the system is shut down for more than two days, a bacteriocide should be circulated through the membranes. At restart, permeate should be diverted to drain until all the bacteriocide is removed.

---

Conclusions

Ultrafiltration will find an increasing application in the production of high purity water. The basic principles outlined here should help in the understanding and use of this technology.

---

Reference

1Gaudet, P.W. "Point-of-use Ultrafiltration of Deionized Water and Effects of Microelectronics Device Quality, American Society for Testing and Materials", 1984.

---

Glossary of Terms

Feed - Liquid to be treated by the ultrafiltration system.

Permeate - Liquid stream that passes through the membrane.

Concentrate - Remaining Portion of the liquid stream after the permeate has been

Recovery - Expressed as percentage, this defines the permeate rate as a fraction of the feed rate. Recovery provides an immediate measure of the maximum concentrations in the system and it affects permeate quality, pump size, power consumption and membrane fouling.

Flux - Permeate flow per unit area of membrane per unit time (gallons/ft²/day)

Rejection - Percent removal of a particular species by the membrane. Expressed as 1-CP CF where CP is the concentration in t he permeate, and CF is the concentration in the feed.

Flow Velocity - Rate at which the liquid goes along the membrane surface, expressed in length per unit time (ft/sec).

---

The Author

Dr. Gil Dhawan is the president of Applied Membranes, Inc. based in Vista, CA. Dr. Dhawan has been involved in the design, development, and marketing of ultrafiltration and reverse osmosis systems for the past 20 years.

Source by http://www.appliedmembranes.com/about_ultrafiltration.htm

TUBE SETTLER

Tube Settler Systems for Clarification

Tube settlers and parallel plates increase the settling capacity of circular clarifiers and/or rectangular sedimentation basins by reducing the vertical distance a floc particle must settle before agglomerating to form larger particles.

Tube settlers use multiple tubular channels sloped at an angle of 60° and adjacent to each other, which combine to form an increased effective settling area. This provides for a particle settling depth that is significantly less than the settling depth of a conventional clarifier, reducing settling times. Tube settlers capture the settleable fine floc that escapes the clarification zone beneath the tube settlers and allows the larger floc to travel to the tank bottom in a more settleable form. The tube settler's channel collects solids into a compact mass which promotes the solids to slide down the tube channel.

Why Tube Settlers?

Tube settlers offer an inexpensive method of upgrading existing water treatment plant clarifiers and sedimentation basins to improve performance. They can also reduce the tankage/footprint required in new installations or improve the performance of existing settling basins by reducing the solids loading on downstream filters. Made of lightweight PVC, tube settlers can be easily supported with minimal structures that often incorporate the effluent trough supports. They are available in a variety of module sizes and tube lengths to fit any tank geometry, with custom design and engineering offered by the manufacturer.

Tube Settler Access

During basin design, consideration should be given to operator access for cleaning and (for larger tanks) access to troughs/weirs, as required, within the tube settler area. Like any type of equipment, tube settlers will require periodic cleaning and maintenance. Basin walkway design or a protective covering above the tube settlers should be provided. A plastic or fiberglass grating is ideal because it not only allows access to the tubes, troughs, and weirs but also adds a protective layer to the tube settlers. Any type of grating must be designed to be structurally sufficient, does not hinder tube settler performance, and will not damage the tube settlers.

PVC MATERIAL TUBE SETTLERS

Under this section of the specification, the contractor shall install a tube settler system in approximately _____ square feet of the sedimentation basins as described herein and as shown in the contract drawings.

It is the intent of the specifications that the equipment, when installed in accordance with the drawings and the manufacturer's recommendations will permit efficient operation at the design flow of ____ GPM. Such equipment shall consist of a number of tube-like channels at least 4.0 square inches in cross sectional are being approximately (2' - 0") (3' - 0") long and orientated at approximately 60° from the horizontal to assure that the settled particles will be purged from the tubes by force of gravity alone.

The tube settler system shall be manufactured by a company that is regularly engaged in the manufacture of this product and who can demonstrate its knowledge of the technology, and its competence and expertise in the field.

All components of the system shall be provided by a single manufacturer. It shall be the sole responsibility of the contractor to coordinate all design, fabrication, assembly, and installation of the components in the clarifier. The manufacturer shall submit detailed drawings and design data on all components for the installation. Design calculations for the support structures shall be submitted and sealed by a registered engineer.

The tube settler modules shall be as manufactured by TUPSET, Inc. of Salt Lake City, Utah.

INFORMATION ON TUBE SETTLER - LAMELLA
Tube settler and plated lamella settling system is employed in the settling part of treatment plants.
Purpose:
This product is used in settling pools in order TO:
Reduce costs by reducing the pool size
Save from space
Reduce chemical dosage
Keep settling efficiency high, and obtain a more transparent water at the outlet

What is Tupset?
It is a system consisting of specially designed cellular plates consisting of hexagonal cells designed for the purpose settling and oil separating pools.

Where can we use TUPSET Tube Settler?

Residential and industrial waste water treatment plant settling pools, Potable water treatment plant settling pools, Oil separators, In rectangular or circular section reinforced concrete pools or sheet construction tanks, Tube Settler Lamella can be used, Shortly, in all treatment plants where a good settling efficiency is expected, For some waste waters, a settling system using TUPSET can become a stand-alone solution. Water parameters can be reduced to required criteria without needing to a chemical dosing, fast or slow mixing.

Why TUPSET Tube Settler?

It is flexible and b since it is in the form of hexagonal cellular honeycomb, Plates are joined by means of ultrasound welding, no different material is employed for installation, It is made of PVC with a smooth surface, sludge forming in the cells slides and very easily falls to the lower part, It has a high settling efficiency with its special design plate and cell size, Settling pools using TUPSET will have smaller size than conventional pools, So, there will be an advantage with regard to material price, construction and installation, transportation etc. No installation and post-installation problems, Can be constructed with UV protection against sun light, for outdoor pools, Operating range is from -5 to +60 degree Celsius. The product can produced at the required sizes, it can be cut angularly, A serious cost advantage, when compared to other alternatives.

Tube Settler Systems for Clarification

Tube settlers and parallel plates increase the settling capacity of circular clarifiers and/or rectangular sedimentation basins by reducing the vertical distance a floc particle must settle before agglomerating to form larger particles.

Tube settlers use multiple tubular channels sloped at an angle of 60° and adjacent to each other, which combine to form an increased effective settling area. This provides for a particle settling depth that is significantly less than the settling depth of a conventional clarifier, reducing settling times. Tube settlers capture the settleable fine floc that escapes the clarification zone beneath the tube settlers and allows the larger floc to travel to the tank bottom in a more settleable form. The tube settler's channel collects solids into a compact mass which promotes the solids to slide down the tube channel.

Why Tube Settlers?

Tube settlers offer an inexpensive method of upgrading existing water treatment plant clarifiers and sedimentation basins to improve performance. They can also reduce the tankage/footprint required in new installations or improve the performance of existing settling basins by reducing the solids loading on downstream filters. Made of lightweight PVC, tube settlers can be easily supported with minimal structures that often incorporate the effluent trough supports. They are available in a variety of module sizes and tube lengths to fit any tank geometry, with custom design and engineering offered by the manufacturer.

Advantages of Tube Settlers

The advantages of tube settlers can be applied to new or existing clarifiers/basins of any size: Clarifiers/basins equipped with tube settlers can operate at 2 to 4 times the normal rate of clarifiers/basins without tube settlers. It is possible to cut coagulant dosage by up to half while maintaining a lower influent turbidity to the treatment plant filters. Less filter backwashing equates to significant operating cost savings for both water and electricity. New installations using tube settlers can be designed smaller because of increased flow capability. Flow of existing water treatment plants can be increased through the addition of tube settlers. Tube settlers increase allowable flow capacity by expanding settling capacity and increasing the solids removal rate in settling tanks. The City of Westminster, CO used alum as their water treatment plant flocculant. After the installation of tube settlers, they cut the alum dosage from 30 ppm to 16 ppm, and the filter influent turbidity was still decreased by 25%. Since the filter influent turbidity had decreased, this enabled a savings of over 27% water used for filter backwashing." TUPSET - PVC Tube Settler Specification S P E C I F I C A T I O N S F O R T U B E S E T T L E R S Y S T PVC MATERIAL TUBE SETTLERS Under this section of the specification, the contractor shall install a tube settler system in approximately _____ square feet of the sedimentation basins as described herein and as shown in the contract drawings.

It is the intent of the specifications that the equipment, when installed in accordance with the drawings and the manufacturer's recommendations will permit efficient operation at the design flow of ____ GPM. Such equipment shall consist of a number of tube-like channels at least 4.0 square inches in cross sectional are being approximately (2' - 0") (3' - 0") long and orientated at approximately 60° from the horizontal to assure that the settled particles will be purged from the tubes by force of gravity alone.

The tube settler system shall be manufactured by a company that is regularly engaged in the manufacture of this product and who can demonstrate its knowledge of the technology, and its competence and expertise in the field.

All components of the system shall be provided by a single manufacturer. It shall be the sole responsibility of the contractor to coordinate all design, fabrication, assembly, and installation of the components in the clarifier. The manufacturer shall submit detailed drawings and design data on all components for the installation. Design calculations for the support structures shall be submitted and sealed by a registered engineer. The tube settler modules shall be as manufactured by TUPSET, Inc. of Salt Lake City, Utah.

MATERIALS AND FABRICATIONS

The materials of construction for the tube settler modules shall be Polyvinyl Chloride (PVC) and shall be b, rigid, resistant to chemical deterioration in an outdoor environment.

PVC MATERIAL
The resin utilized shall be prime grade, virgin, high impact, and have the following physical characteristics:

The tube settler module shall be built up of a number of corrugated sheets with alternately placed flat sheets. The individual sheets shall be solvent welded to provide b continuous bonds and thereby form a durable homogenous structure with each tube being continuos and imperforate, eliminating mixing currents that resuspend settled solids within the tube. The corrugated sheets shall have the inclined channel configurations molded integrally therein, so that all the tubes shall have the same angle of inclination when installed in the basin.

The finished (after thermoforming) thickness of the plastic used in the modules shall not be less than twenty (20) mils, and twenty-five mils for the corrugated and flat sheets respectively. The edges of the plastic shall be clean, smooth, and free from burrs caused by sawing or trimming the parts to minimize the possibility of sludge buildup or growth.

The completed modules shall have a minimum height of (19.5") (30") and be manufactured to the necessary widths and lengths to minimize field modification.

The baffles shall be constructed of corrugated fiberglass reinforced plastic with a structural (aluminum) (stainless steel) frame. The fiberglass panels shall have corrugations of 2.5" pitch and 1/2" depth and be 8 oz. material. The panels shall have the following nominal properties: Glass content 27% (ASTM-2584-86), Barcol hardness 40-60 (ASTM-2583-81), flexural strength 22,000 psi (ASTM-790-81), flexural modulus 900,000 psi (ASTM-D-790-81), tensile strength 11,000 psi (ASTM-D-638-82A).

The structural (aluminum) (stainless steel) frame shall be fabricated of (Type 6061-T6) (304SS) structural shapes. The panels shall be attached by means of 304 stainless steel fasteners to provide an adequate secure attachment.

The collection troughs shall be constructed of 1/4" minimum thickness fiberglass reinforced plastic (FRP) consisting of 24 oz. woven roving and a 10 mil surface matt. The trough shall have a gel coat on all exposed surfaces which is stabilized against attack by sunlight and all chemicals involved in the flow stream.

The FRP shall have a ultimate strength of not less than 16,000 psi, flexural strength of not less than 25,000 psi, flexural modules of elasticity of not less than 838,000 psi, and a glass content of not less than 30%. The above characteristics shall be in conformance with ASTM Standards D638 and D790.

Reinforcing and stiffener sections shall be designed to meet the load design characteristics.

The V-notch weir plates shall be constructed of 1/4" thick cast acrylic material precision machined to the dimensions shown on the contract drawings. The weir plates shall have an ultimate tensile strength of 10,000 psi or greater, flexural strength of not less than 15,000 psi, compressive strength of 17,000 psi, and a modulus of elasticity of not less than 450,000 psi. Test methods on these mechanical characteristics shall be evaluated in accordance with ASTM-D-638, D-695, and D-790 test methods.

The supports shall be constructed of fabricated structural (mild steel) (aluminum) (stainless steel) (fiberglass) members and shall have the proper surface preparation as noted in the Painting Section of the contract specifications.

All fastening hardware and expansion anchors shall be 304 stainless steel of adequate size to meet the design requirements.

STRENGTH

The tube modules shall be capable of supporting their own dead weight [ 2 ft - (3.5 PSF) (5.5 PSF), 3 FT - (5.3 PSF) (8.3 PSF) ] and an additional live load of 10 PSF uniformly distributed. The minimum design for the support structure shall include all dead loads resulting from tube modules, supports, troughs, weirs and baffles, and a uniformly distributed live load of 10 PSF or a moveable concentrated live load of 250 lbs.

INSTALLATION

The tube settler modules, troughs, weirs and baffles shall be provided in such configurations as shown on the contract drawings as to minimize, if not eliminate field trimming, cutting or modifying and are to be installed as shown on the drawings, and in accordance with the manufacturer's recommendations. Care shall be exercised by the contractor in unloading, transporting and placing tube modules. Tube modules shall remain in their containers and protected from environmental extremes until they are installed. Installers may walk on the tube modules to facilitate installation but planking should be used to protect edges of the modules from breakage.

SUBMITTED MATERIALS

The contractor shall submit detailed drawings showing the configuration of the tube settler modules and the manner in which they are to be installed. The installation drawings shall include detailed instructions to the installer regarding any field trimming or other modifications required to fit to the sedimentation basins. Shop drawings shall be fully dimensioned for construction

Source by www.tubesettler.com