Hollow Fiber Membrane Technology at Work

Hollow Fiber Membrane Technology

Hollow-Fiber membranes offer a simple way to purify water by using a non chemical, physical barrier to effectively remove microbiological contaminants from the water. Unlike other filtration methods, Hollow Fiber membranes do not remove the beneficial minerals from the water, waste a great deal of water during the filtration process or require the holding tanks or pumps commonly used in low pressure applications. With the creation of the DownPour Filtration System, OutbackWater has taken HFM technology to a new level.

Hollow-fiber membranes or HFM are micro-thin tubules (resembling long, hollow, spaghetti-like strands) with semi/porous walls. The tubules or fibers are about 1mm thick, with numerous 0.2 micron (or smaller) pores that allow water to flow freely through their fibrous walls.  For perspective, these tiny 1.m tubules are approximately 600-800 times thinner than a human hair.

Brief History of Hollow Fiber Membrane Technology

It is commonly believed that Ultra filtration or Hollow Fiber Membrane technology (HFM) was first developed as early as 1830. But the true emergence for commercial use was during the 1960’s when the technology began to be applied as a method of waste water reclamation and for desalination, gas separation, blood transfusion and cell culture. Over the last 50 years many improvements have been made in both the manufacturing and in the performance of HFM technology creating the ability for manufacturers to offer affordable, personal-use, water purification products.

How do Hollow Fiber Membranes Work?

Hollow fiber membranes are small hollow tubes that resemble a drinking straw (called fibers). Imagine the walls of the straw having thousands even millions of tiny holes or pores. These holes or pores are of a uniform size which allow only clean water to pass from the outside of the tube (straw) to the inside of the tube, physically excluding the passage of microbiological contaminants such bacteria, cysts and virus. A good analogy would be a fly (the microorganism) trying to pass through a closed screen door (the walls of the Hollow Fiber membrane).

The standard pore size of a Hollow Fiber membrane is 0.2 microns although they can be produced with a practical pore size as small as 0.01 microns. The advantage of a larger pore sized membrane is its hydrophilic nature - the larger pore size causing less resistance to the flow of water - making it better suited for specific applications such as a gravity-powered^TM system or straw type filter. Products using membrane material with a 0.01 micron rating require a higher degree of pressure to drive the water through the filter, but DO have the added benefit of addressing virus removal.

How are Hollow Fiber Membranes (HFM) Made?

Hollow fiber membranes are commonly produced using synthetic rather than natural polymers. Synthetic polymers are derived from petroleum products, and are man-made. Nylon, polyethylene, polyester, Teflon, and epoxy are examples of synthetic polymers while natural polymers (polymers extracted from nature) and are often water-based, making them ill-suited for water treatment devices. A few examples of naturally occurring polymers are silk, wool, DNA, cellulose and proteins.

The method used to produce Hollow Fiber membranes is based on the type and molecular weight of synthetic polymer used. The most common production method is referred to a spinning. There are four general types of spinning.

Melt spinning - in which a thermoplastic polymer is melted and extruded through a spinneret, into air, and subsequently cooled.

Dry spinning - in which a polymer is dissolved in an appropriate solvent then extruded through a spinneret into air.

Dry-Jet Wet Spinning - in which a polymer is dissolved in an appropriate solvent then extruded into air and a subsequent coagulant (usually water).

Wet spinning - in which a polymer is dissolved and extruded directly into a coagulant (usually water)

In simple terms, molten polymer is pulled through a specialized apparatus (an extruder) so that a very thin "tube" forms (hollow cylindrical body structure). The tubule material is of variable consistency – some areas are elastic (capable of stretching) and in some areas are crystalline (not capable of stretching).

To achieve effective water filtration it is critical that uniform micro-pores are produced and that their sizing is controlled. Any change in the size of the extruder, adjustment of processing temperature, or a change in the chemical composition of the polymer can affect the pore size and thickness of the membrane.

For those looking for a more in-depth explanation:

Common to each of the four methods of spinning is the use of a spinneret, (a device used to extrude a polymer solution or polymer melt to form fibers.^[1] Streams of viscous polymer exit via the spinneret into air or liquid leading to a phase inversion which allows the polymer to solidify), the spinneret contains a needle through which the solvent is extruded and an annulus (a ring-shaped object, structure, or region) through which a polymer solution is extruded. As the polymer is extruded through the annulus of the spinneret, it retains a hollow cylindrical shape. As the polymer exits the spinneret, it solidifies into a membrane through a process referred to as phase inversion. The properties of the membrane, such as average pore diameter and membrane thickness can be fine tuned by changing the dimensions of the spinneret, temperature and composition of the polymer and solvent solutions. Extrusion of the polymer and solvent through the spinneret can be accomplished either through the use of gas-extrusion or a metered pump. Some of the more common polymers used in the production of Hollow Fiber Membranes (HFM) are cellulose acetate, polysulfone, polyethersulfone, and polyvinylidene fluoride.

 

How Are Hollow Fiber Membrane Filters Made?

The HFM strands are cut to a specific length, gathered in bundles and folded into a 180 degree bend, creating the shape of a loop. This loop shape allows both the exit and the entrance ends of the membrane to be bundled next to each other on one end of the loop. The loop is then inserted into the filter body, which is a premeasured tube, open on both ends. The open ended portion of the loop is potted in a polymer binder/glue, securing it to the end of the tube. This binds the bundle together causing it to maintain a uniform shape and seals it into the filter body. Once the binder/glue has dried the open ends of the membrane are evenly trimmed for uniformity and ease of filter cap installation.

The untreated water now enters the filter from the closed end of the looped bundles, passing through the micro pores in the walls of the HFM and exiting through the tubule openings as clean water.

This is referred to as an "outside–in" flow pattern, which increases the membrane filter's surface area, increasing the products permeability, and reducing premature failure caused by excess debris being trapped on the inside of the membrane. In the case of the HFM Filter, the "outside-in" flow design also creates a product that can be "back-flushed" by flushing the filter with clean water via a reverse flow. Back flushing the filter in this way helps clears it of debris and enables the user to reclaim most of the filter's original performance.

Hollow Fiber Membrane Technology used in Kidney Dialysis Treatment

One of the highly effective uses of Hollow Fiber membranes is in the application of Kidney Dialysis Treatment. Below is a SIMPLE explanation of How Kidney Dialysis works.

Wastes in the bloodstream are primarily removed through a natural process called diffusion. Diffusion happens when fluids are on both sides of a semi permeable membrane, like a tea bag. Semi permeable means the membrane has tiny holes that let tiny particles through, like the tea, but the larger particles like the tea leaves cannot pass.   In dialysis process, the blood cells and protein are too large to pass through the membrane, so they stay in the body during the treatment.

The dialysis process is generally accomplished through one of two membranes:

In peritoneal dialysis (PD), the membrane is the peritoneum, which lines the abdomen.

In hemo dialysis (HD), the membrane is thousands of HOLLOW PLASTIC FIBERS (tubes) called a dialyzer.

On one side of the membrane is the blood. On the other side of the membrane is a fluid called dialysate, or bath. Wastes from the blood diffuse through the membrane and into the bath. Once used, the bath is thrown away.

How does dialysis remove only the wastes? The secret is in the bath.

In nature, diffusion goes on until fluids on both sides of a membrane have the same concentration. Referring back to our tea bag example, until the tea is as strong as it can get - which takes time.

In dialysis the process can be speeded up by creating a gradient (pressure), making the fluid on one side of the membrane stronger (or more concentrated) than on the other side.

Remember that blood has high levels of wastes, so the bath will be created with no wastes. The gradient forces the wastes from the blood to move across the membrane and into the bath, where the both the bath and the waste are discarded.

So How Does a Hollow Fiber Membrane Dialyzer Work?

A dialyzer for HD is a collection of thousands of hollow plastic fiber, each hundreds of times thinner than a hair. The fibers are gathered into a bundle and placed in a clear plastic tube (cylinder). At each end of the tube, the fibers are held in place by a potting material. Blood enters the dialyzer, through the red port on one end of the tube, passes through the inside of the hollow fibers, and leaves through the opposite end of the tube or the blue port. Dialysates enters through the blue port on the SIDE of the dialyzer, flow around the outside of the hollow fibers, and then leaves through the RED port on the side. The wastes from the blood are carried away in the used dialysate.

Drawing of a HFM Dialyzer

 

The Advantages in Using a Hollow Fiber Membrane Filter

Here are a few of the many benefits found in the use of Hollow Fiber Membrane technology

• Removal of microbiological contaminants such as bacteria, cysts and virus
• Does not remove beneficial minerals from the water
• Can be flushed to restore usefulness
• Uses physical filtration Vs chemical filtration (no chemicals are added to the water)
• Does not alter the taste of the water
• Is simple to install either as a point of use or point of entry device
• Can be used as a portable device during recreational activities or/and when traveling
• Is cost effective
• Requires no electricity, batteries or external power supply
• Generates no waste
• Requires no holding or water storage tanks
• Operates well in low water pressure application
• Offers a high flow Vs other membrane that operate using a drip process
• HFM has a long term proven track record as a successful method of water purification
• Offers a wide range of applications