Frequently Asked Questions On Reverse Osmosis

Today, more and more people in highly-urbanized places are choosing reverse osmosis (known also as R/O) to purify their water. However, there are people still in the dark about water purification, filtration, and the like. The following are the answers to some frequently-asked questions on reverse osmosis.

What is reverse osmosis? 

A. Reverse osmosis is a process where water is stripped of minerals and other impurities by forcing it to pass through a semi-porous membrane using pressure. It is the opposite of osmosis, the natural process where water seeps through a semi-porous membrane where liquid is of higher concentration.

What comes out of the reverse osmosis process is fresh, clean water ready for use.

Q. How does industrial reverse osmosis differ from those units used in homes?

A. None, except that the materials used are of industrial strengths for bigger institutions. The big systems usually use spiral wound membranes in high pressure containers. These provide larger surface areas.

Q. Is any pretreatment required?

Ideally, yes. Water should have very low silt (solids) content to keep the membranes from plugging up. This is done by way of pre-filters that remove these solid sediments.

Other TDS (total dissolved solids) like chlorine have to be removed by active carbon filters because it attacks the main R/O membranes. Some, like calcium and magnesium, chokes the membranes, and have to be removed at pretreatment.

Q. How much pressure is required to purify water?

The pressure needed is dependent on the concentration of the sediments and salts on the feedwater (the water to be purified), which is in turn dependent on the pressure from the source.

Q. How pure will the water be?

Purity is determined by two things. One is the reject ratio of the membrane which is pegged at 92% up to 99.5%. The other is the type of TDS (total dissolved solids) present in the feedwater.

However, through length of use, the efficiency (and life span) of the membranes shortens, and leakages occur over time.

Q. How do I clean a system?

Water treated with a cleaning agent is re-circulated on the high pressure side of the system for an hour or so. Then, the membrane is flushed to drain.

Small systems will have to shut down during cleaning, but in larger systems, the individual banks of membranes can be cleaned one by one.

Q. How much maintenance is involved with a system?

A properly set system, with a good pre-treatment in place, usually needs a one-hour cleaning routine once a month. Pre-filters can be checked weekly.

Q. How much does it cost to run a reverse osmosis system?

The cost is dependent on three areas: power, materials, and labor. Check your area on power and labor costs. (Labor is usually low since the system is more or less automated.) Materials are more or less the same everywhere.

Q. What about bugs (bacteria) growing in the water?

Water storage in R/O systems is optionally passed through an UV sterilization system to kill bacteria, usually during cleaning. It is good to have the tanks black or opaque to prevent algae growth.

Q. How long will my reverse osmosis membranes last?

R/O membranes usually last many years. However, they slowly start leaking ions after a time, and have to be replaced once ion levels are not acceptable. Some membranes have been reported in continued use for 20 years.

Comparing Filtration With Reverse Osmosis

Comparing filtration with reverse osmosis is like comparing two brothers with almost the same features, only with different ages perhaps. This is because the principle used by both systems is the same – filtering sediments out of the water.

Let us look first into reverse osmosis.

Reverse osmosis

Because of its present capacity of filtering some of the minutest substances on earth, the process of reverse osmosis is sometimes called hyper filtration. It allows the removal of particles as small as ions from a liquid solution.

The most common use for reverse osmosis process is in purifying water. It is used to remove salts and other impurities to restore the water’s color, taste, and its other properties.

The process is also used to purify other fluids such as ethanol and glycol by removing their contaminants and purify them for better functions.

The heart of the reverse osmosis process is the semi-permeable membrane that allows the fluid being purified to pass through while blocking the contaminants.

Another vital requirement to the system is the driving force that pushes the liquid through the membrane. The most common device used is a pump. The stronger the pressure needed, the bigger the driving force should be.

Nowadays, most reverse osmosis processes now incorporates the use of crossflow, the additional process where the membranes clean themselves during the operations.
As some of the fluid passes through the membrane the rest continues downstream, sweeping the rejected species away from the membrane.

To date, reverse osmosis is capable of straining bacteria, salts, sugars, proteins, particles, dyes, and other chemical constituents.

Filtration - How does it work?

Filtration involves water flowing through a granular bed of sand (or any other suitable filtering medium) at a low speed. The action permits the filtering media to retain most solid materials while permitting water to pass through.

To ensure adequate removal of unwanted particles, the process of filtration is usually repeated. This process is generally known as slow sand filtration. It is the oldest method of filtration but still in use up to this day.

The more modern filtration systems today use carbon as the filtering medium. The carbon is compressed into solid blocks. (This is in direct contrast to the old loose granular sand filters of old.) The carbon filters in solid blocks often include other media substances, and called multimedia filters. This new type of filter works together and in both ways, chemically and physically.

Physically, it duplicates the old work of the slow sand filters: blocking the passage of unwanted materials with molecular structures larger than that of water.

Chemically, it does additional work by the process of absorption. With it, the atomic charge of the carbon and other media encourages unwanted particles to abandon their bond with the water and attach themselves to the filter. The other media included in the filter are designed for more particles to bond with it.

Water is then directed to several stages of carbon and multimedia filters to ensure removal of more unwanted materials. The first removes the most concentrated chemicals (chlorine, etc.) and the other next stages are for the removal of smaller and more hard-to-get chemicals like pesticides.

Some notes on reverse osmosis and filtration

Carbon and multimedia filters possess the same purifying capabilities as reverse osmosis and distillation. All three – filtration, distillation, and reverse osmosis – are all able to remove dangerous chemicals.

The slow process of carbon and multimedia filters does not need expensive energy sources like distillation (heat) and reverse osmosis (force pump), thereby making it cost-effective. It wastes little water in the process, too.

Again, when comparing filtration with reverse osmosis, the issue might all boil down again to cost-effectiveness in maintenance and ownership of either of these water-purifying systems.

Understanding Reverse Osmosis

You probably have heard reverse process as a water purifying process for many times now. You should know that it is a very important and very effective way of purifying water. In general, the process is used not just in residential and commercial water filtration.

Reverse osmosis is the top method used when desalinating seawater so it could be converted into a form that could be commercially drank and consumed by people. In many industries, the process is also used when purifying liquids, wherein water serves as the undesirable impurity that should be taken out (like in the case of ethanol).

To better understand the process, it would be appropriate to review osmosis. Back in the days when you were studying general science, you have discovered that there is diffusion to balance things out. Diffusion happens in the air, while osmosis on water or liquid. The idea is that through osmosis, molecules in water would move to balance two water bodies with different concentration.

For example, if a cell lack concentration of a substance that is abundant in its external, osmosis would work so that molecules from the outside would pass through cell’s semi-permeable membrane to enter the cell. The process goes on until concentration on both the outside and inside of the cell is equal.

Through the years, science has also discovered that the natural process of osmosis could be effectively slowed, stopped, prevented, and even reversed. Reversal of the osmosis process could be possible only if there would be adequate pressure to be applied to the involved membrane coming from the more concentrated side. In some instances, the reverse osmosis is referred to or described as a process of filtration, especially when it is used to purify water.

There is a need to force the solvent in a region where there is higher concentration into getting through the semi-permeable membrane so that it could transfer without much hassle into the region of lower solute concentration. This force is often called osmotic pressure. Membranes used have denser barrier layer to facilitate better separation. Usually, such membranes and designed specifically to allow passage of water only and prevent passing of solutes like in the case of salt ions in seawater desalination. In general, much higher pressure is applied when doing reverse osmosis to purify salt water than used to purify brackish or fresh water.

Reverse osmosis as a water purification process has gotten so popular that many households these days are investing in setups and devices to facilitate the process. Household reverse osmosis devices or units are using much water due to low back pressure.

Though such systems are economical and are convenient, experts assert that they are not as effective as intended. Water recovered is only at 5% to 15%, so the household may need to use more water to be able to fill the desired and required amount. Many people could attest that such devices are yet to be improved to bolster practicality for household use.

The process of reverse osmosis is very promising. But scientists and investors are still underway to discover and develop more effective and efficient ways to prompt and facilitate the process. In the end, this could be a hope to be used and applied in areas where pure and safe drinking water is much of a problem.  

The Uses Of Reverse Osmosis

If necessity is the mother of invention, then, in essence, reverse osmosis is truly a daughter of necessity. It was initially developed by the U.S. Navy to produce drinking water through desalination (getting the salt out of sea water) for submarine crews. Today, the uses of reverse osmosis have branched out to other areas, including home use and uses in other industries.

Basically, reverse osmosis (often referred to as R/O) is a filtration technology that forces water into passing through a semi-porous membrane that filters out unwanted chemicals and solids. In desalination, the water is rid of salt and can be drunk.

In the version adapted for home use, R/O units are fitted with carbon and other mechanical filtration devices to produce highly purified water that tastes good.

The R/O process

Scientists essentially copied what Nature had been doing all the time, with a little modification by way of reversing the process. In Nature, osmosis is the natural process of water seeping through a semi-porous membrane into a solution that is of higher consistency.

In R/O, scientists force higher consistency water solutions (sea water, other water with dissolved minerals and other solids, etc.) through a membrane that strains out all these other non-essentials to produce pure water.

The goal is to provide fresh recycled water for human consumption, especially in progressive and highly-industrialized areas where the use of resources is excessive. Water is one vital resource that’s precariously getting short.

The R/O process is also used in other areas of industry. It is used to squeeze out water from ethanol and glycol so that these can be purified and used for fuel. In others, food concentrates (tomato juice, apple juice) are thickened by squeezing out the extra water.

Still in other industries, the process is also used to harvest dissolved metals (copper, nickel, chromium) and other particulates in liquids for use as metals plating or in the finishing processes.

Development

The research for R/O process began sometime in the 50s when a UCLA scientist first developed “artificial simulation bio selection osmosis membrane”. It was the first time an artificial membrane mimics the natural osmotic process and replicated in a laboratory.

In 1960, the first acetic acid fiber was used as R/O membrane and it was a breakthrough. To date, the U.S. government spent $4 billion in the development of the technique.

At that time, it replaced the only (and very expensive) method of fresh water production from sea water which was distillation.

NASA funded the costs of development to solve the drinking water consumption problems in space. Today, the R/O process is used to purify and recycle used water. In aircraft carriers and submarines, all the fresh water supply is taken from purified sea water.

Today, a home water system using the R/O process can provide 285 liters of drinking water, enough to fulfill the needs of a small office or a regular household.

Nowadays, the water purification systems for home use have three pre-filters. This first stage filters are designed to remove sediments and other solids from the water. The first stage activated carbon filter is used to remove organics, chlorine, and odors.

The osmotic membrane then removes most of the dissolved impurities (metals, chemicals, etc.) in the water. Finally, the last activated carbon filter is used to remove the remaining residue contaminants after the R/O membrane.

Care should be taken to replace old filters and other worn parts. This will ensure that the continued use of your reverse osmosis water filtration system is always in top condition.

What Is Reverse Osmosis

Much of man’s technological innovations today have been borrowed mostly from nature. One of them is a very important process called “reverse osmosis”. It is the opposite of the natural process called osmosis. What, then, is reverse osmosis and what does it do?

As man moves towards progress, water and the other natural resources of the world have become polluted and unsafe for use. Reverse osmosis is one of today’s important methods for processing water to make it clean and safe for use.

Osmosis and reverse osmosis

First, what is osmosis? Osmosis is a natural process where water of different concentrations and separated by a semi-permeable membrane will cause the water from the diluted solution to move into the more concentrated solution.

This is because this semi-permeable membrane (the osmotic membrane) allows only water to pass through and restricts most other elements.

In reverse osmosis, the opposite happens. Applying pressure, water from the more concentrated solution is forced out through the membrane into another container, in effect straining it from impurities. This is the process commonly used today to purify water.

The basis of the whole system is the semi-permeable membrane that acts as a strainer. This membrane is permeable only to water molecules. As the concentrated water (water with dissolved and solid particles of minerals, salts, etc.) is forced through the membrane, only water molecules can pass through the pores.

The most common artificial membranes used today are made from cellulose acetate, cellulose triacetate or aromatic polyamide resins. These are tough to withstand the high water pressures and can last for about two or three years before replacement.

These impurities that cannot pass through the membrane are composed of various mineral salts, heavy metals, other matter particulates, organic molecules, some bacteria, and even some viruses. These are rejected by the membrane based on their molecular or atomic weights.

The membrane removes these salts and other dissolved mineral elements, as well as other impurities like sugar, proteins, dyes, nitrates, and pesticides. All things considered, this improves the taste, color and other natural properties of water.

The process is also known to remove particles as small as ions or charged atoms. Today, it is also used to purify other fluids such as ethanol and glycol which passes through a membrane that rejects other ions and contaminants from passing through.

The most common force used in the procedure is pressure from pumps. The higher the pressure, the bigger the driving force.

In the old mechanical filter systems such as the standard carbon filtration, total dissolved solids (called TDS) cannot be removed. High concentration of TDS usually gives the water an objectionable taste, causes scale buildup in utensils and equipments, and causes poor quality in beverages and ice-making.

According to studies, reverse osmosis removes 95% to 99% of the total dissolved solids in contaminated water. Right now, it is considered one of the best technologies in producing clean water free from TDS and other contaminants and is therefore safe for human consumption.

Some problems

There are some problems, of course, that must be solved while using this system. The amount of contaminants, the size and type of equipment used and the amount of pressure are some factors that contribute to the buildup of materials on the membrane, making it ineffective.

Moreover, chlorine in the water causes damage to some membranes. To prevent this, manufacturers install a carbon pre-filter to reduce the chlorine, as well as sediment pre-filters to prevent other fine particles to clog the membrane. Which, of course, are added costs.

On the whole, however, reverse osmosis is the leading water filtration system used by homes and many industries. So far, it had justified its costs and surpasses expectations from health authorities with regards to safety and cleanliness.