WHAT IS "REVERSE OSMOSIS" SYSTEM ?
Osmosis is a natural process by which water flows into plant and animal cells, through cellular membranes, also known as semi-permeable membranes or osmotic membranes. These membranes have a unique property that pure water can freely flow in either direction, but passage of salt and other dissolved impurities are inhibited. Living cells contain salty solutions, which allows them to absorb water from their natural surroundings, through the osmotic process.
Osmotic pressure is a unique, natural property of all solutions. The osmotic pressure of a solution depends on the concentration of the salt and other dissolved materials. Suspended impurities like silt and bacteria do not contribute to osmotic pressure.
Two chambers are separated by an osmotic membrane. Right-hand compartment has pure water in it. Left-hand compartment has salt solution in it. If left alone, pure water flows in the direction of the arrows from the pure water compartment, into the salt solution compartment. Pressure-head in the salt solution compartment continues to rise, until it reaches a value represented by the osmotic pressure of the solution. Then the flow of water stops.
As the cell absorbs water, it starts building pressure inside the cell. At the same time, the concentration of salt inside the cell decreases, so does its osmotic pressure value. The flow of water into the cell continues, until the pressure inside the cell equals the osmotic pressure value of the remaining salty solution inside the cell. The above diagram explains the concept of osmotic pressure.
Reverse Osmosis works on the same principle as osmosis, but in the reverse direction. If external pressure is applied to the above cell containing salty solution, then pure water is forced out of the cell and the salt is retained. As the water is extracted from the salty solution, its concentration rises, so does its osmotic pressure. The process continues until the osmotic pressure value of the remaining salt solution inside the cell equals the applied external pressure. Reverse osmosis membranes also hold back suspended impurities; such as, silt, colloidal particles and microorganisms by virtue of their ultra-fine pore size. Because reverse osmosis membranes hold back all kinds of impurities, reverse osmosis offers an ideal method of water purification. The diagram below explains the concept of reverse osmosis.
In the same chamber, divided by the osmotic membrane, if increasing pressure is applied on the salt solution compartment, in the direction of the arrow, then the first drop of pure water flows in the direction of the arrows from the solution compartment, into the pure water compartment, when the applied pressure equals the osmotic pressure value of the solution. To obtain any practical flow of water, the applied pressure must be much greater than the osmotic pressure.
WHY REVERSE OSMOSIS ?
There are a number of methods of water treatment, for example, media filtration, activated carbon filtration, water softening, and de-ionization. These methods of water treatment remove one impurity or a selected group of impurities from contaminated water. Unfortunately, water being a universal solvent, it seldom contains only one or a selected group of impurities. Typical natural water contains dozens of impurities from natural sources. Contaminated water may contain an unknown number of impurities, of unknown nature, from both natural and man-made sources including industrial pollution.
The term water purification refers to a process, which selectively extracts pure water from an impure solution, leaving all kinds of impurities behind, regardless of their source or their nature. This is quite different than water treatment described above. There are only three scientifically recognized methods of water purification. These are: Distillation, freeze-thawing, and reverse osmosis ("RO"). Of these, reverse osmosis offers the most practical and economical approach to water purification. The equipment is compact, easy to operate, and it is highly energy-efficient, in comparison with distillation and freeze-thawing equipment.
Whether your goal is to upgrade municipal water for medical treatment or to desalinate sea water for municipal uses, reverse osmosis is the process of choice. Blue Spring Corporation specializes in reverse osmosis technology and holds several patents in reverse osmosis applications
FREQUENTLY ASKED QUESTIONS ABOUT REVERSE OSMOSIS (FAQ)
- What is the difference between water treatment and water purification?
- Does that mean water purification is guaranteed to produce 100% pure water ?
- What do you mean by almost all ?
- How can you remove the small molecular impurities ?
- Does that mean RO can purify any water ?
- What kind of problems can you run into with RO ?
- How can you avoid problems with RO membranes ?
- How can you reverse plugging and scaling of RO?
Water contains a number of impurities, of both known and unknown character.
Treatment of water is designed to remove one specific impurity or a range of selected impurities of known character. For example, water softening treatment removes calcium and magnesium, which are the hardness-causing impurities. Or, de-ionization treatment removes ionic impurities; such as, sodium ion, chloride ion, copper ion. Neither water softening nor de-ionization removes bacteria or many industrial organic chemicals, which are not ionized. You must know what impurities need to be removed, before you can select the proper method of water treatment.
Water purification is a procedure that extracts pure water from impure water, leaving all kinds of impurities behind, regardless of their character. For example, reverse osmosis is able to remove: calcium, magnesium, sodium ion, chloride ion, copper ion, as well as bacteria, and vast majority of industrial organic chemicals that are not ionized. You don't have to know what impurities are present in the contaminated water. You know that the purified water will not contain the impurities.
No. Like any other scientific procedure, water purification procedure has limited efficiency. For example, reverse osmosis will remove 96-99% of elemental ions, 99.5+% of almost all other impurities.
Reverse osmosis removes impurities by two distinct mechanisms. One is based on resistance to passage of ions, due to their electrical charge. This mechanism is responsible for removal of ionic impurities. Even the smallest molecules are rejected if they have ionic charge. The efficiency of removal by this mechanism is in the range of 96-99%, but this depends on the particular membrane and the ionic charge. The other mechanism of impurity removal is based on the ultrafiltration effect, in which the small pores of the reverse osmosis membranes act like molecular filters. The cut-off molecular size is approximately 14-20 nanometers. Any impurity whose molecular size is above the cut-off point will be rejected almost completely. Any impurity whose molecular size is below the cut-off point will pass through almost totally. Few organics have molecular size below 14-20 nanometers. Examples are: methanol, formaldehyde, formic acid, and ethanol. Many gases like carbon dioxide, hydrogen sulfide, methane, and ethane also have small molecules that will pass through reverse osmosis.
One of the most efficient method of removing small molecular impurities is activated carbon treatment. Usually, the carbon treatment precedes reverse osmosis, when such small molecular impurities are present in the raw water.
Reverse osmosis, in combination with activated carbon treatment, provides one of the most definitive means of water purification.
In theory, yes. In practice, no. Certain impurities in water will either plug the RO membranes or they will destroy them chemically. For example, strong oxidizing agents; such as, chlorine and ozone will immediately destroy standard, polyamide-type TFC RO membranes. Water containing these substances must be pre-treated to remove them. Similarly, silica in colloidal form should be avoided, because it will solidify on the surface of the RO membranes, plugging the pores permanently within a short period of time. Certain cationic polymers that are used in water treatment and waste water treatment will also adhere to the RO membranes, thereby reducing their output.
There is also osmotic pressure limitation for RO units. Present day membranes can handle at the most 1,500 psi (100 atm) applied pressure. For practical purposes, the operating pressure limit is 1,100 psi (75 atm). The dissolved impurities in water exert a certain amount of osmotic pressure. If the value of the osmotic pressure exceeds the operating pressure, then water flow will cease and actually, may reverse. This limit corresponds to about 6-8 weight % for salts like sodium chloride and about 15-20 weight % for organics like sugars.
Design of the RO equipment plays an important role in what type of impurities the RO system will handle without problems.
There are two kinds of problems; (1) lowering of output flow, in other words, loss of capacity, and (2) lowering of impurity rejection, in other words, loss of purification efficiency.
Loss of capacity can be due to plugging of the membranes, or due to high impurity concentration in the feed water. Plugging of the membranes can be of reversible type, which is due to normal silt, organic fouling, scaling due to hardness, etc. Or, it can be irreversible type, which is due to the presence of incompatible chemicals in the feed water.
Loss of purification efficiency can be due to scaling of the membranes with certain impurities, or due to chemical damage to the membranes. Scaling can be reversible or irreversible.
First of all, make an inventory of chemicals that are likely to be present in your raw water that will be purified. Have your raw water analyzed for the major constituents and also for the incompatible chemicals.
ext, consult the supplier of the RO equipment regarding the compatibility of the RO membrane with the anticipated impurities in the water. Note: The compatibility of RO membranes with chemicals can be quite different for different types of membranes. Considerable expertise is required to make compatibility determination
Next, in doubtful cases, request the RO equipment supplier to make a trial run with the sample of water. The trial run should last about a week's operation, after which an evaluation can be made.
For removing silt, dirt, fouling by dead bacteria and other organics, use Blue Spring Cat. No. DT special detergent. In some cases, a mild solution of sodium hydroxide works better. Please consult Blue Spring Corporation.
For removing scales due to calcium, magnesium, and iron, use Blue Spring Cat. No. ZS-2 special de-scaling agent. The instructions for use come with the package. In some cases, a mild solution of hydrochloric acid works better. Please consult Blue Spring Corporation.