FREQUENTLY ASKED QUESTIONS ABOUT REVERSE OSMOSIS (FAQ)
- What is the difference between water treatment and water purification?
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.
- Does that mean water purification is guaranteed to produce 100% pure water ?
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.
- What do you mean by almost all ?
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.
- How can you remove the small molecular impurities ?
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.
- Does that mean RO can purify any water ?
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.
- What kind of problems can you run into with RO ?
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.
- How can you avoid problems with RO membranes ?
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.
- How can you reverse plugging and scaling of RO?
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.