Water: Frequently Asked Questions      

1. How Does Reverse Osmosis work

Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration. Osmosis is a special case of diffusion in which the molecules are water and the concentration gradient occurs across a semi-permeable membrane. The semi-permeable membrane allows the passage of water, but not ions (e.g., Na+, Ca2+, Cl-) or larger molecules (e.g., glucose, urea, bacteria). Diffusion and osmosis are thermodynamically favorable and will continue until equilibrium is reached. Osmosis can be slowed, stopped, or even reversed if sufficient pressure is applied to the membrane from the ‘concentrated’ side of the membrane. Reverse osmosis occurs when the water is moved across the membrane against the concentration gradient, from lower concentration to higher concentration. To illustrate, imagine a semi-permeable membrane with fresh water on one side and a concentrated aqueous solution on the other side. If normal osmosis takes place, the fresh water will cross the membrane to dilute the concentrated solution. In reverse osmosis, pressure is exerted on the side with the concentrated solution to force the water molecules across the membrane to the fresh water side.

How Reverse Osmosis works

Semi-permeable refers to a membrane that selectively allows certain species to pass through it while retaining others. In actuality, many species will pass through the membrane, but at significantly different rates. In RO, the solvent (water) passes through the membrane at a much faster rate than the dissolved solids (salts). The net effect is that a solute-solvent separation occurs, with pure water being the product. (In some cases, dewatering is desired to concentrate the salts).                              

2. How does Ion Exchange work

Ion exchange resins are polymers that are capable of exchanging particular ions within the polymer with ions in a solution that is passed through them. The synthetic resins are used primarily for purifying water, but also for various other applications including separating out some elements. A standard dual bed ion exchange unit consists of a strong acid cation resin that exchanges hydrogen for positively charged cations present. The second step is a week or strong basic anion resin that exchanges hydroxide for negatively charged anions present in the water stream. The hydrogen (H+) from the cation exchanger and hydroxide (OH-) from the anion exchanger create the HOH or deionized water to be reused in the process.

There are certain processes that generate an abundance of either a cation or an anion. If this occurs, a more selective weak acid cation can be used to exchange the multi-divalent heavy metals that are in abundance. This “pre-treating” of the heavy metals will assist in the exchanging of the mono-valent cations. A weak basic anion before the strong basic anion can be implemented to assist in pre-treating for hexavalent chrome.

The cation ion exchange resin are regenerated with an acid to exchange the hydrogen back onto the cation resin beads. The anion is regenerated with sodium hydroxide to exchange the hydroxide back onto the anion resin beads.                                          

3. How does Microfiltration work

Cross-flow microfiltration (CFMF), is a process for separation of larger size solutes from aqueous solutions by means of a semi-permeable membrane passed over the filtration medium. Clear liquid permeates the filtration medium and is recovered as the permeate, while the solids accumulate at the filtration barrier to form a fouling layer, or cake. The cake, constituting an increase in hydraulic resistance, decreases the permeate flux. However, the tangential suspension flow tends to limit the growth of the cake. Thus, after an initial rapid increase in cake thickness, cake growth ceases, and the cake thickness becomes limited to some steady-state value.

How does Microfiltration work

Correspondingly, after an initial rapid decrease, the permeate flux levels off and either attains a steady-state, or exhibits a slow, long-term decline with time. Microfiltration is suited to separate larger sizes, such as suspended solids, particulates, and microorganisms. This is accomplished because microfiltration membranes are thought to act like a physical seive. The membranes are highly porous and have discernible pores even when the surface skins are asymetric. Therefore, the separation is based mainly on size. Membrane material is usually made up of ceramics, teflon, polypropylene, or other plastics.                                          

4. How does Ultrafiltration work

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.

How does Ultrafiltration work

Generally flux is expressed as gallons per square foot per day (GFD) or as cubic meters per square meters per day. Ultrafiltration technology has a high removal cabability for bacteria and most viruses, colloids and silt (SDI). The smaller the nominal pore size, the higher the removal cabability. Most materials that are used in UF are polymeric and are naturally hydrophobic. Common polymeric materials used in UF include: Polysulfone (PS), Polyethersulfone (PES), Polypropylene (PP), or Polyvinylidenefluoride (PVDF). Although these materials can be blended with hydrophilic agents, they can reduce the membranes ability to be cleaned with high strength disinfectants such as hypochlorite that impacts removal of bacterial growth.                                          

5. How Nanofiltration works

Nanofiltration Membranes are thin film composite membranes. They have the unique characteristic of rejecting, or defractionation of divalent ions such as Nickel, Copper, Chrome & Cobalt. Nanofiltration Membranes are used on lower system operating pressures because they have very high membrane flux (permeate flow per unit of membrane area).

How Nanofiltration works

A unique characteristic of the Nanofiltration family of thin film composite membranes, is its resistance to fouling. Particularly in acid streams, the membrane requires minimal down time for cleaning. The low molecular weight cut off range of 200-300 makes nanofiltration extremely effective in removing contaminants from Acids & Bases that enable reuse of the commodity.

6. What is pH Neutralization?

The following section describes pH adjustment and the different methods to neutralize pH ranges in solutions.

A brief review of the definition of pH, the pH scale, and some of the chemistry involved in pH Adjustment systems is provided below. For some this may be trivial, yet for many others this may be useful. The information provided below is typical of the background information we provide in our training classes / seminars.

By definition, pH is the measure of free hydrogen activity in water and can be expressed as:

pH= -log[H+]

In more practical terms (although not technically correct in all cases) pH is the measure of acidity or alkalinity of water. Measured on a scale of 0-14, solutions with a pH of less than 7.0 are acids while solutions with a pH of greater than 7.0 are bases. In very simple terms bases are used to neutralize acids, while acids are used to neutralize caustics (the term caustic and base, although not truly synonymous, are often used interchangeably). The by-products are normally salts (which may or may not be soluble) and water.

7. What is ZLD (Zero Liquid Discharge)?

Zero Liquid Discharge is a term used to describe the elimination of wastewater discharge to a sanitary sewer or for offsite disposal. A combination of treatment technologies such as reverse osmosis, ion exchange and closed (to atmosphere) evaporators can be used to recover nearly 100% of your wastewater for reuse as process waters, which helps save on disposal costs. LT Technologies selected recycled technologies can also concentrate hazardous solutions to as little as 5% of your original volume, which will further reduce waste and/or chemical disposal costs.

8. What is (UV) Ultraviolet?

Ultraviolet (UV) light is invisible radiation within a range of the solar spectrum.  UV is similar to the wavelengths that are produced by visible light, but much shorter.  UV radiation is measured in millionths of a millimeter, i.e., Angstrom units (one Angstrom unit wave-length equals one hundred-millionth of a centimeter), and like visible light, it primarily has a surface effect.

Ultraviolet lamp radiation of 2537-Angstrom units (or 254 nanometers) wavelength must hit the microorganism to inactivate it, and each microorganism must absorb a specific amount of energy to be destroyed.

Proteins and nucleic acid, which all microorganisms contain as their main constituents, absorb UV radiation energy.  After absorption, the UV energy destroys or inactivates the DNA (deoxyribonucleic acid), thus preventing the microorganisms from reproducing.

Sterilization of water implies that all life, i.e., bacteria, mold, virus, algae, and protozoa, are destroyed.  Table I gives the absolute amount of UV necessary to kill many of the common types.  Our systems can also supply an 1849A (185NM) ultraviolet lamp that produces ozone (03) disinfection residuals, and in most cases this lamp interchanges with our standard 2537A ultraviolet lamp.

Complete sterilization is not necessary for the production of potable water.  However, the water must conform to the drinking water standards of the EPA or those of the agency governing your supply. Normally, the water must contain less than 2.2 coliforms per 100 ml to be considered safe to drink. The coliform groups of microorganisms are generally associated with fecal matter and indicate that pathogenic (disease-causing) organisms, such as typhoid, may be present.

9. What is the sizing of Ultraviolet Liquid Purification Equipment?

The various factors that must be considered were discussed above.  Assuming a proper voltage source, the ultraviolet liquid purifier can be sized properly if the following are known:

(a) Peak flow rate required in gpm, gph or gpd.
(b) Transmission and physical make-up (absorption coefficient) of the liquid to be treated.
(c) Ultraviolet energy level required for microorganism destruction

10. What is the USP and USP Water?

The United States Pharmacopeia Convention is a private, not-for-profit organization that sets standards for drugs, devices and diagnostics. It publishes two compendia (summary documents). The US Pharmacopeia (USP) contains standards for drug products. The National Formulary (NF) sets standards for drug excipients (inert substances used as carriers or dilutants). The current editions of these standards are USP 23 and NF 18.

The USP monograph lists two waters that are prepared in bulk form: Purified Water (PW), often called USP Purified Water to distinguish it from other purified waters, and Water for Injection (WFI). Purified Water is described in the USP 23; Purified Water is water obtained by distillation, ion-exchange treatment, reverse osmosis, or other suitable process. It is prepared from water complying with the regulations of the U.S. Environmental Protection Agency (EPA) with respect to drinking water. It contains no added substances.

The method of preparation of the various grades of reagent water determines the limits of impurities.

USP 23 Standards

CONDUCTIVITY: 1.3 uS/cm @ 25 C

11. What is A.S.T.M. ELECTRONICS GRADE WATER?

Type I grade of reagent water shall be prepared by distillation or other equal process, followed by polishing with a mixed bed of ion exchange materials and a 0.2-micron membrane
filter. Feedwater to the final polishing step must have a maximum conductivity of 20 microS/cm at 298K (25°C).

Type II grade of reagent water shall be prepared by distillation using a still designed to produce a distillate having a conductivity of less than 1.0 [mu]S/cm at 298 K (25C). Ion
exchange, distillation, or reverse osmosis and organic adsorption may be required prior to distillation if the purity cannot be attained by single distillation.

Because distillation is a process commonly relied upon to produce high purity water, the levels specified for Type II reagent water were selected to represent the minimum quality of water that a distillation process should produce.

Type III grade of reagent water shall be prepared by distillation, ion exchange, reverse osmosis, or a combination thereof, followed by polishing with a 0.45-micron membrane filter.

12. What are the grades of Laboratory (reagent) waters?

Grade 1
Essentially free from dissolved or colloidal ionic and organic contaminants. It is suitable for the most stringent analytical requirements including those of high performance liquid chromatography (HPLC). It should be produced by further treatment of grade 2 water for example by reverse osmosis or ion exchange followed by filtration through a membrane filter of pore size 0.2µm to remove particle matter or re-distillation from a fused silica apparatus.
Grade 2
Very low inorganic, organic or colloidal contaminants and suitable for sensitive analytical purposes including atomic absorption spectrometry (AAS) and the determination of constituents in trace quantities. Can be produced by multiple distillation, ion exchange or reverse osmosis followed by distillation.
Grade 3
Suitable for most laboratory wet chemistry work and preparation of reagent solutions. Can be produced by single distillation, by ion exchange, or by reverse osmosis. Unless otherwise specified, it should be used for ordinary analytical work.                                          

13. What is Ion Exchange?

Ion exchange resins are polymers that are capable of exchanging particular ions within the polymer with ions in a solution that is passed through them. The synthetic resins are used primarily for purifying water, but also for various other applications including separating out some elements. A standard dual bed ion exchange unit consists of a strong acid cation resin that exchanges hydrogen for positively charged cations present. The second step is a week or strong basic anion resin that exchanges hydroxide for negatively charged anions present in the water stream. The hydrogen (H+) from the cation exchanger and hydroxide (OH-) from the anion exchanger create the HOH or deionized water to be reused in the process.

There are certain processes that generate an abundance of either a cation or an anion. If this occurs, a more selective weak acid cation can be used to exchange the multi-divalent heavy metals that are in abundance. This “pre-treating” of the heavy metals will assist in the exchanging of the mono-valent cations. A weak basic anion before the strong basic anion can be implemented to assist in pre-treating for weakly basic anions.

The cation ion exchange resin are regenerated with an acid to exchange the hydrogen back onto the cation resin beads. The anion is regenerated with sodium hydroxide to exchange the hydroxide back onto the anion resin beads.

Process and Water also designs, sells and services innovative EDI (electro-deioniziation, CDI) technologies for high purity water requirements.  

Download EDI datasheet (PDF)

Download DI Water datasheet (PDF)

14. What is pH?

By Definition pH is the measure of free hydrogen activity in water and can be expressed as:

pH= -log[H+]

In more practical terms (although not technically correct in all cases) pH is the measure of acidity or alkalinity of water. Measured on a scale of 0-14, solutions with a pH of less than 7.0 are acids while solutions with a pH of greater than 7.0 are bases. In very simple terms bases are used to neutralize acids, while acids are used to neutralize caustics (the term caustic and base, although not truly synonymous, are often used interchangeably). The by-products are normally salts (which may or may not be soluble) and water.

pH adjustment system is to adjust the pH of the process stream into the defined acceptable discharge range. In the case of an acid neutralization, caustic (NaOH) is added to the effluent stream to pH Neutralize the solution. This neutralization, or titration can be expressed as follows:

HCl + NaOH ==> NaCl + H2O

Download datasheet (PDF)

 15. What Types of Purified Waters ARE there?

USP Purified Water: The United States Pharmacopeia Convention is a private, not-for-profit organization that sets standards for drugs, devices and diagnostics. It publishes two compendia (summary documents). The US Pharmacopeia (USP) contains standards for drug products. The National Formulary (NF) sets standards for drug excipients (inert substances used as carriers or dilutants). The current editions of these standards are USP 23 and NF 18.

The USP monograph lists two waters that are prepared in bulk form: Purified Water (PW), often called USP Purified Water to distinguish it from other purified waters, and Water for Injection (WFI). Purified Water is described in the USP 23; Purified Water is water obtained by distillation, ion-exchange treatment, reverse osmosis, or other suitable process. It is prepared from water complying with the regulations of the U.S. Environmental Protection Agency (EPA) with respect to drinking water.

It contains no added substances.

The method of preparation of the various grades of reagent water determines the limits of impurities.

USP 23 Standards

TOC: 500 ppb Carbon
CONDUCTIVITY: 1.3 uS/cm @ 25 C
0.10 uS/cm @ 20 C
BACTERIA Pure Water: 100 CFU/ml
Water For Injection: 10 CFU/100 ml
BACTERIAL ENDOTOXINS Water For Injection: 0.25 USP EU/ml

A.S.T.M. ELECTRONICS GRADE WATER: Type I grade of reagent water shall be prepared by distillation or other equal process, followed by polishing with a mixed bed of ion exchange materials and a 0.2-micron membrane filter. Feedwater to the final polishing step must have a maximum conductivity of 20 microS/cm at 298K (25°C).

Type II grade of reagent water shall be prepared by distillation using a still designed to produce a distillate having a conductivity of less than 1.0 [mu]S/cm at 298 K (25C). Ion exchange, distillation, or reverse osmosis and organic adsorption may be required prior to distillation if the purity cannot be attained by single distillation.

Because distillation is a process commonly relied upon to produce high purity water, the levels specified for Type II reagent water were selected to represent the minimum quality of water that a distillation process should produce.

Type III grade of reagent water shall be prepared by distillation, ion exchange, reverse osmosis, or a combination thereof, followed by polishing with a 0.45-micron membrane filter.

Grades of Laboratory (reagent) waters:

Grade 1

Essentially free from dissolved or colloidal ionic and organic contaminants. It is suitable for the most stringent analytical requirements including those of high performance liquid chromatography (HPLC). It should be produced by further treatment of grade 2 water for example by reverse osmosis or ion exchange followed by filtration through a membrane filter of pore size 0.2µm to remove particle matter or re-distillation from a fused silica apparatus.

Grade 2

Very low inorganic, organic or colloidal contaminants and suitable for sensitive analytical purposes including atomic absorption spectrometry (AAS) and the determination of constituents in trace quantities. Can be produced by multiple distillation, ion exchange or reverse osmosis followed by distillation.

Grade 3

Suitable for most laboratory wet chemistry work and preparation of reagent solutions. Can be produced by single distillation, by ion exchange, or by reverse osmosis. Unless otherwise specified, it should be used for ordinary analytical work.

16. What Is Reverse Osmosis?

Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration. Osmosis is a special case of diffusion in which the molecules are water and the concentration gradient occurs across a semi-permeable membrane. The semi-permeable membrane allows the passage of water, but not ions (e.g., Na+, Ca2+, Cl-) or larger molecules (e.g., glucose, urea, bacteria). Diffusion and osmosis are thermodynamically favorable and will continue until equilibrium is reached.

Osmosis can be slowed, stopped, or even reversed if sufficient pressure is applied to the membrane from the ‘concentrated’ side of the membrane. Reverse osmosis occurs when the water is moved across the membrane against the concentration gradient, from lower concentration to higher concentration. To illustrate, imagine a semi-permeable membrane with fresh water on one side and a concentrated aqueous solution on the other side. If normal osmosis takes place, the fresh water will cross the membrane to dilute the concentrated solution. In reverse osmosis, pressure is exerted on the side with the concentrated solution to force the water molecules across the membrane to the fresh water side.

Semi-permeable refers to a membrane that selectively allows certain species to pass through it while retaining others. In actuality, many species will pass through the membrane, but at significantly different rates. In RO, the solvent (water) passes through the membrane at a much faster rate than the dissolved solids (salts). The net effect is that a solute-solvent separation occurs, with pure water being the product. (In some cases, dewatering is desired to concentrate the salts).

Download 1-23K RO datasheet (PDF)

Download 30-100K RO datasheet (PDF)