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Fundamentals of Water Purification (Part 2)

by Brian Hagopian, CIPE
April 2000

Reprinted with permission from Plumbing EngineerPart 1 of this article discussed the unique properties of the water molecule. It discussed the five basic groups of contaminants found in water, which are particles, dissolved salts, organics, colloids, and bacteria. Part 1 also provided the reader with an understanding of the importance of water supplies in determining the types of contaminants expected in the water. Part 2 of this article describes the processes used to remove contaminants from a water supply, integrating components into a functional system, and the importance of controls and routine periodic water testing.

Contaminant Removal Technologies

Many contaminants found in water can be described based on their size. Beginning at the periodic table, every element has an atomic weight, which is based on its number of protons and electrons. When combined with other elements to form molecules, each is described by its molecular weight. As molecules increase in size, their molecular weight increases and they become harder to dissolve in water. When they become large enough, they are described by their "micron" size (25 microns = 1/1000 in.). Materials this large are usually insoluble in water and exist as "suspended" solids.

Particle filters remove particles larger than the pore size of a filter. In other words, if a particle is larger than the pore size of the filter, it will not be able to pass through the filter. There are many different types of particle filters, some of which use disposable filter media, while others use permanent media. As water flow increases, or when particle loading is high, the use of a backwashing sand, multimedia or other permanent filter media is recommended over disposable particle filters.

A simple way to classify particle filters is by the size of the particles they remove. It is important to understand that filtering smaller particles usually requires more driving force (pressure). It is also important to note that filters remove a certain percentage of particles at a given particle size, and the percentage removed is usually related to the price of the filter.

Beginning at the coarse end of the particle spectrum are coarse particle filters, which remove suspended solids that are visible to the naked eye. Visible particles are roughly 25 microns and larger in size. Filters used for this purpose include disposable cartridge filters, bag filters, permanent backwashing media filters and a host of other filters beyond the scope of this article.

Absolute membrane filters represent the next step down the particle filtration spectrum. These filters can remove particles down to 0.1 micron in size, including bacteria. Absolute membrane filters are widely used in water purification and also are used to sterile filter pharmaceutical solutions.

Ultrafiltration (UF) represents the next step in the particle filtration spectrum. The particle removal ratings of ultrafilters can be described by a particle size expressed in microns, or a molecular weight cutoff (MWCO), which describes the size of molecules that are removed. From the perspective of water purification, UF membranes are capable of removing colloids along with bacterial decomposition products and are used primarily for these purposes. UF membranes also represent the first filtration process in this spectrum capable of removing materials which are dissolved in water.

Reverse osmosis (RO) represents the finest step in the particle filtration spectrum. RO membranes have particle removal ratings ranging from 100-500 MWCO. These filter membranes are capable of removing nearly all organics and most of the dissolved salts from water. RO represents one of the most powerful water purification technologies available. Residential RO systems operate on incoming city water pressure, whereas industrial units use high pressure pumps to operate at pressures ranging from 150 to 1,000 psi.

As an example, those familiar with RO membranes know that they are capable of removing 90-99 percent of all dissolved sodium from the water supply. Sodium has an atomic weight of 23, which is much smaller than the pore size of RO membranes. There are a number of theories which describe the removal mechanisms for sodium and other molecules, but one of the easiest to understand is to consider the hydrogen bonding properties of water. Sodium, containing a single positive charge, and when in solution, attracts a number of water molecules, which align themselves around the sodium molecule. During any given instant, the number of water molecules aligned around the sodium molecule can change because water is a liquid and its molecules are in constant motion. Increasing the number of water molecules aligned around the sodium molecule increases the "apparent size" of the sodium molecule when it comes into contact with a RO membrane, thereby decreasing its probability of passing through the RO membrane. But, because the number of water molecules surrounding the sodium is constantly changing, some sodium molecules will pass through the RO membrane.

As it turns out, the size of the molecule, coupled with its charge, determine the "apparent size" of the molecule to a RO membrane. Molecules with a higher atomic weight and a greater charge when dissolved in water will be removed with great efficiency.

Hydrogen bonding also explains the penetration of RO membranes by small, nonpolar organic molecules. These molecules do not hydrogen bond with water, so their size does not increase, and their removal efficiency can be much more easily predicted based on molecular weight.

Other Purification Processes

Many other water purification processes exist, which do not completely rely on size as the primary method removing contaminants. Other properties such as charge, polarity, and molecular structure play roles in their removal from water. Some of the more common processes are described below.
Carbon filters use the fine pore structure of activated carbon to remove small dissolved organic and other contaminants. Carbon filters also remove the disinfectants added to drinking water supplies to control bacteria, so careful placement of carbon filters is essential to the water purification process. Carbon filters work very well to complement RO because they remove the small organic contaminants that can pass through RO membranes.

Ultraviolet disinfection processes are used to control bacteria without using disinfecting chemicals. UV units are typically used after disinfectants are removed to control bacterial activity in a pure water system. UV radiation penetrates the cell walls of bacteria and prevents them from replicating.
TOC reducing ultraviolet disinfection processes are similar to the UV units covered above, but also emit a smaller wavelength (higher energy) of light which can be used to control organic and colloidal levels in a water purification system.

Deionization (DI) processes use inert and insoluble materials, called ion exchange resins, to remove dissolved salts from water based on their electrical charge. Two different types of resins -- cation and anion resins -- are used to achieve high purity water. The two most commonly used DI processes are dual bed and mixed bed deionizers. Dual bed DI uses two separate vessels, one for each for the two different DI resins. Mixed bed deionizers use a single vessel containing a mixture of cation and anion resins in order to achieve higher purity levels. DI is typically used after reverse osmosis (RO) to remove the trace dissolved salts that pass through the RO membranes.

Water softeners are commonly used in water treatment. Softeners are actually the cation portion of a deionizer, in a different (sodium) form, used to remove hardness minerals such as calcium and magnesium and replace them with sodium, a more readily dissolved and less scale producing mineral. This information is summarized in Table 1.

 

Coarse Particle Filters

Absolute Membrane Filters Ultrafilters Reverse Osmosis Carbon Filtration Ultraviolet Disinfection
Particles
F
G-E
E
E
N
N
Dissolved Ions
N
N
N
G-E
N
N
Small Organics
N
N
N
F-G
G-E
N
Colloids
N
P
G
E
F
N
Bacteria
P
E
E
E
A
G
N = None P=Poor F=Fair G=Good E=Excellent A=Adds Contaminants to System

Table 1. Removal of various water purification processes

Putting it All Together

There are general guidelines which are employed to put water purification processes into their correct sequence. The contaminants present in the water and the desired level of water purity determine the overall system design. Some contaminants interfere with the removal of other contaminants, so these need to be removed first.

Generally speaking, suspended materials are considered the most undesirable and are usually removed in the first steps of a water purification system. Particle filters are usually the first treatment process in a water purification system. The particles removed at this stage protect the downstream equipment. Water softeners and carbon filters typically appear next. Processes used for removing colloids should be employed here, if required, including UF or RO systems. If either of these processes are used, a storage tank is usually used to store the purified water, and pumps are added to circulate the water.
Once suspended solids are removed, the remaining contaminants may be removed with greater ease. DI processes are used next in order to remove any remaining dissolved salts. UV units, TOC reducing units, and absolute membrane filters typically follow in sequence to destroy and remove bacteria and fine particles from the water.

Control Systems

While a detailed discussion of control systems is beyond the scope of this article, plumbing engineers must understand their importance. Control systems allow the various functions of a number of different devices to be integrated in a way that permits the entire system to operate as a coordinated whole. As examples, controls are needed to turn pumps on and off to fill storage tanks, to alert operators if the system is producing low quality, to report information to a central control center, and various other reasons.

The design of control systems typically is left up to the water system vendor, and the plumbing engineer has had very little say in how the controls actually operate. The net result is a vast array of widely varying control systems. At one end of the spectrum, they perform minimal control functions. At the other end, they appear so comprehensive they are too complicated to understand. Many control systems are user friendly only to the water system vendor technicians and not to the end user, who ultimately has to operate the system.

Because many water system vendors use similar components, it is often the control systems that separate one vendor from another. Vendors have different concepts of what is a necessary control function, and control functions will vary from one industry to the next. With a written specification that focuses on equipment with little emphasis on controls, vendors can provide minimum controls to become the successful (low) bidder on a project. The plumbing engineer, typically involved in the project only until construction is completed, may never know that a substandard control system has been installed. These deficiencies typically manifest themselves after the plumbing engineer is gone, leaving the end user dissatisfied.

Testing the Waters

Water testing is an important part of the picture. Testing is important before the system is designed, to alert the plumbing engineer to any unique local conditions which must be treated. After the system is installed, a comprehensive testing program should be implemented to verify the system is operating properly.

Most testing is performed at the use points in the system, after all steps of water purification, to determine if the water quality meets a certain specification, which varies from industry to industry and client to client. This type of testing is a pass/fail program, intended to determine if the water quality is good enough to use for operations. There is a flaw with this type of testing, which is explained in the example which follows. When a failed test occurs, the end user is usually at a loss as to where to look for the problem. As panic sets in, the water system vendor may receive several threatening phone calls requesting immediate action. Without information, everyone has to make assumptions. At that point, everything that can be changed usually is changed, and the problem may go away. The biggest problem with these events is that nobody really understands why the water failed its test and the end user may have spent much more money than necessary.

It is entirely possible these events could have been prevented by testing the performance of unit processes throughout the system. This helps in developing a profile on the system. End users may use this information to understand where the various contaminants are removed in their system, and can track results and watch for trends. Typically, trends indicate when contaminant levels are increasing, and allow for preventive actions to occur prior to the water going out of specification.

Summary

This article provides a brief introduction to common terms, technologies and equipment used in the water purification industry. It is important to understand that these explanations are generalized and are not intended to cover all technologies or conditions that may be present in a given application.
About the Author

Brian Hagopian has been involved in the water purification industry since 1981. He is a founder and Vice President of Fluid Solutions, Inc., located in Lowell, Mass., a firm specializing in the design, engineering, fabrication, installation, and preventive maintenance of high purity water systems for high technology clients. He maintains affiliations with ISPE, ASPE, AFE, ISA and PDA and regularly presents informational and training sessions for clients. He can be reached at 978/453-9600, fax 978/453-1223, e-mail brianh@fluid-solutions.com. Copies of this article, with illustrations are available upon request.

Page last updated: 5 March 2009