By Lawrence R. ‘Larry’ Zinser
In “Maintaining Drinking Water, Part 1,” which ran in last month’s issue of Water Conditioning & Purification International Magazine, we discussed the natural and man-made contaminants that threaten the earth’s water supplies—a crucial issue, since compatible water supports all life. Further, we discussed one answer to these threats: the employment of water-treatment technologies to remove the contaminants that render a water source unsuitable for consumption. Finally, we defined contaminants that are suspended within water: inorganic suspended solids and organic suspended solids or microbes. In this part, we will address those contaminants that are dissolved within the water.
The distinction between suspended and dissolved solids is based on the effects of the water molecule. The water molecule is polar; it has two opposite charges, plus and minus, similar to a magnet. Consequently, each pole is attracted to its opposite charge. The positive pole is attracted to negative charges, and the negative pole is attracted to positive charges. This simple characteristic has made water the fundamental component of all life on Earth and the key target in the search for life outside Earth.
CONTAMINANT: Dissolved Solids
Unlike suspended solids, dissolved solids exhibit a positive or negative charge. These solids are called ions. Specifically, the positive ions are called cations, and the negative ions are called anions. Ions are formed in water when salts found in minerals dissolve. The most common example is sodium chloride, or table salt. In its solid form, it is a crystal, but in water the crystal dissolves into sodium cations and chloride anions. Other cations include calcium, iron, potassium, and magnesium. Other anions include carbonate, sulfate, nitrate, and bicarbonate. When they are in water, these ions are dissolved solids. Unlike suspended solids, dissolved solids interact with the dipolar water molecules. Importantly, dissolved solids cannot be removed by simple physical filtration; other removal technologies must be employed.
Dissolved-solids contamination is an increasing threat due to long-term highway salting and saltwater intrusion from the oceans, caused by commercial wells along the coast. Dissolved solids are invisible but can be damaging in water. Some dissolved solids, such as calcium, iron, and magnesium, cause scaling in piping and appliances. The scale reduces the effectiveness of pumps and appliances and leaves a deposit of unsightly spotting.
The risk of poisoning from dissolved solids is caused by such contaminants as arsenic, lead, copper, chromium, and cyanide, to name a few. Corrosion of metal surfaces is caused by low pH and chlorides. Other dissolved solids such as sulfates affect the human and animal digestive systems. Some of these contaminants are naturally occurring from the geology surrounding our aquifers. Others are added to our groundwater through the use of fertilizers or from unlawful industrial discharges.
Treating Dissolved Solids
Water treatment of dissolved solids includes ion exchange, oxidation and filtration, and reverse osmosis.
Ion exchange: Ion exchange uses a polymer, or chain of repeating molecules. A weakly charged ion, which is not harmful to humans, is placed on each of the links of this chain. On cation polymers, this ion is negative, and on anion polymers, the ion is positive. In a water solution, since most of the objectionable ions are strongly charged, the weakly charged and harmless ions on the polymer will be displaced by the stronger ions. This process is called ion exchange.
The polymers in ion exchange appear as small plastic beads called resin. For example, in softening resin, the beads are seeded with sodium ions, which are quite weak in relative charge. When water flows through this cation resin, any dissolved cations with a stronger charge—such as calcium, iron, or magnesium—will replace the sodium ions at the charge sites of the resin. As a result, the water exiting the resin will have sodium ions as replacements for the calcium, iron, and magnesium ions, which are left behind attached to the resin.
A wide array of resins, both cation and anion, are available in water treatment. For example, an anion resin with chloride as the seed ion can be used to remove (by ion exchange) nitrates and sulfates from water. Resin manufacturers have designed specialty resins that target specific ions or groups of ions.
However, the target solid must be dissolved and must exhibit a specific electrical charge for the resin to be effective.
Reverse osmosis: One of the fascinating features of living entities is the process of osmosis. When any two waters with different concentrations of dissolved solids are separated by a membrane with a very small pore size—about 0.001 micron—the water molecules will migrate from the side with the lowest concentration of dissolved solids to the side of higher concentration. Only the water molecules, not the dissolved solids, pass the membrane surface. This process is called osmosis and is the basis for all life as we know it. It takes place within the organs of the human body and the cells of any living thing on Earth, whether animal or plant.
The natural force of the migration of the water molecules causes water to flow from the lower to the higher concentrations. The pressure is called osmotic pressure. The larger the difference in dissolved solids concentration, the higher the osmotic pressure.
Humans have capitalized on this natural process by manufacturing artificial membranes with pore size at 0.001 micron and then adding a pump, which generates a pressure higher than the osmotic pressure of the source water. Once that pressure exceeds the natural osmotic pressure, water molecules begin to migrate back to the side with the lower dissolved-solids concentration. This technology, which reduces the concentration of dissolved solids, is called reverse osmosis (RO). Small RO systems for residential points of water use employ the pressure supplied by a well pump or municipal treatment plant. Larger and more-efficient commercial and industrial RO systems use integral high-pressure pumps to create a large pressure differential—as high as 1,000 pounds per square inch.
Just as with the nonliving, inorganic suspended solids, dissolved solids have a category based on living components: dissolved organic compounds. They are distinguished by their composition of carbon and hydrogen atoms, among others.
Many of these solids have a weak polarity due to the structure of the molecule. Because of this, many organics will interact with the polar water molecule and act as dissolved solids. These are referred to as hydrophilic, or water-loving. Examples include alcohol, acetic acid, and sugar.
Organics that do not have this polarity tend to assemble together, or agglomerate, when in water. These are referred to as hydrophobic, or water-fearing. Fuels or vegetable oils are examples. In water treatment, these organics are addressed separately because they have unique characteristics that require unique treatment strategies. Examples include methyl tert-butyl ether (MTBE) and perfluorooctanoic acid (PFOA).
The dangers posed by organics range from a disagreeable odor to life-threatening carcinogenic effects. Advances in manufacturing technology have produced a wide array of ultimately dangerous products, such as MTBE, PFOA, and polychlorinated biphenyls (PCBs). Industrial chemical spills and organic waste discharge from farming and animal husbandry have added to these dangers. The disposal of prescription drugs into our groundwater systems is a recent addition to the list of sources of organic contaminants.
The treatment of organics includes adsorption (bonding to external surfaces), ultrafiltration, and ion exchange.
For the removal of organic contaminants, the most widely used technology is adsorption of the organics by activated carbon. Carbon is marketed in granular form for use in tanks, and in a block form for use in cartridges. The extreme porosity of activated carbon makes it an effective treatment for the removal of most organic contaminants, including many of the prescription medicines and polyfluoroalkyl substances (PFAS) that have been released into our water sources. The removal effectiveness is highly influenced by the contact time (water flow per volume) of the carbon.
Activated carbon is also an effective tool for removing chlorine and chloramine from water. A form of carbon called catalytic carbon acts as both an oxidizer and adsorbent. It is employed primarily as an oxidizing medium. However, it possesses adsorption capacity, as well.
Other adsorption media are available for removing organic or inorganic contaminants. These media are specifically designed to absorb a particular range of contaminants. The oldest of these is activated alumina, sometimes called alum. Other media have a synthetic composition, including some resin-based media. Such media are used to remove contaminants such as arsenic or fluorides from water.
The Way Ahead
The availability of water with contaminant levels acceptable for drinking will continue to be an issue across the globe. In all likelihood, between natural disasters and increased human activity, the situation will become worse. The answer to this challenge lies in curbing the human activity contributing to contamination and the development and application of water-treatment technologies.
Reverse osmosis technology is particularly suited to solve future drinking-water issues through the process of desalination. Approximately 96.5 percent of the earth’s water is in our oceans, and 40 percent of the world’s population lives within 100 miles of an ocean.
Reverse osmosis can turn the ocean’s water into drinking water; we need only the requisite energy to activate the high-pressure pumps. Innovations in membrane technology continue to reduce the amount of energy necessary to perform reverse osmosis. I believe RO technology will someday provide a significant and renewable source of drinking water.
1. United Nations. 2017. “Factsheet: People and Oceans.” https://www.un.org/sustainabledevelopment/wp-content/uploads/2017/05/Ocean-fact-sheet-package.pdf.
About the author
Following an education in chemistry (BS Degree) at Georgetown University, graduate work at Wayne State University and a 27-year career with the US Marine Corps, Lawrence R. ‘Larry’ Zinser has served an additional 27 years in design, manufacture, education and troubleshooting of residential, commercial and industrial water treatment systems. He has provided numerous technical courses throughout the country and internationally, which have been accredited by the Water Quality Association, the Pennsylvania, North Carolina, Maryland, Virginia, and Delaware Ground Water Associations, the American Nephrology Nurses Association and the Lehigh-Carbon County Community College. Zinser can be reached at [email protected] or cell phone, (215) 421-7115.