Cation Exchange, Part 1: Back to the Basics
By Gary Battenberg
In the beginning
Ion exchange water softening is the backbone of our industry, with filtration and reverse osmosis complementing the process when required and based on a thorough water analysis. That will be the focus of this article. The subject of ion exchange water softening has been the focus of much study (and writing of articles, primers and technical manuals) for nearly 100 years. Recorded water treatment history, however, is much older than that.
The three basic methods of water softening include chemical, membrane softening and cation exchange. As early as 1766, Cavendish demonstrated that by adding lime (CaO) to water, calcium and magnesium carbonates were precipitated in the form of what was termed carbonate hardness. Advancements in this process led to the combination of soda ash (Na2CO3) and lime in the mid 1850s for the removal of both carbonate and non-carbonate hardness. This process yielded water that was more suitable for boiler feedstock.
Laundries in Europe were the first to use cation exchange, when softening equipment was rented and rental fees were based on soap savings. This soap-saving approach is still in use today and is used to demonstrate domestic residential water softening systems. The first commercial cation water softening system for boiler water treatment was installed and commissioned in 1910 for and electric generating plant in Moscow, Russia.
The evolution of the water softener (or conditioner, as preferred by some) has seen remarkable technological advancements since the early days of Culligan, Lindsay and Roper, early pioneers of water conditioning. Those early softening systems required large quantities of salt and water to recover a relatively small softening capacity. They were manually operated salt-in-head-type systems, where the water was shut off temporarily to allow the tank cap to be removed to add the salt. After the cap was replaced, regeneration was controlled by a manually operated control valve that diverted the water flow to drain during the brining and rinse cycles and finally back to service. These systems required as much as six hours to regenerate, depending on the size of the softener.
Water softeners come in a variety of configurations, including conventional two-tank models that include the softener and a separate brine tank. Others include a cabinet style that houses the resin tank as well as providing salt storage and the brine well that houses the brine valve and safety float. Still other styles include two softener tanks and a brine tank configured for alternating service, where one tank is on standby while the other is online to the service plumbing to provide uninterrupted soft water service. The true twin-tank configuration has been around for over 30 years and features both tanks online to the service plumbing. This specific type is useful where there is extremely hard water and high TDS by providing longer water contact time with the resin to ensure water with little or no hardness leakage.
While there are many different styles of softeners, one thing they all have in common that does not change is the fundamental requirements of a water softener control valve. Every softener requires the ability to regenerate the resin bed, incorporating the cycles of backwash, brine draw, slow rinse, settle rinse and brine refill. Some of the more advanced softening systems that incorporate up-flow brining eliminate the first backwash, which is supported by resin manufacturers’ application guidelines. (We will discuss this further in Part 2).
Cation exchange is the process by which hard water enters the resin bed via the control valve inlet port. As water flows through the resin bed, calcium and magnesium (the primary hardness minerals) are exchanged for sodium or potassium, depending on which is used for regeneration. Calcium and magnesium are positively charged ions. The resin is negatively charged and is loaded with sodium or potassium ions. The hardness minerals are attracted to the negatively charged resin and an equal amount of sodium or potassium is released into the water, thereby completing the exchange process. To illustrate the exchange process, imagine a magnet inserted into a pail of nails. The magnet will attract and hold only the amount of nails equal to the holding capacity of the magnet. When the magnet can hold no more nails, the maximum holding capacity of the magnet has been achieved. Removing the captured nails restores the original holding capacity of the magnet. Inserting the magnet into the pail of nails again will repeat the holding capacity and can be repeated over and over with the same results.
The holding capacity of the magnet is representative of a resin bed that is fully charged with sodium or potassium. A specific volume of water with a measured amount of total hardness (TH) that is passed through the resin bed will eventually exhaust the capacity of the resin. Unless the resin is regenerated, hard water will then migrate through the softener and into the service plumbing. Regenerating the softener will remove the calcium and magnesium ions and reload the resin with sodium or potassium ions. This process is repeated over and over again, just as in the example of the magnet and nails.
Cation exchange resin has an affinity for positively charged ions. We know that calcium and magnesium are only two cations, but softening resin can and does remove other ions from the water. Iron and manganese in the ferrous or clear water state and in limited quantity are easily exchanged by the cation exchange resin. Affinity for other cations such as barium, copper, zinc, lead and other metals make cation resin a major factor in conditioning water for residential, commercial, industrial, farm and ranch. But caution is urged.
Although a softener can reduce other contaminants from a water supply, its primary purpose is to remove hardness. This requires proper operation and maintenance, which means that a softener must be sized properly for its intended use and be compatible with the water chemistry, hydraulic characteristics of the water supply and of course, environmental conditions, to protect the softener from hostile environmental conditions. These issues will be covered in subsequent articles in this series.
Resin capacity is a reference to the total amount of hardness (Ca, Mg) exchange available in one cubic foot of resin. The amount of salt used per regeneration is expressed in pounds of salt per cubic foot of resin. The typical available capacity of one cubic foot of resin is 30,000 grains when regenerated with 15 pounds (6.8 kg) of salt (NaCL). Practically speaking, typical residential softeners are regenerated with six pounds (2.7 kg) of salt per cubic foot of resin and yield 20,000 grains capacity. Regenerating at 10 pounds (4.5 kg) of salt per cubic foot will yield 25,000 grains capacity. The more salt that is used, the better the regeneration; however, beyond a certain point, the capacity recovered per pound of salt diminishes, thereby reducing the efficiency of the softener. At six pounds of salt per cubic foot, one can see that two-thirds of the available capacity per cubic foot is recovered using only slightly more than one-third of the salt. Doubling the amount of salt does not double the capacity of the resin. Using the six-pound salt setting, it is simple to calculate the available number of gallons of soft water per service run.
Example: Divide the regenerated capacity of 20,000 grains by 10 grains per gallon (gpg) and the result is 2,000 gallons (7,570 liters) net of soft water between regenerations. The amount of soft water between service runs depends on various factors relative to the water chemistry, which we will discuss in further detail later on in this series. The most important factor is the amount of salt used per regeneration and the total hardness of the water supply being treated.
There are two types of regeneration: downflow and upflow. In downflow regeneration, the brine (salt water solution) is injected at the top of the resin bed and flows downward through the bed, releasing the hardness minerals and flushing to the drain. In upflow regeneration, the brine is injected at the bottom of the resin bed and flows upward through the resin bed, releasing the hardness and flushing to the drain. In both cases, the service flow is downflow. Downflow regeneration is the most common, but upflow will be discussed in greater detail later in this series.
In Part 2, we will look more in depth at the basics, including the technical aspects of the fundamental steps required for regeneration of a water softener and the reasons why they are vital to the proper operation and long-term reliability of a water softener. Stay tuned!
About the author
Gary Battenberg is a Technical Support and Systems Design Specialist with the Fluid System Connectors Division of Parker Hannifin Corporation in Otsego, MI. He has 34 years of experience in the fields of domestic, commercial, industrial, high-purity and sterile water treatment processes. Battenberg has worked in the areas of sales, service, design and manufacturing of water treatment systems and processes utilizing filtration, ion exchange, UV sterilization, reverse osmosis and ozone technologies. He may be reached by phone at (269) 692-6632 or by email, gary,email@example.com