Last week, WQP, Water & Wastes Digest (W&WD), and Storm Water Solutions (SWS) editors traveled to Houston to speak with water...
Iron is one of the most troubling contaminants we deal with in traditional water treatment. It can be simple to deal with or very complex.
First let’s review the fundamentals of our water supply. Most of our water used for domestic, commercial or industrial uses comes from either surface water or ground water. Typical surface water supplies are void of iron. The majority of our water supplies is ground water from wells that may be less than 100 feet deep to thousands of feet deep. The water we use today is the same water that has been here since the beginning of time. Water continuously travels through the hydrologic cycle. The hydrologic cycle is nature’s way of purifying water by evaporating surface water from the earth. As it reaches high levels in the atmosphere it condenses, forming clouds that eventually become so saturated that the moisture falls back to the Earth in the form of rain, snow, hail or fog. As this moisture falls on the surface of the Earth, some of it soaks into the ground, and some of it runs off to streams, rivers and oceans.
The water that soaks into the earth percolates through the upper layers of the geological strata and eventually recollects in porous ground strata known as zones of saturation. Wells are drilled into the Earth until they reach these zones. More than 5 percent of the Earth’s upper geological layers contain iron. Water is known to be the universal solvent, it dissolves a little bit of everything it touches. As water percolates through the Earth, it dissolves minerals that are in the soil such as iron, manganese, calcium and magnesium just to name a few. Because the geology of the Earth varies from one region to another, so does ground water. Ground water supplies may have a little iron or extremely high amounts of iron. It may be naturally soft or so hard that it is virtually unusable for domestic purposes. It may be acidic or very alkaline. Out of all of these variables, which are a direct result of the geology of any particular region, iron can be the most troublesome for water use. Iron is considered to be one of the most unstable minerals in our ground water supply.
As water percolates through the ground strata it dissolves the iron from the iron ore deposits as ferrous bicarbonate [Fe (HCO3)2 ], sometimes referred to as “clear water iron.” When iron is dissolved in water you cannot see it. Iron normally wants to revert back to its natural state as iron ore. Iron very easily comes out of solution and precipitates to a solid particle of ferric hydroxide [Fe (OH)3] often referred to as “red water iron.” Simple changes to the water supply such as temperature, pressure or even a change of pH can promote the change from clear water iron to red water iron. The addition of oxygen to a water supply may easily casue this conversion. Generally speaking, the higher the pH, the faster this reaction can take place. Iron will precipitate to a solid particle much faster at a pH of 8 than at a pH of 6. Thus, the pH of the water supply has a major impact on iron precipitation.
The effects of iron in a water supply are numerous. Iron will stain fixtures, water-using appliances or surfaces that the iron-laden water contacts. These stains may vary from a light yellow to a red or light brown color. Iron can give water a metallic taste that may be considered unpalatable. Iron may provide odors that are undesirable for domestic use. Iron can foul water softeners and water-using appliances, and it can plug water pipes or heat exchangers. While none of these effects are hazardous to humans, water processing or the environment, they cause consumers to spend hundreds and even thousands of dollars to clean and maintain appliances, homes and factories every year. In the process of cleaning and maintaining our homes or factories, we quite often use cleaning solutions that may be toxic or hazardous to people and the environment, and all at a substantial expense. In most instances, it is more practical and economical to remove the iron from the water supply before we use it than to deal with the effects of clear and red water iron.
Successful reduction of iron starts with proper identification of iron and other water characteristics that may affect the iron reduction process. Proper testing and analysis of a water supply accomplishes this. At minimum, a water supply should be tested for total hardness, total dissolved solids, pH, alkalinity, iron, manganese, tannins and iron bacteria. Often times iron or iron-related symptoms are misdiagnosed and the wrong equipment may be applied. When this happens, typically the equipment only will work for a short period of time if at all, and then it may become fouled with iron. Most iron-reducing processes are designed to reduce either clear water iron or red water iron. Let’s identify other types of iron that may cause typical symptoms associated with iron.
Iron reduction can be simple once you properly identify the type of iron in a water supply and have a good understanding of the water characteristics. There are two basic technologies that are employed for iron reduction: ion exchange and oxidation/filtration.
Ion Exchange. Ferrous iron is a cation that in limited amounts easily can be removed with an ion exchange process (water softener). As a general guideline, many manufacturers will recommend a limit not to exceed 2 to 5 parts per million of clear water iron (ferrous bicarbonate). Because variables in the water supply and the application have an impact on the success of the ion exchange process, it is best to consult with a professional water treatment dealer in your region to determine if the ion exchange process will work for your application.
Oxidation/filtration typically is employed where the iron levels are high, (greater than 2 to 5 parts per million) or the pH may be high (greater than 8) even when the iron level may be minimal. This process introduces oxygen to the water supply to convert the clear water iron (ferrous bicarbonate) to red water iron (ferric hydroxide). Once the iron is converted to a solid particle, simple filtration will remove it from the water supply. Today, an automatic back-washing filter generally is chosen to filter out the precipitated iron particles. It is important to make sure the filter is backwashed on a regular schedule to prevent the filter bed from becoming fouled. Many manufacturers recommend that intervals between backwash cycles never exceed three days, and more frequent backwashing is recommended in many applications. Backwash intervals are based on specific water supplies, applications and daily usage.
When iron reduction is required, consult with a local professional water treatment dealer to obtain advice on which technology may best fit your application.