Treating Water Contaminants

There are thousands of possible contaminants that can be found in water.  Some are naturally occurring; others are pollutants.  Listed are a few of the major aspects, qualities and contaminants in water, and the methods to treat them.

Total Suspended Solids and Turbidity
TSS (total suspended solids) in the water refers to the amount of filterable material in the water.  Excessive suspended solids in water will cause problems with any water device, including pumps, boilers, cooling towers, storage tanks, distribution lines, spay nozzles and other water treatment equipment.  Levels exceeding 20 mg/l should be treated to a level of 20 microns or less. Removal of TSS is simply done through the use of filters. Disposable cartridge filters are the most common type.  For operations with significant levels of TSS, bag filters or backwash filters can be used.  The filter rating is also important.  Most cartridge filters will have a rating of 1, 5, 20 or 50 microns which refers to the size of the solids that are removed.  A 20 micron filter will protect most mechanical water treatment devices.

Hardness (Calcium & Magnesium)
Many water supplies contain dissolved solids such as calcium and magnesium.  These ions are typically referred to as “hardness” in the water. Levels exceeding 40 mg/l are deemed hard, and water treatment should be considered as these elements can cause excessive wear on equipment such as water heaters, boilers, steamers, pumps humidifiers and other water aided devices. By removing the hardness from the water, the “softened” water lengthens the life of many appliances.  The most common method of softening water is through an ion exchange process.  Through the use of manmade resins, the ion exchange process has become a very efficient means of removing hardness.  In this process, resins are used to remove calcium and magnesium from the water.  As a part of the process, the resin releases sodium or potassium into the water as a part of the exchange process.  After the resin becomes exhausted, a regeneration sequence is used to remove the concentrated deposit of harness from the resin bed. The hardness is replaced with soft ions such as sodium or potassium. This regeneration process typically uses sodium chloride (table salt) for the regeneration; however, potassium chloride may also be used.

Silica
Silica in water can be found either as a colloidal precipitant or as a dissolved contaminant.  Since the solubility of silica is very low, in most cases it will be precipitated.  In a precipitated form, it acts as an abrasive in the water, greatly increasing wear on mechanic systems using water. Levels exceeding a few parts can be detrimental. For some high efficiency boilers, requirements for silica removal extend to the 5 ppb level.  Silica can be removed through either reverse osmosis or deionization; however, if ppb reduction is required, deionization is the preferred and more reliable method.

Iron
Similar to hardness, iron in the water can also have a detrimental effect on water processing equipment. Levels exceeding 0.1 mg/l should be treated.  In addition to removing hardness, a water softener can also serve the function of removing iron.  It is important when using a softener for the removal of these contaminants to minimize the oxidation potential in the water. This means keeping as little air from contacting the water as possible.  If oxidizers are introduced into the stream, the iron will precipitate, making for an unfavorable environment for the ion exchange process to take place.  If oxidation is significant, the recommended treatment method would be to use a filter system.  In order to remove precipitated iron, a filter with at least a 5 micron rating is required.

Chlorine
Chlorine is a strong oxidant and is added to municipal water supplies as a residual disinfectant.  Most municipalities look to provide a residual of 1 mg/l of Cl2 at the furthest point in their distribution network.  This means that locations close to the processing plant will have significantly higher values.  Residual chlorine can have a significant corrosive effect on materials not designed to handle it.

A typical treatment method for Cl2 removal is the use of activated carbon.  The carbon can be incorporated into a filter cartridge; however, this type of filter will require constant monitoring and replacement.  Most commercial applications will use the carbon in a contact media tank.  These types of systems will be able to operate between 9-12 months before the media will need to be replaced. Another way of removing the Cl2 is to use a retention pond.  Since Cl2 levels will diminish with time and exposure to atmosphere, facilities using retention ponds will typically have Cl2 levels diminish without additional treatment.

Bacteria/Algae
Biological contaminants can cause problems with any water processing systems.  By coating surfaces, they significantly reduce efficiencies of either a cooling tower or boiler.  Growth to any level is detrimental; however, treatment is advised when growth creates a visible biological volume.  Chemical treatment, using biocides and oxidants to destroy any biological contaminant, is the most common form of treatment. Both ozone and ultraviolet disinfecting are growing trends in treatment since their end products are less destructive to machinery than their chemical counterparts.  One of the newest methods is the use of filters that are used as positive barriers that bacteria cannot penetrate.  This is the most effective way of reducing bacteria levels without chemical additions or electricity.

TDS (Total Dissolved Solids)
The total dissolved solids load of the water includes sodium, magnesium, calcium, iron, chloride, sulfate, nitrate and many others.  The TDS of water varies greatly across the US.  Some parts of the Northeast have water supplies with TDS less than 50, while some Southwestern areas can have levels exceeding 5,000.  Elevated TDS levels increase corrosion levels and wear on the hardiest designed systems.  This is especially true for both boilers and cooling towers where water contaminants can be further concentrated due to evaporation.

Although there are a few treatment methods for the reduction of TDS, the most common to the industry is reverse osmosis. RO technology uses a membrane as a barrier to dissolved salts, inorganic molecules and organic molecules with a molecular weight greater than approximately 100.  Water molecules, on the other hand, pass freely or “permeate” through the membrane, creating a purified product stream.  The capability of most commercial membranes is to “reject” more than 95 percent of the contaminants.  This means that the membrane will prevent 95 percent of the salts from permeating through the membrane and let the other 5 percent pass through.  A constant drain or reject flow is required with any RO system.  This reject flow contains all of the non-permeated contaminants. The efficiency or “recovery” of the RO system is determined by the amount of reject flow there is in comparison to permeate flow.  Typical commercial RO designs will provide an efficiency of about 50 percent, meaning for every one gallon processed, 50 percent (0.5 gallons) is permeated into good water while the other 50 percent is processed to drain.

pH
The pH of the water will give a measurement of reactivity.  Most water in the US is near neutral, measured as a pH of 7. A pH lower than 7 is considered acidic, while a pH higher than 7 is alkaline. To maintain pH in the proper range for the work water, chemical addition is the typical choice.  With this type of configuration, acid is injected into the system if the pH is too high, while a caustic material is used if the pH is too low.  These adjustments can be done manually, by sampling the solution, calculating the amount of chemical required, then adding the chemical.

Alkalinity
Alkalinity in water is a measurement used in determining how much “neutralization” capacity is in the water.  Items including carbonate, bicarbonate and hydroxide ions comprise water’s alkalinity.  The processing concerns related with alkalinity do not come from alkalinity itself but from a byproduct formed as water goes through either temperature or chemical changes.  Carboxyl acid is formed from a break down of carbonate and bicarbonate in the water.  Treatment for alkalinity can be done through either chemical additions of sequestering agents designed to prevent the formation of carboxyl acid, or through the removal of alkalinity through a selective ion exchange processes used to dealkalize the water.  Treatment is based on the application’s potential to convert alkalinity, then based on the actual level in the water. Treatment is required for applications exceeding 100 mg/l of alkalinity.

Chlorides
Depending on the application, chlorides in water can be reactive to metals they come in contact with.  In rinsing, low chlorides are recommended to prevent etching, pitting and corrosion. The effect of chloride corrosion is even more exaggerated with depressed pH levels.  Typically, treatment for chloride removal is based on application corrosion issues and not a designated Cl level.  One common treatment method includes maintaining an alkaline pH, where reactivity issues are minimized. Another treatment calls for the removal of chlorides either through reverse osmosis or deionization technologies.