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.

Suspended Solids
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-using device, including transfer pumps, boilers, cooling towers, storage tanks, distribution lines, spay nozzles. Certain types of water treatment equipment are sensitive to excessive levels of suspended solids such as reverse osmosis, ion exchange and nanofiltration. 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. The type of filter recommended will depend on the solids being filtered and the flow volumes required.

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-using devices. By removing the hardness from the water, the “softened” water lengthens the life of many appliances. A common process for softening water is ion exchange.

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, ion exchange systems can also remove iron. It is important to minimize the potential for oxidation in the water when using an ion exchanger to remove iron. This means keeping as little air from contacting the water as possible. If oxidizers are introduced into the stream, the iron will precipitate and make an unfavorable environment for the ion exchange process. 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.

Bacteria/Algae
Chlorine, a strong oxidant, is used as a residual disinfectant for municipal water supplies. Most municipalities look to provide a residue of 0.5 mg/l of Cl2 at the furthest point in their distribution network; therefore, 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.

TDS (Total Dissolved Solids)
Total dissolved solids include 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 with even the hardiest of system designs.

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 for 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 prevent or “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 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 that is lower than 7 is considered acidic, while a pH that is 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 and 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 by-product formed as water goes through either temperature or chemical changes. As alkalinity is consumed in the water, typically a by-product is dissolved carboxyl acid. This 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 to prevent the formation of carboxyl acid, or through the removal of alkalinity through a selective ion exchange process to dealkalize the water. Treatment is first based on the application’s potential to convert alkalinity, and then it is based on the actual level in the water, with treatment required for applications exceeding 100 mg/l of alkalinity.