Following is a brief overview of water properties and water chemistry as applicable to drinking water and the treatment of drinking water.
pH is a measure of how acidic or basic water is. The range goes from 0 to 14, with 7 being neutral. pH of less than 7 indicates acidity, while pH of greater than 7 indicates a base.
pH is really a measure of the relative amounts of free hydrogen (H+) or free hydroxyl (OH-) ions in the water. Water that has more free hydrogen ions is acidic, while water that has more free hydroxyl ions is basic. Where the concentration of hydrogen ions equals the concentration of hydroxyl ions the water is neutral, and the pH is 7. In nature, such pure, neutral water does not exist; there will be gases or minerals in the water affecting the ion concentration.
pH is reported on a logarithmic basis – each number represents a ten-fold increase in acidity/basicness. For instance, water with a pH of 5 is ten times more acidic than water having a pH of 6.
Excessively high and low pH can be detrimental to the use of water. For instance, high pH can cause a bitter taste and depresses effectiveness of chlorine. Low pH will corrode or dissolve metals and other substances.
pH is very important for water treating because the pH determines the solubility of a number of chemical constituents, including those considered undesirable or harmful in water for public use, and impacts effectiveness of treatment chemicals such as coagulants.
Pure water has a pH of 7. Pure water put in equilibrium with the atmosphere has a pH of approximately 5.2; the reduction is due to dissolved carbon dioxide. Groundwater can have a range of pH, and are most commonly found in the range of 6 to 8.5. While carbon dioxide content has a major influence on water pH, acidity in natural waters may also occur from mineral acids such as H2SO4 and HCl. Carbon dioxide/carbonic acid cannot drive the pH below 4.5, so natural water with pH below 4.5 is due to mineral acids.
In open water, carbon dioxide content is limited by the low partial pressure of CO2
In the atmosphere. However, groundwater under soil is not so limited and CO2 concentrations are often in the range of 30 to 50 mg/l. Water with this relatively high CO2 content percolating through soils containing calcium or magnesium will dissolve these minerals, producing water with high calcium and/or magnesium content; calcium bicarbonate being very common.
Alkalinity is a measure of the capacity of the water to resist a change in pH that would make the water more acidic. Alkalinity comes from a high concentration of carbon-based mineral molecules in the solution. In natural water supplies, the ions of interest include carbonates and bicarbonates. Bicarbonates are most common – almost allnatural water supplies have a measurable quantity of bicarbonates.
Alkalinity can be thought of as buffering capacity, or as the capacity of the water to neutralize acid. Water with zero alkalinity will see an immediate pH drop with added acid, but alkaline water will not see this initial drop, but will resist until the buffering capacity is overloaded.
Alkalinity in groundwater comes from the soil and bedrock through which the water passes. The main sources for natural alkalinity are rocks which contain carbonate, bicarbonate and hydroxide compounds.
Water with high alkalinity is often said to be hard; water with low alkalinity is said to be soft. However, it should be noted that measure of hardness and alkalinity are not necessarily identical. For the typical CaCO3, the hardness is a measure of the calcium, while the alkalinity is a function of the CO3 carbonate.
In nature, alkalinity is especially important for fish and aquatic life because the alkalinity buffers against rapid pH changes, from rain, for instance.
EPA standards do not directly regulate alkalinity but do regulate indirectly through standards for total dissolved solids and pH in their Secondary Drinking Water Standards.
Total alkalinity is determined by measuring the amount of acid (for instance sulfuric acid) needed to bring the water sample to a pH of 4.2 (Titration of all alkalinity is complete at about pH 4.5). In this fashion, it is the measure of bases that can be titrated by a strong acid. In most natural waters, all anions except HCO3-and CO3-2 have low concentrations, so water alkalinity is typically carbonate (HCO3- and CO3-2) alkalinity.
Corrosivity of water is typically a function of both alkalinity and pH.
Hardness refers to the metal ions, typically calcium and magnesium, that are dissolved in the water. Other ions that contribute to hardness include aluminum, barium, iron, manganese, and zinc. Hardness is typically described as milligrams of calcium carbonate equivalent per liter. As mentioned above, hardness is related to alkalinity.
The common classification for hardness of water, as determined by equivalent calcium carbonate:
The main issue associated with water hardness in residential use is the potential for precipitates to form, in particular when heated, in a water heater for instance. The phrase “temporary hardness” is sometimes used to describe this effect of calcium and magnesium salts precipitating out when heated.
Historically, water hardness described the capacity for water to react with a soap; hard water requires considerably more soap to produce a lather. This occurs because the sodium/potassium soap is converted into an insoluble non-detergent. This soap scum is the reaction of calcium with the soap; spotting of dishes from a dishwasher is another hard water residue.
As to consumption, both calcium and magnesium are essential minerals, beneficial to human health in several respects. While the primary human source is foods, water can make significant contributions to total intake of these nutrients for some populations and subgroups.
Dissolved minerals contribute to the taste of drinking water to varying degrees. The acceptability of a given level of minerals will usually depend on the individual’s familiarity and taste.
Corrosion and scaling is a function of hardness, pH and alkalinity. Hard water has deposition concerns, and excessively hard water can also have corrosion tendencies.
As to EPA regulations on hardness, it is not directly regulated, but can be indirectly regulated by standards for total dissolved solids in the Secondary Drinking Water Standards.
Indices to Address Water Scaling and Corrosion
At this point, the issue of assessing potential for water scaling and corrosion will be reviewed. The potential for water to scale or corrode is a complicated function of pH, temperature, alkalinity, calcium concentration and TDS. Several empirical indices have been developed to assess a water’s potential for scale or corrosion based on data for pH, temperature, calcium concentration and TDS. These indices include the Langelier Index, the Ryzner Stability index, and the Aggressive Index.
pH control is the most common means of corrosion and scale control for potable water distribution systems.
The Langelier Index is an approximate indicator of the degree of saturation of calcium carbonate in water. It is sometimes referred to as the Langelier Saturation Index. In the calculation, it is the difference between the actual pH and a calculated pH and can be interpreted as the pH change required to bring the water to equilibrium.
The Langelier Index is an important characteristic in potable water systems:
The Langelier Index is calculated using pH, alkalinity, calcium concentration, total dissolved solids and water temperature
The Langelier Index increases with temperature, and therefore, if positive, will become more scale forming with higher temperature.
Ryzner Stability Index
The Ryzner Stability Index was developed in the 1940’s as an attempt to improve on the Langelier index for prediction of calcium carbonate scale.
The index uses values for TDS, temperature, calcium hardness, m-alkalinity and pH. As with the Langelier Index, it is the difference between a calculated and the actual pH.
In the Ryzner index, a value above 6 indicates the water is likely to form calcium carbonate scale, while a value below 6 indicates the water will dissolve scale and be corrosive.
The Aggressive index was developed as a means to assess the corrosive tendency of water on asbestos cement pipe, though it is also used more generally. It is part of AWWA Standard C-400. It is another substitution for the Langelier Index. It is simpler and more convenient than the Langelier Index, but also less accurate. The Aggressive Index uses pH, calcium hardness, and alkalinity. It does not use temperature and TDS.
An AI above 12 indicates nonaggressive, non-corrosive water; an AI below 10 indicates extremely aggressive (corrosive) water. Values between 10 and 12 would be considered moderately aggressive.
Turbidity is the measure of relative clarity in a liquid; in water it is a cloudy appearance caused by small suspended particles, often referred to as colloidal or non-settleable solids. Turbidity is measured by shining a light through water and measuring the intensity of scattered light reflected back to the sensor.
Turbidity is aesthetically displeasing and can also represent a health concern, either due to the nature of the suspended particles, or because these particles provide food and shelter for pathogens.
Turbidity is required to be controlled under the EPA National Primary Drinking Water Standards. For systems that use conventional or direct filtration, at no time can turbidity go higher than 1 Nephelometric Turbidity Unit (NTU), and samples for turbidity must be less than or equal to 0.3 NTU’s in at least 95% of samples in any month. Systems that use filtration other than conventional or direct filtration must follow state limits, which must include turbidity at no time exceeding 5 NTU’s.
water, as well as certain organic acids and neutral salts, but is primarily related to contaminants of vegetable origin. As such, color is typically more of an issue with surface water rather than groundwater (though some shallow wells can have coloring issues).
An arbitrary standard scale has been developed for measuring color intensity in water samples, related to color developed by concentrations of potassium chloroplatinate in water.
Excessive color is aesthetically unappealing and can cause staining in laundry.
Color is regulated by EPA under their Secondary Drinking Water Standards, with a Secondary Maximum Contaminant limit (SMCL) of 15 color units.
In general, color is reduced or removed by coagulation, settling and filtration techniques.
Natural Water Impurities
The water contaminants that can be found in natural water can be grouped into five different categories:
For water contaminants/impurities, it can be useful to think of them as being in one of three physical forms:
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