What Is Lime Softening Used For?

… and,  why do water treatment plants seem to have different lime doses?

Groundwater or well water is a common source for municipal drinking water in Iowa. Water with high hardness is also common, especially with deeper wells. Hardness is a common term associated with a high mineral content in water, specifically in calcium and magnesium.

While dissolved minerals can be a benefit to the water’s taste, too much of any particular mineral can also be a problem. If there is too much hardness in the water, scaling can occur on plumbing fixtures and can lead to partial blockages in piping, coffee makers, and some appliances which use water jets coming out of nozzles or sprayers. Too much magnesium has been identified as a cause of shortened life cycles in water heaters due to scaling.

Solids Contact Unit cutaway

Solids Contact Unit cutaway

It is common to use a treatment goal in the range of 40 -65 mg/L as CaCO3 as a start for dosing calculations. Some water systems produce water with higher magnesium levels, based on site-specific factors.

Hardness is specifically defined as the sum of the calcium and magnesium concentrations in the water.

To simplify the expression of the chemical reactions involved with lime softening, all concentrations are expressed in terms of calcium carbonate or CaCO3.

To express the concentration of calcium and magnesium in terms of calcium carbonate, the following multiplication factors are used:

Calcium, mg/L as CaCO3 = 2.5 * Calcium, mg/L as Calcium

Magnesium, mg/L as CaCO3 = 4.1* Magnesium, mg/L as Magnesium

Converting to calcium carbonate equivalents is the first step in lime dose determination.

Higher hardness concentrations require more soap to wash clothes or dishes. Decades ago, more basic soaps were used to measure hardness and thus determine an appropriate level of hardness removal.

Modern detergents do a better job at lathering and cleaning so the water chemistry constituents are now used to determine appropriate doses. Regardless of the cleaning efficiency of modern detergents, the detergent must still overcome hardness as a barrier to laundering, and higher hardness requires a higher detergent use [Water Quality & Treatment, 6th Ed., Randtke].

Hardness is not a contaminant which must be removed to protect human health, such as those constituents in water which have a federally mandated maximum contaminant level (MCL). Federal drinking water standards do not have a suggested treatment goal for hardness, even for aesthetic reasons.

Calcium and magnesium, the two constituents which are used to quantify hardness in drinking water, are generally known to provide health benefits to humans. Further, certain levels of hardness provide a protective coating to the interior of steel or iron pipes. This coating prevents corrosion of the interior surface of the piping and pipe joint materials. Soft, or corrosive water can leach lead and copper out of metallic piping and solder materials at concentrations which exceed the MCL, exposing consumers to a higher health risk.

Therefore, the water treatment plant must strike a balance between leaving enough hardness in the drinking water for human health benefits and corrosion control, and removing enough hardness to prevent problems with plumbing fixtures and high detergent use.

Treatment technologies for hardness removal include lime softening, ion exchange softening, electrodialysis reversal (EDR), and membrane filtration (reverse osmosis or nanofiltration).

Batch slaker tank

Batch slaker tank

Treatment by ion exchange may not be a good fit for communities due to the cost of the brine needed for ion exchange, or the high levels of chlorides in ion exchange treatment residuals (backwash/regeneration water).

Treatment by membrane filtration generates high quantities of treatment residuals (reject water), which some municipal infrastructure or receiving streams simply cannot handle or accept due to water quality regulations. For these cities, lime softening continues to be a viable and effective treatment to reduce hardness and produce high quality drinking water.

Based on conversations with current clients and while presenting the topic of lime dosing at American Water Works Association (AWWA) Iowa Section conferences, treatment plant operators and managers have different lime doses and finished water hardness levels. The amount of lime to treat the water varies in ways which appear random upon a quick review. Further, and adding to the challenge, it is not uncommon for finished water hardness to be different for each well used by one particular water treatment plant.

The treatment goal is more of a range of concentrations. In Iowa, this range seems to be from 125 mg/L to 150 mg/L as CaCO3. For most treatment plants, the finished hardness is generally what fits best for all treatment goals, which falls within a typical range or what has historically been acceptable to the consumers. Most consumers cannot taste a difference between a hardness of 130 and 140 mg/L as CaCO3. This flexibility allows the treatment plant to target removal of some other constituents which may be present in low concentrations, but still are a concern.

Other treatment goals factor into a lime dose. First, there may be other constituents to remove. Lime softening can remove lead, cadmium, silver, chromium(+3), barium, arsenic, and selenium, which have MCLs. Treating for one of these inorganic contaminants may require a higher lime dose to reach a pH of 10 or 11. Another constituent which can be a problem with fixtures is silica. Silica removal can also be more effective with lower magnesium levels.

Some water treatment plants can have high costs associated with lime residual disposal. In Iowa, lime residuals are typically land-applied to row-cropped farm land. When land application sites are not located very close to the water treatment plant, higher hauling costs result (which can be exacerbated when fuel prices are high). This factor can cause a treatment plant manager to use less lime and remove less hardness. Lastly, there may be consumer pressure to keep treatment costs low, which may cause the water treatment plant to use less lime.

The presentation provided by FOX at recent AWWA conferences and regional meetings of water treatment professionals focused on the methods that FOX engineers use to estimate the lime dose. Some municipal operators only know the hardness and lime dose which has historically been provided, and are hesitant to change it. These operators appreciate that the chemistry of hardness removal can be complicated since the lime dose is not proportional to hardness removed. In other words, adding one more pound of lime does not remove one more pound of hardness.

The FOX presentation reviewed the water quality data needed in order to perform dosing calculations, followed by three methods for determining a lime dose. Most sets of water quality data include the following constituents:

  • Total Alkalinity
  • Calcium
  • Magnesium
  • Dissolved Carbon Dioxide
  • Temperature
  • Total Dissolved Solids
  • Chloride
  • Sulfate

Operators can take a representative sample of their well water and test it in house, and/or send it to their preferred lab to find the concentrations of each of these constituents.

The FOX presentation provided examples to illustrate all three methods for one given set of raw water quality data. These methods include a traditional calculation involving definitions of the types of hardness and approximately how much lime is needed to treat for each type, a computer model, and solubility plots/nomographs. The traditional calculation provides an estimate of the lime dose needed to meet the treatment goal, but has a lot of simplifying assumptions to make the math easy and has multiple steps. In comparison to the traditional calculation, the computer model is faster and more accurate.

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The computer model is a mathematical solution to the solubility plots/nomographs but due to the somewhat complex math functions involved with solubility, sometimes the computer model outputs lime dose answers which make no sense. It is in these situations where the water chemistry expertise of a seasoned professional and the last method are useful. The last method uses graphical procedures which illustrate the water chemistry in a way that allows one to see the effect of increasing/decreasing the chemical dose without doing the blind guessing or trial and error which seem to occur with the other two methods. Unless a person has experience with the solubility plots/nomograph method, it is not necessarily intuitive. Using multiple methods, FOX Engineering can assist the water treatment plant staff with understanding what an ideal lime dose should be.

 

FOX Engineering is an environmental engineering firm based in Ames, Iowa. We specialize in water and wastewater solutions for our diverse municipal and industrial clients. Our work varies in size and scope and can be found throughout the Midwest and beyond.