Lime softening

Lime softening, also known as Clark's process,^[1] is a type of water treatment used for water softening.

In the USA, it is used primarily in the Midwest, Florida and Texas. It utilizes the addition of limewater (calcium hydroxide) to remove hardness (calcium and magnesium) ions by precipitation. The process is also effective at removing a variety of microorganisms and dissolved organic matter.^[2]

History

Lime softening was first used in 1841 to treat Thames River water. The process expanded in use as the bactericidal effect of the process was discovered. Lime softening greatly expanded in use during the early 1900s as industrial water use expanded. Lime softening provides soft water that can, in some cases, be used more effectively for heat transfer and various other industrial uses.

Chemistry

Softening can be achieved by adding lime in the form of limewater, Ca(OH)₂, which, in a carbonatation reaction with CO₂, forms calcium carbonate precipitate, reacts next with multivalent cations to remove carbonate hardness, then reacts with anions to replace the non-carbonate hardness due to multivalent cations with non-carbonate hardness due to calcium. The process requires recarbonation through the addition of carbon dioxide to lower the pH which is raised during the initial softening process.^[3]

As lime is added to raw water, the pH is raised and the equilibrium of carbonate species in the water is shifted. Dissolved carbon dioxide (CO₂) is changed into bicarbonate (HCO₃^-) and then carbonate (CO₃^2-). This action causes calcium carbonate to precipitate due to exceeding the solubility product. Additionally, magnesium can be precipitated as magnesium hydroxide in a double displacement reaction.

The process is interesting in that both the calcium (and to an extent magnesium) in the raw water as well as the calcium added with the lime are both precipitated. This is in contrast to ion exchange softening where sodium is exchanged for calcium and magnesium ions. In lime softening, there is a substantial reduction in total dissolved solids (TDS). In ion exchange softening (sometimes referred to as zeolite softening), there is no significant change in the level of TDS.

Lime softening can also be used to remove iron, manganese, radium and arsenic from water.

Future uses

While lime softening continues to be used at existing facilities, newer treatment plants often use membrane softening such as nanofiltration or reverse osmosis. These processes remove calcium and magnesium ions as well as other dissolved components by way of passage through a semipermeable membrane. Membrane processes generally produce a larger volume of waste — up to 20% of the input stream, but use substantially fewer chemicals and produce little, if any, solid waste.

Lime softening is now often combined with newer membrane processes to reduce waste streams. Lime softening can be applied to the concentrate (or reject stream) of membrane processes, thereby providing a stream of substantially reduced hardness (and thus TDS), that may be used in the finished stream. Also, in cases with very hard source water (often the case in Midwestern USA ethanol production plants), lime softening can be used to pre-treat the membrane feed water.

Waste products

Lime softening produces large volumes of calcium carbonate and magnesium hydroxide sludge. This residual stream can substantially increase costs associated with the process. The sludge can be used as an agricultural soil amendment (it is an alkalinity supplement) and can also be used as a filler material in certain cementitious materials such as low strength concrete.

References

1. ↑ Mellor, J W, Intermediate Inorganic Chemistry, Longmans, Green & Co, London, 1941, p. 202
2. ↑ usbr.gov
3. ↑ "Lime Softening". Retrieved 4 November 2011.