Calcium hydroxide
(endorsed 2005)
Calcium hydroxide (hydrated lime) is used to raise pH and adjust alkalinity for coagulation optimisation, corrosion control and water softening. It can also be used to dewater sludge.
General description
Calcium hydroxide, (also known as lime or hydrated lime), adds hydroxide ions to water, thereby increasing its pH and alkalinity. It is a soft, white, crystalline powder.
The hydrated lime available commercially is a powder that contains mainly calcium hydroxide, or a mixture of calcium hydroxide and magnesium hydroxide. Pure hydrated lime has a specific gravity of 2.3ā2.4. The bulk density of commercial lime varies from 450 to 560 kg/, and it usually contains 80ā96% calcium hydroxide. Its solubility at 20Ā°C is 0.165% (or 0.165 g/100 g of saturated solution). Hydrated lime can be stored in rubber-lined containers or in fibreglass-reinforced plastic, polyethylene, polyvinyl chloride, cast iron or steel.
Chemistry
Calcium hydroxide is obtained by hydrating quicklime with sufficient water to satisfy its chemical affinity for water. Quicklime is the product of the calcination of limestone, and consists mainly of the oxides of calcium (CaO) and magnesium (MgO). Calcium hydroxide is added to water to provide hydroxide ions to raise pH and alkalinity, and to neutralise free carbon dioxide or carbonic acid. It reacts with carbon dioxide to form calcium bicarbonate.
To remove carbonate hardness, hydroxide ions are used to raise the pH of water. This causes precipitation, as bicarbonate ions are converted to the carbonate (pH > 10), precipitating calcium carbonate.
Typical use in Australian drinking water treatment
In production of drinking water, calcium hydroxide is used:
at the start of the water treatment process, to adjust pH and boost alkalinity, to assist coagulation
at the end of the treatment process, to adjust final pH and alkalinity, and to minimise corrosion
to soften hard waters by raising the pH, and thus precipitating calcium carbonate
with carbon dioxide, to increase soft waterās resistance to pH changes during distribution and to decrease its corrosivity
to reduce the moisture content of sludge ā if the concentration of calcium hydroxide is sufficiently high it will collapse the sludge structure, helping to reduce the water content of the sludge.
Lime is usually made up as a solution or as a slurry of up to 10% concentration; a slurry with a concentration of 1ā5% is most commonly employed.
Typical lime concentrations used in drinking water treatment depend on the quality of the water to be treated and the purpose of the treatment (water softening, pH adjustment, alkalinity increase). Lime concentrations can vary from 5 to 500 mg/L, and the appropriate concentration should be determined by laboratory trials.
Poor mixing, poor pipe design, lime scaling and impurities often lead to blockages in lime dosing systems. To overcome such problems, the design of the system should minimise areas of solids accumulation, and the dosing system should be flushed each time it is turned off with water, chlorinated water or weak acid. Regular cleaning of the batch and dosing tanks using a solution of weak acid is also recommended.
Contaminants
The purity of chemicals used in Australia for the treatment of drinking water varies, depending on the manufacturing process. The following chemical contaminants may be present in calcium hydroxide, depending on the source of the raw materials (JECFA, KIWA 1994, NRC 1982):
aluminium
arsenic
barium
cadmium
chromium
fluoride
iron
lead
magnesium
manganese
mercury
nickel
selenium
silica
silver
Residual and by-product formation in drinking water
When employed in drinking water treatment, calcium hydroxide should be used in such a way that any contaminant or by-product formed by the use of the chemical does not exceed guideline values in the Australian Drinking Water Guidelines.
Adding lime to water can significantly raise the turbidity. It can also increase the concentrations of iron, aluminium and manganese. Thus, it is often best to add lime at the start of the water treatment process, so that any impurities added with the lime can be removed during the treatment process.
The sludge resulting from water softening consists mainly of calcium carbonate, or a mixture of calcium carbonate and magnesium hydroxide. This sludge is generally dense, stable and inert; dries well; has a solids content of about 5% from the clarifier (although it can range from 2 to 30%); and has a pH greater than 10.5.
Status
Calcium hydroxide was endorsed by the NHMRC for use as a drinking water treatment chemical in 1983. The revision undertaken in 2003 did not change the status of this chemical for the treatment of drinking water.
References
ANSI (American National Standards Institute)/AWWA (American Water and Wastewater Association) Standard no B202-02. AWWA CD-ROM (April 2003). Available at <www.awwa.org>
Clesceri LS, Greenberg AE and Eaton AD (eds) (1998). APHA Method 2340B, C, Hardness. In: Standard Methods for the Examination of Water and Wastewater, 20th edition., American Public Health Association, Washington, DC.
JECFA (Food and Agriculture Organization of the United Nations (FAO)/World Health Organization (WHO) Joint Expert Committee on Food Additives). Compendium of Food Additive Specifications. FAO Food and Nutrition Papers 52 (two volumes). Available at https://www.who.int/foodsafety/publications/jecfa/en/
KIWA (1994) Guideline quality of materials and chemicals for drinking water supplies. Inspectorate of Public Health and Environmental Planning, Publication 94-01. Rijswijk, The Netherlands.
National Lime Association (1992). Chemical Lime Facts, 6th edition. National Lime Association, Washington, DC.
National Lime Association (1995). Lime: Handling, Application and Storage, 7th edition. National Lime Association, Arlington, Virginia.
NRC (National Research Council) (1982). Water Chemicals Codex. Committee on Water Treatment Chemicals, Food and Nutrition Board, Assembly of Life Sciences, National Research Council.
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