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The functions of lime in water treatment

Lime in all its forms – quicklime and hydrated lime, as well as calcined dolomite – are the most effective and cost efficient alkaline products used in the making of drinking water, as well as in the treatment of waste water and sludge.  In fact, most of the lime that is produced is used to improve the quality of drinking water as well as the water used by industry. Accordingly, lime allows water to be softened, purified, have its cloudiness eliminated, its acidity to be neutralized and its impurities to be eliminated, etc.

Because of all these advantages, explained in more detail below, calcium oxide (CaO) and calcium hydroxide (Ca(OH)2) are used in drinking water purification plantsCalcined dolomite (CaO MgO), can also be used in water treatment plants, for example to eliminate silica from water. This is because the magnesium in the dolomite acts as the main reactive agent for the elimination of silica. The use of quicklime and slaked lime is much less common.

Thanks to the use of lime in water treatment plants an improvement in the taste, smell and colour of the water is achieved, among other benefits, because suspended matter is eliminated, and therefore any cloudiness is eliminated. Moreover, manganese, fluorides and organic tannins are also eliminated along with the afore-mentioned silica.

One of the most important features in this type of application is that of disinfection. When lime is added to water to take the pH up to between 10.5-11.00 for a period of between 24 to 72 hours it is possible to lower the quantity of bacteria and viruses in it, at the same time most of the heavy metals dissolved in it are also eliminated.

In the same way, the pH of the water can be adjusted, via the use of lime milk, until it becomes suitable for its subsequent treatment in the drinking water plant.

Soft water and hard water

Continuing with the subject of the treatment of drinking water, lime provides another advantage worth mentioning because it allows water quality to be modified, either re-mineralizing water that is too soft or on the contrary, decarbonating water that is too hard.

The former is a case of water with an acidic nature, that is with a low pH. This could be because of the local geology, but can also be caused by human influence. To avoid possible side-effects from the consumption of this type of soft-water, it is necessary to adjust the calco-carbonic balance, that is to control the pH with simultaneous softening.

On the other hand, if it is necessary to lower the hardness of the water it can be subjected to a process of decarbonation, avoiding precipitates and the incrustation of calcium-carbonate. For this application, the chemical products used are calcium-oxide (CaO) or calcium hydroxide (Ca(OH)2), added in suspension (lime milk) or dissolved (lime water), bringing about the precipitation of the calcium as calcium-carbonate and magnesium as hydroxide, achieving a reduction in the number of calcium cations and the dissolved magnesium concentration.

Waste water

So far, we have only spoken about the use of lime in the treatment of drinking water, but it also has several applications in waste water treatment.

In the case of conventional chemical treatment of industrial inorganic waste waters, lime has several advantages: control of the pH, neutralization of acidic waste water, a reduction in the concentration of oxidizable organic pollutants, clarification, precipitation of dissolved pollutants as well as flocculation and coagulation of colloidal particles.

Por su parte, el empleo de la cal en las plantas de aguas residuales para el tratamiento de lodos orgánicos comenzó a extenderse hace más de 100 años. El producto cálcico más utilizado en las Estaciones de Depuración de Aguas Residuales (EDAR) es el hidróxido de calcio (Ca(OH)2), con una pureza superior al 90%.

The use of lime in waste water treatment plants for the elimination of organic sludge became widespread over 100 years-ago. Nowadays the most widely used lime product in Waste Water Treatment Plants (WWTP) is calcium hydroxide (Ca(OH)2), with a purity of more than 90%.

Several advantages are provided by the application of lime in these plants, both in the water line (precipitating toxic metals and lowering the level of suspended solids) as well as in the sludge line (conditioning all type of sludge, stabilizing it without it becoming contaminated, eliminating bad smalls and destroying pathogenic agents, making it more hygienic).

Such are the advantages of treating sludge with lime that it is specifically prescribed in the USA by the Environmental Protection Agency (EPA) for use in Waste Water Treatment Plants. In Europe there are several decrees that regulate the treatment of sludges, in the manual used by the industry “Good practices in sludge treatment” lime and calcium hydroxide are included as suitable products for the treatment and cleaning of sludges.