Water treatment

Lime product applications in water treatment vary greatly from the processing of drinking water to the purification of wastewater, the water treatment processes of natural water as well as mine water and to the stabilisation of sludge. Lime products are also used for the neutralisation of industrial wastewater.

In drinking water and wastewater treatment, lime products are used to raise the pH level of the water and to adjust alkalinity. In wastewater treatment, lime is also used to precipitate phosphorus and metals and to stabilise the sludge build up.

Water utilities can use either Nordkalk QL products i.e. quicklime or Nordkalk SL 90 products i.e. slaked lime. In the production of drinking water, originally dry products are dosed into the process as lime water according to pH control. Lime water stands for clear liquid, which does not contain lime as a solid form.

When using lime water, the reaction in the actual deacidification process is faster and more even as the dissolving process has already taken place. Lime water is made from a suspension of slaked lime. Big water utilities can use quick lime as the original material and it will be slaked in situ before dosing.

The main goal of wastewater treatment is to minimize the burden on the receiving watercourses by removing all organic, oxygen-consuming compounds and nutrients. In municipal wastewater treatment plants, waters are mostly treated with physical, biological chemical processes, where lime products are used for controlling the pH and alkalinity, precipitating nutrients, handling sludge and removing odors.

The pH and alkalinity of water can be raised with lime products. Lime products differ from each other chemically and affect the process and the end result in different ways.

Calcium carbonate is slightly caustic and it dissolves slowly. The product works best when ground fine and when the pH of the wastewater is less than 6.5. The product dosage is controlled by the flow rate. Calcium oxide and calcium hydroxide are strongly caustic. With these products the pH level of wastewater can easily be regulated to the desirable level. The chemical reactions are fast, therefore the pH signal can be used for controlling the dosage.

The different phosphorus compounds can be precipitated from waste water efficiently with calcium ions. The high pH level required for precipitation can be reached by using calcium oxide or calcium hydroxide.

Nordkalk’s QL and SL products i.e. quicklime and slaked lime are mainly used for the neutralisation of industrial wastewaters. In some cases the more slowly reacting limestone powder is needed.

Industrial wastewaters are treated either at a company’s own wastewater treatment plant or they are prepared before being led into the municipal sewer system. Lime products are used for neutralising acidic waters, the precipitation of dissolved impurities and removing colour. The biggest industrial wastewater treatment plants with different biological phases function just like the municipal plants, where lime products are used for controlling pH levels and alkalinity.

Lime products are used e.g. at steel mills and treatment plants for the neutralisation of different acid baths and the precipitation of metals. When the precipitation is done right at the start of the process, harmful heavy metals will not end up in the water ways or in the municipal water treatment plant’s sludge to impede the recycling of the sludge.

Lime products are good for precipitating different metal ions such as aluminium (AI), copper (Cu), chrome (Cr), iron (Fe), Zinc (Zn), lead (Pb), nickel (Ni) and cadmium (Cd). Lime products can also be used for removing harmful anions such as sulphates, fluoride and phosphates. Metals normally precipitate when the pH level is caustic.