Question 1

What is hydrated lime used for in water treatment in Oman?

Accepted Answer

Hydrated lime (calcium hydroxide, Ca(OH)₂) is used in water treatment for pH correction of desalinated or acidic water, lime softening (removing hardness by precipitating calcium and magnesium), phosphorus co-precipitation in wastewater treatment, and heavy metal precipitation in industrial effluent. In Oman's municipal water treatment, hydrated lime is one of the primary chemicals for remineralising RO permeate — raising pH from the naturally acidic 5.5–6.5 of desalinated water to the target distribution pH of 7.5–8.0. It is also widely used in Oman's construction sector for soil stabilisation and as an alkaline supplement in concrete and mortar, particularly relevant to Vision 2040 infrastructure projects across the country.

Question 2

How is lime dosed for pH adjustment in municipal water treatment?

Accepted Answer

Hydrated lime is dosed as a slurry (lime milk) prepared by mixing lime powder with water in a slurry tank at 5–15% concentration. The slurry is then dosed into the treated water stream using a peristaltic or progressive cavity pump. Automated pH control systems with inline pH sensors provide feedback to the dosing pump to maintain target pH. Lime slurry systems require agitation to prevent settling and pipe blockage — a consideration in Oman where temperature fluctuations can affect slurry consistency. Typical dosing rates for RO permeate pH correction are 2–8 mg/L as Ca(OH)₂, depending on the initial pH and alkalinity of the desalinated water and the target distribution pH.

Question 3

What is the difference between quicklime and hydrated lime for water treatment?

Accepted Answer

Quicklime (calcium oxide, CaO) is the anhydrous form produced by calcining limestone, while hydrated lime (calcium hydroxide, Ca(OH)₂) is produced by reacting quicklime with water (slaking). Quicklime has a higher available lime content per tonne (90–95% CaO vs. 68–74% CaO equivalent in Ca(OH)₂) and is more economical for large-scale applications. However, quicklime requires on-site slaking equipment, generates significant heat during slaking, and is more hazardous to handle. Hydrated lime is safer to handle, dissolves more easily, and is preferred for smaller to medium water treatment plants in Oman that lack slaking equipment. For large PAEW treatment plants with the necessary infrastructure, quicklime is often used to reduce chemical costs.

Question 4

How should hydrated lime be stored and handled safely?

Accepted Answer

Hydrated lime is a mildly alkaline irritant (pH 12.4 in suspension) that absorbs CO₂ from air, gradually converting to calcium carbonate (carbonation) and losing reactivity. Bags must be stored in a dry, sealed warehouse to prevent moisture absorption and carbonation. In Oman's humid coastal climate, opened bags should be resealed and used promptly. PPE requirements include dust mask (P2 rated), chemical-resistant gloves, and eye protection to avoid dust inhalation and eye irritation. Lime dust can cause respiratory irritation on prolonged exposure. Storage areas must be ventilated. When preparing lime slurry, add lime to water (not water to lime) to control heat generation. Keep away from acids and ammonium compounds.

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