Water and Wastewater Treatment

Primary treatment removes coarse solids through physical processes such as screening, grit removal and sedimentation. Secondary treatment breaks down dissolved organic matter using biological systems such as activated sludge or MBR. Tertiary treatment removes phosphorus, heavy metals and pathogens through coagulation, filtration and disinfection.

Lime is used in wastewater treatment to raise pH, neutralize acids, precipitate heavy metals, remove phosphorus and support coagulation. Hydrated lime, Ca(OH)₂, is the backbone chemical of this work. It is typically fed by dosing pumps, and the right amount depends on the wastewater's character and on the discharge limits the plant must meet.

Hydrated lime (Ca(OH)₂) is ready for direct use and is frequently preferred for neutralizing acidic waters. Quicklime (CaO), on the other hand, is brought into liquid form (milk of lime) in large-scale plants by mixing it in on-site slaking units, and its transportation is more economical.

Wastewater containing heavy metals is treated by adding hydrated lime, Ca(OH)₂, until the pH reaches the 9-11.5 range. At these levels, metal ions such as chromium, copper, nickel, zinc and lead precipitate as insoluble hydroxides, which are then easily filtered out in solid-liquid separation basins. The resulting sludge must be handled and disposed of properly.

Odor control is achieved by neutralizing sulfurous gases such as H₂S and mercaptans with lime-based gas-removing chemical absorbers. These products bind the odor-causing compounds before they spread, which also protects worker health. Odor emissions are one of the significant problems of treatment plants, so this step is usually planned together with the rest of the process.

The lime dosage is determined with 'Jar Test' analyses carried out in the laboratory, based on the raw water's pH value, alkalinity and heavy metal load. Dosage should not be guessed. After the laboratory result, pH and turbidity meters at the plant outlet allow the dosing pumps to be adjusted in real time as conditions change.

Lime softening is the process of adding hydrated lime to drinking or process water so that the calcium and magnesium ions causing carbonate hardness precipitate as insoluble compounds and are removed from the system. The precipitated solids settle and are separated, leaving softer water. The dosage depends on the raw water's hardness and alkalinity.

Because discharging wastewater outside the permitted pH window can harm receiving waters and treatment systems, environmental laws in Türkiye and around the world generally require pH to be kept between 6.5 and 8.5. The most economical way to achieve this is to add lime to acidic waters, with the dose adjusted automatically according to pH measurements.

Lime stabilizes sludge by raising its pH to around 11.5, which halts the proliferation of pathogenic (disease-causing) organisms. This high-pH environment also prevents odor formation and enables the sludge to be disposed of safely. The required amount depends on the sludge's composition and moisture content, so it is normally set through testing at the plant.

The hyper-alkaline treatment method is applied to mining or metallurgy wastewater with a serious potential for heavy metal toxicity. The pH is deliberately raised above 11 with lime to achieve strong precipitation of the metals, which are then removed as sludge. Standard neutralization is usually sufficient for ordinary acidic wastewater, so this method is reserved for harder cases.