How can alkalinity be increased in water treatment?

Increasing alkalinity in water treatment is primarily achieved through the addition of compounds that can provide hydroxide ions (OH-) or bicarbonate/carbonate ions (HCO3-/CO3^2-). The correct approach involves using substances such as caustic soda (sodium hydroxide), soda ash (sodium carbonate), or lime (calcium hydroxide). Each of these substances contributes to the chemical composition of the water by raising the pH and supplying alkaline ions, which effectively increases the alkalinity.

Caustic soda adds hydroxide ions directly, which elevates the water's alkalinity and pH. Soda ash can increase both alkalinity and pH by supplying carbonate ions. Lime, when dissolved, increases alkalinity by generating hydroxide ions through its dissociation in water.

In contrast, the other choices do not correctly address the method of increasing alkalinity. Decreasing water temperature is unrelated to alkalinity adjustments; it typically has an effect on solubility and reaction rates but does not add alkaline substances. Filtering through sand mainly serves to remove particulates rather than alter chemical properties like alkalinity. The introduction of alum or ferric salts is generally aimed at coagulation and flocculation processes, which can lower p