What is considered a caustic substance used in wastewater treatment?

A caustic substance is one that has alkaline properties and can cause chemical reactions that neutralize acids or break down contaminants. Calcium hydroxide, commonly known as lime, is widely used in wastewater treatment processes. It acts as a strong alkaline agent, helping to raise the pH of acidic wastewater and facilitating the precipitation of heavy metals and other impurities. By increasing pH, it helps to stabilize the waste material and make it easier to handle or treat further.

In the context of wastewater treatment, the effective use of calcium hydroxide can enhance sedimentation processes and promote the removal of odors produced by organic matter. Its reactivity allows it to convert dissolved solids into solid forms that can be more easily filtered out.

Other substances listed do not have the same caustic properties. Sodium chloride is primarily used for salinity control and disinfection rather than as a caustic agent. Alum serves as a coagulant, helping to bind particles together but is not classified as caustic. Ferrous chloride is used for removing phosphates and as a coagulant as well, but again, it does not possess aggressive alkaline properties like calcium hydroxide.