Tennessee Wastewater Operator Practice Exam

What happens to alkalinity when either alum or ferric chloride is added to wastewater?

1. Alkalinity is produced

2. Alkalinity remains unchanged

3. Alkalinity is consumed

4. Alkalinity increases

The correct answer is: Alkalinity is consumed

When alum or ferric chloride is added to wastewater, the chemical reactions involved consume alkalinity. This occurs because both of these coagulants react with the bicarbonate and carbonate ions present in the wastewater, which are key components of alkalinity. Specifically, when these coagulants are introduced, they promote the formation of floc that binds to suspended solids and other impurities for removal. In this process, hydroxide ions are produced leading to a decrease in the concentration of carbonate and bicarbonate ions, thus consuming the available alkalinity. As a result, the overall alkalinity of the wastewater is reduced due to these chemical interactions. Understanding this reaction is crucial for wastewater operators, as maintaining proper levels of alkalinity is important for effective treatment processes, especially in maintaining pH levels and optimizing coagulation and flocculation operations.