Which of the following systems is rarely used as a primary treatment method?

The deionization tank is typically used as a secondary treatment method rather than a primary one. This is because deionization is a process that effectively removes ionic impurities from water, but it is often employed after primary treatment methods have already been used to address larger particulates and contaminants. For instance, water might first undergo filtration and some form of primary treatment to reduce turbidity and organic matter before passing through a deionization system to ensure that it meets specific purity standards.

In practical use, systems like carbon tanks and brine tanks serve critical roles in primary treatment by addressing contaminants like chlorine, organic compounds, and hardness in water, respectively. Chemical feed systems are also vital for primary treatment as they can adjust water chemistry by adding necessary chemicals to correct imbalances or remove contaminants. The limited use of deionization as a primary treatment method stems from its focused application on achieving high-purity water, which is not the initial goal in most water treatment processes.