Before using tertiary filters, such as sand filters, what is a necessary action?

Before employing tertiary filters like sand filters, backwashing the filters is a critical step to ensure optimal performance. Backwashing is the process of reversing the flow of water through the filter media to dislodge and remove any accumulated solids, debris, and contaminants that have been trapped during the filtration process. This action helps to clean the media, restore its filtering capacity, and prevent clogging, which can lead to decreased efficiency or even failure of the filtration system.

When filters are clean and functioning properly, they are more effective at removing finer particles and impurities, thus improving the overall quality of the wastewater being treated. Skipping this step could result in insufficient treatment of the effluent, which can impact subsequent treatment processes and compliance with regulatory standards.

In the context of the other choices, increasing the flow rate may lead to poor filtration performance if the filters are compromised. Adding additional media could be necessary in some circumstances, but it typically doesn't precede the basic cleaning action of backwashing. Calibrating pressure gauges is an essential maintenance task but only needs to be done after ensuring that the filters are clean and operational. Therefore, backwashing is a necessary preparatory step to guarantee that the tertiary filtration system operates effectively.