Why is a rapid sand filter backwashed at a lower rate at the beginning?

The correct answer is based on the principle of ensuring that the rapid sand filter operates efficiently. When backwashing a filter, particularly at the beginning of the backwash cycle, it's important to purge trapped air that may have accumulated in the filter media. If air is present, it can lead to uneven flow patterns and impede the effectiveness of the backwash process.

By starting the backwash at a lower rate, it allows for the gradual release of air bubbles and helps in preventing air entrapment, thereby ensuring a smoother backwash that can effectively dislodge particulate matter from the filter media. This step is critical because a well-functioning filter relies on proper hydraulic conditions to effectively clean itself.

Other options relate to different functions that backwashing serves but do not directly explain the importance of starting at a lower rate. Rinsing off chemicals, maintaining filtration rates, and preventing clogging highlight other aspects of filtration management but do not specifically address the venting of air during the initial phase of backwashing. Understanding the role of air purging in the backwash cycle is essential for maintaining the overall performance and longevity of the filtration system.