What is the purpose of having a screen on the vent in a sand filter?

The purpose of having a screen on the vent in a sand filter primarily serves to prevent insects and other small animals from entering the system. This function is essential because if insects, such as mosquitoes, were allowed access, they could breed within the filter and potentially introduce contaminants. This could lead to malfunctions in the system and compromise its effectiveness in treating wastewater.

While enhancing airflow and filtering out debris are important functions in various contexts of wastewater treatment, the specific design choice of adding a screen to the vent is explicitly focused on excluding unwanted pests. Gauging pressure is also related to system monitoring, but it is not typically associated with the vent's role. Thus, the correct answer highlights the necessity of keeping the sand filter system clean and functional by preventing infestations.