The rock in trickling filters is placed?

In trickling filter systems, the rock or media is typically arranged on top of system tiles that sit above the under drains. This setup is essential for the efficient collection of the effluent that filters through the media. The system tiles help to distribute the flow evenly and minimize the risk of clogging, which ensures that the wastewater can effectively pass through the rocks and receive treatment due to contact with the biofilm that forms on the media.

Ensuring that the rock is placed on system tiles also promotes proper drainage and aeration, which are critical for the biological processes taking place within the trickling filter. This arrangement allows for optimal contact between the wastewater and the microorganisms, which ultimately enhances the treatment efficiency.

In contrast, the other choices do not accurately represent best practices for installing media in a trickling filter. For instance, uniform placement of the rock, while it might sound practical, does not account for the need for adequate drainage and airflow. Arranging the media from small to large or with large voids is not a standard approach either, as these methods could lead to uneven water distribution and inadequate treatment. Thus, placing the rock on system tiles under drains is the most effective and correct answer.