Trickling Filters: Used to remove organic matter from wastewater using biofilm and aerobic microorganisms attached to a medium. Common in packed bed reactors (bio-towers). Normally known as attached-growth processes, but if microorganisms are in liquid it is called suspended-growth processes. 

The top view of a trickling filter

A basic overview of a trickling filter, the actual filtering layers will vary.

Energy efficiency recommendations at this step would be hard to define unless the facility has specific concerns about their trickling filter.