What constitutes a significant indicator of the organic load on a trickling filter?

A significant indicator of the organic load on a trickling filter is the presence of various observable characteristics in the wastewater and the system's performance. When considering the options provided, each contributes to understanding the filter's operation and the organic load it experiences.

The presence of insects can indicate organic material that attracts certain pests, which may suggest a higher organic load. Insects typically thrive in environments with significant decomposing matter, which can reflect the biological activity within the trickling filter.

The color of wastewater is another crucial aspect. Variations in color can indicate different levels of organic compounds. For instance, darker-colored effluent may suggest higher concentrations of organic materials or pollutants, which relate to the organic load the system is processing.

Ponding problems can also serve as an indicator. When a trickling filter experiences ponding, it often means that the filter media is becoming clogged due to a buildup of organic material, reducing the filter’s effectiveness to treat wastewater efficiently. This issue typically arises when the organic load exceeds the filter's designed capacity.

Each of these factors—presence of insects, color of wastewater, and ponding issues—is relevant to assessing the organic load in trickling filters. Thus, the option that encompasses all these indicators accurately reflects the overall indicators of