What indicates that a septic tank is in need of cleaning?

The indication that a septic tank requires cleaning is supported by all the factors presented in the options. Each of these signs points to a problem with the tank's ability to effectively treat and manage wastewater.

A deep sludge depth suggests that a significant amount of solids has settled at the bottom of the tank, which can reduce the available volume for wastewater, hinder bacteria's ability to break down solids, and lead to untreated wastewater being released into the drain field.

The presence of a thick fog layer indicates a large accumulation of scum on the surface of the wastewater, which can also limit the space for wastewater treatment by creating a barrier that prevents gases from escaping. This can lead to odors and potential backup issues.

Seeing particles of solids in the effluent is a strong sign of a malfunctioning system. This occurrence means that the solids are not being effectively decomposed and stored in the tank, and are instead being discharged into the drain field, potentially leading to clogs and environmental contamination.

Given that each of these conditions highlights either a significant accumulation of waste or an inability of the septic system to function properly, it is essential to address them. Therefore, identifying all of these indicators together (as indicated in the correct answer) emphasizes a comprehensive understanding of septic system maintenance and