Define the term 'sludge' in the context of wastewater treatment.

In the context of wastewater treatment, the term 'sludge' refers to the solid material that settles at the bottom of a treatment tank after the liquid waste has been processed. During the treatment process, wastewater is typically subjected to physical, chemical, and biological processes that promote the separation of solids from the liquids. The heavier solids, which include organic matter, microorganisms, and non-biodegradable materials, settle out of the liquid effluent to form a layer of sludge.

This sludge typically requires further treatment before disposal or can be processed into biosolids for land application, depending on regulatory standards and the level of treatment applied. Understanding the composition and characteristics of sludge is crucial in wastewater management, as it affects not only the performance of treatment facilities but also the environmental impact of wastewater disposal practices.

The other options mentioned do not accurately describe 'sludge' as they refer to different aspects of the wastewater treatment process, such as effluent, microorganisms, and disinfecting chemicals.