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

The term "sludge" in the context of wastewater treatment refers to the solid byproduct that settles at the bottom of treatment tanks. This byproduct primarily consists of organic and inorganic matter that remains after the liquid wastewater has undergone various treatment processes. As wastewater is treated, suspended solids and biological materials, such as bacteria and microorganisms, aggregate and form sludge.

This is an essential aspect of wastewater treatment as it requires further processing, often involving digestion and stabilization, before it can be disposed of or reused. The accumulation of sludge is a key indicator of the treatment plant's performance and can impact the efficiency of the treatment processes if not managed properly.

Other options presented do not accurately describe sludge. For instance, liquid waste treated by chemicals does not represent the solid material that accumulates; dissolved oxygen pertains to a critical factor in aerobic treatment but is not related to the solids that settle; and filtered water after treatment is the effluent that has undergone treatment, which is the opposite of sludge.