In environmental engineering and wastewater treatment, the semi solid residue that settles out or is produced during the treatment of sewage and industrial effluents is commonly known as what?

Introduction / Context:
Wastewater treatment plants are designed to remove contaminants from sewage and industrial effluents before the water is discharged back into the environment. During these treatment processes, not only is the water purified, but various solid and semi solid wastes are also separated from the liquid. One important by product is a thick, mud like material that accumulates in settling tanks and other units. This question asks you to identify the common term used for this semi solid residue in environmental engineering.

Given Data / Assumptions:

• We are talking about residues produced during the treatment of sewage and industrial wastewater.
• The residue is semi solid in nature, thicker than water but not fully solid, and often accumulates at the bottom of settling tanks.
• The options are sedge, toxic waste, sludge and scrubber.
• We follow standard terminology used in wastewater and environmental engineering.

Concept / Approach:
In wastewater treatment, suspended solids and some dissolved substances are removed by processes such as sedimentation, biological treatment and chemical precipitation. The material that settles out as a semi solid mass is called sludge. Sludge can be primary, formed from settling of raw sewage solids, or secondary, formed from biological growths in treatment processes. It is usually thick, dark and contains organic matter, microorganisms and various pollutants. Sedge is a type of plant, toxic waste is a hazard classification rather than a specific type of treatment residue, and scrubber refers to equipment used to remove pollutants from gases, not to the residue in wastewater tanks. Therefore, the correct term for the semi solid residue described is sludge.

Step-by-Step Solution:
Step 1: Identify that the question is about semi solid material produced in wastewater treatment plants. Step 2: Recall that in settling tanks and clarifiers, solid particles settle out of the wastewater and form a thick mass at the bottom. Step 3: Recognise that this thick mass is referred to as sludge in environmental engineering terminology. Step 4: Consider option a, sedge, and note that sedges are grass like plants, not treatment residues. Step 5: Understand that toxic waste describes hazardous property, and scrubber is a gas cleaning device, so neither of these names fits the semi solid sewage residue. Therefore, sludge is the correct term.

Verification / Alternative check:
Engineering textbooks on wastewater treatment describe process flow diagrams in which influent sewage passes through screens, grit chambers and primary clarifiers. They show that the solids settling in primary and secondary clarifiers form primary and secondary sludge. Additional treatment steps such as thickening, digestion and dewatering are applied specifically to sludge to stabilise it before final disposal or reuse. Environmental regulations and guidelines also refer to sewage sludge and industrial sludge as materials requiring controlled handling. These consistent references confirm that sludge is the standard term used for the semi solid residue produced by wastewater treatment processes.

Why Other Options Are Wrong:
Option a, sedge, refers to a family of plants that resemble grasses and grow in wetlands and other habitats, not to waste material from treatment plants.

Option b, toxic waste, is a broad term that can refer to any hazardous waste, including liquids, solids or gases, and does not specifically describe the semi solid residue from wastewater treatment.

Option d, scrubber, is a device used in air pollution control to remove pollutants from exhaust gases, not the name of the residue in water treatment units.

Common Pitfalls:
Some learners may choose toxic waste because they know that sludge can contain pollutants and be hazardous, but the question is asking for the specific engineering term for the semi solid residue, not for a hazard category. Others might be unfamiliar with the word sedge and mistakenly think it is related to sediment. To avoid such confusion, it is helpful to remember that sludge is the official name given to the thick, mud like material that collects at the bottom of treatment tanks in sewage and industrial effluent treatment plants.

Final Answer:
The semi solid residue produced during wastewater treatment is called Sludge.