Understanding Substances in Sewage

When we talk about sewage, which is essentially wastewater containing waste from homes, schools, and industries, it contains many different types of materials. These materials can be dissolved or suspended. The suspended materials can either float on the surface or settle down at the bottom.

Identifying Floating Substances in Sewage: Scum

The question asks about the greasy and other substances that float on the surface of sewage. This specific layer of floating material has a technical name in wastewater treatment. Let's look at the options provided:

• Sludge: This refers to the solid material that settles down at the bottom of the sewage during treatment processes. It is the heavier material, not the floating layer.
• Scum: This is the layer that forms on the surface of the sewage. It typically consists of lighter materials like greases, oils, fats, plastics, and other buoyant debris that do not settle.
• Sewage: This is the general term for the wastewater itself, containing all the dissolved and suspended materials. It is not the name of the floating substances specifically.
• Aerobic: This term describes a condition or process that requires the presence of oxygen. For example, aerobic bacteria are used in some stages of sewage treatment. It is not a type of substance found in sewage.

Based on the definitions, the greasy and other substances floating on the surface of sewage are specifically termed as scum.

Comparing Scum and Sludge in Sewage Treatment

In primary sewage treatment tanks (like sedimentation tanks), sewage is allowed to sit for a period. During this time, the heavier solids settle to the bottom (forming sludge), and the lighter materials float to the surface (forming scum). Both scum and sludge are removed separately as part of the treatment process.

Therefore, the correct term for the greasy and other substances floating on the surface of sewage is scum.

Revision Table: Sewage Components

Additional Information: Sewage Treatment Processes

The removal of scum and sludge is a critical step in primary sewage treatment. This physical process separates the easily settleable solids and floatable materials from the main body of wastewater before further biological or chemical treatment.

• Primary Treatment: This initial stage uses screens to remove large debris and then sedimentation tanks where gravity separates scum (floats) and sludge (settles).
• Secondary Treatment: This stage uses biological processes, often involving microorganisms, to remove dissolved and suspended organic matter that wasn't removed in primary treatment.
• Tertiary Treatment: An advanced stage that removes remaining pollutants, such as nutrients (nitrogen and phosphorus) and pathogens, before discharging the treated water.

Understanding the different components like scum, sludge, and the processes involved helps in comprehending how wastewater is cleaned before being released back into the environment.