In sewage and wastewater treatment, the process specifically used to remove floating oil and grease from wastewater is known as what?

Introduction / Context:
This question connects environmental chemistry with civil engineering and public health. Sewage and wastewater contain various types of impurities, including suspended solids, dissolved substances, and floating materials like oil and grease. Different treatment steps are designed to remove different categories of pollutants. You are asked to identify the specific process used to remove floating oil and grease from wastewater, which is an important pre-treatment step before further purification.

Given Data / Assumptions:

• The focus is on removal of oil and grease from sewage or wastewater.
• Options include skimming, filtration, screening, sedimentation and chlorination.
• Oil and grease are usually lighter than water and tend to float on the surface.
• Standard wastewater treatment terminology is assumed.

Concept / Approach:
Skimming is the process by which floating oil, grease and scum are removed from the surface of wastewater. In a skimming tank or chamber, wastewater is allowed to stand or flow slowly so that oils and greases rise to the surface due to their lower density. Mechanical scrapers or simple overflow systems then remove this floating layer. Screening, in contrast, removes larger solid objects; filtration removes fine suspended particles by passing water through porous media; sedimentation allows heavier solids to settle; and chlorination is used for disinfection. Therefore, the appropriate technique for removing floating oil and grease is skimming.

Step-by-Step Solution:
Step 1: Recognise that oil and grease are less dense than water and therefore tend to float on the surface of sewage. Step 2: In a treatment plant, wastewater is allowed to flow through a chamber where flow velocity is reduced, giving time for oils and greases to rise to the surface. Step 3: The surface layer of oil and grease is then removed mechanically; this removal process is called skimming. Step 4: Screening refers to the removal of large floating debris such as sticks, leaves, plastics and rags using metal screens, not specifically oil. Step 5: Filtration is used at later stages to remove finer suspended particles and is not the primary method for oil and grease removal. Step 6: Sedimentation removes heavier suspended solids that settle at the bottom, which is the opposite behaviour to floating oil and grease. Step 7: Chlorination is a disinfection step using chlorine and does not physically remove oil and grease. Thus skimming is the correct process.

Verification / Alternative check:
Descriptions of standard sewage treatment plants often mention primary treatment including screening, grit removal, sedimentation and skimming. Skimming tanks are specifically designed for the removal of grease and oil. They may be located after bar screens and grit chambers. Textbook diagrams show skimming arms or mechanisms that sweep the surface to collect foam, oil and scum. In contrast, filtration units and chlorination facilities are located downstream, after most physical impurities have been removed. This alignment of processes confirms that skimming is the named method for removing oil and grease.

Why Other Options Are Wrong:
Screening removes large floating solids, not emulsified or free oil and grease. Filtration is aimed at finer suspended material and will clog quickly if used to remove large amounts of oil. Sedimentation is used to settle out heavy particles like sand and silt that sink, not those that float. Chlorination kills microorganisms but does not physically eliminate floating oils and greases from the water. Therefore, these options do not describe the specific process used to handle oil and grease in sewage treatment.

Common Pitfalls:
Students sometimes confuse screening and skimming because both occur early in treatment and involve removing materials from wastewater. The key difference is that screening uses bars or meshes to catch large solids, whereas skimming removes a floating scum layer from the surface. Another mistake is to think that sedimentation covers all physical removal, ignoring the fact that light materials float instead of sink. To avoid such confusion, remember: heavy particles settle (sedimentation), large debris is caught (screening), and floating oils and greases are taken off the top (skimming).

Final Answer:
The process used to remove floating oil and grease from sewage is called skimming.