Understanding Sewage Treatment Processes

Sewage treatment is a multi-stage process designed to remove pollutants from wastewater before it is discharged into the environment. These stages typically include primary, secondary, and sometimes tertiary treatment.

Primary Sewage Treatment

Primary treatment is the first stage and primarily involves the physical removal of large solids and suspended particles from sewage. This stage removes approximately 50-60% of suspended solids. Key processes in primary treatment include:

• Screening: Removing large objects like rags, sticks, and debris.
• Grit Removal: Settling of heavier inorganic materials like sand and gravel.
• Sedimentation (Primary Clarification): Allowing suspended organic solids to settle out as sludge or lighter materials (like fats, oils, and grease) to float to the surface as scum.

Skimming tanks are used in the primary sedimentation stage to efficiently remove the floating materials (fats, oils, grease) from the surface of the wastewater. This is a physical separation process.

Secondary Sewage Treatment Methods

Secondary treatment follows primary treatment and focuses on removing dissolved and colloidal organic matter that was not removed physically. This stage primarily uses biological processes involving microorganisms to break down organic pollutants. Common methods employed in secondary treatment include:

• Activated Sludge Process: Using a suspension of microorganisms in an aerated tank.
• Trickling Filters: Wastewater is trickled over a bed of media (like rocks or plastic) where microorganisms form a biofilm and biologically degrade organic matter.
• Rotating Biological Contactors (RBCs): Microorganisms grow on discs that rotate through the wastewater.
• Contact Beds (also known as Intermittent Sand Filters): Beds of sand or gravel are intermittently flooded with wastewater, allowing microorganisms growing on the media to treat the pollutants aerobically when drained. While sometimes considered a form of tertiary treatment due to the high effluent quality, older designs function primarily as biological secondary treatment.
• Sand Filters: Similar to contact beds but often operated continuously or intermittently for polishing effluent. They rely on biological activity on the sand surface and interstitial spaces.

Trickling filters, contact beds, and sand filters are all examples of fixed-film biological reactors commonly used in secondary (or sometimes advanced secondary/tertiary) sewage treatment processes.

Identifying Processes NOT in Secondary Sewage Treatment

Based on the descriptions above, Skimming tanks are used for the physical removal of floating matter like fats, oils, and grease during the primary treatment stage. They do not involve the biological degradation of dissolved or colloidal organic matter, which is the hallmark of secondary treatment.

Therefore, skimming tanks are NOT employed in the secondary treatment of sewage.

Revision Table: Key Sewage Treatment Stages

Additional Information on Biological Treatment

Biological treatment methods in secondary sewage treatment rely on the activity of heterotrophic microorganisms, primarily bacteria. These microbes consume the organic pollutants (measured as Biochemical Oxygen Demand - BOD) in the wastewater as their food source and energy. This process reduces the BOD significantly, typically by 85-95%.

The two main types of biological systems are:

• Suspended Growth Systems: Microorganisms are kept in suspension in the wastewater, such as in activated sludge tanks. Air or oxygen is typically supplied to maintain aerobic conditions.
• Fixed-Film Systems: Microorganisms grow as a biofilm attached to a stationary medium, such as rocks, plastic media (in trickling filters), or sand (in sand filters/contact beds). Wastewater flows over the biofilm, allowing the microbes to absorb and degrade the organic matter.

Trickling filters, contact beds, and sand filters fall under the category of fixed-film biological treatment systems.