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tertiary treatment
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Diagram illustrating tertiary wastewater treatment, showing two tanks: one for filtering, where water passes through filter media to produce treated water, and one for backwashing, where air and water clean the filter using sand, gravel, and anthracite layers. Key components and flow directions are labeled.
How is sewage and industrial wastewater reclaimed for use?
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tertiary treatment

sanitation engineering
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Diagram illustrating tertiary wastewater treatment, showing two tanks: one for filtering, where water passes through filter media to produce treated water, and one for backwashing, where air and water clean the filter using sand, gravel, and anthracite layers. Key components and flow directions are labeled.
tertiary treatment of wastewater (Left) During the filtering step, wastewater from secondary treatment, still containing suspended solids, pours from a trough and percolates through a filter bed made of porous media such as sand, gravel, and anthracite. The filtered water is then piped away for disposal. (Right) In the backwashing step, entrained solids are periodically flushed from the filter media by pumping filtered water back through the assembly. The backwash water, carrying suspended solids, is returned to the beginning of the wastewater treatment process.

Tertiary treatment, also known as advanced wastewater treatment, is the final stage in purifying sewage. It is employed when stringent environmental standards require the removal of more than 85% of solids and biochemical oxygen demand (BOD), or when specific nutrients like nitrates and phosphates must be reduced.1234

This level of treatment can produce effluent of nearly drinking-water quality but is significantly more expensive than secondary treatment, often doubling the cost. Tertiary processes can include:1234

  • Nutrient Removal: Chemical precipitation is used to remove phosphorus, while biological processes like nitrification-denitrification convert ammonia and nitrates into nitrogen gas, which escapes into the atmosphere.1
  • Effluent Polishing: Granular media filters or microstrainers are used to remove any remaining suspended solids and BOD.2

Following tertiary treatment, disinfection is the final step to destroy any remaining pathogens before the treated water is discharged.1234

Summary created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
4 Britannica Sources
  1. wastewater treatment - Removal of plant nutrients
  2. wastewater treatment - Effluent polishing
  3. wastewater treatment - On-site septic tanks and leaching fields
  4. wastewater treatment - Activated sludge

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