Tertiary Treatment Explained: What Happens After Secondary Wastewater Treatment

Recent Trends in Tertiary Treatment

Regulatory pressure and water scarcity have pushed tertiary treatment from a niche upgrade to a common requirement in many regions. Recent interest centers on polishing effluent for potable reuse, with municipalities exploring advanced oxidation, membrane filtration, and nutrient removal beyond secondary capabilities. Utilities are also retrofitting existing plants to meet tighter discharge limits on phosphorus and nitrogen.

• Growing adoption of membrane bioreactors (MBRs) combined with reverse osmosis for high-quality reuse.
• Increased use of ultraviolet (UV) disinfection and advanced oxidation for pathogen reduction.
• Focus on real-time monitoring of effluent quality to optimize chemical dosing and energy use.

Background: The Role of Tertiary Treatment

Secondary treatment typically removes around 85–90% of organic matter and suspended solids. Tertiary—or advanced—treatment targets the remaining pollutants, including nutrients, trace organics, pathogens, and residual solids. The specific processes depend on the intended reuse or discharge requirements. Common steps include filtration (sand, cloth, or membrane), disinfection (chlorine, UV, ozone), and nutrient polishing (biological or chemical phosphorus removal, denitrification).

• Polishing filtration: Removes fine particles escaped from secondary clarifiers; typical turbidity reduction to below 2 NTU.
• Nutrient removal: Lowers total nitrogen to under 3 mg/L and total phosphorus to below 0.1 mg/L for sensitive receiving waters.
• Disinfection: Achieves fecal coliform levels below regulatory limits for recreational or irrigation uses.

User Concerns and Operational Considerations

Operators of wastewater treatment plants often face trade-offs between cost, performance, and reliability when adding tertiary steps. Key concerns include energy consumption (particularly for membranes and UV), chemical costs (coagulants, polymers, disinfectants), and maintenance of advanced equipment. Sludge handling and disposal can also become more complex as tertiary processes increase solids production or change sludge characteristics.

• Initial capital investment: Can range from 10–30% higher than a secondary-only plant of similar capacity.
• Operational complexity: Requires trained staff for membrane cleaning, UV lamp replacement, and chemical feed adjustments.
• Regulatory compliance: Permits may demand quarterly or monthly reporting on additional parameters; failure can lead to fines or discharge limits.

Likely Impact on Water Quality and Reuse

When designed and operated correctly, tertiary treatment produces effluent that meets strict standards for non-potable reuse (agriculture, landscaping, industrial cooling) and can support indirect potable reuse after environmental buffer or advanced treatment. Typical improvements include reduction of biochemical oxygen demand (BOD) to below 5 mg/L, total suspended solids (TSS) below 5 mg/L, and near-complete removal of pathogens. Nutrient removal helps prevent algal blooms in receiving waters.

• Increased water recycling potential: Communities can reduce freshwater withdrawals by 30–50% with appropriate reuse schemes.
• Protection of aquatic ecosystems: Lower nutrient loads reduce eutrophication risks in lakes, rivers, and coastal zones.
• Long-term cost savings: Reused water for non-potable needs lowers demand on potable supplies and can defer new source development.

What to Watch Next

As water scarcity intensifies, tertiary treatment is expected to become standard for new plants in arid and semi-arid regions. Innovations in low-energy membranes, electrochemical nutrient removal, and smart automation may reduce operational hurdles. Policy shifts toward water reuse regulations—such as the U.S. EPA’s updated Water Reuse Guidelines—could accelerate adoption. Watch for pilot projects combining tertiary treatment with decentralized systems and for cost reductions in advanced oxidation technologies.

• Emerging standards for direct potable reuse may require multiple barriers including reverse osmosis and advanced oxidation.
• Artificial intelligence and digital twins are being tested for real-time optimization of tertiary processes.
• Funding programs (e.g., state revolving funds) increasingly prioritize projects that include advanced treatment for reuse.