Why is tertiary wastewater treatment more expensive than secondary treatment?

Tertiary wastewater treatment is often significantly more expensive than secondary treatment—sometimes doubling the total cost—because it requires additional, complex processes to remove impurities that secondary treatment cannot address.1

The increased costs are primarily driven by the following factors:

• Infrastructure Requirements: Tertiary processes, such as nitrification-denitrification, necessitate the construction of additional aeration and settling tanks to facilitate the biological removal of nutrients like nitrogen.2
• Chemical and Operational Demands: Removing phosphorus through chemical precipitation increases the volume and weight of sludge, which requires further handling and disposal. Other methods, such as ammonia stripping, involve chemical additives and specialized towers, though these may be less efficient in certain climates.2
• Advanced Purification Goals: While secondary treatment removes over 85 percent of suspended solids and biochemical oxygen demand, tertiary treatment is designed to remove more than 99 percent of impurities to protect vulnerable ecosystems or meet stringent water quality standards.31

Because of these resource-intensive requirements, tertiary treatment is typically reserved for special circumstances where high-quality effluent is strictly necessary.1