Metro's CaRRB Diet - Centrate Treatment Process Tackles Big Challenges in a Small Package

In 2004, the Metro Wastewater Reclamation District (Metro) initiated an expansion project to increase capacity, improve process performance, and ensure process stability at their 380,000 m3/day (100 mgd) north secondary complex (NSEC). All of the nitrogen-rich centrate from the solids generated in both the NSEC and south secondary complex (SSEC) is returned to the NSEC. The project was to add two new aeration basins and secondary clarifiers to the existing complex of 12 aeration basins and secondary clarifiers. As an alternative approach, Metro evaluated the concept of centrate treatment with RAS in a reaeration basin. This approach afforded several important advantages over the original configuration, including increased capacity at lower cost, reduction in required mixed liquor return pumping, and improved nitrification. The facility was constructed in 2009, and performance since startup has exceeded expectations and provided confirmation of the benefits offered by this process.