Densification Specific Considerations for Secondary Clarifier Assessments

Intensification is a major innovation for water resource recovery facilities (WRRFs) to realize cost-effective use and deployment of infrastructure. One of the promising technologies and approaches for process intensification is densified activated sludge (DAS). Secondary clarifiers are a key unit process to ensure retention of the solids inventory and permit compliance. The performance of clarifiers depends on several interrelated processes and factors including hydrodynamics, settling, turbulence, flocculation, sludge rheology, atmospheric conditions, tank geometry, internal features and loading conditions. Several tools and design “rules of thumb” are available from regulatory loading guidelines such as Ten State Standards, sludge specific assessment tools such as state point analysis, and calibrated, site-specific tools such as 2D and 3D computational fluid dynamics (CFD) modeling. These tools have been in use for decades with traditional sludge settling characteristics. However, the industry is still working through methods and procedures to assess the impacts of DAS settling characteristics on traditional secondary clarifier operations and loading guidelines. This paper will highlight the use of clarifier evaluation tools for DAS applications, including key considerations, observed performance, potential loading rates, and other relevant properties. Case studies are highlighted to demonstrate potential impacts at full-scale WRRF sites.