Overcoming Hydraulic Limitations of the Integrated Fixed–Film Activated Sludge (IFAS) Process

In 2011 more than 600 moving bed biofilm reactors (MBBRs) were operational in 50 different countries. Approximately 20 systems incorporating an integrated fixed-film activated sludge (IFAS) process were operational in 2011, 15 of which exist in the United States of America. Similar design standards are applied to MBBR and IFAS process mechanical components: maximum plastic biofilm carrier volumetric fill, plastic biofilm carrier retention screens, air diffuser grid (in aerobic zones), and mechanical mixers (in anoxic zones). Each of these process mechanical components influences the hydraulic throughput of MBBR and IFAS systems. The authors are aware of IFAS systems that have been subject to a hydraulic failure that resulted in plastic biofilm carrier loss. Hydraulic failures are an engineering problem that can be solved. While hydraulic failures that result in plastic biofilm carrier loss have occurred in less than 1percent of the known existing MBBR and IFAS systems, the failures were public. Potential negative public perception may perpetuate utility reluctance to implement this technology, which has numerous benefits. This paper presents examples of IFAS processes that have lost plastic biofilm carriers as a result of hydraulic incidents. In addition, hydraulic failure mechanisms that result in plastic biofilm carrier loss are described. Finally, design features for overcoming hydraulic limitations inherent to IFAS and MBBR processes are described. Methods to overcome these hydraulic limitations include 1) proper selection of media geometry, 2) suitable fill fraction given the hydraulic design criterion, 3) redirecting flow perpendicular to traditional basin flow scheme, 4) bypassing (low-pollutant load) wet-weather flows around the IFAS zone, 5) splitting internal mixed-liquor recirculation (IMLR) flow with a series of pumps, and 6) process instrumentation and control to optimize bypass and IMLR operations.