Use of total suspended solids in characterizing the impact of spent filter backwash recycling

Recycling of spent filter backwash water is a widely practiced residual management approach throughout the United States for drinking water utilities. The United States Environmental Protection Agency (USEPA), under the 1996 Safe Drinking Water Act Amendments, has recently proposed regulations governing the recycle of this waste stream. Considering this new regulation, a comprehensive study was conducted by researchers at Colorado State University, and a suspended solids mass balance model was developed to characterize the impact of backwash water recycling on the overall treatment process. Online particle count data indicated that certain recycle practices could impact the overall treatment process. Data from pilot-scale experiments showed that total suspended solids (TSS) is a useful tool for characterizing the impacts of the backwash recycle processes. TSS can be used to assess whether solids loading or suboptimal coagulation conditions are the cause of recycle related issues. For the study described here, filter breakthrough occurred at about the same total influent solids load, regardless of the manner in which backwash recycling was performed, indicating that recycle of backwash solids did not impact the overall treatment process.