Troubleshooting the Design and Start-Up of Wastewater Granular Media Filters

The Alexandria Sanitation Authority's wastewater treatment facility in Alexandria, Virginia, implemented a program to improve tertiary treatment capacity. This included improving tertiary clarifier and filter capacity. During the start-up of the filters, significant levels of boiling during the backwash cycle led to excessive media loss. A number of mechanisms were investigated to attempt to relieve the problems:Was there a failure during the start-up that disturbed the support gravel system?Was there a mechanical problem in the underdrain design or installation that caused the problem?Was there a fouling problem in the underdrain that created the problem?Was there was a fundamental incompatibility between the filter media selection and the underdrain design?Was there a wastewater characteristic that created the problem?It was found that:It is important that the headloss across the underdrain system be substantially higher than the headloss through the filter media during backwash. The current trend in minimizing the filter underdrain headloss has a limit, and the practitioner should place at least some focus on maintaining a certain minimum headloss across the underdrain in a filter system.Effluent foaming can cause significant problems in filter backwash performance. The practitioner should take special care in using granular media filter at plants where effluent foaming is significant.At the Alexandria Wastewater Treatment Facility, it was not possible to determine exactly the reasons for the problems. However, evidence indicates that the foamy wastewater (high Methylene Blue Active Substances levels) created problems in the underdrain and resulted in the backwashing problems. Once the foamy water went away (as a result of the new bioreactor coming online) the serious boiling and filter media loss reduced to levels more typical of normal filter operation.