Attrition of granular activated carbon during air scour followed by water backwash and the impact on filter performance

Backwashing of GAC has developed from backwash regimes used for sand, which has greater density and more resistance to attrition. Attrition arises from lack of grain strength and washout arises from low grain density. Loss of GAC by either means can increase costs, filter downtime and cause other operational maintenance problems. Carbon fines produced by attrition may remain in the filter bed pores and pass into the supply during filtration. To investigate the attrition of GAC during backwashing, a laboratory scale filter column system was assembled and operated. The attrition of GAC and subsequent release of fines during filtration were assessed by using a series of backwashing and filtration cycles. A particle counter was used to quantify particles generated during backwash and filtration. The average GAC attrition after 8 cycles of backwashes was approximately 0.33%. Results showed that fines were released into backwash and filtration effluents throughout the experiment.