Spent filter backwash water contaminants and treatment strategies

Twenty-five representative water treatment plants were surveyed to compare differences in microbial, physical, and chemical water quality of raw waters and spent filter backwash waters (SFBWs). On the basis of the observed Giardia and Cryptosporidium levels, it was determined that a recycle ratio of 5-10% would be sufficient to minimize the impact of protozoa on ambient raw water. Using the cell culture-polymerase chain reaction method, infectious Cryptosporidium oocysts were observed in raw water and SFBW samples. The presence of particles in the SFBW samples increased the potassium permanganate demand by a factor of 5.5 and the chlorine dioxide demand by a factor of 4.0. However, the presence of particles in SFBWs did not affect the inactivation of Cryptosporidium. Conventional treatment of SFBW with polymers resulted in excellent removal of turbidity, particles, end microorganisms. A backwashable depth filter can be used in place of a conventional filter for control of Cryptosporidium in clarified SFBW.