(545af) An Investigation of Flow Obstructions to Minimize Media Loss in Simultaneous Air/Water Backwash Operations in Gray Water Filtration Systems

Simultaneous air/water backwash in water filtration systems has been shown to be more effective in cleaning filtration media than water-only backwash or sequential water and air backwash. However, simultaneous air/water backwash operations can result in unacceptable media loss. This work is an investigation of troughs as flow obstructions in gray water filtration tanks during backwash operations. These flow obstructions can generate quiescent zones near the liquid surface and allow for the settling of media particles thus decreasing media loss. Media loss during simultaneous air/water backwashing in a pilot scale (D= 42 in) vertical system was investigated as a function of air/water flow rates. Flow velocity profiles near the trough were measured to obtain velocities in two directions and a measure of turbulence in the system. Industrial scale water and air flow rates of 3 GPM/sq.ft. and 5 GPM/sq.ft. and 0.8 scfm/sq.ft. and 1.5 scfm/sq.ft. respectively were used. Commercial anthracite and Greensand PlusTM were used as filtration media. The rationale for the trough design used will be discussed in the context of the complexity of these systems. These results will be compared to laboratory scale measurements and the effect of system and air/water flow rate scales will be discussed.