Although special accommodations may be made for other types, the chain-and-flight or indexing grid sludge collection system is frequently employed because the settler modules would interfere with any of the other collection mechanisms. The indexing grid system (Figure7-15) on page 7-24 consists of a series of concave-faced triangular blades rigidly connected to glide bars. The glide bars ride on top of polyethylene wear strips anchored to the floor of the tank. The system operates at the bottom of the sludge layer. The collector operates by gently pushing the grid system and the sludge in front of the grid at a speed between 0.6 and 1.2 m/min. When the hydraulic cylinder that drives the grid reaches the end of its stroke, the grid system reverses at two to three times the forward speed. During the return movement, the triangular shaped blades slide under the sludge so that there is minimum disturbance to the sedimentation process. The result is a continuous