The riser pipe as developed in the early 1960s went through a number of design iterations for sludge removal. Due to the lack of concern for uniform floor pick up by the original designers, the original riser pipes were such that each pipe was of the same diameter and was adjustable to vary the return sludge flow. The design allowed the operator to adjust each pipe for the perceived conditions that might be found at various points within the clarifier. This flexibility to remove sludge from the various areas in the basin was thought to be paramount. Although this flexibility was and is a good design feature, other problems associated with the pick up of the sludge manifested themselves in the field.
One of the major drawbacks was that low velocities caused solids deposition and blockage within the pipes and elbows at low flow conditions. This became a major problem for some of the installations that utilized the riser pipe design. Also, there were problems due to the increased headloss that was required for some flow control devices (particularly the telescopic sludge valve type). Since it was most economical to go with a single size for the withdrawal pipes and space them equally on the radius of the basin, uniform pick up of the sludge was compromised.
