A NOVEL SLUDGE SUCTION RETROFIT OF THE METROPOLITAN WASTEWATER TREATMENT PLANT'S FINAL SETTLING TANKS

Brown and Caldwell designed a stationary sludge suction header as part of a retrofit of 3 of 24 rectangular final settling tanks, or clarifiers, for demonstration at the Metropolitan Wastewater Treatment Plant (Metro) in St. Paul, Minnesota. The sludge suction header replaced a cross collector system that contributed to re-suspension of settled solids and the formation of transverse currents. The retrofit was completed in December 1996 at a cost of only 115,000 per clarifier.During the following year, Metro operators conducted a supplemental sampling program, consisting of sludge settling, flocculation, and compaction testing, that would provide the basis for a performance comparison between modified and unmodified clarifiers. Improved effluent quality of the modified over the unmodified clarifiers was evident. Statistical analysis of sludge characterization data linked improved performance to the new sludge suction header. Based on these results, the remaining 21 final clarifiers at Metro will be identically retrofitted.