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Upgrades and improvements to the Metro plant have also resulted in large reductions in effluent loading of suspended solids and nitrogen. TSS loading has dropped by 95 percent from the peak loading rate of 219,000 lb/day in 1968 to 10,000 lb/day by 1998; effluent concentration declined from 122 mg/L in 1968 to 5.7 mg/L by 1998 (Figure 12-10). Based on effluent data from monitoring that began in 1971, TKN loading has dropped by 78 percent from the peak loading rate of 36,500 lb/day in 1982 to 7,800 lb/day by 1998; effluent concentration has been reduced from 21 mg/L in 1982 to 4.5 mg/L by 1998 (Figure 12-11). Prior to the upgrade to advanced secondary with nitrification, toxicity-based water quality standards for the un-ionized portion of ammonia were frequently violated in the Upper Mississippi River. After upgrading the plant to nitrification with ammonia removal in 1984, effluent discharges of ammonia-N declined considerably. Using effluent data collected since 1975, ammonia-N loading has dropped by 90 percent from the peak loading rate of 25,500 lb/day in 1982 to 2,600 lb/day by 1998. The effluent concentration of ammonia-N has been reduced from 14.7 mg/L in 1982 to 1.5 mg/L by 1998 (Figure 12-11).

Beginning in the 1920s through the 1970s, the major water quality issues for the Upper Mississippi River have been bacterial contamination and depletion of DO from sewage discharges and combined sewer overflows. Historical DO data sets collected since 1926 illustrate the dramatic change in long-term trends in the spatial distribution of DO recorded 5 miles downstream of the confluence with the Minnesota River near St. Paul (UM milepoint 840) to Lock and Dam No. 3 at Red Wing, Minnesota (UM milepoint 797) (Figure 12-12). These historical data sets clearly illustrate the adverse impacts of wastewater loading and the effectiveness of upgrades in waste-

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