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As discussed in Sections 3.2 and 4.5.8, estimates of N and P fluxes from sewage treatment plants from the MART (2006b) report were much lower for both total N and total P than in the Integrated Assessment. As pointed out in the MART report, much of this decline is thought to be due to the values assigned for total N and P concentrations in sewage treatment plants effluent. Few measured data were used, but rather estimated values were applied to most. Most estimates were made using a “typical pollutant concentration” (TPC) for N or P based on the level of treatment. These TPC’s were from an update of values compiled in a report by Tetra Tech (1998). The 2006 MART report assumed that sewage treatment plants with advanced wastewater treatment had TPC values of 5.6 and 0.82 mg/L for total N and total P, respectively. The MART report then applied these assumed values to estimated daily discharges to calculate an estimated daily flux. The MART report further assumed that plants that had less than advanced wastewater treatment had TPC values of 11.2 and 2.02 mg/L for total N and total P, respectively, applied to estimated daily discharges to calculate an estimated daily flux. The Panel is not comfortable with these assumptions and instead believes that most wastewater treatment plants in the MARB had TPC’s applied that were too low. The Panel, therefore, adjusted the database, by using TPC’s of 11.2 and 2.02 mg/L for total N and total P, respectively, for plants with advanced wastewater treatment, and TPC’s of 15 and 4 mg/L for total N and total P, respectively, for plants with less than advanced treatment.
As an example of how these adjustments changed estimates, the Panel examined seven Chicago plants (Stickney is the largest sewage treatment plant in the basin) and one in Champaign-Urbana, IL, where measured flux data were available (daily to weekly measurements of total N and total P and flow were made at each plant). From this analysis, it is clear that the TPCs used in the MART report were not appropriate and gave substantially lower flux estimates (Table 20) than the actual measured values. The MART report indicated that each of these plants had advanced treatment, and therefore applied their estimated TPCs of 5.6 mg/L total N and 0.82 mg/L total P respectively. Most plants in the MARB do not have treatment processes (either biological or chemical) to remove P, and much of the advanced treatment is to nitrify ammonium to nitrate, because most are permitted for ammonia in effluent.
Table 20: Comparison of MART estimated sewage treatment plant annual effluent loads of total N and P and values from measurements at each plant for 2004.
* All plants are in Illinois. Also shown in red is the recalculated MART value as described below, except for Stickney, where actual values were used because of plant size and concentration considerations.
This analysis supports the Panel’s use of increased TPC’s when estimating point source loads. Therefore, all plants that were labeled as advanced treatment and used the Clean Water Needs Survey data for load estimates were recalculated using total N and P concentrations of 11.2 and 2.02 mg/L, respectively (this included most plants, and some were estimated using the permit compliance system data and were not recalculated). These concentrations were much closer to the values reported by the plants in Table 20 (red values in table), although there still was considerable variability, and included 2,080 point sources (the total database has 33,302 point sources of all types). For plants identified as receiving secondary treatment, total N and P concentrations of 15 and 4 mg/L, respectively, were applied (there were 4,480 plants of this type). For the seven plants in Table 20 recalculated this way, total N and P fluxes were 113 and 91% of measured values, respectively, much closer to the measured values than the original MART values. The Panel’s discussion of point sources in the MARB utilizes these adjustments to the MART values. Finally, the Panel again emphasizes that measured data are generally not available in these large databases, so that many assumptions need to be made.
Intensification of animal feeding operations
Current census information shows that there has been an 18% increase in the number of pigs in the U.S. over the last 10 years along with a 72% decrease in the number of farms. Over the same 10 years, the number of dairies has decreased by 40%, but herd size has increased by 50%. A similar trend in the poultry and beef industries has also occurred, with 97% of poultry production in the U.S. coming from operations with more than 100,000 birds and over a third of beef production from <2% of the feedlots (Gardner, 1998). Fattened cattle numbers remained fairly constant from 1982 to 1997 but the number of fattening operations decreased over 50 percent (Kellogg et al., 2000). Overall, cattle, pig, and poultry numbers have increased 10% to 30%, while the number of farms on which they were reared has decreased 40% to 70% over the last 10 years (Gardner 1998).
The large-scale consolidation has created much larger animal feeding operations, which makes economical utilization and re-distribution of manure to croplands difficult and has profound consequences for farm and regional nutrient transfer and management within the MARB. For example, the accumulation of nutrients is first evident at the farm scale, where N and P management is affected by daily operation decisions and the long-term goals of each farmer. For example, the potential for P and N surplus on farms with AFOs can be much greater than in cropping systems where nutrient inputs become dominated by feed rather than fertilizer (Table 21). With a greater reliance on imported feeds, only 30% of N and 29% of P in purchased feed for a 1280-hog operation on a 30-ha farm could be accounted for in farm outputs. These nutrient budgets clearly show that animal feed is the largest input of nutrients to farms with AFOs, and thus is the primary source of on-farm nutrient excess, for which a resolution will require innovative management. Current animal number and estimated manure N and P production within the MARB is given in Table 22.
Table 21: Farming System and Nutrient Budget.
a 30 hectare cash crop farm growing corn and alfalfa.
b 40 hectare farm with 65 dairy Holsteins averaging 6600 kg milk cow-1 yr-1, 5 dry cows and 35 heifers. Crops were corn for silage and grain and alfalfa and rye for forage.
c 30 hectare farm with 1280 hogs; surplus includes 36 kg P and 140 kg N ha-1 yr-1 manure exported from the farm.
d 12 hectare farm with 74,000 poultry layers; surplus includes 180 kg P and 720 kg N ha-1 yr-1 manure exported from the farm.
Table 22: Number of animals and amount of manure produced and N and P excreted within the MARB states based on information from the 1997 U.S. Census of Agriculture (data obtained from USDA-ERS, http://ers.usda.gov/data/MANURE/).
† Manure in dry state, as excreted adjusted for water content.
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