Agricultural Utilization of Sewage Sludge

Executive Summary

The Rosemount Watershed study was initiated in 1973 on land of the University of Minnesota’s Agricultural Experiment Station in Rosemount, Minnesota. The project was run by the USDA-Agricultural Research Service from 1974 to 1985. It was then run by the University of Minnesota Department of Soil, Water and Climate (formerly the Soil Science Department) from 1986 to 1993. The primary goal of the study was to increase knowledge of the effects of liquid sewage sludge on surface and ground water quality, crop yield and quality, and soils over a period of 20 years.

The long-term study at the Rosemount Watershed is a good example of a detailed analysis of sludge application to land. Sludge was applied at rates to meet the nitrogen (N) needs of high producing corn (Zea mays L.) and reed canarygrass (Phalaris arundinacea L.). Control treatments receiving commercial fertilizers without sludge were also included.

The 20 years of research have shown there are many benefits in using sludge as a plant nutrient source. Historically, yields on the sludge-applied land have been slightly better than on the fertilized control areas within the watershed. Reed canarygrass yields averaged nearly 11 Mg ha-1 (4.9 T A-1) and corn grain 8.6 Mg ha-1 (151 bu A-1). Grass was harvested for hay three times per year and corn for grain or ensilage.

Information was also gathered on the amount of nutrients removed by the crops. In the period from 1973-1984, reed canarygrass was more efficient than corn in removing N, phosphorus (P), and potassium (K) from sludge-treated soil. This indicated that the choice of crop is an important consideration in the land application of sludge.

During the course of the study the variety of corn planted changed with time. This affected the removal rates of N, P, and K. Different corn varieties removed N, P, K and metals at different rates. However, little difference was found between tissue concentrations of N, P, and K in crops grown on sludge versus control areas, when averaged over the life of the experiment. In earlier years of the study, commercial K fertilizers were added to the sludge areas because very low levels of K were provided in the sludge. In later years, based on soil test analyses, no additional K was applied to the sludge terraces.

Large amounts of N and P supplied by sludge did not adversely affect crop yields by creating imbalances of nutrients within the plants. In addition, sludge supplied many of the other nutrients required for plant growth. Because the amounts of N and P removed from sludge and fertilizer control treatments were nearly equal for the corn crop, lower amounts of sludge were applied to the corn area for the 1980 growing season and subsequent years. The lower application rates increased the efficiency of nutrient use while continuing to meet the nutrient needs of the crop.

This study also answered questions concerning sludge application and its effect on the environment. Trace metal levels found in corn tissues grown in sludge areas were not significantly different from the low levels found in corn plants grown with commercial fertilizers, except for zinc (Zn), which was found in slightly elevated concentrations in the corn stover. Corn used the trace elements available in the native soil (or from commercial fertilizers), but did not take up excessive amounts of these elements when they were added in sludge. Only copper (Cu) and chromium (Cr) levels on grass tissues from sludge treatments were slightly elevated over that of grass supplied by commercial fertilizer treatments, possibly a result of surface contamination from rain splash erosion.

From a water quality viewpoint, the Rosemount Watershed study showed that sludges can be applied in an environmentally safe manner. Soil erosion was reduced and surface water quality was protected by constructing terraces on the sloping ground.

More nutrients moved in surface runoff from grass terraces than from corn areas for two reasons. First, the fertilizer and sludge were not incorporated into the soil on the grass areas, so surface runoff had more contact with these materials. Second, grass tissues breakdown over winter losing their nutrients to snow melt runoff. Winter spreading of sludge on sloped land should be discouraged because significant increases in nutrient and trace metal contents were found in snow melt runoff.

It was shown that excessive fertilizer and sludge application rates affected the nitrate concentration in shallow ground water. Nitrate levels peaked in 1981 to 1984 and decreased steadily once N application rates were reduced.

The Rosemount Watershed site has a slowly permeable layer of glacial till 5 to 19 feet below the soil surface. This prevents the nitrate from reaching the ground water aquifer. At other sites where such a slowly permeable layer in these silt loam soils does not exist, excess nitrate could directly affect ground water quality. This points to the importance of monitoring the amount of nutrients, particularly N, applied to sludge-treated sites. Balancing the amount of N applied in the sludge with the amount needed for plant growth given specific soils and crops is an important management technique.

The 1993 cropping season culminated 19 years (1974 to 1992) of sewage sludge application and 20 years (1974 to 1993) of environmental monitoring at the Rosemount Watershed. Sludge was applied to the terraces every year except two. These 20 years of data helped determine the long-term effects of nutrient and metal additions from sewage sludge applications on crop production. Extensive soil, plant, and water sampling and analyses at this site have provided results to show that long-term sludge utilization on agricultural land can be accomplished in an environmentally safe and effective manner.

The long duration of the Rosemount Watershed study represents a unique and valuable site, possibly the only one in the United States with such a detailed database. Continual collaboration among the University of Minnesota, U.S. Department of Agriculture-Agricultural Research Service (USDA-ARS), and the Metropolitan Council Wastewater Services (form- erly Metropolitan Waste Control Commission of Minneapolis/St. Paul) is a prime example of the positive relationship involved in agricultural utilization of sewage sludge.

Produced by Communication and Educational Technology Services, University of Minnesota Extension.

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