University of Nebraska Cooperative Extension in Lancaster County

Land Application of Wastewater Solids:
Is Sludge Safe?

Barb Ogg, Extension Educator

014-96
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 In May, 1992, the first truckload of waste-water solids (i.e., sludge) from Lincoln's Theresa Street Wastewater Facility was delivered to farmland in Lancaster County. This event culminated a decade of planning by Lincoln's sanitary engineers to dispose this organic waste in a more environmentally sound manner, rather than burial in the landfill. This program may be new to Lincoln, but land application programs in some American cities (Chicago, Minneapolis, and Milwaukee, are examples) are now entering their third decade or more. Some of these municipalities have also creatively (and successfully) marketed processed wastewater solids to users other than farmers. For example, Milorganite®, marketed as a soil enhancer/fertilizer for the home gardener, is nothing more than heat-dried high-quality organic waste from the sewers of Milwaukee. (Yes, it is true. It just shows you that, with the correct product and processing, a terrific name, and marketing, you can make a silk purse out of a sow's ear!) The Metropolitan Denver Sewage Disposal District markets several products under the name of METROGROTM. All of these products are sold as a soil conditioners, either for horticultural or agricultural use. The METROGROTM program is another successful marketing strategy to recycle waste and give it a positive image. 

Some alternatives to land application are environmentally unfriendly and may also have other problems associated with them. Landfilling sludge has become expensive because of the high costs associated with burial in properly constructed landfills. Landfilling also concentrates organic wastes and may result in point-source contamination for future generations to deal with. Incinerators, properly constructed to prevent air pollution, can be expensive, as is the removal of enough water to allow the solids to burn. Ocean disposal, mostly done by New York City in the Atlantic Ocean, was very environmentally unsound. In the last decade, these practices have decreased in favor of land application programs, partly from encouragement from the U.S. Environmental Protection Agency (EPA). 

Land application programs usually involve application to agricultural cropland, although applications to forests and turf have been used. Sludge applications increase the organic matter and fertility of poor soils and are especially beneficial to reclaimed land damaged from mining, other excavation activities, or soils damaged by erosion. Much of the nitrogen in wastewater solids (and other organic manures) is bound up with organic molecules which undergo chemical changes before becoming available to plants. Because these chemical changes take place over time, the nitrogen in these organic solid becomes available for the plants for several years, similar to a slow-release encapsulated fertilizer. 

The risks associated with land application of wastewater solids are twofold, and both have been recently addressed by regulations by the EPA in Standards for the Disposal and Utilization of Sewage Sludge, Part 503. The first part of the risk equation is related to concentrations of pathogens (like bacteria) and other undesirable substances that might be present in sludge. Sludge that is applied to agricultural land must be processed to reduce the amounts of pathogens to safe levels. At the Theresa Street Wastewater Facility, heat processing in giant egg-shaped anaerobic digesters significantly reduces pathogens in the sludge. 

The EPA has also issued ceiling concentrations for 10 metals that must be low in wastewater solids that are applied to agricultural land. EPA's 503 regulations were determined using a sophisticated risk analysis based on many scientific studies examining potential health hazards to humans. Fourteen pathways, each examining a possible route of exposure, were carefully studied. Some of these pathways are very complicated. For example, one pathway examined the effect of applying contaminated wastewater solids to soil grown to forage plants that were fed to livestock that ultimately ended up as human food. To understand this pathway better, scientists studied the uptake of metal-contaminated soil by different crops that might be fed to livestock. In other experiments, scientists fed contaminated feed directly to livestock animals and examined the meat, milk, and body organs of the livestock. Results of these and many other experiments were used to determine the maximum concentrations of metals that can be present in sludge and soil to minimize human health risks and environmental contamination. 

The second part of the risk equation involves proper application of wastewater solids to prevent contamination of the environment and to minimize exposure to humans. According to EPA regulations, sludge applied to land must be applied at agronomic rates, based on the nitrogen need of the next crop. Understanding the proper application rate means that each field must be intensively evaluated using soil tests before sludge is applied to prevent over application. The EPA also has restricted the application of wastewater solids close to wells, rivers or streams, and public water supplies. In addition, there are restrictions that prohibit sludge application on horticultural crops used for human consumption. 

All of these EPA regulations mean that municipalities undertaking a land application project must have a rigorous sampling and analysis program to show that the sludge is in compliance with the pathogen and metal restrictions. In addition, careful site selection and monitoring of application are necessary to ensure the safe use of this material. Within the last couple years, there have been some disparaging articles in farm magazines about land application of sludge, but these negative stories should not imply that all municipal sludge is contaminated or that environmental contamination will inevitably result when waste-water solids are applied to farmland. There are hundreds of successful land application programs throughout the U.S., with few examples of health problems or environmental contamination. When sludge meets EPA regulations regarding pathogen and metal standards and when responsible application practices are followed, the benefits of sludge far outweigh the likelihood of health problems or environmental contamination. Municipalities, like the city of Lincoln, are working hard with careful monitoring to make sure that wastewater solids are safe and applied in a responsible manner.


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