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Biosolids Basics

What are biosolids?
Biosolids are a nutrient-rich organic material that meets regulated standards for application to land as a fertilizer and soil conditioner. Biosolids are a byproduct of the wastewater treatment process. Because wastewater is collected from area industries as well as residences and businesses, the industries must meet discharge standards that protect the quality of the biosolids. Pathogen treatment and pollutant limitation requirements allow biosolids to be safely utilized as a resource.

The Missouri Department of Natural Resources (DNR) estimates that 250,000 dry tons of biosolids are generated annually by wastewater treatment plants in the state. Approximately 60% of this total is incinerated, 30% is applied to agricultural land, 7% is deposited into holding lagoons, and 3% is taken to landfills. Locally, 7,141 dry metric tons of biosolids were generated at the Northwest and Southwest Treatment Plants in 2001. Collaboration with area farmers has allowed 100% of this amount to be land-applied and recycled back into the soil.

Why are biosolids recycled?
Land application is the disposal option preferred by the MO DNR and the U.S. Environmental Protection Agency (EPA). Incineration requires a large amount of energy and produces concentrated ash that must be disposed of carefully. Disposing of biosolids in a landfill takes up already shrinking space. These disposal methods are more common in densely populated or non-agricultural areas. Land application is the most cost-effective option and has beneficial properties for the environment. Farmers use the biosolids to replenish topsoil that has been depleted of nutrients or eroded. Biosolids are used at the Springfield Sanitary Landfill as a soil amendment that is then planted with grasses and vegetation. Disturbed sites such as lead and zinc mines are also restored in this manner. Figure 1 shows the percentage of use for each of these applications.

Biosolids treatment process
Biosolids begin as microorganisms that are separated from the wastewater by settling. The waste solids are thickened from approximately 1% to 5% solids by using water-soluble polymers and gravity belt thickeners. The waste solids then undergo anaerobic digestion for stabilization. This digestion process fulfills the federal requirement for Class B pathogen reduction. The digested biosolids are dewatered additionally to approximately 23% solids using high-speed centrifuges. Waste solids that meet all regulations for land application can then be termed biosolids. See Southwest Wastewater Treatment Plant Biosolids Handling.

Land Application Process
Currently, biosolids generated at the Northwest Treatment Plant are hauled to the Southwest Plant for processing and proper application. Prospective application sites undergo a geologic and soil assessment. These assessments determine the amount of biosolids that can be applied and indicate the existence of sinkholes, losing streams, or other situations which will require buffer zones. Farmers or other individuals responsible for the land usage are informed of this assessment and relevant management practices. The City biosolids staff delivers and applies the biosolids to the site.

Local Perspectives

“Applying biosolids to my fields has increased the nutrient levels and productivity of the soil. My crop yield has increased by about 50%. I have seen no adverse effects to my cattle who graze on these forage crops. Testing of the soil before and after application of the biosolids assures me of the safety of this practice. The staff from the Southwest Treatment Plant did an excellent job of applying biosolids to my fields and informing me of the necessary management practices.”

- Tom Dewitt, local farmer and Area Resource Soil Scientist with the Natural Resource Conservation Service, USDA  

Currently this year, about 3000 acres on 38 farms in Greene and Christian counties have been fertilized with biosolids generated at the Southwest and Northwest Treatment Plants. Feedback from these local farmers indicates advantages to using biosolids over commercial fertilizers. Biosolids are offered at no cost, thus reducing farmers’ fertilizer bills dramatically. Farmers report higher crop yields, with an average reported increase of 51%. The increases can be attributed to nutrients and trace minerals, such as zinc and magnesium, that are present in biosolids and are needed for plant growth. Improved soil structure holds in these nutrients and retains moisture.

The main concern mentioned by farmers using biosolids is the creation of ruts in the soil and the compaction of grasses by the heavy spreading trucks. The City will be acquiring new application equipment with flotation tires which will eliminate this problem. Neighboring residents have made some complaints about odors from the biosolids. Processes at the Southwest Plant are now producing mostly dewatered biosolids instead of the liquid form used in the past. Also, additional digestion capacity has recently been provided. These changes will minimize odors. Using dewatered biosolids is also more efficient in that less volume is needed to fulfill the recommended application amount. Less trucks and manpower are required per field, saving fuel cost and allowing more sites to be handled.

The use of biosolids as soil cover at the Springfield Sanitary Landfill and at the depleted mine sites in Jasper county have been successful in transforming areas that would otherwise be eyesores on the landscape. Grasses and vegetation flourish, minimizing the runoff of any contaminants from these areas.