Biosolids are captured in the primary clarification process and fed to two anaerobic digesters that operate in series.  The anaerobic digesters are filled with trillions of acid and methane producing microscopic bacteria.  Anaerobic digestion is one of nature’s greatest tools because the result is a net reduction of biosolids.  This means that at the end of the process there are less biosolids than originally went into the system.  The methane byproduct produced by the bacteria is captured and recycled as natural gas to heat their environment to the required 95°F.  The biosolids are broken down and digested by the bacteria for over 60 days in the anaerobic digesters.  

After digestion, the sludge is pumped to the lagoon system.  The biosolids are further digested and broken down to a safe pathogen free product.  The water is removed and treated separately at the Wastewater Treatment Plant before discharge in the Missouri River.  The biosolids remain in the lagoons until fall, when they are sampled and tested for metal pollutants, nitrogen, phosphorous and potash. 

The Wastewater Treatment Plant staff then sample the soil of the agricultural property that will receive the biosolids.  The soil is sampled in each soil profile to a depth of five feet in one foot increments to test each layer of soil for the background nitrogen, phosphorous, and potash levels in the field.

Once staff receives the test results for the biosolids and the soils, the agronomic rate is calculated according to the background soil nutrient levels, the biosolids nutrient levels, and based on the next year's planned crop type (each crop has a unique nutrient uptake need).   A 10% safety factor is calculated into the agronomic rate for the biosolids application.

The land application of biosolids is carried out annually by a private contractor selected by public bid process.  Waterwater Treatment Plant staff are responsible for oversight and ensuring the correct procedure and application of the biosolids to the agricultural land. 

All of these activities are completed under the regulation of the South Dakota Department of Environment and Natural Resources, South Dakota Water Pollution Control Act, and with the appropriate permits.  The land application of biosolids is carefully regulated, and the City of Yankton’s Biosolids Management Plan under Permit No SDL023396 provides additional details about the testing and application process.

Kyle Goodmanson, Director of Environmental Services 
315 West Riverside Drive
PO Box 176
Yankton, SD 57078
Phone: (605) 668-5270

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