SiouxperGrow Biosolids Program

In 1995 the City of Sioux Falls created their SiouxperGrow program.  The SiouxperGrow program is the reuse of the byproducts of the treatment of wastewater.  The SiouxperGrow program focuses on the beneficial reuse of these byproducts. 

Generating Electricity

The first byproduct produced is Biogas.  Biogas is the gas produced by the microbiology in the (Digesters) biosolids stabilization process.  The City of Sioux Falls utilizes anaerobic digestion to reduce pathogens in the wastewater sludge, and the production of Biogas is one of the byproducts.  This Biogas is used for cogeneration of electricity.  The microbes make approximately 250,000 cubic feet of gas (biogas).  The biogas is then used as fuel (electricity) for the electric generators that produce about 40% of the electricity to operate the water reclamation facility.

What are Biosolids?

The other byproduct created is called Biosolids.  Biosolids are the bacteria that have reached the end of their life cycle and need to be removed from the anaerobic digesters.  The City of Sioux Falls currently has two biosolids storage lagoons, to which the Biosolids are pumped from the digesters.  Each of these storage lagoons is 2.5 acres and can store 8.4 million gallons of biosolids, for a total of 16.8 million gallons of storage. 

Regulatory Requirements

Once the bacteria is pumped to the lagoons it is now considered Biosolids.  In the lagoons the Biosolids are allowed to separate from the water and settle to the bottom of the lagoon.  The water on the surface is called decant is returned to the headwork’s of the Treatment facility to be retreated and discharged as clean effluent.  The Biosolids that settled to the bottom of the lagoon can remain in the lagoon for up to two years before being removed and applied to farm fields. The City of Sioux Falls produces a Class B Biosolid as defined by EPA CFR 40 PART 503.  To be classified as a Class B Biosolid, the Biosolids must be stabilized to the point that insects and rodents (called vectors) are not attracted to the agricultural fields where the biosolids are land applied.  The number of bacteria are also reduced so the biosolids can be safely land applied.  Under the Biosolids Permit issued by SD DENR the City is required to submit an annual Biosolids Report.  The Biosolids Report contains detailed information regarding the Biosolids at the Water Reclamation Facility.  

Land Application of the Biosolids

The City of Sioux Falls utilizes a liquid application method.  The operators remotely control an 8 foot wide 25 foot long dredge.  The dredge is a barge with an auger and a pump on a shaft that can be raised and lowered in order to remove the Biosolids.  The dredge is capable of pumping up to 1,200 gallons per minute, but is normally run at 600 gallons per minute due to the thickness of the Biosolids.    From the dredge the Biosolids are pumped to one of three 6,500 gallon semi tankers.  These tankers then transport the Biosolids to a field approved by South Dakota Department of Environment and Natural Resources (SD DENR).  Once in the field the tankers are off loaded to a slurry tank pulled by a four wheel drive tractor. Now that the Biosolids are in the slurry tank they are applied to the land at the Agronomic Rate prescribed by the Residue Coordinator.  To calculate the Agronomic Rate the Residue Coordinator must have current soil sample results, the planned crop, anticipated yield of the next crop being planted, and the nutrient value of the Biosolids.  To determine the nutrient value of the Biosolids, samples of the Biosolids are collected throughout the week and the Water Reclamation lab staff perform the required testing.  After land application is completed the Residue Coordinator notifies the farmer of the quantity of the nutrients that were applied to his field.  The field is also marked with signage informing the public that Biosolids have been applied by the City of Sioux Falls.

Disposal of Biosolids

Land application isn’t the only method of disposal for Biosolids.  The City of Sioux Falls can also dispose of Biosolids into the Sioux Falls Regional Landfill.  However, there are additional SD DENR requirements that must be met for this option.  One of the additional requirements is that the Biosolids have to pass a Paint Filter Test to ensure that there isn’t too much water remaining in the Biosolids.  The City of Sioux Falls only uses this option when absolutely necessary because it is not as environmentally conscious.    The Biosolids can also be sent to another Wastewater Treatment Facility to be used as seed material.  In this option another facility is in need of our Biosolids to start up a new process or restart an old process.  This is a benefit to both facilities, the City of Sioux Falls is removing Biosolids from our facility and the facility receiving the Biosolids can start up much quicker.

Overview:

The City of Sioux Falls Water Reclamation Facility generates approximately 2,800 tons of Biosolids a year from the cleaning of wastewater.  These Biosolids are a very good source of nutrients for plants, mainly Nitrogen (N) and Phosphorus (P) with traces of other nutrients.  The City of Sioux Falls works in cooperation with area farmers, the South Dakota DENR, and the US EPA to provide this nutrient rich fertilizer to local farms in the safest way possible.