The most common method of biosolids disposal has been burial in a landfill. Some of our biosolids are disposed of in this method, but we also use our biosolids in another way, land application. This method recycles the organic matter into the soil. It improves water retention, reduces acidity, adds bulk to thin soil, and helps with erosion. Biosolids with enough nutrients in them can be used to improve the yields of farmlands, while mixing biosolids with leaves, shredded paper, or wood chips in a composting operation create a product that can be used on lawns, in parks, and on golf courses.
In many areas, biosolids are marketed to farmers as fertilizer. Federal regulation (40 CFR Part 503) defines minimum requirements for such land application practices, including contaminant limits, field management practices, treatment requirements, monitoring, record keeping, and reporting requirements. Properly treated and applied biosolids are a good source of organic matter for improving soil structure and help supply nitrogen, phosphorus, and micro-nutrients that are required by plants. Biosolids have also been used successfully for many years as a soil conditioner and fertilizer, and for restoring and re-vegetating areas with poor soils due to construction activities, strip mining, or other practices.
At the end of our digestion process, there are leftover solids that did not get broken down and need to be removed from the system and disposed of. We run these solids through centrifuges to dewater them. A centrifuge is a long drum that spins at 2600 RPM. The solids enter on one end and using the centrifugal forces, are pulled to the outside of the drum and the water is left to drain out of the center on the opposite end. The water, known as centrate, is returned back into the liquid stream of the treatment plant.
Approximately 30 wet tons of a 24% solid, known as “cake” are produced each day. These cake solids are conveyed out to four semi-trailers. Weather and season permitting, they are hauled to farm fields, where they are applied and incorporated into the soil. If not land applied, they are hauled to a landfill
The Fond du Lac WTRRF annually provides local farmers with over 4,500 wet tons of cake biosolids fertilizer. The land application of the biosolids provides a slow-release, quality nitrogen and phosphorus product at no cost to the farmers.
Application of the biosolids is either done by incorporation or surface application on our permitted sites. When biosolids are incorporated into a field, they are injected into the soil at a depth of approximately 7 inches. Prior to land application, the contents of the biosolids are analyzed by an independent lab. The results of the analysis are then used to determine application rates at each site. The EPA and WI DNR specify the parameters that need to be analyzed and the frequency of analysis. We base our biosolids application rates on the nitrogen requirement of the crop grown. Information on biosolids quality and metal application rates are provided to the WI DNR and participating farmers annually. During land application, a great amount of consideration for safety, human health, and the environment are taken into account. Certain setback distances from homes, wells, and environmentally sensitive areas are maintained to provide a high level of safety.
Fields, where our biosolids can be land applied, must meet the following criteria:
- Fields with crops not grown for human consumption
- Fields with low slopes
- Fields low in phosphorus
- Fields must be within 20 miles of Fond du Lac
- Fields with early Spring access before planting
- Fields that are accessible in Summer (Alfalfa)
- Fields that are ready after the Fall harvest
Services that are provided to the farmer:
- Soil testing is provided every four years on fields that are DNR approved
- Nitrogen is supplied at the agronomic rate for the crop season
- Lost organic matter is replenished and the biosolids addition will increase soil water holding capacity
- A report is provided to each farmer with amounts of nutrients supplied per field