Introduction
Biosolids are nutrient-rich organic materials produced during the biological treatment of wastewater. The Oregon Wastewater Treatment Plant uses the same biological and physical processes by which water is cleaned in nature.
Microorganisms (primarily bacteria) biologically break down the sewage in the facility’s aeration tanks. These microorganisms, called activated sludge, utilize the organic material in sewage as a food source. Air is pumped into aeration tanks to provide oxygen and mixing.
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Biosolids are created by the treatment of activated sludge in the aerboic digesters. |
The sewage provides a constant food supply, which causes the bacteria to continually reproduce. To maintain the proper balance of Food to Microorganisms in the aeration tanks, a portion of the activated sludge is removed and sent to the aerobic digestion tanks. Once in the aerobic digesters, the activated sludge is dewatered to increase the solids content, and stabilized to reduce pathogens and odors. Only after it has been through this treatment process can it be called biosolids. |
Biosolids Recycling
The Oregon Wastewater Treatment Plant recycles 100% of the biosolids it produces. The biosolids are applied, primarily to agricultural land, as fertilizer to improve and maintain productive soils and to stimulate plant growth.
| Biosolids contain significant amounts of nitrogen (N), phosphorous (P), and a trace of potassium (K). The majority of the nitrogen is organic nitrogen, which is released slowly throughout the growing season and becomes available as the crop grows and takes it up. Biosolids also contain micronutrients that are essential for crop growth. These include boron, calcium, copper, iron, magnesium, molybdenum, sulfur and zinc, among others. |
Biosolids are removed from the aerobic digesters and loaded into a field applicator. |
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The field applicator injects the biosolids beneath the soil surface. |
Biosolids contain more than 50% organic matter. Organic material conditions the soil by improving the soil structure, porosity and increasing nutrient and water retention. Organic material improves the soil’s cation exchange capacity (CEC) and helps maintain healthy levels of soil bacteria. |
Biosolids Benefits
- Returns resources back to nature’s cycle
- Adds nutrients to the soil that may not be added by commercial fertilizers
- Improves soil structure and soil fertility
Regulations
Oregon only applies biosolids to sites approved and inspected by the Ohio Environmental Protection Agency.
Oregon applies its biosolids at or below the agronomic rate of the nitrogen requirements of the crop being grown.
Biosolids application is closely monitored and regulated to ensure that it presents no increased risk to people, animals or the environment.
Metals concentrations in Oregon’s Biosolids are well below regulatory levels:
|
Metal |
USEPA Monthly Limit mg/kg |
Oregon 2008 Average mg/kg |
Oregon 2007 Average mg/kg |
|
Arsenic |
41 |
8 |
10 |
|
Cadmium |
39 |
2 |
2 |
|
Copper |
1,500 |
340 |
350 |
|
Lead |
300 |
44 |
42 |
|
Mercury |
17 |
1.8 |
1.2 |
|
Nickel |
420 |
24 |
24 |
|
Selenium |
100 |
7 |
7 |
|
Zinc |
2,800 |
730 |
750 |
Pathogen levels in Oregon’s Biosolids are consistently below the Class B 2,000,000 fecal coliform bacteria per dry gram of sludge limit. In 2008, the average fecal coliform level was 12,000 fecal coliform bacteria per dry gram of sludge. The highest 2008 fecal coliform bacteria level was 35,000.
To further reduce the risk of pathogens, farmers must wait before harvesting crops on land that has received biosolids. This allows time for the pathogens to die in the soil.
Resources
City of Oregon Wastewater Treatment
Contact Todd Smith
Telephone: 419-698-7133
Ohio Environmental Protection Agency
Northwest District Office
347 North Dunbridge Road
Bowling Green, Ohio 43402-9398
Telephone: 419-352-8461
OEPA Biosolids
www.epa.state.oh.us/dsw/sludge/biosolid.html
National Biosolids Partnership
601 Wythe Street
Alexandria, Virginia 22314-1994
Telephone: 703-684-2400
NBP
U.S. Environmental Protection Agency Region 5
77 West Jackson Boulevard
Chicago, IL 60604
Telephone: 312-886-6106
