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Biosolids?? What Are Biosolids?

by Barb Ogg, PhD, UNL Extension Educator

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The widespread collection, treatment and disposal of wastewater effluent evolved in the mid-to late 19th century. Before this time, wastes were often discharged into surface waters and polluted streams and rivers. As the technology of waste systems has improved, the threat to our environment has been reduced because wastewater effluent has become cleaner and cleaner. However, in the process of producing clean effluent, the process itself accumulates residues or solids. The older, commonly used term for these solids is sewage sludge.

The word biosolids is a new term used to describe municipal treatment plant solids which are tested and determined to be safe for land application. The word sludge is a generic term that most people use to refer to some type of unprocessed waste material. There is still some interchange of these two words, but biosolids always refers to processed wastewater solids that have met specific criteria and are suitable for land application.

Lincoln's Biosolids: What's In It?

Biosolids from a specific city wastewater treatment facility are somewhat unique because the content is based the complement of industries that contribute to the wastestream. The constituents in biosolids represents an agglomeration of substances originally added to the wastewater. It is normal to have concentrations of substances in biosolids that fluctuate somewhat throughout the year because the disposal of substances to the wastestream is not constant. Frequent sampling is important. 

In the final stages of biosolids production through the treatment plant, a polymer is added to the wastewater which causes the solids suspended in the water to precipitate. This allows water to be removed by pressing the liquid through a series of belt presses. The final product is about 20% solids, which is dry enough for it to be applied with a conventional manure spreader. 

Theresa street biosolids are high in organic nitrogen, phosphorus and have significant levels of potassium and sulfate. Iron, copper, manganese, molybdenum and zinc, essential elements necessary for plant growth are also found in this material. A chemical analysis of Lincoln's biosolids is found in the tables below.

Table 1. Analysis of Biosolids Components: Theresa St. Wastewater Plant, 2002 Annual Average

*Because the organic matter in Theresa Street biosolids increases the water holding capacity of the heavy clay soils found in our area, it reduces erosion.

Table 2. Nitrogen Found in Theresa Street Biosolids: 2002

Biosolids are primarily organic N, which means that the nitrogen becomes available for plants slowly through a process called mineralization. This slow-release nitrogen makes it a more environmentally sound source of nitrogen.

Table 3. Other essential plant nutrients found in Theresa Street Biosolids, 2002

Regulated Metals:

Biosolids from the Theresa Street Wastewater Facility meets all federal requirements for regulated metals. These EPA regulations, called Standards for the Disposal and Utilization of Sewage Sludge, Part 503, define the maximum concentrations of undesirable metals that might be found in sewage sludge. These regulations were implemented in 1993 after two decades of research examined the hazards and long-term consequences of land application of sewage sludge. For more information about other aspects of the Part 503 Rule, check out Regulations for Using Biosolids Safely.

Table 4. Concentrations of regulated metals in biosolids from Lincoln's Theresa Street Wastewater Facility in 2002. 

Even though these elements are deemed undesirable by the EPA, some of these regulated substances, like copper, molybdenum and zinc are micronutrients, actually required by plants in small quantities. Zinc, usually found in the upper soil layers, is often deficient in soils that have been eroded or excavated. Other elements, like arsenic, lead and cadmium are not needed by plants for plant growth, but at low levels, these elements will not harm plants. These elements often occur naturally in many soils.

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PHOTO Credit: Vicki Jedlicka