Part 1, Section 9: Nontraditional Soil Amendments
Nontraditional Soil Amendments
BIOSOLIDS (PROCESSED MUNICIPAL SEWAGE SLUDGE AND DOMESTIC SEPTAGE)
Biosolids result from the stabilization of solid, mainly organic materials produced during the biological treatment of municipal wastewater or sewage. Biosolids also may refer to domestic septage, the solid material that is pumped from septic tanks. Depending on how they are processed, biosolids may be a dilute suspension of solids in water that is sprayed, dribbled, or injected into the soil; a cake-like material that is spread like manure; or a very dry granular or pelletized material. Biosolids are the most extensively regulated of any nontraditional soil amendment. Regulations pertaining to the use of biosolids as well as more complete information regarding biosolids production, characteristics, and agronomic usage are more fully explained in series of four Penn State fact sheets entitled Land Application of Sewage Sludge in Pennsylvania. The individual fact sheets in this series are subtitled A Plain English Tour of the Regulations; What is Sewage Sludge and What Can Be Done With It?; Use of Biosolids in Crop Production; and Biosolids Quality. It is the responsibility of the biosolids producer and applicator, not the farmer, to ensure that all applicable regulations are followed. Nevertheless, farmers considering using biosolids should familiarize themselves with the regulations, with biosolids characteristics, and with recommendations and possible problems with agronomic use of biosolids.
Most biosolids available in Pennsylvania result from one of four stabilization methods: anaerobic digestion, aerobic digestion, alkaline stabilization, and composting. Biosolids typically are 50 to 60 percent organic matter. The organic fraction of alkaline-stabilized biosolids is somewhat lower due to the addition of lime. Each of these biosolids, as well as septage (which is usually stabilized by adding lime to the septage) has differing characteristics that affect its use and management. The major benefits of biosolids for crop production are the nitrogen and phosphorus they supply, as well as the addition of organic matter. Alkaline-stabilized biosolids contain excess lime and will neutralize soil acidity and supply calcium. These biosolids, therefore, also provide a liming value if the soil test indicates a need for lime.
Biosolids generally are applied to supply the nitrogen required by the next crop to be grown on a field. Total N content of most biosolids is in the range of 3 to 5 percent. The total N content of a particular biosolid will vary depending on the type of biosolids and the particular operating parameters used at the wastewater treatment plant. Application rates for these biosolids are determined similarly to those of manures. Most of the N in biosolids is in an organic form that is only available for crop uptake as it is mineralized. Availability of organic N in various biosolids during the first year after application is estimated to be 30 percent for aerobically digested and alkaline-stabilized biosolids, 20 percent for anaerobically digested biosolids, and 10 percent for composted biosolids. Biosolids contain some inorganic N as ammonium and nitrate. The inorganic N is assumed to be 100 percent crop-available. Farmers using alkaline- stabilized biosolids should monitor soil pH to ensure it does not go too high. Soil pH above 8 could lead to micronutrient deficiencies and may alter herbicide activity.
In addition to nutrients, organic matter, and lime, biosolids also contain low levels of trace elements, organic chemicals, and disease-causing organisms. The levels of these contaminants and their implications are discussed more fully in the fact sheets listed above. The regulations developed by EPA that govern the use of biosolids in agriculture were designed to prevent any adverse effects from these contaminants on human health and crop and animal production. The adequacy of the biosolids regulations is continuously being studied, and some changes can be expected in the future.