Biosolids Benefiting Soils
Modern farming practices make maintaining health soil a challenge. Tillage, intensive cropping and removal of crop residues stress the productivity of soils. Farmers bring to their croplands the craft of affordably matching fertilizers, manures and other amendments to their crop’s needs for nutrients and a good growth media. Human waste have been used as a fertilizer ingredient on agricultural lands for as long as mankind has shifted from hunting and gathering to farming. Biosolids have been used also as a soil amendment and fertilizer for as long as mankind has deployed sewage treatment plants to manage human wastes. It is the most obvious of natural cycles, and for the past nearly 25 years this cycle has been managed according to science-based national standards set by the US EPA. According to this agency, a little more than half of the solids entering treatment plants in the USA are put to beneficial use as biosolids on land.
Biosolids and the Wastewater Treatment Plant
When sewage from homes and businesses enters a wastewater treatment plant, solids and liquids are separated and both are treated in a way to allow their safe discharge back to the environment.The solids from wastewater treatment plants can follow three major paths:
- Co-disposal with trash in a municipal solid waste landfill. Landfilling is the easiest disposal option and when managed correctly poses minimal risk to public health. The opponents of landfilling say landfilling wastes the valuable nutrients in the material such as organic matter, nitrogen and phosphorous. In addition landfilling poses a greater issue because it is not a sustainable option considering the global impact landfills have on the environment.
- Thermal oxidation in a sewage sludge incinerator. Incinerating sludge reduces the volume of material leaving the wastewater treatment facility. The incineration process is not self sustaining so very little energy is produced and the construction costs make this disposal option very costly. In addition, incineration does not make use of the beneficial nutrients in the solids.
- Land application to soil for the nutrient and organic matter. Application of biosolids to soil reuses the organic matter and nutrients found in solids. Sludges that are settled out of waste water in the treatment plant are processed into biosolids to reduce the amount of water, to manage odors and to kill human pathogens. The resulting material is known as biosolids. Biosolids can be applied on agricultural, forest and reclamation sites. The process is highly regulated to control public health and environmental risks (1).
Benefits of Recovering the Resource Value of Biosolids
Biosolids contain “macro” nutrients essential for plant growth, specifically nitrogen and phosphorus. They also have an ideal proportion of micronutrients that are essential to healthy plant growth; the array of nutrients that are included in a vitamin tablet, like magnesium, calcium, iron, zinc, copper, and many other elements. According to a joint report recently released by an international collection of biosolids associations, “If the total biosolids production would be applied on land it would fertilize the equivalent of 24 million hectares for nitrogen and 76 million ha for phosphate (1).” The amount for phosphate represents nearly half of the European Union’s consumption.
Recycling of biosolids to farmlands is made part of a larger soil management effort. The regulatory system for biosolids typically calls for full implementation of approved conservation plans, incorporating current best practices for soil and erosion control. It also involves preparing a nutrient management plan, to ensure a proper match of available nutrients and crop requirements. The application of biosolids is measured and reported to record compliance with the nutrient plan. Management practices that are a part of the biosolids program include setbacks from property lines, wells, steep slope, drainage swales, and such landscape features that are prudent to avoid during biosolids fertilization.
Use of biosolids not only is a sound way of making use of a recovered nutrient source, but the practice encourages good agricultural practices. What is more, there is a saving in reducing the energy needed to extract those nutrient resources and deliver them to farmers, thereby also decreasing CO2 emissions.
Biosolids application rates are often determined by the nitrogen requirements of the farmer’s crop. Biosolids contain nitrate-N, ammonium-N and organic N, but nitrate-N is only found in small amounts so more focus is given to ammonium-N and organic N. Ammonium-N is available to plants immediately after land application, while organic N is slow releasing (2). The amounts present change depending on the treatment process the biosolids undergo.
Ammonium-N can release into the atmosphere after initial application. To reduce that loss, biosolids can be incorporated into the soil. Biosolids with higher water content hold ammonium longer in the soil.
Organic-N remains in soil longer than ammonium-N when biosolids are land applied. The nitrogen is converted into ammonium or nitrate by soil microorganisms and is available to plants in a process called mineralization. This process works more rapidly in the three to six weeks following biosolids application and in warm moist conditions conducive to microbial activity (2).
Phosphorus is an essential nutrient for the plant’s energy system and cells, and reacts within the soil to form minerals that become a long lasting reservoir for plant needs. Managing phosphorus levels added with biosolids requires insight into:
- Existing P (phosphorus) levels in the soil,
- Type of cultivation and tillage practices on the farm, and presence of subsurface tile drains.
- Topographic character of the watershed affecting erosion and drainage.
Synthetic fertilizers used on farmland contain inorganic P that is highly soluble. Biosolids can be a safer source of P because a smaller fraction of the total P in biosolids is soluble. The lower solubility of P in biosolids decreases the potential of P runoff, without decreasing P availability to the roots of growing plants. In addition, some wastewater treatment facilities add forms of aluminum, iron, or calcium which further decrease solubility of P (2).
Micronutrients, pH and Soil Quality
Levels of Potassium, Calcium, Magnesium, Sulfur, soil PH and the soil quality also change with biosolids application. When land applying biosolids, soils benefit from the plant-available sulfides (S) as well as slow-release S from decomposition of biosolids organic matter.
The pH of biosolids changes depending on the treatment process in the wastewater treatment plant. Some facilities mix alkaline materials during the biosolids processing, thereby increasing the pH of the biosolids and, when added to the croplands, supplementing other liming sources (2). It is important to know the current soil pH of application sites and the pH conditions most beneficial to the plant life at the site. Biosolids that are not stabilized with alkaline materials before land application can cause the pH of the receiving soils to become lower, meaning that the farmer will need to add other liming agents more frequently than is the case with conventional fertilizers.
Organic matter in soils can increase with biosolids applications. Though the quantities are relatively small, biosolids-borne carbon is already in a stable form, so it is retained in the soil after biosolids application. When soil management practices such as reduced tillage or use of cover crops are used by the farmer, the soil organic matter can significantly rise (2). Experience with long-term biosolids use shows that soils show greater water-holding capacity, improved tilth, reduced soil erosion, increase cation exchange capacity, micronutrient deficiencies correction and increased earthworm and microbial activity.
When appropriately applied and managed, biosolids serve as an effective soil amendment and fertilizer. Their composition provide nutrient releases that may outperform synthetic fertilizers, and importantly at a fraction of the economic to the farmer and the local wastewater agency, and at lower total cost to the environment.
To learn more about the benefits biosolids have to soils check out:
- Biosolids Land Application and Food Crop Quality Assurance Scheme Blog Highlight
- Fertilizing with Biosolids
- Biosolids Benefits
- Land application of sewage sludge and biosolids
- Land Application of Biosolids : a Review of Research Concerning Benefits, Environmental Impacts, and Regulations of Applying Treated Sewage Sludge