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Land application of biosolids: benefits, risks and cost

Nikolaidis Nikolaos, Saru Maria-Liliana, Leeda Demetropoulou

Πλήρης Εγγραφή

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Περίληψη

Governmental agencies, municipalities and industries around the world have been seeking viable alternatives for disposing of biosolids generated from wastewater treatment since landfill capacity diminishes and waste disposal costs increase. Biosolids are composed of compounds valuable for agricultural use (organic matter, nitrogen, phosphorous, potassium and micro-nutrients such as calcium, sulphur and magnesium), and pollutants that include heavy metals, organic compounds and pathogens. While they can be a valuable resource, potential risks associated with the nutrients, disease causing microorganisms, chemical contaminants and unstabilized material that biosolids include need to be appropriately managed. Alternative disposal methods for biosolids that have been used in the past include dumping at sea, landfilling, incineration and land application. The amount of generated sludge that needs to be disposed of in EU is exceeding 10 M dry tons per year while the US has to dispose approximately 7 M dry tons per year. Given that approximately half of the operating cost of waste water treatment plants is related to the treatment and disposal of sludge, the magnitude of treatment and disposal has promoted regulatory actions during the past 20 years focusing mostly on land application of biosolids and spurred the development of composting and other lower cost technologies that ensure not only their disposal, but also their reuse. The objective of this manuscript is to provide an overview of the potential benefits and risks of land application of biosolids with special emphasis on rural environments as well as on the management cost of disposal. Special focus will be given to sludges produced in rural areas such as the island of Crete and on closing the nutrient loop between urban and peri-urban areas. Sewage sludge has to be treated (dewatered and stabilized) before land application either in terms of soil fertilization or land reclamation. There are several treatment methods available: lime stabilization, composting, anaerobic digestion and thermal drying. Land application of biosolids has significantly positive effect on soil fertility ensuring their use in a sustainable manner. Their relatively high and readily available nutrient content has high fertilization value for the plants. In addition to the high nutritional value, biosolids improve the physico-chemical properties of the soil (soil conditioning) by increasing soil organic matter, decrease bulk density, increase soil particle aggregation, soil structure and porosity. Organic matter addition stimulates microbial activity and enzymatic production in the soil that are essential biocatalyst for plant growth. Finally, sludge amendments to agricultural lands have increased crop production and yields typically exceed inorganic fertilization yields. The risk associated with land application of biosolids depends on the origin of the pollution loads entering the waste water treatment plants (municipal versus industrial loads). Human and animal health, soil quality, plant growth and water quality can be seriously affected by uncontrolled pollutants and pathogens and excessive or improperly application of N and P that may be contained in biosolids. Excessive rates of land application can negatively impact groundwater quality because of nutrient leaching. However, this risk is substantially reduced if appropriate stabilization methods have been applied and risk bazed, site-specific land application loads have been incorporated in current regulations. This would be especially true in rural areas where the quality of sludges is of superior quality compared to major urban centres that have significant industrial contributions. The cost of treatment (both capital cost and operating cost) can be significantly reduced if we view sludge treatment and disposal as part of a holistic system of separation and recycling of organic matter from all sources (including solid waste -organic fraction recycling, agricultural wastes, livestock etc) in a urban-peri urban system and in this way closing the nutrient loop and returning the organic matter back to the cultivated soil, restoring in this way soil fertility.

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