Biosolids can provide essential plant nutrients, improve soil structure and tilth, add organic matter, enhance moisture retention, and reduce soil erosion. Sewage sludge contains organic matter and plant nutrients beneficial to soil health and crop production. Applying sewage sludge to agricultural lands fits well with the current concepts of resource recycling and sustainable agriculture. However, because sewage sludge contains contaminants, such as trace elements, that may decrease the quality of agricultural soils, careful control of contaminant levels in soils receiving sludge is needed.
Contamination of soil by trace elements is a concern because they are persistent and may affect plant, animal, and human health. Application to agricultural soils is a beneficial method of managing sewage sludge, but adding contaminants to soil in this, and other, waste materials must be controlled. The trace element content of sewage sludge is the main factor that still restricts its agricultural use.
However, steps can be taken to decrease the availability of trace elements in sludge-amended soils. Trace element content in plants is strongly influenced by pH and sludge treatment. Increasing the pH of a sewage-amended soil or heat-treating it can reduce the plant metal uptake. Manipulating the soil pH is the most effective and rapid method of controlling the availability of trace elements in sludged soil. Liming the soil to a pH of 6.5 to 7 can reduce the mobile fraction of most of the trace elements in soil.
The trace elements in sludge are present in various chemical forms that differ in respect to solubility in water, bioavailability, and stability [166,167]. The utilization of sewage sludge as fertilizer depends not only on the total content of trace elements but also on the chemical forms in which they are present. The form in which a metal exists significantly affects its biological activity availability, toxicity, and mobility in the soil environment. It is well known that vegetation grown on sludge-amended soils poses a direct entry for trace elements into human food chain. India has no administration or legislative measures to control the continued application of sludge as a manure.
Most regulations in India concerning trace elements in food, occupational health, and environment are based on the total element contents and are frequently given as maximum limits or guideline levels. In contrast, few regulations pay attention to the molecular species (speciation) in which the elements are bound. There is a great need for the development of more species-specific analytical and toxicological data and modified regulations are necessary if India wants to enter the global market in the area of food, agriculture, and environment. Also, there is a need for creating a different speciation scheme to study the toxicity, mobility, and phytoavailability of these trace elements for the development of a model to predict the soil-to-cell movement of metal pollutants. The model representing the pathways of trace elements in the environment and prediction about the fate of these pollutants based on the speciation studies will help in reducing the risk due to the application of sewage sludge as manure.
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