Keshavan (2024) Analysis of biosolids and biochar from sewage sludge with its application on soil in Norwegian context. NTNU. https://ntnuopen.ntnu.no/ntnu-xmlui/handle/11250/3130822

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With increasing global population and environmental pressures, sustainable waste management has become a priority. One area of focus is the treatment of sewage sludge, a byproduct of wastewater treatment processes. This study, conducted by Pranav Keshavan for a Master’s thesis at the Norwegian University of Science and Technology (NTNU), explores the potential of biosolids and biochar derived from sewage sludge to improve soil quality and reduce environmental impacts, particularly in Norway.

The Problem with Sewage Sludge

Sewage sludge is rich in organic and inorganic matter, including nutrients beneficial for soil but also containing harmful contaminants like heavy metals and persistent organic pollutants (POPs). Traditional disposal methods such as landfilling and incineration pose environmental risks, prompting a shift towards more sustainable practices like resource recovery and reuse.

Research Objectives

The primary objective of this study is to analyze the efficacy of biosolids and biochar in soil applications. This involves assessing the removal efficiency and fate of various contaminants, particularly heavy metals and emerging organic pollutants like PFAS (Per- and Polyfluoroalkyl Substances) and OPFRs (Organophosphorus Flame Retardants).

Methodology

Data Collection

Samples were collected from 18 wastewater treatment plants across Norway. The study focused on four treatment scenarios:

1. Lime stabilization followed by dewatering.
2. Anaerobic digestion (AD) followed by dewatering.
3. AD followed by drying and pyrolysis.
4. Direct drying and pyrolysis without AD.

Analysis

The study measured the concentration of contaminants in the produced biosolids and biochar, examining their mobility, toxicity, and bioavailability. Pyrolysis temperatures ranged from 500-800°C to evaluate the effects on biochar characteristics and contaminant behavior.

Key Findings

Contaminant Retention and Removal

1. Heavy Metals: Biochar showed a higher retention rate for heavy metals compared to biosolids. Metals like Cr, Cu, Cd, and Pb were found to accumulate in the soil, with Cu and Zn showing higher leaching potential.
2. PFAS and OPFRs: Pyrolysis effectively reduced the concentration of these pollutants. Biochar produced at higher temperatures had lower contaminant levels, making it a safer option for soil application.

Soil Quality and Environmental Impact

1. Bioavailability and Toxicity: Biochar reduced the bioavailability and toxicity of heavy metals in the soil. For instance, Pb, As, Zn, and Cd were found to be more bioavailable in biosolids than in biochar-amended soils.
2. Leaching and Mobility: Biochar produced at low temperatures without AD showed less leaching and lower mobility of contaminants, making it a more stable amendment for soil.

Regulatory Compliance and Recommendations

The study highlighted the need for comprehensive monitoring and regulation of treated sludge applications to ensure environmental protection. Recommendations include:

1. Implementing thermal hydrolysis and co-pyrolysis to enhance treatment efficiency.
2. Promoting biochar amendment in agricultural practices due to its lower environmental risks compared to biosolids.

Conclusion

This study underscores the potential benefits of using biochar derived from sewage sludge in improving soil quality and mitigating environmental risks. Biochar, particularly when produced at higher temperatures, demonstrates superior performance in reducing the mobility, leaching, and toxicity of contaminants. These findings are crucial for developing sustainable sewage sludge management practices, especially in countries like Norway with stringent environmental regulations.

Future Research

Further studies are needed to explore the long-term effects of biochar on soil health and crop productivity. Additionally, more research on the interaction between various contaminants within biosolids and biochar is essential for optimizing treatment processes and ensuring safe soil application.

Practical Implications

The practical implications of this research are significant. By converting sewage sludge into biochar, wastewater treatment plants can not only reduce waste but also produce a valuable soil amendment. This aligns with circular economy principles, promoting resource recovery and sustainable agriculture.

In summary, this study provides valuable insights into the application of biosolids and biochar in soil management. It highlights the importance of choosing the right treatment processes to minimize environmental risks and enhance the benefits of waste-derived products in agriculture.