Pyrolysis of Sewage Sludge as a Method of Carbon Sequestration into Soil

Bc. Michaela Majorová

Bachelor's thesis

Abstract:

One of the strategies to improve soil quality and resilience, lessen the effects of land use on the environment, and help mitigate climate change is soil carbon sequestration - rebuilding the reserves of soil organic carbon in croplands. Organic matter is key for retaining soil structure, water-holding capacity, and nutrient availability. However, the efforts to accumulate organic matter encounter an inevitable dilemma that the benefits of organic matter arise not only from its presence, but from its decay and continual turnover in soils. Pyrolysis of organic matter into biochar is currently being promoted as a technical fix for increasing the content of organic carbon in soils and as a strategy for mitigating climate change by carbon sequestration into soils in a form resistant to decomposition. Pyrolysis of sewage sludge into biochar also serves as a waste disposal strategy. In order to make biochar production an economically viable enterprise compared to demand for materials and energy use of biomass, methodologies have been devised by organizations, which make the carbon sequestered in biochar tradable for carbon credits on carbon markets. These methodologies use decay models to estimate the permanence of biochar in soil or in construction materials, which is then counted as avoidance of future emissions from biomass decomposition, and the biomass combustion during the process is assumed to be carbon neutral, as is the accounting consensus. Companies purchase carbon credits to offset their emissions, allowing them to make marketing claims of carbon neutrality and, in jurisdictions where permissible, to meet the imposed emission limits in emission trading systems. The field experiment included in this thesis studies the effect of sewage sludge biochar and wood biochar on the growth of perennial ryegrass (Lolium perenne L.), and studies the effect of the soil amendments on the soil carbon content under vegetation and on bare fallow. The methodology of the experiment includes two separate harvests during the vegetation season and an analysis of soil total carbon content (CTOT) and oxidizable carbon (COX), which usually serve as indicators of soil quality and are tracked over time. According to the results of the field experiment, biochar amendments did not lead to significant changes in biomass yields and there was no significant difference between the yields on plots with sewage sludge biochar and wood biochar. Biochar amendments did lead to increases in soil carbon content detectable after the vegetation season; however, the distinction of biochar carbon content, quantification of its degradation, or its effect on the native soil carbon content is challenging in soil analysis. To determine whether carbon sequestration in biochar also represents a climate benefit requires a comparison with alternative uses of the resources and energy used for its production, an assessment of environmental impacts and benefits, and a life-cycle assessment of the whole technology process.

Keywords

biochar, biosolids, carbon offsets, carbon credits

Language used: English

Date on which the thesis was submitted / produced: 28. 4. 2024

Thesis defence

• Date of defence: 28. 5. 2024
• Supervisor: Ing. Filip Mercl, Ph.D.
• Reader: Jan Habart, externi

Czech University of Life Sciences Prague

Faculty of Agrobiology, Food and Natural Resources

Bachelor programme:
Landscape Conservation and Use of Natural Resources