AgroChar

Problem

Currently, inappropriate farming techniques, indiscriminate use of fertilizers and pesticides lead to soil degradation, a decrease in soil organic matter, and adverse effects on plant growth. Among the methods contributing to soil improvement, the use of sewage sludge has become increasingly popular in recent years. Sewage sludge, a waste product derived from sewage treatment in wastewater treatment plants, has agriculturally suitable levels of N (%1.5-4.0), P (%0.3-1.2), and organic carbon, making it a soil conditioner that has been used for centuries to sustain productivity in agricultural soils. However, sewage sludges have diverse sources, and their contents vary. The quantity of sewage sludge depends on the degree and characteristics of wastewater pollution, the processes applied in the treatment plant (physical, physiochemical, biological, etc.), and the quality of the treatment. Nevertheless, the processes applied for the treatment and disposal of sewage sludge in treatment plants are often inadequate. This inadequacy may arise from either the failure to construct the units envisaged during the planning stage or the unhealthy operation of existing units. Additionally, the application of untreated sewage sludge, along with pathogens, raises health concerns due to organic and inorganic pollutants. Various studies have identified that such sludge containing heavy metals like lead (Pb), arsenic (As), and chromium (Cr) pose a carcinogenic risk, especially for children. Due to these reasons, the direct use of sewage sludge as a soil conditioner in agriculture is an insufficient and not recommended solution. While sewage sludge is hardly used for agricultural purposes in Belgium, the Netherlands, and Sweden, in Germany, the United Kingdom, France, and Portugal, 30-70% of the produced sludge is used for agricultural purposes. As of the end of 2018, the number of wastewater treatment plants serving approximately 83% of the Turkish population, equivalent to 54,484 tons of dry sludge daily, has reached 991 Newly emerging pollutants such as pharmaceuticals, personal care products, flame retardants, and micro-plastics have further heightened concerns regarding the agricultural use of sewage. Processes like composting and anaerobic digestion can, at best, only partially address the associated health issues

Solution

In response to the serious attention given to this issue by the government in recent times, our team envisions contributing by transforming sewage sludge accumulated in sewer systems and septic tanks into biochar. This biochar can then be utilized as soil fertilizer in agricultural lands. Thermal processes, incineration, and pyrolysis are alternative treatment methods. Pyrolysis is a method that breaks down a substance into products under high temperatures and anaerobic (oxygen-free) conditions. It sterilizes the material and reduces the concentration of organic toxins (e.g., poly-cyclic aromatic hydrocarbons (PAHs), poly-chlorinated biphenyls (PCBs), and dioxins), pharmaceuticals (e.g., hormones), and other micro-components (plastics, surfactant). Additionally, it passive immobilizes potentially toxic elements like Cu, Cd, and Cr found in the raw material, reducing the risk of leakage and plant uptake. Unlike incineration, pyrolysis retains most of the carbon in the sewage sludge. Contrary to additives like calcium (Ca), iron (Fe), and magnesium (Mg) in pristine biochar, which have been shown to reduce the presence of P, we believe using potassium (K) as a pre-pyrolysis additive can reverse the situation. Potassium phosphate salts exhibit very high solubilities, and our hypothesis suggests that if K is abundantly present, pyrolysis could result in the preferred formation of potassium phosphate, thereby increasing the presence of P in biochar. We used a potassium salt as a pre-pyrolysis additive due to its widespread use and low cost ($25 per ton) for general applications like de-icing. The aim is to produce a more complete carbon-sequestering fertilizer with well-balanced nutrient availability. Considering both financial and feasibility aspects, we believe this solution, which we find highly effective and practical, also supports recycling or reuse, which are fundamental initiatives of the modern approach. If pyrolysis vapors and liquids are used to feed the process or renewable energies, atmospheric carbon removal, storage, and environmental impact-wise, pyrolysis for sewage sludge can be considered the most sustainable solution.

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