Evaluation of the Application Suitability of Hydrochar Produced from Sewage Sludge and Cow Manure as a Soil Amendment

In this study, hydrothermal carbonization (HTC) was employed to recycle organic waste with high moisture content, specifically sewage sludge and cow manure. Hydrochar was produced by mixing sewage sludge and cow manure in a 7:3 ratio and was analyzed to contain 58% ash, 29% volatile matter, and 8% fixed carbon. To evaluate the applicability of the produced hydrochar as a soil conditioner, it was mixed with standard sand at various blending ratios. The mixed soil was then analyzed for moisture content, pH, electrical conductivity (EC), exchangeable cations, cation exchange capacity (CEC), total nitrogen (T-N), and available phosphorus (A-P). The results were categorized as superior, intermediate, lower, or poor based on the general planting base soil evaluation grades from the Ministry of Land, Infrastructure, and Transport (MLIT)'s landscaping design standards. The analysis results revealed that hydrochar was rated as superior in all categories, while standard sand was classified as lower in all categories except for pH and EC. However, the addition of hydrochar demonstrated a notable improvement, particularly in exchangeable cations and CEC, which were rated as intermediate, indicating positive results for nutrient availability and soil fertility for plant growth. This improvement was attributed to the high ash content in hydrochar, which induced a liming effect. Therefore, hydrochar made from sewage sludge and cow manure is considered suitable for use as a soil conditioner. However, it is recommended that long-term observations of soil changes can be conducted in the future.