(729h) Hydrothermal Carbonization for Nutrient Recovery

A variety of macro- and micronutrients are required for plant’s healthy growth and maximum crop production. The most common macronutrients associated with commercial fertilizers are nitrogen, phosphorous, and potassium (NPK), each of which is commercially available in large quantities for modern farming. Until now, NPK containing fertilizers are produced by either accompanying chemical processes (e.g., Haber-Bosch process) or physical processing of mineral ores (e.g. potash). Although, most of the common waste streams (e.g. sewage sludge, dairy manure etc.) are very rich in macro-and micronutrients, the lack of economically and environmentally benign processes is hindering the use of nutrients from wastes.   

Hydrothermal carbonization (HTC or wet torrefaction) is a pretreatment process, which makes a homogenized, carbon and nutrient rich solid product, often called hydrochar, from underutilized various wet feedstocks, including dairy manure and sewage sludge. Compared to raw biomass, hydrochar is more hydrophobic and more biologically inert, which favors better storage properties for further applications. In this pretreatment method, biomass is treated with hot compressed water in an inert environment in the temperature range of 180-260 °C. The ionic constant of water is highest in this temperature range. As a result, the oxygen containing volatiles are removing from the solid, leaving a stable carbon and nutrient rich solid hydrochar.

In this study, macronutrients especially NPK recoveries from various wastes were evaluated by HTC. Both dairy manure and sewage sludge were hydrothermally carbonized at 180-260 °C for 5-30 min. Physico-chemical properties of both solid and liquid products were determined by means of ICP-OES, Lachat, TOC/TN, and elemental CHONS analyses. Total nitrogen and NH₄-N in the HTC process liquid was increasing with HTC temperature and time. However, more than half of the starting nitrogen was recovered in the hydrochar. Phosphorus recovery in the solid hydrochar was also increasing with HTC severity but potassium was found entirely in the process liquid. In conclusion, HTC can be applied to extent for nutrient recovery especially for nitrogen and phosphorus in the solid product.