This study focuses on the conversion of biosolids to biochar and its further use in adsorbing per- and polyfluoroalkyl substances (PFASs) from contaminated water.

The elevated conditions needed for hydrothermal carbonization of biomass require a special high-pressure reactor which makes it expensive and time-consuming. The soft computing approaches as proposed here can predict the conversion of any feedstock based on the composition and operating conditions without the need for any kinetic modeling. In this study, Extreme Gradient Boosting method (XGBoost), Multilayer Perceptron Artificial Neural Network (MLPANN) and Support Vector Machine (SVM) were trained in python programming language using the data available from the literature for hydrothermal carbonization of different biomass. Statistically, XGBoost showed a higher accuracy among all studied approaches with R² of 0.999 and 0.964 for training and testing data, respectively. The conversion was sensitive to temperature, time, lignin, moisture content, cellulose and hemicellulose, respectively, for the range of conditions applied. It was also revealed that none of the parameters were negligible, however operating conditions were more influential followed by lignin content. This proposed approach can be extended to include liquefaction and gasification processes, where the distribution of products can be estimated for any lignocellulosic biomass.

Thermochemical treatment is rapidly emerging as an alternative method for the management of stabilised sewage sludges (biosolids) to effectively reduce waste volume, degrade contaminants, and generate valuable products, particularly biochar and hydrochar. Biosolids-derived char has a relatively high concentration of heavy metals compared with agricultural chars but is still applied to land due to its beneficial properties and ability to retain metals. However, non-agricultural applications can provide additional economic and environmental benefits, promote sustainability and support a circular economy. This review identifies extensive non-agricultural opportunity for biosolids biochar, including adsorption, catalysis, energy storage systems, biological process enhancement, and as additives for rubber compounding and construction. Biosolids chars have received limited attention vs agricultural char, and we draw on both areas of literature, as well as evaluating differences between agricultural and biosolids chars. A key opportunity for biosolids biochar in comparison with other materials and agricultural chars is its sustainable and low-cost nature, relatively high metals content, improving catalyst properties, and ability to modify in various stages to tune it to specific applications. The specific opportunities for hydrochar have only received limited attention. Research needs to include better understanding of the benefits and limitations for specific applications, as well as adjacent drivers, including society, regulation, and market and economics.