This paper is about the conversion of wet waste stream into valuable products via thermal processing. Hydrothermal carbonization of sewage sludge was carried out at 200°C and 2.1？MPa in a closed reactor for 1–6？h. Main products were in solid and liquid phases. The resulting hydrochar was shown to have H/C and O/C ratios moving towards natural lignite, improved energetic content, and adsorption property in terms of iodine number. The aqueous solution was found to contain high concentration of plant food nutrients, especially nitrogen and potassium. They may be desirable for subsequent fuel and chemical production as well as applications in agriculture. The study shows that valuable products can be generated successfully from sewage sludge using hydrothermal carbonization. 1. Introduction Sewage sludge is generated as an inevitable by-product of wastewater treatment activity. Their production is expected to rapidly increase with urbanization and industrial development. In Thailand, for example, about 4,000 tons per day of dry, treated sewage sludge solids is generated. Sewage sludge consists of mostly offensive and toxic substances. It must be disposed of or managed properly, otherwise, serious effects on humans and ecological systems will occur. Several methods can be adopted for the management of sewage sludge, such as landfill disposal, incineration, and utilization in agriculture, but each of these options has important limitations. Both incineration and landfill are troubled by their lowly public image. There are always concerns regarding associated costs and emissions. Application of sewage sludge to agricultural lands is restricted due to possible contamination of the soil and vegetation as well as hazardous consequences for animals and human. The decline of the traditional disposal routes for sewage sludge has created a strong demand for more cost effective and environmentally acceptable alternatives. This has motivated the research community to search for innovative and beneficial use of sewage sludge for years [1, 2]. A very appealing method is hydrothermal carbonization (HTC), also known as subcritical water or hot compressed water carbonization. It is ideal for high moisture content material such as sewage sludge. HTC can be described as a thermochemical process for converting an organic feedstock into value-added products, at moderate temperatures (180–350°C) and pressures (2–10？MPa) in the presence of liquid water. Hydrothermal degradation of organic matter and synthesis of basic chemicals and fuels have recently gained considerable
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