Application of hydrothermal carbonization for sewage sludge and food waste valorization

Sewage sludge (SS), which is the main by-product of a Water Resources Recovery Facility (WRRF), is produced worldwide in large amounts. The rapid population growth, together with the progressive urbanization, has led to the generation of an increasing amount of SS, which is expected to continuously increase in the next future (up to an annual production of 150 – 200 million tons on a dry basis by 2050). Further, the production of other wastes, such as food waste (FW), are going to increase in the coming years. In fact, more than 2 billion tons of municipal waste have been generated worldwide and the production is estimated to increase by 70% to 2050. Sewage sludge and food waste must be properly disposed of using the best available technologies, in accordance with the current legislation and the circular economy principles. Both wastes can be generally treated via composting, anaerobic digestion (AD), incineration, and landfilling. However, all these treatments suffer from various limitations (e.g., long treatment time, pre-treatments, and inhibition). Therefore, thermochemical technologies, such as hydrothermal carbonization (HTC), are becoming increasingly attractive to treat different types of wet biomasses (such as SS, and FW). Indeed, HTC is able to transform the feedstock in three fractions: a solid fraction (hydrochar, HC), a liquid phase (process water, PW), and a small gaseous phase. This aim of this work was to investigate the application of HTC to treat SS or FW. In particular, the carbonization of SS has been extensively studied (Chapter 1 – 5), whereas only Chapter 6 has been devoted to the treatment of FW through HTC. Chapter 1 studies the influence of HTC reaction conditions (i.e., temperature, time, and solid content) on products (HC and PW) characteristics. Secondary SS derived from San Colombano WRRF (Florence, Italy) was collected and further treated via HTC by changing the operating conditions. Both HC and PW were characterized, pointing out that there was a relationship between HTC conditions and products characteristics. Further, the dewaterability of SS after HTC treatment was tested, showing better filtration performance than raw sludge. Chapter 2 investigates the recovery of phosphorous (P) from HC by acid leaching. Two acids (HNO3, and H2SO4) were tested, using both process water derived from SS carbonization and demineralized water as solution. Phosphorous yield (P yield) and ash content were selected as responses, with the goal to find the optimal conditions to maximize P yield while minimizing ash content. H2SO4 favoured P yield, but at the same time increased ash content in HC after leaching. In Chapter 3, three possible valorization pathways of PW derived from HTC on SS are proposed. Six SS samples (three anaerobically digested, and three aerobically stabilized) were collected from six WRRFs in Tuscany (Italy). The potential applications of PW as fertilizer on soils, as a substrate in AD, and as an effluent to be recirculated into the WRRF were further investigated. Process water was studied both in terms of chemical characterization and of biodegradability in anaerobic and aerobic conditions. The result was that PW has potential for a future use in soils, and that a correlation between anaerobic and aerobic biodegradability can be found. Chapter 4 is focused on the continuous anaerobic treatment of HTC-derived PW of the digested SS. For this purpose, an upflow anaerobic sludge blanket reactor (UASB) was set up for continuous treatment of PW. The reactor was first started up with only glucose, and subsequently a progressively increasing percentage of PW was added, reaching the 100 % of PW after 113 Days. Various parameters were monitored over time (e.g., biogas volume and composition, chemical oxygen demand (COD), and volatile fatty acids). A soluble COD removal up to 73 % was observed, while a specific CH4 production equal to 202 (33) mL STP CH4 g-1CODfed was achieved. Chapter 5 addresses the integration between the existing SS treatment line of San Colombano WRRF and HTC through Life Cycle Assessment analysis. HC was assumed to be energetically valorized as a solid fuel, while different treatments were proposed for PW (recirculation into the WRRF, or anaerobic digestion). In addition, phosphorous recovery from HC was also included in two of five Scenarios. Results showed that more environmental benefits occurred when including HTC into the SS treatment line (excluding three impact categories). Phosphorous recovery negatively affected the environmental performances of the proposed configurations, indicating that this process should be optimised. Finally, Chapter 6 investigates the application of HTC on FW. HC was chemically characterized, and since its properties resulted to fulfil the requirements of ISO/TS 17225-8, its application as biofuel was proposed. Further, PW was used as substrate in AD. The process was monitored over time in terms of soluble COD, total ammonia nitrogen, pH, alkalinity, and volatile fatty acids. The trend of recalcitrant compounds before and after AD was studied, observing that AD promoted the removal of specific refractory compounds. In addition, an energetic and economical balance of the process was carried out, evaluating the benefits produced by HTC technology.