Search Results (20)

The hydrothermal dewatering (HTD) of lignite results in noticeable variations in the low-temperature oxidation process. Consequently, this study was made on the gas release and temperature change characteristics to investigate the oxidation kinetics and mechanism of HTD coal samples. In this study, a lignite from Inner Mongolia in China was upgraded by HTD. N₂ adsorption, SEM, FT-IR, and chemical titration experiments were also carried out on raw and HTD coal samples to relate the physico-chemical structure properties with low-temperature oxidation characteristics. Results show that HTD coal samples have higher low-temperature oxidation activities and lower critical ignition temperatures compared with raw coal. According to the change in activation energy by kinetic analysis, the low-temperature oxidation process in the temperature range 35–140 °C could be divided into the stage I (oxygen adsorption stage) and stage II (accelerated oxidation stage). The correlation analysis indicates that the oxygen adsorption stage is controlled by the aliphatic and surface structures, while the accelerated oxidation stage is jointly affected by the competition of physico-chemical structures. The oxygen adsorption stage promotes the progress in accelerated oxidation stage. Full article

With the acceleration of urbanization, the disposal of dewatered sewage sludge (DSS) has become an urgent environmental issue worldwide. Hydrothermal conversion (HC) of DSS is an important method for sludge sustainable utilization due to its combination of efficiency and economic and environmental advantages. This study investigates the product distribution and composition of products during the HC of DSS under subcritical and supercritical water conditions (200–450 °C, 5–90 min), with a particular focus on the formation and conversion mechanisms of steroid compounds. The results indicate that increasing temperature and reaction time leads to a rise in gas-phase products (GPs) and a corresponding decline in solid-phase products (SPs), with phenolic compounds identified as the predominant constituents. In the water-soluble products (WSPs), nitrogen-containing cyclic compounds are the major products. Saturated nitrogen heterocycles dominate at lower temperatures (200 °C), while at elevated temperatures (300–350 °C), saturated azapolycyclic compounds emerge, ultimately transitioning into unsaturated aromatic nitrogen heterocycles at 450 °C. Steroids are primarily concentrated in the oil-phase products (OPs). The conversion process involves the initial conversion of lipids in the DSS to long-chain olefins at 200 °C, which are then converted to steroids at 250–350 °C. At higher temperatures (400–450 °C), these steroids might decompose into gaseous products or undergo polymerization to form char. This suggests the potential for steroids to act as precursor compounds in the process of char formation. This work could contribute to a deeper understanding of the HC mechanism of DSS and provide valuable technical insights for improving bio-oil quality. Full article

Hydrothermal carbonization (HTC) is a promising thermochemical process to convert residues into hydrochar. While conventional HTC utilizes one type of residue as raw material only, Co-HTC generally combines two. By mixing dry and wet wastes, Co-HTC can advantageously avoid water addition. Therefore, this work investigated the potential of hydrochar derived from the Co-HTC of sawdust and non-dewatered sewage sludge as a dye (methylene blue) adsorbent and evaluated the toxicity of the resulting Co-HTC process water (PW) on Daphnia magna. Three hydrochars were produced by Co-HTC at 180, 215, and 250 °C and named H-180, H-215, and H-250, respectively. For methylene blue adsorption, H-180 and H-215 had a better performance than H-250. Both H-180 and H-215 presented a maximum adsorption capacity of approximately 70 mg·g⁻¹, which was superior compared with the adsorption of methylene blue by other hydrochars in the literature. Moreover, the removal percentage obtained with H-180 remained satisfactory even after five cycles. Regarding the toxicological assays of the PWs, raising the Co-HTC temperature increased the variety of substances in the PW composition, resulting in higher toxicity to D. magna. The EC₅₀ values of PW-180, PW-215, and PW-250 were 1.13%, 0.97%, and 0.51%, respectively. This highlights the importance of searching for the treatment and valorization of the PW. Instead of viewing this by-product as an effluent to be treated and disposed of, it is imperative to assess the potential of PWs for obtaining other higher added-value products. Full article

With the rapid development of urbanization and the widespread adoption of wastewater treatment facilities, the volume of sludge produced has steadily increased. Hydrothermal oxidation (HTO) technology offers an effective solution for sludge reduction, harmless disposal, and resource recovery, making it a highly promising method for sludge treatment. In recent years, HTO has attracted significant attention due to its efficiency and environmental benefits. This paper provides a detailed explanation of the fundamental principles of HTO in sludge treatment, with a focus on the removal of organic pollutants, nitrogen transformation, and phosphorus recovery. The influence of key operational parameters, such as reaction temperature, time, initial oxygen pressure, and pH, on the performance of HTO treatment is also explored. In addition, the research status of HTO sludge treatment and an example of product recovery after treatment are also discussed. It examines the challenges associated with scaling up HTO for large-scale sludge treatment, along with potential research directions for future work. Special attention is given to the innovation of catalysts, with the goal of achieving self-catalysis in sludge treatment. Moreover, considering that ammonia nitrogen (NH₃-N) is a major intermediate product in HTO, its removal, as well as the prediction and planning of other unintended products, remains a key issue. Further areas of interest include improving sludge dewatering performance and enhancing the production of valuable single carboxylic acids, which can boost resource recovery efficiency. This paper also highlights the diversification of sludge applications after HTO treatment. By providing insights into future development trends, this review offers valuable references for further research and practical applications. The ultimate goal is to support the development of HTO as a sustainable and efficient solution for sludge treatment, addressing environmental concerns while maximizing resource recovery opportunities. Full article

One of the processes that can serve to valorise low-quality biomass and organic waste is hydrothermal carbonization (HTC). It is a thermochemical process that transpires in the presence of water and uses heat to convert wet feedstocks into hydrochar (the solid product of hydrothermal carbonization). In the present experimental study, an improvement consisting of an increased hydrophobic character of HTC-treated biomass is demonstrated through the presentation of enhanced mechanical dewatering at different pressures due to HTC valorisation. As part of this work’s scope, flashing-off of low-quality steam is additionally explored, allowing for the recovery of the physical enthalpy of hot hydrochar slurry. The flashing-off vapours, apart from steam, contain condensable hydrocarbons. Accordingly, a membrane system that purifies such effluent and the subsequent recovery of chemical energy from the retentate are taken into account. Moreover, the biomethane potential is calculated for the condensates, presenting the possibility for the chemical energy recovery of the condensates. Full article

Sewage sludge management at wastewater treatment plants is becoming a more and more challenging task. Here, an innovative integrated modeling approach is developed to investigate the optimization of a municipal wastewater treatment plant (MWWTP) by the inclusion of hydrothermal carbonization (HTC). To this aim, two alternative plant layouts have been considered: (i) a conventional activated sludge-based treatment plant, i.e., based on thickening, stabilization, conditioning, and dewatering; (ii) additional hydrothermal carbonization and integrated treatment of the spent liquor in the sludge line. An Italian MWWTP has been selected as a case study, and three different scenarios have been implemented in the process simulation software World Wide Engine for Simulation Training and Automation (WEST) by considering the effect of the different digestion times in the aerobic reactor. Then, according to the Design of Experiment (DoE) methodology applied both on simulated and experimental data, and by the use of a Python code, the desired models have been developed and compared. Finally, a Life Cycle Assessment (LCA) study has been carried out to estimate the impacts on human health, ecosystems, and resources. The integration of HTC corresponds to the generation of a valuable product (the hydrochar), whereas the conventional layout is associated with high disposal costs of the sewage sludge. According to LCA results, a sludge age of 40 days is recommended due to the lowest impacts estimated, both with and without a HTC section. This has been ascribed mainly to the electricity demand of the sludge line, which increases with the excess sludge flow rate, i.e., as the sludge age decreases. Full article

Sludges from the papermaking industry represent a challenging residue stream that is difficult to dewater using conventional processes. The successful development and scale-up of innovative processes from lab- to pilot- to industrial-scale are required to tackle challenges for waste treatment, including paper sludges. Biological paper sludge was treated via a mild hydrothermal carbonization process (TORWASH^®) to improve dewaterability of the sludge, including long-duration, continuous testing. Initial lab-scale experiments indicated the optimal treatment temperature for sludge dewatering was 190 °C. Dewaterability improved with increasing temperature, but the obtained solid yield decreased. Scaling-up to a continuous flow pilot plant required a temperature of 200 °C to achieve optimum dewatering. Pilot-scale hydrothermal treatment and dewatering resulted in solid cakes with an average dry matter content of 38% and a solid yield of 39%. This study demonstrates the benefits of hydrothermal carbonization for the dewatering of biological paper sludge without the use of dewatering aids such as fiber sludge or polyelectrolytes. The results also demonstrate the successful adaptation of a lab-scale batch process to a pilot-scale continuous flow process for hydrothermal carbonization of industrial wastewater sludge. Full article

Phosphorus (P) recovery from alternative P-rich residues is essential to meet the growing demands of food production globally. Despite sewage sludge being a potential source for P, its direct application on agricultural land is controversial because of the obvious concerns related to heavy metals and organic pollutants. Further, most of the available P recovery and sludge management technologies are cost-intensive as they require mandatory dewatering of sewage sludge. In this regard, hydrothermal carbonization (HTC) has gained great attention as a promising process to effectively treat the wet sewage sludge without it having to be dewatered, and it simultaneously enables the recovery of P. This study was conducted to analyse and compare the influence of acid (H₂SO₄) addition during and after HTC of sewage sludge on P leaching and the characteristics of hydrochar. The obtained results suggested that despite using the same amount of H₂SO₄, P leaching from solid to liquid phase was significantly higher when acid was used after the HTC of sewage sludge in comparison with acid utilization during the HTC process. After HTC, the reduction in acid-buffering capacity of sewage sludge and increase in solubility of phosphate precipitating metal ions had a greater influence on the mobilization of P from solid to liquid phase. In contrast, utilization of H₂SO₄ in different process conditions did not have a great influence on proximate analysis results and calorific value of consequently produced hydrochar. Full article

Microalgae have attracted significant attention worldwide as one of the most promising feedstock fossil fuel alternatives. However, there are a few challenges for algal fuels to compete with fossil fuels that need to be addressed. Therefore, this study reviews the R&D status of microalgae-based polyculture and biocrude oil production, along with wastewater treatment. Mixotrophic algae are free to some extent from light restrictions using organic matter and have the ability to grow well even in deep water-depth cultivation. It is proposed that integrating the mixotrophic microalgae polyculture and wastewater treatment process is the most promising and harmonizing means to simultaneously increase capacities of microalgae biomass production and wastewater treatment with a low land footprint and high robustness to perturbations. A large amount of mixotrophic algae biomass is harvested, concentrated, and dewatered by combining highly efficient sedimentation through flocculation and energy efficient filtration, which reduce the carbon footprint for algae fuel production and coincide with the subsequent hydrothermal liquefaction (HTL) conversion. HTL products are obtained with a relatively low carbon footprint and separated into biocrude oil, solid, aqueous, and gas fractions. Algae biomass feedstock-based HTL conversion has a high biocrude oil yield and quality available for existing oil refineries; it also has a bioavailability of the recycled nitrogen and phosphorus from the aqueous phase of algae community HTL. The HTL biocrude oil represents higher sustainability than conventional liquid fuels and other biofuels for the combination of greenhouse gas (GHG) and energy return on investment (EROI). Deep water-depth polyculture of mixotrophic microalgae using sewage has a high potential to produce sustainable biocrude oil within the land area of existing sewage treatment plants in Japan to fulfill imported crude oil. Full article

A number of technological challenges need to be overcome if algae are to be utilized for commercial fuel production. Current economic assessment is largely based on laboratory scale up or commercial systems geared to the production of high value products, since no industrial scale plant exits that are dedicated to algal biofuel. For macroalgae (‘seaweeds’), the most promising processes are anaerobic digestion for biomethane production and fermentation for bioethanol, the latter with levels exceeding those from sugar cane. Currently, both processes could be enhanced by increasing the rate of degradation of the complex polysaccharide cell walls to generate fermentable sugars using specifically tailored hydrolytic enzymes. For microalgal biofuel production, open raceway ponds are more cost-effective than photobioreactors, with CO₂ and harvesting/dewatering costs estimated to be ~50% and up to 15% of total costs, respectively. These costs need to be reduced by an order of magnitude if algal biodiesel is to compete with petroleum. Improved economics could be achieved by using a low-cost water supply supplemented with high glucose and nutrients from food grade industrial wastewater and using more efficient flocculation methods and CO₂ from power plants. Solar radiation of not <3000 h·yr⁻¹ favours production sites 30° north or south of the equator and should use marginal land with flat topography near oceans. Possible geographical sites are discussed. In terms of biomass conversion, advances in wet technologies such as hydrothermal liquefaction, anaerobic digestion, and transesterification for algal biodiesel are presented and how these can be integrated into a biorefinery are discussed. Full article

This research paper evaluates hydrothermal carbonization (HTC) as a possible treatment for sewage sludge, including phosphorus recycling. German governmental requirements force a high number of wastewater treatment plants (WWTP) to recover phosphorus from sewage sludge above limits of 20 g kg_TS⁻¹ before further disposal (e.g., co-incineration). The results show that pH reduction has a positive effect on shifting phosphorus to the liquid phase during HTC. Although the experimental results of this research do not yet achieve the necessary phosphorus reduction, various calculations are made to achieve this goal in future experimental studies. In order to be able to assess the energy benefits of HTC, Aspen Plus modeling was used to show the positive impact of implementing this technology in a WWTP. It is shown that the mechanical dewaterability of sewage sludge (SS) increases after HTC enabling energy savings by means of subsequent thermal drying. A heat optimized HTC is able to cut energy expenses by half, further providing a phosphorus-depleted hydrochar for extensive energetic use. Full article

As the use of sewage sludge as a fertilizer in agriculture is increasingly restricted in the European Union, other ways to utilize this waste stream need to be developed. Sewage sludge is an ideal input material for the process of hydrothermal carbonization, as it can convert wet biomass into a solid energy carrier with increased mechanical dewaterability. Digested sewage sludge was hydrothermally carbonized at 160–200 °C for 30–60 min with initial pH levels of 1.93–8.08 to determine optimal reaction conditions for enhanced dewaterability and phosphorus release into the liquid phase. Design of experiments was used to develop response surface models, which can be applied to optimize the process conditions. For optimal dewaterability and phosphorus release, low initial pH values (pH 1.93) and mild temperatures around 170 °C are favorable. Because holding time had no statistically relevant effect, a dependency of reaction time was investigated. Though it did not yield substantially different results, it could be included in investigations of short reaction times prospectively. Low reaction temperatures and short holding times are desirable considering economic reasons for scale-up, while the high acid consumption necessary to achieve these results is unfavorable. Full article

In this work, dewatered waste activated sludge (DWAS) was subjected to hydrothermal carbonization to obtain hydrochars that can be used as renewable solid fuels or activated carbon precursors. A central composite rotatable design was used to analyze the effect of temperature (140–220 °C) and reaction time (0.5–4 h) on the physicochemical properties of the products. The hydrochars exhibited increased heating values (up to 22.3 MJ/kg) and their air-activation provided carbons with a low BET area (100 m²/g). By contrast, chemical activation with K₂CO₃, KOH, FeCl₃ and ZnCl₂ gave carbons with a well-developed porous network (BET areas of 410–1030 m²/g) and substantial contents in mesopores (0.079–0.271 cm³/g) and micropores (0.136–0.398 cm³/g). The chemically activated carbons had a fairly good potential to adsorb emerging pollutants such as sulfamethoxazole, antipyrine and desipramine from the liquid phase. This was especially the case with KOH-activated hydrochars, which exhibited a maximum adsorption capacity of 412, 198 and 146 mg/g, respectively, for the previous pollutants. Full article

Conventional activated sludge systems, still widely used to treat wastewater, produce large amounts of solid waste that is commonly landfilled or incinerated. This study addresses the potential use of Hydrothermal Carbonization (HTC) to valorize sewage sludge residues examining the properties of hydrochars depending on HTC process conditions and sewage sludge withdrawal point. With increasing HTC severity (process residence time and temperature), solid yield, total Chemical Oxygen Demand (COD) and solid pH decrease while ash content increases. Hydrochars produced from primary (thickened) and secondary (digested and dewatered) sludge show peculiar distinct properties. Hydrochars produced from thickened sludge show good fuel properties in terms of Higher Heating Value (HHV) and reduced ash content. However, relatively high volatile matter and O:C and H:C ratios result in thermal reactivity significantly higher than typical coals. Both series of carbonized secondary sludges show neutral pH, low COD, enhanced phosphorous content and low heavy metals concentration: as a whole, they show properties compatible with their use as soil amendments. Full article

Agriculture affects both the quantity and the quality of water available for other purposes, which becomes problematic, especially during increasingly frequent severe droughts. This requires tapping into the resources that are typically neglected. One such resource is a by-product of anaerobic digestion, in which moisture content typically exceeds 90%. Application of hydrothermal carbonization process (HTC) to this residue could partially remove organic and inorganic material, improve dewatering, decrease the overall solid mass, sanitize the digestate, change its properties, and eliminate problems related with emissions of odors from the installation. However, a significant gap still exists in terms of the dewatering of the hydrochars and the composition of the effluents. This work presents results of experimental investigation focused on the removal of organic compounds from the HTC effluent. Results of qualitative and quantitative analysis of liquid by-products of HTC of the agricultural digestate showed that acetic acid, 3-pyridinol, 1-hydroxyacetone, and 1,3-propanediol were the main liquid organic products of the process. Application of ultrafiltration process with the use of 10 kDa membrane for liquid HTC by-product treatment allows for the reduction of chemical oxygen demand up to 30%, biological oxygen demand up to 10%, and dissolved organic carbon up to 21%. Full article