Liquefaction Of Sewage Sludge Research Articles

Machine learning and experiments on hydrothermal liquefaction of sewage sludge: Insight into migration and transformation mechanisms of phosphorus

Sewage sludge (SS), as a phosphorus (P)-rich wet biomass, can generate a large number of P-containing high-value by-products after converted to biofuel by hydrothermal liquefaction (HTL). However, the potential mechanisms of migration and transformation of phosphorus in this process are unclear due to the complexity of SS composition and HTL processes. In this work, Gradient Boosting Regression (GBR) algorithm was employed in machine learning (ML) models to investigate the impacts of input features on the phosphorus content and forms during HTL. Machine learning models showed good performance with an average training R2 > 0.95 and an average testing R2 > 0.85. Further, representative HTL conditions including temperature (270–350 °C) and residence time (20–60 min) were selected for experiment validation of studied mechanisms. The results indicated that with temperature and time increasing, phosphorus recovery (R_P) increased from 86.85 % to 92.65 % and the relative content of apatite inorganic phosphorus in hydrochar (APRC_HC) increased from 33.67 % to 39.06 %. The relative content of inorganic phosphorus in hydrochar (IPRC_HC) stabilized at 98 % at above 270 °C. 85 % of phosphorus in SS migrated to hydrochar after HTL, and most of P was in the form of IP. Ca3(PO4)2 and Ca4P6O19 were detected by X-ray diffraction (XRD). This information can provide significant theoretical support for phosphorus resource recovery in hydrochar derived from HTL of SS.

Multiphase Numerical CFD Simulation of the Hydrothermal Liquefaction Process (HTL) of Sewage Sludge in a Tubular Reactor

This article presents multiphase numerical computational fluid dynamics (CFD) for simulating hydrothermal liquefaction of sewage sludge in a continuous plug-flow reactor. The discrete particle method (DPM) was used to analyze the solid particles’ interaction in liquid–solid high shear flows to investigate coupling computational fluid dynamics (CFD). Increasing solid particles’ interactions were observed with the increasing liquid velocity. The study examined the influence of parameters such as flow rate, temperature, and residence time on the efficiency of bio-oil production. An increase in temperature from 500 to 800 K caused an increase in the amount of biocrude oil produced from 12.4 to 32.9% within 60 min. In turn, an increase in the flow rate of the suspension from 10 to 60 mL/min caused a decrease in the amount of biocrude oil produced from 38.9 to 12.9%. This study offers insights into optimizing the flow channel of tubular reactors to enhance the HTL conversion efficiency of sewage sludge into biocrude oil. A parametric study was performed to investigate the effect of the slurry flow rate, temperature, and the external heat transfer coefficient on the biocrude oil production performance. The simulation data will be used in the future to design and scale up a large-scale HTL reactor.

Continuous wet air oxidation of aqueous phase from hydrothermal liquefaction of sewage sludge

The conversion of organic waste into advanced fuels through hydrothermal liquefaction (HTL) is a promising route. However, for the commercialisation of HTL technology, treatment and valorisation of the generated aqueous by-product are required for its economic viability. This study investigates subcritical non-catalytic continuous wet air oxidation (WAO) for the treatment of the aqueous phase from hydrothermal liquefaction (HTL-AP) of sewage sludge. A custom-made tubular reactor was used at temperatures and residence times similar to the HTL process (300–350 °C; 6–50 min), with excess air equivalent to 1.5–2 times the oxygen demand. The WAO process removed up to 95.3 % of the chemical oxygen demand and 91.8 % of the total organic carbon, generating volatile fatty acids, mainly acetic acid, as intermediary compounds from the oxidation reactions, and CO2 and H2O from the pollutant’s complete degradation. The process effectively eliminated almost all of the organonitrogen compounds, generating NH4+, which reached values of up to 97 % of the total nitrogen. Comprehensive molecular characterization performed by FT−Orbitrap MS analysis revealed that the main basic organonitrogen compounds in HTL-AP are distributed in the O1N1, N2, O1N2, N1 classes. For neutral to acidic organonitrogen species, O3N1 is the most representative chemical class. The energy consumption for heating the WAO reactor was up to 40 % lower using HTL-AP compared to blanks experiments. Despite good process water clean-up, autothermal operation was not achieved, and improvements in the reactor design are suggested.

Anaerobic digestion of wastewater from hydrothermal liquefaction of sewage sludge and combined wheat straw-manure

Hydrothermal liquefaction (HTL) shows promise for converting wet biomass waste into biofuel, but the resulting high-strength process water (PW) requires treatment. This study explored enhancing energy recovery by anaerobic digestion using semi-batch reactors. Co-digesting manure with HTL-PW from wheat straw-manure co-HTL yielded methane (43–49% of the chemical oxygen demand, COD) at concentrations up to 17.8 gCOD·L-1, whereas HTL-PW from sewage sludge yielded methane (43% of the COD) up to only 12.8 gCOD·L-1 and complete inhibition occurred at 17 gCOD·L-1. Microbial community shifts confirmed inhibition of methanogenic archaea, while hydrolytic-fermentative bacteria were resilient. Differences in chemical composition, particularly higher levels of N-containing heterocyclic compounds in PW of sewage sludge, likely caused the microbial inhibition. The considerable potential of combining HTL with anaerobic digestion for enhanced energy recovery from straw-manure in an agricultural context is demonstrated, yet sewage sludge HTL-PW requires more advanced approaches to deal with methanogenesis inhibitors.

Hydrothermal liquefaction of sewage sludge: use of HCOOH and KOH to improve the slurry pumpability in a continuously operated plant

We studied the hydrothermal liquefaction (HTL) of digested sewage sludge (DSS) as model of waste biomass in batch and continuous reactors. HCOOH and KOH were used to improve the slurry pumpability.HTL experiments were conducted at the same kinetic severity factor in a batch reactor of 25 mL of volume and in a continuously operated tubular reactor with 350 mL of volume. The observed outcomes suggested that it was not possible to achieve the pumpability of native DSS when a high concentrated stream of suspended solid particles has been fed to the HTL continuous plant. Using acidic or basic homogeneous additives, as potassium hydroxide or formic acid, it was possible to enhance the pumpability of a concentrated slurry of DSS in the continuous plant achieving yields of heavy oil (fraction of biocrude) similar to those obtained in the batch reactor and with higher H/C ratios. Hence, we found that HCOOH and KOH are promising additives for the practical implementation of a continuous HTL process.

The construction of a biomass component interaction model based on research into the hydrothermal liquefaction of sewage sludge.

Sewage sludge (SS), a hazardous solid waste with a high water and pollutant content, should be disposed of correctly. Hydrothermal liquefaction (HTL) shows tremendous potential to treat organic matter with substantial water content like SS. In this paper, we examined the impact of key factors on the characteristics and yield of bio-oil during HTL of SS. We clarified the impacts of each component on the yield through model compounds based on that and constructed a component additivity model for forecasting the bio-oil yield from biomass with complex component composition. In the reactions of the model compounds, cellulose showed synergistic interaction with protein and alkali lignin in the bio-oil yield but lipids showed antagonistic effects with protein and alkaline lignin. The co-HTL results of these binary mixtures improved our model and further clarified the reaction mechanism of HTL of SS.

Understanding the fate of nitrogen during catalytic hydrothermal liquefaction of sewage sludge

In this study, the fate of nitrogen during catalytic hydrothermal liquefaction (HTL) of sewage sludge (SS) is investigated using three different catalysts (CuNi/SiO2, HCOOH, CuSO4) with 5 wt% loading. The bio-crude yields obtained from HTL experiments catalyzed with CuNi/SiO2 are similar to those obtained through non-catalyzed experiments. HCOOH slightly increases the bio-crude yields, while maximum yields of 24.5 wt% is obtained in the presence of CuSO4, which also reduces the nitrogen content by 15 % and enhances the hydrocarbons compared to the non-catalyzed HTL. Mechanistic investigations regarding the interaction of amino acids and carbohydrates by Maillard reactions are carried out using model compounds, namely lysine and lactose. CuSO4 effectively increases the yield with 50 % and reduces the nitrogen content by 24 % in the bio-crude during HTL of lysine alone. In the case of the model mixtures, bio-crude yields, nitrogen content, and Maillard reactions products behaved similarly both for catalytic and non-catalytic HTL, a slight reduction of amines was found in the presence of CuSO4. Hydro-char and some organic compounds are assumed to act highly reductant during catalytic HTL. Coke deposition and adsorbed poisoning by Maillard reaction products are proposed as the main reasons for the deactivation of catalysts.

Hydrothermal liquefaction of sewage sludge and model compound: Heavy metals distribution and behaviors

Hydrothermal liquefaction (HTL) is an attractive thermochemical method for the conversion of wet sewage sludge (SS) into bio-oil. However, the bio-oil suffers from the contamination of heavy metals (HMs). In this study, the effects of reaction conditions on the HMs (Cu, Zn, Cd, Cr and Pb) migration during HTL of SS was investigated. Soybean protein was used as the typical model compound of SS to reveal the interactions between HMs and organic matters in the HTL process. Results showed that higher temperatures and longer reaction times promoted HMs enriched into the solid residues with more stable forms, while the proportion of HMs in the bio-oil was also increased. XPS analysis showed that the coordinate bond was formed between HMs and N during HTL of soybean protein at 180 °C. At higher temperature of 280 °C, metal oxides appeared in residues, while Cu and Cd led to the formation of nitrates. GC–MS analysis showed that Cu reduced the aromatic content by 52.8 %, while other HMs (Zn, Cd, Cr and Pb) reduced the oxygenated compounds content by 21.6 % at 180 ℃. In addition, the HMs had little effects on the bio-oil compositions at 280 °C due to the more stable forms of HMs at higher temperatures.

Sequential Extraction and Characterization of Nitrogen Compounds after Hydrothermal Liquefaction of Sewage Sludge.

Organic solid wastes such as sewage sludge are potential feedstocks for the production of drop-in biofuels. Hydrothermal liquefaction (HTL) is a process that converts the wet sewage sludge into an organic biocrude. To fulfill industrial fuel standards, the considerable heteroatom content of the biocrude needs to be lowered by downstream processes. Nitrogen (N) contained in several compounds poses a challenge and yet, the complex chemical composition of HTL-biocrude samples has hindered detailed analysis and understanding. In particular, N-containing aromatic substances appear very persistent in biocrude. In the present work, two alkaline (NaHCO3 and NaOH) and one acidic (HCL) aqueous solutions were subsequently applied to extract and recover polar N-containing compounds from the biocrude matrix with an N-content of 3.8 wt %. Gas chromatography-mass spectrometry, atmospheric pressure chemical ionization in positive mode, and Fourier transform ion cyclotron resonance mass spectrometry were applied for their characterization and results show that a large share of N-compounds with an aromatic, pyridinic structure was found in the acidic extracted fraction with an N-content of 9.5 wt %. Aliphatic N-compounds were less affected by the separation and ended in the residual fraction. N-compounds with multiple oxygen functionalizations are enriched in the alkaline extracted fractions. This showed that N-compounds with an aromatic structure are strongly affected by polar groups and can potentially be extracted by downstream processes with appropriate solvents.

Biobased additives for asphalt applications produced from the hydrothermal liquefaction of sewage sludge

Sewage sludge from wastewater treatment plants is a large source of organic waste with suboptimal disposal solutions available. Current common handling solutions include disposing of it as fertilizer on arable land, or direct discharge in the sea. This work investigates the valorization of sewage sludge into biocrude oils using hydrothermal liquefaction (HTL). Biocrude oils are bitumen-like materials with potential applications as green additives for asphalt binder, one of the most used materials in infrastructure. Here, we study the links for sewage sludge between feedstock (digested versus non-digested sludge), HTL conditions (temperature, biomass loading to the reactor and reaction time) and yields of biocrude oil. Our data suggests that non-digested sewage sludge leads to higher biocrude oil yields (30–40 wt%) at temperatures of 300–320 °C and biomass loadings of 20 wt%. Furthermore, we use density functional theory (DFT) calculations to study the reactivity and clustering mechanisms of asphaltenes – a key molecular component of asphalt binder, and largely responsible for its mechanical performance. Biobased asphaltenes are present in biocrude oil, and our aim was to understand their differences with fossil asphaltenes derived from petroleum. Our data suggests that biobased asphaltenes are similar to petroleum-based ones in terms of thermodynamic stability and π-π stacking, despite the higher content in polar chemical functionalities in biobased asphaltenes. Overall, the chemical features and intermolecular interactions indicate that biocrude oils produced from sewage sludge via HTL are promising candidates for application as asphalt additives.

Hydrothermal liquefaction of sewage sludge into biocrude: Effect of aqueous phase recycling on energy recovery and pollution mitigation

Hydrothermal liquefaction (HTL) of sewage sludge is facing the challenges of low biocrude yield, a large number of intractable aqueous phase and heavy metals pollution. In this study, the aqueous phase produced by HTL was recycled as solvent with an aim to improve the biocrude yield and mitigate potential pollution. Results showed that the recycling of aqueous phase increased the biocrude yield from 17.9 to 30.5% and the energy recovery ratio from 40.3 to 61.7%. The recycling could increase the contents of Zn, Cu, Pb and Cr in solid residues by 2.7−3.0 times. It is worth emphasizing that the recycling reduced the COD of aqueous phase by 24.9%. However, the enhanced protein hydrolysis process reduced the calorific value of biocrude from 36.4 to 28.5 MJ/kg, and promoted the migration of the nitrogen to the aqueous phase, which was not environmentally favourable for the direct usage in diesel engines. Analysis showed that the ketones and the phenols in aqueous phase participated in HTL process as reactants, and the acids promoted the hydrolysis of protein in the sludge. Overall, the recycling of aqueous phase effectively improved the energy recovery and alleviated pollution of the sludge HTL.

Comparative investigation on the value-added products obtained from continuous and batch hydrothermal liquefaction of sewage sludge

Hydrothermal liquefaction (HTL) can be considered a promising route for the energy valorisation of waste sewage sludge (SS). However, not much information is available on continuous flow processing. In this study, the mixed SS was subjected to HTL at 350°C for 8 min in a continuous reactor with loadings of 10 wt% in the feed flow. The results show that the mass recovery reached 88%, with a biocrude yield of 30.8 wt% (3.9 wt% N content). The recovered biocrude yields are highly dependent on the selection of the recovery solvent for extraction, and dichloromethane can contribute an additional 3.1 wt% biocrude from aqueous phase, acetone can extract some pyrrole derivatives into the trapped phases. Comparable results were also achieved by performing batch reactions under the same conditions: a slightly higher biocrude yield (33.1 wt%) with an N content of 4.3 wt%. The higher N content observed in the biocrude from the batch process indicates that interactions and chelation between intermediates are enhanced during heating up and cooling period, which lead to more N-containing compounds.

Hydrothermal liquefaction of sewage sludge for biofuel application: A review on fundamentals, current challenges and strategies

Scientific studies have demonstrated that hydrothermal liquefaction (HTL) can be suggested as a cost-effective and eco-friendly solution for the valorization of sewage sludge in the fields of waste management and renewable energy production. On-going efforts are continued extensively to improve conversion efficiency in order to transform this technology into industrial application. The present review focuses on state-of-the-art of HTL of sewage sludge, systematically, the HTL fundamentals, the influences of the essential operation parameters, main existing challenges and integrated upgrading are evaluated in detail. Opportunities and strategies including pre-treatment, co-liquefaction, post-application of aqueous and solid phases are addressed. Additionally, some available results regarding the continuous-flow tests of sewage sludge are summarized. Finally, further considerations and recommendations are made for improving or completing the economic feasibility of HTL process.

Effect of transition metals and homogeneous hydrogen producers in the hydrothermal liquefaction of sewage sludge

Hydrothermal liquefaction (HTL) of sewage sludge (SS) was performed in the presence of metallic Zn, Fe and Ni to investigate their effect on the performances of the process in terms of product yields and quality. Experiments were performed in subcritical water at 350 °C for 10 min using each metal individually and in the presence of homogenous hydrogen producers like formic acid (FA) and KOH. Interesting results were obtained with Zn that when used alone or in the presence of KOH, increased the cumulative biocrude (BC) yields and resulted in energy recoveries (ER) higher than 100% with respect to initial energy content of the biomass, thus indicating that HTL of SS is globally endothermic. Moreover, when Zn was used with FA it strongly enhanced gas yield leading to a significant hydrogen production that was determined to be generated mainly from SS or water. Ni chips were used in two consecutive HTL experiments with no decrease in BC yields and ER. Collected results suggest that metallic powders can be interesting catalyst to improve yield and quality of BC in particular when FA acid was used as hydrogen vector.

Production of marine biofuels from hydrothermal liquefaction of sewage sludge. Preliminary techno-economic analysis and life-cycle GHG emissions assessment of Dutch case study

The aim of this paper is to evaluate the costs and GHG emissions of advanced biofuels production through hydrothermal liquefaction (HTL) of sewage sludge in The Netherlands targeting the marine fuels market. The process evaluated consists of a distributed configuration of regional HTL plants co-located with wastewater treatment plants, with centralized hydrotreating co-located with an existing refinery at the Port of Rotterdam. The process is simulated in ASPEN + based on published experimental data and the mass and energy balances are used as input for techno-economic and environmental evaluation. Lifecycle GHG emissions of the HTL and hydrotreating processes are estimated using consequential modelling principles and background data from the Ecoinvent database and compared with the business-as-usual scenario of sludge mono-incineration and fossil marine fuels production. The results indicate that the HTL + hydrotreating configuration has potential to deliver on-spec marine biofuels at a minimum fuel selling price between 410 and 1300 EUR/t, being at least 3 times more beneficial compared to the business-as-usual scenario from a GHG emissions perspective. Future work is recommended to optimize the size and location of the HTL plants in order to decrease capital costs and to address uncertainties regarding the sludge gate fee and the costs associated with the aqueous and solid by-products treatment. The results indicate the potential of such configuration in locations with relatively high population density and good transport infrastructure. This can be the case of port areas around the North Sea with access to offshore renewable electricity for hydrogen production, where drop-in marine biofuels are expected to play a role with the increasing share of renewables in the marine fuels mix.

Catalytic hydrothermal liquefaction of sewage sludge over alumina-based and attapulgite-based heterogeneous catalysts

In this study, Co/γ-Al2O3, Co-Mo/γ-Al2O3, Co/attapulgite (ATP) and Co-Mo/ATP catalysts were prepared and applied for hydrothermal liquefaction (HTL) of sewage sludge (SS) to produce bio-oil. The properties of the catalysts were characterized by various techniques (N2 physisorption, ICP, XRD and XPS). The mesoporous nature of synthesized catalysts was conformed. Beside, Co2+ and Mo6+ particles were responsible as the major active species for catalytic HTL reactions. The effect of carrier and metal type, loading of catalyst on product yields and composition of bio-oil was studied. The highest bio-oil yield of 31.36 wt% was achieved in the presence of 10 wt% catalyst loading. The main components of bio-oil were phenols, N-containing compounds and hydrocarbons, and the increased selectivity to hydrocarbons with Co-Mo/ATP was observed. Meanwhile, TGA results showed that Co-Mo/ATP increased the low boiling point fraction (<410 °C) of bio-oil to 81.19 wt%. Additionally, catalytic HTL of SS resulted in higher carbon recovery, increasing from 55.61 to 92.24 wt%. Finally, the potential reaction mechanism of the catalysts on HTL of SS was proposed. Overall, this study provides valuable information for bio-oil production from sewage plant and further HTL implementation with ATP based catalyst, improving energy efficiency and alleviating the energy shortage.

Catalytic hydrothermal liquefaction of sewage sludge: Effect of metal support heterogeneous catalysts on products distribution

Industrial wastes and natural mixed oxide materials were evaluated as inexpensive heterogeneous catalysts supports for catalytic hydrothermal liquefaction (HTL) of biomass. In this study, Palygorskite (ATP) clay, a hydrated magnesium aluminosilicate clay mineral supported metal catalysts (Co, Mo and Co–Mo/ATP) and Al2O3 supported (Co, Mo, Co–Mo/Al2O3) catalysts were examined for the sewage sludge (SS) hydrothermal liquefaction (HTL). The highest biocrude yield of 34.2 wt% was obtained with Co–Mo/ATP bimetal catalyst. Biocrude characterization revealed that significantly different families of molecules were formed in the presence and absence of catalysts, implying that the main role of the catalyst is to promote rates of thermal reactions, leading to biocrude production. The improved catalytic HTL performance with Co–Mo metal catalyst as attributed to the synergistic effects of both metal sites present on catalyst surfaces. The main components of bio-oil were phenols, N-containing compounds and hydrocarbons, and the increased selectivity to hydrocarbons with Co–Mo/ATP was observed. Maximum yield of aromatic hydrocarbon (20% observed in case of HTL with metal supported ATP catalyst. presence of Co–Mo metal catalysts promoted the deoxygenating by reduction of oxygen in the form of carbon dioxide or water. The distribution of biocrude samples indicates that most compounds in biocrude were light-weighted and had low boiling points. There were more light-weight compounds in biocrude by adding catalyst.

Effect of Transition Metals and Homogeneous Hydrogen Producers in the Hydrothermal Liquefaction of Sewage Sludge

Effect of Transition Metals and Homogeneous Hydrogen Producers in the Hydrothermal Liquefaction of Sewage Sludge

Towards the Hydrothermal Liquefaction of Sewage Sludge: The Fate of Nitrogen During Continuous Processing

Towards the Hydrothermal Liquefaction of Sewage Sludge: The Fate of Nitrogen During Continuous Processing

Hydrothermal liquefaction of sewage sludge – effect of four reagents on relevant parameters related to biocrude and PFAS

Catalytic hydrothermal liquefaction (HTL) can convert wet biomass into crude-like oil and has potential to degrade pollutants during the thermal depolymerization process. To avoid negative environmental impacts from per- and polyfluoroalkyl substances (PFAS) in sewage sludge after its final disposal, we comprehensively evaluated catalytic destruction of PFAS in sludge through HTL. The results showed that adding red mud to HTL was beneficial for the biocrude yield and significantly increased the carbon conversion efficiency and energy recovery of the hydrothermal process. HTL at 300 °C for 2 h degraded > 96% of spiked perfluorooctanoic acid (PFOA). The mass of spiked perfluorobutanesulfonic acid (PFBS) and perfluorooctanesulfonic acid (PFOS) and pre-existing perfluorohexanoic acid (PFHxA) and perfluoroheptanoic acid (PFHpA), however, increased in the HTL product streams. The remaining PFAS after HTL mainly stayed in the biocrude phase. Our findings indicated that HTL under the studied condition was not able to degrade all PFAS existed in sewage sludge. Although adding red mud could improve the HTL performance with respect to biocrude, this reagent did not improve the removal of perfluoroalkyl acids (PFAAs). As a matter of fact, the hydrothermal process amended with the selected reagents resulted in increased mass of PFBS, PFHxA, and PFHpA in the HTL products due to degradation of PFAA precursors. Thus, other treatment approaches for PFAS removal from sewage sludge need to be identified to prevent possible contamination of environments receiving PFAS-containing sludge.