Effects of hydrothermal treatment on organic compositions, structural properties, dewatering and biogas production of raw and digested sludge

Thermal hydrolysis-induced molecular transformations in sludge: Implications for photochemical reactivity and dissolved antibiotics photodissipation

Thermal hydrolysis is an effective technique for enhancing the solubilization of sewage sludge and improving the safety of biosolids for disposal or reuse. However, the release of various dissolved organic matter (DOM) at different TH temperatures, along with their properties that may influence intrinsic photochemical characteristics, remains poorly understood. This study investigates the temperature-dependent molecular evolution of sludge DOM (90-220°C) and its impact on antibiotic photodegradation. FT-ICR MS and ETC analysis were employed to explore the structural evolution, redox properties, and reactive oxygen species generation of DOM. The results reveal that 150°C represents a critical threshold for optimal photochemical activity. At this temperature, proteinaceous substances undergo decarboxylation and denitration, reducing polar functional groups and enhancing electron donor capacity (30.424 μmol e⁻ (mg C)⁻¹ ). Simultaneously, this molecular transformation facilitates the generation of excited triplet states (³DOM*) and significantly enhances the production efficiency of key reactive oxygen species (ROS), such as ¹O₂ and ·O2-. These properties significantly improved sulfamethoxazole photodegradation (kobs=0.2587 h-1). Below 150°C, limited DOM release and reduced ROS production hinder photochemical activity, whereas above 180°C, the increased aromaticity and molecular stability of humic-like substances inhibited photochemical reactivity due to light-shielding effects. This study offers a theoretical basis for optimizing sludge thermal hydrolysis conditions and links DOM molecular structures to the fate of dissolved antibiotics during photodegradation.

Physical conditioning methods for sludge deep dewatering: A critical review

Sludge is an inevitable waste product of sewage treatment with a high water content and large volume, it poses a significant threat of secondary pollution to both water and the atmosphere without proper disposal. In this regard, dewatering has emerged as an attractive method in sludge treatment, as it can reduce the sludge volume, enhance its transportability and calorific value, and even decrease the production of landfill leachate. In recent years, physical conditioning methods including non-chemical conditioners or energy input alone, have been extensively researched for their potential to enhance sludge dewatering efficiency, such as thermal treatment, freeze-thaw, microwave, ultrasonic, skeleton builders addition, and electro-dewatering, as well as combined methods. The main objective of this paper is to comprehensively evaluate the dewatering capacity of various physical conditioning methods, and identify key factors affecting sludge dewatering efficiency. In addition, future research anticipated directions and outlooks are proposed. This work is expected to provide valuable insights for developing efficient, eco-friendly, and low-energy consumption techniques for deep sludge dewatering.

Applying sludge hydrolysate as a carbon source for biological denitrification after composition optimization via red soil filtration

Sludge hydrolysate, the byproduct generated during sludge hydrothermal treatment (HT), is a potential carbon source for biological denitrification. However, the refractory organic matters and the nutrient substances are unfavorable to the nitrogen removal. In this study, effects of HT conditions on the hydrolysate properties, and the hydrolysate compositions optimization via red soil (RS) filtration were investigated. At HT temperature of 160-220 °C and reaction time of 1-4 h, the highest dissolution rate of organics from sludge to hydrolysate achieved 70.1 %, while the acetic acid dominated volatile fatty acids (VFAs) was no more than 5.0 % of the total organic matter content. The NH4+-N and dissolved organic nitrogen (DON) were the main nitrogen species in hydrolysate. When the hydrolysate was filtered by RS, the high molecular weight organic matters, DON, NH4+ and PO43- were effectively retained by RS, while VFAs and polysaccharide favorable for denitrification were kept in the filtrate. When providing same COD as the carbon source, the filtrate group (Fi-Group) introduced lower concentrations of TN and humic substances but higher content of VFAs. This resulted in TN removal rate (57.0 %) and denitrification efficiency (93.6 %) in Fi-Group higher than those in hydrolysate group (Hy-Group), 39.4 % and 83.7 %, respectively. It is noticeable that both Hy- and Fi- Groups up-regulated most of denitrification functional genes, and increased the richness and diversity of denitrifying bacteria. Also, more denitrifying bacteria genera appeared, and their relative abundance increased significantly from 3.31 % in Control to 21.15 % in Hy- Group and 31.31 % in Fi-Group. This indicates that the filtrate is a more suitable carbon source for denitrification than hydrolysate. Moreover, the pH rose from 4.6 ± 0.14 to 6.5 ± 0.05, and the organic carbon, TN, TP and cation exchange capacity (CEC) of RS increased as well after being filtered, implying that the trapped compounds may have the potential to improve soil quality. This study provides a new insight for hydrolysate application according to its composition characteristics, and helps make the most use of wasted sludge.

Improved solid/liquid separation performance of hydrochar from sludge via hydrothermal carbonization

A solid-liquid separation process is crucial for the utilization of hydrochar from biomasses through hydrothermal carbonization (HTC). This study aimed to evaluate the separation performance of hydrochar from coking sludge (CS) and municipal sludge (MS) via HTC and propose its mechanistic insights. The results indicated that the separation performance of hydrochar was enhanced, and relatively severe hydrothermal temperatures exerted a relatively stronger dewatering effect (53.25% and 77.05% reductions in the total water contents in CS and MS, respectively) and a superior separation efficiency (the specific resistance to filtration of products obtained from CS and MS decreased form 7.21 × 1011 and 2.46 × 1012 to 1.92 × 1010 m/kg and 2.14 × 1011 m/kg, respectively). Mechanism investigation involved analyzing the surface functional groups of hydrochar and changes in organic components. It was demonstrated that the improvement in dewatering effect predominantly resulted from the release of bound water due to the decomposition of carbohydrates and proteins. Specifically, the release of bound water depended on the decomposition of carbohydrates at 180 °C, while it relied on the decomposition of proteins in the 210-300 °C range. Compared with particle size, the hydrophilicity of the particle played a more important role in improving the separation efficiency. The HTC reaction eliminated hydrophilic groups, such as hydroxyl and carboxyl groups, and induced the formation of aromatic structures, thus reducing the hydrophilicity of hydrochar particles. Moreover, it was found that the lower heating value of hydrochar from CS and MS increased from 3.51 to 1.94 to 8.32 and 4.60 MJ/kg due to the improvement of the separation efficiency. These comprehensive findings provide valuable mechanistic insights into the solid-liquid separation process and controlling the dewaterability of hydrochar.

Optimal deployment of thermal hydrolysis and anaerobic digestion to maximize net energy output based on sewage sludge characteristics

Thermal hydrolysis (TH) is widely employed in combination with anaerobic digestion (AD) to efficiently treat primary sludge and waste-activated sludge in municipal wastewater treatment plants. In this study, four different scenarios-conventional AD (S1), TH-AD (S2), AD-TH-AD (S3), and characteristics-based AD-TH-AD (S4, primary AD only for primary sludge)-were evaluated to determine the optimal deployment of TH and AD for treating primary sludge and waste-activated sludge to maximize net energy output. The maximum net energy output of 4899 MJ/t-TSfed (per ton total solids of sludge fed) was achieved in S4 when assuming the recovered heat was only used for AD heating and surplus heat was wasted, and the net energy output of S4 was 70.8 % higher than that of S1 and 48.6 % higher than that of S2. This remarkable improvement was attributed to a reduction of > 15.2 % in refractory compounds, resulting in a 17 % increase in methane yield. Importantly, this study provides the first comparison of refractory compounds between inter-thermal hydrolysis (inter-TH) and pre-thermal hydrolysis (pre-TH) using a simulated A2O process. Overall, this study provides innovative insights and strategies for enhancing the TH and AD process performance based on the specific characteristics of sewage sludge derived from wastewater treatment plants.

Bidirectional Catalysis Disintegration and Mineral Polymerization via Endogenous Iron(III) from Iron-Rich Sludge in Synergy with Waste Incineration Fly Ash

To enhance the utilization of solid waste in cement kiln co-processing, this study analyzed the multifaceted synergy of pyrolysis and mineralization processes of iron-rich sludge (SS) and waste incineration fly ash (FA) at optimal blending ratios. Based on the physicochemical properties of SS and co-pyrolysis experiments, it was found that Fe acted as a positive catalyst in pyrolysis between 700 and 1000 °C, while the endogenous polymerization effect of Fe(III) mineral groups dominated above 800 °C. Additionally, the study investigated the solidification and migration of heavy metals and the transformation of harmful elements (S, Cl, and P). Results indicated that the best mixture ratios for SS and FA were 6:4 and 9:1, respectively, and synergistic pyrolysis and mineral co-curing effects were observed in the pyrolysis temperature range of 50-1000 °C. The synergy between SS and FA allowed for the decomposition and solidification of harmful organic components and heavy metals, reducing environmental risks. Furthermore, in actual production, by mixing 100 tons of SS and FA with Portland cement with a daily output of 2500 tons, the compressive strength during early hydration stages can reach 34.52 MPa on the third day, exceeding the highest performance of Portland cement (62.5R) strength index specified in the standard.

Release characteristics of hydrochar-derived dissolved organic matter: Effects of hydrothermal temperature and environmental conditions

Much research has been done on the preparation and application of hydrochars, but research on the release characteristics of hydrochar-derived dissolved organic matter (HDOM) is very limited; clarifying the release characteristics of HDOM is important for understanding and adjusting the environmental behaviour of hydrochar. Herein, the potential release of HDOM from rice straw-derived hydrochars prepared at different hydrothermal temperatures was investigated under various potential environmental conditions for the first time. The total release quantity and humification degree of HDOM decreased with increasing hydrothermal temperature. The critical dividing line for various hydrothermal reactions, decomposition and polymerization, was in the range of 240 °C-260 °C. Alkaline condition increased the HDOM release amount (up to 299 mg g^-1), molecular weight (as high as 423 Da) and molecular diversity (8857 compounds) from rice straw-derived hydrochars. The unique substances of HDOM released under alkaline condition were mainly distributed in lipids-like substances, CRAM/lignins-like substances, aromatic structures, and tannins-like substances, while few unique substances were found under acidic condition. Additionally, CRAM/lignins-like substances were the most abundant in all HDOM samples, reaching 82%, which were relatively stable and could achieve carbon sequestration in different environments. The findings provided a new insight on understanding the potential environment behaviors of hydrochar.

Combined hydrothermal and biological treatments for valorization of fruit and vegetable waste into liquid organic fertilizer

This study investigated the effects of hydrothermal treatment, biological treatment and their combination on nutrients recovery from fruit and vegetable waste (FVW) and evaluated the feasibility of fruit and vegetable waste juice (FVWJ) from the combined treatment as liquid organic fertilizer. In this study, following conditions were determined suitable for FVW treatment: the temperature of 165 °C and retention time of 45 min for hydrothermal treatment, 20 h for biological treatment, and Weissella, as the dominant microbial genus present in FVW, was suggested as inoculum for biological treatment. In the combined treatment, based on the above conditions of hydrothermal and biological treatments, the yield of FVWJ was 93.03 g out of 100 g FVW, and concentrations of organic matter (1.45%, w/w), primary nutrients (0.51%, w/w), and toxic components in the FVWJ complied with the requirements for use concentration in both Chinese and European standards for liquid organic fertilizer. The economic analysis showed the net saving of 13.60 USD per ton FVW, indicating that it is an economical approach to valorize fruit and vegetable waste into liquid organic fertilizer through the combined treatment.

What are the determinants of wastewater discharge reduction in China? Decomposition analysis by LMDI

Wastewater discharge reduction (WDR) is a key breakthrough point for China's environmental protection. Based on China's 30 provincial data from 2011 to 2017, this paper applied the logarithmic mean Divisia index (LMDI) method to clarify the determinants of WDR at national, regional, and provincial levels. Except for wastewater discharge factor, economic development, and total population, four innovative factors, total water application intensity, water environment cost, water treatment industry development level, and drainage infrastructure investment scale were first proposed in this study. The results indicated that from 2011 to 2017, at the national level, total water application intensity and water treatment industry development level were dominant contributors to WDR, while other factors all inhibited WDR. At the regional level, the results of wastewater discharge factor, economic development, and water environment cost were similar to the national level. The drainage infrastructure investment scale had a positive effect on WDR in Northeast and South China while having a negative effect on other regions. And except for Northeast China, the water treatment industry development level promoted WRD, while the total population inhibited WDR. Finally, the determinants of WDR at the provincial level were investigated. On this basis, targeted corresponding policies were provided in this paper.

Promoting the Circular Economy on an Island: Anaerobic Co-Digestion of Local Organic Substrates as a Possible Renewable Energy Source

The local waste co-digestion is an interesting option to tackle in reduced and isolated areas like the islands. The islands have limited territory and scarce fuel production. Moreover, organic waste can create serious environmental problems in soil, water and air. Anaerobic co-digestion (AcoD) is a technology fulfilling the concept of waste-to-energy (WtE) based on local resources. The valorisation of organic waste through AcoD on an island would prevent environmental impacts, while being a source of renewable energy. In this study, cow manure (outdoor and indoor), pig slurry, bird manure, kitchen waste, sewage sludge and oily lacteous waste produced on Island Terceira (Portugal) were tested in mesophilic -35 °C- Biochemical Methane Potential (BMP) co-digestion assays. The goals were to analyse the recalcitrant and high potential produced waste and to estimate the energetic supply source on the island. The cow manure and pig slurry were used as inocula and specific methanogenic activities (SMAs) were carried out. The results showed that both substrates have a significant methanogenic activity-SMA 0.11 g-COD/(g-VSS.d) and 0.085 g-COD/(g-VSS.d), respectively. All the studied combinations were feasible in AcoD, showing TS removals in the range of 19-37%; COD removals in the range 67-78% and specific methane yields from 0.14 to 0.22 L/gCOD removed, but some differences were found. The modified Gompertz model fitted the AcoD assays (R² 0.982-0.998). The maximum biogas production rate, Rmax. was highest in the AcoD of Cow+Pig+Oily and in the Cow+Pig+Sludge with 0.017 and 0.014 L/g-VS_added.day, respectively, and the lowest in Cow+Pig+Bird with 0.010 L/g-VS_added. In our AcoD studies, the bird manure limited the performance of the process, since it was recalcitrant to anaerobic degradation. On the other hand, the oily lacteous waste showed a great potential in the anaerobic digestion. The estimated biogas production, from the best-studied condition, could cover the 11.4% of the energy supply of the inhabitants. These preliminary results would prevent the environmental impact of organic waste on the island and promote the use of local waste in a circular economy scenario.