Alkali-solubilized organic matter from sludge and its degradability in the anaerobic process

Neglected sludge solid phase in sludge pretreatment process: Physicochemical characterization and mechanism study of its role in anaerobic degradation

The low anaerobic digestion efficiency of the solid phase separated from pre-treated sludge indicates the need to explore other suitable resource utilization pathways for sludge solid phase. However, there is a lack of comprehensive and in-depth research on the physicochemical properties of sludge solid phase. This study comprehensively analyzes the characteristics of sludge solid phase and elucidates the mechanism of sludge solid phase in the anaerobic degradation of toxic wastewater. The results show that the surface free energy of sludge solid phase after different pre-treatments is mainly contributed by Lewis acid-base hydration free energy. The distribution of proteins on the surface of sludge solid phase plays a major role in the adhesion between sludge solid particles. Metal ions in the sludge solid phase are mainly present in the exchange state, followed by the carbonate state and the organics-bound state. The sludge solid phase obtained by sludge pH 12 + 150 °C treatment has the highest conductivity (1.36 mS/m) and capacitance (25.51 μF/g), mainly due to the presence of melanoidins in the sludge solid phase, which has similar semiquinone radicals to humic acids, thus increasing conductivity. The addition of sludge solid phase promotes an increase in cumulative methane production and rate of methane production. The sludge solid phase might play a role of an auxiliary carbon source acting as an adsorbent to buffer against toxicity inhibition and facilitate electron transfer. This study reveals the characteristics of sludge solid phase and its role in anaerobic digestion, providing theoretical guidance for finding suitable resource utilization pathways for sludge solid phase.

Characterizing properties and environmental behaviors of organic matter in sludge using liquid chromatography organic carbon detection and organic nitrogen detection: A mini-review

The presence of organic matter in sludge plays a significant role in sludge dewatering, anaerobic sludge digestion, resource (i.e., protein) recovery and pollutants removal (i.e., heavy metals) from sludge, as well as post-application of sludge liquid and solid digestate. This study summarized the current knowledge on using liquid chromatography organic carbon detection and organic nitrogen detection (LC-OCD-OND) for characterization and quantification of organic matter in sludge samples related with sludge treatment processes by fractionating organic matter into biopolymers, building blocks, humic substances, low molecular weight (LMW) acids, low LMW neutrals, and inorganic colloids. In addition, the fate, interaction, removal, and degradation of these fractions in different sludge treatment processes were summarized. A standardized extraction procedure for organic components in different extracellular polymeric substances (EPS) layers prior to the LC-OCD-OND analysis is highly recommended for future studies. The analysis of humic substances using the LC-OCD-OND analysis in sludge samples should be carefully conducted. In conclusion, this study not only provides a theoretical foundation and technical guidance for future experiments and practices in characterizing sludge organic matter using LC-OCD-OND, but also serves as a valuable resource for consulting engineers and other professionals involved in sludge treatment.

Anaerobic digestion of process water from hydrothermal treatment processes: a review of inhibitors and detoxification approaches

Integrating hydrothermal treatment processes and anaerobic digestion (AD) is promising for maximizing resource recovery from biomass and organic waste. The process water generated during hydrothermal treatment contains high concentrations of organic matter, which can be converted into biogas using AD. However, process water also contains various compounds that inhibit the AD process. Fingerprinting these inhibitors and identifying suitable mitigation strategies and detoxification methods is necessary to optimize the integration of these two technologies. By examining the existing literature, we were able to: (1) compare the methane yields and organics removal efficiency during AD of various hydrothermal treatment process water; (2) catalog the main AD inhibitors found in hydrothermal treatment process water; (3) identify recalcitrant components limiting AD performance; and (4) evaluate approaches to detoxify specific inhibitors and degrade recalcitrant components. Common inhibitors in process water are organic acids (at high concentrations), total ammonia nitrogen (TAN), oxygenated organics, and N-heterocyclic compounds. Feedstock composition is the primary determinant of organic acid and TAN formation (carbohydrates-rich and protein-rich feedstocks, respectively). In contrast, processing conditions (e.g., temperature, pressure, reaction duration) influence the formation extent of oxygenated organics and N-heterocyclic compounds. Struvite precipitation and zeolite adsorption are the most widely used approaches to eliminate TAN inhibition. In contrast, powdered and granular activated carbon and ozonation are the preferred methods to remove toxic substances before AD treatment. Currently, ozonation is the most effective approach to reduce the toxicity and recalcitrance of N and O-heterocyclic compounds during AD. Microaeration methods, which disrupt the AD microbiome less than ozone, might be more practical for nitrifying TAN and degrading recalcitrant compounds, but further research in this area is necessary.

Structure and composition of dissolved organic matters in sludge by ultrasonic treatment

The leaching of dissolved organic matter (DOM) from the sludge into the liquid phase is induced by ultrasound. However, there is limited investigation into the structure and molecular composition of sludge DOM in this process. The molecular structure and composition of sludge DOM in ultrasonic treatment were comprehensively elucidated in this study. The sludge dissolved organic carbon (DOC) and three-dimensional fluorescence spectroscopy (3D-EEM) image had most significant change at 15-min ultrasonic time and 1.2 W/mL ultrasonic density, respectively. Gas Chromatography-Mass Spectrometry (GC-MS) analysis indicated that ultrasonic treatment of sludge reduced the macromolecules to small molecules in DOM. Then, electrospray ionization Fourier-transform ion cyclotron resonance mass spectrometry (ESI FT-ICR-MS) analysis revealed that lignin, tannins, and carbohydrates were the main components of sludge DOMs after ultrasound treatment. analysis revealed that lignin, tannins, and carbohydrates were the main components of sludge DOMs after ultrasound treatment. Furthermore, through the Van Krevelen analysis, the major components were CHO (48.50%) and CHOS (23.20%) in the DOM of ultrasonicated sludge. This research provides the basis for the practical application of ultrasonic treatment of sludge and provides basic information for DOM components.

Near-infrared reflectance spectroscopy for rapid prediction of biochemical methane potential of wastewater wasted sludge

The information of biochemical methane potential (BMP) of wasted sludge is essential to ensure the stable operation of sludge management processes. However, conventional anaerobic digestion (AD) approach for BMP test is time-consuming and labour-intensive. Currently, the technique of Near Infrared Spectroscopy (NIRS) is gaining prominence in the biogas production within AD process. Previous studies mostly focused on predicting BMP values for fibrous plant biomass and solid waste, with only a limited number of studies attempting to apply NIRS to obtain BMP values across a wide array of wasted sludge types. To obtain BMP values for this diverse range of wasted sludge efficiently and accurately, it is imperative to develop precise models for assessing BMP values using NIRS. In this study, the possibility of using NIRS to predict the BMP values of wasted sludge was evaluated. A total of 70 sludge samples from different sources were investigated to develop a BMP-prediction model by correlating the measured BMP values with the obtained NIR spectra. As a result, a reliable and successful BMP-prediction model was established with the determination coefficient of 0.90, residual prediction deviation of 3.50 and low root mean square error of prediction of 36.8 mL CH4/g VS. This BMP-prediction model is satisfactory for predicting BMP values of various types of sludge. It could provide support for plant operators to make decisions rapidly, thereby improving the process efficiency and optimizing sludge management procedures.

Effect and mechanism of perfluorooctanoic acid (PFOA) on anaerobic digestion sludge dewaterability

Perfluorooctanoic acid (PFOA) as nonbiodegradable organic pollutant, its presence and risks in wastewater treatment system has aroused wide concern. This study investigated the effect and underlying mechanism of PFOA on anaerobic digestion sludge (ADS) dewaterability. Long-term exposure experiments were set up to investigate the effect with various concentration of PFOA dosed. Experimental results suggested that the existence of high concentration PFOA (over 1000 μg/L) could deteriorate ADS dewaterability. The long-term exposure to 100,000 μg/L PFOA of ADS increased specific resistance filtration (SRF) by 81.57%. It was found that PFOA promoted the release of extracellular polymeric substances (EPS), which was strongly associated with sludge dewaterability. The fluorescence analysis revealed that the high PFOA concentration could significantly improve the percentage of protein-like substances and soluble microbial by-product-like content, and then further deteriorated the dewaterability. The FTIR results showed that long-term exposure of PFOA caused loose protein structure in sludge EPS, which led to loose sludge floc structure. The loose sludge floc structure aggravated the deterioration of sludge dewaterability. The solids-water distribution coefficient (K_d) decreased with the increase of initial PFOA concentration. Moreover, PFOA significantly affected microbial community structure. Metabolic function prediction results showed significant decrease of fermentation function exposed to PFOA. This study revealed that the PFOA with high concentration could deteriorated sludge dewaterability, which should be highly concerned.

Stress response and signalling of a low-temperature bioaugmentation system in decentralized wastewater treatment: Degradation characteristics, community structure, and bioaugmented mechanisms

Low temperatures present challenges for stable wastewater treatment operations in cold regions. Low-temperature effective microorganisms (LTEM) were added as a bioaugmentation strategy at a decentralized treatment facility to improve performance. The effects of a low-temperature bioaugmentation system (LTBS) with LTEM at low temperatures (4 °C) on organic pollutant performance, microbial community changes, and the metabolic pathways of functional genes and functional enzymes were studied. To explore the bioaugmentation mechanism of LTBS based on stress response and signalling. The results showed that the start-up time of the LTBS (S2) with LTEM was shorter (8 days) and that it removed COD and NH₄⁺-N at higher rates (87 % and 72 %, respectively) at 4 °C. LTEM effectively degraded complex macromolecular organics into small molecular organics, and decomposing sludge flocs and the changing the extracellular polymeric substances (EPS) structure removed more organics and nitrogen. LTEM and local microbial communities (nitrifying and denitrifying bacteria) improved the ability of organic matter degradation and denitrification of the LTBS and formed a core microbial community dominated by LTEM (Bacillus and Pseudomonas). Finally, based on the functional enzymes and metabolic pathways of the LTBS, a low-temperature strengthening mechanism consisting of 6 cold stress responses and signal pathways under low temperatures was formed. This study demonstrated that the LTEM-dominated LTBS could provide an engineering alternative for future decentralized wastewater treatment in cold regions.

Accelerated stabilization of high solid sludge by thermal hydrolysis pretreatment in autothermal thermophilic aerobic digestion (ATAD) process

Autothermal thermophilic aerobic digestion (ATAD) is a rapid biological treatment technology for sludge stabilization. To improve digestion efficiency and shorten stabilization time, thermal hydrolysis pretreatment was employed before ATAD of high solid sludge. The results showed that accelerated stabilization of high solid sludge (total solid = 10.1%) was achieved by thermal hydrolysis pretreatment with volatile solid removal efficiency of 40.3% after 8 days of ATAD, 11 days earlier than unpretreated sludge. The enhanced release and hydrolysis of intracellular organics resulted in a solubilization degree of 45.3%. The reduced sludge viscosity and improved fluidity after thermal hydrolysis facilitated mixing, aeration and organics degradation during ATAD. Excitation emission matrix analysis indicated that the fluorescence intensity of soluble microbial byproduct and tyrosine-like protein increased markedly after thermal hydrolysis and decreased after ATAD. The proportion of high molecular weight (MW > 10 kDa) substances in the supernatant increased significantly after thermal hydrolysis, while the low MW (MW < 1 kDa) substances decreased after ATAD. The significant difference in microbial composition between the pretreatment and control groups elucidated the accelerated sludge stabilization under thermal hydrolysis. This work provides an efficient and practical strategy to achieve rapid stabilization of high solid sludge.

Accelerating anammox nitrogen removal in low intensity ultrasound-assisted ASBBR: Performance optimization, EPS characterization and microbial community analysis

Efficient enrichment of slow-growing anammox species is essential for rapid start-up and stable operation of high-rate anammox reactors. Herein, a low intensity ultrasound (LIU) was introduced into anaerobic sequencing batch biofilm reactors (ASBBRs) to enhance anammox nitrogen removal from nitrogen-rich wastewater. Operation results demonstrated that the maximum total nitrogen (TN) removal efficiency of 91.5% were achieved under the optimal ultrasonic parameters (32.7 °C water temperature, 0.18 W/cm² ultrasonic intensity and 25.7 min ultrasonication time). Moreover, significant increases of extracellular polymeric substances (EPS) components and contents were observed via the ultrasonication stimulation. A close correlation between nitrogen removal and shifts in transformation and intensity of spectrum peaks was also verified by three-dimensional excitation-emission matrix spectroscopy (3D-EEM) analysis. High-throughput sequencing revealed that the relative abundance of Candidatus Kuenenia as the key anammox consortium significantly increased after applying optimal ultrasonication condition. Furthermore, enhancement mechanisms and future prospect of the LIU-assisted anammox process was elucidated and discussed. This research provides a viable and promising acceleration strategy for anammox-based process in practice.

"Humic substances" measurement in sludge dissolved organic matter: A critical assessment of current methods

The role of humic substances (HS) during sludge treatment has been the focus in recent years. Quantification of HS in sludge dissolved organic matter (DOM) and the chemical and structural characterization of HS data are the prerequisite for understanding their role during different sludge treatment processes. Currently, a number of published articles inadequately acknowledge fundamental principles of analysis methods both in terms of experimental approach and data analysis. Therefore, a more comprehensive and detailed description of the experimental methods and the data analysis are needed. In this study, the current used methods for HS quantification in DOM of sludge had been tested for different calibration and sludge DOM samples. The results indicated that the current methods showed overestimated and contradictory results for HS quantification in sludge DOM. To be specific, using the modified Lowry method, different values were obtained depending on the humic acids used for calibration, and false negative results were observed for some sludge samples. By using the relative amount of HS (based on dissolved organic carbon (DOC)) to total sludge DOM (based on DOC), variations among the results of different analysis methods for the same sample were high. According to the calculated Bray-Curtis dissimilarity indexes, the results for HS quantification obtained by three-dimensional excitation emission matrix (3D-EEM), either with spectra analysis methods by peak picking, fluorescence region integration (both region volume and area integration), or PARAllel FACtor analysis showed higher degrees of dissimilarity to those quantified by size exclusion liquid chromatography or XAD-8 method. The selection of fluorescence regions for HS seemed to be the determining factor for overestimation obtained by the 3D-EEM technique. In future work, strategies, like a consistent terminology of HS, the use of an internal standard sample, and the related standardized operation for HS quantification in sludge DOM need to be established.

Quantification of DOM effects on tetracyclines transport during struvite recovery from swine wastewater

Struvite (MgNH₄PO₄·6H₂O) recovered from livestock wastewater may impose a pharmacological threat to the environment, due to the extensive existence of antibiotics in the wastewater. In this study, tetracyclines (TCs) were selected as the typical antibiotics, and the individual processes of dissolved organic matters (DOM) evolution and their effects on TCs migration in struvite recovery from swine wastewater were discriminated and quantified. Results revealed that TCs transport was contributed by the adsorption of pure struvite crystals, struvite adsorbing DOM-TCs complex and DOM aggregation, which occupied 2.29-6.53%, 23.53-34.66%, and 59.09-74.19% of the total TCs migration amounts, respectively. A tangential flow filtration system was employed to divide DOM into five fractional parts on the basis of molecular weight cut-offs. Experimental results indicated that under alkaline conditions of struvite crystallization, DOMs with larger molecular weights, hydrolyzed to DOMs with smaller molecular weights, which consequently promoted TCs re-distribution in DOMs from higher molecular weights to those with lower molecular weights. Furthermore, a distribution model was developed to characterize TCs transport in struvite recovery by describing TCs distribution among various phases, including struvite adsorption, DOM-TCs complexing, DOM aggregation, and free state in the solution, respectively. These outcomes provided new understanding on DOM evolution and effects on antibiotics transport in phosphate recovery from wastewater.

Conventional bioretention column with Fe-hydrochar for stormwater treatment: Nitrogen removal, nitrogen behaviour and microbial community analysis

An FeCl3-modified rice husk hydrochar ('Fe-hydrochar') was used as the filler in a conventional bioretention column to remove nitrogen from synthetic stormwater. When the ammonia nitrogen (NH4-N) and nitrate nitrogen (NO3-N) concentrations of the influent were both 20 mg/L, the average removal rates of NH4-N and total nitrogen (TN) were approximately 97% and 50%, respectively. Nitrogen was mainly removed by microbial nitrification and denitrification, with 25% of NH4-N being adsorbed by the Fe-hydrochar. The remaining NH4-N was converted into NO3-N by nitrification in the upper layer, and NO3-N was mainly converted to nitrogen gas (N2) by denitrification in the lower layer. The organic matter released by the Fe-hydrochar was degraded and used as the carbon source for denitrification. The dominant bacteria were Pseudomonas, Rhizobium, and Flavobacterium at the genus level. Pseudomonas and Rhizobium were responsible for heterotrophic nitrification-aerobic denitrification, while Flavobacterium was related to the degradation of organic matter.

Underestimated humic acids release and influence on anaerobic digestion during sludge thermal hydrolysis

Humic-like acids (HAs) are abundant in sewage sludge but mainly bonded with solids. Thus, their influences are often neglected in conventional digestion processes. Currently thermal hydrolysis pretreatment (THP) has been widely adopted in sludge anaerobic digestion (AD) to enhance hydrolysis of complex matters and further to improve methane production. However, the impacts of enhanced release of HAs and the mechanisms involved are not well understood and need to be further investigated because the substantial amounts of HAs present in AD could severely threaten the sludge AD processes. Results in the present study indicated that the concentration of soluble HAs in sludge was elevated by 90 times due to the THP, from 8 mg/L in raw sludge to 727 mg/L in the pretreated sludge hydrolyzed at 180 °C. Moreover, the structural characteristics of soluble HAs, including aromatic condensation degree, elemental composition and functional group, also showed substantial differences with the increased temperature of the THP. Furthermore, the release of HAs presented significant influences on sludge digestion. Acidification rate was inhibited by over 50% with 0.4 g/L of HAs, whereas methanogenesis was improved by nearly 200% with 0.8 g/L HAs and inhibited about 50% with 2.0 g/L. The activities of proteinase and co-enzyme F₄₂₀ were decreased by 20% and increased by 19%, respectively, under HAs stress at 0.6 g/L for 5 days. Moreover, molecular structural changes of soluble HAs also contributed to the influences. Especially, the E4/E6 value representing the degree of HAs aromatic condensation and C/N ratio of soluble HAs were closely correlated with their inhibition degree to sludge hydrolysis. The findings of this study demonstrate that the influences of HAs are evident and also vary to the different steps of anaerobic digestion processes, which shall not be negligible during the sludge digestion that is with THP. Due to the rate-limiting step was methanogenesis in the AD process of pretreated sludge by thermal hydrolysis, HAs concentration was recommended at low level, for example around 1.0 g/L, to accelerate or not limit methanogenesis.

A novel strategy for enhancing the partial denitrification to treat domestic wastewater by feeding sludge fermentation liquid

Partial-denitrification (PD; NO₃^-→NO₂^-) has recently been proposed to be a feasible choice of NO₂^--N supply for anammox bacteria. In this study, an aerobic/anoxic process for treating domestic wastewater was operated for 176 days to evaluate the feasibility of using sludge fermentation liquid for partial denitrification of the wastewater. Results show that, with the ratio of C/N (COD/ NO₃^--N) increased at anoxic stage, the average NO₂^--N concentration in the effluent and nitrate-to-nitrite transformation ratio (NTR) at anoxic stage showed relative growth. High-throughput sequencing analysis demonstrated that the enhancement of PD can be explained by the increases of Thauera, Paracoccus and Enterobacteriaceae. Moreover, Candidatus_Brocadia (0.13%) was detected as the predominant anammox bacteria. Ex-situ isotopic tracing technique analysis assessed that the ratio of anammox role (ra%) was 7.29%. This study has a great potential for being coupled with the anammox bacteria for advanced nitrogen removal.

Insights into thermal hydrolyzed sludge liquor - Identification of plant-growth-promoting compounds

This study proposes a new path to utilize thermal hydrolyzed sludge (TH sludge) as fertilizer given high value chemical compounds that can promote plant growth were identified in the liquid fraction of TH sludge (TH liquor). Together with micro- and macro-nutrients released/synthesized during thermal hydrolysis, the feasibility of using TH liquor as organic fertilizer was evaluated. Besides high contents of N, P and K, total free amino acids (FAAs) and plant-growth-promoting FAAs (including glutamic acid, leucine and cystine) also presented in high concentration (4.98-6.48 and 1.12-2.73 g/100 g) in the TH liquor. For the first time, phytohormone compound, indole-3-acetic acid, was observed and the content was the highest in TH liquor with 165 °C treatment (165 °C TH liquor). Meantime, 165 °C TH liquor did not have negative impact on the growth of soil microbes, and this product, instead, demonstrated stimulating effect on the plant growth. These results suggest that 165 °C TH liquor has a great potential to be an organic fertilizer. The remaining solids of TH sludge could be converted to valuable biochar. The holistic approach of using TH liquor as organic fertilizer and producing biochar could realize nearly zero-waste discharge in sludge management.

Variation of dissolved organic matter during excess sludge reduction in microbubble ozonation system

Sewage sludge is the major by-product of wastewater treatment plants, and about 30% readily biodegradable organic matters might be reused through the mass reduction process, which could be also reduced the disposal fee. In this study, the microbubble ozonation (MB-O₃) was employed to improve the oxidation efficiency for sludge solubilization. At 160 mgO₃/gSS, the maximum mixed liquor suspended solids (MLSS) reduction ratio was 37.5% and the protein and polysaccharide contents increased to 31.6 and 138.6 mg/L, respectively. It was proposed that aromatic protein and soluble microbial in sludge were oxidized preferentially by MB-O₃, and the dissolved organic matter (DOM) fractions (mainly humic-acid-like substances) exhibited low degradability according to the variations of fluorescence excitation-emission spectrum coupled with fluorescence regional integration. MB-O₃ could enhance the settleability, but deteriorate sludge dewaterability at low dosage (< 160 mgO₃/gSS) due to a reduction in particle size from 61.7 to 47.5 μm. MB-O₃ has a good performance on the mass reduction of sludge through the improvement of the radical generated.

Changes in the characteristics of dissolved organic matter during sludge treatment: A critical review

Dissolved organic matter (DOM) of sludge is a heterogeneous mixture of high to low molecular weight organic substances which is including proteinaceous compounds, carbohydrates, humic substances, lipids, lignins, organic acids, organic micropollutants and other biological derived substances generated during wastewater treatment. This paper reviews definition, composition, quantification, and transformation of DOM during different sludge treatments, and the complex interplay of DOM with microbial communities. In anaerobic digestion, anaerobic digestion-refractory organic matter, particularly compounds showing polycyclic steroid-like, alkane and aromatic structures can be generated after pretreatment. During dewatering, the DOM fraction of low molecular weight proteins (< 20,000 Dalton) is the key parameter deteriorating sludge dewaterability. During composting, decomposition and polymerization of DOM occur, followed by the formation of humic substances. During landfill treatment, the composition of DOM, particularly humic substances, are related with leachate quality. Finally, suggestions are proposed for a better understanding of the transformation and degradation of DOM during sludge treatment. Future work in sludge studies needs the establishment and implementation of definitions for sample handling and the standardization of DOM methods for analysis, including sample preparation and fractionation, and data integration. A more detailed knowledge of DOM in sludge facilitates the operation and optimization of sludge treatment technologies.

Development of a denitrification system using primary sludge as solid carbon source - Potential to couple with anammox process

Heterotrophic denitrification is a robust and reliable process for nitrogen removal from wastewater. However, wastewater often faces the issue of lacking carbon source. In this study, the feasibility of using primary sludge, a by-product of wastewater treatment plants, to support denitrification of high-strength nitrite wastewater was investigated. Results suggest the desired performance can be achieved with the influent nitrite concentration of 400 to 1200 mg N/L, and the optimal primary sludge dosage for the complete nitrite removal was 3.6 g VSS/g N. Ammonium removal was also detected along with nitrite removal. Microbial analysis reveals various types of denitrifying bacteria and a large number of macromolecular organics degrading bacteria existed in the microbial community. Notably, anammox bacteria, Candidatus Brocadia, was also identified with an abundance of 0.1%. The slow kinetics of carbon source release from primary sludge was likely the reason for the existence of anammox process. This study developed a promising nitrogen removal process using an alternative carbon source for denitrification, and it shows great potential to couple denitrification with anammox to reduce ammonium residue.

Synergistic Partial-Denitrification, Anammox, and in-situ Fermentation (SPDAF) Process for Advanced Nitrogen Removal from Domestic and Nitrate-Containing Wastewater

This study presents a new method for energy-efficient wastewater treatment that synergizes the partial-denitrification, anammox, and in-situ fermentation (SPDAF) processes in an up-flow reactor. Nitrate-containing wastewater and actual domestic sewage were fed into this SPDAF system, which was operated for 180 days without the addition of external carbon sources and aeration. The total inorganic nitrogen (TIN) removal efficiency reached 93.1% with a low C/N ratio of 1.6, a NO₃^--N/NH₄⁺-N ratio of 1.13 and a TIN concentration of 92.5 mg N/L. The contribution of anammox to nitrogen removal accounted for 95.6%. Batch tests demonstrated that the partial-denitrification process was able to use organics from either the influent or those produced by fermentation, thus providing nitrite for anammox. Significantly, fermentation played a key role in using the slowly biodegradable organics and provided adequate electron donor for partial-denitrification. Metagenomic sequencing analysis showed that the genera related to partial-denitrification, anammox, and fermentation bacteria were coexisted in this SPDAF system. The key functional genes of anammox bacteria (Hzs, 3986 hits; Hdh, 2804 hits) were highly detected in this study. The abundances of cytoplasmic nitrate reductase (58 706 hits) and periplasmic nitrate reductase (70 540 hits) were much higher than copper nitrite reductase (16 436 hits) and cytochrome cd₁ nitrite reductase (14 264 hits), potentially contributing to the occurrence of partial-denitrification. Moreover, different abundances of genes involved in fermentation metabolism suggested that fermentation likely generated easily biodegradable organics for partial-denitrification.

Liquid and solids separation for target resource recovery from thermal hydrolyzed sludge

This study proposed an integrated process for biogas generation and biochar production from thermal hydrolysis pretreated sludge (THP sludge). In this study, the liquid and solids fractions of THP sludge were separately processed for the first time. The liquid fraction of THP sludge (THP-L) reached the biodegradability (262.6 ± 5.1 mL CH₄/g tCOD_feed) on the 15th day during anaerobic treatment, while the solids fraction of THP sludge (THP-S) only contributed 31.0% to the total methane production and required more than 30 days digestion time. We investigated the feasibility to convert THP-S into biochar to realize the higher value of the solids fraction. The results prove the produced biochar can be used as slow-release fertilizer. Preliminary energy analysis was performed to evaluate the energy efficiency of the integrated approach, namely, methane generation from THP-L coupled with biochar production from THP-S. The process realized energy surplus of 0.81 MWh/tonne dry sludge. In addition, THP-L digested sludge showed better dewaterability, lower yield stress and reduced viscosity during digestion. The proposed new sludge treatment process therefore has lower operating cost and higher value returns.

Development of an alkaline/acid pre-treatment and anaerobic digestion (APAD) process for methane generation from waste activated sludge

Anaerobic sludge digesters are biorefineries for energy recovery from waste activated sludge (WAS) via methane production, in which disintegration of floc structure and microbial cells is a major challenge in releasing extracellular polymeric substances (EPS) and cytoplasmic macromolecules for subsequent hydrolysis and fermentation. Here, we developed a new process combining alkaline/acid pre-treatments and anaerobic digestion (APAD) to improve sludge digestion. Both alkaline and acid pre-treatments effectively disintegrated the floc structure and microbial cells to release sludge organic contents. Under the optimized alkaline/acid pre-treatment condition, carbon removal achieved 52.8 ± 1.7% in APAD digesters, in contrast to 30.9 ± 2.2% and 42.4 ± 1.6% in anaerobic digesters fed with fresh WAS (control-AD) and thermal pre-treated sludge (thermal-AD), respectively. Both alkaline/acid and thermal pre-treatments largely shifted sludge community composition and function, but in distinct ways, possibly due to their different sludge constitutes (i.e., dissolved organic matter and NaCl). Correspondingly, microbial network analysis identified three modules with varied keystone taxa and interaction patterns in the three digesters. Life cycle assessment showed the comparable environmental impacts of APAD, thermal-AD and control-AD. In all, this study provided a new solution for WAS treatment and insights into impact of sludge pre-treatments on sludge digestion microbiome.

Molecular composition of hydrothermal liquefaction wastewater from sewage sludge and its transformation during anaerobic digestion

Anaerobic digestion (AD) has shown potential to convert hydrothermal liquefaction wastewater (HTLWW) into biogas in previous studies. However, the identification of refractory components and further insights into the molecular transformations of organics in HTLWW are essential for developing more efficient AD processes. In this study, two HTLWWs were obtained from the temperature-derived hydrothermal liquefaction of sewage sludge at 170 ℃ and 320 ℃. Their molecular compositions, as well as their modifications in the subsequent AD process, were characterized using a suite of advanced molecular tools. The dissolved organic matter (DOM) in the high temperature-derived HTLWW was lower in molecular weight, less saturated, less oxidized, and enhanced in nitrogenous substances. During the AD process, most of the volatile compounds and low molecular weight (LMW) neutrals were removed, while biopolymers were the most refractory. Carboxylic-rich alicyclic molecules (CRAM), particularly those containing 3 to 5 N for low temperature-derived DOM and 1 to 3 N for high temperature-derived DOM, were resistant to anaerobic biodegradation. Meanwhile, compounds with fewer nitrogens and more carboxyl groups were preferentially produced. This molecular characterization of HTLWW-derived DOM and examination of its transformation during AD will contribute to the development of efficient methods for HTLWW treatment in the future.

Novel insights into integrated fermentation and nitrogen removal by free nitrous acid (FNA) serving as treatment method

Free nitrous acid (FNA) has only been studied as the pretreatment of waste activated sludge (WAS). Integrated fermentation and nitrogen removal using FNA as a primary means of treatment are seldom investigated. WAS fermentation was characterized under various FNA concentration. The production of COD, protein, and carbohydrate increased with FNA concentration (in the range of 0.197-1.97 mg/L) before the denitrification process. Volatile fatty acids (VFA) were only produced after complete denitrification. Potential FNA impact on fermentation step found FNA facilitated both solubilization and hydrolysis but inhibited acidification, acetogenesis, and methanogenesis processes. The types of fermentation were determined using threedimensional excitation-emission matrix (EEM) fluorescence spectroscopy. Protein-like substances and Tyrosine/Tryptophan were the most dominant dissolved organic matters (DOMs). The cell decay rate increased from 0.044 to 0.102/d based on the nonlinear fitting for the FNA concentration of 0.197-1.97 mg/L. The microbial biomass mortality reached 92.7% when the FNA in tight extracellular polymeric substances (T-EPS) exceeded 0.04 mg/L. In addition, the microbial diversity and microbial structure were substantially reduced by FNA during long-term operation, while the bacterial abundance associated with hydrolysis and acidification increased significantly.

The characterization of dissolved organic matter in alkaline fermentation of sewage sludge with different pH for volatile fatty acids production

Sewage sludge (SS) alkaline fermentation (especially at pH 10) can efficiently enhance volatile fatty acids (VFAs) production. VFAs are considered an excellent carbon source for the biological nutrient removal (BNR) process. Dissolved organic matter (DOM) in fermentation liquid is the direct substrate used for producing VFAs and can greatly influence the effluent quality of BNR process. However, knowledge of DOM characteristics in sludge alkaline fermentation is limited. This study focused on the functional groups, fluorescent components and molecular features of DOM as well as molecular weight of proteins in SS alkaline fermentation (at pH uncontrolled, 7, 8, 9 and 10). Results showed a significantly improved generation of tryptophan-like and tyrosine-like substances as well as molecular weight <1 kDa proteins (381.5 ± 38.4 mg/L) was observed at pH 10 (p < 0.05). Further analysis of DOM molecular characteristics indicated that pH 10 resulted in the highest molecular diversity and the generation or degradation of easily biodegradable lipid-like and proteins/amino sugars-like formulas. The improved solubilization of DOM contributed to VFAs production. Meanwhile, increasing pH to 10 also promoted the release of hard-biodegradable organic matter, e.g., humic-like and lignin-like substances. Additionally, a high diversity of resistant N-containing organic molecules was generated at pH 10. Fermentation of SS at pH 10, is favored to enhance VFAs production and, can also result in a higher content of refractory DOM. This study helps to achieve a comprehensive understanding of SS alkaline fermentation and provides fundamental information for further treatment.

The presence of ferrihydrite enhances greenhouse gas-methane emission in the environment

Aquatic system is the major source of atmospheric methane. This study explored the influences of ferrihydrite, which is widely existed in natural aquatic system, on methane emission. Results showed that the presence of ferrihydrite led to 26.4% more methane emission. By tracking the transformation of organic compounds, it is revealed that the enhanced methane emission was attributed to greater hydrolysis and degradation of refractory compounds. More specifically, the remaining humic-like substances (HS) in ferrihydrite group (46.4 mg/L-C) were only half of that in control group (80.1 g/L-C) after 30-day incubation. The X-ray photoelectron spectroscopy spectrum confirmed the more active oxidation of organics occurred in ferrihydrite group. It was also found that ferrihydrite aided in sustaining microbial activity at stationary and starvation phases. Further study on microbial communities found that ferrihydrite promoted the enrichment of both functional and electroactive genera. This study provides insights into the greenhouse gas emission in natural environment.

New insights on the sludge fermentation liquid driven denitrification: Evaluation of the system performance and effluent organic matter (EfOM)

Sludge fermentation liquid (FL) is a promising and sustainable carbon source for biological denitrification. In this study, four types of FLs were prepared using different chemical treatments to enhance denitrification. The profile of effluent organic matter (EfOM) were evaluated. Results indicate that the FL_NaOH (NaOH treatment at pH 10) contained the maximum amount of soluble organics, but led to the worst nitrate removal performance. Nitrate removal efficiency with FL_control was much higher than that of FL_CaO2, FL_Ca(OH)2, and FL_NaOH. On the other hand, it was found some organics in FL was hard to be utilized by denitrifiers. In particular, EfOM with FL_NaOH presented the highest building blocks, high molecular weight (HMW) protein and HMW polysaccharide. It also contained much higher C-(C/H) group. This study provided a new insight on the FL driven denitrification. The necessarity of pursuing the alkali pretreatment to produce carbon source were discussed and evaluated.

Accelerating waste sludge hydrolysis with alkyl polyglucose pretreatment coupled with biological process of thermophilic bacteria: Hydrolytic enzyme activity and organic matters transformation

A novel pretreatment method combining thermophilic bacteria (TB) with alkyl polyglucose (APG) was employed to pretreat waste sludge for enhancing the sludge hydrolysis. TB combined with APG pretreatment was effective in the releasing of soluble chemical oxygen demand (SCOD), protein and carbohydrate in extracellular polymeric substances (EPS) when the dosage of APG was below 0.1 g/g TSS. The enhancement of SCOD, carbohydrates and protein in dissolved organic matter (DOM) was promoted by the synthetic effect of APG and TB, which provides more carbon and energy source to the subsequent biochemical processes in sludge digestion. Excitation-emission matrix (EEM) fluorescence spectroscopy revealed that the combined pretreatment was beneficial for the decrease of non-biodegradable materials and the increase of biodegradable materials in DOM, resulting in the enhancement of the biodegradation of waste sludge. The combined use of TB and 0.4 g/g TSS APG achieved the maximal activities of protease (1.8) and α-glucosidase (1.9), and the activities of protease and α-glucosidase were positively correlated to the dosage of APG. The combined pretreatment was advantageous for the sludge reduction and sludge stabilization.

Organic matter rather than salinity as a predominant feature changes performance and microbiome in methanogenic sludge digesters

Due to low digestibility and long retention time of anaerobic sludge digestion, pre-treatment with alkaline/acid has been widely employed to enhance the rate and extent of sludge digestion. Nonetheless, effects of gradient concentrations of alkaline/acid pre-treatments and resulting salinity on digestion performance and sludge microbiome remain poorly understood. To elucidate these effects, both batch- and reactor-experiments were setup with varied feeding sludge. Significant digestion improvement and sludge microbiome changes were observed with alkaline/acid sludge pre-treatment, compared to non-pretreatment controls, e.g., ˜88% increase of carbon removal in sludge digesters. Surprisingly, with the same concentration of influent sludge, no notable change in digestion performance and sludge microbiome was observed in digesters when increasing alkaline/acid concentrations from 0.25 to 0.8 mol/L, and in batch serum bottles with or without NaCl amendment. Consequently, organic compounds dissolved in sludge pre-treatment could be a predominant selective pressure driving the performance and microbiome changes. By contrast, salinity as a consequence of the alkaline/acid pre-treatment could only enrich specific lineages, without altering the overall community profile and function. Together, this study provided insights into specific impacts of major factors on digester performance and sludge microbiome, and shed lights on optimization of sludge digestion.

Characterization of the refractory dissolved organic matters (rDOM) in sludge alkaline fermentation liquid driven denitrification: Effect of HRT on their fate and transformation

Enhanced biological denitrification for nitrogen removal using sludge alkaline fermentation liquid (SAFL) as an alternative carbon source has been widely reported in previous studies, while limited studies focused on the degradation of the organics presented in SAFL. In this study, an SAFL driven anoxic denitrification sequencing batch reactor (SBR) was established, the mechanism of organics utilization was characterized and the refractory dissolved organic matters (rDOM) was identified. Denitrification could rapidly proceed with the presence of volatile fatty acids (VFAs) initially, while the denitrification rate largely decreased after the VFAs depleted. A great deal of rDOM, which was hard to be utilized by denitrifying microorganism, was found in the effluent. A prolonged hydraulic retention time (HRT) led to the further transformation of particles and colloids to smaller colloids and soluble organics. Extended HRT promoted the degradation of soluble microbial by-product (SMP), but had minor effect on the removal of humic-like, and fulvic acid-like substances. The characterization of the effluent demonstrated the building blocks, were dominated in the rDOM (43.79%-48.78%), followed by high molecular weight protein (HMW-PN) (13.37%-17.39%), HMW polysaccharide (HMW-PS) (12.84%-15.9%), low molecular weight (LMW) neutrals (11.28%-13.65%), and hydrophobic dissolved organic carbon (HO-DOC) (8.0%-12.62%). Moreover, it was found that the building blocks were relatively easy to be degraded with the extension of HRTs, followed by LMW-PS, LMW-PN, LMW neutrals, HMW-PN, and HMW-PS. However, further extended HRT >24 h could not improve the removal of building blocks, LMW-PS and LMW neutrals. This study, for the first time, provided insights into the transformation of organic matters produced by SAFL in a denitrification system and acted as a guide for the subsequent advanced treatment.

Impact of molecular structure and charge property of chitosan based polymers on flocculation conditioning of advanced anaerobically digested sludge for dewaterability improvement

Anaerobically digested sludge is generally difficult to dewater due to the release of sticky soluble microbial products in anaerobic digestion. Traditional flocculation processes have the disadvantages of high chemical dosing and solid increase, thus affecting subsequent land application. Therefore, it is desirable to develop low-cost, biodegradable, nontoxic and environmentally friendly sludge conditioners. In this work, the chitosan (CTS) was chemically modified by incorporating functional groups (amino group and the carboxyl group) to improve its water solubility and flocculation efficiency, and the anaerobically digested sludge conditioning effectiveness of different chitosan based flocculants were comparatively investigated. Results indicated that aminated chitosan (CTS-DMDAAC) and CTS performed well in sludge dewatering improvement in terms of specific resistance to filtration (SRF) and Capillary suction time (CST), which decreased to a minimum when the concentration of conditioner reached to 35 mg/g TSS. Flocs conditioned by CTS-DMDAAC were more compact and aggregated more efficiently than that flocculated with CTS and C-CTS (carboxylated chitosan). CTS-DMDAAC and CTS interacted with extracellular polymeric substance (EPS) by charge neutralization and complexation adsorption, which caused the densification of gel-like structure and enhancement of floc strength of sludge. Scanning electron microscope (SEM) analysis showed that after CTS-DMDAAC treatment, there were plentiful large pores distributed on floc surface, which provided channels for water release under pressure filtration. Confocal laser scanning microscopy (CLSM) confirmed that protein-like substances were agglomerated under flocculation conditioning, which was responsible for the promotion of sludge dewatering performance. This study provides a green and promising solution for the improvement of anaerobically digested sludge dewatering performance.

Enhancing the hydrolysis of saline waste sludge with thermophilic bacteria pretreatment: New insights through the evolution of extracellular polymeric substances and dissolved organic matters transformation

Recently, the treatment and utilization of saline waste sludge has drawn growing attention because large amounts of saline waste sludge were generated with the increase of saline wastewater discharge. In this study, thermophilic bacteria (TB) pretreatment was applied to accelerate the hydrolysis of saline waste sludge and the efficiency of hydrolysis at different salinities was evaluated. Compared with the group without salinity, the releasing of carbohydrate (up to a 67.0% decrease) in extracellular polymeric substances (EPS) was inhibited at the salinity ranging from 1.0% to 2.5%, and the releasing of protein (up to a 17.6% decrease) was inhibited under salinity conditions. Excess salinity (4.0%) caused the cell lysis, and the content of soluble chemical oxygen demand (SCOD), soluble carbohydrate and protein in dissolved organic matter (DOM) increased by 44.9%, 38.8% and 20.8% than that obtained without salinity, respectively. According to the excitation-emission matrix (EEM) fluorescence spectroscopy, the biodegradability of sludge was improved at 2.0% salinity. At 2.0% salinity, the maximum fluorescence intensity of soluble microbial byproduct substances (76,358.9 (au)) and the minimum fluorescence intensity of humic acid-like substances (173,424 (au)) were obtained. The increased salinity was beneficial for the sludge stabilization and was disadvantageous for the sludge reduction.

Spectroscopic study of the effects of dissolved organic matter compositional changes on availability of cadmium in paddy soil under different water management practices

It is well established that water management can influence the availability of Cd in paddy soil but the role of dissolved organic matter (DOM) characteristics in this process is still unclear. Here, we measured and compared the DOM quantity and quality between flooded and wetted treatments by spectroscopic and chemometric analysis and applied correlation analysis to relate DOM characteristics with availability concentrations of Cd. Ultraviolet-visible showed that aromaticity and hydrophobicity of DOM significantly decreased with time in wetted paddy soil (p < 0.05) but had no significant difference in flooded paddy soil (p > 0.05). According the results from two-dimensional correlation spectroscopy analytical method, humic- and protein-like substances had fast response during cultivation process. Two humic-like substances (C1, C2) and two protein-like substances (C3, C4) were identified from paddy soil-derived DOM by combining emission and excitation matrix spectroscopy with parallel factor. Compared to component C1, C3, and C4, component C2 has stronger aromaticity and hydrophobicity and higher molecular size (665-1000 Da). Its proportion declined markedly during the wetting periods but increased slightly during flooding. Pearson correlation analysis illustrated that flooding was more helpful in immobilizing Cd than wetting due to the aromatic, hydrophobic, and high molecular weight constituents remained in flooded treatments and the substantial decomposition of component C2 in wetted treatments. These results suggested that spectroscopic and chemometric methods are helping to further explain the impacts of DOM quality on Cd availability under different water management practices.

Effects of return sludge alkaline treatment on sludge reduction in laboratory-scale anaerobic-anoxic-oxic process

Three alkaline treatments (pH 10, 11, and 12 for 1 h) were used to treat return sludge alone to reduce sludge production in laboratory-scale anaerobic-anoxic-oxic processes. After 99 days of operation, alkaline treatments at pH 10 and 11 led to accumulative excess sludge production and sludge yield reduction of 18.8%-31.7% and 14.7%-27.8%, respectively. However, alkaline treatment at pH 12 led to system breakdown because of sludge bulking. The alkaline treatment at pH 10 did not affect the chemical oxygen demand and NH₄⁺-N removals of the system and sludge volume index (SVI) of aerobic activated sludge. However, alkaline treatments at pH 11 and 12 obviously deteriorated the wastewater treatment efficiencies and sludge SVI. Although the three treatments increased the effluent pH by 0.08 to 0.38, the effluent pH of three systems were all lower than 9.00. The treatments at pH 10 and 11 increased the specific oxygen uptake rate of activated sludge, whereas the treatment at pH 12 decreased this rate.

Disintegration of waste activated sludge with composite ferrate solution: Sludge reduction and settleability

Sludge reduction has been a key issue in waste activated sludge (WAS) treatment. In this study, composite ferrate solution (CFS) has been used to disintegrate WAS. The results showed that CFS could effectively disrupt sludge flocs and cells and caused the release of intracellular matter such as SCOD, cations and organic acids. These results showed that the sludge disintegration process could be divided into a rapid reaction stage (0-2 h) and a slow reaction stage (2-24 h). It was determined that at a CFS dosage of 50 mg Fe/g SS and a reaction time of 24 h, the sludge reduction was 55.4% and SV and SVI were reduced by 12.1% and 46.4%, respectively. The Fe(VI), ClO^- and OH^- in CFS all played important roles in sludge decomposition, but they did not have synergistic effects. The small-particle sludge, in situ formed Fe³⁺ and released Ca²⁺ could improve the sludge settleability.

Transformation of dissolved organic matters produced from alkaline-ultrasonic sludge pretreatment in anaerobic digestion: From macro to micro

Soluble organic compounds released by alkaline (ALK), ultrasonic (ULS) and combined alkaline-ultrasonic (ALK-ULS) pretreatment as well as their transformation in the anaerobic digestion systems were investigated. The maximum methane production of 197.1 ± 3.0 mL CH₄/g tCOD_feed was observed with ALK-ULS pretreated sludge (pH 12 and specific energy input of 24 kJ/g TS). The combined treatment likely enhanced the sludge solubilization and produced more low molecular weight (LMW) substances, which were beneficial to improve the biogas generation rate. However, such pretreatment released not only easily biodegradable substances but also more recalcitrants, such as humic substances (HS) and complex high molecular weight (HMW) proteins. Thus, more residual dissolved organic matters (DOMs) were detected after digestion, which may pose adverse effects on the downstream water treatment. Refractory HS and hydrophobic dissolved organic carbon (HO DOC) were the main components of the residual DOMs, which accounted up to 35.0% and 22.3% respectively. At the molecular level, a large amount of residual polycyclic steroid-like matters, alkanes and aromatics were identified. Specific higher MW residual compounds, e.g. polar metabolites (like dipeptide, benzene and substituted derivatives), and non-polar lipids (like diacylglycerols, long chain fatty acids, alkenes, flavonoids, sphingolipids, glycerolipids, glycerophospholipids and their derivatives) were also identified. The results indicate that further polishing steps should be considered to remove the remaining soluble recalcitrant compounds. This study helps to understand the insight of sludge treatment from macro to micro level.

Insights into anaerobic transformation of key dissolved organic matters produced by thermal hydrolysis sludge pretreatment

The detailed dissolved organic matters (DOMs) profile by thermal hydrolysis pretreatment and their transformation during anaerobic digestion (AD) were investigated. Among the temperature tested, 172 °C treatment showed the best sludge solubilization and the maximum methane production. The study revealed that high temperature sludge pretreatment mainly improved the release of low molecular weight (LMW) proteins, LMW neutrals and LMW polysaccharides. Notably, the effluent from thermal treated sludge digesters contained more DOMs residues. The predominant residual DOMs were humic substances, LMW proteins and LMW neutrals. At the molecular level, over 50% of the residual LMW components were slowly biodegradable or nonbiodegradable steroid-like compounds and aromatics. Further profiling of the higher MW compounds detected the recalcitrant or inhibitory compounds, e.g. benzenoids, flavonoids, pyridines and their derivatives. It is recommended that polishing step should be considered to further reduce the refractory residues in AD liquor.

Influences of dissolved organic matters on tetracyclines transport in the process of struvite recovery from swine wastewater

Due to the extensive existence of tetracyclines (TCs), struvite (MgNH₄PO₄·6H₂O) recovery from swine wastewater will pose TCs-pharmacological threats to the agricultural planting and environment. This study investigated the influences of dissolved organic matters (DOM), as an important medium in the wastewater, on TCs transport during struvite recovery from swine wastewater. Compared to TCs concentrations of 1.49-2.16 μg/g in the solids obtained from synthetic wastewater, the existence of DOM significantly enhanced TCs contents in the products with the values of 360-742 μg/g. DOM was fractionated into four size fractions with different molecular weight cut-off, i.e. FDOM1 (30 kDa-0.45 μm), FDOM2 (5-30 kDa), FDOM3 (1-5 kDa) and FDOM4 (<1 kDa). Results revealed that the destabilization and aggregation of FDOM1 and FDOM2 contributed major roles to TCs transport from the aqueous phase to the solid products. Meanwhile, the hydrolysis of certain parts of FDOM1 and FDOM2 led to the aqueous TCs re-partition among various DOM constituents, which presented a false appearance that FDOM4 with smaller molecular weight posed significant influences on TCs transport. Increasing pH values from 8.5 to 10.5 resulted with a stepwise increase of precipitated DOM, thereby enhancing TCs concentrations from 94.5 to 292.4 μg/g to 627.2-825.0 μg/g in the recovered solids. The outcomes provide a better understanding on the capability of DOM on TCs transport and abatement in the phosphate recovery process.

Enhancing denitrification with waste sludge carbon source: the substrate metabolism process and mechanisms

Using waste sludge internal carbon source for nitrogen removal in wastewater has drawn much attention, due to its economic advantages and sludge reduction. In this study, the performance of enhanced denitrification with waste sludge thermal hydrolysate and fermentation liquid as carbon sources at different SCOD/N (soluble chemical oxygen demand/NO₃^--N) was investigated. The optimum SCOD/N was 8 for sludge thermal hydrolysate and 7 for fermentation liquid, with NO₃^--N removal efficiency of 92.3 and 98.9%, respectively, and no NO₂^--N accumulation. To further understand the fate of sludge carbon source during denitrification, the changes of SCOD, proteins, carbohydrates, and volatile fatty acids (VFAs) were analyzed, and three-dimensional fluorescence excitation-emission matrix (EEM) spectroscopy with fluorescence regional integration (FRI) analysis was introduced. The utilization of SCOD was consistent with NO₃^--N reduction, and the utilization efficiency of different organic matter was as follows: VFAs > proteins > carbohydrates. The soluble organic-like materials (region IV) were the most readily utilized organic matter according to three-dimensional fluorescence EEM spectroscopy. Regarding denitrification mechanisms, the denitrification rate (V_DN), denitrification potential (P_DN), heterotroph anoxic yield (Y_H), and the most readily biodegradable COD (S_S) were also investigated.

Enhanced sludge solubilization and dewaterability by synergistic effects of nitrite and freezing

In this study, nitrite was added into sludge during freezing process to evaluate its role in waste activated sludge (WAS) solubilization and effect on sludge dewatering characteristics. The results showed that the introduction of 100 mg L^-1 of nitrite could increase dissolved organic carbon (DOC) concentration from 29.5 to 48.8 mg DOC g^-1 VSS under freezing conditions. More DOC was released with the increase of nitrite concentration. Freezing temperature, or freezing speed, also played a role in sludge solubilization. It was found that some readily-biodegradable low molecular weight (LMW) compounds, e.g. LMW protein, LMW polysaccharide, LMW neutrals, building blocks and LMW acids, were mainly released during the freezing process with the presence of nitrite. Interestingly, nitrite could also improve the sludge filterability at the lower nitrite concentration as a result of the increased sludge particle size. However, electrolytes (sodium nitrite) addition effects may mask such enhancement when nitrite concentration was high (800 mg L^-1). The rheological characteristics of sludge could be well modeled by Herchel-Bulkley model and the introduction of nitrite into freezing process further increased sludge flowability and decreased sludge viscosity. These results indicated that freezing with the presence of suitable concentration of nitrite could promote sludge solubilization and dewaterability. As such, good liquid and solid separation can be achieved with the recovery of liquid stream as carbon source.

Comparison of thermophilic bacteria and alkyl polyglucose pretreatment on two-stage anaerobic digestion with waste sludge: Biogas production potential and substrate metabolism process

To gain a better understanding of the influence on two-stage anaerobic digestion of waste sludge with thermophilic bacteria (TB) and alkyl polyglucose (APG) pretreatment, changing of soluble chemical oxygen demand (SCOD), carbohydrate and protein in extracellular polymeric substances (EPS) and dissolved organic matters (DOM) were analyzed. The excitation-emission matrix (EEM) with fluorescence regional integration (FRI) was also used to investigate compositional and structural characteristics of DOM. The highest hydrogen and methane yield of TB pretreated sludge was 12.2 ml/g VS (volatile suspended solid) and 124.7 ml/g VS, and that of APG pretreated sludge was 28.3 ml/g VS and 19.9 ml/g VS. The VS removal of TB pretreated sludge (36.7%) was higher than APG pretreated sludge (27.1%) in the two-stage anaerobic digestion. The APG pretreatment could inhibit the activity of methanogens and the substrate (such as volatile fatty acids (VFAs), protein and soluble microbial materials) was accumulated compared with TB pretreatment.

Long solid retention time (SRT) has minor role in promoting methane production in a 65°C single-stage anaerobic sludge digester

In this study, a thermophilic (65°C) single-stage wasted activated sludge (WAS) digester was established and the effects of solid retention time (SRT) on the reactor performance were investigated. The result showed that the optimum SRT was 6days with methane yield of 186.16mL/g VS. It was found that SRT had little effect on the hydrolysis and volatile solids (VS) destruction, and the high temperature employed seemed sufficient to achieve maximum hydrolysis and VS destruction performance. Longer SRT, however, promoted the release of recalcitrant compounds and impaired acidification, leading to the low methane yield. The microbial community analysis revealed that the dominant pathway for methane production was through syntrophic activity of acetate oxidizing bacteria and hydrogenotrophic methanogens while acetoclastic methanogens were absent in the system.

Enhanced volatile fatty acids (VFAs) production in a thermophilic fermenter with stepwise pH increase - Investigation on dissolved organic matter transformation and microbial community shift

In this study, a mixture of primary and wasted activated sludge was fermented in a semi-continuous reactor aiming for enhanced volatile fatty acids (VFAs) production. The reactor was subjected to a stepwise pH increase from 7 to 10 during approximately 130 days of operation. The result revealed that the maximum acidification was obtained at pH 8.9 (21%) resulting in the maximum production of VFAs (423.22 ± 25.49 mg COD/g VSS), while the maximum hydrolysis efficiency was observed at pH 9.9 (42%). The high pH was effective in releasing dissolved organic matter (DOM) including protein, carbohydrate, building blocks and low molecular weight (LMW) neutrals. More LMW DOMs were released than high molecular weight (HMW) DOMs fractions at higher pH. pH 9.9 favored hydrolysis of HMW DOMs while it did not enhance the acidogenesis of LMW DOMs. The microbial community analysis showed that the relative abundance of phyla Actinobacteria and Proteobacteria increased with the increased pH, which may lead to the maximum hydrolysis at pH 9.9. At pH 8.9, class Clostridia (59.16%) was the most dominant population where the maximum acidification (21%) was obtained. This suggested that the dominance of Clostridia was highly related to acidification extent. The relative abundance of Euryarchaeota decreased significantly from 58% to 2% with increased pH.

Evaluating filterability of different types of sludge by statistical analysis: The role of key organic compounds in extracellular polymeric substances

An investigation was conducted for 20 different types of sludge in order to identify the key organic compounds in extracellular polymeric substances (EPS) that are important in assessing variations of sludge filterability. The different types of sludge varied in initial total solids (TS) content, organic composition and pre-treatment methods. For instance, some of the sludges were pre-treated by acid, ultrasonic, thermal, alkaline, or advanced oxidation technique. The Pearson's correlation results showed significant correlations between sludge filterability and zeta potential, pH, dissolved organic carbon, protein and polysaccharide in soluble EPS (SB EPS), loosely bound EPS (LB EPS) and tightly bound EPS (TB EPS). The principal component analysis (PCA) method was used to further explore correlations between variables and similarities among EPS fractions of different types of sludge. Two principal components were extracted: principal component 1 accounted for 59.24% of total EPS variations, while principal component 2 accounted for 25.46% of total EPS variations. Dissolved organic carbon, protein and polysaccharide in LB EPS showed higher eigenvector projection values than the corresponding compounds in SB EPS and TB EPS in principal component 1. Further characterization of fractionized key organic compounds in LB EPS was conducted with size-exclusion chromatography-organic carbon detection-organic nitrogen detection (LC-OCD-OND). A numerical multiple linear regression model was established to describe relationship between organic compounds in LB EPS and sludge filterability.

Variations in physical, chemical and biological properties in relation to sludge dewaterability under Fe (II) - Oxone conditioning

The mechanism of Fe (II) - oxone conditioning to improve sludge dewaterability was investigated in this study. Five different types of sludge were tested, including raw sludge (Group 1: mixed primary and secondary sludge, waste activated sludge and anaerobic digested sludge) and pretreated sludge with prior solubilisation (Group 2: ultrasonic or thermal pretreated sludge). After Fe (II) - oxone conditioning, the concentrations of dissolved organic carbon, protein and polysaccharide of soluble extracellular polymeric substances (SB EPS) increased for Group 1, but decreased for Group 2. For all types of sludge investigated, the related organic compounds of loosely bound (LB) and tightly bound (TB) EPS decreased with Fe (II) - oxone conditioning, and increased sludge filterability showed strong and positive correlation with the removal of low molecular weight protein and neutrals in LB EPS. Fe (II) - oxone was very effective in disintegrating cell membrane and caused potential cell lysis, as indicated by increased percentage of damaged microbial cells. From this study, the mechanism of Fe (II) - oxone conditioning was proposed and can be divided into two steps: (1) Oxidation step - sulfate radicals degraded organic compounds in LB and TB EPS in sludge and transformed bound water to free water that was trapped in TB and LB EPS; It also damaged cells membrane and may help to release intracellular water content. Sludge flocs were broken into smaller particles; (2) Coagulation step - Fe (III), generated from the oxidation step can act as a coagulant to agglomerate smaller particles into larger ones and reduce the repulsive electrostatic interactions. Combined effects from above two steps can greatly improve sludge filterability.

Structural and functional properties of organic matters in extracellular polymeric substances (EPS) and dissolved organic matters (DOM) after heat pretreatment with waste sludge

The effects of heat pretreatment on waste sludge hydrolysis were investigated in this study. Heat pretreatment was conducted at 65°C, 80°C, 100°C and 121°C for 5min, 10min, 15min, 20min, 25min and 30min. Not only analyzed the changes of SCOD (Soluble chemical oxygen demand), carbohydrate and protein, but also evaluated the structural and functional properties of organics in extracellular polymeric substances (EPS) and dissolved organic matters (DOM) by using three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy with fluorescence regional integration (FRI) analysis. The SCOD in DOM increased with pretreated temperatures. The optimal heat hydrolysis temperature and time were selected by further studying the biodegradable and non-biodegradable components. After treated at 80°C for 25min, the fluorescence intensity and percent fluorescence response (Pi,n) of easily biodegradable soluble microbial by-product substance were higher than others, and little non-biodegradable fulvic acid-like substance was accumulated.