Soluble carbon source recovery using preconditioning coagulants for applicable short-term fermentation of waste activated sludge in WWTPs.

A huge production of waste activated sludge (WAS) has been a burden for wastewater treatment plants (WWTPs) with high disposal cost and little benefit back to wastewater purification. The short-chain fatty acids (SCFAs) produced by a short-term acidogenic fermentation of WAS before methane production have been proven to be a high-quality carbon source available for microbial denitrification process. The dual purpose of full recovery of fermentation liquid products and facilitating disposal of residual solid waste necessitate an efficient solid-liquid separation process of short-term fermentation liquid. The transformation and loss of various soluble carbon sources between solid and liquid are very important issues for carbon recovery efficiency when combining short-term fermentation and sludge dewatering in WWTPs. Here we testified the three conventional preconditioning coagulants, Polyferric Sulfate (PFS), Poly Aluminum Chloride (PAC) and Polyacrylamide (PAM), to improve the efficiency of subsequent solid-liquid separation. The results show that conversion yield of SCFAs in the liquid phase of sludge after short-term fermentation was 195 mg COD/g VSS, when using the coagulants PFS, PAC, and PAM for recovery, the recovery ratio was 79.5%, 82.0%, and 85.9%, respectively, while the dewaterability could be improved after preconditioning short-term fermentation sludge. The complexation of Al3+/Fe3+ in metal coagulants with carboxyl groups of SCFA demonstrated by Density Functional Theory calculation led to small part of soluble carbons co-migration to the solid phase, mainly a loss of high molecular weight organic compounds (carbohydrate, proteins, humic acids), while the application of PAM had little impact on carbon recovery. Economic calculations further showed PAM preconditioning short-term fermentation liquid of WAS could achieve higher recovery benefits.

Understanding the coordination behavior of antibiotics: Take tetracycline as an example.

Gaining insight into the occurrence states of residual antibiotics is crucial to demystify their environmental behavior. However, the complexation of heteroatoms functioned on antibiotic molecules to metal ions in the water environment is not fully understood. This study reports that a fluorescence response was unexpectedly triggered by tetracycline (TC) and Al3+, serving as solid evidence to visualize the Al3+-TC coordination reaction. Differential electron absorption spectroscopy shows a quantifiable signal of the redshifted n-π* transition with a coordination reaction, which is also proportional to the fluorescence. The occurrence of Al3+-complexed TC also caused a split in retention time in liquid chromatogram. The TC ligands were re-released in the presence of stronger ligands competing for central Al3+. The complex ratio of Al3+-TC is confirmed to be 1:1 using Job's plot with a stability constant of 1.01 × 106. Quantum chemical computations coupled with Gibbs free energy analysis simulated the formation of octahedral Al3+-TC configuration through a spontaneous bidentate chelation. This study helps convey a broad consensus and opens a new door in the mechanistic study of metal-involved antibiotic transformation process, leading to a better understanding that can ultimately be essential to reach the final goal of alleviating the antibiotic crisis.

Biostimulants in dissolved organic matters recovered from anaerobic digestion sludge with alkali-hydrothermal treatment: Nontarget identification by ultrahigh-resolution mass spectrometry.

Recovering high-value biomaterials from anaerobic digestion sludge (ADS) has attracted considerable attention. However, the molecular features and biological effects of abundant dissolved organic matters (DOMs) in ADS are still unclear, which limits the efficient recycling and application of these bioproducts. This study investigated the molecular composition and transformation of DOMs recovered from ADS through a mild-temperature alkali-hydrothermal treatment (AHT) with ultrahigh-resolution mass spectrometry and energy spectroscopy, and the fertilizing effects of DOMs were evaluated by rice hydroponics. The results indicated that AHT processes significantly promoted the solubilization and release of DOMs from ADS, where most of DOMs molecules remained unchanged and mainly consisted of N-containing compounds with 1-3 N atoms, featuring aromatic or N-heterocyclic rings. Furthermore, AHT processes at pH of 9-10 induced the hydrolysis of partial protein-like substances in DOMs, which was accompanied by formation of heterocyclic-N compounds. Under AHT at pH of 11-12, protein-like and heterocyclic-N substances were increasingly decomposed into amino-N compounds containing 1 or 5 N atoms, while numerous oxygenated aromatic substances with phytotoxicity were degraded and removed from DOMs. Rice hydroponic test verified that ADS-derived DOMs recovered by AHT process at pH of 12 exhibited the highest bioactivity for rice growth, which was attributed to the abundance of amino compounds and humic substances. This study proposed a novel process for the recovery of high-quality liquid organic fertilizer from ADS through AHT process, which can further enrich the technical options available for the safe utilization of sludge resources.

Carbon source recovery from waste sludge reduces greenhouse gas emissions in a pilot-scale industrial wastewater treatment plant.

Carbon cycle regulation and greenhouse gas (GHG) emission abatement within wastewater treatment plants (WWTPs) can theoretically improve sustainability. Currently, however, large amounts of external carbon sources used for deep nitrogen removal and waste sludge disposal aggravate the carbon footprint of most WWTPs. In this pilot-scale study, considerable carbon was preliminarily recovered from primary sludge (PS) through short-term (five days) acidogenic fermentation and subsequently utilized on-site for denitrification in a wool processing industrial WWTP. The recovered sludge-derived carbon sources were excellent electron donors that could be used as additional carbon supplements for commercial glucose to enhance denitrification. Additionally, improvements in carbon and nitrogen flow further contributed to GHG emission abatement. Overall, a 9.1% reduction in sludge volatile solids was achieved from carbon recovery, which offset 57.4% of external carbon sources, and the indirect GHG emissions of the target industrial WWTP were reduced by 8.05%. This study demonstrates that optimizing the allocation of carbon mass flow within a WWTP has numerous benefits.

Molecular composition and biotoxicity effects of dissolved organic matters in sludge-based carbon: Effects of pyrolysis temperature.

Sludge pyrolysis carbonization has shown potential to convert sludge biomass into multifunctional carbon materials. However, ecological risks of dissolved organic matters (DOMs) with obscure molecular characteristics retaining in sludge-based carbons (SBCs) have received little attention. This study investigated the impact of pyrolysis temperatures on the molecular conversion and biotoxicity effects of DOMs in SBCs. The results revealed that DOMs in SBCs300-400 were mainly derived from depolymerization of biopolymers and the polycondensation and cyclization of small intermediate molecules, which mainly consisted of aromatic CHON compounds with 1-3 N atoms, featuring high unsaturation and molecular weights. High-temperature pyrolysis (500-800 °C) promoted the decomposition and ring-opening of aromatic CHON compounds into saturated aliphatic CHO compounds with 2-4 O atoms in SBCs500-800. Noteworthily, SBCs300-400-derived DOMs showed relatively strong biotoxicity on the growth and development of wild-type zebrafish embryos, pakchoi seeds, and Vibrio qinghaiensis Q67, which was significantly related to aromatic amines, phenols, and heterocyclic-N compounds in DOMs of SBCs300-400. SBCs500-800-derived DOMs were mainly straight-chain fatty acids and showed no observable acute biotoxicity. This study highlights the negative impact of DOMs in SBCs on the ecological environment, and provides the theoretical basis for controlling toxic byproducts in sludge pyrolysis process.

Obtaining high-value nitrogen-containing carbon nanosheets with ultrahigh surface area from waste sludge for energy storage and wastewater treatment.

Recovering high value-added resources from waste activated sludge (WAS) is a potential way for the sustainable wastewater treatment. In this study, hydrothermal treatment at 180 °C was used to simultaneously improve sludge dewaterability and recover sludge organic matters (SOMs). The recovered SOMs were subsequently employed as precursors to prepare nitrogen-doped porous carbon nanosheets via a facile stepwise synthesis method. The as-prepared optimal carbon (AP-SOM800) was characterized with an ultrahigh specific surface area (3473 m2/g), appropriate porosity (1.77 cm3/g), and abundant heteroatoms (1.47% N and 7.44% O). AP-SOM800 exhibited a high specific capacitance (409 F/g at 0.25 A/g), low resistance (0.52 Ω), and superior cyclic stability (only 9.09% loss after 10,000 cycles) in 6 M KOH aqueous electrolyte. Furthermore, AP-SOM800 demonstrated an extraordinary adsorption capacity (1528 mg/g for methyl orange (MO) and 1265 mg/g for tetracycline (TC)) that can be maintained (˃ 1200 mg/g) over a wide range of pH conditions. Specifically, 80.97% of MO and 66.67% of TC were rapidly absorbed through AP-SOM800 within 10 min, and 90.27% of MO and 81.24% of TC were eventually removed from wastewater after 60 min. The adsorption processes fit closely with the pseudo-second-order kinetic (R2 > 0.999) and Langmuir models (R2 > 0.914), revealing that the adsorption processes were dominated by a monolayer chemical adsorption reaction. This study suggests that high value-added materials can be obtained from the WAS through improving and extending the traditional sludge treatment processes, which will enrich the technical options available for future sustainable sludge treatment and disposal.

Molecular mechanisms of interaction between enzymes and Maillard reaction products formed from thermal hydrolysis pretreatment of waste activated sludge.

Thermal hydrolysis pretreatment (THP) is often used to improve the anaerobic digestion performance of waste activated sludge (WAS) in wastewater treatment plants (WWTPs). During the THP process, the proteins and polysaccharides in the biomass will undergo hydrolysis and Maillard reaction, producing biorefractory organic substances, such as recalcitrant dissolved organic nitrogen (rDON) and melanoidins. In this study, a series of spectroscopy methods were used to quantitatively analyze the Maillard reaction of glucose and lysine, and the interaction mechanisms of the Maillard reaction products (MRPs) and lysozyme were investigated. Results showed that the typical aromatic heterocyclic structures in MRPs, such as pyrazine and furan, were found to quench molecular fluorescence of lysozyme, resulting in an unfolding of standard protein structure and increase in lysozyme hydrophobicity. Significant loss of enzyme activity was detected during this process. Thermodynamic parameters obtained from isothermal titration calorimetry (ITC) confirmed that the interaction between MRPs and lysozyme occurred both exothermically and spontaneously. Density functional theory (DFT) calculations suggested that the molecular interactions of MRPs and protein included parallel dislocation aromatic stacking, T-shaped vertical aromatic stacking, H-bond and H-bond coupled to aromatic stacking.

Preparation of ultrahigh-surface-area sludge biopolymers-based carbon using alkali treatment for organic matters recovery coupled to catalytic pyrolysis.

In this work, we employed waste activated sludge (WAS) as carbon source to prepare ultrahigh specific surface area (SSA) biopolymers-based carbons (BBCs) through alkali (KOH) treatment coupled to pyrolysis strategy. Before the pyrolysis process, the involvement of KOH made a great recovery of soluble biopolymers from WAS, resulting in highly-efficient catalytic pyrolysis. The Brunner-Emmett-Teller and pore volume of BBCs prepared at 800°C (BBC800) reached the maximum at 2633.89 m2·g-1 and 2.919 m3·g-1, respectively. X-ray photoelectron spectroscopy suggested that aromatic carbon in the form of C=C was the dominant fraction of C element in BBCs. The N element in BBCs were composed of pyrrolic nitrogen and pyridinic nitrogen at 700°C, while a new graphitic nitrogen appeared over 800°C. As a refractory pollutant of wastewater treatment plants, tetracycline (TC) was selected to evaluate adsorption performance of BBCs. The adsorption behavior of BBCs towards TC was conformed to the pseudo-second-order kinetic and the Langmuir models, signifying that chemisorption of monolayers was dominant in TC adsorption. The adsorption capacity of BBC800 reached the maximum at 877.19 mg·g-1 for 90 min at 298 K. Thermodynamic analysis indicated that the adsorption process was endothermic and spontaneous. Hydrogen bonding and π-π stacking interaction were mainly responsible for TC adsorption, and interfacial diffusion was the main rate-control step in adsorption process. The presence of soluble microbial products (SMPs) enhanced TC removal. This work provided a novel strategy to prepare bio-carbon with ultrahigh SSA using WAS for highly-efficient removal of organic pollutants.

Mechanistic insights into the effects of biopolymer conversion on macroscopic physical properties of waste activated sludge during hydrothermal treatment: Importance of the Maillard reaction.

In this study, the molecular transformation of sludge biopolymers during hydrothermal treatment with the temperature ranging from 25 °C to 200 °C was examined and was seen to significantly affect the macrophysical properties (dewaterability and rheological property) of sludge. The results showed that the sludge dewaterability and flow ability under high shear stress deteriorated by a hydrothermal process at 25 °C to 120 °C, but the deterioration alleviated above the temperature threshold of 120 °C. The consistence of changes in sludge dewaterability and rheological property in HT process was mainly attributed to the variation in gel properties of soluble biopolymer. Two-stage changes in biopolymer transformation were identified, beginning with a solubilization stage from 25 °C to 120 °C in which a biopolymer with a gel-like network structure was released into liquid phase, creating flow resistance under high shear stress such that sludge dewaterability deteriorated. The second stage was identified as a conversion stage (120 °C-200 °C) in which proteins and polysaccharides hydrolyzed and experienced a Maillard reaction, leading to the degradation of the biopolymer network structure. The newly formed recalcitrant Maillard products showed weak flow response to high shear stress, allowing for an improvement in sludge dewaterability. The pathways of a Maillard reaction were identified via gas chromatography-mass spectrometer (GC-MS), 1H nuclear magnetic resonance spectroscopy (1H NMR) and two-dimensional correlation spectral analysis (2D-COS) of Fourier-transform infrared spectrometer (FTIR), etc. Three-dimensional excitation-emission matrix (3D-EEM) proved to be an applicable method for tracking Maillard reaction in sludge hydrothermal process due to the distinctive fluorescence characteristics of Maillard products. This study further clarifies the obscure process of sludge hydrothermal treatment and will help improve the accuracy of subsequent research.

Effect of extracellular polymer substances on the tetracycline removal during coagulation process.

In this study, the effect of extracellular polymer substances on the tetracycline removal under hydroxyl aluminium treatment was investigated, and the molecular mechanisms of extracellular polymeric substances mediated coagulation of tetracycline were also explored. The results show that the presence of extracellular polymeric substances could significantly enhance the removal efficiency of tetracycline in hydroxyl aluminium coagulation. Findings suggest that tyrosine and tryptophan in extracellular proteins acted as binding sites to capture tetracycline. Evidences provided by the density functional theory calculations in combination with spectroscopy analysis indicated that two main mechanisms accounted for tetracycline removal in the presence of extracellular polymeric substances and polyaluminum chloride: (1) amino group in proteins and carbonyl in tetracycline were bridged by Al3+; (2) benzene rings in tryptophan and tyrosine were π-π stacked with tetracycline, and the amino group in complexes were further coordinated with Al3+.

Characterization of changes in extracellular polymeric substances and heavy metal speciation of waste activated sludge during typical oxidation solubilization processes.

Biopolymer solubilization is considered to be the rate-limiting stage of anaerobic digestion of waste activated sludge (WAS). Oxidation processes have been proven to be effective in disrupting sludge flocs and causing solubilization of the solid biopolymers. In this study, WAS was treated by NaNO2 or H2O2 oxidation at pH of 2. The changes in extracellular polymeric substances properties and the speciation of heavy metals were investigated. The results revealed that both NaNO2 and H2O2 treatments were effective in solubilizing organics in WAS, while the conversion of biopolymers in the two treatment processes was different. Free nitrous acid destroyed the gel network structure of EPS, and organic materials were released from the solid phase to the supernatant. Indigenous peroxidase catalyzed H2O2 to produce hydroxyl radicals which caused significant solubilization of biopolymers, and the protein-like substances were further degraded into micro-molecule polypeptides or amino acids at high dosages of H2O2. During the oxidation processes, Zn, Cd and Cu, with excellent mobility, tended to migrate to the supernatant, and thus were easy to remove through the liquid-solid separation process. Ni and As showed moderate migration ability, of which the residual fraction tended to transform into reducible and soluble fractions. With poor mobility, Cr and Pb mainly existed in the forms of residual and oxidizable fractions, which were difficult to dissolve and remove from WAS. Both NaNO2 and H2O2 treatment resulted in the enhancement of sludge solubilization efficiency and heavy metal mobility in WAS, but different heavy metals showed distinct migration and transformation behaviors.

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.

Enhancement of anaerobic digestion sludge dewatering performance using in-situ crystallization in combination with cationic organic polymers flocculation.

Anaerobic digestion (AD) has been widely used in sludge treatment for biogas recovery, organic fraction stabilization, and sludge reduction. However, after AD, sludge filterability is extremely deteriorated due to the release of biopolymers and the formation of fine particles. AD sludge is generally rich in nutrients, mainly ammonium nitrogen and phosphates, that result from biopolymer degradation. We designed a conditioning process that combines the in-situ crystallization of magnesium ammonium phosphate (MAP), as a skeleton builder, with organic polymer flocculation. We show that crystallized MAP can bind with extracellular polymeric substance fractions to increase sludge floc density. The molecular structure and electrical charge of organic polymers importantly influence sludge particle flocculation and aggregation. We found that cationic polyacrylamides form flocs that aggregate with branching structures which are characterized by a larger size and a more compact structure. Simultaneous crystallization and flocculation produced by a magnesium-organic polymer gel improved AD dewaterability more than the separate addition of magnesium ions and organic polymers. The method of sludge conditioning that we propose was tested by extensive use in different AD sludge conditioning protocols. The method reduces the ammonium nitrogen load in AD liquor and increases the suitability of the biosolids for use as land fertilizer.

Highly effective enhancement of waste activated sludge dewaterability by altering proteins properties using methanol solution coupled with inorganic coagulants.

Proteins are the dominant organic component of extracellular polymeric substances (EPS) in waste activated sludge (WAS), and play an important role during sludge dewatering processes. Methanol is a polar hydrophilic reagent and can denature proteins, which suggested to us that the modification of protein configurations with methanol could improve sludge dewatering performance. In this study, methanol was used to precondition WAS prior to adding inorganic coagulants for dewatering enhancement. The morphology and EPS properties (especially of proteins) were investigated to analyze and explain the effects of methanol in the sludge conditioning process. The results show that methanol performed much better than traditional inorganic coagulants in improving sludge dewaterability in term of specific resistance to filtration (SRF) and cake solid content (CSC). Extractable proteins in EPS increased to a maximum when the concentration of methanol reached 40% (w/w) because cell membranes were destroyed and intracellular substances and water were released. Floc protein content was reduced with the further increase in methanol concentration due to protein precipitation. Confocal laser scanning microscopy analysis indicated that proteins precipitated and formed larger aggregates because methanol destroyed both the hydration shell and the hydrophobic clusters of proteins and expanded the protein tertiary structure to release interstitial water and bound water. The combination treatment of methanol and inorganic coagulants (PAC or FeCl3) showed significant synergetic effects on enhancing sludge dewatering and cake drying. In practical applications, methanol from the dewatering sludge can be returned to the biochemical pool and used as the carbon source for nitrogen removal in the denitrification process. This integrated process is appropriate for sludge final disposal technologies that have high energy demands, such as incineration and pyrolysis. This paper describes a novel approach to improving sludge dewaterability through the alteration of protein properties by use of physiochemical techniques.

Wastewater sludge dewaterability enhancement using hydroxyl aluminum conditioning: Role of aluminum speciation.

Chemical conditioning is one of the most important processes for improve the performance of sludge dewatering device. Aluminum salt coagulant has been widely used in wastewater and sludge treatment. It is generally accepted that pre-formed speciation of aluminum salt coagulants (ASC) has an important influence on coagulation/flocculation performance. In this study, the interaction mechanisms between sludge particles and aluminum salt coagulants with different speciation of hydroxy aluminum were investigated by characterizing the changes in morphological and EPS properties. It was found that middle polymer state aluminum (Alb) and high polymer state aluminum (Alc) performed better than monomer aluminum and oligomeric state aluminum (Ala) in reduction of specific resistance to filtration (SRF) and compressibility of wastewater sludge due to their higher charge neutralization and formed more compact flocs. Sludge was significantly acidified after addition Ala, while pH was much more stable under Alb and Alc conditioning due to their hydrolysis stability. The size of sludge flocs conditioned with Alb and Alc was small but flocs structure was denser and more compact, and floc strength is higher, while that formed from Ala is relatively large, but floc structure was loose, floc strength is relatively lower. Scanning environmental microscope analysis revealed that sludge flocs conditioned by Alb and Alc (especially PAC2.5 and Al13) exhibited obvious botryoidal structure, this is because sludge flocs formed by Alb and Alc were more compact and floc strength is high, it was easy generated plentiful tiny channels for water release. In addition, polymeric aluminum salt coagulant (Alb, Alc) had better performance in compressing extracellular polymeric substances (EPS) structure and removing sticky protein-like substances from soluble EPS fraction, contributing to improvement of sludge filtration performance. Therefore, this study provides a novel solution for improving sludge dewatering property by controlling aluminum speciation.