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

A review on the recovery of humic substances from anaerobic digestate of sludge as a potential fertilizer: Quantification, efficiency and interaction with pollutants

This study reviewed the recovery of humic substances (HS) from anaerobic digestate of sludge as a potential fertilizer, focusing on the quantification of HS, the efficiency of HS recovery, and its interaction with pollutants. The potential pitfalls of current misunderstanding for HS quantification in sludge were pointed out. HS present in sludge showed potential to be used as a fertilizer, which solubilized insoluble phosphates for enhanced soil fertility. Pretreatment prior to anaerobic sludge digestion and composting of anaerobic digestate of sludge (ADS) improved the amount of HS in anaerobic digestate. But the amount of HS extracted from the retentate of ADS was much lower than the level required for the common HS fertilizer. Therefore, an additional concentration was required to process the retentate of ADS into HS liquid fertilizer. The quinone moieties in HS accepted electrons from anaerobic microbial respiration and their role in the degradation of organic pollutants were summarized. By binding with HS, the speciation of metals in sludge was changed from water-soluble and exchangeable to organic- and sulfide-bound fractions. Future research should focus on the novel application of machine learning for quantifying HS within sludge, offering a practical approach to interpret complex fluorescence data and enhance understanding of HS characteristics and distribution. Further studies should explore the application of hydrothermal humification to enhance HS content in ADS, offering an energy-efficient method for rapid fertilization in sustainable agriculture. This study offers a window into HS recovery from ADS.

Deciphering the dead zone on anammox system in biofilters

In an anammox biofilm reactor, long-term operation inevitably leads to the repeated formation of localized dead zones. Once these dead zones (DZs) occur, the anammox reactor's nitrogen removal efficiency is severely reduced. However, the mechanisms and intrinsic reasons for the transformation of DZs remain unexplored. In this study, the pilot-scale biofilters were classified into biologically active zones (BZs), transition zones (TZs), and DZs. The results indicated that microbial communities undergo accelerated succession from the TZ. Biofilms respond to environmental stress from the DZs by altering the levels of signaling molecules, triggering a series of cascading reactions. These reactions alter the abundance of genes involved in nitrogen removal, promote substance transformation, and speed up the succession of microbial communities. This study demonstrates the objectives and self-healing mechanisms of the anammox biofilm process in the presence of dead zones, which could support the long-term application of anammox technology.

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.

New insights into the improved contaminants removal in SBR by intermittently weak ultrasound

The combination of intermittently weak ultrasound and sequencing batch reactor was thoroughly investigated to elucidate the relationship between enhanced contaminants removal and activated sludge characteristics, microbial composition, and regulation of differentially expressed genes (DEGs). At 12 °C, irradiation with an ultrasound intensity of 9.68 W/L, an irradiation time of 10 min, and an interval time of 24 h led to significant increases in COD, NH4+-N, and TP removals with the rates of 93.10 ± 1.51%, 95.75 ± 0.76%, and 92.52 ± 0.95%, respectively. The intermittently weak ultrasound enhanced contaminants removal was primarily attributed to the stimulated microbial metabolism, in which the mechanical oscillation rather than free radical oxidation facilitated the loosening of activated sludge flocs and promoted microorganism proliferation. Elevating the ultrasound intensity or irradiation time could weaken the effect of enhancing ammonia-oxidizing bacteria activity and suppressing nitrite-oxidizing bacteria activity. The results revealed that intermittently weak ultrasound primarily affected the extracellular polymeric substances (EPS), with protein nitrogen playing a more significant role than polysaccharide within EPS against ultrasound-induced stress. Furthermore, ultrasound irradiation elevated the energy barrier in total-binding EPS interaction energy curves, thereby inhibiting activated sludge aggregation. Over prolonged operation, the relative abundance of the prevalent denitrifying genus Thauera increased by 90.3%, whereas that of the fully aerobic denitrifier and nitrite producer Dokdonella increased by 68.7%. The intermittently weak ultrasound induced enhancement of microbial metabolism-related DEGs pathways, which served as the main contributor to the improved contaminants removal. These findings provide novel insights into the mechanisms by which intermittently weak ultrasound enhances the effectiveness of biological wastewater treatment.

Chemical characteristics of dissolved organic matter in effluent from sludge alkaline fermentation liquid-fed sequencing batch reactors

Sludge alkaline fermentation liquid (SAFL) is a promising alternative to acetate for improving biological nitrogen removal (BNR) from wastewater. SAFL inevitably contains some refractory compounds, while the characteristics of dissolved organic matter (DOM) in effluent from SAFL-fed BNR process remain unclear. In this study, the molecular weight distribution, fluorescent composition and molecular profiles of DOM in effluent from SAFL and acetate-fed sequencing batch reactors (S-SBRs and A-SBRs, respectively) at different hydraulic retention time (12 h and 24 h) was comparatively investigated. Two carbon sources resulted in similar effluent TN, but a larger amount of DOM, which was bio-refractory or microorganisms-derived, was found in effluent of S-SBRs. Compared to acetate, SAFL increased the proportion of large molecular weight organics and humic-like substances in effluent DOM by 74.87%-101.3% and 37.52%-48.35%, respectively, suggesting their bio-refractory nature. Molecular profiles analysis revealed that effluent DOM of S-SBRs exhibited a more diverse composition and a higher proportion of lignin-like molecules. Microorganisms-derived molecules were found to be the dominant fraction (71.51%-72.70%) in effluent DOM (<800 Da) of S-SBRs. Additionally, a prolonged hydraulic retention time enriched Bacteroidota, Haliangium and unclassified_f_Comamonadaceae, which benefited the degradation of DOM in S-SBRs. The results help to develop strategies on reducing effluent DOM in SAFL-fed BNR process.

Carbon dots/TiO2 enhanced visible light-assisted photocatalytic of leachate: Simultaneous effects and Mechanism insights

The persistence and potential fouling risks associated with humic substances and bacteria present in leachate have gained increasing attention. Therefore, developing efficient and environmentally compatible technologies for their removal is essential. This study presented the hydrothermal synthesis of a photocatalyst by coupling carbon dots (CDs) and bulk TiO2 (P25). The incorporation of CDs increased the photocatalytic performance by enhancing visible light absorption and facilitating the separation of electrons/holes. Compared to P25, the CDs/P25 exhibited optimal photocatalytic activity for humic acid (HA), fulvic acid (FA), and leachate, with 1.64, 1.02, and 1.12 times higher activity, respectively. Remarkedly, the CDs/P25 accelerated the conversion of large HA molecules into small molecules at a faster rate and higher amount than the bulk P25, due to the increase of hydroxyl radicals, monoclinic oxygen radicals, and superoxide radicals. Additionally, the CDs/P25 demonstrated better bacterial-deactivation ability than the P25, with dead bacteria percentages of 83.3% and 34.6%, respectively. This study provides a promising strategy for efficiently applying CDs/P25 photocatalysis to leachate treatment.

A Method of Water COD Retrieval Based on 1D CNN and 2D Gabor Transform for Absorption-Fluorescence Spectra

Chemical Oxygen Demand (COD) is one of the indicators of organic pollution in water bodies. The rapid and accurate detection of COD is of great significance to environmental protection. To address the problem of COD retrieval errors in the absorption spectrum method for fluorescent organic matter solutions, a rapid synchronous COD retrieval method for the absorption-fluorescence spectrum is proposed. Based on a one-dimensional convolutional neural network and 2D Gabor transform, an absorption-fluorescence spectrum fusion neural network algorithm is developed to improve the accuracy of water COD retrieval. Results show that the RRMSEP of the absorption-fluorescence COD retrieval method is 0.32% in amino acid aqueous solution, which is 84% lower than that of the single absorption spectrum method. The accuracy of COD retrieval is 98%, which is 15.3% higher than that of the single absorption spectrum method. The test results on the actual sampled water spectral dataset demonstrate that the fusion network outperformed the absorption spectrum CNN network in measuring COD accuracy, with the RRMSEP improving from 5.09% to 1.15%.

Spectroscopic fingerprints profiling the polysaccharide/protein/humic architecture of stratified extracellular polymeric substances (EPS) in activated sludge

Extracellular polymeric substances (EPS), with a stratified structure including tightly-bound EPS (TB-EPS), loosely-bound EPS (LB-EPS), and soluble EPS (S-EPS) surrounding the microbial cells, are known to vitally affect the physicochemical and biological functions of activated sludge in wastewater treatment. Polysaccharides (PS), proteins (PN), and humic acids (HA) are key components of EPS but their roles in constructing the multi-layer architecture are still unclear. This study explored the EPS characteristics in relation to the components using spectroscopic fingerprinting techniques. Ultraviolet-visible (UV-vis) spectra demonstrated stark difference between TB-EPS and other EPS. Fluorescence excitation-emission matrix (FEEM) and apparent quantum yield revealed further detailed differences. Fluorescence quotient analysis highlighted the dominance of TB-EPS, LB-EPS, and S-EPS in the excitation/emission wavelength (Ex/Em) region of Em = 350-400 nm, Em > 400 nm, and low-Stokes shift band (Em - Ex < 25 nm), respectively. Wavelength-wise prediction of the FEEM intensity was achieved through multiple linear regression against the chemical composition and variance partitioning analysis witnessed binary interactions of PS×HA and PS×PN in S-EPS, PN×HA and PS×PN in LB-EPS, and ternary interaction of PS×PN×HA in TB-EPS as well as the wavelength-specific fluorescence responses of these interactions. Further, X-ray photoelectron spectroscopy, infrared spectra, and circular dichroism spectra corroborated the differences in primary, secondary, and tertiary structures across the EPS layers. Ultrahigh-performance liquid chromatography-mass spectrometry detected molecular fragments confirming the multi-component hybridization among PS, PN, and HA. This study demonstrates a spectroscopic approach to sensitively fingerprint the fine structure of EPS, which has the potential for rapid monitoring of EPS and related sludge properties in wastewater treatment systems.

Deciphering the structural characteristics and molecular transformation of dissolved organic matter during the electrolytic oxygen aerobic composting process

Electrolytic oxygen aerobic composting (EOAC) effectively treats organic solid waste by using in-situ electrolytic oxygen for aeration. However, the fundamental mechanism of compost maturity is still unclear. Therefore, we comprehensively characterized dissolved organic matter (DOM) transformation closely related to compost maturity during EOAC. Excitation-emission matrix-parallel factor (EEM-PARAFAC) and Fourier transform infrared (FTIR) analysis confirmed that EOAC quickly decreased organic matter and increased humus substances, accelerating the compost humification process compared with conventional aerobic composting. Electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis reveals that the double bound equivalent and aromaticity index during EOAC are higher than in conventional aerobic composting (CAC), suggesting more aromatic compounds in EOAC. DOM's detailed transformation investigation suggested that low O/C and high H/C compounds were preferentially decomposed during EOAC. Our investigation firstly extends the in-depth molecular mechanisms of humification during EOAC, and reveals its practical engineering applications.

Effects of Sewage Sludge Application on Plant Growth and Soil Characteristics at a Pinus sylvestris var. mongolica Plantation in Horqin Sandy Land

The application of domestic sewage sludge (SS) may affect plant growth and soil quality through altering nutrient availability. However, the effect of SS application on the plant–soil system in sandy soils is poorly understood. In this study, we established SS application treatment plots (SL, 25 t ha−1) and control treatment plots without sewage sludge application (CK, 0 t ha−1). SS was applied to the soil surface of a Mongolian pine (Pinus sylvestris var. mongolica) plantation in Horqin Sandy Land, Inner Mongolia, China, to assess its potential effects on plants and soil. We analyzed tree growth performances (tree height, basal diameter, and diameter at breast height), understory traits (species diversity, coverage, and aboveground biomass), soil physical and chemical parameters (nutrient content, dissolved organic carbon, soil water content, bulk density, pH), and proxies of ecosystem services (soil organic carbon and total nitrogen stocks). The results showed that SS addition not only significantly increased soil nutrient contents, but also markedly enhanced aboveground productivity and plant coverage. Specifically, SS addition decreased soil bulk density and increased concentrations of soil organic carbon, total nitrogen, and total phosphorus and mineral nitrogen, and it also increased soil carbon and nitrogen stocks. Furthermore, the addition of SS significantly increased soil dissolved organic carbon contents and enhanced the fluorescence intensities of dissolved organic carbon components (humic acid-like and UV fulvic acid-like) in the topsoil (0–5 cm). This study provides evidence that SS is an acceptable, and possibly preferred organic fertilizer for improving the soil quality and tree–grass growth of Mongolian pine plantations.