Minimization of excess sludge production by in-situ activated sludge treatment processes — A comprehensive review

Molecular choreography of sludge extracellular polymeric substances-From biomolecule identification to energetics and assembly dynamics

Extracellular polymeric substances (EPS) shape the immediate environment for microbial survival and biofilm formation. Dynamic agglomeration of EPS dominates the formation kinetics and structural properties of activated sludge flocs as a consequence of biopolymer interactions across the wastewater treatment process. Current partial understanding and imprecise modeling of the structure hinder the comprehensive elucidation of the dynamic reorganization of clusters as component interactions change, causing a gap in the fundamental knowledge of EPS generation and functions. Here, biopolymer models of aerobic activated sludge and anaerobic digestion sludge (ADS) were constructed through molecular screening, and the dynamic landscape of EPS multicomponent clusters was then captured by an extensive set of molecular dynamics simulations. Biopolymer chains are assembled hierarchically driven by interactions between polar functional groups and stabilized by hydrogen bonding and van der Waals forces after several substates to obtain the final conformation. Electrostatic repulsion induced by carboxylic groups causes the rugged energy landscape of the process. Biopolymer molecular arrangement governed by polar interactions determines the nonuniform distribution of functional groups and characteristic regions, resulting in the microscopic heterogeneity of EPS clusters. The structure of alpha-helices enhances protein aggregation efficacy by facilitating more polar interactions compared with other residues. Meanwhile, the flexible branched structure and amphiphilic unit improve the energetic contribution of polysaccharides to EPS structural stabilization. Higher humic substance and carboxyl groups content primarily weaken the structural strength of ADS EPS. In general, this study proposes a powerful approach for investigating the molecular choreography within EPS, utilizing atomic simulations based on solved structures to explore the contribution of specific biopolymer features to structural energetics, providing theoretical insights to guide EPS-engineered regulation in wastewater treatment processes.

Effect of pre-chlorination on bioelectricity production and stabilization of excess sludge by microbial fuel cell

Microbial fuel cell (MFC) is a technology that can generate electricity while degrading excess sludge. However, the complex components, intricate biological structures, and inhibitory compounds in sludge limit the application of MFC. Therefore, this study utilized chlorination as a sludge pretreatment method to improve the comprehensive performance of MFC in sludge treatment. Results showed that pre-chlorination at a dose of 0.2 mg/L increased output voltage of MFC by 500 % from approximately 100 mV to around 600 mV, and power density by 15.60 % from 3.15 W/m³ to 3.64 W/m³, and simultaneously increased the degradation of sludge MLSS (mixed liquor suspended solids), MLVSS (mixed liquor volatile suspended solids), EPS (extracellular polymeric substances) polysaccharide and protein by 9.64 %, 47.07 %, 18.63 % and 16.26 %, respectively. Molecular composition analysis of EPS in sludge by three-dimensional excitation emission matrix fluorescence spectroscopy (3D-EEM), Fourier transform infrared spectroscopy (FTIR) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) indicated pre-chlorination significantly promoted the molecular transformation in MFC. The microbiome analysis of anode biofilm in MFC by scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM), metagenomics and metametabolomics revealed that pre-chlorination facilitated the development of biomass, enrichment of electricity-producing bacteria (EPB), enhancement of electricity-producing activity and metabolic activity. Moreover, the sludge EPS was the importance source for the microbial metabolites in MFC was validated by the joint analysis of FT-ICR-MS and metametabolomics.

Resource recovery from wastewater by directing microbial metabolism toward production of value-added biochemicals

Dynamic oxygen fluctuations in activated sludge were investigated to enhance valuable biochemical production during wastewater treatment. Batch experiments compared constant aeration with rapid cycling between oxygen-rich and oxygen-poor states. Fluctuating oxygen concentrations (0-2 mg/L) significantly increased production of valuable biochemicals compared to constant oxygen concentration (2 mg/L). Continuous oxygen perturbations increased free amino acids by 35.7 ± 7.6 % and free fatty acids by 76.4 ± 13.0 %, while intermittent perturbations with anoxic periods enhanced free amino acids by 42.4 ± 8.1 % and free fatty acids by 39.3 ± 7.7 %. Fourteen standard amino acids showed significant increases, and most fatty acids had carbon chain lengths between C12-C22. Mechanistically, oxygen perturbations activated FNR and ArcA regulons, resulting in lower relative abundances of TCA cycle enzymes and higher abundances of amino acid and fatty acid biosynthetic enzymes. These findings demonstrate that controlled oxygen fluctuations in wastewater treatment can enhance the biochemical value of activated sludge with minimal process modifications, facilitating resource recovery.

Circular Economy Applied to Sludge Minimization: The STAR Project

The management of biological sludge from wastewater treatment plants (WWTPs) poses a significant environmental challenge due to increasing sludge production and the presence of emerging pollutants. This study investigates an innovative solution by integrating a thermophilic aerobic membrane reactor (TAMR) into the sludge treatment line of a medium-size WWTP, aiming to minimize biological sludge output while enhancing resource recovery. The study involved a six-month monitoring of an industrial-scale TAMR system, assessing the reduction in volatile solids (VSs) in thickened sludge and evaluating the compatibility of TAMR residues with conventional activated sludge (CAS) systems. The TAMR unit, which achieved up to a 90% reduction in VSs, was combined with traditional CAS processes, forming the STAR (Sludge Treatment and Advanced Recycling) configuration. This configuration reduced sludge output to just 10% of conventional levels while enabling the recirculation of nutrient-rich liquid effluents. Both batch and continuous respirometric tests demonstrated the biological treatability of TAMR residues, highlighting their potential reuse as external carbon sources and their positive impact on CAS system performance. The findings suggest that integrating mesophilic and thermophilic systems can significantly improve sludge management efficiency, lowering both operating costs and environmental impacts.

Combination of a membrane bioreactor with a rotating biological contactor holding several diverse metazoans can reduce excess sludge with fouling mitigation

In a membrane bioreactor (MBR) system, in situ sludge reduction techniques induce membrane fouling. To address this challenge, we incorporated a rotating mesh carrier, which can adsorb organic matter and provide a habitat for metazoans, into the anoxic tank of a conventional anoxic/oxic-MBR (A/O-MBR) system, termed rotating biological contactor-MBR (RBC-MBR), and evaluated treatment performance. Over 151 days, lab-scale RBC-MBR and A/O-MBR were used to treat municipal sewage. Both reactors showed similar COD and NH4+ removal rates. However, RBC-MBR reduced excess sludge by approximately 45 % compared with A/O-MBR. Microscopic observation and 18S rRNA gene-based microbial analysis revealed the persistence of microfauna and metazoans (oligochaetes, nematodes, and rotifers) in RBC, which are typically absent in activated sludge. Additionally, the metazoan's population in the RBC-MBR membrane tank was two-fold that of A/O-MBR, indicating enhanced sludge reduction through predation. Despite these reductions, the increase in transmembrane pressure was similar between RBC-MBR and A/O-MBR, suggesting that sludge holding by RBC mesh media degrade fouling substances, such as proteins and polysaccharides and improves sludge filterability, resulting in membrane fouling mitigation. Microbial communities in both reactors were similar, indicating that the installation of RBC did not alter the microbial community of sludge. Network analysis suggested potential symbiotic or prey-predator relationships between bacteria and metazoans. This study reveals that RBC-MBR effectively reduced the excess sludge while mitigating membrane fouling, highlighting one of the promising technology for applying metazoan predation into MBR.

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.

Comprehensive study on pilot nitrification-sludge fermentation coupled denitrification system with extended sludge retention time

The sludge fermentation-coupled denitrification process, utilized for sludge reduction and nitrogen removal from wastewater, is frequently hindered by its hydrolysis step's efficacy. This study addresses this limitation by extending the sludge retention time (SRT) to 120 days. As a result, the nitrate removal efficiency (NRE) of the nitrification-sludge fermentation coupled denitrification (NSFD) pilot system increased from 67.1 ± 0.2 % to 96.7 ± 0.1 %, and the sludge reduction efficiency (SRE) rose from 40.2 ± 0.5 % to 62.2 ± 0.9 %. Longer SRT enhanced predation and energy dissipation, reducing intact cells from 99.2 % to 78.0 % and decreasing particle size from 135.2 ± 4.6 μm and 19.4 ± 2.1 μm to 64.5 ± 3.5 μm and 15.5 ± 1.6 μm, respectively. It also created different niches by altering the biofilm's adsorption capacity, with interactions between these niches driving improved performance. In conclusion, extending SRT optimized the microbial structure and enhanced the performance of the NSFD system.

Towards zero excess sludge discharge with built-in ozonation for wastewater biological treatment

In this study, a biological treatment process, which used a built-in ozonation bypass to achieve sludge reduction, was built to treat the industrial antifreeze production wastewater (mainly composed of ethylene glycol). The results indicated there is a positive correlation between ozone dosage and sludge reduction. At the laboratory level, the MLSS in the system can be stably controlled at around 3400 mg MLSS L-1 under the dosage of 0.18 g O3 g-1 MLSS. Ozonation can increase the compactness of sludge flocs (fractal dimension increased from 1.89 to 1.92). Ozone destroys microbial cell membranes and alters the structure of sludge flocs through direct oxidation through electrophilic reactions. It leads to the release of intracellular polysaccharides, proteins, and other biological macromolecules in microorganisms, thereby promoting the implicit growth of microbial populations. Some bacteria such as g_Pseudomonas, g_Gemmobacter, etc. have strong ethylene glycol degradation ability and tolerance to ozonation. The removal of ethylene glycol includes the glyoxylate cycle, glycine serine carbon cycle, and the glutamate-cysteine ligase pathway of assimilation. Gene KatG and gpx may be key factors in improving microbial tolerance to ozonation. The comprehensive evaluation from the perspectives of cost and carbon emission shows that choosing ozone cracking-implicit growth in wastewater treatment systems has significant cost advantages and application value.

In-situ sludge reduction based on Mn2+-catalytic ozonation conditioning: Feasibility study and microbial mechanisms

To improve the sludge conditioning efficiency without increasing the ozone dose, an in-situ sludge reduction process based on Mn2+-catalytic ozonation conditioning was proposed. Using ozone conditioning alone as a control, a lab-scale sequencing batch reactor coupled with ozonated sludge recycle was evaluated for its operating performance at an ozone dose of 75 mg O3/g VSS and 1.5 mmol/L Mn2+ addition. The results showed a 39.4% reduction in MLSS and an observed sludge yield of 0.236 kg MLSS/kg COD for the O3+Mn2+ group compared to the O3 group (15.3% and 0.292 kg MLSS/kg COD), accompanied by better COD, NH4+-N, TN and TP removal, improved effluent SS and limited impact on excess sludge properties. Subsequently, activity tests, BIOLOG ECO microplates and 16S rRNA sequencing were applied to elucidate the changing mechanisms of Mn2+-catalytic ozonation related to microbial action: (1) Dehydrogenase activity reached a higher peak. (2) Microbial utilization of total carbon sources had an elevated effect, up to approximately 18%, and metabolic levels of six carbon sources were also increased, especially for sugars and amino acids most pronounced. (3) The abundance of Defluviicoccus under the phylum Proteobacteria was enhanced to 12.0% and dominated in the sludge, they had strong hydrolytic activity and metabolic capacity. Denitrifying bacteria of the genus Ferruginibacter also showed an abundance of 7.6%, they contributed to the solubilization and reduction of sludge biomass. These results could guide researchers to further reduce ozonation conditioning costs, improve sludge management and provide theoretical support.

Adaptation mechanisms of microalgal-bacterial granular sludge to outdoor light-limited conditions

Microalgal-bacterial granular sludge (MBGS) has attached attention for sustainable wastewater treatment, but it remains elusive whether it can adapt to outdoor light-limited conditions. This paper investigated the biological adaptation mechanisms of MBGS to outdoor light-limited diel conditions using real municipal wastewater. The results indicated that MBGS still had excellent pollutants removal performance, and that both the extracellular polymeric substances and glycogen content of MBGS increased significantly. The main functional microalgae and bacteria were revealed to be Leptolyngbyaceae and Rhodanobacteria, respectively. Further analyses indicated that the abundance of genes encoding PsbA, PsbD, PsbE, PsbJ, PsbP, Psb27, Psb28-2, PsaC, PsaE, PsaL, PsbX, PetB, PetA, and PetE increased in photosystem. Meanwhile, the abundance of gene encoding Rubisco decreased but the gene abundance regarding to crassulacean acid metabolism cycle increased. These suggested that MBGS could adjust the photosynthetic pathway to ensure the completion of photosynthesis. This study is anticipated to add fundamental insights for the MBGS process operated under outdoor light-limited conditions.

Combination of sequencing batch reactor activated sludge process with sludge lysis using thermophilic bacterial community for minimizing excess sludge

Sludge reduction is a major challenge in biological wastewater treatment. Hydrolytic enzymes secreted by thermophilic bacteria can lyse sludge and thus achieve sludge reduction, and the indigenous thermophilic community in sludge can lyse sludge more effectively. In this study, the feasibility of combining a sludge lysis reactor based on thermophilic bacteria community (LTBC reactor, 75 °C) with a conventional sequencing batch activated sludge reactor (SBR) for sludge reduction (i.e., LTBC-SBR process) was systematically investigated first time. The effect of lysed sludge returning to the biochemical tank on pollutant removal efficiency, sludge flocculation, sludge settling, and microbial community and function of the LTBC-SBR process was studied. In the LTBC1-SBR process, a sludge growth rate of 0.71 g TSS/day was observed when the lysed sludge reflux ratio (LRR) was 1, and the sludge generation was reduced by 81.5% compared to the conventional SBR reactor. In the LTBC1-SBR process, the removal efficiencies of chemical oxygen demand and total nitrogen were 94.0% and 80.5%, respectively. There was no significant difference in the sludge volume index from the SBR to the LTBC1-SBR stage, however, the effluent suspended solids concentration increased from 35.2 ± 2.1 mg/L to 80.1 ± 5.3 mg/L. This was attributed to the reflux of sludge lysate. In addition, the changes in extracellular polymers content and composition resulted in poor sludge flocculation performance. Heterotrophic bacteria associated with Actinobacteria and Patescibacteria enriched in LTBC1-SBR with relative abundance of 28.51 ± 1.25% and 20.01 ± 1.21%, respectively, which decomposed the macromolecules in the refluxed lysed sludge and contributed to the sludge reduction. Furthermore, due to the inhibition of nitrite-oxidizing bacteria, the nitrite concentration in the effluent of the LTBC1-SBR system reached 4.7 ± 1.1 mg/L, and part of the denitrification process was achieved by short-cut nitrification and simultaneous denitrification. These results indicate that in-situ sludge reduction technology based on lyse sludge lysing by thermophilic community has considerable potential to be widely used in wastewater treatment.

Efficient dye removal from industrial wastewater using sustainable activated carbon and its polyamide nanocomposite derived from agricultural and industrial wastes in column systems

Sugar beet crown (SBC) waste was employed to produce sustainable activated carbon (AC) by a thermo-chemical activation procedure using a fixed ratio of H3PO4/SBC (1 : 1 w/w ratio) at 550 °C/2 h. An activated carbon/polyamide nano-composite (AC/PA) was also prepared through the polymerization of the fabricated AC (90%) with polyamide (PA, 10%) synthetic textile waste using a proper dissolving agent at a specified w/w ratio with the employed polymer (formic acid/PA = 82/18%). Both AC and its derivative AC/PA were employed in the remediation of dyes from industrial wastewater in column systems, and their efficiencies were compared at various applied experimental conditions. The adsorption of the industrial dye waste (IDW) was a pH-, flow rate-, and bed thickness-controlled process by the regarded adsorbents. Kinetic studies confirmed the suitability of the Thomas equation over the Yoon and Nelson model in predicting the dynamic adsorption process of IDW by AC and AC/PA as was assured by the close agreement among the calculated and experimental uptake capacities of both adsorbents at the same applied flow rates, suggesting the chemisorption nature of IDW adsorption. Additionally, electrostatic attraction was the leading mechanism of IDW adsorption by AC and AC/PA composite with some advantages of the former over the latter.

Valorization of thermally hydrolyzed sludge with clay for sintering of ceramic tiles

The disposal of wastewater sludge is one of the most challenging environmental problems for large cities. Wastewater sludge may be utilized as a feasible substitute for clay to sinter ceramics, given their similar mineralogical composition. However, the organics in sludge will be wasted, while their release during sintering will leave cracks in the ceramic products. In this research, after the thermal treatment for effective organic recovery, the thermally hydrolyzed sludge (THS) is incorporated with clay for the sintering of construction ceramics. The experimental results showed that a THS dosing ratio up to 40 % can be achieved for mixing with montmorillonite clay to make ceramic tiles. The sintered tiles (THS-40) had an intact shape and structure, and the tile performance was close to that made from single montmorillonite (THS-0), with water absorption of 0.4 % vs. 0.2 %, compressive strength of 136.8 vs. 140.7 MPa, and undetected heavy metal leaching. Further addition of THS would lead to a considerable deterioration of the quality of the tiles to a compressive strength of as low as 5.0 MPa for the THS only product (THS-100). Comparing with the tiles incorporated with raw sludge (RS-40), the THS-40 tiles had a more intact and denser structure with a 10 % improved compressive strength. Cristobalite, aluminum phosphate, mullite, and hematite dominated in the THS-born ceramics, which are typical components of ceramics, and the amount of hematite increased with the THS dosing ratio. Sintering at a high temperature of 1200 °C enabled efficient phase transformation from quartz to cristobalite and from muscovite to mullite, which ensured the toughness and compactness of the THS-born ceramic tiles.

Enhanced volatile fatty acid production from waste activated sludge by urea hydrogen peroxide: performance and mechanisms

Anaerobic acidogenesis of waste activated sludge (WAS) presents significant potential for resource recovery and waste treatment. However, the slow hydrolysis of WAS limits the efficiency of this approach. In this study, we applied urea hydrogen peroxide (UHP) pretreatment to enhance WAS hydrolysis and investigated the effects of operating parameters on volatile fatty acid (VFA) production and the associated mechanisms. Results demonstrated that UHP significantly improved WAS hydrolysis and VFA production, with a three-fold increase in soluble chemical oxygen demand (SCOD) compared to the control group. UHP dosage emerged as the most critical factor for VFA production, with the maximum VFA concentration increasing from 1127.6 to 8800.9 mg COD per L as UHP dosage ranged from 0 to 6 mmol g-1 VSS (Volatile suspended solids). At an optimal UHP dosage of 4 mmol g-1 VSS, both the unit oxidant promotion efficiency (ΔVFAs/ΔUHP) and the maximum VFA concentration reached relatively high levels, at 35.3 mg COD per mmol and 7527.3 mg COD per L, respectively. UHP pretreatment generated alkaline conditions, H2O2, ·OH and free ammonia, which collectively disrupted the extracellular polymeric substances (EPS) structure, transforming unextractable EPS into extractable forms and promoting the release of organic matter during both the pretreatment and fermentation stages. Excitation-emission matrix (EEM) analysis revealed that UHP increased the concentration of easily utilizable organic matter, providing more substrates for acidogenic bacteria and enhancing VFA production. Furthermore, weak alkaline conditions and high free ammonia concentrations in the UHP group facilitated VFA accumulation by preventing rapid acidification and suppressing methanogen activity. This study offers valuable insights into the potential of UHP pretreatment for enhancing WAS hydrolysis and VFA production, with promising applications in wastewater treatment and resource recovery.

Treatment of industrial ferric sludge through a facile acid-assisted hydrothermal reaction: Focusing on dry mass reduction and hydrochar recyclability performance

Large amounts of Fenton sludge and waste activated sludge (WAS) are mixed as ferric sludge (FS) in most industrial wastewater treatment plants. The treatment of such waste represents a challenge and quantity-dependent cost, so that a reliable way for FS waste reduction is required. In this study, we develop a facile acid-assisted hydrothermal treatment (HT) for the cost-efficient treatment of hazardous FS waste. Sulfuric acid was dosed at 0.25 mL/g dry solid (DS) to the HT process, which significantly increased the total solid mass reduction (TMR) by 25.1 % and dry mass reduction (DMR) by 104.4 %. The participation of sulfuric acid during the HT process changed the HT reaction pathway from dehydration to demethylation based on the analysis of the derivative thermogravimetric and Van Krevelen diagram. The addition of sulfuric acid improved the release of Fe from FS by 52.9 %, which contributed to the DMR. During the acid-assisted HT, Fe(III) was effectively reduced to Fe(II) within the produced hydrochar, which can be recycled for the Fenton reaction during the degradation of actual industrial wastewater such as pharmaceutical wastewater. Moreover, Sulfuric acid facilitated the generation of sulfonated hydrochar, which was efficient as an adsorbent for the complete removal of some metals such as Cu(II) - cation metal (98.8 %) and Cr(VI) - anion metal (99.9 %). This study firstly provides a novel and reliable approach for hazardous FS reduction and pointed out the recycling of hydrochar as the supplement for the Fenton reaction and adsorbents for some hazardous heavy metals.

Burkholderia vietnamiensis G4 as a biological agent in bioremediation processes of polycyclic aromatic hydrocarbons in sludge farms

Polycyclic aromatic hydrocarbons (PAHs) are one of the main pollutants generated by the refining and use of oil. To search bioremediation alternatives for these compounds, mainly in situ, considering the biotic and abiotic variables that affect the contaminated sites is determinant for the success of bioremediation techniques. In this study, bioremediation strategies were evaluated in situ, including biostimulation and bioaugmentation for 16 priority PAHs present in activated sludge farms. B. vietnamiensis G4 was used as a biodegradation agent for bioaugmentation tests. The analyses occurred for 12 months, and temperature and humidity were measured to verify the effects of these factors on the biodegradation. We used the technique GC-MS to evaluate and quantify the degradation of PAHs over the time of the experiment. Of the four treatments applied, bioaugmentation with quarterly application proved to be the best strategy, showing the degradation of compounds of high (34.4% annual average) and low (21.9% annual average) molecular weight. A high degradation rate for high molecular weight compounds demonstrates that this technique can be successfully applied in bioremediation of areas with compounds considered toxic and stable in nature, contributing to the mitigation of impacts generated by PAHs.

In-Situ Sludge Reduction Performance and Mechanism in Sulfidogenic Anoxic-Oxic-Anoxic Membrane Bioreactors

The excess sludge generated from the activated sludge process remains a big issue. Sustainable approaches that achieve in situ sludge reduction with satisfactory effluent quality deserve attention. This study explored the sludge reduction performance of sulfidogenic anoxic-oxic-anoxic (AOA) membrane bioreactors. The dynamics of the microbial community and metabolic pathways were further analyzed to elucidate the internal mechanism of sludge reduction. Compared with the conventional anoxic-oxic-oxic membrane bioreactor (MBR_control), AOA_S150 (150 mg/L SO₄^2- in the membrane tank) and AOA_S300 (300 mg/L SO₄^2- in the membrane tank) reduced biomass production by 40.39% and 47.45%, respectively. The sulfide reduced from sulfate could enhance the sludge decay rate and decrease sludge production. Extracellular polymeric substances (EPSs) destruction and aerobic lysis contributed to sludge reduction in AOA bioreactors. The relative abundance of Bacteroidetes (phylum), sulfate-reducing bacteria (SRB, genus), and Ignavibacterium (genus) increased in AOA bioreactors compared with MBR_control. Our metagenomic analysis indicated that the total enzyme-encoding genes involved in glycolysis, denitrification, and sulfate-reduction processes decreased over time in AOA_S300 and were lower in AOA_S300 than AOA_S150 at the final stage of operation. The excess accumulation of sulfide in AOA_S300 may inactive the functional bacteria, and sulfide inhibition induced sludge reduction.

Harnessing the microbiome to prevent global biodiversity loss

Global biodiversity loss and mass extinction of species are two of the most critical environmental issues the world is currently facing, resulting in the disruption of various ecosystems central to environmental functions and human health. Microbiome-targeted interventions, such as probiotics and microbiome transplants, are emerging as potential options to reverse deterioration of biodiversity and increase the resilience of wildlife and ecosystems. However, the implementation of these interventions is urgently needed. We summarize the current concepts, bottlenecks and ethical aspects encompassing the careful and responsible management of ecosystem resources using the microbiome (termed microbiome stewardship) to rehabilitate organisms and ecosystem functions. We propose a real-world application framework to guide environmental and wildlife probiotic applications. This framework details steps that must be taken in the upscaling process while weighing risks against the high toll of inaction. In doing so, we draw parallels with other aspects of contemporary science moving swiftly in the face of urgent global challenges.

In-Situ Sludge Reduction in Membrane-Controlled Anoxic-Oxic-Anoxic Bioreactor: Performance and Mechanism

Conventional and advanced biological wastewater treatment systems generate excess sludge, which causes socio-economic and environmental issues. This study investigated the performance of membrane-controlled anoxic-oxic-anoxic (AOA) bioreactors for in-situ sludge reduction compared to the conventional anoxic-oxic-oxic membrane bioreactor (MBR_control). The membrane units in the AOA bioreactors were operated as anoxic reactors at lower sludge recirculation rates to achieve hydrolysis of extracellular polymeric substances (EPS) and extensive endogenous respiration. Compared to MBR_control, the AOA bioreactors operated with 90%, and 80% recirculation rates reduced the sludge growth up to 19% and 30%, respectively. Protein-like components were enriched in AOA bioreactors while fulvic-like components were dominant in MBR_control. The growth of Dechloromonas and Zoogloea genra was promoted in AOA bioreactors and thus sludge reduction was facilitated. Metagenomics analysis uncovered that AOA bioreactors exhibited higher proportions of key genes encoding enzymes involved in the glycolysis and denitrification processes, which contributed to the utilization of carbon sources and nitrogen consumption and thus sludge reduction.

Analysis of the Effect of Sludge Reduction and Energy Production from Introducing Anaerobic Digest Plant into a Sewage Treatment Plant

As the amount of sewage sludge (SS) is increasing worldwide, anaerobic digesters (ADP) are being distributed to reduce it and treat it in an environmentally friendly way. In Korea, 20 years have passed since the introduction of ADP, but the number of sewage treatment plants (STP) installed with ADP is only about 10% of the total STP. Accordingly, problems and improvements were examined through material flow analysis targeting STP, and the effect of introducing anaerobic digestion (AD) was analyzed as a way to solve them. As a result of the analysis, the amount of SS generated by the STP (SS-ADP + Co-ADP) installed and operated by an AD was 0.54 kg/m3, and the facility without it was 0.77 kg/m3, showing a reduction effect of about 31%. The SS reduction effect of the facility that only used SS-AD was found to be reduced by 21–24% TOE (Ton Of Equivalent))/day, which has the effect of reducing 794,867 kg-CO2/day of greenhouse gas. In terms of energy efficiency, Co-AD was 86% higher than SS-AD, and biogas production yield was 90% higher than that of SS-AD. Therefore, it is necessary to introduce Co-AD into STP in terms of reducing SS, energy production, and greenhouse gas reduction.

Sustainable Sewage Sludge Management Technologies Selection Based on Techno-Economic-Environmental Criteria: Case Study of Croatia

The management and disposal of sewage sludge is becoming a growing concern at the global level. In the past, the main goal was to completely eliminate sewage sludge since it was deemed a threat to humans and the environment, but recently different possibilities for energy generation and material recovery are emerging. Existing technologies such as incineration or direct application in agriculture contribute to quantity reduction and nutrient recovery but are unable to fully exploit the potential of sewage sludge within the frameworks of circular economy and bioeconomy. This paper developed a model within the PROMETHEE method, which analyses technologies for the sustainable management of sewage sludge, which could make the most from it. For the empirical part of the study, the Republic of Croatia was used as a country in which sewage sludge is increasing in quantity as a result of recent upgrades and expansions in the wastewater system. Incineration, gasification, anaerobic digestion, and nutrient recovery were analyzed as treatment concepts for the increased amounts of sewage sludge. The model reveals that the best solution is the material recovery of sewage sludge, using the struvite production pathway through analysis of selected criteria.

Application of Integrated Fixed-Film Activated Sludge in a Conventional Wastewater Treatment Plant

It is often only at the operation stage of a wastewater treatment plant that there is a need to adjust the treatment process in terms of variable hydraulic capacity, increased pollutant load, high/low concentration of suspended biomass, or the unfavorable phenomenon of reduced sedimentation capacity of the activated sludge. One of the ways to improve the treatment process efficiency is to increase the biologically active surface by using bio-carriers in the form of fibers, materials, or bio-balls. This paper presents the results of a wastewater treatment plant operation during the period of six months after the implementation of the integrated fixed-film activated sludge (IFAS) technology. The research showed that microorganisms developed both in the activated sludge and on the fibers, positively influencing the activated sludge condition. During the start-up of the IFAS process, ciliates predominated over the other species. However, as oxygen content was high (2 mg/dm3 and more) and textile beds were used, the protozoan population developed intensively, and small metazoans became increasingly common. Throughout the research period, nitrifying and phosphorus-accumulating bacteria were observed both in the activated sludge and on the fibers. Between the 59th and 184th day of operation, numerous microorganisms were detected on the fibers and in the activated sludge, testifying to low biological oxygen demand, good aerobic conditions for nitrification, and long sludge age. However, the process seemed to break down after day 72, when the occurrence of metazoan led to reduced sludge production; after day 88, chemical oxygen demand and total suspended solids in the outflow increased, and oligochaetes and rotifers dominated the suspended sludge and fibers. Results also showed that the textile bed and low ammonia concentration became an excellent substrate for the development of Stentor sp. With regard to chemical and biological oxygen demand, total nitrogen- and total phosphorus-effluent concentrations were mostly within the legally permissible limits throughout the 184 days of operation.

Monitoring the Process of Anaerobic Digestion of Native and Microwave Pre-Treated Sludge by Dielectric and Rheological Measurements

The anaerobic digestion (AD) of wastewater sludge presents a promising and efficient method of wastewater and sludge treatment, as it can lower the organic content of sludge while producing a renewable source of energy—biogas—at the same time. However, using native, non-treated industrial wastewater sludge as a substrate for AD may present difficulties, hence we focused our research primarily on the applicability of measuring techniques—viscosity and dielectric measurement. One of our research goals was to investigate the presence of any connection between the aforementioned properties and the overall biogas production, in order to prove whether these measurement techniques were capable of monitoring the process of AD. Our other aim was to investigate how microwave (MW) pre-treatment affected the anaerobic fermentation. Our results revealed that in terms of total biogas yield, microwave irradiation could enhance fermentation by 14%, and also reduced the viscosity of the fermentation media by 13%. However, microwave irradiation did not change the overall tendencies of the AD process regarding these aspects. Moreover, it was discovered that dielectric and absolute viscosity measurements were effective methods for monitoring the fermentation. Additionally, a correlation was found between the accumulating biogas yield, the dielectric constant and loss factor, and the absolute viscosity of the digested media—each of these share a similarity in tendency and can indicate the occurrence of different phases during batch anaerobic fermentation.

Responses of Nitrogen Removal, Extracellular Polymeric Substances (EPSs), and Physicochemical Properties of Activated Sludge to Different Free Ammonia (FA) Concentrations

To investigate the effect of free ammonia (FA) on the nitrogen removal performance, extracellular polymeric substances (EPSs), and physicochemical properties of activated sludge, four laboratory-scale sequencing batch reactors (SBRs) were operated at FA concentrations of 0.5, 5, 10, and 15 mg/L (R0.5, R5, R10, and R15, respectively). Results showed that nitrogen removal and the production of EPSs and their components (including polysaccharides, proteins, and nucleic acid) significantly increased with the increased FA concentration from 0.5 to 10 mg/L; however, they decreased with a further increase in FA to 15 mg/L. Moreover, the capillary suction time (CST), specific resistance of filtration (SRF), and sludge volume index (SVI) decreased when FA concentration increased, indicating that better settleability and dewaterability of activated sludge was obtained. Additionally, a path diagram showed that Nitrosomonas was positively correlated, while Denitratisoma was negatively correlated with EPSs and their components. Thauera was positively correlated, while Zoogloea was negatively correlated with the settleability and de-waterability of activated sludge.

Effects of the metabolic uncoupler TCS on residual sludge treatment: Analyses of the microbial community and sludge dewaterability potential

Residual sludge is a by-product with a large volume and complex composition from wastewater treatment plants. It is significant to reduce sludge volume to decrease the negative effects of sludge on environmental pollution and needless land use. We investigated the effects of uncoupler 3, 3', 4', 5-tetrachlorosalicylanilide (TCS) on the properties of sludge. After adding 0.12 g TCS/g VSS with 24 h mixing, the sludge concentration and total ATP content decreased by 51.1% and 60.8%, respectively. At the same time, the microbial community also changed significantly, leading to the decrease of richness and diversity. Additionally, the secretion of extracellular polymeric substances (EPS) reduced approximately 43% under the addition of 0.12 g/g VSS compared with the control. The decrement of EPS may be explained by the decreased relative abundance of functional bacteria (i.e. Chloroflexi reduced about 60% and Nitrospirota reduced about 31%). Notably, the addition of TCS before coagulation conditioning (FeCl₃) promoted the adhesion of sludge flocs according to the theory of Extended Derjaguin Landau Verwey Overbee (XDLVO), leading to the increased hydrophobicity of the residual sludge. Therefore, energy uncoupling has the potential of improving sludge dewaterability.

Insight investigation of the on-site activated sludge reduction induced by metabolic uncoupler: Effects of 2,6-dichlorophenol on soluble microbial products, microbial activity, and environmental impact

Metabolic uncoupling technology was one of the methods widely used to on-site control the production of excess sludge in wastewater treatment processes. However, the uncoupler effects on soluble microbial products (SMP), microbial activity, and environment impact have few been reported. This study showed that sludge yield was reduced by 33.3% at 2,6-dichlorophenol (2,6-DCP) concentrations of 10 mg/L. The addition of 2,6-DCP also reduced the content of polysaccharide and protein in SMP, and the three-dimension excitation emission matrix (3D-EEM) suggested that the fluorescence intensities of humic acid-like, fulvic acid-like, and tryptophan protein-like substances decreased, proving that 2,6-DCP addition will weaken the interaction between microorganisms and the environmental matrix. Moreover, 2,6-DCP addition will change the microbial morphology and community of activated sludge. The active or respiring bacteria portion was lessened, and sludge flocs become dispersed, but it will not affect its settling performance. Surprisingly, 2,6-DCP has certain biodegradability and could be used as an environmentally friendly metabolic uncoupler under low-concentration dosing conditions. This study systematically evaluated the effect of 2,6-DCP on sludge production, SMP contents, microbial morphology, microbial community, demonstrating the environmental impact and application feasibility in the wastewater treatment systems.

Biomethane Production from Anaerobic Co-Digestion of Selected Organic Fraction of Municipal Solid Waste (OFMSW) with Sewage Sludge: Effect of the Inoculum to Substrate Ratio (ISR) and Mixture Composition on Process Performances

The aim of this study was to evaluate the effect of the inoculum to substrate ratio (ISR) and the mixture ratio between organic fraction of municipal solid waste (OFMSW) and sewage sludge (SS) on the methane production potential achievable from anaerobic co-digestion (AcoD). Biochemical Methane Potential (BMP) assays at mesophilic temperature were used to determine the best AcoD configuration for maximizing methane yield and production rate, as well as to address possible synergistic effects. The maximum methane yield was observed at ISR of 1 and 60% OFMSW: 40% SS as co-digestion mixture, whereas the highest methane production rate was achieved at ISR of 2 with the same mixture ratio (207 mL/gVS/d). Synergistic effects were highlighted in the mixtures having OFMSW below 60%, determining an increase of approximately 40% in methane production than the OFMSW and SS digestion as a sole substrate. The experimental data demonstrated that co-digestion of OFMSW and SS resulted in an increase in the productivity of methane than anaerobic digestion using the sole substrates, producing higher yields or production rates while depending on the ISR and the mixture ratio.

Recovery of polymeric substances from excess sludge: Surfactant-enhanced ultrasonic extraction and properties analysis

The recovery of polymeric substances from excess sludge is gaining significant research interest in future wastewater treatment technologies. We present a surfactant-enhanced ultrasonic method to extract mixed polymeric substances with typical functional groups from excess sludge. Four potential reasons were revealed for the higher efficiency upon ultrasonication with surfactant: low surface tension, damage of non-covalent bonds between extracellular polymeric substances and cells, enhanced dissolution of polymeric substances, and release of intracellular polymeric substances caused by cell lysis. The increase in extraction efficiency after the addition of cetyltrimethylammonium bromide and sodium dodecyl sulfate reached the maximum of 76.5% and 53.1%, respectively. The contents of polysaccharides, proteins, and DNA were approximately 50% of the total polymeric substances, and the content of protein was higher than that of polysaccharide; the concentration change of the surfactant had a minimal effect on these contents. For the polymeric substances extracted via ultrasonication with surfactant, the size was smaller than that for the non-surfactant extraction; moreover, the contents of metals decreased significantly (Al: 0.18% → 0%; Na: 0.15% → 0%; Ca: 0.24% → 0.11%), which was probably caused by the interaction between the surfactant and metal ions in the excess sludge. The surfactant had a negligible effect on the properties of polymeric substances, adsorption capacity of polymeric substances for heavy metal ions, and dewatering performance of sludge. The recycled polymeric substances may be used as a substitute for commercial adsorbents of heavy metal ions. Thus, the obtained results provide further insight into the recovery of polymeric substances from excess sludge via the surfactant-enhanced ultrasonic method.

Microplastics in Wastewater and Drinking Water Treatment Plants: Occurrence and Removal of Microfibres

Microplastics (MPs), and specifically microfibres (MPFs), are ubiquitous in water bodies, including wastewater and drinking water. In this work, a thorough literature review on the occurrence and removal of MPs, and specifically MPFs in WWTPs and DWTPs, has been carried out. When the water is treated, an average microfiber removal efficiency over 70% is achieved in WWTPs and DWTPs. These high percentages are still inefficient for avoiding the presence of a large number of microfibres in treated wastewater and also in tap water. RSF, DAF, oxidation ditch and CAS processes have been described as the most efficient treatments for eliminating MPFs from wastewater treatment. It is remarkable the wide range of the data reported on this topic; for example, treated wastewater contains between not detected and 347 MPFs/L, whereas tap water contains between not detected and 168 MPFs/L. Microfibres constitute more than half of the MPs found in treated wastewater and sewage sludge, whereas in DWTP effluents the percentage of MPFs is around 32%. Nevertheless, the relative amount of MPFs reported in tap water is notably higher (71%). Microfibres from WWTPs are discharged to the environment, being a source of MP pollution. Additionally, MPs released by DWTPs directly enter the drinking water lines, which constitute a direct route for MP human consumption, so that it has been estimated that an adult may ingest an average value of 7500 MPFs per year only via tap water. Thus, this review provides an update on the performance of WWTPs and DWTPs in removing MPs from water, which is an issue of great interest.

Dewaterability improvement and environmental risk mitigation of waste activated sludge using peroxymonosulfate activated by zero-valent metals: Fe0 vs. Al0

The stabilization and dewaterability of waste activated sludge (WAS) are essential factors for downstream disposal or reuse. Herein, two types of zero-valent metals, zero-valent iron (Fe⁰) and zero-valent aluminum (Al⁰), were compared for their ability to activate peroxymonosulfate (PMS) during the WAS conditioning process, with the effects of PMS activation by these two metals on WAS dewaterability and the potential environmental risks evaluated. Results showed that compared to Al⁰/PMS treatment, Fe⁰/PMS treatment achieved superior WAS dewaterability and reduced operational costs. Using PMS combined with Fe⁰ and Al⁰ treatments under optimal conditions, the water content (Wc) of dewatered sludge decreased to 55.7 ± 2.7 wt% and 59.4 ± 1.3 wt%, respectively. Meanwhile, application of the Fe⁰/PMS treatment system reduced the total annual cost by approximately 33.1%, compared to the Al⁰/PMS treatment. Analysis of the dewatering mechanism demonstrated that in the Fe⁰/PMS treatment, Fe³⁺/Fe²⁺ flocculation played an important role in the enhancement of WAS dewatering, while sulfate radical (SO₄^•-) oxidation was the dominant factor for WAS dewaterability improvement in Al⁰/PMS treatment. The greater enhancement of WAS dewaterability by Fe⁰/PMS treatment, was mainly attributed to more efficient reduction of hydrophilic extracellular polymeric substances (EPS) and an increase in surface charge neutralization. Environmental risk evaluation results indicated that Fe⁰/PMS and Al⁰/PMS treatments both effectively alleviated the environmental risks of heavy metals and faecal coliforms in dewatered sludge. Overall, this study proposes a novel perspective for the selection of an optimal PMS activator in sludge treatment.

Elemental sulfur as electron donor and/or acceptor: Mechanisms, applications and perspectives for biological water and wastewater treatment

Biochemical oxidation and reduction are the principle of biological water and wastewater treatment, in which electron donor and/or acceptor shall be provided. Elemental sulfur (S⁰) as a non-toxic and easily available material with low price, possesses both reductive and oxidative characteristics, suggesting that it is a suitable material for water and wastewater treatment. Recent advanced understanding of S⁰-respiring microorganisms and their metabolism further stimulated the development of S⁰-based technologies. As such, S⁰-based biotechnologies have emerged as cost-effective and attractive alternatives to conventional biological methods for water and wastewater treatment. For instance, S⁰-driven autotrophic denitrification substantially lower the operational cost for nitrogen removal from water and wastewater, compared to the conventional process with exogenous carbon source supplementation. The introduction of S⁰ can also avoid secondary pollution commonly caused by overdose of organic carbon. S⁰ reduction processes cost-effectively mineralize organic matter with low sludge production. Biological sulfide production using S⁰ as electron acceptor is also an attractive technology for metal-laden wastewater treatment, e.g. acid mine drainage. This paper outlines an overview of the fundamentals, characteristics and advances of the S⁰-based biotechnologies and highlights the functional S⁰-related microorganisms. In particular, the mechanisms of microorganisms accessing insoluble S⁰ and feasibility to improve S⁰ bio-utilization efficiency are critically discussed. Additionally, the research knowledge gaps, current process limitations, and required further developments are identified and discussed.

Reactivation of Frozen Stored Microalgal-Bacterial Granular Sludge under Aeration and Non-Aeration Conditions

In this paper, reactivation of microalgal-bacterial granular sludge (MBGS) stored at −20 °C for 6 months was investigated under respective aeration (R1) and non-aeration (R2) conditions. Results showed that the granular activity could be fully recovered within 21 days. The average removal efficiency of ammonia was higher in R1 (92.78%), while R2 showed higher average removal efficiencies of organics (84.97%) and phosphorus (85.28%). It was also found that eukaryotic microalgae growth was stimulated under aeration conditions, whereas prokaryotic microalgae growth and extracellular protein secretion were favored under non-aeration conditions. Sequencing results showed that the microbial community underwent subversive evolution, with Chlorophyta and Proteobacteria being dominant species under both conditions. Consequently, it was reasonable to conclude that the activity and structure of frozen stored MBGS could be recovered under both aeration and non-aeration conditions, of which aeration-free activation was more feasible on account of its energy-saving property. This study provides important information for the storage and transportation of MBGS in wastewater treatment.

Reduction of excess sludge production by membrane bioreactor coupled with anoxic side-stream reactors

While cleaning wastewater, biological wastewater treatment processes such as membrane bioreactors (MBR) produce a significant amount of sludge that requires costly management. In the oxic-settling-anoxic (OSA) process, sludge is retained for a temporary period in side-stream reactors with low oxygen and substrate, and then it is recirculated to the main reactor. In this way, excess sludge production is reduced. We studied the influence of the rate of sludge exchange between MBR and side-stream anoxic reactors on sludge yield reduction within MBR. Two MBRs, namely, MBR_OSA and MBR_control, each coupled with separate external anoxic side-stream reactors, were run in parallel for 350 days. Unlike MBR_control, MBR_OSA had sludge exchange with the external reactors connected to it. During the investigation over a sludge interchange rate (SIR) range of 0-22%, an SIR of 11% achieved the highest sludge reduction (58%). Greater volatile solids destruction i.e., bacterial cell lysis and extracellular polymeric substance (EPS) destruction occurred at the SIR of 11%, which helped to achieve the highest sludge reduction. The enhanced volatile solids destruction was evident by the release of nutrients in the external anoxic reactors. It was confirmed that the sludge yield reduction was achieved without compromising the wastewater treatment quality, sludge settleability and hydraulic performance of the membrane in MBR.

A quantitative analysis method to determine the amount of cellulose fibre in waste sludge

A novel quantitative analysis method for cellulose fibre was developed to understand its behaviour in biological wastewater treatment and waste sludge processes. The method developed in this study was designed using Pseudomonas aeruginosa to remove it by dissolving all the organic components except cellulose from the sludge due to needing the solubilisation of bacteria occupied almost of sludge matrix and quantifying the amount of remaining cellulose. The results of this study indicated that a combined treatment process that employed 2,000 U/L protease, 2 M hydrogen peroxide, and 2 mM potassium hydroxide after pre-treatment for floc dispersion with an ultrasonic treatment at 26 W for 1 min resulted in a solubilisation of 96% of P. aeruginosa without losing the cellulose fibre. When it was applied to the cellulose fibre added in the sludge from a municipal wastewater treatment facility, 99.5% of the cellulose fibre was recovered by using the high-speed centrifuge.

Simulating a combined lysis-cryptic and biological nitrogen removal system treating domestic wastewater at low C/N ratios using artificial neural network

In this study, a combined alkaline (ALK) and ultrasonication (ULS) sludge lysis-cryptic pretreatment and anoxic/oxic (AO) system (AO + ALK/ULS) was developed to enhance biological nitrogen removal (BNR) in domestic wastewater with a low carbon/nitrogen (C/N) ratio. A real-time control strategy for the AO + ALK/ULS system was designed to optimize the sludge lysate return ratio (R_SLR) under variable sludge concentrations and variations in the influent C/N (⩽ 5). A multi-layered backpropagation artificial neural network (BPANN) model with network topology of 1 input layer, 3 hidden layers, and 1 output layer, using the Levenberg-Marquardt algorithm, was developed and validated. Experimental and predicted data showed significant concurrence, verified with a high regression coefficient (R² = 0.9513) and accuracy of the BPANN. The BPANN model effectively captured the complex nonlinear relationships between the related input variables and effluent output in the combined lysis-cryptic + BNR system. The model could be used to support the real-time dynamic response and process optimization control to treat low C/N domestic wastewater.

The Enhancement of Energy Efficiency in a Wastewater Treatment Plant through Sustainable Biogas Use: Case Study from Poland

The improvement of energy efficiency ensuring high nutrients removal is a great concern for many wastewater treatment plants (WWTPs). The energy balance of a WWTP can be improved through the application of highly efficient digestion or its intensification, e.g., through the introduction of the co-substrates with relatively high energy potential to the sewage sludge (SS). In the present study, the overview of the energetic aspect of the Polish WWTPs was presented. The evaluation of energy consumption at individual stages of wastewater treatment along with the possibilities of its increasing was performed. Additionally, the influence of co-digestion process implementation on the energy efficiency of a selected WWTP in Poland was investigated. The evaluation was carried out for a WWTP located in Iława. Both energetic and treatment efficiency were analyzed. The energy balance evaluation of this WWTP was also performed. The obtained results indicated that the WWTP in Iława produced on average 2.54 GWh per year (7.63 GWh of electricity in total) as a result of the co-digestion of sewage sludge with poultry processing waste. A single cubic meter of co-substrates fed to the digesters yielded an average of 25.6 ± 4.3 Nm3 of biogas (between 18.3 and 32.2 Nm3/m3). This enabled covering the energy demand of the plant to a very high degree, ranging from 93.0% to 99.8% (98.2% on average). Importantly, in the presence of the co-substrate, the removal efficiency of organic compounds was enhanced from 64% (mono-digestion) to 69–70%.

The effect of microwave pretreatment on anaerobic co-digestion of sludge and food waste: Performance, kinetics and energy recovery

This paper studied the effect of microwave (MW) pretreatment on anaerobic co-digestion of sludge (SS) and food waste (FW). Using SS and FW as digestive substrates, the MW pretreatment method was used to determine the changes in the substrate matrix by means of batch anaerobic digestion at 37 °C. The kinetics of methane production were calculated, and the changes in organic matter during anaerobic co-digestion, the properties of the anaerobic-digested effluent, and the net energy output of the co-digestion system were determinated. The results showed that MW pretreatment was beneficial to the dissolution of organic matter, conversion of protein to NH₄⁺-N, cumulative methane production, unit biomethane yield, and reaction rate of methane production in the SS and FW anaerobic co-digestion system. The highest cumulative methane production in the co-digestion system reached 3446.3 ± 172.3 mL (35 days), which was 19.93% higher than that of the control. Furthermore, MW pretreatment significantly increased the accumulation of VFAs and the content of butyric acid in the anaerobic-digested effluent, which was beneficial to the methanogenesis process. The MW pretreatment of all co-digested substrates produced a greater net energy output than the control, and the MW-SS + MW-FW group yielded the highest net energy output, which was 76.25 kJ/g Fed VS. The results indicated that MW pretreatment prior to SS and FW anaerobic co-digestion is an effective way to improve the anaerobic digestion efficiency and energy recovery rate.

Bioelectrochemical system for the enhancement of methane production by anaerobic digestion of alkaline pretreated sludge

An increasing interest is devoted to combined microbial electrolysis cell-anaerobic digestion (MEC-AD) system which could convert waste activated sludge into biogas. In this study series tests were initially conducted to study the effect of alkaline pretreatment on AD system and the results showed that alkaline pretreatment could promote the dissolution of organic matters in the sludge and thus improve the methane production. Then, the methane production in combined MEC-AD system fed with alkaline-pretreated sludge was investigated. The results indicated that the methane productions increased by 37% and 42% when applied voltage was 0.5 V and 0.8 V. The microbial electrochemical system strongly promoted the growth of Euryarchaeota (Methanosaeta and Methanobacterium). Meanwhile, the abundance of Paraclostridium increased from 17.9% to 38.5% when applied voltage was 0.8 V, suggesting an enhanced fermentation and acetogenesis process. The results of energy balance estimation indicated that MEC-AD system at 0.5 V could achieve higher net energy output.

Ex-situ treatment of sediment from a black-odor water body using activated sludge

Sediment containing numerous nutrients and pollutants has become an important consideration when treating black-odor water. Excessive activated sludge produced in wastewater treatment plants contains a large number of microorganisms, which is beneficial for removing organics and nutrients from the black-odor sediment. In this study, three types of sludge from a secondary sedimentation tank (SST), a digestion tank (DT), and an aerobic tank treating landfill leachate (AT_leachate) were used to treat black-odor sediment, respectively. All the three types of activated sludge enhanced the treatment performance of sediment. The SST sludge worked the best with the optimal dosage of 2.56 g/(kg sediment), and the removal of nitrogen and organics reached 57.03 and 28.14%, respectively. Illumina MiSeq sequencing revealed that the activated sludge significantly affected the microbial community of the sediment. In particular, SST sludge resulted in significant increase in the number of microorganisms related to nitrification and sulfur metabolism to 10.68 and 10.97%, respectively. This was found to be important for degrading organics and promoting nitrogen removal. This study provides an efficient strategy for the treatment of black-odor sediment, and also realizes the complete utilization of waste activated sludge.

Revealing hydrodynamic effects on flocculation performance and surface properties of sludge by comparing aeration and stirring systems via computational fluid dynamics aided calculation

The effects of aeration and stirring systems on the physical properties of sludge were analyzed using a computational fluid dynamics (CFD) model. The aims of this study were to (1) compare the effects of aeration and stirring on sludge properties using the same turbulent mixing intensity, and (2) to reveal the relationship between sludge properties and hydrodynamic indicators to determine how hydrodynamic conditions influence sludge flocculation. Mixing experiments with stirring and aeration were carried out in 2-L beakers with the average velocity gradient (G) set to 90, 190, or 280 s^-1. The sludge flocculation performance, zeta potential, and Gibbs free energy (ΔG) were analyzed and the flow velocity, turbulence energy, turbulence dissipation rate, and Kolmogorov microscale were calculated as hydrodynamic parameters. The average flow velocity and the turbulence dissipation rate were obviously higher in the stirring system than in the aeration system at the same G. However, the turbulence energy and Kolmogorov microscale in the aeration system were much higher than those in the stirring system. Both the zeta potential and ΔG were lower in the aeration system than the stirring system. The zeta potential and ΔG results for the two systems suggest that aeration is more beneficial for sludge flocculation than stirring even though the sludge flocculation performance F/F₀ in the stirring and aeration systems showed no obvious differences. Significant relationships between hydrodynamic parameters calculated based on the CFD model and average values of sludge properties in the stable phase showed that the Kolmogorov microscale, average flow velocity, and turbulence energy were appropriate hydrodynamic parameters for evaluating the flocculation performance F/F₀, zeta potential, and ΔG, respectively.

Combination of the OSA process with thermal treatment at moderate temperature for excess sludge minimization

This study investigated the chance to couple the conventional Oxic Settling Anaerobic (OSA) process with a thermic treatment at moderate temperature (35 °C). The maximum excess sludge reduction rate (80%) was achieved when the plant was operated under 3 h of hydraulic retention time (HRT). Compared with the conventional OSA system, the thermic treatment enabled a further improvement in excess sludge minimization of 35%. The observed yield coefficient decreased from 0.25 gTSS gCOD^-1 to 0.10 gTSS gCOD^-1 when the temperature in the anaerobic reactor was increased to 35 °C, despite the lower HRT (3 h vs 6 h). Moreover, the thermic treatment enabled the decrease of filamentous bacteria, thereby improving the sludge settling properties. The thermic treatment enhanced the destruction of extracellular polymeric substances and the increase of endogenous decay rate (from 0.64 d^-1 to 1.16 d^-1) that reduced the biomass active fraction (from 22% to 4%).

Characterization of sludge reduction and bacterial community dynamics in a pilot-scale multi-stage digester system with prolonged sludge retention time

Sludge reduction performance and bacterial community dynamics in a pilot-scale multi-stage digester system with prolonged sludge retention time were characterized. Throughout the operation period of 281 days, the total loading sludge and the total digested sludge were 4700 and 3300 kg-MLSS. After 114 days of operation, the residual MLSS (RMLSS) in the reactors for sludge treatment was maintained at 18-25 kg-RMLSS m^-3, and the sludge reduction efficiency achieved 95% under the F/M ratio (kg-loading MLSS kg-RMLSS^-1) of less than 0.018. Also, among the sludge components, both fixed suspended solids and volatile suspended solids were reduced. Based on the sludge reduction performance and the RNA-based bacterial community characteristics, the combined action of the maintenance metabolism, lysis-cryptic growth, and particulate inorganic matter is proposed as the sludge reduction mechanism in the multi-stage sludge treatment process.

Formation of microbial products by activated sludge in the presence of a metabolic uncoupler o-chlorophenol in long-term operated sequencing batch reactors

Metabolic uncouplers are widely used for reducing excess sludge in biological wastewater treatment systems. However, the formation of microbial products, such as extracellular polymeric substances, polyhydroxyalkanoate and soluble microbial products by activated sludge in the presence of metabolic uncouplers remains unrevealed. In this study, the impacts of a metabolic uncoupler o-chlorophenol (oCP) on the reduction of activated sludge yield and formation of microbial products in laboratory-scale sequencing batch reactors (SBRs) were evaluated for a long-term operation. The results show the average reduction of sludge yield in the four reactors was 17.40%, 25.80%, 33.02% and 39.50%, respectively, when dosing 5, 10, 15, and 20 mg/L oCP. The oCP addition slightly reduced the pollutant removal efficiency and decreased the formation of soluble microbial products in the SBRs, but stimulated the productions of extracellular polymeric substances and polyhydroxyalkanoate in activated sludge. Furthermore, the significant reduction of electronic transport system activity occurred after the oCP addition. Microbial community analysis of the activated sludge indicates dosing oCP resulted in a decrease of sludge richness and diversity in the SBRs. Hopefully, this study would provide useful information for reducing sludge yield in biological wastewater treatment systems and behaviors of activated sludge in the presence of uncouplers.