Unveiling the dual faces of chitosan in anaerobic digestion of waste activated sludge

Natural flocculant chitosan inhibits short-chain fatty acid production in anaerobic fermentation of waste activated sludge

Chitosan (CTS) serves as an excellent natural flocculant in wastewater purification and sludge conditioning, but its potential impact on anaerobic fermentation of waste-activated sludge is unclear. The current study investigated the role of CTS in short-chain fatty acids (SCFAs) generation via sludge alkaline anaerobic fermentation. The results showed a drastic reduction in SCFA production with CTS, showing a maximum inhibition of 33 % at 6 mg/g of total suspended solids. CTS hindered sludge solubilization through flocculation, and acted as a humus precursor, promoting humus formation, and consequently reduced the amount of available substrates. Further, CTS promoted free ammonia production, posing a challenge to enzymes and cell viability. Additionally, CTS increased the population of Rikenellaceae sp. and weakened the dominance of hydrolyzing and acidifying bacteria. This study deepens the understanding of the potential impact of CTS on anaerobic fermentation and provides a theoretical basis for reducing the risk of polymeric flocculants.

Stormwater runoff treatment through electrocoagulation: antibiotic resistant bacteria removal and its transmission risks

Recently, increasing attention has been paid to antibiotic resistant bacteria (ARB) in stormwater runoff. However, there were little data on ARB removal through electrocoagulation (EC) treatment. In this study, batch experiments were conducted to investigate key designs for ARB removal, role of SS, effects of water matrix, and potential risks after EC treatment under the pre-determined conditions. EC treatment with 5 mA/cm2 of current density and 4 cm of inter-electrode distance was optimal with the highest ARB removal (3.04 log reduction for 30 min). The presence of SS significantly improved ARB removal during EC treatment, where ARB removal increased with the increase of SS levels when SS less than 300 mg/L. Large ARB removal was found under particles with size lower than 150 μm with low contribution (less than 10%) of the settlement without EC treatment, implying that the enhancement of ARB adsorption onto small particles could be one of the reasonable approaches for ARB removal through EC treatment. ARB removal increased firstly and then decreased with the increase of pH, while had proportional relationship with conductivity. After the optimal condition, there were weak conjugation transfer but high transformation frequency (5.5 × 10-2 for blaTEM) for target antibiotic resistance genes (ARGs), indicating that there could be still a risk of antibiotic resistance transformation after EC treatment. These suggested that the combination of EC and other technologies (like electrochemical disinfection) should be potential ways to control antibiotic resistance transmission through stormwater runoff.

Unveiling the mechanisms of Fe(III)-loaded chitosan composite (CTS-Fe) in enhancing anaerobic digestion of waste activated sludge

Anaerobic digestion (AD) of waste activated sludge (WAS) is usually limited by the low generation efficiency of methane. Fe(III)-loaded chitosan composite (CTS-Fe) have been reported to effectively enhanced the digestion of WAS, but its role in promoting anaerobic sludge digestion remains unclear. In present study, the effects of CTS-Fe on the hydrolysis and methanogenesis stages of WAS anaerobic digestion were investigated. The addition of CTS-Fe increased methane production potential by 8%-23% under the tested conditions with the addition of 5-20 g/L CTS-Fe. Besides, the results demonstrate that the addition of CTS-Fe could effectively promote the hydrolysis of WAS, evidenced by lower protein or polysaccharides concentration, higher soluble organic carbon in rector adding CTS-Fe, as well as the increased activity of extracellular hydrolase with higher CTS-Fe concentration. Meanwhile, the enrichment of Clostridia abundance (iron-reducing bacteria (IRBs)) was observed in CTS-Fe adding reactor (8.9%-13.8%), which was higher than that in the control reactor (7.9%). The observation further suggesting the acceleration of hydrolysis through dissimilatory iron reduction (DIR) process, thus providing abundant substrates for methanogenesis. However, the presence of CTS-Fe was inhibited the acetoclastic and hydrogenotrophic methanogenesis process, which could be ascribed to the Fe(III) act as electron acceptor coupled to methane for anaerobic oxidation. Furthermore, coenzyme F420 activity in the CTS-Fe added reactor was 34.9% lower than in the blank, also abundance of microorganisms involved in hydrogenotrophic methanogenesis was decreased. Results from this study could provide theoretical support for the practical applications of CTS-Fe.

Polydimethyldiallylammonium chloride inhibits dark fermentative hydrogen production from waste activated sludge

Polydimethyldiallylammonium chloride (PDDA) is an excellent flocculant for wastewater purification and sludge dewatering, but whether it poses a threat to hydrogen production from waste activated sludge is not known. In this study, the effect and underlying mechanism of PDDA on the dark fermentation of sludge was investigated. The results showed that PDDA reduced cumulative hydrogen production from 3.8±0.1 to 2.4±0.1 mL/g volatile suspended solids at 40 g/kg total suspended solids. PDDA impeded the dark fermentation process by inhibiting the activity of key enzymes, presenting a stronger inhibitory effect on the hydrogen production process than the hydrogen consumption process. Additionally, PDDA inhibited Firmicutes by enriching other microorganisms, thereby impeding hydrogen production via the acetate pathway. This study deepens the understanding of the potential effects of PDDA on sludge treatment and provides a theoretical basis for alleviating the negative effects of quaternary ammonium-based cationic flocculants.

Anaerobic co-digestion of sewage sludge pretreated by thermal hydrolysis and food waste: gas production, dewatering performance, and community structure

Anaerobic co-digestion can effectively break the limitations of mono-digestion. However, there are still some problems such as long residence time, unsatisfactory methane yield, and unstable performance for co-digestion of sewage sludge (SS) and food waste (FW). Therefore, the SS in the reactor treating co-digestion of SS and FW is considered to be pretreated by thermal hydrolysis. In this work, the anaerobic co-digestion of SS of thermal hydrolysis pretreatment (THP) and FW significantly improved the stability, methane production of the digestive reactor, and dewaterability of the digested sludge. The R6 obtained the most cumulative methane production (315.76 mL/g VS). In addition, compared to R3, the cumulative methane production and maximum methane production rate of R5 increased by 9.93% and 14.56%, respectively. The dewaterability of R4, R5, and R6 was improved, while the dewatering performance of the R3 decreased to a greater extent. The results of the kinetic model fitting were consistent with the experimental results. Among them, the hydrolysis constants (Kh) of anaerobic co-digestion of THP-SS and FW were 0.121, 0.130, and 0.114 d-1, respectively, which were higher than those of other groups. And the estimated lag time (λ) of co-digestion was also lower than that of mono-digestion groups. Microbial community analysis indicated that the bacterial diversity and richness of anaerobic co-digested groups of THP-SS and FW were enhanced, while the methanogens with acetoclastic pathway became the main methanogenic microorganisms. This work provides essential information on anaerobic co-digestion containing different THP-SS contents.

Distinct effects of chemical- and bio- flocculants on the sludge acidogenic fermentation for volatile fatty acids production by affecting the acidogenic steps, microbial community structure and metabolic functions

Flocculants play crucial roles in sludge treatment, while the specific impact of chemical and bio-flocculants on sludge anaerobic fermentation was unknown. This study unveiled the contrasting effects of chitosan (CTS) and poly‑aluminum chloride (PAC) on volatile fatty acids (VFAs) generation during sludge fermentation. CTS supplementation resulted in 17.2 % increase in VFAs production, while PAC exposure led to 7.6 % reduction compared to the control. Further investigation revealed that CTS facilitated sludge solubilization and hydrolysis, thus providing sufficient organic substrates for VFAs generation. Additionally, environmental-friendly CTS exposure positively influenced the abundance and activity of functional anaerobes, as well as the expression of genes associated with VFAs biosynthesis. In contrast, PAC exposure resulted in the formation of larger sludge flocs, which hindered WAS solubilization and hydrolysis. Meanwhile, its potential microbial toxicity also impeded the microbial metabolic activity (i.e., genetic expressions), resulting in unsatisfactory VFAs production.

Characterization of melanoidins in thermal hydrolysis sludge and effects on dewatering performance

Thermal hydrolysis pretreatment (THP) of sludge can form the refractory brown melanoidins due to the occurrence of the Maillard reaction, which adversely involves the subsequent sludge anaerobic digestion (AD) process. However, details of the generation pattern of melanoidins and how they affect the sludge dewaterability remain largely unknown. This work aims to determine an approach to characterize and quantify the melanoidins created by THP of sludge. On this basis, the effect of melanoidins on sludge dewatering performance was revealed by adding synthetic melanoidins to the mixed sludge. Experimental results showed that three-dimensional fluorescence-region integration (3DEEM-FRI) could effectively distinguish melanoidins from other organic substances and achieve semi-quantitative characterization in sludge. The melanoidins significantly deteriorated the sludge dewaterability, and the lowest solids content of the filter cake (TS) was only 17.78% at the addition of 480 mg (g TS)-1, which was a drop of about 20% compared to the control group. The mechanism investigations indicated that the internal structure of sludge becoming particularly complicated and the opportunities for molecules to collide with each other enlarged because of the contribution of melanoidins, resulting in the increment of the sludge apparent viscosity and consistency coefficient (k), a decline of the flow behavior index (n) and a weakening of flowability. Melanoidins could capture massive water molecules and carry negative charges with the decrease of sludge particle size and zeta potential value, which enhanced the electrostatic repulsion between sludge particles and abated the flocculation ability, thus further aggravating the sludge dewatering performance.

Enhanced dewaterability of sewage sludge by grafted cationic lignin-based flocculants

Lignin-based flocculants are widely used for wastewater purification, but their application in sludge dewatering has not yet been documented. In this study, a novel cationic lignin-based flocculant named LS-g-CPA was prepared by grafting cationic polyacrylamide (CPA) synthesized from methacryloyloxy ethyltrimethyl ammonium chloride (DMC) and acrylamide (AM) onto sodium lignosulfonate (LS), and its roles and underlying mechanisms in sludge conditioning were investigated. The results showed that LS-g-CPA effectively improved the dewaterability of sludge, reducing the filtration resistance and filter cake moisture content of sludge from 0.61 ± 0.05 × 1012 m/kg to 0.14 ± 0.02 × 1012 m/kg and 85.64 ± 0.25 % to 76.84 ± 0.41 %, respectively. The dewatering performance of LS-g-CPA was positively correlated with the DMC/AM ratio. The quaternary ammonium groups brought by DMC disrupted the reticular structure of extracellular polymeric substances, exposing hydrophobic residues and releasing bound water. Nevertheless, the key to LS-g-CPA for improving sludge dewatering lies more in the amphoteric flocculant properties that enhance sludge flocculation and the octopus-type structure that provides good drainage channels. This study reveals that lignin-based flocculants are effective in improving the dewaterability of sludge, which provides direct evidence for their application in sludge dewatering.

Dose-response and type-dependent effects of antiviral drugs in anaerobic digestion of waste-activated sludge for biogas production

In the context of the COVID-19 pandemic, antiviral drugs (AVDs) were heavily excreted into wastewater and subsequently enriched in sewage sludge due to their widespread use. The potential ecological risks of AVDs have attracted increasing attention, but information on the effects of AVDs on sludge anaerobic digestion (AD) is limited. In this study, two typical AVDs (lamivudine and ritonavir) were selected to investigate the responses of AD to AVDs by biochemical methane potential tests. The results indicated that the effects of AVDs on methane production from sludge AD were dose- and type-dependent. The increased ritonavir concentration (0.05-50 mg/kg TS) contributed to an 11.27-49.43% increase in methane production compared with the control. However, methane production was significantly decreased at high lamivudine doses (50 mg/kg TS). Correspondingly, bacteria related to acidification were affected when exposed to lamivudine and ritonavir. Acetoclastic and hydrotropic methanogens were inhibited at a high lamivudine dose, while ritonavir enriched methylotrophic and hydrotropic methanogens. Based on the analysis of intermediate metabolites, the inhibition of lamivudine and the promotion of ritonavir on acidification and methanation were confirmed. In addition, the existence of AVDs could affect sludge properties. Sludge solubilization was inhibited when exposed to lamivudine and enhanced by ritonavir, perhaps caused by their different structures and physicochemical properties. Moreover, lamivudine and ritonavir could be partially degraded by AD, but 50.2-68.8% of AVDs remained in digested sludge, implying environmental risks.

Responses of Bacillus sp. under Cu(II) stress in relation to extracellular polymeric substances and functional gene expression level

The production and composition of extracellular polymeric substances (EPS), as well as the EPS-related functional resistance genes and metabolic levels of Bacillus sp. under Cu(II) stress, were investigated. EPS production increased by 2.73 ± 0.29 times compared to the control when the strain was treated with 30 mg L^-1 Cu(II). Specifically, the polysaccharide (PS) content in EPS increased by 2.26 ± 0.28 g CDW^-1 and the PN/PS (protein/polysaccharide) ratio value increased by 3.18 ± 0.33 times under 30 mg L^-1 Cu(II) compared to the control. The increased EPS secretion and higher PN/PS ratio in EPS strengthened the cells' ability to resist the toxic effect of Cu(II). Differential expression of functional genes under Cu(II) stress was revealed by Gene Ontology pathway enrichment analysis and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis. The enriched genes were most obviously upregulated in the UMP biosynthesis pathway, the pyrimidine metabolism pathway, and the TCS metabolism pathway. This indicates an enhancement of EPS regulation-related metabolic levels and their role as a defense mechanism for cells to adapt to Cu(II) stress. Additionally, seven copper resistance genes were upregulated while three were downregulated. The upregulated genes were related to the heavy metal resistance, while downregulated genes were related to cell differentiation, indicating that the strain had initiated an obvious resistance to Cu(II) despite its severe cell toxicity. These results provided a basis for promoting EPS-regulated associated functional genes and the application of gene-regulated bacteria in heavy metal-containing wastewater treatment.

Microbial Behavior and Influencing Factors in the Anaerobic Digestion of Distiller: A Comprehensive Review

Anaerobic digestion technology is regarded as the most ideal technology for the treatment of a distiller in terms of environmental protection, resource utilization, and cost. However, there are some limitations to this process, the most prominent of which is microbial activity. The purpose of this paper is to provide a critical review of the microorganisms involved in the anaerobic digestion process of a distiller, with emphasis on the archaea community. The effects of operating parameters on microbial activity and process, such as pH, temperature, TAN, etc., are discussed. By understanding the activity of microorganisms, the anaerobic treatment technology of a distiller can be more mature. Aiming at the problem that anaerobic treatment of a distiller alone is not effective, the synergistic effect of different substrates is briefly discussed. In addition, the recent literature on the use of microorganisms to purify a distiller was collected in order to better purify the distiller and reduce harm. In the future, more studies are needed to elucidate the interactions between microorganisms and establish the mechanisms of microbial interactions in different environments.

Enhanced hydrogen production from sludge anaerobic fermentation by combined freezing and calcium hypochlorite pretreatment

A novel and high-efficiency sludge pretreatment method by combination of freezing and calcium hypochlorite (CH) for promoting the anaerobic fermentation performance was reported in this work. Experimental results indicated that a maximum biohydrogen production of 18.18 ± 0.43 mL/g volatile suspended solids (VSS) was realized by freezing (-5 °C) combined with CH (0.12 g/g VSS) pretreatment, which was 1.19, 4.05 and 11.36 times to that from the sole CH, sole freezing and control fermenters, respectively. Mechanism study showed that freezing + CH pretreatment efficiently disintegrated sludge flocs, producing abundant substrates for anaerobic fermentation. Model substances degradation experiment showed that the biochemical processes were all suppressed by freezing + CH method, but the suppressive degrees for hydrogen-consuming processes were greater than hydrogen-producing processes. 16S rRNA analysis revealed that the microbial community in freezing + CH treated reactor was more beneficial to hydrogen generation than that in the control, because the abundance of functional microbes was enriched from 6.81 % to 34.95 % by the co-treatment. Furthermore, sludge dewatering performance, including settleability, dewaterability and filterability, was enhanced by freezing + CH pretreatment.

Conductive materials as fantastic toolkits to stimulate direct interspecies electron transfer in anaerobic digestion: new insights into methanogenesis contribution, characterization technology, and downstream treatment

Direct interspecies electron transfer (DIET) stimulated by conductive materials (CMs) enables intercellular metabolic coupling that can address the unfavorable thermodynamical dilemma inherent in anaerobic digestion (AD). Although the DIET mechanism and stimulation have been extensively summarized, the methanogenesis contribution, characterization techniques, and downstream processes of CMs-led DIET in AD are surprisingly under-reviewed. Therefore, this review aimed to address these gaps. First, the contribution of CMs-led DIET to methanogenesis was re-evaluated by comparing the effect of various factors, including volatile fatty acids, free ammonia, and functional enzymes. It was revealed that AD systems are usually intricate and cannot allow the methanogenesis stimulation to be singularly attributed to the establishment of DIET. Additionally, considerable attention has been attached to the characterization of DIET occurrence, involving species identification, gene expression, electrical properties, cellular features, and syntrophic metabolism, suggesting the significance of accurate characterization methods for identifying the syntrophic metabolism interactions. Moreover, the type of CMs has a significant impact on AD downstream processes involving biogas purity, sludge dewaterability, and biosolids management. Finally, the central bottleneck consists in building a mathematical model of DIET to explain the mechanism of DIET in a deeper level from kinetics and thermodynamics.

Phase separated pretreatment strategies for enhanced waste activated sludge disintegration in anaerobic digestion: An outlook and recent trends

A significant ecological problem was developed on disposing the enormous amounts of waste activated sludge (WAS) produced by traditional wastewater treatment. There have been various attempts recently originated to develop innovative methods for substantial sludge treatment. The most frequently used approach for treating sludge to produces methane and reduces sludge is anaerobic treatment. The hydrolysis phase in WAS limits the breakdown of complex macrobiotic compounds. The presence of extracellular polymeric substances (EPS) in biomass prevents the substrate from being hydrolyzed. Enhancing substrate hydrolysis involves removal of EPS preceded by phase separated pretreatment. Hence, a critical assessment of various phase separated pretreatment that has a remarkable effect on the anaerobic digestion process was documented in detail. Moreover, the economic viability and energy requirement of this treatment process was also discussed. Perspectives and recommendations for methane production were also provided to attain effectual sludge management.

Yak rumen fluid inoculum increases biogas production from sheep manure substrate

Hydrolytic bacteria are essential for the degradation of lignocellulose to produce biogas and organic fertilizers. In this study, sheep manure was used as substrate, and sheep manure slurry, yak rumen fluid and slurry from a biogas reactor (SBR) were used as inocula in single-stage anaerobic digestion. The SBR and rumen fluid inocula increased biogas production by 23% and 43%, respectively, when compared to solely sheep manure in the single-stage anaerobic digestion. The two-stage anaerobic digestion, with yak rumen fluid as inoculum in the hydrolytic reactor, increased the biogas production by 59, 86, and 58% compared with the control. Microbial analysis of the effluent revealed that yak rumen fluid contained hydrolytic bacteria such as Proteiniphilum, Jeotgalibaca, Fermentimonas, and Atopostipes to enhance the degradation of sheep manure and increase biogas production. It was concluded that yak rumen fluid, rich in hydrolytic bacteria, increases the degradability of sheep manure and improves production of volatile fatt acids and biogas.

Uncovering the effect of polyethyleneimine on methane production in anaerobic digestion of sewage sludge

The potential effect of polyethyleneimine as a flocculant on anaerobic digestion of sludge was investigated. Polyethyleneimine above 12 g/kg total suspended solids inhibited the entire anaerobic digestion process including solubilization, hydrolysis, acidification, and methanogenesis. The addition of 24 g/kg total suspended solids polyethyleneimine reduced methane production from 167 ± 5 L/kg volatile suspended solids in the control reactor (without polyethyleneimine) to 141 ± 5 L/kg volatile suspended solids. Polyethyleneimine bound to extracellular polymeric substances, thus enhancing sludge agglomeration and hindering the release of organics. Meanwhile, the reduction of cytochrome C impeded electron transport, consequently curbed direct interspecies electron transfer. The adsorption of carbon dioxide by amine groups also hampered methane conversion. This study elucidated the concept that polyethyleneimine reduces mass transfer in anaerobic digestion, providing new insights into the potential behavior of flocculants in sludge treatment.

Synergistic effect of chitosan-based sludge aggregates CS@NGS inoculum accelerated the start-up of biofilm reactor treating aquaculture effluent: Insights into performance, microbial characteristics, and functional genes

The moving bed bioreactor (MBBR) process has drawn more attention as a promising biological wastewater treatment process. Nevertheless, achieving quick start-up and microbial biofilm formation remains a significant challenge. Consequently, the present study investigated a novel chitosan-based natural sludge (CS@NGS) seeding strategy for the accelerated start-up of MBBR. Three identical bioreactors were employed; the first bioreactor was without sludge seed as the control (BR1), the second was inoculated only with sludge (BR2), and the third was inoculated with CS@NGS according to the proposed seeding method (BR3). All bioreactors were utilised to treat simulated recirculating aquaculture systems (RAS) effluent. Resultantly, the CS@NGS shortened the start-up period from over twenty to seven days due to the enhanced initial microbial adhesion and biofilm formation. Under optimal conditions, the ammonium removal in BR3 approached 100%, which was relatively higher than BR2 (96.35 ± 1.12%) and BR1 (92.56 ± 2.17%). Moreover, a low nitrite accumulation was exhibited in the effluents, approximately ≤0.03 mg L^-1. The process performance correlated positively with core bacteria from the genera Nakamurella, Hyphomicrobium, Nitrospira, Paenarthrobacter, Rhodococcus, and Stenotrophobacter. The quantitative polymerase chain reaction (qPCR) results demonstrated that the CS@NGS enhanced the expressions of amoA, nxrB, nirK, nirS, narG, and napA nitrogen metabolism-related functional genes to varying degrees. The present study findings can assist the rapid start-up of aquaculture biofilters utilised to solve high nitrite and ammonia accumulation in recirculated water from industrial RAS.

Newly isolated lysozyme-producing strain Proteus mirabilis sp. SJ25 reduced the waste activated sludge: Performance and mechanism

To promote aerobic digestion of sludge, a lysozyme-producing strain was screened and identified as Proteus mirabilis sp. SJ25. The results of response surface methodology (RSM) showed that at the temperature of 30.8 °C, pH of 6.69, and the inoculum amount of 2.81%, the sludge reduced by 26.58%. Compared with the control group, the removal efficiency of suspended solids (SS) from sludge in the experimental group increased by 14.60%, the release of soluble chemical oxygen demand (SCOD) increased by 2.21 times, and the release of intracellular substances increased significantly. Actinobacteriota, Chloroflexi, Proteobacteria, Bacteroidota, and Firmicutes were the main phyla involved in the sludge reduction process. Strain SJ25 enhanced the degradation rate of sludge by releasing lysozyme lysis to lyse bacteria, enhancing the metabolism and membrane transport of carbohydrates and amino acids. This study provides a new perspective in the field of efficient degradation of waste sludge.