Enhanced methane production from anaerobic digestion of waste activated sludge through preliminary pretreatment using calcium hypochlorite

Biogas production prediction model of food waste anaerobic digestion for energy optimization using mixup data augmentation-based global attention mechanism

Achieving rapid, efficient, and cost-effective anaerobic digestion (AD) of food waste is a key means to improve the efficiency of food waste treatment. However, in view of the shortage of historical anaerobic digestion data, the limitation of general neural networks in predicting biogas production, and its sensitivity to abnormal variation points, achieving accurate prediction of biogas production is not easy. This paper proposes a novel biogas production prediction model of food waste AD for energy optimization based on the mixup data augmentation integrating an improved global attention mechanism long short-term memory (LSTM). Taking the AD data of the actual factory as samples, the mixup data augmentation is introduced to generate virtual samples with the similar distribution as original samples. Then original samples and generated virtual samples are used as the input of the global attention mechanism LSTM to establish the food waste AD biogas production prediction model. Finally, the proposed method is applied in the biogas production prediction of actual food waste treatment plants. Compared with other industrial modeling models, the experimental results show that the proposed method has the highest prediction accuracy of 0.988, which performs well in predicting biogas production and can effectively guide and timely adjust feed configuration of AD plants.

Natural zeolite enhances anaerobic digestion of waste activated sludge: Insights into the performance and the role of biofilm

Anaerobic digestion is widely employed for the treatment of waste activated sludge (WAS) due to its advantages like simultaneous energy recovery and sludge stabilization, promoting carbon-neutral operation of wastewater treatment plants. Natural zeolite, a low-cost and eco-friendly additive, has the potential to improve methane production from anaerobic digestion. This study investigated the effects of natural zeolite on anaerobic digestion when the substrate was WAS. It was found that methane production potential in response to natural zeolite was dosage-dependent. The optimal dosage was 0.1 g zeolite/g volatile suspended solids (VSS), with a methane yield of 181.89 ± 6.75 mL/g VSS, which increased by 20.1% compared to that of the control. Although the methane yields with other dosages of natural zeolite were higher than that of control, they were lesser than that with 0.1 g zeolite/g VSS. Natural zeolite affected transfer and conversion of proteins much more than polysaccharides in liquid phase and extracellular polymeric substances. In anaerobic digestion, natural zeolite had with little effects on WAS solubilization, while it improved hydrolysis, acidification, and methanogenesis. The dosages of natural zeolite did have significant effects on bacterial communities in biofilm rather than suspension, while the archaeal communities in biofilm and suspension were all greatly related to natural zeolite dosages. The developed biofilms promoted richness and functionality of microbial communities. The syntrophic metabolism relationships between methanogens and bacteria were improved, which was proved by selective enrichment of Methanosarcina, Syntrophomonas, and Petrimonas. The findings of this work provided some new solutions for promoting methane production from WAS, and the roles of natural zeolite in anaerobic digestion.

Contribution identification of hydrolyzed products of potassium ferrate on promoting short-chain fatty acids production from waste activated sludge

Potassium ferrate (K2FeO4) has been extensively employed to promote short-chain fatty acids (SCFAs) production from anaerobic fermentation of waste activated sludge (WAS) because of its potent oxidizing property and formation of alkaline hydrolyzed products (potassium hydroxide, KOH and ferric hydroxide, Fe(OH)3). However, whether K2FeO4 actually works as dual functions of both an oxidizing agent and an alkalinity enhancer during the anaerobic fermentation process remains uncertain. This study aims to identify the contributions of hydrolyzed products of K2FeO4 on SCFAs production. The results showed that K2FeO4 did not execute dual functions of oxidization and alkalinity in promoting SCFAs production. The accumulation of SCFAs using K2FeO4 treatment (183 mg COD/g volatile suspended solids, VSS) was less than that using either KOH (192 mg COD/g VSS) or KOH & Fe(OH)3 (210 mg COD/g VSS). The mechanism analysis indicated that the synergistic effects caused by oxidization and alkalinity properties of K2FeO4 did not happen on solubilization, hydrolysis, and acidogenesis stages, and the inhibition effect caused by K2FeO4 on methanogenesis stage at the initial phase was more severe than that of its hydrolyzed products. It was also noted that the inhibition effects of K2FeO4 and its hydrolyzed products on the methanogenesis stage could be relieved during a longer sludge retention time, and the final methane yields using KOH or KOH & Fe(OH)3 treatment were higher than that using K2FeO4, further confirming that dual functions of K2FeO4 were not obtained. Therefore, K2FeO4 may not be an alternative strategy for enhancing the production of SCFAs from WAS compared to its alkaline hydrolyzed products. Regarding the strong oxidization property of K2FeO4, more attention could be turned to the fates of refractory organics in the anaerobic fermentation of WAS.

Mechanistic insights into Fe3O4-modified biochar relieving inhibition from erythromycin on anaerobic digestion

Anaerobic digestion (AD) of antibiotic manufacturing wastewater to degrade residual antibiotics and produce mixture of combustible gases has been investigated actively in the past decades. However, detrimental effect of residual antibiotic to microbial activities is commonly faced in AD process, leading to the reduction of treatment efficiency and energy recovery. Herein, the present study systematically evaluated the detoxification effect and mechanism of Fe3O4-modified biochar in AD of erythromycin manufacturing wastewater. Results showed that Fe3O4-modified biochar had stimulatory effect on AD at 0.5 g/L erythromycin existence. A maximum methane yield of 327.7 ± 8.0 mL/g COD was achieved at 3.0 g/L Fe3O4-modified biochar, leading to the increase of 55.7% compared to control group. Mechanistic investigation demonstrated that different levels of Fe3O4-modified biochar could improve methane yield via different metabolic pathways involved in specific bacteria and archaea. Low levels of Fe3O4-modified biochar (i.e., 0.5-1.0 g/L) led to the enrichment of Methanothermobacter sp., strengthening the hydrogenotrophic pathway. On the contrary, high levels of Fe3O4-modified biochar (2.0-3.0 g/L) favored the proliferation of acetogens (e.g., Lentimicrobium sp.) and methanogen (Methanosarcina sp.) and their syntrophic relations played vital role on the simulated AD performance at erythromycin stress. Additionally, the addition of Fe3O4-modified biochar significantly decreased the abundance of representative antibiotic resistant genes (ARGs), benefiting the reduction of environmental risk. The results of this study verified that the application of Fe3O4-modified biochar could be an efficient approach to detoxify erythromycin on AD system, which brings high impacts and positive implications for biological antibiotic wastewater treatment.

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.

Calcium hypochlorite-coupled aged refuse promotes hydrogen production from sludge anaerobic fermentation

This work investigated the effect of calcium hypochlorite (CH) coupled aged refuse (AR) treatment on the enhanced hydrogen generation from sludge anaerobic dark fermentation (SADF). The enhanced mechanism was systematically revealed through sludge disintegration, organic matter biotransformation, and microbial community characteristics, etc. The experimental data showed that CH coupled AR increased the hydrogen yield to 18.1 mL/g, significantly higher than that in the AR or CH group alone. Mechanistic analysis showed that CH-coupled AR significantly promoted sludge disintegration and hydrolysis processes, providing sufficient material for hydrogen-producing bacteria. Microbiological analysis showed that CH-coupled AR increased the relative abundance of responsible hydrogen-producing microorganisms. In addition, CH-coupled AR was very effective in reducing phosphate content in the fermentation liquid and fecal coliforms in the digestate, thus facilitating the subsequent treatment of fermentation broth and digestate. CH coupled AR is an alternative strategy to increase hydrogen production from sludge.

Successive choline addition enhancing the methanogenesis of waste activated sludge anaerobic digestion: Insight from hydrophilicity, electrochemical performance and microbial community

Anaerobic digestion (AD) is a promising technology to treat waste-activated sludge, previous study proved that methane production could be enhanced with the addition of choline, this work aimed to solve the problem of rapid biodegradability of choline in the AD process by changing its dosing method. With 0.75 g/L as the optimal choline dosing concentration, experimental results showed that successive choline dosing during the first 3-6 days of AD (experimental groups, EGs) performed better than the single dosing. The accumulative biogas production in EGs was increased by 35.55-36.73%, which could be caused by the simultaneous promotion of hydrolysis-acidification and methanogenesis processes. Especially, the electron exchange capacity of digested sludge in EGs was increased by 16.71-34.58%. In addition, the surface Gibbs free energy (△G_SL) of sludge in EGs was 105.51-172.21% higher (corresponding to stronger hydrophilicity and repulsion), which might help disperse sludge flocs and improve mass transfer efficiency, and the △G_SL values were positively correlated with the accumulative methane production (R² = 0.7029). Microbiological analysis showed that microbial communities in EGs were richer and Methanosaeta was regarded as the dominant species with 15.93-30.08% higher relative abundance with choline addition. According to Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis, EGs were found to be more active in metabolism clusters. Collectively, these findings demonstrated that successive choline dosing during the first 3-6 days is an effective and novel method to enhance methane production in AD process.

Ca(ClO)2 pretreatment enhancing suspended solids removal through flocculation from digested dairy wastewater and its mechanisms

Intensive animal farming produces large volume of digested liquid, and overdose application often causes the pollution of surface water and groundwater. Therefore, post-treatment is very necessary for the discharging of surplus digested liquid, but the removal of high concentrations of suspended solids (SS) in the digested liquid is a challenge. In this study, the effect of Ca(ClO)₂ pretreatment on SS flocculation removal of digested dairy wastewater was investigated. The results showed that, without Ca(ClO)₂ pretreatment, the flocculation by polyacrylamide (PAM), polyferric sulfate (PFS) or polymeric aluminum chloride (PAC) only removed 42.6 %-50.4 % SS from anaerobic digested liquid. With the combination of Ca(ClO)₂ pretreatment and PAC flocculation together, the SS removal efficiency can reach 80 %. The total chemical oxygen demand (TCOD) removal had a similar trend with SS removal, but soluble chemical oxygen demand (SCOD) removal was less affected by the pretreatment and flocculation. More than 75 % of orthophosphate (SRP) and total soluble phosphorus (TSP) was removed after Ca(ClO)₂ pretreatment and flocculation with PFS or PAC. Ca(ClO)₂ pretreatment also effectively inactivated fecal bacteria. The mechanisms of Ca(ClO)₂ pretreatment enhancing SS flocculation removal were further elucidated. The SS removal was the action of ClO^- and Ca²⁺ together. The function of ClO^- was to break down suspended particles, change the surface, and decrease the absolute Zeta potential, while the function of Ca²⁺ was to form precipitation. This result indicates that Ca(ClO)₂ pretreatment can effectively enhance the SS flocculation removal of anaerobic digested liquid.

Freezing method assists calcium hypochlorite for synergistically promoting methane production from sludge anaerobic digestion

Anaerobic digestion is widely considered to be a promising technology for waste activated sludge (WAS) treatment, by which sludge stabilization and resource recovery are simultaneously achieved. The poor reaction efficiency however hinders the large-scale applications of WAS anaerobic digestion technology. This study reported an efficient sludge pretreatment method by combining freezing with calcium hypochlorite (Ca(ClO)₂) for enhancing the anaerobic digestion efficiency. Experimental data showed that the optimal combination was freezing at -20 °C coupled with 0.075 g/g VSS (volatile suspended solids) Ca(ClO)₂, by which the maximum biomethane production of 274.4 ± 8.2 mL/g VSS was realized, 1.62 times higher than that of the control. Model-based analysis demonstrated that higher potential and rate for methane production were attained by the combined pretreatment. Mechanism analysis revealed that the extracellular polymeric substances (EPS) and microbial cells were both effectively destructed when treated by combined freezing and Ca(ClO)₂, and more dissolved organics were generated in consequence. Microbial analysis demonstrated that the co-treated reactor enriched more functional microbes (such as Methanosaeta, Methanosarcina and Candidatus_Methanofastidiosum) responsible for biomethane generation than that of the control. Furthermore, the number of fecal coliform was largely reduced in co-treated reactor. As the correlation between sludge anaerobic digestion performance and numerous pretreatment parameters was systematically revealed, this study can provide important references for engineers when applying the combined freezing and Ca(ClO)₂ technology in practical engineering.

Potassium permanganate pretreatment effectively improves methane production from anaerobic digestion of waste activated sludge: Reaction kinetics and mechanisms

As a powerful oxidizing agent, potassium permanganate (KMnO₄) has attracted widespread interest in sludge treatment and contaminant removal. However, its effect on the anaerobic digestion of waste activated sludge (WAS) is ambiguous. This investigation was designed to provide perspectives into this problem. In comparison with the control, 0.3 g KMnO₄/g TSS pretreatment enhanced the methane production by 78.82 %. Model analysis demonstrated that the KMnO₄ pretreatment enhanced the biochemical methane potential (B₀) of WAS. Mechanistic studies elucidated that the KMnO₄ pretreatment process generated reactive radicals such as ·OH, ·O₂^- and ¹O₂, which contributed to sludge disintegration and biodegradation process of dissolved substances, thus resulting in more substances available for subsequent methane generation. Enzyme activity analysis indicated that KMnO₄ pretreatment facilitated the activities of key enzymes associated with anaerobic digestion to various degrees. Microbial analysis illustrated that the relative abundance of functional microorganisms was significantly elevated after KMnO₄ pretreatment, which was conducive to methane production.

Anaerobic digestion of thickened waste activated sludge under calcium hypochlorite stress: Performance stability and microbial communities

Hypochlorite pretreatment has been proven effective in enhancing waste activated sludge (WAS) anaerobic digestion performances recently. In this study, two semi-continuous anaerobic sequencing batch reactors (ASBRs), one fed with Ca(ClO)₂ pretreated thickened WAS (TWAS) and one with raw TWAS, were operated at mesophilic conditions (35 °C) for 145 days. Three loading shocks were introduced to each reactor to compare the performance stability and resilience between the digestion of Ca(ClO)₂ pretreated TWAS and untreated TWAS. Microbial community shifts were quantified to reveal the microbiome responses to disturbances. The results suggested that 1% Ca(ClO)₂ enhanced the digestion of TWAS by inactivating and transforming the biomass to more easily digested substrates. Co-occurrence network analysis revealed that the strongest interactions in the microbial community occurred in the steady state of TWAS anaerobic digestion.

Enhanced biogas biological upgrading from kitchen wastewater by in-situ hydrogen supply through nano zero-valent iron corrosion

The in-situ hydrogen supply by nano zero-valent iron (nZVI, nFe⁰) corrosion provided a feasible way to improve the efficiency of biogas biological upgrading. This work studied the effects of nZVI at different dosages (0, 2, 4, 6, 8 and 10 g/L) on anaerobic digestion of kitchen wastewater by two buffer systems 2-[4-(2-hydroxyethyl) piperazin-1-yl] ethanesulfonic acid (HEPES) and sodium hydrogen carbonate (NaHCO₃). The addition of nZVI improved the content of methane (CH₄) and stability of anaerobic digestion process. In HEPES buffer system, the CH₄ was all increased and the maximum reached 90.51% with 10 g/L nZVI, higher than 32.25% compared to the control. The maximum hydrogen enrichment (HE) was 113 ppb after nZVI addition, indicating the mass transfer efficiency of hydrogen (H₂) was improved. Microbial community analysis showed that the total relative abundance of Methanobacterium and Methanolinea at 10 g/L nZVI was 53.72%, which was 1.62 times of the control group. However, in the NaHCO₃ buffer system with 10 g/L nZVI addition, the content of CH₄ and the loosely bound extracellular polymeric substances (LB-EPS) was lower than the control. The results indicated that the addition of nZVI was feasible for biogas upgrading, and the bidirectional effect of nZVI on the promotion or inhibition of bio-methanation might be related to the buffer system of the anaerobic process.

Enhanced methane yield through sludge two-phase anaerobic digestion process with the addition of calcium hypochlorite

This study investigated the effects of calcium hypochlorite (Ca(ClO)₂) on biomethane generation from sludge two-phase anaerobic digestion system. In first (acidogenic) phase, volatile fatty acids (VFAs) were largely generated when pretreated by Ca(ClO)₂, while the methane yield was severely inhibited. In second (methanogenic) phase, the methane yield was observably enhanced by Ca(ClO)₂. Further calculation showed that the total methane yield from the two phases was firstly promoted from 156.0 ± 4.5 to 269.9 ± 5.2 mL when Ca(ClO)₂ dosage enhanced from 0 to 1.6 g/L, which then reduced to 235.4 ± 5.5 mL when Ca(ClO)₂ content reached 2.0 g/L. Mechanism analysis showed that the suppression of Ca(ClO)₂ on coenzyme F₄₂₀ activity was relieved in methanogenic phase, and the abundances of functional microbes in methanogenic phase were enriched when added with Ca(ClO)₂. The Ca(ClO)₂-based method well realized the balance between efficacy and economy, possessing outstanding potential for large-scale applications.

Digested Sludge Quality in Mesophilic, Thermophilic and Temperature-Phased Anaerobic Digestion Systems

Anaerobic digestion (AD) technology is commonly used to treat sewage sludge from activated sludge systems, meanwhile alleviating the energy demand (and costs) for wastewater treatment. Most often, anaerobic digestion is run in single-stage systems under mesophilic conditions, as this temperature regime is considered to be more stable than the thermophilic one. However, it is known that thermophilic conditions are advantageous over mesophilic ones in terms of methane production and digestate hygienisation, while it is unclear which one is better concerning the digestate dewaterability. Temperature-phased anaerobic digestion (TPAD) is a double-stage AD process that combines the above-mentioned temperature regimes, by operating a thermophilic digester followed by a mesophilic one. The aim of this study is to compare the digestate quality of single-stage mesophilic and thermophilic AD and TPAD systems, in terms of the dewaterability, pathogenic safety and lower calorific value (LCV) and, based on the comparison, consider digested sludge final disposal alternatives. The research is conducted in lab-scale reactors treating waste-activated sludge. The dewaterability is tested by two methods, namely, centrifugation and mechanical pressing. The experimental results show that the TPAD system is the most beneficial in terms of organic matter degradation efficiency (32.4% against 27.2 for TAD and 26.0 for MAD), producing a digestate with a high dewaterability (8.1–9.8% worse than for TAD and 6.2–12.0% better than for MAD) and pathogenic safety (coliforms and Escherichia coli were not detected, and Clostridium perfringens were counted up to 4.8–4.9 × 103, when for TAD it was only 1.4–2.5 × 103, and for MAD it was 1.3–1.8 × 104), with the lowest LCV (19.2% against 15.4% and 15.8% under thermophilic and mesophilic conditions, respectively). Regarding the final disposal, the digested sludge after TAD can be applied directly in agriculture; after TPAD, it can be used as a fertilizer only in the case where the fermenter HRT assures the pathogenic safety. The MAD digestate is the best for being used as a fuel preserving a higher portion of organic matter, not transforming into biogas during AD.