Potassium ferrate addition as an alternative pre-treatment to enhance short-chain fatty acids production from waste activated sludge

Study on short-chain fatty acids production from anaerobic fermentation of waste activated sludge pretreated by alkali-activated ammonium persulfate

As a sustainable method for carbon recovery from waste activated sludge (WAS), anaerobic fermentation to produce short-chain fatty acids (SCFAs) is often limited by disintegration of WAS. A novel pretreatment method of alkaline-activated ammonium persulfate (AP/Alk), employing an initial pH of 10 and an ammonium persulfate dosage of 1 mM/g VSS (mmol per gram volatile suspended solids), was proposed in this study to enhance disintegration of WAS and yield of SCFAs. It was compared with one control and five pretreatment groups including alkali, persulfate, free ammonia, ammonium persulfate, alkali-activated sodium persulfate to elucidate the synergistic effects of free ammonia and radicals in WAS dissolution and acidogenesis within the AP/Alk system. The highest sludge disintegration degree with 30.3 % and maximum SCFAs production with 295.4 mg COD/g VSS were achieved by using the method. Comparative analysis showed that free ammonia primarily disrupted microbial cells to release intracellular organics, while radicals preferentially degraded tightly bound extracellular polymeric substances (TB-EPS) proteins. The synergistic effects of free ammonia and radicals accelerated accumulation of soluble proteins and polysaccharides, improved selectively enrichment of hydrolytic-acidogenic genera (e.g., Macellibacteroides, Proteiniclasticum, Desulfobulbus), and upregulated antioxidant genes to alleviate oxidative stress, but suppressed SCFAs consumers (e.g., unclassified_f__Comamonadaceae) including methanogens (e.g., Methanosaeta), thereby promoting the accumulation of SCFAs and acetic acid proportion. AP/Alk offers a sustainable strategy for WAS utilization and energy recovery.

Poly (sodium acrylate-acrylamide) hydrogels for enrichment and purification of microalgal biomass in an open system: performance optimization and mechanistic analysis

Super adsorbent polymer gels can be utilized in microalgal culture systems to concentrate and harvest microalgal biomass through water absorption. In open microalgal culture systems, however, bacteria and other non-algal impurities may affect the water absorption efficiency of the hydrogels and the quality of harvested microalgae. This study prepared and tested hydrogels synthesized with varying sodium acrylate (SA) and acrylamide (AM) ratios in open systems to evaluate their biomass harvesting efficacy. Results showed that when WSA:WAM = 10:0, the chlorophyll a (Chl-a) concentration in the harvested microalgal biomass increased by 417.9 %, the Chl-a/VSS ratio increased by 3.7 %, and the concentration of extracellular polymeric substances (EPS) decreased by 9.5 % compared to the pre-harvest period. Additionally, the number of bacteria adsorbed in the hydrogel particles also significantly increased. It indicates that the poly (sodium acrylate-acrylamide) (PSA-AM) hydrogel absorbed both water and non-algal impurities, achieving both concentration and purification of microalgal biomass. Mechanistic analysis revealed that the pore size and ratio of the PSA-AM hydrogel acted as a sieve, separating microalgal cells from other substances such as water, EPS, and bacterial cells. Given that EPS and bacterial cells are more hydrophilic compared to microalgal cells, it may explain that the hydrogel particles absorbed water while also capturing EPS and bacterial cells. Moreover, the PSA-AM hydrogel exhibited superior reusability. In conclusion, this study provides valuable data and a theoretical basis for the application of PSA-AM hydrogel in open microalgal culture systems, which could further promote the purification of PSA-AM in microalgal biomass utilization by optimizing the preparation of hydrogels.

An integrated strategy for sequential nitrite removal and methane recovery: Sludge fermentation driven by nitrite reduction

Although the treatment of sludge with free nitrous acid can effectively recover short chain fatty acids, the feasibility of sequential nitrite reduction and methane recovery without acidic pH adjustment is still scarcely studied. Therefore, this study aimed to provide insights into the effect of nitrite at different levels on nitrite reduction and methane production. The results showed that the nitrite concentrations of 100, 200, 400 and 800 mg/L were completely reduced in 1, 2, 2 and 4 days, respectively. The nitrite reduction process stimulated the fermentation of sludge to produce more organic matters, which served as electron donors for denitrification. With the nitrite concentrations increasing from 200 to 800 mg/L methane production decreased from 128.7 to 0 mg/L at the digestion time of 15 d. The toxicity of nitrite to methanogenic microorganisms and the nitrite reduction process competing with methanogens for carbon sources may lead to the inhibition of methane production by excessive nitrite. Moreover, the methane production reached 184.4 mL with 100 mg/L nitrite reduction, which was increased by 83.2 % compared with that without nitrite addition (101.1 mL). Nitrite reduction stimulated hydrolysis without negatively impacting acetogenesis, thereby providing more substrates for subsequent methanogenesis. Model-based analysis indicated that nitrite reduction enhanced the maximum methane yield and methane production rate, aligning with the aforementioned analysis. 16S rRNA analysis unraveled that the bacterial abundance associated with hydrolysis increased. This anaerobic digestion technique driven by nitrite reduction is both environmentally and economically attractive for increasing methane production.

Enhancing concurrent production of volatile fatty acids and phosphorus minerals from waste activated sludge via magnesium ferrate pre-oxidation

This study investigated the strategy of magnesium ferrate (MF) pre-oxidation to enhance acidogenic fermentation of waste activated sludge (WAS), targeting the simultaneous production of volatile fatty acids (VFAs) and phosphorus-minerals (struvite and vivianite). Results showed that such fermentation produced a high-value liquid within four days, achieving a peak VFA content of 241 ± 4 mg COD/ g VSfeed, ammonia nitrogen levels below 350 mg/L of and PO43-P under 3 mg/L. Further investigation revealed that the MF pre-oxidation raised the pH, enhanced key hydrolases activity, enriched acidogens and iron-reducing bacteria for driving the concurrent production of VFAs and phosphorus-minerals. The MF pre-oxidation promoted VFAs and phosphorus-minerals formation by enhancing the cooperation among the hydrolyzing bacteria of Acinetobacter and Proteocatella, acidogens of Fusibacter and Tissierella_Soehngenia, and iron-reducing bacteria of Dechloromonas and Thauera. This study provided an effective strategy for realizing concurrent production of high-purity VFAs along with struvite and vivianite from WAS.

Alkalinity enhanced hydrolysis of primary sludge for carbon source recovery and its impact on denitrification in wastewater treatment

Primary sludge can serve as an internal carbon source for denitrification in wastewater treatment plants (WWTPs). This study explores the use of alkaline treatment to produce a fermentation broth from primary sludge, which predominantly contains short-chain volatile fatty acids (VFAs), with acetic acid and propionic acid making up over 65% of the total VFAs. The performance of this fermentation broth as a sole carbon source for denitrification was compared with that of sodium acetate, acetic acid, methanol, and ethanol in both biofilm and activated sludge systems. The results revealed that the denitrification rate achieved using the fermentation broth was as high as 2.1661 mg NO3--N/(g MLSS·h), which was slightly lower than that of sodium acetate and acetic acid but higher than that of methanol and ethanol. The fermentation broth demonstrated a high heterotrophic yield (0.7183), an equivalent specific carbon requirement for denitrification as acetic acid and sodium acetate, and a rapid denitrification start-up. Moreover, variations in the VFAs/SCOD ratios in the fermentation broth did not significantly impact the denitrification rate or substrate biodegradation rate. However, the yield coefficient and specific carbon requirement for denitrification were found to vary significantly depending on the carbon source used. This study concludes that with appropriate treatment, fermented broth from primary sludge can be an effective carbon source comparable to commercial external carbon sources, significantly reducing carbon emissions.

Microplastics in anaerobic digestion: occurrence, impact, and mitigation strategies

Microplastic pollution has emerged as a global environmental concern, with pervasive contamination in terrestrial and aquatic ecosystems. This review paper delves into the intricate dynamics of microplastics within anaerobic digestion systems, addressing their occurrence, impact, and potential mitigation strategies. The occurrence of microplastics in anaerobic digesters is widespread, entering these systems through diverse inputs, such as sewage sludge, organic waste, and etc. Microplastics in anaerobic digestion have been associated with potential adverse impacts on biogas production, process performance, microbial communities, and degradation processes, though the relationship is complex and context dependent. This review highlights the urgent need for comprehensive research into the fate of microplastics within anaerobic digesters. Mitigation strategies offer promise in alleviating microplastic contamination, with advanced separation methods, innovative techniques such as magnetic micro-submarines, photocatalytic micro-motors, membrane bioreactors combined with activated carbon filters, rapid sand filtration, or conventional activated sludge, and disintegration-oriented techniques such as electrocatalysis, biodegradation, and thermal decomposition. Nonetheless, there is a significant knowledge gap that necessitates further research into the fate and long-term effects of microplastics in digestate. Collaborative efforts are crucial to addressing this emerging concern and ensuring the sustainability of anaerobic digestion systems in the face of microplastic challenges.

Combination of magnetite and sodium percarbonate to enhance acetate-enriched short-chain fatty acids production during sludge anaerobic fermentation

Large amounts of waste activated sludge are generated daily worldwide, posing significant environmental challenges. Anaerobic fermentation is a promising method for sludge disposal, but it has two technical bottlenecks: the availability of short-chain fatty acids (SCFAs)-producing substrates and SCFAs consumption by methanogenesis. This study proposes a pretreatment strategy combining sodium percarbonate (SPC) and magnetite (Fe3O4) to address these issues. Under optimized conditions (20 mg Fe3O4/g TSS and 15 mg SPC/g TSS), SCFAs production increased to 3244.10 ± 216.31 mg COD/L, about 3.06 times the control (1057.29 ± 35.06 mg COD/L) and surpassing reported treatments. The combined pretreatment enhanced the disruption of extracellular polymeric substances, increased the release of biodegradable matters, improved acidogenesis enzyme activities, and inhibited methanogenesis. Additionally, it increased NH4+-N release in favor of the recovery of phosphorus from sludge residual. This study demonstrates an efficient pretreatment for high SCFAs production and resource recovery from WAS.

Temperature-dependent microbial mechanism and accumulation of volatile fatty acids in primary sludge pretreated with peroxymonosulfate

For revealing the influence of temperature on volatile fatty acids (VFAs) generation from primary sludge (PS) during the anaerobic fermentation process facilitated by peroxymonosulfate (PMS), five fermentation groups (15, 25, 35, 45, and 55 °C) were designed. The results indicated that the production of VFAs (5148 mg COD/L) and acetic acid (2019 mg COD/L) reached their peaks at 45 °C. High-throughput sequencing technology disclosed that Firmicutes, Proteobacteria, and Actinobacteria was the dominant phyla, carbohydrate metabolism and membrane transport were the most vigorous at 45 °C. Additionally, higher temperature and PMS exhibit synergistic effects in promoting VFAs accumulation. This study unveiled the mechanism of the effect of the pretreatment of PS with PMS on the VFAs production, which established a theoretical foundation for the production of VFAs.

Effect of hydraulic retention time on denitrification performance and microbial communities of solid-phase denitrifying reactors using polycaprolactone/corncob composite

This study investigated the effect of hydraulic retention time (HRT) on the denitrification performance and microbial composition of reactors, packed with composite polycaprolactone and corncob carbon sources, during the treatment mariculture wastewater. The optimal HRT was 3 h, and average nitrogen removal efficiency was 99.00 %, 99.07 %, and 98.98 % in the HRT =3, 5, and 7 h groups, respectively. However, the 3 h group (DOC 2.91 mg/L) was the only group with a lower DOC concentration than that of the influent group (3.31 mg/L). Moreover, species richness was lower at HRT =3 h, with a greater proportion of denitrification-dominant phyla, such as Proteobacteria. The abundance of the NarG, NirK, and NirS functional genes suggested that the HRT =3 h group had a significant advantage in the nitrate and nitrite reduction phases. Under a short HRT, the composite carbon source achieved a good denitrification effect.

Enhanced short-chain fatty acid production from sludge anaerobic fermentation by combined pretreatment with sodium pyrophosphate and thermal hydrolysis

The slow breakdown of sludge is the primary obstacle hindering the conversion of waste-activated sludge to short-chain fatty acids (SCFAs) by anaerobic fermentation. This study proposed a novel method incorporating sodium pyrophosphate and thermal hydrolysis (SP-TH) for sludge pretreatment and evaluated its effectiveness regarding SCFA production. The combined pretreatment of SP at 0.4 g/g of total suspended solids and TH at 140 °C enhanced SCFA production from 2,169 ± 208 to 4,388 ± 184 mg chemical oxygen demand/L. SP strips extracellular polymeric substances, and the subsequent TH decomposes cells in the sludge, thus promoting sludge hydrolysis. SP-TH pretreatment promoted SCFA accumulation by enhancing enzyme activity and enriching acidifying bacteria. This study demonstrated that SP-TH pretreatment can effectively promote acid production from sludge, providing a new avenue for organic matter recovery through sludge anaerobic fermentation pretreatment.

Application of calcium peroxide in promoting resource recovery from municipal sludge: A review

The harmless disposal, resource recovery, and synergistic efficiency reduction of municipal sludge have been the research focuses for the last few years. Calcium peroxide (CaO2) is a multifunctional and safe peroxide that produces an alkaline oxidation environment to promote the fermentation of municipal sludge to produce hydrogen (H2) and volatile fatty acids (VFAs), thus realizing sludge resource recovery. This review outlines the research achievements of CaO2 in sludge resource recovery, improvement of sludge dewaterability, and removal of pollutants from sludge in recent years. Meanwhile, the mechanism of CaO2 and its influencing factors have also been comprehensively summarized. Finally, the future development direction of the application of CaO2 in municipal sludge is prospected. This review would provide theoretical reference for the potential engineering applications of CaO2 in improving sludge treatment in the future.

Deep groundwater irrigation altered microbial community and increased anammox and methane oxidation in paddy wetlands of Sanjiang Plain, China

The over-utilizing of nitrogen fertilizers in paddy wetlands potentially threatens to the surrounding waterbody, and a deep understanding of the community and function of microorganisms is crucial for paddy non-point source pollution control. In this study, top soil samples (0-15 cm) of paddy wetlands under groundwater's irrigation at different depths (H1: 6.8 m, H2: 13.7 m, H3: 14.8 m, H4: 15.6 m, H5: 17.0 m, and H6: 17.8 m) were collected to investigate microbial community and function differences and their interrelation with soil properties. Results suggested some soil factor differences for groundwater's irrigation at different depths. Deep-groundwater's irrigation (H2-H6) was beneficial to the accumulation of various electron acceptors. Nitrifying-bacteria Ellin6067 had high abundance under deep groundwater irrigation, which was consistent with its diverse metabolic capacity. Meanwhile, denitrifying bacteria had diverse distribution patterns. Iron-reducing bacteria Geobacter was abundant in H1, and Anaeromyxobacter was abundant under deep groundwater irrigation; both species could participate in Fe-anammox. Furthermore, Geobacter could perform dissimilatory nitrate reduction to ammonia using divalent iron and provide substrate supply for anammox. Intrasporangium and norank_f_Gemmatimonadacea had good chromium- and vanadium-reducting potentials and could promote the occurrence of anammox. Low abundances of methanotrophs Methylocystis and norank_f_Methyloligellaceae were associated with the relatively anoxic environment of paddy wetlands, and the presence of aerobic methane oxidation was favorable for in-situ methane abatement. Moisture, pH, and TP had crucial effects on microbial community under phylum- and genus-levels. Microorganisms under shallow groundwater irrigation were highly sensitive to environmental changes, and Fe-anammox, nitrification, and methane oxidation were favorable under deep groundwater irrigation. This study highlights the importance of comprehensively revealing the microbial community and function of paddy wetlands under groundwater's irrigation and reveals the underlying function of indigenous microorganisms in agricultural non-point pollution control and greenhouse gas abatement.

Comprehending the impact of berberine on anaerobic digestion of waste activated sludge

Berberine is a natural isoquinoline alkaloid performing wide-spectrum antimicrobial and antiviral effects like antibiotics. Its production generates berberine containing wastewater, and berberine adsorbed on waste activated sludge (WAS) will unavoidably enter the anaerobic digestion (AD) system while its impact on the AD process is unknown. Our research found that berberine of 20 mg/L (BBR20) slightly enhanced the methane yield (4.2 ± 0.6%) under mesophilic condition (35.0 ± 1.0 °C). However, 100 and 500 mg/L (BBR100 and BBR500) depressed methane production by 17.3 ± 4.3% and 83.2 ± 0.4%; meanwhile more soluble chemical oxygen demand (SCOD) including volatile fatty acid (VFA), protein, and polysaccharide were left in the fermentation broth, which led to an increase in sludge reduction. 88.3 ± 0.09%-99.1 ± 0.04% of berberine was distributed in the sludge phase and could be efficiently removed even under a high berberine level of 500 mg/L during the AD process. Exposure to different berberine concentrations promoted sludge dissolution and triggered more sludge extracellular polymeric substances (EPS) being dissolved. Lower berberine concentration (20 mg/L) enhanced acidification and methanogenesis steps, resulting in a final methane generation increase. While hydrolysis, acidification and methanogenesis processes were all inhibited by 100 and 500 mg/L berberine. Microbial analysis revealed that the main acid-producing bacteria genera were changed as Bacteroidetes vadinHA17 dominated in control, BBR20 and BBR100 groups, was replaced by Petrimonas in BBR500. Additionally, Methanosaeta, as a strict acetoclastic methanogen, was suppressed under exposure to 100 and 500 mg/L berberine. Accordingly, the declined abundance of archaea genera consuming acetic acid caused more VFA accumulation and less methane production in BBR100 and BBR500 groups.

Optimization of biogas production from straw wastes by different pretreatments: Progress, challenges, and prospects

Lignocellulosic biomass (LCB) presents a promising feedstock for carbon management due to enormous potential for achieving carbon neutrality and delivering substantial environmental and economic benefit. Bioenergy derived from LCB accounts for about 10.3 % of the global total energy supply. The generation of bioenergy through anaerobic digestion (AD) in combination with carbon capture and storage, particularly for methane production, provides a cost-effective solution to mitigate greenhouse gas emissions, while concurrently facilitating bioenergy production and the recovery of high-value products during LCB conversion. However, the inherent recalcitrant polymer crystal structure of lignocellulose impedes the accessibility of anaerobic bacteria, necessitating lignocellulosic residue pretreatment before AD or microbial chain elongation. This paper seeks to explore recent advances in pretreatment methods for LCB biogas production, including pulsed electric field (PEF), electron beam irradiation (EBI), freezing-thawing pretreatment, microaerobic pretreatment, and nanomaterials-based pretreatment, and provide a comprehensive overview of the performance, benefits, and drawbacks of the traditional and improved treatment methods. In particular, physical-chemical pretreatment emerges as a flexible and effective option for methane production from straw wastes. The burgeoning field of nanomaterials has provoked progress in the development of artificial enzyme mimetics and enzyme immobilization techniques, compensating for the intrinsic defect of natural enzyme. However, various complex factors, such as economic effectiveness, environmental impact, and operational feasibility, influence the implementation of LCB pretreatment processes. Techno-economic analysis (TEA), life cycle assessment (LCA), and artificial intelligence technologies provide efficient means for evaluating and selecting pretreatment methods. This paper addresses current issues and development priorities for the achievement of the appropriate and sustainable utilization of LCB in light of evolving economic and environmentally friendly social development demands, thereby providing theoretical basis and technical guidance for improving LCB biogas production of AD systems.

Percarbonate-strengthened ferrate pretreatment for enhancing short-chain fatty acids production from sewage sludge

Sewage sludge management poses a pressing environmental challenge, demanding the implementation of sustainable solutions to facilitate resource recovery. Short-chain fatty acids (SCFAs) serve as valuable chemicals and renewable energy sources, underscoring the importance of maximizing their production to achieve sustainable waste management. Therefore, this study proposes a novel and green strategy, i.e., percarbonate-strengthened ferrate pretreatment to enhance SCFAs synthesis from sewage sludge, because percarbonate could activate ferrate oxidation through providing (bi) carbonate and hydrogen peroxide. Results show that percarbonate largely reduces the required ferrate dosage for fermentation improvement, and their combination exhibits obvious synergistic effects on SCFAs accumulation and sludge reduction. Under the optimal pretreatment conditions, SCFAs production is promoted to 3670.2 mg COD/L, representing a remarkable increase of 5512.4 %, 156.0 % or 395.1 % compared to the control, percarbonate alone or ferrate alone, respectively. Mechanism explorations demonstrate that percarbonate-strengthened ferrate pretreatment significantly enhances sludge solubilization, elevates substrate biodegradability, and alters the physiochemical properties of sludge to favor organics fermentation. The synergistic effects on solid organics release and sludge properties can be attributed to the combined mechanisms of enhanced oxidation and alkaline hydrolysis. Further investigations on metabolic pathways reveal that the combination substantially improves key enzyme activities associated with hydrolysis and SCFAs formation, while severely inhibits that of SCFAs consumption. These findings are further supported by the functional genes coding relevant enzymes. Moreover, the combination alters microbial structures and compositions, leading to the screening and enrichment of key microbes that facilitate SCFAs accumulation. This innovative strategy holds significant promise in advancing sewage sludge management towards a more circular and resource-efficient paradigm.

Impact analysis of operating conditions on carbon dioxide reduction in microbial electrosynthesis: Insight into the substance utilization and microbial response

Microbial electrosynthesis (MES) is facing a series of problems including low energy utilization and production efficiency of high value-added products, which seriously hinder its practical application. In this study, a more practical direct current power source was used and the anaerobic activated sludge from wastewater treatment plants was inoculated to construct the acetic acid-producing MES. The operating conditions of acetic acid production were further optimized and the specific mechanisms involving the substance utilization and microbial response were revealed. The optimum conditions were the potential of 3.0 V and pH 6.0. Under these conditions, highly electroactive biofilms formed and all kinds of substances were effectively utilized. In addition, dominant bacteria (Acetobacterium, Desulfovibrio, Sulfuricurvum, Sulfurospirillum, and Fusibacter) had high abundances. Under optimal conditions, acetic acid-forming characteristic genera (Acetobacterium) had the highest relative abundance (Biocathode-25.82 % and Suspension-17.24 %). This study provided references for the optimal operating conditions of MES and revealed the corresponding mechanisms.

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.

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.

Insights into the roles and mechanisms of a green-prepared magnetic biochar in anaerobic digestion of waste activated sludge

Methane is one of the most promising renewable energies to alleviate energy crisis or replace fossil fuels, which can be recovered from anaerobic digestion of bio-wastes. However, the engineering application of anaerobic digestion is always hindered by low methane yield and production rate. This study revealed the roles and mechanisms of a green-prepared magnetic biochar (MBC) in promoting methane production performance from waste activated sludge. Results showed that the methane yield reached 208.7 mL/g volatile suspended solids with MBC additive dosage of 1 g/L, increasing by 22.1 % compared to that in control. Mechanism analysis demonstrated that MBC could promote hydrolysis, acidification, and methanogenesis stages. This was because the properties of biochar were upgraded by loading nano-magnetite, such as specific surface area, surface active sites, and surface functional groups, which made MBC have greater potential to mediate electron transfer. Correspondingly, the activity of α-glucosidase and protease respectively increased by 41.7 % and 50.0 %, and then the hydrolysis performances of polysaccharides and proteins were improved. Also, MBC improved the secretion of electroactive substances like humic substances and cytochrome C, which could promote extracellular electron transfer. Furthermore, Clostridium and Methanosarcina, as well-known electroactive microbes, were selectively enriched. The direct interspecies electron transfer between them was established via MBC. This study provided some scientific evidences to comprehensively understand the roles of MBC in anaerobic digestion, with important implications for achieving resource recovery and sludge stabilization.

Insights into response mechanism of anaerobic digestion of waste activated sludge to particle sizes of zeolite

Anaerobic digestion has been proved as one promising strategy to simultaneously achieve resource recovery and environmental pollution control for biosolid treatment, and adding exogenous materials is a potential alternative to promote the above process. This study investigated response mechanisms of anaerobic digestion of waste activated sludge (WAS) to particle sizes of zeolite. Results showed that the methane production reached 186.75 ± 7.62 mL/g volatile suspended solids (VSS) with zeolite of the particle size of 0.2-0.5 mm and the additive dosage of 0.1 g/g VSS, which increased by 22% compared to that in control. Mechanism study revealed that zeolite could improve hydrolysis, acidification, and methanogenesis stages. Rapid consumption rates of soluble polysaccharides and proteins were observed, correspondingly, the accumulation of SCFAs were enhanced, and the compositions of SCFAs were optimized. Moreover, the activities of F420 increased by 28% with zeolite, and the syntrophic metabolism between bacteria and methanogens were promoted.

Role of potassium ferrate in anaerobic digestion of waste activated sludge: Phenotypes and genotypes

The specific effects of potassium ferrate (PF) on acid and methane production in anaerobic digestion need further exploration. This study comprehensively investigated the role of PF in organic matter conversion in waste activated sludge (WAS) digestion. Due to the high pH produced by PF self-decomposition, the hydrolysis of organic matter was promoted, whereas the methanogenesis was inhibited. PF could further directly oxidize protein and polysaccharides released by hydrolysis to produce volatile fatty acids (VFAs) and involve in the transformation of ammonia nitrogen. PF could induce the enrichment of functional genes related to fermentation pathways and lessen those related to methanogenesis, and the phylum resistant to PF oxidation and the strains capable of producing VFAs were enriched, resulting in VFAs accumulation. This study analyzed the participation way of PF in anaerobic digestion and provided a theoretical basis for the application of PF in promoting VFAs recovery from sludge digestion.

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.

Evaluation of sludge anaerobic fermentation driving partial denitrification capability: In view of kinetics and metagenomic mechanisms

Partial denitrification (PD) provides a promising approach of efficient and stable nitrite (NO₂^--N) generation for annamox. In this study, the feasibility of short-term sludge anaerobic fermentation driving PD was evaluated. It was found that a higher NO₂^--N accumulation in nitrate (NO₃^--N) reduction was obtained with the 5-days fermented sludge compared to 8 and 15-days fermentation. Moreover, compared to acetate as carbon source, sludge fermentation products (SFPs) induced the higher NO₂^--N production with nitrate-to-nitrite transformation ratio (NTR) nearly 100 %. Denitrification activity of fermented sludge were obviously improved with SFPs as electron donor. Metagenomic analysis revealed that Thauera was the dominant bacteria, which was assumed to be the key contributor to PD performance by harboring the highest narGHI and napAB but much lower nirS and nirK. Under the conditions of SFPs and fermented sludge, Thauera was speculated to have higher resistance than other denitrifiers attributed to versatile carbon metabolic capabilities utilizing SFPs with the significantly improved genes for metabolism of complex organic carbon via glycolysis after anaerobic fermentation. A novel integration of sludge fermentation driving PD and anammox for mainstream wastewater treatment and sidestream polishing was proposed to offer a promising application with reduced commercial carbon source consumption and waste sludge reduction.

Enhancing methane production from waste activated sludge with heat-assisted potassium ferrate (PF) pretreatment: Reaction kinetics and mechanisms

This work proposed a novel strategy via heat-assisted potassium ferrate (PF) pretreatment to enhance methane production from waste activated sludge (WAS) during anaerobic digestion. In this research, five dosages of PF (i.e., 0, 0.05, 0.1, 0.3 and 0.5 g/g VSS) at two temperatures (i.e., 25 °C and 55 °C) were explored. Biochemical methane potential experiments illustrated that heat-assisted PF pretreatment improved cumulative methane production with the maximum yield up to 163.93 mL CH₄/g VSS, 149.0 %, 119.6 % and 121.0 % of that in the control, individual 0.5 g PF/g VSS and individual heat (i.e., 55 °C) pretreatment digesters, respectively. The maximum methane potential (B₀) was promoted by 63.2 % with heat-assisted PF pretreatment compared to the control, while the hydrolysis rate (k) changed slightly. Mechanism analysis revealed that heat-assisted PF pretreatment accelerated WAS solubilization and enhanced the biodegradability of released substances, providing more available matrix for bacteria during the following anaerobic digestion processes. Microbial community analysis exhibited that several microbes such as Proteiniclasticum sp., Sedimentibacter sp. and Methanosaeta sp. associated with hydrolysis, acidification and methanogenesis respectively were improved after heat-assisted PF pretreatment. In addition, the relative bioactivities of protease, butyrate kinase and acetate kinase were also increased. Furthermore, variation of dominant genes associated with methane production indicated that acetate-dependent methanogenesis was the main pathway while CO₂-dependent methanogenesis pathway was inhibited by heat-assisted PF pretreatment.

Spatial distribution of fecal pollution indicators in sewage sludge flocs and their removal and inactivation as revealed by qPCR/viability-qPCR during potassium ferrate treatment

Sludge reuse and utilization is one of important routines of disseminating fecal pollution to surface water and groundwater. However, it remains unclear the spatial distribution of fecal pollution indicators in sludge flocs and their reductions during sludge treatment processes. In this study, the abundances of fecal pollution indicators including cross-assembly phage (crAssphage), JC and BK polyomavirus (JCPyV, BKPyV), human adenovirus (HAdV), the human-specific HF183 Bacteroides (HF183) and Escherichia coli (EC) in soluble extracellular polymeric substances (S-EPS), loosely-bound EPS (LB-EPS), tightly-bound EPS (TB-EPS), and pellets of sludge flocs were determined, and the effect of potassium ferrate (PF) treatment on their removal and inactivation was investigated by using both qPCR and viability-qPCR. Results showed that all investigated indicators were detected in each fraction of sludge flocs. The PF treatment led to a great migration of indicators from sludge pellets to sludge EPS and some extent of their inactivation in each fraction of sludge flocs. The overall reductions of human fecal indicators in sludge determined by qPCR were 0-1.30 logs, which were 0-2 orders of magnitude lower than those of 0.69-2.39 logs detected by viability-qPCR, implying their inactivation by PF treatment to potentially alleviate the associated human health risks.

Ferrate pretreatment-anaerobic fermentation enhances medium-chain fatty acids production from waste activated sludge: Performance and mechanisms

The rupture of cytoderm and extracellular polymeric substances (EPS), and competitive inhibition of methanogens are the main bottlenecks for medium-chain fatty acids (MCFAs) production from waste activated sludge (WAS). This study proposes a promising ferrate (Fe (VI))-based technique to enhance MCFAs production from WAS through accelerating WAS disintegration and substrates transformation, and eliminating competitive inhibition of methanogens, simultaneously. Results shows that the maximal MCFAs production attains 8106.3 mg COD/L under 85 mg Fe/g TSS, being 58.6 times that of without Fe (VI) pretreatment. Mechanism exploration reveals that Fe (VI) effectively destroys EPS and cytoderm through electron transfer, reactive oxygen species generation (i.e., OH, O₂^- and ¹O₂) and elevated alkalinity, resulting in the transfer of organics from solid to soluble phase and from macromolecules to intermediates. Generation and transformation of intermediates analyses illustrate that Fe (VI) facilitates hydrolysis, acidification and chain elongation (CE) but suppresses methanogenesis, promoting the targeted conversion of intermediates to MCFAs. Also, Fe (VI) pretreatment provides potential electron shuttles for chain elongation. Microbial community and functional genes encoding key enzymes analysis indicates that Fe (VI) screens key microorganisms and up-regulates functional genes expression involved in CE pathways. Overall, this technology avoids methanogens inhibitor addition and stimulates vivianite synthesis during MCFAs production from WAS.

A novel strategy and mechanism for high-quality volatile fatty acids production from primary sludge: Peroxymonosulfate pretreatment combined with alkaline fermentation

To obtain high-quality VFAs production from primary sludge, a novel strategy that combined peroxymonosulfate (PMS) pretreatment and alkaline fermentation (i.e., PMS & pH9) was proposed in the study. The results showed that PMS & pH9 was efficient in sludge solubilization and hydrolysis, resulting in a maximal VFAs yield of 401.2 mg COD/g VSS, which was 7.3-, 2.1-, and 8.8-fold higher than the sole PMS, sole pH9, and control, respectively. Acetate comprised 87.6% of VFAs in this integration system. Mechanism investigations revealed that sulfate and free radicals produced by PMS play roles in improving VFAs yield under alkaline conditions. Besides, sulfate also aided in C3∼C5 VFAs converting to acetate under alkaline conditions depending on the increase of incomplete-oxidative sulfate-reducing bacteria (iso-SRB) (i.e., Desulfobulbus and Desulfobotulus). Moreover, the relative abundances of acid-forming characteristic genera (i.e., Proteiniborus, Proteinilcasticum, and Acetoanaerobium) were higher in PMS & pH9.

Ferrous-Iron-Activated Sulfite-Accelerated Short-Chain Fatty Acid Production from Waste-Activated Sludge Fermentation: Process Assessment and Underlying Mechanism

To break the bottlenecks of slow hydrolysis and low acid production efficiency of waste-activated sludge (WAS) in the traditional anaerobic fermentation process, this study investigated the employment of ferrous-iron (Fe(II))-activated sulfite to produce hydroxyl, sulfate, and other highly oxidizing radicals on WAS floc cracking and short-chain fatty acid (SCFAs) production during anaerobic fermentation. The effect of the dosage ratio of Fe(II)/S(IV) was also studied. Results showed that the combined pretreatment of Fe(II)-activated sulfite significantly promoted the exfoliation of extracellular polymers and the subsequent SCFAs production. The highest concentration of SCFAs reached 7326.5 mg COD/L under the optimal dosage of 1:2 for Fe(II)/S(IV), which was 1.1~2.1 times higher than that of other research groups. Meanwhile, the analysis by 3D fluorescence spectroscopy and EPR (electron paramagnetic resonance) showed that Fe(II)-activated sulfite had a synergistic effect on the rupture of sludge cells and the stripping of extracellular polymers, with SO4− and OH as the key radicals generated and being much stronger in the 1:1 and 1:2 groups. High-throughput sequencing showed that the Fe(II)-activated sulfite system significantly changed the functional microbial diversity. The anaerobic fermentation bacteria and sulfate-reducing bacteria were significantly enriched. The underlying mechanism of Fe(II)-activated sulfite oxidation and molecular ecological network of key microbiomes were unveiled.

Role of extracellular polymeric substances in methane production from waste activated sludge induced by conductive materials

Conductive materials have been widely used to establish direct interspecies electron transfer (DIET) for enhancing methane production potential from anaerobic digestion of waste activated sludge (WAS). However, the roles of extracellular polymeric substances (EPSs) affected by conductive materials on anaerobic digestion have been rarely reported. This study selected four widely used conductive materials, i.e., granular active carbon (GAC), biochar (BC), zero-valent iron (ZVI), and magnetite (Mag), to reveal the roles of EPSs. Results showed that methane production potentials were increased by BC, ZVI and Mag compared to that of control, with increase ratios of 13.4 %, 22.2 %, and 12.2 %, while a decrease was observed by GAC. The contents, components and characteristics of EPSs were all affected by conductive materials. The contents of EPSs were increased by ZVI and Mag, while they were decreased by BC and GAC. The ratios between proteins and polysaccharides (PN/PS) in loosely bound EPSs (LB-EPSs) were reduced in all groups, while they were similar in tightly bound EPSs (TB-EPSs) of ZVI and Mag groups. In addition, the cytochrome C and redox properties were remarkably promoted in suspension rather than in LB- and TB-EPSs. It was found that the correlation relationships between the maximal methane production potential (Pmax) and PN/PS in EPSs were positive, as well as fluorescent substances, especially tyrosine-like and tryptophan-like substances, with R² of 0.96 and 0.98. Furthermore, the correlation relationships also existed between EPSs and microbial communities. Clostridium and Methanobacterium, potential DIET partners, presented significant positive correlation relationships (P < 0.05) with Pmax, PN/PS and fluorescent substances in EPSs. The findings may provide some new insights for mechanism investigation of anaerobic digestion induced by conductive materials.

Moderate potassium ferrate dosage enhances methane production from the anaerobic digestion of waste activated sludge

The annual increase of waste activated sludge (WAS) has become an urgent problem to be solved in sewage plants worldwide. Anaerobic digestion (AD) of WAS is an attractive choice to maximize the resource utilization rate. Nevertheless, the disintegration of sludge complex polymers is difficult, resulting in a low bioconversion rate. Potassium ferrate (PF), as a green oxidant with strong oxidizing property, has attracted great attention in WAS pretreatment recently. The effects of PF pretreatment on WAS hydrolysis and its dosage-response on methane production were investigated in the present study. Results show that as PF dosage raise from 0 to 50 g-K2FeO4/ kg-TS (total solids), the methane yield enhanced significantly by 40.3% from 0.083 to 0.12 L/g-VSadded (volatile solids). Nevertheless, the further increase in PF dosage resulted in decreased methane production. Especially with the PF dosage of 500 g-K2FeO4/ kg-TS, methane production is even slightly lower than the control reactor without PF oxidation. The mechanism analysis showed that although the dissolution of polysaccharides and proteins was enhanced with the high dosage of PF, the accompanying released humic-like substances and high concentration of ferric ions should be the main reasons inhibiting methane production.

Effects of enzymes on organic matter conversion in anaerobic fermentation of sludge to produce volatile fatty acids

Sludge hydrolysis is a vital step in anaerobic digestion of sludge. This study compared the efficacy of free versus immobilized enzymes at different concentrations in promoting sludge disintegration. Pretreatment with 1,000 mg/L immobilized enzymes was more efficient in promoting sludge disintegration than free enzymes at the same concentration. Under the optimized conditions, volatile fatty acids (VFAs) were produced at 10.6 g/L, accounting for 85 % of total soluble chemical oxygen demand. Improved VFA production was attributed to the release of large amounts of polysaccharides and proteins from the enzymatically pretreated sludge. Released organic matter are the substrates for VFAs generated by the determined microbial community of Firmicutes, Proteobacteria, Bacteroidetes, Actinobacteria, and Chloroflexi. In this study, anaerobic fermentation was used to successfully convert organic matter in sludge into high-value-added VFAs. Therefore, this process can be selected as a strategy to reduce carbon emissions from wastewater treatment plants (WWTPs).

Effect of Different Acid and Base Potassium Ferrate Pretreatment on Organic Acid Recovery by Anaerobic Digestion of Sludge

Potassium ferrate has strong oxidation in both acid and alkali environments, which has attracted extensive attention. However, the impact of the pH environment on this coupling process with the goal of resource recovery has not received attention. Under the goal of the efficient recovery of organic acid, the changes of solid-liquid characteristics of sludge after acid and alkaline ferrate pretreatment and during anaerobic digestion were discussed. The results showed that compared with blank control groups, after alkaline ferrate pretreatment, the volatile suspended solids (VSSs) decreased the most, reaching 28.19%. After being pretreated with alkaline ferrate, the sludge showed the maximum VFA accumulation (408.21 COD/g VSS) on the third day of digestion, which was 1.34 times higher than that of the acid ferrate pretreatment. Especially in an alkaline environment, there is no need to add additional alkaline substances to adjust the pH value, and the effect of sludge reduction and acid production is the best.

Enhanced Methane Production from Sludge Anaerobic Digestion with the Addition of Potassium Permanganate

This work aims to reveal the effect of potassium permanganate (KMnO₄) on the sludge anaerobic digestion process, as well as the relevant mechanisms. Experimental data showed that the biomethane production was gradually increased from 159.3 ± 3.0 to 211.5 ± 5.1 mL/g VSS (volatile suspended solids) when the KMnO₄ content was increased from 0 to 0.08 g/g VSS, with an increasing rate of 32.8%. A further increase in the KMnO₄ dosage however resulted in the decline of the methane yield. First-order kinetic model analysis indicated that higher methane production potentials and hydrolysis rates were achieved in KMnO₄-added reactors than in the control. Mechanism analysis demonstrated that KMnO₄ not only efficiently disintegrated the sludge flocs, which resulted in the increased contents of dissolved organics, but also enhanced the proportion of biodegradable substances in the sludge liquor. Meanwhile, the biodegradabilities of recalcitrant humus and lignocellulose substances were found to be promoted by KMnO₄ treatment as higher methane yields were attained from KMnO₄-treated model substrates. 16S rRNA analysis illustrated that the functional microbes participated in anaerobic digestion were largely enriched in the KMnO₄-pretreated digestor. Furthermore, efficient inactivation of the fecal coliform was achieved by KMnO₄ pretreatment.

Response of anaerobic digestion of waste activated sludge to types of alkalis: Contribution identification of metal ions

Alkaline pretreatment is one promising strategy for promoting anaerobic digestion of waste activated sludge (WAS). This study selected three types of alkalis with monovalent (NaOH and KOH), divalent (Ca(OH)₂ and Mg(OH)₂), and trivalent (Fe(OH)₃ and Al(OH)₃) cations to reveal the roles of metal ions on short chain fatty acids (SCFAs) production. The enhanced production potentials of SCFAs were reduced by order of alkalis with monovalent, divalent, and trivalent cations. Na⁺, K⁺, Ca²⁺, and Mg²⁺ did no contributions on SCFAs production, while Fe³⁺ and Al³⁺ performed better than control, especially the latter. The mechanism analysis proved that Na⁺, K⁺, Ca²⁺, and Mg²⁺ did no significant effects on solubilization, hydrolysis, acidification and methanogenesis stages, while the first three stages were improved by Fe³⁺ and Al³⁺ and the methanogenesis stage was inhibited. The findings may provide some new insights when using alkalis or residual metal ions to improve anaerobic digestion of WAS.

Synergistic effect of yeast integrated with alkyl polyglucose for short-chain fatty acids production from sludge anaerobic fermentation

An efficient strategy for short-chain fatty acid (SCFA) production from sludge anaerobic fermentation was proposed with the combination of yeast and alkyl polyglucose (APG). It revealed that the synergetic effect of yeast and APG could boost the SCFA concentration to the maximum value of 2800.34 mg COD/L within 9 days at 0.20 g/g suspended solids (SS) yeast and 0.20 g/g SS APG, which was significantly higher than that of its counterparts. Interestingly, the sludge solubilization, the biodegradability of fermentation substrate, as well as the acidification of hydrolyzed products, was evidently improved in the coexistence of APG and yeast. The activities of hydrolytic enzymes and acetate kinase were also stimulated, whereas the coenzyme F420 was inhibited. The synergetic effect of yeast and APG used in this work enriches the study of carbon resource recovery from sludge anaerobic fermentation.

Synergic role of ferrate and nitrite for triggering waste activated sludge solubilisation and acidogenic fermentation: Effectiveness evaluation and mechanism elucidation

Enhancing anaerobic treatment efficiency of waste activated sludge (WAS) toward preferable resource recovery would be an important requirement for achieving carbon-emission reduction, biosolids minimization, stabilization and security concurrently. This study demonstrated the synergic effect of potassium ferrate (PF) and nitrite on prompting WAS solubilisation and acidogenic fermentation toward harvesting volatile fatty acids (VFAs). The results indicated the PF+NaNO₂ co-pretreatment boosted 7.44 times and 1.32 times higher WAS solubilisation [peak soluble chemical oxygen demand (SCOD) of 2680 ± 52 mg/L] than that by the single nitrite- and PF-pretreatment, respectively, while about 2.77 times and 2.11 times higher VFAs production were achieved (maximum VFAs accumulation of 3536.25 ± 115.24 mg COD/L) as compared with the single pretreatment (nitrite and PF)-fermentations. Afterwards the WAS dewaterability was improved simultaneously after acidogenic fermentation. Moreover, a schematic diagram was established for illustrating mechanisms of the co-pretreatment of PF and nitrite for enhancing the VFAs generation via increasing key hydrolytic enzymes, metabolic functional genes expression, shifting microbial biotransformation pathways and elevating abundances of key microbes in acidogenic fermentation. Furthermore, the mechanistic investigations suggested that the PF addition was conducive to form a relatively conductive fermentation environment for enhancing electron transfer (ET) efficiency, which contributed to the VFAs biotransformation positively. This study provided an effective strategy for enhancing the biodegradation/bioconversion efficiency of WAS organic matters with potential profitable economic returns.

Potassium ferrate pretreatment promotes short chain fatty acids yield and antibiotics reduction in acidogenic fermentation of sewage sludge

During the acidogenic fermentation converting waste activated sludge (WAS) into short-chain fatty acids (SCFA), hydrolysis of complex organic polymers is a limiting step and the transformation of harmful substances (such as antibiotics) during acidogenic fermentation is unknown. In this study, potassium ferrate (K₂FeO₄) oxidation was used as a pretreatment strategy for WAS acidogenic fermentation to increase the hydrolysis of sludge and destruct the harmful antibiotics. Pretreatment with K₂FeO₄ can effectively increase the SCFA production during acidogenic fermentation and change the distribution of SCFA components. With the dosage of 0.2 g/g TS, the maximum SCFA yield was 4823 mg COD/L, which is 28.3 times that of the control group; acetic acid accounts for more than 90% of the total SCFA. The higher dosage (0.5 g/g TS) can further increase the proportion of acetic acid, but inhibit the overall performance of SCFA production. Apart from the promotion of hydrolysis and acidogenesis, K₂FeO₄ pretreatment can also simultaneously oxidizes and degrades part of the antibiotics in the sludge. When the dosage is 0.5 g/g TS, the degradation efficacy of antibiotics is the most significant, and the contents of ofloxacin, azithromycin, and tetracycline in the sludge are reduced by 69%, 42%, and 50%, respectively. In addition, K₂FeO₄ pretreatment can also promote the release of antibiotics from sludge flocs, which is conducive to the simultaneous degradation of antibiotics in the subsequent biological treatment process.

pH regulated potassium ferrate oxidation promotes acetic acid yield and phosphorous recovery rate from waste activated sludge

To improve the dose efficiency of K₂FeO₄ in waste activated sludge (WAS) treatment, pH regulation on K₂FeO₄ pretreatment and acidogenic fermentation was investigated. Four pretreatments were compared, i.e. pH3 + 50 g/kg-TS, pH10 + 50 g/kg-TS, neutral pH + 50 g/kg-TS and neutral pH + 100 g/kg-TS (without pH adjustment). The higher short chain fatty acids (SCFAs) yield and phosphorous dissolution rate was found under the condition of pH 10.0. In pH10 + 50 g/kg-TS, the maximum concentration of SCFAs was 5591 mg-COD/L, which yield was 22.6 times higher than that of the neutral pH + 50 g/kg-TS (237 mg COD/L). The acidogenic fermentation period could be shortened to 5 days and acetic acid accounted for 70 % of SCFAs. Furthermore, PO₄^3--P in the hydrolysate (346.5 mg/L) accounted for 47.59 % of TP, which is easier to be recovered by chemical precipitation. Therefore, a more economical and feasible utilization mode of potassium ferrate was proposed.

Comparison of primary and secondary sludge carbon sources derived from hydrolysis or acidogenesis for nitrate reduction and denitrification kinetics: Organics utilization and microbial community shift

Seeking available and economical carbon sources for denitrification process is an intractable issue for wastewater treatment. However, no study compared different types of waste sludge as carbon source from denitrification mechanism, organics utilization and microbial community aspects. In this study, primary and secondary sludge were pretreated by thermophilic bacteria (TB), and its hydrolysis or acidogenic liquid were prepared as carbon sources for denitrification. At C/N of 8-3, the variations of NO₃^--N and NO₂^--N were profiled in typical cycles and denitrification kinetics was analyzed. Primary sludge achieved a competitive NO_X-N removal efficiency with less dosage than secondary sludge. Fourier transform infrared (FTIR) spectroscopy was introduced to analyze organic composition from functional-group perspective and the utilization of organic matters in different sludge carbon sources was investigated. To further analyze the microbial community shift in different denitrification systems, high-throughput sequencing technology was applied. Results showed that denitrifier Thauera, belonging to Proteobacteria, was predominant, and primary sludge acidogenic liquid enriched Thauera most intensively with relative abundance of 47.3%.

Rhamnolipid increases H2S generation from waste activated sludge anaerobic fermentation: An overlooked concern

Rhamnolipid (RL), one representative biosurfactant, is widely regarded as an economically feasible and environmentally beneficial additive to improve fermentation efficiency and resource recovery from waste activated sludge (WAS). However, its potentially detrimental impact on WAS fermentation such as H₂S generation was overlooked previously. This study therefore aims to fill the gap through exploring whether and how the presence of RL affects H₂S generation from WAS anaerobic fermentation. Experimental results showed that when RL increased from 0 to 40 mg/g total suspended solids (TSS), the cumulative H₂S yield enhanced from 323.6 ×  10^-4 to 620.3 ×  10^-4 mg/g volatile suspended solids (VSS). Mechanism analysis showed that RL reduced WAS surface tension, which benefited transformations of organic sulfurs (e.g., aliphatic-S and sulfoxide) and inorganic sulfate from solid to liquid phase. The presence of RL not only reduced the ratio of α-helix/(β-sheet + random coil) and damaged the hydrogen bonding networks of organic sulfurs but also promoted substrate surface charges and cell membrane permeability. These facilitated the contact between hydrolase and organic sulfurs, thereby increasing sulfide production from organic sulfurs hydrolysis. Further investigations showed that RL promoted the expression of key genes (e.g., aprA/B and dsrA/B) involved in the dissimilatory sulfate reduction, which accelerated the reaction of adenosine 5'-phosphosulfate (APS)→ sulfite→ sulfide. Meanwhile, RL inhibited the corresponding key genes such as CysH, and Sir, responsible for assimilatory sulfate reduction (APS→3'-phosphoadenosine-5'phosphosulfate→organosulfur), which reduced substrate competition in favor of H₂S production from dissimilatory sulfate reduction. Besides, RL decreased the fermentation pH, which benefited the transformation of HS^- to H₂S.

Potassium ferrate followed by alkali-stripping treatment to achieve short-chain fatty acids and nitrogen recovery from waste activated sludge

Waste activated sludge (WAS) is a potential resource to achieve carbon-neutral goal of wastewater treatment plant. However, the solubilization is always the rate-limiting step for resource recovery from anaerobic digestion of WAS. This study reported a novel strategy, i.e., potassium ferrate (PF) followed by alkali-stripping treatment, to achieve short-chain fatty acids (SCFAs) and nitrogen recovery from WAS. Results showed that whether the stripping process was conducted under alkaline condition or not, the SCFAs production potential was increased rather than reduced. The promoted SCFAs production was due to the accelerated solubilization and hydrolysis stages but the inhibited methanogenesis stage. The SCFAs yield reached 258 mg chemical oxygen demand (COD)/g volatile suspended solids (VSS), and the carbon source, including SCFAs, soluble polysaccharides and proteins, reached 384 mg COD/g VSS. The potentially recovered nitrogen was about 8.71 mg NH₄⁺-N/g VSS. This work may provide some new solutions for enhancing resource recovery from WAS.

Persulfate-based strategy for promoted acesulfame removal during sludge anaerobic fermentation: Combined chemical and biological effects

The acesulfame (ACE) degradation in waste activated sludge (WAS) via direct anaerobic fermentation is difficult and the efficient elimination techniques are imperative for the ultimate safe WAS disposal. Persulfate (PS)-based approach was developed to promote the ACE removal during WAS anaerobic fermentation. Results demonstrated the effectiveness of PS-based treatments on ACE degradation, and the ACE removal efficiency was respectively 48.2% and 96.2% in the PS and PS/Fe-treated reactors while it was only 6.0% in the control reactor. Mechanism explorations revealed that the active free radicals (i.e. OH• and SO₄^•-) generated in the PS-based reactors were the key oxidative species for the ACE degradation. However, such effects were interfered by the released soluble substrates (i.e. protein, carbohydrate and inorganic ions) during anaerobic fermentation by competing and/or quenching free radicals, which caused the deceleration of the ACE removal efficiency. Moreover, the PS-based treatment facilitated the enrichment of functional microorganisms (i.e. Phyllobacteriaceae and Bradyrhizobiaceae) and upregulated the critical genes (i.e. pncB and nadE) involved in the ACE degradation. Based on the density functional theory (DFT) and metabolic intermediates analysis, the hydroxylation and oxidative ring-opening were the two main proposed metabolic pathways for ACE degradation. Overall, the combined chemical and biological metabolism effects collectively contributed to the efficient ACE degradation, and it provided a novel and effective strategy for refractory pollutants removal during WAS anaerobic fermentation.

Simultaneous recovery of vivianite and produce short-chain fatty acids from waste activated sludge using potassium ferrate as pre-oxidation treatment

Recovery resources from waste active sludge (WAS) is an effective way to alleviate the predicament of WAS disposal, and it is also conducive to the carbon neutralization of wastewater treatment systems. This study discussed the strategy of WAS anaerobic fermentation after pre-oxidation with potassium ferrate (K₂FeO₄, PF), which can simultaneously recover vivianite and enhance SCFAs production. The results showed that PF pre-oxidation considerably shortened the fermentation time of SCFAs to 2 days, and the main Fe-P mineral was vivianite. The optimal PF dosage of 0.06 g Fe (VI)/g TSS for pre-oxidation WAS resulted in the maximum SCFAs production and vivianite recovery rate of 3698.2 ± 118.98 mg COD/g VSS and 32.39%, respectively. The mechanism analysis showed that the oxidizing properties of PF significantly accelerated the disintegration of tight EPS, release of protein and sludge acidification efficiency. Moreover, the PF strengthened the transfer of P to the solid phase, forming the Fe-P mineral and unsaturated coordination state of phosphate group. Then the key microorganism Geobacter reduced the Fe³⁺ in Fe-P state to Fe²⁺ and combined unsaturated phosphate to form vivianite. This study provides an alternative method for resource recovery and environmentally friendly disposal of WAS and contributes to the carbon neutrality of urban water systems.

Stepwise freezing-thawing treatment promotes short-chain fatty acids production from waste activated sludge

Waste activated sludge (WAS), as the byproducts of wastewater treatment plants, has been greatly produced. With high cost and environmental risk of WAS disposal, to explore a low-cost and environment-friendly technology has been a great challenge. Considering that WAS is a collection of organic matters, anaerobic fermentation has been selected as a sustainable way to simultaneously recover resources and reduce environmental pollution. To recover short-chain fatty acids (SCFAs) has gained great concern because of the high value-added application and high-efficiency production process. Considering the temperature in some areas of the world can reach to below 0 °C, this study proposed an efficient strategy, i.e., stepwise freezing and thawing treatment, to promote SCFAs production. The maximal production of SCFAs, i.e., 246 mg COD/g volatile suspended solid, was obtained with the shortened retention time of five days. Mechanistic studies showed that the solubilization of both extracellular polymeric substances (EPSs) and microbial cells could be accelerated, with the EPSs removal of 58.3% for proteins and 59.0% for polysaccharides. Also, the hydrolysis process was promoted to provide more substrates for subsequent acidogenisis, and the functional microorganisms, such as Romboutsia, Paraclostridium, Macellibacteroides and Conexibacter, were greatly enriched, with a total abundance of 26.2%. Moreover, compared to control, methanogenesis was inhibited at a shortened sludge retention time (e.g., five days), which benefited to the accumulation of SCFAs, but the methane production was increased by 25.2% at a longer sludge retention time (e.g., ten days). Thus, these findings of this work may provide some new solutions for the enhanced resource recovery from WAS, and further for carbon-neutral operation of wastewater treatment plants.

Role and significance of water and acid washing on biochar for regulating methane production from waste activated sludge

Methane recovered from anaerobic digestion of waste activated sludge (WAS) can be used as the energy supplement of the wastewater treatment plant, benefiting to its carbon-neutral operation. In order to enhance methane production, biochar (BC) has been widely selected as conductive material to build direct interspecies electron transfer (DIET) in anaerobic digestion of WAS. However, the role and significance of washing strategies, including water and acid washing, on BCs for regulating methane production have not been reported. This study selected the frequently used woody- (W) and straw (S)-BCs as mode. Compared to raw W-BC, water and acid washing W-BC increased the methane yields by 19.1% and 15.7%, respectively. Differently, the methane yields among raw, water and acid washing S-BCs were similar. Mechanism study showed that both the two washing strategies optimized the properties of raw W-BC for promoting methane production. Water and acid washing W-BCs increased the electron transfer functional groups, such as ketones and quinones, which were not observed in S-BCs. Moreover, the electron-active microorganisms were enriched with the presence of water and acid washing W-BCs, and the predominant pathway for methane production shifted from hydrogentrophic to acetotrophic and DIET methanogenesis, while the microbial communities, including bacteria and archaea, were similar with the presence of raw, water and acid washing S-BCs. These findings of this work provide some new insights for production improvement regulation of methane from anaerobic digestion of wastes induced by BCs.

Enhanced aerobic granular sludge by static magnetic field to treat saline wastewater via simultaneous partial nitrification and denitrification (SPND) process

Saline wastewater poses a threat to biological nitrogen removal. This study investigated whether and how static magnetic field (SMF) can improve the salt-tolerance of aerobic granular sludge (AGS) in two simultaneous partial nitrification and denitrification (SPND) reactors. Results confirmed that the SMF improved the mean size and settleability of granules, stimulated secretion of extracellular polymeric substances with high protein content, in turn enhancing the aerobic granulation. Although high salt stress inhibited functional microorganisms, the SMF maintained better SPND performance with average COD removal, TN removal and nitrite accumulation ratio finally recovering to 100%, 72.9% and 91.1% respectively. High throughput sequencing revealed that functional bacteria evolved from Paracoccus to halotolerant genera Xanthomarina, Thauera, Pseudofulvimonas and Azoarcus with stepwise increasing salinity. The enhanced salt-tolerance may be because the SMF promoted the activity of these halotolerant bacteria. Therefore, this study proposes an economic, effective and environmental biotechnology for saline wastewater treatment.

Effect of potassium persulphate addition on sludge disintegration of a mesophilic anaerobic fermentation system

Persulphates, an advanced oxidation process, has been recently used as an alternative pretreatment method to enhance short-chain fatty acids (SCFAs) yield from waste-activated sludge (WAS) anaerobic fermentation (AF). But so far, the effects of peroxydisulphate (PDS) dosages on mesophilic anaerobic fermentation are still not studied fully. Herein, we explored the influences of potassium PDS addition on mesophilic AF of WAS. Notably, the addition of PDS could drastically accelerate WAS solubilization and hydrolysis, which was proportional to the amount of PDS. The maximal total SCFAs yield of 249.14 mg chemical oxygen demand/L was obtained with 120 mg PDS/g suspended solids addition at 6 days of AF, which was 2.2-fold that of the control one. Tightly bound extracellular polymeric substances (EPSs) were transformed into loosely bound EPS and dissolved organic matters, and aromatic proteins and humic-like substances of EPSs were disintegrated, which were caused by the devastating effects of PDS treatments on EPSs disruption. The intracellular constituents of microbial cells in the sludge were released accordingly. As a result, there was release of soluble substrates derived from the disintegration of both EPSs and cells, the amounts of which were proportional to the dose of PDS. Moreover, microbial diversity and richness were both decreased in the presence of PDS, and the relative abundance of phyla Actinobacteria increased with the increase of the PDS dosage. In addition, the stability of sludge flocs was destroyed in the presence of PDS, the distribution of particle size tended to be small and dispersive, and dewaterability of the sludge was deteriorated.

Short-Chain Fatty Acids Production from Anaerobic Fermentation of Sewage Sludge: The Effect of Higher Levels Polyaluminium Chloride

With the annual increase in the sludge production in China's sewage treatment plants, the problem of sewage sludge treatment and disposal is becoming more and more serious. Anaerobic fermentation can convert complex organic matter in sewage sludge into short-chain fatty acid, hydrogen, methane and other resources and is an effective method for sewage sludge treatment and disposal. At the same time, sewage sludge often contains flocculants, which will inevitably affect the effect of anaerobic fermentation. As a high-performance flocculant, polyaluminum chloride (PAC) is widely used in wastewater treatment and sewage sludge dewatering processes. Previous studies indicated that lower levels of PAC inhibit the effect of the anaerobic fermentation process of sewage sludge; on the other hand, it is necessary to understand the effects of higher levels of PAC in anaerobically fermented sewage sludge. The results showed that higher levels (0.2-1 g Al/g total solids (TS)) of PAC could promote acid production from anaerobically fermented sewage sludge. Moreover, mechanism studies suggest that higher levels (0.2-1 g Al/g total solids (TS)) of PAC caused excessive adsorption of the charge on the surface of the sewage sludge colloid and reversed the charge. The sewage sludge colloid was stabilized again, which increases the concentration of soluble proteins, polysaccharides, and soluble extracellular polymers (S-EPS) in the fermentation broth, thereby improving the anaerobically fermented sewage sludge efficiency. The results provided from this study may act as technical reference and guidance for the engineering application of sewage sludge anaerobic fermentation.