Disintegration Of Waste Activated Sludge Research Articles

Development of oriented multi-enzyme strengthens waste activated sludge disintegration and anaerobic digestion: performance, components transformation and microbial communities

Low methane production and long retention time are the main dilemmas in current anaerobic digestion (AD) of waste activated sludge (WAS). This work used WAS as only substrate to prepare oriented multi-enzyme (ME) that directly used for WAS pretreatment. Under the optimal parameters, the highest activities of protease and amylase in ME could respectively reach 16.5 U/g and 580 U/g, and the corresponding methane production attained 197 mLCH4/g VS, which was increased by 70.4% compared to blank group. It was found that ME pretreatment could strengthen WAS disintegration and organic matters dissolution, lead to the soluble chemical oxygen demand (SCOD) was increased from the initial 486 mg/L to 2583 mg/L, and the corresponding volatile suspended solid (VSS) and extracellular polymeric substances (EPS) were reduced by 27% and 73.8%, respectively. The results of three-dimensional excitation-emission matrix (3D-EEM) and Fourier transform infrared spectroscopy (FTIR) indicated that protein disintegration may be the critical step during the process of WAS hydrolysis with ME, of which the release of tyrosine-like proteins achieved the better biodegradability of WAS, while the results of X-ray photoelectron spectroscopy (XPS) showed that the formation of protein derivatives was the main harmful factor that could extend the lag phase of AD process. Microbial communities analysis further suggested that ME pretreatment facilitated the enrichment of acetogenic bacteria and acetotrophic methanogens, which caused the transition of the methanogenesis pathway from hydrogenotrophic to acetotrophic. This study is expected to furnish valuable insight for ME pretreatment on enhancing WAS disintegration and methane production.

Effects of sulfite and freezing co-pretreatment on the volatile fatty acids production of waste activated sludge in anaerobic fermentation

Volatile fatty acids (VFAs) produced during the anaerobic fermentation of waste activated sludge (WAS) are renewable energy sources. However, the biodegradability of untreated WAS is significantly low, thereby imposing a substantial constraint on the generation of VFAs during anaerobic fermentation of WAS. In this study, the experimental results showed that freezing and sulfite co-pretreatment had a synergism on the disintegration of WAS and the production of VFAs. The optimal freezing temperature for VFAs production were −20 °C, 8 h and sulfite dosage of 1400 mg/L resulted in the VFAs reaching a highest concentration of 627.58 mg/L, which was 17.41 times higher than that of the blank group. In addition, the disintegration effect of WAS has been improved, the concentration of soluble chemical oxygen demand (SCOD), proteins and polysaccharides were 7.21, 5.56 and 2.58 times than the blank group, respectively. The microbial community analysis revealed an increase of the OUT of co-pretreatment. Also showed that co-pretreatment could increase the abundance of Levilinea, Unclassified_Anaerolineaceae and Blvii28_wastewater_sludge_group, which might increase VFAs production. Furthermore, compared to sulfite pretreatment or freezing pretreatment, the co-pretreatment significantly enhanced the disintegration efficiency of WAS; shortened the hydrolysis required time for WAS; provided the higher concentration of substrates for VFAs production; and improved the overall efficiency of sludge anaerobic fermentation.

Feasibility and mechanism exploration of enhancing short chain fatty acid production assisted by nitrate photolysis during sludge fermentation

Seeking for the eco-friendly and low-cost pretreatment technology has become the prerequisite for the efficient release of the embedded organics in waste activated sludge (WAS), further for the higher production of short chain fatty acids (SCFAs) via anaerobic fermentation. Nitrate, which can produce reactive oxygen species (ROS) and reactive nitrogen species (RNS) during photolysis, may have the positive effect on WAS disintegration and organics degradation, while its performance and mechanism for WAS treatment is still unknown. This study investigated the feasibility of the nitrate photolysis on WAS disintegration and SCFAs production, and elucidated the underlying mechanism. Results showed that the highest acidification rate (26.6 h−1) and short-chain fatty acid (SCFAs) production (5370.6 mg COD/L) were achieved at 8 d in the nitrate photolysis group (Nitrate-UV) during the anaerobic fermentation, significantly promoted by 2.3 and 12.7 folds than that of Control (raw WAS). Acetic acid (HAc) and propionic acid (HPr) accounted for 89.1 % and 90.1 % in Nitrate-UV and Nitrate group (sole nitrate addition), which were much higher than that obtained in UV and Control. HO• and O2•− were the main contributors compared with NO2•, ONOO– and NO• on the release and bioconversion of the soluble organics. Microbial community and metagenomics revealed the promotion of nitrate photolysis on anaerobic fermentation bacteria (AFB) and nitrate reducing bacteria (NRB), as well as the gene abundance related to the metabolic pathways of glycolysis, amino acid, acetate and nitrogen metabolism, which jointly devoted to the SCFAs enhancement during the anaerobic fermentation.

Insights into the effect of nitrate photolysis on short-chain fatty acids production from waste activated sludge in anaerobic fermentation system: Performance and mechanisms

Nitrate photolysis has become an efficient, low-cost and promising technology for emerging contaminants removal, while its performance and mechanism for waste activated sludge (WAS) treatment is still unknown. This study innovatively introduced nitrate photolysis for WAS disintegration, and investigated the effect of nitrate addition (150–375 mg N/L) for short-chain fatty acids (SCFAs) production during anaerobic fermentation (AF). The results showed that nitrate photolysis significantly promoted the SCFAs production from WAS, and peaked at 280.7 mg/g VSS with 7-d fermentation with 150 mg N/L addition (150N-UV), which increased by 8.8–35.0 % and 10.7–23.3 % compared with other photolysis groups and sole nitrate groups. Effective release of the soluble organics was observed in the nitrate photolysis groups during AF, especially soluble proteins, reaching 1505.4 mg COD/L at 9 d in 150N-UV group, promoted by 7.0∼15.7 % than nitrate/nitrate photolysis groups. The model compounds simulation experiment further demonstrated the positive effect of nitrate photolysis on organics hydrolysis and SCFAs accumulation. The result of the radical capture and quenching verified the reactive oxygen species contributed more compared with reactive nitrogen species. Functional group analysis confirmed the effective bioconversion of the macromolecular organics during the fermentation. Moreover, the nitrate photolysis enhanced the enrichment of the functional consortia, including anaerobic fermentation bacteria (AFB), e.g., Fnoticella, Romboutsia, Gracilibacter and Sedimentibacter, and nitrate reducing bacteria (NRB), e.g., Acinerobacter and Ahniella. The macrogenetic analysis further revealed that glycolysis, amino acid metabolism, acetate metabolism and nitrogen metabolism were the dominating metabolic pathways during fermentation, and the abundance of the relevant genes were enhanced in 150N-UV group.

Enhancing acidogenic fermentation of waste activated sludge via urea hydrogen peroxide pretreatment: Performance and mechanisms

Improving the anaerobic treatment performance of waste activated sludge (WAS) to achieve resource recovery is an indispensable requirement to reduce carbon emissions, minimize and stabilize biosolids. In this study, a novel strategy by using urea hydrogen peroxide (UHP) to enhance SCFAs production through accelerating WAS disintegration, degrading recalcitrant substances and alleviating competitive suppression of methanogens. The SCFAs production and acetate proportion rose from 436.9 mg COD/L and 31.3% to 3102.6 mg COD/L and 54.1%, respectively, when UHP grew from 0 to 80 mg/g TSS. Mechanism investigation revealed that OH, O2 and urea were the major contributors to accelerate WAS disintegration with the sequence of OH>O2 > urea. Function microbes related to acidification and genes associated with acetate production ([EC:2.3.1.8] and [EC:2.7.2.1]) were upregulated while genes encoding propionic acid production ([EC:6.4.1.3] and [EC:6.2.1.1]) were downregulated. These results raised the application prospects of UHP in WAS resource utilization.

Fe-loaded alginate hydrogel beads activating peroxymonosulfate for enhancing anaerobic fermentation of waste activated sludge: Performance and potential mechanism

The recovery of volatile fatty acids (VFAs) through anaerobic fermentation (AF) is usually restricted by the poor biodegradability of waste activated sludge (WAS). This study proposed a novel strategy, i.e. peroxymonosulfate (PMS) activated by Fe-loaded sodium alginate hydrogel beads (Fe-SA), to enhance AF performance. Experimental results demonstrated that the as-synthesized Fe-SA and PMS co-pretreatment synergistically enhanced WAS solubilization and VFAs production. The maximal VFAs yield of 2013 mg COD/L was achieved at the Fe-SA dosage of 4.0 mM/g TSS, which was 93.7% higher than that with sole PMS addition and 8.82 times higher than that of the control. Mechanistic studies elucidated that the generation of reactive radicals such as SO4•− and •OH from PMS was greatly induced by Fe-SA, which contributed to WAS disintegration and degradation of refractory compounds. Additionally, analysis of the key enzyme activities indicated that the Fe-SA could strengthen biological hydrolysis and acidogenesis of sludge during AF. Microbial analysis illustrated that Fe-SA evidently improved the abundances of fermentative microorganisms as well as functional gene expression via creating a favorable environment for microbial growth. This study demonstrated the applicable potential of Fe-SA hydrogel beads activating PMS for VFAs production and provides an important reference for developing advanced oxidation processes-based application in AF.

Thiosulfate pretreatment enhancing short-chain fatty acids production from anaerobic fermentation of waste activated sludge: Performance, metabolic activity and microbial community

A novel strategy based on thiosulfate pretreatment for enhancing short-chain fatty acids (SCFAs) from anaerobic fermentation (AF) of waste activated sludge (WAS) was proposed in this study. The results showed that the maximal SCFA yield increased from 206.1 ± 4.7 to 1097.9 ± 17.2 mg COD/L with thiosulfate dosage increasing from 0 to 1000 mg S/L, and sulfur species contribution results revealed that thiosulfate was the leading contributor to improve SCFA yield. Mechanism exploration disclosed that thiosulfate addition largely improved WAS disintegration, due to thiosulfate serving as a cation binder for removing organic-binding cations, especially Ca2+ and Mg2+, dispersing the extracellular polymeric substance (EPS) structure and further entering into the intracellularly by stimulated carrier protein SoxYZ and subsequently caused cell lysis. Typical enzyme activities and related functional gene abundances indicated that both hydrolysis and acidogenesis were remarkably enhanced while methanogenesis was substantially suppressed, which were further strengthened by the enriched hydrolytic bacteria (e.g. C10-SB1A) and acidogenic bacteria (e.g. Aminicenantales) but severely reduced methanogens (e.g. Methanolates and Methanospirillum). Economic analysis confirmed that thiosulfate pretreatment was a cost-effective and efficient strategy. The findings obtained in this work provide a new thought for recovering resource through thiosulfate-assisted WAS AF for sustainable development.

Optimizing waste activated sludge disintegration by investigating multiple electrochemical pretreatment conditions: Performance, mechanism and modeling

The complex and rigid floc structure often limits the reutilization of waste activated sludge (WAS). Electrochemical pretreatment (EPT) is one of the most effective technologies that can enhance WAS disintegration. But a comprehensive investigation into how multiple EPT conditions work was rarely reported. The study evaluated the effects of multiple EPT conditions, i.e., different electrolytes (NaCl, Na2SO4, and CaCl2), electrolytes dosage (0 g/L, 0.5 g/L, 1.0 g/L, and 3.0 g/L), EPT current (0 A, 0.5 A, 1.0 A, and 3.0 A) and EPT time (0 min, 30 min, 60 min, and 90 min) on WAS disintegration. The results showed that NaCl was outstanding from other electrolytes in promoting more WAS disintegration. Besides, a relatively higher NaCl dosage, a higher EPT current, and a longer EPT time promoted more reactive chlorine species (RCS), thus enhancing WAS disintegration in terms of extracellular polymeric substances (EPS) structure destruction and biodegradable organic matter release. After EPT for 60 min at NaCl dosage of 1.0 g/L and current of 1.0 A, the EPS multilayer structure destruction, biodegradable organic matters release, and soluble chemical oxygen demand (SCOD) increase in the supernatant were enhanced by 17.2 %, 130.5 %, and 238.7 %, respectively. Then a predictive quadratic model was established and the impact significance of the above EPT factors for enhancing WAS disintegration followed dosage of NaCl > current > EPT time. Furthermore, response surface methodology (RSM) suggested NaCl dosage of 2.75 g/L, current of 2.0 A, and EPT time of 30 min were the optimal EPT conditions, bringing a 42.0 % increase in the net economic benefit of WAS treatment compared to without EPT.

Novel Insights into the Mechanisms of Periodate-Based Pretreatment in Enhancing Short-Chain Fatty Acids from Waste Activated Sludge

Resource recovery in the form of short-chain fatty acids (SCFAs) from waste activated sludge (WAS) is usually limited due to the quantity and quality of SCFAs. To mitigate these restrictions, this study provides a novel method by using periodate (PI) pretreatment. With an increase in the PI concentration from 0 to 50 mg/g total solids (TS), the SCFA yield increased from 1486 to 3109 mg COD/L and acetic acid production was enhanced by more than twice. Further increase of PI had an adverse effect on SCFA production. Electron spin resonance analysis confirmed that hydroxyl radicals (•OH) and superoxide radicals (•O2–) might play crucial roles in the pretreatment process. Further mechanism studies revealed that PI pretreatment improved the disintegration of WAS and decomposition of humus as well as lignocellulose, thereby providing more substrates for SCFA production. The model-based synthetic wastewater tests combined with enzymatic analyses revealed that the stronger suppression of SCFA consumers compared to SCFA generators causes SCFA accumulation; specifically, the inhibitory order was methanogenesis > hydrolysis > acetogenesis > acidogenesis. The suppressed gene expression of acetate-dependent (such as cdh [EC: 2.1.1.245]) and CO2-type methanogenesis (such as fwd [EC: 2.3.1.101] and mch [EC: 1.5.98.1]) further triggered SCFA accumulation with PI addition. Moreover, PI pretreatment caused the microbial community to shift toward the favorable hydrolytic and acidification-associated (e.g., Longilinea, Acinetobacter, and Macellibacteroides) microorganisms. This study is expected to broaden an unknown application of PI and provide an alternative strategy for resource recovery in some specific circumstances.

Ultrasonic radiation enhances percarbonate oxidation for improving anaerobic digestion of waste activated sludge

Anaerobic digestion of waste activated sludge (WAS) has always been restricted by the low rate of sludge hydrolysis, due to the complex substrates structured by cell walls, intracellular constituents and extracellular polymeric substances (EPS), leading to a long retention time and a low methane output. This study reported a novel ultrasonic (US)-enhanced percarbonate oxidation to improve anaerobic WAS digestion. Results showed that US enhanced sodium percarbonate (SPC) oxidation and reduced the SPC dosage required for WAS disintegration as compared to sole SPC groups, with the cumulative methane yield being enhanced by 46.8 % after the optimal pretreatment, i.e., US + 0.15 g SPC/g TSS. Kinetic analysis demonstrated that US-enhanced SPC pretreatment boosted both the hydrolysis rate and biochemical methane potential of anaerobic WAS digestion, simultaneously. Mechanistic explorations revealed that US-enhanced SPC significantly promoted substrates solubilization, hydrolysis and acidification during the pretreatment stage, providing huge amounts of reaction intermediates directly available for subsequent methanation. Meanwhile, US-enhanced SPC improved biodegradability of soluble substrates, reduced particle size and increased specific surface area of WAS, which created preferable conditions for anaerobic biotransformation. Further analysis suggested that US-enhanced SPC boosted the metabolic activity associated with key bioprocesses in digestion system, in accord with enhanced methane production. Likewise, microbial community analysis illustrated the functional microbes (e.g., Romboutsia sp.) participating in key bioprocesses were enriched by US-enhanced SPC pretreatment, with their total abundances increasing from 13.3 % to 23.1 %.

Performance and mechanism of sodium citrate pretreatment to promote waste activated sludge disintegration and short-chain fatty acid production during anaerobic fermentation

Waste activated sludge (WAS) contains massive organic matter that can be used for resource recovery. Anaerobic fermentation to produce short-chain fatty acids (SCFAs) is recognized as an expedient and efficient measure to recycle sludge resource. Complexing agents have been reported to be effective in disintegrating extracellular polymeric substances (EPS) and increasing the rate of sludge hydrolysis. This study investigated the performance and mechanism of sodium citrate (SC) pretreatment to promote the disintegration of WAS and the production of SCFAs by anaerobic fermentation. The results showed that when SC dosage was 0.25 g/g TSS, both EPS and cell membrane were destroyed. Among them, the soluble protein and polysaccharide in the supernatant reached 1 072 ± 15 mg COD/L and 233 mg COD/L, and DNA reached 175 ± 3 mg/L at the fifth hour. Damage of EPS is mainly reflected in S-EPS and LB-EPS, while it is not obvious in TB-EPS. The SC pretreated group was much higher in SCFAs production, and the SC dosing of 0.25 g/g TSS group reached the peak acid production of 7 677 ± 350 mg COD/L on the third day, which was 8.3 times higher than that of the control group. Moreover, SC was able to increase the hydrolytic enzyme activity and inhibit the coenzyme F420 activity, which increased the production of SCFAs and decreased its consumption rate.

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.

Heat-assisted potassium ferrate pretreatment enhancing short-chain fatty acids production from waste activated sludge: Performance and mechanisms

This work presented an innovative strategy for short-chain fatty acids (SCFAs) production from anaerobic waste activated sludge (WAS) fermentation via heat assisted potassium ferrate (PF) pretreatment. Results showed that under optimal pretreatment condition (55 °C + 0.1 g PF/g TSS), SCFAs production achieved 4068.4 mg COD/L with acetic acid reaching 1766.1 mg COD/L, significantly higher than those of the control (77.39 mg COD/L, only acetic acid contained) as well as individual heat (2930.8 and 1203.5 mg COD/L) and individual 0.1 g PF/g TSS (1970.1 and 982.7 mg COD/L) pretreatment groups. Mechanism analysis revealed that heat-assisted PF pretreatment aggravated WAS disintegration by facilitating the disruption of EPS and cells, hence accelerating organics release from sludge flocs to the supernatant. Besides, the biodegradability of released substances was also improved thereby providing quantities of directly available substrates for hydrolysis and acidification process. Analysis of key enzymes involved in SCFAs accumulation showed that heat-assisted PF pretreatment promoted the relative bioactivities of protease, butyrate kinase (BK) and acetate kinase (AK) by 51.6%, 39.2% and 155.6% compared to the control respectively, while suppressing the bioactivity of coenzyme F420. Microbial community analysis demonstrated the enrichment of Proteobacteria and Firmicutes, which were related to SCFAs accumulation. Meanwhile, in genus level, hydrolytic bacteria (e.g., Acinetobacter sp.), acidification bacteria (e.g., Petrimonas sp.) and iron-reducing bacteria (e.g., Romboutsia sp.) were enriched while SCFAs consuming bacteria (e.g., Candidatus_Competibacter sp.) were inhibited.

Unveiling the Mechanism of Urine Source Separation-Derived Pretreatment on Enhancing Short-Chain Fatty Acid Yields from Anaerobic Fermentation of Waste Activated Sludge.

A novel strategy employing urine wastewater derived from source separation technology, to pretreat waste activated sludge (WAS) for promoting yields of short-chain fatty acids (SCFAs), has been proposed in this study. It was found experimentally that SCFA production could ascend up to 305.4 mg COD/g VSS (volatile suspended solids) with a urine volumetric proportion of 1:2 to the whole reaction system, being 8.8 times that produced in the control. Exploration of the mechanism indicated that WAS disintegration was significantly enhanced due to the synergistic effect of urea and free ammonia (FA). Degradation rates of model organic substrates and measurements of critical enzymatic activities demonstrated that hydrolysis and acidogenesis were inhibited under high urine content (urine proportion of 1:2), while not significantly affected under low urine content (i.e., 1:4), which might be attributed to metal ions existing in urine wastes alleviating the inhibition induced by FA. In contrast, methanogenesis was negatively suppressed by any urine concentration owing to its higher sensitivity to the environmental variations. Shift of microbial population further elucidated the abundance of hydrolytic and acidogenic microbes were enriched in the fermenters with urine addition. The findings provide a new thought for recovering resources from wastes, potentially reducing the pressure of sewage and sludge treatment in wastewater treatment plants.

Enhanced anaerobic digestion of waste activated sludge with periodate-based pretreatment

The potential of periodate (PI) in sludge anaerobic digestion is not tapped, although it has recently attracted great research interest in organic contaminants removal and pathogens inactivation in wastewater treatment. This is the first work to demonstrate significant improvement in methane generation from waste activated sludge (WAS) with PI pretreatment and to provide underlying mechanisms. Biochemical methane potential tests indicated that methane yield enhanced from 100.2 to 146.3 L per kg VS (VS, volatile solids) with PI dosages from 0 to 100 mg per g TS (TS, total solids). Electron spin resonance showed PI could be activated without extra activator addition, which might be attributed to the native transition metals (e.g., Fe2+) in WAS, thereby generating hydroxyl radical (•OH), superoxide radicals (•O2−), and singlet oxygen (1O2). Further scavenging tests demonstrated all of them synergistically promoted WAS disintegration, and their contributions were in the order of •O2− > •OH > 1O2, leading to the release of substantial biodegradable substances (i.e., proteins and polysaccharides) into the liquid phase for subsequent biotransformation. Moreover, fluorescence and ultraviolet spectroscopy analyses indicated the recalcitrant organics (especially lignocellulose and humus) could be degraded by reducing their aromaticity under oxidative stress of PI, thus readily for methanogenesis. Microbial community analysis revealed some microorganisms participating in hydrolysis, acidogenesis, and acetoclastic methanogenesis were enriched after PI pretreatment. The improved key enzyme activities and up-regulated metabolic pathways further provided direct evidence for enhanced methane production. This research was expected to broaden the application scope of PI and provide more diverse pretreatment choices for energy recovery through anaerobic digestion.

Thiosulfate-assisted Fe2+/persulfate pretreatment effectively alleviating iron dose and enhancing biotransformation of waste activated sludge into high-value liquid products.

Ferrous-based acidogenic fermentation (AF) as a means to treat waste activated sludge (WAS) and produce short-chain fatty acids (SCFAs) has drawn increasing attention, but the massive amount of "iron sludge" that it produces not only significantly increases costs and difficulty of disposal but also poses risks to the environment and human health. This study explored a novel approach to not only reduce the iron dosage required by AF but also to improve its performance by introducing a thiosulfate (TS)-assisted Fe2+/persulfate (TAFP) pretreatment. Effects of the TAFP pretreatment on WAS disintegration and biodegradability, SCFA production, and microbial community structure with different persulfate-Fe2+-thiosulfate molar ratios at 4:4:0 (R1), 4:3:1 (R2), 4:2:2 (R3) and 4:1:3 (R4) were investigated. The results showed that the TAFP pretreatment by a remarkable margin promoted the disintegration of WAS as well as the biodegradability of the organics released, owing to the production of robust free radicals (SO4•- and •OH) triggered by the thiosulfate and Fe2+ cycles. 48-day AF tests further showed maximum SCFA production, ranging roughly between 1283 and 1395mg COD/L in the TAFP pretreated samples, much higher than Control (<120mg/L) and R1 (around 593mg COD/L). At the meantime, the Fe2+ dosage was reduced by 50% in R3 than that of R1. However, a prolonged lag phase of SCFA generation was observed between days 7 and 25, which was ascribable to the acidic conditions (pH<4.5) closely related to impaired metabolic activities as well as electron transfer efficiencies and limited activities of acidogenic enzymes (i.e., pyruvate-ferredoxin oxidoreductase). Despite such lag phase, the economic and environmental assessment of AF of TAFP-pretreated WAS had a higher net SCFA yield and less "iron sludge" than either without any pretreatment or with Fe2+/persulfate-only pretreatment.

Enhanced volatile fatty acid production from anaerobic fermentation of waste activated sludge by combined sodium citrate and heat pretreatment

In this study the combination pretreatment of sodium citrate (SC) and heating was investigated to enhance the volatile fatty acid (VFA) production during anaerobic fermentation of waste activated sludge (WAS). The results showed that the combined pretreatment (0.3 g-SC/g-TSS + at 121 ℃ for 30 min) resulted to the maximum VFA yield of 354.5 mg-COD/g-VS, which was 16.4, 6.0 and 2.0-fold of that of control, with sole SC and sole heat pretreatment, respectively. Meanwhile, the combined pretreatment greatly improved the proportion of acetate and butyrate, while the production of propionate significantly decreased. Mechanism analysis indicated that the combined pretreatment effectively promoted WAS disintegration and solubilization. Moreover, the heat pretreatment exhibited superior performances to the SC pretreatment on release of organic matters and enhancement of VFA production in the combined pretreatment. The microbial analysis revealed that with the combined pretreatment the microbial community shifted toward hydrolysis-acidification, such as Enterobacter and Macellibacteroides.

Electrochemical oxidation process on palm oil mill effluent waste activated sludge: optimization by response surface methodology.

Biological-based treatment as the conventional treatment for palm oil mill effluent (POME) in open-ponding system face a well known rate-limiting step which is hydrolysis. In this study, electrochemical oxidation (EO) by a ruthenium oxide-coated titanium (Ti/RuO2) electrode was introduced as a pre-treatment for POME waste activated sludge (WAS). Surface morphology and elemental analysis were investigated using field emission scanning electron microscopy and energy dispersive X-ray spectroscopy, respectively. Response surface methodology type central composite design was used in this study to understand the relationship between the independent and dependent variables. Analysis of variance (ANOVA) was used to validate the model of the studied variables. The correlation coefficients (R2) indicated a close agreement between the experimental results and the predicted values, with high R2 values of 0.9044-0.9773. Multiple response optimization suggested that the range of current density (17-27 mA/cm2) and electrolysis time (55-75 min) at a fixed concentration of sodium chloride (10 g/L), resulted in mixed liquor volatile suspended solids (MLVSS) removal >20%, capillary suction timer (CST) reduction >43%, extracellular polymeric substances (EPS) increment <19% and soluble chemical oxygen demand (sCOD) increment >25%. EO appears to be an efficient pre-treatment as well as practical way to improve the POME WAS disintegration and dewaterability.

Ferric chloride aiding nitrite pretreatment for the enhancement of the quantity and quality of short-chain fatty acids production in waste activated sludge

The production of short-chain fatty acids (SCFAs) via anaerobic fermentation of waste activated sludge (WAS) is often limited with poor quality of SCFAs and long fermentation time. To overcome these issues, we provided an efficient strategy by using ferric chloride (FC) to aid nitrite pretreatment. Experimental results showed that the maximal SCFAs production of 211.3 ± 3.1 mg COD/g VS was achieved with 4 mmol/L of FC integrated with 250 mg/L of nitrite pretreatment on day 5, which was 4.1-fold higher than that of the blank control (52 ± 5 mg COD/g VS, day 7). Besides, the enrichment of acetic acid was observed in the combined system, which accounted for 54.6 ± 3.5% of total SCFAs, while the proportion was only 31.5 ± 4.9% in the blank control. Propionic acid, isobutyric acid, n-butyric acid, n-valeric acid and isovaleric acid accounted for 14.7 ± 1.5%, 6.9 ± 1.4%, 7.4 ± 1.5%, 13.1 ± 1.0%, and 3.3 ± 1.5% of total SCFAs in the combined system and 22.8 ± 4.0%, 11.9 ± 3.0%, 6.7 ± 3.1%, 17.6 ± 2.0%, and 9.5 ± 3.9% of total SCFAs in the blank control, respectively. It was found that soluble proteins and carbohydrates in the combined system were higher than those in the blank control, suggesting that FC and nitrite pretreatment was beneficial for WAS disintegration. The fluorescence spectrum results suggested that FC and nitrite pretreatment improved the biodegradability of released organics, which provided more biodegradable substances for the subsequent SCFAs production. This was because the addition of FC induced the formation of free nitrous acid from nitrite. Besides, FC-induced iron reduction also promoted the conversion of recalcitrant organics to biodegradable organic matter. Microbial community structure analysis demonstrated that the functional bacteria involved in acetogenesis process such as Enterococcus, Proteiniclasticum, and Petrimonas were highly enriched due to the pretreatment of FC and nitrite, indicating this method could improve the relative abundance of SCFAs producers. Overall, this study revealed that the pretreatment of FC and nitrite promoted the formation of free nitrous acid and increased the yield of SCFAs, which provided a novel method for wastewater treatment plants to ameliorate the sewage treatment craft and rationally use the existing substances in WAS to enhance resource recovery.

Impact of magnesium ions on lysozyme-triggered disintegration and solubilization of waste activated sludge

Lysozyme can efficiently accelerate solubilization and hydrolysis of waste activated sludge (WAS) for anerobic digestion. However, the effect of lysozyme was easily to be inhibited by metal ions in WAS. The impact of magnesium ions (Mg2+) on lysozyme catalyze WAS disintegration was investigated in this study. The effect of lysozyme on WAS hydrolysis could be hindered by Mg2+. Relatively high concentrations (>50 mg/L) of Mg2+ in sludge significantly reduced the release of soluble polysaccharides and proteins from WAS, while sulfate ions or chloride ions caused no such effect. Proteins were difficult to be extracted from extracellular polymeric substances (EPS) of WAS in the presence of Mg2+ (>10 mg/L) due to the divalent cation bridging (DCB) behavior, while the extraction of polysaccharides was not significantly affected. The polysaccharides and proteins in the inner EPS layer were transferred to the outer layer during the lysozyme treatment, and total quantities of both components maintained constantly. At least 23.1% lysozymes were trapped in the liquid phase of 100 mg Mg2+/L in the first hour. Mg2+ could significantly affect the transfer of lysozyme from liquid phase to the inner layer of sludge. Mg2+ neutralized the negative surface charge of the sludge particles, which hindered the absorption of positively charged lysozyme molecules by sludge flocs from the liquid phase. The proteins of TB-EPS had higher ratios of α-helixes and tighter structures than those in LB-EPS, which could impede the lysozyme transfer to the cell wall.