Waste Activated Sludge Hydrolysis 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.

Integration of hydrothermal liquefied sludge as wastewater for anaerobic treatment and energy recovery: Aqueous phase characterization, anaerobic digestion performance and energy balance analysis

For the hydrothermal liquefaction (HTL) process as the final method for waste activated sludge, the liquid by-product of the process, i.e., the HTL aqueous solution, poses the greatest bottleneck due to its large volume and contaminant levels. In this study, the effects of hydrothermal liquefaction pretreatment (HTL) on the performance of anaerobic digestion and energy balance of the waste-activated sludge (WAS) aqueous phase were investigated. The results showed that HTL could greatly increase the hydrolysis of WAS and enhance the protein, polysaccharide and volatile fatty acid contents in the aqueous phase. The anaerobic digestion showed that the optimal gas and methane production efficiency was achieved at 160 °C. Positive net energy gain was obtained in each process (ΔE) and the optimal HTL heat recovery rate was obtained at 160 °C. HTL was found to improve the hydrolyzing bacterial community in anaerobic digestion but had little effect on the archaeal community.

Enhancement of waste activated sludge hydrolysis and decomposition by combined free nitrous acid and sodium citrate pretreatment

Hydrolysis of waste activated sludge (WAS) is the limiting step in the anaerobic fermentation process. In view of this, this study evaluated the feasibility of a combination of free nitrous acid (FNA) and sodium citrate (SC) pretreatment to promote WAS hydrolysis and anaerobic fermentation. The results showed that the combined treatment of FNA and SC could promote the decomposition of WAS. Under the conditions of 2.504 mg/L FNA and 0.25 g/g total suspended solids of SC pretreatment for 5 h, the soluble chemical oxygen demand release rate (DDSCOD) was 1.86 and 1.42 times that of FNA and SC pretreatment alone. Meanwhile, the contribution rate of intracellular organic compounds reached 84.18 %. The maximum short-chain fatty acids yield was obtained at the 8th day of fermentation, which was 580.34 mg COD/g volatile suspended solids. The protein utilization rate of combined pretreatment was increased to 74.10 %, which was much higher than that of single treatment. Moreover, FNA-SC also changed the activities of key enzymes, increasing the activities of protease and α-glucosidase but inhibiting the activities of coenzyme F420. Microbial analysis showed that the combined pretreatment significantly changed the composition of microbial community and effectively enriched acid-producing bacteria (such as Petrimonas and Macellibacteroides).

Integrated microbial electrolysis with high-alkali pretreated sludge digestion: Insight into the effect of voltage on methanogenesis and substrate metabolism

Integrated microbial electrolysis with anaerobic digestion is proved to be an effective way to improve methanogenesis efficiency of waste activated sludge (WAS). WAS requires pretreatment for efficient improvement of acidification or methanogenesis efficiency, but excessive acidification may inhibit the methanogenesis. In order to balance these two stages, a method for efficient WAS hydrolysis and methanogenesis has been proposed in this study by high-alkaline pretreatment integrated with microbial electrolysis system. The effects of pretreatment methods and voltage on the normal temperature digestion of WAS have also been further investigated with emphasis on the effects of voltage and substrate metabolism. The results show that compared to low-alkaline pretreatment (pH = 10), high-alkaline pretreatment (pH > 14) can double the SCOD release and promote the VFAs accumulation to 5657 ± 392 mg COD/L, but inhibit the methanogenesis process. Microbial electrolysis can alleviate this inhibition effectively through the rapid consumption of VFAs and speeding up of the methanogenesis process. The optimal methane yield of the integrated system is 120.4 ± 8.4 mL/g VSS at the voltage of 0.5 V. Enzyme activities, high-throughput and gene function prediction analysis reveal that the cathode and anode maintain the activity of methanogens under high substrate concentrations. Voltage positively responded to improved methane yield from 0.3 to 0.8 V, but higher than 1.1 V is found to be unfavorable for cathodic methanogenesis and results in additional power loss. These findings provide a perspective idea for rapid and maximum biogas recovery from WAS.

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

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

Insights into the Effect of Rhamnolipids on the Anaerobic Fermentation and Microalgae Lipid Production of Waste Activated Sludge: Performance and Mechanisms

In the last few years, the production of waste activated sludge (WAS) has increased dramatically, and the treatment and disposal have attracted widespread attention. In this study, rhamnolipids were used to improve the efficiency of hydrogen production from WAS, and the fermentation solution was used as a substrate by lipid-producing microalgae to recover resources and generate clean energy. Results showed that rhamnolipids had the best effect among several surfactants in promoting hydrogen production, and the hydrogen production of WAS added with 0.4 g/g TS rhamnolipids reached 178.48 ± 6.07 mL/g TS, which improved by 13.53 times compared to the control group. The content of SCOD, soluble protein, and soluble polysaccharide in WAS fermentation solution with the addition of 0.4 g/g TS rhamnolipid increased by 9.19, 11.46, and 42.28 times, respectively, indicating that rhamnolipid could improve the hydrolysis of WAS, which led to an increase in the efficiency of hydrogen production. And three-dimensional excitation–emission matrix fluorescence spectroscopic analysis showed that the intensity of dissolved organic matter increased, suggesting a rise in soluble microbial metabolites. The analysis of the composition of volatile fatty acids in the fermentation solution showed that the type of hydrogen production was mainly butyric acid-type fermentation hydrogen production with high concentrations of n-butyric acid (9330.14 ± 525.33 mg/L) and acetic acid (6324.07 ± 80.67 mg/L). In addition, analysis of the microbial community structure of the WAS fermentation hydrogen production system showed that the dominant genus was Clostridium sensu stricto. Microalgae can utilize acetic acid, ethanol, NH4+-N, PO43–-P, dissolved proteins, and dissolved polysaccharides in the fermentation solution to grow and produce lipids. The biomass and lipid productivity of microalgae cultured by fermentation solution reached 3.16 ± 0.58 g/L and 356.86 ± 4.91 mg/L/day, respectively. The microalgae contained 84.57 ± 4.33% saturated fatty acid content, which was more favorable for biodiesel production. This study not only successfully utilized rhamnolipids to promote hydrogen production from WAS but also coupled microalgae lipid production system, further laying the foundation for realizing the resource utilization of waste resources and clean energy production.

Enhancing short-chain fatty acids recovery through anaerobic fermentation of waste activated sludge with cation exchange resin assisted lysozyme pretreatment: Performance and mechanism

Short-chain fatty acids (SCFAs) biosynthesis through anaerobic fermentation has attracted increasing attention for achieving economic benefits and carbon recovery in waste activated sludge (WAS) management. Lysozyme has been reported as a promising biological pretreatment for WAS hydrolysis and SCFAs generation, whereas the low efficiency and high dosage limited its application. In this study, cation exchange resin (CER) was innovatively employed for extracellular polymer substances (EPS) disruption, expecting to promote the lysozyme-induced enzymolysis and solubilization of dissolved organic matters (DOMs). With 3 g/g SS CER and 0.10 g/g SS lysozyme, 5404 mg/L SCOD (i.e. 36.5% of TCOD) and 43.1% VSS reduction rate were realized in the coupled pretreatment, which were 2.63 and 2.09 times than those with sole lysozyme pretreatment. Large amounts of DOMs were released attributed to EPS disruption and cell lysis, leading to 3760 mg COD/L SCFAs accumulation mainly composed of acetic and propionic acids (i.e. occupying 59.7–65.3%). The coupled pretreatment boosted the enrichment of hydrolysis and SCFAs production bacteria, accompanied by the corresponding metabolic functions enhancement (i.e. organic matters metabolism, microbial proliferation and membrane transport). The new sight into the potential mechanism between lysozyme and CER was proposed, and the engineering implication was demonstrated. The findings provided an environmental and economical solution to realize carbon recovery in WAS management.

The characteristics of stepwise ultrasonic hydrolysates of sludge for enhancing denitrification

The disposal of waste activated sludge (WAS) accounts for approximately 60 % of wastewater treatment plant operating costs. In this study, according to the reaction time and water quality parameters, ultrasonic hydrolysis of WAS is divided into three stages, including floc-disintegration (0–25.2 kJ/g TS), cell-disruption (25.2–36 kJ/g TS), and cell-degradation (over 36 kJ/g TS). The results show that more than 70 % carbon distributes inside the cell, which also contains 63.8 % protein enhancing denitrification capacity. Moreover, cell-degradation hydrolysate has a higher proportion of readily biodegradable COD, indicating that intracellular organic matter is more capable of denitrification than extracellular. Therefore, the optimal ultrasonic operating range is Es = 36–72 kJ/g TS as carbon source, and obtain the hydrolysate with high ratio of soluble chemical oxygen demand to total nitrogen for denitrification. Furthermore, this study supports the comprehensive interpretation of ultrasonic hydrolyzed WAS and the characteristics of hydrolysate as carbon source for enhancing denitrification.

In-situ sulfite treatment enhanced the production of short-chain fatty acids from waste activated sludge in the side-stream anaerobic fermentation

Sulfite-based technology could enhance methane production from anaerobic sludge digestion. However, its potential for in-situ direct sludge treatment without anaerobic sludge addition in the side-stream remains unclear. This study investigated the feasibility of using in-situ sulfite treating sludge for short-chain fatty acids (SCFAs) production via anaerobic fermentation of waste activated sludge (WAS) as a side-stream treatment. In-situ sulfite direct sludge treatment enhanced SCFAs and acetic acid production by 2.03 and 4.89 times at 500 mg S/L compared to the control. With in-situ sulfite treatment, WAS hydrolysis and acidification were enhanced while methanogenesis was spontaneously hindered. The in-situ sulfite treatment inactivated pathogens and improved the sludge dewatering properties. The relative abundances of SCFAs-production microbial were stimulated, facilitating the sludge bioconversion. The produced SCFAs from in-situ sulfite side-stream treatment could be applied as an “internal carbon source” to enhance biological nutrient removal to improve economic and environmental value from sludge treatment.

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 low bioconversion rate. Potassium ferrate (PF), as a green oxidant with strong oxidizing property, has been 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 of PF dosage resulted in the decreased methane production. Especially with the PF dosage of 500 g-K2FeO4/ kg-TS, methane production even slightly lower than the control reactor without PF oxidation. The mechanism analysis showed that although the dissolution of polysaccharides and proteins were 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.

Enhancing anaerobic fermentation of waste activated sludge by investigating multiple electrochemical pretreatment conditions: Performance, modeling and microbial dynamics

Electrochemical pretreatment (EPT) is an efficient technology to improve volatile fatty acids (VFAs) production during anaerobic fermentation of waste activated sludge (WAS). This study investigated the co-effects of different current intensities, electrolyte NaCl dosage and pretreatment time for promoting VFAs production. The results showed that it was considerably enhanced by 51.6 % when EPT was performed at 1.0 A, 1.0 g/L and 60 min, and response surface methodology strategy adjusted the optimal EPT conditions to 1.0 A, 1.2 g/L and 66 min. The potential mechanisms were proposed that EPT at optimal conditions greatly enhanced solubilization and hydrolysis of WAS and selectively inactivated methanogens, causing the enrichment of acidogenic bacteria (i.e., Lactobacillus, Saccharimonadales, Tetrasphaera and Prevotella) due to generated reactive chlorine species. Finally, the economic analysis indicated the promising application potential with the profit of EPT at optimal conditions increasing by 36.0 %.

Potential of anaerobic co-digestion of acidic fruit processing waste and waste-activated sludge for biogas production

Abstract The potential of acidic fruit processing waste (FPW) and waste-activated sludge (WAS) co-digestion for methane production was investigated using batch and continuous experiments. First, batch experiments showed the co-digestion of FPW and WAS solved acid accumulation and increased cumulative biogas production. When the volatile solid (VS) ratio of FPW to WAS was 2:1, the cumulative biogas production was the highest (4,695.47 mL), which increased by 5.2% and 10.5% compared with the VS ratios of 3:1 and 1:1, respectively. Methane production was the rate-limiting step when the FPW percentage was high, and hydrolysis was the rate-limiting step when the WAS percentage was high. Second, the continuous experiments showed fermentation was the most stable when the VS ratio was 2:1, without acid accumulation or excessive total alkalinity consumption. Additionally, the ammonia nitrogen content in the system was above 138.34 mg·L−1, which solved the slow WAS hydrolysis rate and low nitrogen supply efficiency. Microbial community structure of the reactor was correlated with substrate composition greatly. On the 180th day, the relative abundance levels of Bacteroides, Paludibacter, Litorilinea, Levilinea, and Smithella were higher than those on the 120th day and the 240th day. The enrichment of those bacterial groups was beneficial to improve the substrate hydrolysis rate and reduce the influence of organic acids on the anaerobic system.

Chronic effects of benzalkonium chlorides on short chain fatty acids and methane production in semi-continuous anaerobic digestion of waste activated sludge

As an emerging pollutant, benzalkonium chlorides (BACs) potentially enriched in waste activated sludge (WAS). However, the microbial response mechanism under chronic effects of BACs on acidogenesis and methanogenesis in anaerobic digestion (AD) has not been clearly disclosed. This study investigated the AD (by-)products and microbial evolution under low to high BACs concentrations from bioreactor startup to steady running. It was found that BACs can lead to an increase of WAS hydrolysis and fermentation, but a disturbance to acidogenic bacteria also occurred at low BACs concentration. A noticeable inhibition to methanogenesis occurred when BAC concentration was up to 15 mg/g TSS. Metagenomic analysis revealed the key genes involved in acetic acid (HAc) biosynthesis (i.e. phosphate acetyltransferase, PTA), β-oxidation pathway (acetyl-CoA C-acetyltransferase) and propionic acid (HPr) conversion was slightly promoted compared with control. Furthermore, BACs inhibited the acetotrophic methanogenesis (i.e. acetyl-CoA synthetase), especially BAC concentration was up to 15 mg/g TSS, thereby enhanced short chain fatty acids (SCFAs) accumulation. Overall, chronic stimulation of functional microorganisms with increasing concentrations of BACs impact WAS fermentation.

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.

Hydrolyzation characteristics of waste actived sludge and antibiotics release when improved with alkyl polyglycoside

The purpose of this study was to find the optimal dosage of APG and initial pH to accelerate the hydrolysis of the waste activated sludge (WAS) and promote the release of antibiotics. In this study, the effects with APG dosage from 0.1 to 0.5 g/gSS and initial pH from 4.0 to 12.0 on WAS hydrolysis were investigated. Samples were regularly collected from different reaction conditions to determine SCODcr, protein, polysaccharide, antibiotics, antibiotic resistance genes (ARGs) and microbial diversity. The results showed that the combined action of APG+pH greatly promoted the solubilization of organic matter, and promoted the release of antibiotics. Compared with untreated sludge, when the optimal APG dosage was 0.3 g/gSS with pH 11.0 was combined, the release of organic matter was significantly increased, and the concentrations of the SCODcr, protein and polysaccharide reached 5650.4, 1386.4 and 794.2 mg/L, respectively within 2 h. At the same time, antibiotics (tetracyclines, sulfonamides, quinolones and macrolides) was also transferred from solid phase to liquid phase, among them roxithromycin (ROM) was 72.6 times higher than that in untreated control group, accordingly, the tetracycline ARGs tetB was 774.2 times higher than that in the control group. Meanwhile, APG greatly reduced the microbial diversity in the WAS samples, and greatly increased the relative abundance of proteobacteria, in which Gammaproteobacteria was the main microorganism.

Reconsidering hydrolysis kinetics for anaerobic digestion of waste activated sludge applying cascade reactors with ultra-short residence times

Hydrolysis is considered to be the rate-limiting step in anaerobic digestion of waste activated sludge (WAS). In this study, an innovative 4 stages cascade anaerobic digestion system was researched to (1) comprehensively clarify whether cascading configuration enhances WAS hydrolysis, and to (2) better understand the governing hydrolysis kinetics in this system. The cascade system consisted of three 2.2 L ultra-short solids retention times (SRT) continuous stirred tank reactors (CSTRs) and one 15.4 L CSTR. The cascade system was compared with a reference conventional CSTR digester (22 L) in terms of process performance, hydrolytic enzyme activities and microbial community dynamics under mesophilic conditions (35 °C). The results showed that the cascade system achieved a high and stable total chemical oxygen demand (tCOD) reduction efficiency of 40–42%, even at 12 days total SRT that corresponded to only 1.2 days SRT each in the first three reactors of the cascade. The reference-CSTR converted only 31% tCOD into biogas and suffered process deterioration at the applied low SRTs. Calculated specific hydrolysis rates in the first reactors of the cascade system were significantly higher compared to the reference-CSTR, especially at the lowest applied SRTs. The activities of several hydrolytic enzymes produced in the different stages revealed that protease, cellulase, amino peptidases, and most of the tested glycosyl-hydrolases had significantly higher activities in the first three small digesters of the cascade system, compared to the reference-CSTR. This increase in hydrolytic enzyme production by far exceeded the increase in specific hydrolysis rate, indicating that hydrolysis was limited by solids-surface availability for enzymatic attack. Correspondingly, high relative abundances of hydrolytic-fermentative bacteria and hydrogenotrophic methanogens as well as the presence of syntrophic bacteria were found in the first three digesters of the cascade system. However, in the fourth reactor, acetoclastic methanogens dominated, similarly as in the reference-CSTR. Overall, the results concluded that using multiple CSTRs that are operated at low SRTs in a cascade mode of operation significantly improved the enzymatic hydrolysis rate and extend in anaerobic WAS digestion. Moreover, the governing hydrolysis kinetics in the cascading reactors were far more complex than the generally assumed simplified first-order kinetics.

Aged landfill leachate enhances anaerobic digestion of waste activated sludge

Anaerobic digestion (AD) is considered as a sustainable pathway to recover energy from organic wastes, but the digestive efficiency for waste activated sludge (WAS) is not as expected due to the limitations in WAS hydrolysis. This study proposes an effective strategy to simultaneously treat WAS and landfill leachate, aiming to promote WAS hydrolysis and enhance organics converting to methane. The effects of landfill leachate on the four stages (i.e., solubilization, hydrolysis, acidogenesis, and methanogenesis) of AD of WAS, as well as the effect mechanisms were investigated. Results showed that adding appropriate amounts of landfill leachate could promote the steps of solubilization, hydrolysis and acidogenesis of WAS, but had no-effect on methanogenesis. The hydrolysis and acidogenesis efficiency in the leachate added digesters were 2.0%–8.4% and 35.2%–72.7% higher than the control digester. Mechanism studies indicated that humic acid (HA) contained in the leachate was conducive to the processes of both hydrolysis and acidogenesis, but detrimental to the methanogenesis. Effects of heavy metals (HMs) on AD of WAS was also dose-dependent. Digestive performance was inhibited by excessive HMs but promoted by moderate dosages. Humic acid and metal ions tend to interact to form complexes, and thus relieve their each inhibition effects. It is also found that the stability of sludge flocs was reduced by the leachate through reducing both apparent activation energy (AAE) and median particle size (MPS) of the sludge. Microbial community and diversity results revealed that the relative abundance of microbes responsible for hydrolysis and acidogenesis increased when landfill leachate was present. This research provides a more technically and economically feasible approach to co-treating and co-utilizing WAS and landfill leachate.

Aerobic sludge digestion is distinguishingly affected by the different entering pathways of zinc oxide nanoparticles

Zinc oxide nanoparticles (ZnO NPs) are widespread emerging pollutants raising global concerns about their influences on biological wastewater treatment processes. However, the impacts of ZnO NPs on aerobic sludge digestion that is a major sludge treatment process remain unknown. Herein, this study comprehensively investigated the key influences of ZnO NPs on aerobic digestion of waste activated sludge (WAS) and the potential mechanisms involved. Two different entering pathways, i.e., ZnO NPs directly entered into aerobic sludge digester and ZnO NPs initially entered into wastewater bio-treatment reactor, were tested to evaluate the different impacts. Compared to the control, ZnO NPs initially entering into wastewater bioreactor inhibited WAS degradation by 18.2 ± 0.1%, whereas ZnO NPs immediately entered into digester inhibited it by 29.7 ± 0.1%. This was accompanied by a similar decrease in inorganic nitrogen production and oxygen consumption. ZnO NPs exposure in wastewater bioreactor changed WAS characteristics in favor of solubilization in aerobic digestion. Modelling analysis indicated that ZnO NPs inhibited WAS hydrolysis, especially for their direct entering into aerobic digester. Correspondingly, microbial community was shifted in the direction against aerobic digestion by the ZnO NPs. Excessive oxidative stress and Zn2+ release represented the primary toxicity factors for the inhibition.

Polyamide 6 microplastics facilitate methane production during anaerobic digestion of waste activated sludge

Polyamide 6 (PA6) as a typical emerging microplastic pollutant has frequently been featured in wastewater treatment plants (WWTPs), yet its associated impact on waste activated sludge (WAS) anaerobic digestion process have not been fully understood. This study identified the key role of PA6 microplastics (5–50 particles/g TS) in methane production during anaerobic digestion through biochemical methane production testing and model-based analysis, and explored the mechanism involved by monitoring the conversion of metabolic intermediates, the activity of key enzymes, and the effects of leachate caprolactam (CPL). Unlike other microplastics that normally inhibit anaerobic digestion, PA6 significantly enhanced methane production due to the effect of CPL. The presence of 10 particles PA6/g TS improved methane production by 39.5% (from 124 ± 6 to 173 ± 8 L CH4/kg VS). Model-based analysis showed that PA6 promoted methane production potential and volatile solids destruction. PA6 offered marginal effect on the solubilization and hydrolysis of WAS, the leaching of CPL improved acidification and methanogenesis due to the promotion of key enzyme activities, which is the main reason why PA6 increased methane production. These findings indicate that PA6 is less toxic to WAS anaerobic digestion than other microplastics, which prompts an understanding of the dual role of microplastics in WWTPs.

Insight into efficient phosphorus removal/recovery from enhanced methane production of waste activated sludge with chitosan-Fe supplementation

Fe(III)-loaded chitosan (CTS-Fe) composite was used for the first time to remove and recover phosphorus (P) from waste activated sludge (WAS) via anaerobic digestion (AD). The P transformation pathway and the effect of CTS-Fe addition on the AD process were investigated using batch experiments. The P fractionation results indicate that non-apatite inorganic phosphorus (NAIP) reduction in the solid phase of sludge at 20 g/L of CTS-Fe addition (6.72 mg/g-SS) was 2.4 times higher than that in the control (2.77 mg/g-SS, no CTS-Fe addition). This is probably brought about by the added CTS-Fe enhanced the reduction of Fe(III)-P compounds in the sludge with phosphate released into the liquid phase. CTS-Fe can efficiently recover 95% of P from the liquid digestate of WAS. Notably, partial Fe(III) on the CTS-Fe was reduced and effectively combined with P to form vivianite crystals on the CTS-Fe surface during the AD process. Characterization analysis demonstrated that ligand exchange and chemical precipitation were the dominant mechanisms for P removal/recovery. Furthermore, the addition of CTS-Fe increased methane production by 11.9 - 32.2% under the tested conditions, likely attributable to the enhanced hydrolysis of WAS under CTS-Fe supplementation. As the P-loaded CTS-Fe particles can be easily separated and recovered from the AD system and further reutilized in agriculture, this study could provide a new approach for simultaneous P removal/recovery and enhanced methane production from AD of WAS.