Free Nitrous Acid Pretreatment Research Articles

Impacts of free nitrous acid on stabilizing food waste and sewage sludge for anaerobic digestion

This work investigated the effectiveness of free nitrous acid (FNA) in enhancing organic waste solubilization to improve biogas production in anaerobic digestion (AD). The results indicated that FNA pretreatment can enhance soluble organic content and control H2S odor in tested organic wastes, including food waste, sewage sludge, and their combination. However, a significant decrease (>50 %) in FNA concentration was found in the reactors, possibly due to denitrifier-driven NO2– consumption. Biochemical methane potential (BMP) tests showed a 25 ± 8 % enhancement in CH4 production in the reactors fed with mixed substrate pretreated with 2.9 mg FNA-N/L. However, the presence of NO2– (325.6–2368.0 mg N/L) in some BMP reactors, due to carryover from FNA pretreatment, adversely affected CH4 production (>55 %) and prolonged lag time (>4.2 times). These findings are valuable for researchers and practitioners in waste management, offering insights for implementing FNA pretreatment to enhance the biodegradability of organic wastes in AD.

Nitrite acts as the key “switch” governing the free nitrous acid pretreatment in anaerobic digestion: A comprehensive mechanism of abiotic, bioinformatics and bioenergetics effects

Free nitrous acid (FNA) pretreatment has been an effective and environmentally-friendly strategy to improve anaerobic digestion (AD) performance, and the mechanism was generally ascribed to the abiotic role of enhancing substrate solubilization. However, a comprehensive understanding of FNA is lack considering the different biotic roles on various steps, microbes and functional genes, as well as the variation of thermodynamic conditions. Herein, FNA pretreatment significantly enhanced the AD performance of food waste (FW), and the maximum increment of methane yield (36.45 %) was observed for FNA level of 1.42 mg HNO2 − N/L. Nevertheless, FNA exhibited an inhibition-to-enhancement effects as the AD process proceeds, and the introduced nitrite was the key “switch”. Initially, the abiotic effects of FNA reduced the molecular weight of components in FW, benefiting the solubilization and formation of available organics (glucose and amino acid). The individual steps experiment confirmed that residual nitrite severely inhibited methanogenesis rather than hydrolysis and acidogenesis, which resulted in the decrement of methane yield and accumulation of VFAs. Meanwhile, the methanogens (Methanothrix and Methanobacterium) and genes involved in methanogenesis metabolisms (aceticlastic and hydrogenotrophic pathways) were downregulated. However, with the elimination of nitrite, the microbial activities and gene expressions restored and were even enhanced, and Gibbs-energy-based analysis suggested that accumulated VFAs provided favorable thermodynamic conditions for subsequent methanogenesis. This study firstly proved that the abiotic breakdown of macromolecule components, selective and reversible inhibition on microbes and functional genes induced by nitrite and varying thermodynamic conditions together governed the FNA affecting the AD process of FW.

Enhanced methane yield in anaerobic digestion of waste activated sludge by combined pretreatment with fungal mash and free nitrous acid

This study explores a novel approach for enhancing anaerobic digestion of waste activated sludge (WAS) through the combined pretreatment of fungal mash and free nitrous acid (FNA). Aspergillus PAD-2, a fungal strain with superior hydrolase secretion, was isolated from WAS and cultivated in-situ on food waste to produce fungal mash. The solubilization of WAS by fungal mash achieved a high soluble chemical oxygen demand release rate of 548 mg L−1 h−1 within first 3 h. The combined pretreatment of fungal mash and FNA further improved the sludge solubilization by 2-fold and resulted in a doubled methane production rate of 416±11 mL CH4 g−1 volatile solids. The Gompertz model analysis revealed a higher maximum specific methane production rate and shortened lag time by the combined pretreatment. These results demonstrate that the combined fungal mash and FNA pretreatment offers a promising alternative for fast anaerobic digestion of WAS.

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).

Free nitrous acid-assisted asymmetrical alternating current electrochemistry (FNA-AACE) for multi-heavy metals decontamination in waste activated sludge

Heavy metal contamination of waste activated sludge (WAS) is a key factor limiting the land application of sludge for nutrients recovery. This study proposes a novel free nitrous acid (FNA)-assisted asymmetrical alternating current electrochemistry (FNA-AACE) process to achieve high-efficiency decontamination of multi-heavy metals (Cd, Pb, and Fe) in WAS. The optimal operating conditions, the heavy metal removal performance of FNA-AACE, and the related mechanisms for maintaining the high performance were systematically investigated. During the FNA-AACE process, FNA treatment was optimal with an exposure time of 13 h at a pH of 2.9 and an FNA concentration of 0.6 mg/g TSS. Then the sludge was washed with EDTA in a recirculating leaching system under asymmetrical alternating current electrochemistry (AACE). The 6-h working and the following electrode cleaning were defined as a working circle of AACE. After three cycles of working-cleaning periods in AACE treatment, the cumulative removal efficiency of the toxic metals Cd and Pb reached over 97% and 93%, respectively, whilst that of Fe was greater than 65%. This surpasses most previously reported efficiencies and possesses a shorter treatment duration and sustainable EDTA circulation. The mechanism analysis suggested that FNA pretreatment provoked the migration of heavy metals for leaching enhancement, as well as reduced the demand for EDTA eluent concentration and increased conductivity, which can improve the AACE efficiency. Meanwhile, the AACE process absorbed the anionic chelates of heavy metals and reduced them to zero-valent particles on the electrode, regenerating the EDTA eluent and maintaining its high extraction efficiency for heavy metals. In addition, FNA-AACE could provide different electric field operation modes, allowing it to have flexibility for the real application processes. This proposed process is expected to be coupled with anaerobic digestion in wastewater treatment plants (WWTPs) for high efficiency of heavy metal decontamination, sludge reduction, and resource/energy recovery.

Effects of free nitrous acid combined with alkyl polyglucoside on short-chain fatty acids production from waste activated sludge anaerobic fermentation and fermentation liquor for polyhydroxyalkanoates synthesis

Free nitrous acid (FNA) pretreatment and alkyl polyglucoside (APG) addition are environmentally-friendly methods for enhancing waste activated sludge (WAS) anaerobic fermentation (AF) performances. The short-chain fatty acids (SCFAs) produced are available substrates for polyhydroxyalkanoates (PHAs) synthesis to reduce the C-source cost. This study aims to investigate the optimal fermentation conditions of FNA + APG treatment and the performance of PHAs synthesis by the SCFAs from AF system. Through investigating the FNA + APG combined treatment effects, key enzyme activities, and microbial community variation, the AFL from optimal fermentation condition was utilized as a C-source for PHAs synthesis. Experimental results indicated that 0.653 mg/L FNA + 0.075 g/g VSS APG treatment could synergistically achieve the maximum total SCFAs yield of 324.94 mg COD/g VSS at day 5 of AF, which was much more than those of sole FNA or APG treatment. Mechanism analysis indicated that FNA + APG treatment could promote WAS solubilization, hydrolysis and acidogenesis, while severely inhibiting methanogenesis. Microbial community analysis results demonstrated that SCFA producers, including Enterococcus and Clostridium, became the dominant genera in the FNA + APG reactor. Finally, AF liquor after ammonia removal was applied for batch-mode PHAs synthesis, and PHA yields increased to a maximum of 44.44 % of biomass (w/w) after five operation periods.

Novel free nitrous acid and ultrasonication pretreatment enhanced sludge biodegradability

ABSTRACT The main goal of this study was to investigate the novel combined Ultrasonication and Free Nitrous Acid (FNA) pretreatment on biodegradability and kinetics of thickened waste-activated sludge (TWAS). Partial factorial design with four levels of (0, 600, 1500, and 3000 KJ/Kg) for ultrasonication and 0, 0.7, 1.4, and 2.8 mg HNO2-N/L for FNA dose were examined creating 16 different combinations. Results revealed that combined pretreatment could significantly improve solubilization and solid destruction compared to solo pretreatments. The highest organic matter solubilization of 25.6% and volatile suspended solids destruction of 21.7% were observed when 2.8 mg HNO2-N/L and 1500 KJ/Kg were combined. Moreover, combining the pretreatments further enhanced biodegradability up to the highest percentage of 50.3% when pretreatment of 3000 KJ/Kg and 2.8 mg HNO2-N/L was applied. Also, the experimental data from a biochemical methane potential test was fitted well into First Order Kinetic and Modified Gompertz models, given that the coefficients of determination, R2, for models at all treatment levels were above 99%.

A novel strategy for efficiently transforming waste activated sludge into medium-chain fatty acid using free nitrous acid

The global energy crisis is approaching due to rapid population growth and overexploitation of fossil fuels. Therefore, the development and use of new and renewable energy sources is already in the extreme urgency. This work developed a novel technology to efficiently produce renewable liquid bioenergy from discarded wastes, by effectively transforming sewage sludge into high-value medium chain fatty acids (MCFA). The maximum MCFA yield in the anaerobic sludge fermentation was revealed to be 10.6 times of control when utilizing sewage sludge with 1.78 mg-N/L free nitrous acid (FNA) pretreatment. The carbon flow from sewage sludge into MCFA in the fermentation system was significantly enhanced with appropriate levels (0.71–1.78 mg-N/L) of FNA pretreatment. Compared to FNA pretreatment, however, its direct addition severely inhibited total products (i.e., carboxylates and complex alcohols) generation because of the toxicity on live cells (decreasing to 8.3 %–13.9 %) in sludge. Kinetic models (one-substrate and two-substrate) were utilized to investigate the mechanism of MCFA promotion by FNA pretreatment on anaerobic sludge fermentation, in which linear relationship analysis between FNA-derived organic release and the fitted parameters were also performed. The results indicated that the conversion of refractory materials into rapidly bioavailable substrates for MCFA production contributed to increasing MCFA production rate and potential. Moreover, the relative abundances of functional microorganisms related to hydrolysis-acidification and chain elongation process increased under FNA pretreatment, further favoring the MCFA production. This study provides a novel and effective technology of sludge energy recovery that can achieve the next-generation sustainable sewage sludge management.

Free nitrous acid (FNA) pretreatment enhances biomethanation of lignocellulosic agro-waste (wheat straw)

Agro-residues having lignocellulosic biomass are considered the most effective source (heating value 16 MJ/kg) for energy production through anaerobic digestion (AD). However, complex lignocellulosic fraction in agro-residue obstructs its biotransformation and is a rate-limiting step of the AD process. This study presents a novel free nitrous acid (FNA) pretreatment to enhance the biomethanation of wheat straw (WS). The effect of FNA pretreatment was evaluated through anaerobic digestion test and model-based evaluation performed in two phases, i.e., mono-digestion (phase I) and co-digestion of wheat straw with cattle manure and food waste (phase II). For phase I, the first-order, transference, and dual pool first-order models showed the best model fit with R2 > 0.98, but the dual pool first-order model (DP-FOM) showed the least root mean square error (RMSE) of 1.88–62.10. The DP-FOM analysis of assays from anaerobic digestion test showed the biogas yields of 576, 564, and 565 mL/g VSadded at 1.77, 3.54, and 5.31 mg HNO2–N/L FNA concentrations and 12 h reaction time, respectively. The corresponding increment in biogas yields was 10.87, 8.85 and 8.80%. In phase II, DP-FOM analysis revealed the best model fit with R2 > 0.99 and the least RMSE of 8.88–13.04. The DP-FOM analysis from assays from anaerobic digestion test for 12 h FNA pretreatment showed the peak biogas yields of 715, 648, and 654mL/gVSadded at 1.77, 3.54 and 5.31 mg HNO2–N/L FNA concentrations, respectively. The corresponding improvement in biogas yields was 32.24, 19.81, and 20.90%. The FNA pretreatment can potentially improve the biogas yield from wheat straw effectively under mono- and co-digestion conditions. Even co-digestion could achieve enhanced biogas yield over mono-digestion.

Enhanced two-phase anaerobic digestion of waste activated sludge by combined free nitrous acid and manganese dioxide

Anaerobic digestion (AD) is a mature and reliable technology for sludge treatment, but it still faces many technical problems including slow hydrolysis rate and low methane yield. Free nitrous acid (FNA) pretreatment was previously confirmed in enhancing sludge hydrolysis and extracellular polymeric substances (EPS) disruption in anaerobic digestion. It was hypothesized that, due to the manganese dioxide (MnO2) addition during methanogenic stage, direct interspecies electron transfer (DIET) facilitated the biological activities, thus boosting the methane production in FNA-pretreated two-phase AD model. The results showed that the methane production of the combination of FNA pretreatment and MnO2 addition was improved by 18.64% and 22.23%, compared with reactors that solely treated by either MnO2 addition or FNA pretreatment. The changes of microbial metabolism activity were evaluated by measuring the coenzyme F420 and electron transfer system (ETS). The activities of coenzyme F420 and ETS were increased to 156.26% and 134.71% in two-phase AD model, respectively. Meanwhile, the obvious microbial community succession was found with the enrichment of methanogens such as Methanosarcina and Methanobacterium. Overall, the combination of FNA pretreatment and MnO2 addition avoided the inhibition of FNA pretreatment on methanogenesis in the early stage, and showed positively synergistic effect on methane production. The enhancement of microbial metabolism was responsible for, promoting methane production in the two-stage AD model. This research provides an alternative strategy for efficiency improvement of anaerobic sludge digestion as well as methane production.

Improving sludge dewaterability by free nitrous acid and lysozyme pretreatment: Performances and mechanisms

Reducing the water content of waste activated sludge (WAS) is critical for sludge treatment and disposal in wastewater treatment plants (WWTPs). In this study, a new combined conditioning processes by using lysozyme (LZM) and free nitrous acid (FNA) were proposed and demonstrated to enhance the dewaterability of WAS. The water content of sludge cake dropped from 82.82 % to 68.42 % (1 h FNA treatment + 1 h LZM treatment) and 69.52 % (6 h FNA treatment + 1 h LZM treatment) with the combined FNA and LZM treatment; and the corresponding capillary suction time (CST) reduction efficiency increased 49.29 % (1 h FNA treatment + 1 h LZM treatment) and 52.98 % (6 h FNA treatment + 1 h LZM treatment). A comprehensive investigation conducted in this study revealed the underlying mechanism of dewaterability improvement lies in the transformations of extracellular polymeric substances (EPS). The combined conditioning led to enhanced hydrophobicity in the sludge, as suggested by FTIR protein secondary structure and interfacial free energy. The reduced zeta potential and the potential barrier indicated the reduction of the repulsive force of sludge particles and the bound water content in the conditioned floc. The hydrophobicity, flow permeability and flocculability were enhanced after combined treatment, leading to the release of bound water.

Combining high pressure homogenization with free nitrous acid pretreatment to improve anaerobic digestion of sewage sludge.

Single pretreatment of sewage sludge, either physical, chemical or biological, has its own drawbacks in term of poor sanitization, energy intensity and high operational and capital cost. To tackle these drawbacks, combined high pressure homogenization (HPH) and free nitrous acid (FNA) pretreatment for sludge solubilization and further biodegradation in anaerobic digestion was investigated. Synergistic effect of combined HPH (40MPa) and FNA (2.49mg/L) pretreatment (HPH-FNA) for improving anaerobic digestion was evaluated, and its effect on archaeal and bacterial community structure was analyzed. Compared with single HPH and FNA pretreatments, HPH-FNA pretreatment efficiently solubilized wasted activated sludge (WAS), subsequently improved anaerobic digestion. Cumulative biogas production from sewage sludge pretreated with HPH-FNA was 154%, 108% and 284% more than that with single pretreatment of FNA, HPH and raw sludge, respectively. In addition, volumetric biogas production of combined pretreatment system (815ml) was more than the sum from single pretreatment (710ml). Methane content in biogas for raw sludge, FNA, HPH and HPH-FNA pretreated sludge was 45%, 51%, 55% and 65%, respectively. Illumina MiSeq sequencing analysis revealed that HPH-FNA pretreatment promoted bacterial growth of phyla Bacteroidetes, Firmicutes and Synergistetes and archaeal genera Methanospirillum and Methanosaeta. Overall, combined HPH-FNA pretreatment of sewage sludge, prior to anaerobic digestion, is an environmentally-friendly and potentially economic technology.

Increasing the removal efficiency of antibiotic resistance through anaerobic digestion with free nitrous acid pretreatment

Swine manure is a significant reservoir for antibiotic resistance. Anaerobic digestion (AD) is a common biological process used to treat swine manure but still faces low efficiencies in biogas production and antibiotic resistance removal. It is here shown that AD with free nitrous acid pretreatment (FNA) was effective in reducing antibiotic resistance genes (ARGs) in swine manure. FNA pretreatment (nitrite =250 mg N/L, pH=5.0, temperature=20 ± 1 °C) simultaneously reduced antibiotics (Tetracyclines, Quinones and Sulfonamides), inactivated antibiotics resistance bacteria (ARB) by 0.5–3 logs, and decreased ARGs tet, sul and qnr by 1–2, 1–3 and 0.5 logs, respectively. In the following AD step, the total residual ARGs was reduced to ~3.49 × 107 gene copies/g dry total solids (TS), ~1 log lower than that in the AD without pretreatment (3.55 ×108 gene copies/g dry TS). Microbial community and network analyses revealed that the ARG removal was mainly driven by the direct FNA effect on reducing ARGs and antibiotics, not related to ARB. Besides, the FNA pretreatment doubled biochemical methane production potential from swine manure. Together these results demonstrate that AD with FNA pretreatment is a useful process greatly facilitating swine manure management.

Improving Sludge Dewaterability by Free Nitrous Acid and Lysozyme Pretreatment: Demonstrations and Mechanisms

Improving Sludge Dewaterability by Free Nitrous Acid and Lysozyme Pretreatment: Demonstrations and Mechanisms

Optimization and system energy balance analysis of anaerobic co-digestion process of pretreated textile dyeing sludge and food waste

Anaerobic digestion treatment can transform textile dyeing sludge (TDS) to resources, but direct anaerobic digestion of TDS products only little biogas. In order to improve the effect of anaerobic digestion, pre-treatment and co-anaerobic digestion were performed in this study. Two pre-treatment methods, which are thermal pre-treatment and free nitrous acid (FNA) pre-treatment, were selected for comparison. Then, food waste (FW), as the co-digestion substance, co-digested with thermally pre-treated TDS to explored the optimized ratio of pre-treated TDS and FW. Results indicated that both the two pre-treatment methods have a certain solubilization effect on TDS, but the effect of thermal pre-treatment is better than that of FNA. The optimal mixing ratio of thermally pre-treated TD and FW is 1:1. The removal rates of soluble chemical oxygen demand and aromatic amines were the highest (75% and 64%, respectively). The cumulative methane yield reached 479.9 mL CH4/g VS, which was increased by 20–40% compared to others ratios and 66.6% to the TDS. Furthermore, system energy balance analysis was evaluated. Based on the dyeing wastewater treatment plant with a daily treatment capacity of 15000 t, electricity produced by biogas can completely satisfy the electric consumption of the wastewater treatment plant and the anaerobic co-digestion system with 57.69% and 41.78% for the respective. It was found that TDS can be reused resourcefully by the AcoD and costs can be significantly reduced.

Enhancing anaerobic digestion using free nitrous acid: Identifying the optimal pre-treatment condition in continuous operation

Free Nitrous Acid (FNA) pre-treatment is a promising technology demonstrated effective in improving waste activated sludge degradability and anaerobic digestion (AD) performance. Pre-treatment conditions including FNA concentration and treatment duration determine operational and capital cost of full-scale implementation, which have not been studied in long-term experiments. The knowledge of FNA pre-treatment conditions improving the AD performance is urgently required to determine suitable conditions for the technology implementation. In this work, five different FNA concentrations (2.2, 4.4, 7.2, 12 mgN/L and nitrite only without pH adjustment) and three treatment durations (8, 24 and 48 h) were studied in four lab-scale semi-continuous AD reactors for over 300 days. FNA pre-treatment was shown under all tested conditions effective in enhancing AD performances, while its effectiveness and resulted benefits varied substantially amongst different pre-treatment conditions. The long-term experiment demonstrated that the methane production, sludge reduction and digested sludge viscosity of AD are positively correlated with the FNA concentration and durations, until an optimal condition is reached, which was identified in this work to be FNA concentration of 7.2 mgN/L and treatment duration of 24 h. Microbial community changes supported the apparent observation of enhanced sludge degradation at elevating FNA concentrations applied during pre-treatment. The short-term sludge solubilization results were inconsistent with the long-term AD performance, which was potentially caused by inhibitions from stringent FNA pre-treatment conditions applied (FNA = 12 mgN/L with 24-hour treatment & FNA = 7.2 mgN/L with 48-hour treatment). Overall, results suggested FNA pre-treatment at the optimized condition is highly beneficial to WWTPs and competitive with other pre-treatment technologies, e.g., thermal hydrolysis pre-treatment. This work comprehensively evaluated the key design parameters of FNA pre-treatment process, reached a major milestone in the development and applications of FNA technologies.

The origin of waste activated sludge affects the enhancement of anaerobic digestion by free nitrous acid pre-treatment

Anaerobic digestion is a common stabilization method for treating primary sludge (PS) and waste activated sludge (WAS). However, its application is often limited by the degradation of WAS. Recent studies have demonstrated FNA to be an effective pre-treatment for enhancing WAS degradability, while having limited effect on PS degradability. WAS characteristics are impacted by wastewater treatment plant (WWTP) configuration and this study is the first to compare the effectiveness of FNA pre-treatment on WAS from WWTP with and without primary treatment. In this study, WAS samples were collected from four full-scale WWTPs with or without primary treatment. Sludge characterization, biomethane potential tests and mathematical modeling were conducted to assess the impacts of FNA pre-treatment on anaerobic digestion. The results showed that FNA pre-treatment was consistently effective for WAS from different WWTPs, while the extent of enhancement varied between WWTPs. For WAS from WWTPs without primary treatment, FNA pretreatment increased the rate of hydrolysis by 54–66% compared to 22–33% increase for WAS without primary treatment. In contrast, WAS from WWTPs with primary treatment experienced greater increases in methane potential (22–24%) compared to WAS from WWTPs without primary treatment (14–16%). These variances could be associated with primary treatment impacting the wastewater COD/N ratio and thus portion of extracellular polymetric substances (EPS) and cells in WAS. FNA pre-treatment targets the destruction of polymetric substances and cells, therefore WAS with a higher proportion of cells (i.e., WAS with primary treatment) experienced greater improvements in methane yield. Similarly, greater improvements in hydrolysis rate were observed for WAS from WWTP without primary sedimentation which contain higher proportions of large EPS molecules. Despite its consistent effectiveness on WAS samples, FNA pre-treatment was ineffective for improving the digestibility of high-rate activated sludge (HRAS).

Combined hydrothermal and free nitrous acid, alkali and acid pretreatment for biomethane recovery from municipal sludge.

This study focused on investigating the effect of combined chemical and hydrothermal pretreatment (HTP) on the anaerobic digestibility of thickened waste activated sludge (TWAS). Three different combined pretreatment conditions of HTP+free nitrous acid (FNA), HTP+Acid, and HTP+Alkaline were applied to TWAS. To control and compare the effect of combined pretreatments and a single pretreatment, Acid, Alkaline, FNA and HTP pretreatments were applied done prior to AD. The results of this study revealed that combined pretreatments have higher potential to improve methane production yield and rate but not in the solubilization of COD. The highest methane yield of 275mL CH4/g TCOD added was achieved for the combined pretreatment with FNA and HTP. HTP+FNA pretreatment was found to produce higher methane yields compared to the combination of other typical acid and alkaline reagents with hydrothermal pretreatment. Methane yields of 594, 527, and 544 L CH4/g VSS added, were achieved for HTP+FNA, HTP+ALK, and HTP+ACID pretreatments, respectively. The preliminary economic analysis showed that out of the combined pretreatment, only combining HTP with FNA is economically feasible.

Quorum sensing shaped microbial consortia and enhanced hydrogen recovery from waste activated sludge electro-fermentation on basis of free nitrous acid treatment

In this study, the feasibility of free nitrous acid (FNA) pretreatment coupled with quorum sensing (QS) was investigated to enhance hydrogen recovery from waste activated sludge (WAS) via electro-fermentation (EF). 3-oxo-hexanoyl-homoserine lactone (3OC6-HSL), as the signal molecule, was only added in the first three cycles of sludge inoculation at the phase of microbial electrolysis cells (MECs) startup. Results showed that QS combined FNA (AHL-FMEC) enabled highest hydrogen yield and current (4.3 mg/g VSS and 4.5 mA), while that generated from sole FNA/QS treated WAS (FMEC/AHL-RMEC) were only 3.5/3.0 mg/g VSS and 1.5/1.5 mA, respectively. Fourier transform infrared (FT-IR) spectra illustrated the effective conversion of organics in AHL-FMEC, the utilization efficiencies of proteins and carbohydrates achieved to 75.0% and 79.7%, respectively. Besides, the internal resistance decreased from 34.5 Ω (FMEC) to 22.9 Ω (AHL-RMEC), further to 18.0 Ω, indicating the promoted bioelectrochemical activity of electroactive bacteria (EAB) in AHL-FMEC. Correspondingly, both EAB (21.7%), e.g., Geobacter (9.3%) and Pseudomonas (3.2%) and anaerobic fermentation bacteria (AFB, 28.6%), e.g., Proteiniclasticum (14.2%) and Petrimonas (3.6%) enriched to peaks in AHL-FMEC. Moreover, molecular ecological network (MEN) analysis revealed the underling relationships among AFB, EAB and homo-acetogen in EF system, suggesting the possible cooperative QS has been constructed. The results obtained in this study may provide a new insight for efficient hydrogen recovery from electro-fermentation of WAS.

Extracellular polymeric substance decomposition linked to hydrogen recovery from waste activated sludge: Role of peracetic acid and free nitrous acid co-pretreatment in a prefermentation-bioelectrolysis cascading system

Free nitrous acid (FNA) has been recently reported to be an effective and eco-friendly inactivator for waste activated sludge (WAS), while the limited decomposition of the extracellular polymeric substance (EPS) matrix hampers resource recovery from WAS. This work employed peracetic acid (PAA) to assist FNA and explored the contribution of co-pretreatment to hydrogen recovery in a prefermentation-bioelectrolysis cascading system. The results showed that co-pretreatment led to approximately 8.8% and 20.4% increases in the exfoliation of particulate proteins and carbohydrates, respectively, from tightly bound EPS (TB-EPS) over that of sole FNA pretreatment. Electron paramagnetic resonance analysis verified that the synergistic effect of FNA, PAA and various generated free radicals was the essential process. This effect further promoted the accumulation of volatile fatty acids (VFAs) after 96 h of prefermentation, and the peak concentration in co-pretreated WAS (AD-FPWAS) was approximately 2.5-fold that in sole FNA-pretreated WAS (AD-FWAS). Subsequently, the cascading utilization of organics in the bioelectrolysis step contributed to efficient hydrogen generation. A total of 10.8 ± 0.3 mg H2/g VSS was harvested in microbial electrolysis cells (MECs) fed with AD-FPWAS, while 6.2 ± 0.1 mg H2/g VSS was obtained from AD-FWAS. X-ray photoelectron spectroscopy (XPS) revealed the effective decomposition of the phospholipid bilayer in the cytomembrane and the transformation of macromolecular organics into VFAs and hydrogen in the cascading system. Further microbial community analysis demonstrated that co-pretreatment enhanced the accumulation of functional consortia, including anaerobic fermentative bacteria (AFB, 28.1%), e.g., Macellibacteroides (6.3%) and Sedimentibacter (6.9%), and electrochemically active bacteria (EAB, 57.0%), e.g., Geobacter (39.0%) and Pseudomonas (13.6%), in the prefermentation and MEC steps, respectively. The possible synergetic and competitive relationships among AFB, EAB, homo-acetogens, nitrate-reducing bacteria and methanogens were explored by molecular ecological network analysis. From an environmental and economic perspective, this promising FNA and PAA co-pretreatment approach provides new insight for energy recovery from WAS biorefineries.