Evaluation of anaerobic digestion of food waste and waste activated sludge: Soluble COD versus its chemical composition

New insight into enhanced carbon recovery from anaerobic fermentation of waste activated sludge with cation exchange resin coupled with NaCl pretreatment

Carbon recovery from waste activated sludge has been attracting considerable attention. However, the migration and transformation patterns of carbon sources between the phases have rarely been reported. In this study, a novel strategy using cation exchange resin (CER) coupled with sodium chloride (NaCl) to enhance carbon recovery through anaerobic fermentation (AF) was proposed. The results demonstrated that CER coupled with NaCl destroyed OH and CO stretching in amide I while promoting the formation of β-sheet and random coil structures, leading to sludge disintegration. This significantly improved the kinetics of endogenous carbon release, resulting in the release of 1146.33 mg/L of carbon from the solid sludge into the liquid phase. Approximately 75.61 % of the initial carbon source was bio-transformed into short-chain fatty acids. Correspondingly, carbon recovery was significantly increased up to 852.23 mg C/L, 4.57 times that of the control. Mechanism exploration revealed that carbon source recovery was significantly elevated by the synergistic effect of CER and NaCl. CER effectively removed high-valence cations from extracellular polymeric substance (EPS), weakening its bridging and adsorption-electro neutralization capabilities, promoting protein deflocculation, and triggering EPS disruption to release extracellular carbon sources. NaCl disrupted the ionic strength and distribution inside and outside microbial cells, creating an osmotic pressure difference that resulted in cell plasmolysis and lysis, ultimately inducing the release of intracellular carbon sources. Economic and carbon emission reduction benefit analyses verified that the CER coupled with NaCl pretreatment is a cost-effective sludge treatment strategy. This study illustrates the carbon source migration and transformation pathways in the CER coupled with NaCl-assisted AF process, providing guidance for sustainable sludge management.

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.

Investigating functional mechanisms in the Co-biodegradation of lignite and guar gum under the influence of salinity

The biodegradation of guar gum by microorganisms sourced from coalbeds can result in low-temperature gel breaking, thereby reducing reservoir damage. However, limited attention has been given to the influence of salinity on the synergistic biodegradation of coal and guar gum. In this study, biodegradation experiments of guar gum and lignite were conducted under varying salinity conditions. The primary objective was to investigate the controlling effects and mechanisms of salinity on the synergistic biodegradation of lignite and guar gum. The findings revealed that salinity had an inhibitory effect on the biomethane production from the co-degradation of lignite and guar gum. The biomethane production declined with increasing salinity levels, decreasing from 120.9 mL to 47.3 mL. Even under 20 g/L salt stress conditions, bacteria in coalbeds could effectively break the gel and the viscosity decreased to levels below 5 mPa s. As salinity increased, the removal rate of soluble chemical oxygen demand (SCOD) decreased from 55.63% to 31.17%, and volatile fatty acids (VFAs) accumulated in the digestion system. High salt environment reduces the intensity of each fluorescence peak. Alterations in salinity led to changes in microbial community structure and diversity. Under salt stress, there was an increased relative abundance of Proteiniphilum and Methanobacterium, ensuring the continuity of anaerobic digestion. Hydrogentrophic methanogens exhibited higher salt tolerance compared to acetoclastic methanogens. These findings provide experimental evidence supporting the use of guar gum fracturing fluid in coalbeds with varying salinity levels.

Study on the mechanism of action of methane production by co-fermentation of sludge and lignite

To improve the methanogenic efficiency of lignite anaerobic fermentation and explore innovative approaches to sludge utilization, a co-fermentation technique involving lignite and sludge was employed for converting biomass into biomethane. Volatile suspended solids were introduced as a native enrichment of the sludge and mixed with lignite for fermentation. The synergistic fermentation mechanism between sludge and lignite for biomethane production was analyzed through biochemical methane potential experiments, measurement of various parameters pre- and post-fermentation, observation of bacterial population changes during the peak of reaction, carbon migration assessment, and evaluation of rheological characteristics. The results showed that the addition of sludge in the anaerobic fermentation process improved the microorganisms' ability to degrade lignite and bolstered biomethane production. Notably, the maximum methane production recorded was 215.52 mL/g-volatile suspended solids, achieved at a sludge to coal ratio of 3:1, with a synergistic growth rate of 25.37%. Furthermore, the removal rates of total suspended solids, and total chemical oxygen demand exhibited an upward trend with an increasing percentage of sludge in the mixture. The relative abundance and activity of the methanogens population were found to increase with an appropriate ratio of sludge to lignite. This observation confirmed the migration of carbon between the solid-liquid-gas phases, promoting enhanced system affinity. Additionally, the changes in solid-liquid phase parameters before and after the reaction indicated that the addition of sludge improved the system's degradation capacity. The results of the study hold significant implications in realizing the resource utilization of sludge and lignite while contributing to environmental protection endeavors.

Comparison of anaerobic digestion of starch- and petro-based bioplastic under hydrogen-rich conditions

To identify an economically viable waste management system for bioplastics, thermoplastic starch (TPS) and poly(butylene adipate-co-terephthalate) (PBAT) were anaerobically digested under hydrogen (H2)/carbon dioxide (CO2) and nitrogen (N2) gas-purged conditions to compare methane (CH4) production and biodegradation. Regardless of the type of bioplastics, CH4 production was consistently higher with H2/CO2 than with N2. The highest amount of CH4 was produced at 307.74 mL CH4/g volatile solids when TPS digested with H2/CO2. A stepwise increased in CH4 yield was observed, with a nominal initial increment followed by accelerated methanogenesis conversion as H2 was depleted. This may be attributed to a substantial shift in the microbial structure from hydrogenotrophic methanogen (Methanobacteriales and Methanomicrobiales) to heterotrophs (Spirochaetia). In contrast, no significant change was observed with PBAT, regardless of the type of purged gas. TPS was broken down into numerous derivatives, including volatile fatty acids. TPS produced more byproducts with H2/CO2 (i.e., 430) than with N2 (i.e., 320). In contrast, differential scanning calorimetry analysis on PBAT revealed an increase in crystallinity from 10.20 % to 12.31 % and 11.36 % in the H2/CO2- and N2-purged conditions, respectively, after 65 days of testing. PBAT surface modifications were characterized via Fourier transform infrared spectroscopy and scanning electron microscopy. The results suggest that the addition of H2/CO2 can enhance the CH4 yield and increase the breakdown rate of TPS more than that of PBAT. This study provides novel insights into the CH4 production potential of two bioplastics with different biodegradabilities in H2/CO2-mediated anaerobic digestion systems.

Enhanced anaerobic digestion of human feces by ferrous hydroxyl complex (FHC): Stress factors alleviation and microbial resistance improvement

Anaerobic digestion (AD) offers a reliable strategy for resource recovery from source-separated human feces (HF), but is limited by a disproportionate carbon/nitrogen (C/N) ratio. Ferrous hydroxyl complex (FHC) was first introduced into the HF-AD system to mediate methanogenesis. Mono-digestion of undiluted HF was inhibited by high levels of volatile fatty acids (VFAs), ammonia, and hydrogen sulfide (H2S). FHC addition at optimum dosage (500-1000 mg/L) increased the cumulative methane (CH4) yield by 22.7%, enhanced the peak value of daily CH4 production by 60.5%, and shortened the lag phase by 24.7%. H2S concentration in biogas was also greatly decreased by FHC via precipitation. FHC mainly facilitated the hydrolysis, acidification, and methanogenesis processes. The production and transformation of VFAs were optimized in the presence of FHC, thus relieving acid stress. FHC elevated the activities of alkaline protease, cellulase, and acetate kinase by 32.3%, 18.2%, and 30.3%, respectively. Microbial analysis revealed that hydrogenotrophic methanogens prevailed in mono-digestion at high HF loading but were weakened after FHC addition. FHC also enriched Methanosarcina, thereby expanding the methanogenesis pathway and improving the resistance to ammonia stress. This work would contribute to improving the methanogenic performance and resource utilization for HF anaerobic digestion.

Effects of microbubble pretreatment on physiochemical and microbial properties of excess activated sludge

Fast increased amount of excess activated sludge (EAS) from wastewater treatment plants has aroused universal concerns on its environmental risks and demands for appropriate treatments, while effective treatment is dependent upon proper pretreatment. In this study, air-supplied microbubbles (air-MBs) with generated size of 25.18 to 28.25 μm were used for EAS pretreatment. Different durations (30, 60, 90, and 120 s) yielded sludge with varied physiochemical conditions, and 60 s decreased sludge oxidation status and significantly increased adenosine triphosphate (ATP) content. Soluble, loosely-bound, and tightly-bound extracellular polymeric substances (SEPS, LB-EPS, and TB-EPS) were extracted from the sludge through a stepwise approach and examined through three-dimensional excitation-emission matrix (3D-EEM) and quantitative analysis. The results showed that 60- and 120-s treatments generated stronger fluorescence intensities on dissolved organic matters (DOMs) of protein-like and fulvic acid in LB-EPS and TB-EPS, which indicated the decrease of counterparts in EAS, and therefore facilitated sludge dewaterability and reduction. The dominant microbial communities in EAS, including Proteobacteria, Bacteroidota, Chloroflexi, and Actinobacteriota, were not significantly affected by MB pretreatment. The results collectively revealed the effects of MB pretreatment on EAS and indicated that MBs could be an effective pretreatment technique for EAS treatment process.

Efficient derivation of transgene-free porcine induced pluripotent stem cells enables in vitro modeling of species-specific developmental timing

Sus scrofa domesticus (pig) has served as a superb large mammalian model for biomedical studies because of its comparable physiology and organ size to humans. The derivation of transgene-free porcine induced pluripotent stem cells (PiPSCs) will, therefore, benefit the development of porcine-specific models for regenerative biology and its medical applications. In the past, this effort has been hampered by a lack of understanding of the signaling milieu that stabilizes the porcine pluripotent state in vitro. Here, we report that transgene-free PiPSCs can be efficiently derived from porcine fibroblasts by episomal vectors along with microRNA-302/367 using optimized protocols tailored for this species. PiPSCs can be differentiated into derivatives representing the primary germ layers in vitro and can form teratomas in immunocompromised mice. Furthermore, the transgene-free PiPSCs preserve intrinsic species-specific developmental timing in culture, known as developmental allochrony. This is demonstrated by establishing a porcine in vitro segmentation clock model that, for the first time, displays a specific periodicity at ∼3.7 h, a timescale recapitulating in vivo porcine somitogenesis. We conclude that the transgene-free PiPSCs can serve as a powerful tool for modeling development and disease and developing transplantation strategies. We also anticipate that they will provide insights into conserved and unique features on the regulations of mammalian pluripotency and developmental timing mechanisms.

Effect of thermal hydrolysis pretreatment on the occurrence of N-acyl homoserine lactone during anaerobic digestion of waste activated sludge

This study sought to investigate the relationship between N-acyl homoserine lactones (AHLs) and methanogenic microorganisms, focusing on endogenous AHLs in the anaerobic digestion (AD) process. By analyzing waste activated sludge (WAS) samples, we examine the changes in microbial communities and the AHLs-methanogens connection. The Mantel test and Spearman correlation analysis were conducted to gain novel insights into the AD process. Our findings demonstrate that thermal hydrolysis pretreatment (THP) modifies AHL concentrations during AD, thereby enhancing methanogenic bacteria activity and regulating social interactions among microorganisms. In the Eth group (AD of THP samples labeled Eth), Methanobacterium and Methanosarcina accounted for over 80% of the methanogenic bacteria, with correlation coefficients greater than 0.5 between these bacterial taxa and N-hexyl-l-homoserine lactone (C6-HSL) and N-enanthyl-l-homoserine lactone (C7-HSL).

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.

The migration regularity and removal mechanism of antibiotic resistance genes during in situ enzymatic hydrolysis and anaerobic digestion of food waste

This study developed a high efficiency compound enzyme (fungal mash) produced in situ from food waste (FW) used for improving hydrolysis and anaerobic digestion (AD) efficiency of FW. Results showed that the soluble COD and methane yield were respectively increased by 67.80% and 16.58% after 24 h in situ enzymatic hydrolysis of food waste by fungal mash. Furthermore, most of target ARGs in FW were also reduced by 45-94% after 24 h in situ enzymatic hydrolysis, while the total tested ARGs and intI1 were respectively further removed by 44-55% and 21-73% in subsequent AD process. In-depth analysis showed that fungal mash could effectively reduce potential hosts and control the horizontal transfer of ARGs during the in situ enzymatic hydrolysis and AD process. Ultimately, correlation analysis and redundancy analysis indicated that the evolution of bacterial communities and changes in intI1 where the common driving forces for the fate of ARGs.

Pilot-scale two-phase anaerobic digestion of deoiled food waste and waste activated sludge: Effects of mixing ratios and functional analysis

Anaerobic co-digestion of deoiled food waste (dFW) and waste activated sludge (WAS) can address the challenges derived from mono-digestion of FW. In the present study, a pilot-scale methanogenic bioreactor of a two-phase anaerobic digestion system was developed to explore the impact of dFW/WAS volatile solids ratios on the overall performance, microbial community, and metabolic pathways. Besides, the tech-economic of the system was analyzed. The results showed that the degradation efficiency of soluble chemical oxygen demand (SCOD) was more than 84.90% for all the dFW/WAS ratios (v/v) (1:0, 39:1, 29:1, 19:1 and 9:1). Moreover, the dominant genus of bacteria and archaea with different ratios were Lactobacillus (66.84-98.44%) and Methanosaeta (53.66-80.09%), respectively. Co-digestion of dFW and WAS (29: 1 in v/v ratios) obtained the highest yield of methane (0.41 L CH₄/L_added) with approximately 90% of SCOD being removed. In the pilot-scale experiment, the co-digestion of FW and WAS makes positive contribution to reusing solid waste for improving solid management.

Different rapid startups for high-solid anaerobic digestion treating pig manure: Metagenomic insights into antibiotic resistance genes fate and microbial metabolic pathway

High-solid anaerobic digestion (HSAD), as an emerging disposal technology for swine manure, was commonly hampered by the long lag phase and slow startup, resulting in poor performance. Rapid startups by different leachate reflux forms can solve the problem, but related study was scarcely reported. Therefore, metagenomic analysis was used to exploit the effects of different rapid startups on the biogas performance, antibiotic resistance genes (ARGs) removal and microbial metabolic pathway during HSAD. Compared anaerobic digestion with natural start (T1), three different rapid startups were set, including with autologous leachate reflux (T2), with water reflux (T3) and with exogenous leachate reflux (T4). The results showed that rapid startups (T2-T4) enhanced biogas yield and the cumulative methane yield was increased by 3.7-7.3 times compared with the control. Totally, 922 ARGs were found, most of which belonged to multidrug and MLS ARGs. About 56% of these ARGs could be reduced in T4, while just 32% of ARGs were reduced in T1. Antibiotic efflux pump is the main mechanism of microbial action, which could be decreased largely by these treatments. Moreover, all the rapid startups (T2-T4) made Methanosarcina content (9.59%-75.91%) higher than that in the natural startup of T1 (4.54%-40.27%). This is why these fast-startups helped methane production fast. Network analysis showed that microbial community and environmental factors (pH and VFAs) both contributed to the spread of ARGs. The reconstructed methane metabolic pathway by different identified genes showed that all methanogenesis pathways existed but acetate metabolic pathway was dominant. And the rapid startups made the abundance of acetate metabolic (M00357) higher than the natural startup.

Effects of Metal and Metal Ion on Biomethane Productivity during Anaerobic Digestion of Dairy Manure

To overcome major limiting factors of microbial processes in anaerobic digestion (AD), metal and metal ions have been extensively studied. However, there is confusion about the effects of metals and metal ions on biomethane productivity in previous research. In this study, Zn and Zn2+ were selected as representatives of metals and metal ions, respectively, to investigate the effects on biomethane productivity. After the metals and metal ions at different concentrations were added to the batch AD experiments under the same mesophilic conditions, a Zn dose of 1 g/L and a Zn2+ dose of 4 mg/L were found to cause the highest biomethane production, respectively. The results indicate that metal (Zn) and metal ion (Zn2+) have different mechanisms to improve AD performance. There may be two possible explanations. To act as conductive materials in interspecies electron transfer (IET), relatively high doses of metals (e.g., 1 g/L of Zn, 10 g/L of Fe) are needed to bridge the electron transfer from syntrophic bacteria to methanogenic archaea in the AD process. As essential mineral nutrients, the AD system requires relatively low doses of metal ions (e.g., 4 mg/L of Zn2+, 5 mg/L of Fe2+) to supplement the component of various enzymes that catalyze anaerobic reactions and transformations. This research will provide clear insight for selecting appropriate amounts of metals or metal ions to enhance biomethane productivity for industrial AD processes.

The Preparation Processes and Influencing Factors of Biofuel Production from Kitchen Waste

Kitchen waste is an important component of domestic waste, and it is both harmful and rich in resources. Approximately 1.3 billion tons of kitchen waste are produced every year worldwide. Kitchen waste is high in moisture, is readily decayed, and has an unpleasant smell. Environmental pollution can be caused if this waste is treated improperly. Conventional treatments of kitchen waste (e.g., landfilling, incineration and pulverization discharge) cause environmental, economic, and social problems. Therefore, the development of a harmless and resource-based treatment technology is urgently needed. Profits can be generated from kitchen waste by converting it into biofuels. This review intends to highlight the latest technological progress in the preparation of gaseous fuels, such as biogas, biohythane and biohydrogen, and liquid fuels, such as biodiesel, bioethanol, biobutanol and bio-oil, from kitchen waste. Additionally, the pretreatment methods, preparation processes, influencing factors and improvement strategies of biofuel production from kitchen waste are summarized. Problems that are encountered in the preparation of biofuels from kitchen waste are discussed to provide a reference for its use in energy utilization. Optimizing the preparation process of biofuels, increasing the efficiency and service life of catalysts for reaction, reasonably treating and utilizing the by-products and reaction residues to eliminate secondary pollution, improving the yield of biofuels, and reducing the cost of biofuels, are the future directions in the biofuel conversion of kitchen waste.

Role of humic substances and alkaline in phosphorus release from sludge pre-treated by (alkali-) hydrothermal

The scarcity of phosphorus (P) resources makes the recovery of P urgent. Sludge is a secondary resource rich in P, and the release of P from it is a key step for recovery. Hydrothermal (HT) is currently a popular method for sludge pretreatment, and its combination with alkaline (alkali-hydrothermal, AHT) could reduce the energy consumption in treatment. This study tried to compare their P release profiles in treating activated sludge in which organic P (OP) and non-apatite inorganic P (NAIP) were co-existence. Apart from the OP release in cell lysis, P release from NAIP brought by the joint effect of OH^- and humic substances (HS) formed in treatment was focused. The results showed that, compared to HT treatment, more P was released when OH^- participated (AHT), and the peak P release was observed at 160 °C. Variation of P distribution in the treated sludge revealed that more P was released from NAIP in AHT than in HT. HS formed in treatments was extracted and characterized. The amount and the structure of the HS varied significantly with the treatment conditions, and there was a linear correlation ship between PO₄^3--P release and the humic acid (HA) amount in HS. Mechanism study indicated there was a synergism between HS and OH^- in promoting PO₄^3--P release from NAIP. This study linked HS produced by sludge with P release, which provided a new perspective for subsequent P recovery from sludge.

Mechanism of dielectric barrier plasma technology to improve the quantity and quality of short chain fatty acids in anaerobic fermentation of cyanobacteria

The recycling of high value carbon resources from cyanobacteria has become a research hotspot. This work investigated the possibility of dielectric barrier discharge (DBD) plasma pretreatment to improve the anaerobic fermentation performance of cyanobacteria. The maximum accumulations of short-chain fatty acids (SCFAs) and acetic acid in DBD group were 3.30 and 1.49 times of that in control group. The physical effects of DBD plasma and the oxidative stress response of cyanobacteria cells could improve the solubilization of cyanobacteria polymer. The destruction of humus by DBD plasma can reduce the negative impact of humus on the early stage of anaerobic fermentation, thus facilitating the rapid start of anaerobic fermentation. The contents of Bacteroidetes, Firmicutes and Chloroflexi in DBD group were higher than those in control group, while the content of Proteobacteria was on the contrary, which was conducive to the hydrolysis and acidification process. The decrease of Methanosaeta sp. and Methanosarcina sp. abundance in DBD group might be another reason for the increase of acetic acid ratio. Under the joint action of plasma chemical oxidation and microbial degradation, the degradation effect of microcystin-LR in the anaerobic fermentation supernatant of DBD group was better than that of the control group, which was conducive to the recycling of cyanobacteria anaerobic fermentation supernatant. Therefore, DBD pretreatment was conductive to recycling valuable carbon source from cyanobacteria and can be further developed as a potential new pretreatment technology.

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 (40 MPa) and FNA (2.49 mg/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 (815 ml) was more than the sum from single pretreatment (710 ml). 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.

Influence of organic loading rate and temperature fluctuation caused by solar energy heating on food waste anaerobic digestion

Anaerobic digestion, one of the most currently remarkable techniques for biogas production, has provided a method of high organic solid waste disposal. Operating temperature, especially in the winter of northern city, makes biomass degradation less efficient. The microorganisms that take on the role of gas production are greatly affected by temperature. In our study, solar energy was selected for anaerobic digestion and winter was selected as the experimental environment. Anaerobic digestion was performed with solar heating and electric heating separately. Parameters were tested (pH, soluble chemical oxygen demand, total ammonia nitrogen, total volatile fatty acids), and microbial structure was monitored. The volume of methane produced was measured over 60 days. The methane yield differed by 15.92% under different conditions. It is clearly shown that methane yield can be improved by a steady temperature environment. Nevertheless, dominant bacteria and microbial structure did not seem to be much different. This study may provide more energy-saving ideas for winter anaerobic digestion projects in northern regions.

Polycyclic aromatic hydrocarbons stimulate acidogenesis, acetogenesis and methanogenesis during anaerobic co-digestion of waste activated sludge and food waste

Polycyclic aromatic hydrocarbons (PAHs) have been reported to influence acetic acid production during anaerobic treatment. However, investigations of the impacts of PAHs on the anaerobic co-digestion of waste activated sludge and food waste are limited. Therefore, the effects of PAHs on anaerobic co-digestion were explored in this study. Four kinds of PAHs all exhibited positive contributions to methane production, especially phenanthrene. Mechanism exploration revealed that acidogenesis, acetogenesis, and methanogenesis were improved in the presence of phenanthrene, and acetotrophic methanogenesis had the greatest improvement with 69.4%. Dominant bacteria and archaea related to acetic acid and methane accumulation were changed by phenanthrene. Moreover, extracellular polymeric substances, coenzyme F₄₂₀, and McrA gene copy number were promoted by phenanthrene, which was beneficial for the generation of acetic acid and methane. Overall, this study provides new insights into the role of organic pollutants in the anaerobic co-digestion of solid wastes.

Correlating black soldier fly larvae growths with soluble nutrients derived from thermally pre-treated waste activated sludge

Black soldier fly larvae (BSFL) have been deployed to valorize various organic wastes. Nonetheless, its growth rate whilst being offered with waste activated sludge (WAS) is not promising, likely by virtue of the presence of extracellular polymeric substances' structure in WAS. In this work, the WAS were first thermally pre-treated under different treatment temperatures and durations before being administered as the feeding substrates for BSFL. The results showed the thermal pre-treatment could improve WAS palatability and subsequently, enhance the growth of BSFL especially after the pre-treatments at 75 °C and above. The highest larva weight gained was recorded at 2.16 mg/larva for the WAS sample being pre-treated at 90 °C and 16 h. Furthermore, the samples pre-treated above 75 °C also achieved higher degradation rates, indicating that the 75 °C was a threshold temperature to effectively hydrolyze the WAS. The changes of WAS characteristics, namely, (i) soluble chemical oxygen demand (SCOD), (ii) soluble carbohydrate, (iii) soluble protein, (iv) humic substances and (v) total soluble protein and humic substances, after the thermal pre-treatments were also studied in correlating with the BSFL growth. Accordingly, a model was successfully developed with the highest R² value attained at 0.95, evidencing the SCOD was the most suitable WAS characteristic to accurately predict the BSFL growth behavior.

Effects of Increasing Concentrations of Enrofloxacin on Co-Digestion of Pig Manure and Corn Straw

Enrofloxacin (ENR) is one of the most commonly used antibiotics in pig farms. In this study, using fresh pig manure and corn straw powder as substrates, the effects of different concentrations of ENR (2.5, 10, and 20 mg/L) on anaerobic digestion in completely mixed anaerobic reactors were investigated. A relatively low concentration of ENR (2.5 mg/L) increased methane production by 47.58% compared with the control group. Among the volatile fatty acids (VFAs) in the reactors, the propionic acid content was the lowest, and the concentrations of acetic acid kinase and coenzyme F420 were highest in the first seven days during peak gas production. However, methane production in the reactors with 10 mg/L and 20 mg/L ENR decreased by 8.59% and 20.25%, respectively. Furthermore, the accelerated hydrolysis of extracellular polymeric substances causes a significant accumulation of VFA levels. The microbial community in anaerobic reactors was analyzed by 16S rRNA sequencing. Proteiniphilum was the dominant bacterial genus. In addition, ENR at 2.5 mg/L effectively increased the abundance and diversity of anaerobic microorganisms, whereas a high concentration of ENR (10 and 20 mg/L) significantly decreased these parameters. This study demonstrated that different concentrations of ENR had significantly different effects on anaerobic digestion.

Hydrolysis kinetics for solubilizing waste activated sludge at low temperature thermal treatment derived from multivariate non-linear model

Low temperature thermal pre-treatment is a low-cost method to break down the structure of extracellular polymeric substances in waste activated sludge (WAS) while improving the sludge biodegradability. However, previous models on low temperature thermal pre-treatment did not adequately elucidate the behaviour of sludge hydrolysis process for the duration ranging from 5 to 9 h. Therefore, this work had developed an inclusive functional model to describe the kinetics of sludge hydrolysis for a wide range of treatment conditions (30 °C-90 °C within 0 and 16 h). As compared with treatment duration, the treatment temperature played a greater impact in solubilizing WAS. Accordingly, the 90 °C treatment had consistently produced WAS with the highest degree of solubility. Nonetheless, the mediocre discrepancies between 90 °C and 75 °C may challenge the practicality of increasing the treatment temperatures beyond 75 °C. The effects of treatment duration on soluble chemical oxygen demand, soluble carbohydrate and soluble protein were only significant during the first 4 h, except for humic substances release that continued to increase with treatment duration. Finally, a good fit with R² > 0.95 was achieved using an inclusive multivariate non-linear model, substantiating the functionality to predict the kinetics of sludge hydrolysis at arbitrary treatment conditions.

Enhanced biomethanation of lipids by high-solid co-digestion with food waste: Biogas production and lipids degradation demonstrated by long-term continuous operation

Biomethanation of lipids by high-solid co-digestion with food waste (FW) was investigated with a long-term operation under the mesophilic condition. The experiment was conducted with a continuous stirred-tank reactor (CSTR) at a hydraulic retention time (HRT) of 30 days and an influent TS concentration of 10% by varying influent lipid/TS ratio from 10% to 60%. As lipid/TS ratio increased from10% to 50%, it was found that the biogas production rate improved by 30% and the content of CH₄ in the biogas increased by up to 67%. When the lipid/TS ratio reached 60%, the whole system collapsed. It was inferred that when the lipid/TS ratio was less than 50%, sufficiently high concentration of microorganisms ensured by high-solid co-digestion maintained relatively high average reaction rates for hydrolysis, acidogenesis, acetogenesis and methanogenesis. The rapid degradation of lipid reduced the adsorption to the surface of microbes, thereby ensuring the stability of the system.

Ineffectiveness of ultrasound at low frequency for treating per- and polyfluoroalkyl substances in sewage sludge

Concerns have been raised about per- and polyfluoroalkyl substances (PFAS) in sewage sludge given the urgent need of finding suitable disposal methods for sludge. In this study, we evaluated the effect of ultrasonication on PFAS changes in sewage sludge. It was revealed that although ultrasonication at 20 kHz increased the soluble chemical oxygen demand (SCOD) of treated sewage sludge, this technique was ineffective for degrading perfluoroalkyl acids (PFAAs) and their precursors. Ultrasonic treatment for longer time (>15 min) led to concentration increase of perfluorooctanoic acid (PFOA), perfluoroheptanoic acid (PFHpA), and perfluorohexanoic acid (PFHxA) in the liquid phase, possibly due to their release from disrupted sludge flocs during cavitation. Adding permanganate (10 mM) to the ultrasonic system could also enhance the disruption of sludge particles, resulting in higher concentrations of PFOA and PFHxA in the solid phase and PFOA, PFHpA, PFHxA, and perfluorobutanesulfonic acid (PFBS) in the liquid phase. Overall, ultrasonic pretreatment at 20 kHz and 0.7 W/mL is unlikely to remove PFAS from sewage sludge. Instead, it could increase the risk of PFAS pollution upon final sludge disposal. Effective treatment technologies are thus demanded if PFAS in sludge are regulated.

Responses of simultaneous anammox and denitrification (SAD) process to nitrogen loading variation: Start-up, performance, sludge morphology and microbial community dynamics

The effects of loading variation on the efficiency, EPS, sludge morphology and microbial population of simultaneous anammox and denitrification (SAD) were thoroughly investigated with the low-abundance SAD sludge. Results indicated that the first stage lasted the longest (33d), and the average removal rate of TN can be maintained above 95%. The specific anammox activity (SAA), specific denitrification activity and PN/PS continued to increase, but the excessive loading caused the effluent to deteriorate rapidly, and SAA and PN/PS also decreased slightly, but it could be recovered quickly. The contribution rate of anammox and denitrification to N removal reached 87.6% and 12.4% eventually, respectively. The abundance of AnAOB was 10.68%-18.01%, 9.01%-15.54%, 5.74%-12.88% in the upper, middle and lower layers, respectively. Candidatus Kuenenia was always the dominant AnAOB, especially after high loading inhibition. The abundance of denitrifying bacteria (mainly Bacillus, Comamonas and Denitratisoma) gradually became the highest.

High-solids anaerobic digestion of organic fraction of municipal solid waste: Effects of feedstock to inoculum ratio and percolate recirculation time

The effects of feedstock to inoculum (F:I) ratio and percolate recirculation time (PRT) were studied for the high-solids anaerobic digestion (HSAD) of the organic fraction of municipal solid waste (OFMSW). Six mesophilic HSAD systems were operated at different F:I ratios (1 to 3 kg VS/kg VS; PRT = 2.5 h/d) and PRTs (1.5 to 3.5 h/d; F:I = 2 kg VS/kg VS). The F:I ratio of 1 provided up to 86% of the theoretical methane potential of OFMSW. In contrast, F:I ratio of 3 provided only 34% methane recovery due to volatile fatty acids (VFAs) accumulation and pH drop. Despite F:I ratio of 2 could provide 70% methane recovery, it could enable almost 45% higher organics processing capacity (VS basis) and lower solids washout during percolate recirculation, as compared to the F:I ratio of 1. However, different examined PRTs showed marginal impacts on methane yields with comparable changes in profiles of percolate characteristics.

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.

Minimizing hazardous impact of food waste in a circular economy - Advances in resource recovery through green strategies

Recent trends in food waste and its management have increasingly started to focus on treating it as a reusable resource. The hazardous impact of food waste such as the release of greenhouse gases, deterioration of water quality and contamination of land areas are a major threat posed by food waste. Under the circular economy principles, food waste can be used as a sustainable supply of high-value energy, fuel, and nutrients through green techniques such as anaerobic digestion, co-digestion, composting, enzymatic treatment, ultrasonic, hydrothermal carbonization. Recent advances made in anaerobic co-digestion are helping in tackling dual or even multiple waste streams at once with better product yields. Integrated approaches that employ pre-processing the food waste to remove obstacles such as volatile fractions, oils and other inhibitory components from the feedstock to enhance their bioconversion to reduce sugars. Research efforts are also progressing in optimizing the operational parameters such as temperature, pressure, pH and residence time to enhance further the output of products such as methane, hydrogen and other platform chemicals such as lactic acid, succinic acid and formic acid. This review brings together some of the recent progress made in the green strategies towards food waste valorization.

Co-digestive performance of food waste and hydrothermal pretreated corn cob

Anaerobic co-digestion of lignocellulosic biomass and food waste (FW) has been extensively applied. However, whether hydrothermal pretreatment (HTP) of lignocellulosic biomass can enhance the performance in co-digestion deserves further investigation. In this study, corn cob (CC) was adopted as a typical lignocellulosic biomass for co-digestion with FW at different VS ratios of 1:3 (S1-S4) and 1:6 (S5-S8), attempting to evaluate the effect of HTP of CC at different temperature gradients (125, 150 and 175 °C) on the co-digestion performance. The emphasis was placed on hydrolysis, acidification and methanogenesis for different feedstock conditions. Results illustrated that the HTP had a certain destroying effect on the lignocellulose structure in CC and the crystallinity of cellulose decreased, significantly facilitating its co-digestion with FW. For FW/CC co-digestion at the VS ratio of 1:3, the S3 group (CC was pretreated at 150 °C) reached the maximum cumulative biogas yield (CBY) of 4660 mL and the maximum specific methane yield (SMY) of 316.9 mL/g·VS. Moreover, at 1:6, S7 group (pretreated at 150 °C) exhibited the optimal CBY of 4100 mL while achieving the SMY of 277.6 mL/g·VS among the digesters, indicating that the co-digestion of pretreated CC and FW could achieve higher methane production, and 150 °C refers to the optimal pretreatment temperature. Moreover, the peak values of the accumulated VFAs in digesters S1-S4 (2000-3000 mg/L) is higher than that in digesters S5-S8 (800-1500 mg/L). As suggested from microbial community and diversity date, the HTP expedited the enrichment of system hydrolyzing and acidogenic bacteria. These results are significant and provide certain guidance for optimizing the co-digestion of FW and CC in actual engineering.

Synergistic digestion of banana pseudo-stems with chicken manure to improve methane production: Semi-continuous manipulation and microbial community analysis

This study aimed to investigate the interactions between banana pseudo-stems (BPS) and chicken manure (CM) during anaerobic co-digestion (AcoD) in batch and semi-continuous experiments. The batch experiments results showed that the methane yield was the highest (193.7 mL/g VS) in AcoD with BPS: CM ratio of 4:1, which was increased by 57.2% and 66.1%, respectively. Semi-continuous experiments revealed that AcoD resulted in higher methane production. Monitoring of the system parameters indicated that AcoD could better adapt to the increasing organic loading rate, with better system stability and methane production efficiency. The microbial analysis illustrated that AcoD increased the relative abundance of hydrolytic bacteria such as Firmicutes, Patescibacteria, and Bacteroidetes. With regard to archaea, AcoD improved the abundance of Methanosaeta, the major acetoclastic methanogens. These changes in the microbial flora allowed AcoD to remain stable while efficiently producing methane and improved the BPS and CM processing efficiency.

Bioenergy recovery from methanogenic co-digestion of food waste and sewage sludge by a high-solid anaerobic membrane bioreactor (AnMBR): mass balance and energy potential

To support smart city in terms of municipal waste management and bioenergy recovery, a high-solid anaerobic membrane bioreactor (AnMBR) was developed for sewage sludge (SeS) and food waste (FW) treatment in this study. COD mass balance showed that 54.1%, 66.9%, 73.5%, 91.4% and 93.5% of the COD input was converted into methane at the FW ratio of 0, 25%, 50%, 75% and 100%, respectively. The corresponding net energy balance was 13.6, 14.1, 17.1, 22.9 and 27.4 kJ/g-VS, respectively. An important finding of this investigation was that, for the first time, the relationship between net energy balance and carbon to nitrogen (C/N) ratio was revealed and the established sigmoid-type function was shown to be capable of predicting energy balance at different C/N ratios regardless of the region. The outcomes of this study show the potential of high-solid AnMBRs in SeS and FW treatment for supporting smart cities in the future.

Food waste valorization: Energy production using novel integrated systems

Management of food waste (FW) is a global challenge due to increasing population and economic activities. Presently, landfill and incineration are the keyways of FW management, while economical and environmental sustainability have been an issue. Therefore, the biological processes have been investigated for resource and energy recovery from FW. However, these biological approaches have certain drawbacks and cannot be a complete solution for FW management. Therefore, this review aims to offer a detailed and complete analysis of current available technologies to achieve environmental and economical sustainability. In this context, zero solid waste discharge for resource and energy recovery has been put into view. Corresponding to which several innovative technologies using integrated biological methods for resource and energy recovery from FW have been elucidated.

Insight into sludge anaerobic digestion with granular activated carbon addition: Methanogenic acceleration and methane reduction relief

In this study, the multiple effects of granular activated carbon (GAC) on sludge anaerobic digestion at ambient (16-24 °C), mesophilic (35 °C) and thermophilic (55 °C) temperature were investigated. After GAC addition, although the methane yields of raw sludge were reduced by 6.5%-36.9%, the lag phases of methanogenesis were shortened by 19.3%-30.6% and the reductions of methane yields were declined to only 5.9%-8.1% simultaneously for pretreated sludge. The inhibitory substances like phenols that generated by thermal pretreatment were reduced after GAC addition, which were demonstrated to be responsible for the methanogenic acceleration. Meanwhile, the methane reduction due to the non-selective adsorption by GAC could be mitigated by pretreatment and elevated temperature. Thus, a strategy coupling thermal pretreatment with detoxification by GAC was proposed to improve the methane production rate and avoid the negative effects during sludge anaerobic digestion with GAC addition.

Advanced methanogenic performance and fouling mechanism investigation of a high-solid anaerobic membrane bioreactor (AnMBR) for the co-digestion of food waste and sewage sludge

Disposal of the increasingly huge amounts of sewage sludge (SeS) has become an impending problem worldwide. To solve this problem, a high-solid anaerobic membrane bioreactor (AnMBR) was used for the anaerobic co-digestion (AcoD) of SeS and food waste (FW). This study investigated the effects of SeS ratio on the methanogenic performance of the AcoD with a gradual increase value from 0 to 25%, 50%, 75% and 100% (total solids based). The results showed that the highest methanogenic performance was achieved at mono FW digestion. As for the co-digestion, the optimal FW/SeS ratio for methanogenic performance was 75%:25% among all the mixing ratios. The COD based biogas yield and methane yield were 0.498 L-biogas/g-COD_fed and 0.295 L-CH₄/g-COD_fed at this optimal mixing ratio, which were 67.7% and 67.6% higher than those of the mono SeS digestion, respectively. The upgraded values were attributed to the improved hydrolysis ratio (by 8.14%) and the balanced carbon-to-nitrogen (C/N) ratio by co-digestion with FW, which synergistically stimulated methanogenesis ratio by 81.0%. The continuous membrane filtration property was investigated and four typical trans-membrane pressure (TMP) variation curves at different fouling degrees were determined. The membrane could sustainably operate at a flux of 6 L/m²/h (LMH) at the mixed liquor total solids (MLTS) concentration of 25 - 30 g/L. The combination of continuous membrane filtration property, particle size distribution of the mixed liquor and the external forces analysis was firstly applied to unravel the membrane fouling mechanism of a high-solid AnMBR. The result of this study will contribute to the establishment of an efficient FW and SeS treatment strategy.

Hydrolase activity and microbial community dynamic shift related to the lack in multivalent cations during cation exchange resin-enhanced anaerobic fermentation of waste activated sludge

The correlation of the lack in multivalent cations with hydrolase activity and microbial community in anaerobic fermentation of waste activated sludge was investigated in this study. It was demonstrated that considerable solid phase reduction of 41 % (7.87 g/L) was achievable through a cation exchange resin-enhanced anaerobic fermentation of 4 days. The protease and α-glucosidase, especially α-glucosidase, were easily influenced by a lack in multivalent cations. Furthermore, species abundance and diversity of microbial community gradually decreased. Meanwhile, the bacteria community structure presented obvious dynamic shifts. Ruminococcaceae_UCG_009, Bacteroides and Macellibacteroides responsible for organic matter biodegradation and SCFAs production became dominant bacteria in cation exchange resin-enhanced anaerobic fermentation, which was less influenced by the lack in multivalent cations, while the SCFA consumers (e.g. methanogens) were inhibited with reduced abundances due to their susceptibility to the lack in multivalent cations. Redundancy analysis revealed that the lack in multivalent cations were responsible for the microbial community evolution, which was proved by the high Grey relational coefficients (0.747-0.820) and significant negative Spearman coefficients (-0.5798 to -0.9429) between multivalent cation and microbial community. Obviously, the cation exchange resin-induced removal of multivalent cations reduced enzyme activity and modified microbial community structure, which created a beneficial environment for enhancing anaerobic fermentation.

Calcium peroxide eliminates grease inhibition and promotes short-chain fatty acids production during anaerobic fermentation of food waste

Deterioration of anaerobic fermentation can occur with the presence of grease in food waste, but little information on eliminating this deterioration is currently available. In this study, it was found that the presence of 10 g/L grease decreased SCFAs production from 16.97 to 13.32 g COD/L and prolonged the optimal fermentation time to 7 days, but could be respectively recovered to 39.10 g COD/L and 4 days with 0.02 mg/g VS (volatile solids) calcium peroxide addition. Mechanism investigations indicated that calcium peroxide facilitated biodegradable organics release and improved grease degradation, thereby providing enough nutrients and better growth environments to microbes for SCFAs-producing, which could be further supported by the elevated enzymes activities responding to hydrolysis and acidification process. Further investigations revealed that among the main derivates of calcium peroxide, OH^- and Ca²⁺ played vital role in SCFAs production promotion, O₂^- and OH radicals were the main contributors to grease degradation.

Cation-exchange resin regeneration waste liquid as alternative NaCl source for enhancing anaerobic fermentation of waste activated sludge: Compositions of dissolved organic matters and chemical conditioning performance

This study reported the NaCl-enhanced anaerobic fermentation with cation-exchange resin regeneration waste liquid (CRWL) as alternative NaCl source for waste activated sludge (WAS) disposal and carbon recovery. Through 4-day CRWL-enhanced anaerobic fermentation at Na⁺ concentration of 0.34 mol/L, the Na⁺-caused sludge disintegration triggered numerous release of dissolved organic matters (DOMs), i.e. 371.6 mg/g VSS, with composition distribution: acetic acid (28.2%) > butyric acid (16.1%) ≈ valeric acid (17.8%) ≈ proteins (16.4%) > propionic acid (14.4%) > unknown (3.2%) > carbohydrates (3.9%). Satisfying chemical conditioning performance for the fermented sludge was observed at the FeCl₃ dosage of 0.3 g/g DS, attributing to the roles of double-layer compression and electric neutralization. The capillary suction time (CST) and sludge cake moisture content were decreased to 60.3 s and 75.1%, against those of 607 s and 93.5% before conditioning, respectively. Such "treating waste by waste" strategy could provide numerous environmental and economic benefits.

Food-processing wastes

Literature published in 2018 and literature published in 2019 related to food-processing wastes treatment for industrial applications are reviewed. This review is a subsection of the Treatment Systems section of the annual Water Environment Federation literature review and covers the following food-processing industries and applications: general, meat and poultry, fruits and vegetables, dairy and beverage, and miscellaneous treatment of food wastes. PRACTITIONER POINTS: This article summarizes literature reviews published in 2018 and in 2019 related to food processing wastes treatment for industrial applications are reviewed. This review is a subsection of the Treatment Systems section of the annual Water Environment Federation literature review and covers the following food processing industries and applications: general, meat and poultry, fruits and vegetables, dairy and beverage, and miscellaneous treatment of food wastes.

Enhanced anaerobic fermentation of waste activated sludge by NaCl assistant hydrolysis strategy: Improved bio-production of short-chain fatty acids and feasibility of NaCl reuse

This study developed an economical approach for enhancing short-chain fatty acids (SCFAs) production from waste activated sludge (WAS) by NaCl assistant anaerobic fermentation. With NaCl addition at 20 g/L, sludge disintegration with extracellular polymeric substance (EPS) disruption and cell lysis were induced owing to the attack of osmotic pressure, which facilitated WAS solubilization with release of biodegradable organic matters. The SCOD sharply increased to 4092 mg/L (SCOD/TCOD = 23.9%) after 2-day hydrolysis, against 1462 mg/L in the control. After 4-day anaerobic fermentation, considerable SCFAs production of 288.2 mg COD/g VSS was achievable. More than 60% of the SCFAs was composed of acetic and propionic acids. The feasibility of bio-electrogenesis in microbial fuel cell (MFC) utilizing fermentative liquid was assessed. As such, the produced SCFAs could be consumed with energy recovery, thereby the used NaCl was reusable, which created environmental and economic benefits, e.g. reduced NaCl consumption and cost, negligible residual NaCl.

An innovative alkaline protease-based pretreatment approach for enhanced short-chain fatty acids production via a short-term anaerobic fermentation of waste activated sludge

This study reported a novel pretreatment approach with combination of alkaline protease (AP) and pH 10 for enhancing short-chain fatty acids (SCFAs) production from waste activated sludge (WAS). Through the AP-based pretreatment, WAS flocs were disintegrated with cell lysis, leading to release of biodegradable organic matters. At the external AP dosage of 5%, SCOD of 5363.7 mg/L (SCOD/TCOD = 32.5%) was achievable after 2-h pretreatment. More than 66% of SCOD was composed of proteins and carbohydrates. Considerable SCFAs of 607 mg COD/g VSS was produced over a short-term anaerobic fermentation of 3 days, which was 5.4 times higher than that in the control. Acetic and propionic acids accounted for 74.1% of the SCFAs. The AP-based approach increased endogenous protease and α-glucosidase activities, facilitating biodegradation of dissolved organic matters and SCFAs production. Such approach is promising for WAS disposal and carbon recovery, the produced SCFAs might supply 60% of carbon gap in wastewater.

Cation exchange resin-induced hydrolysis for improving biodegradability of waste activated sludge: Characterization of dissolved organic matters and microbial community

This study reported an efficient and green approach towards facilitating hydrolysis of waste activated sludge (WAS) using cation exchange resin (CER) as a recyclable additive. Through CER-mediated removal of multivalent cations, WAS flocs were disintegrated into small particles with extracellular polymeric substance (EPS) solubilization. At CER dosage of 1.75 g/g SS, SCOD increased to 2579 mg/L (SCOD/TCOD = 15.9%) after 8-h hydrolysis. Afterwards, CER displayed further sludge hydrolysis performance lasting 2 days, i.e. SCOD/TCOD = 34.2%. Meanwhile, proteins, carbohydrates and other organics in dissolved organic matters (DOMs) were major contributors for volatile fatty acids (VFAs) accumulation, with composition percentage: VFAs (58.9%) > proteins (21.8%) > other organics (8.8%) > humic acids (5.9%) > carbohydrates (4.4%). The biodegradable tryptophan-like and tyrosine-like proteins were major proteins, while other organics included amino acids, aliphatic and metabolic intermediates. More than 85.2% of DOMs were easily biodegradable. Moreover, CER-induced hydrolysis modified microbial community structure through inhibiting VFAs-utilizing microbes, while hydrolytic-acidogenic bacteria were enriched, responsible for DOMs biodegradation.

Effects of humic matter on the anaerobic digestion of sewage sludge: New insights from sludge structure

A sludge structure perspective of the effects of humic matter (HM) on the anaerobic digestion (AD) of sewage sludge was proposed and investigated in this study. It was observed that the net cumulative methane production of sludge was significantly lower in the presence of HM. The fractal dimension values of sludge samples were increased by the addition of HM, whilst their total surface energy values were decreased, indicating that HM reinforced the stability of the sludge structure. Analyses of the physicochemical properties of different sludge samples revealed that HM enhanced the structural stability of sludge organic substances and restricted its organic solubility by interacting with the biopolymers to form a dense granular aggregate, thereby reducing the random-coil degree of extracellular organic substances and its active sites for enzymes, which suggested that HM could inhibit AD by changing the sludge structure. It was further confirmed by the biochemical methane potential (BMP) assay process. The BMP tests and corresponding kinetic analyses revealed that HM can restrict the potentials of sludge organic solubilisation, hydrolysis and acidification processes, but not necessarily the rates of these processes by enhancing the structural stability of high molecular weight readily biodegradable organic matter (RBOM) and restricting the mobility of low molecular weight RBOM (i.e., hydrolysis products). These findings can enrich understanding of the AD of sludge and may thus aid in the development of more effective methods to improve sludge treatment.

Enhanced hydrolysis of lignocellulose in corn cob by using food waste pretreatment to improve anaerobic digestion performance

This study aims to enhance hydrolysis and anaerobic digestion of corn cob (CC) by using food waste (FW) pretreatment. FW, which tends to be acidification in fermentation, was applied in this process as an acid-like agent to accelerate lignocellulose hydrolysis, aiming to promote methane yield in further digestion process. The effect of FW pretreatment on pH, soluble chemical oxygen demand (SCOD), volatile fatty acids (VFAs), cellulose/hemicellulose contents and cellulose crystallinity are specially focused. FW:CC = 1:3 based on volatile solid (VS) was found to be the optimal mixing ratio in pretreatment and its hydrolysis efficiency was 28% higher than the control group. An increase of 13.2% in cellulose reduction and a decrease of 6.7% in cellulose crystallinity was achieved at this ratio. Supplementation of FW increased VFA concentrations in slurry mixture that directly change the activities of enzymes and microorganisms. In the stage of methane production, the digester A3 (FW:CC = 1:6 based on VS) with higher hydrolysis efficiency presented the best performance in methane production with a specific methane yield of 401.6 mL/g·VS, due to the recovery of the pH in this digester to the optimal pH range for methanogens' metabolism (pH 6.3-7.2). Kinetics studies of cellulose/hemicellulose degradation indicated that the pretreatment of FW could improve the degradation of cellulose. Three-dimensional excitation emission matrix (3DEEM) results further confirmed that FW play an important role in lignocellulose hydrolysis. In addition, variations of lignocellulosic textures during the pretreatment were also cleared by using field emission-scanning electron microscopy (FE-SEM) analysis.

Enhanced biomethane recovery from fat, oil, and grease through co-digestion with food waste and addition of conductive materials

In this study, the effect of conductive additives on co-digestion of fat, oil, and grease (FOG) and food waste (FW) was evaluated. Initially, biochemical methane potential (BMP) test was conducted for optimization of mixing ratio of FW and FOG. The optimal methane production (800 L (kg VS)^-1) was obtained from co-digestion of 70% FW + 30% FOG (w/w), which was 1.2 times and 12 times of that obtained from mono-digestion of FW and FOG, respectively. This optimal mixing ratio was used for subsequent fed-batch studies with the addition of two conductive additives, granular activated carbon (GAC) and magnetite. The addition of GAC significantly shortened the lag phase (from 7 to 3 d), reduced accumulation of various volatile fatty acids (VFAs), and enhanced methane production rate (50-80% increase) compared to the control and magnetite-amended bioreactor. Fourier transformation infrared (FTIR) analysis suggested that the degradation of lipids, protein and carbohydrates was the highest in GAC amended reactor, followed by magnetite and control reactors. GAC addition also enriched more abundant and diverse bacteria and methanogens than control. Magnetite addition also showed similar trends but to a lesser degree. The substantial enrichment of syntrophic LCFA β-oxidizing bacteria (e.g. Syntrophomonas) and methanogenic archaea in the GAC-amended bioreactor likely attributed to the superior methanogenesis kinetics in GAC amended bioreactor. Our findings suggest that the addition of GAC could provide a sustainable strategy to enrich kinetically efficient syntrophic microbiome to favor methanogenesis kinetics in co-digestion of FW and FOG.

Expanded granular sludge blanket reactor treatment of food waste at ambient temperature: Analysis of nitrogen compositions and microbial community structure

The influent and effluent nitrogen compositions of an expanded granular sludge blanket (EGSB) reactor employed for treating food waste (FW) operated under ambient temperature was evaluated. Additionally, dynamic changes in the bacterial community structures and its metabolic functions were investigated. Results show that the EGSB reactor had a good effect on FW disposal and well resistance to variations in the organic loading rate. Furthermore, the COD concentration in the influent increased to about 10,000 mg/L and the COD removal rate stabilized at about 95%. The dissolved ammonia nitrogen (d-ammonia) content was the largest, accounting for approximately 70-80% of the dissolved nitrogen in the effluent. The amount of particulate organic nitrogen (PON) decreased by about 25%-33%. Amino acid, carbohydrate and lipid metabolism decreased at high organic loading rate (OLR). Meanwhile, the abundance of Methanothrix increased from 30.82% to 70.25%, whereas Methanobacterium decreased from 66.14% to 14.49%.