Synergistic effects of rice straw and rice bran on enhanced methane production and process stability of anaerobic digestion of food waste

Thermophilic and mesophilic digestion of high-solid oily food waste: How to ensure long-term and stable continuous operation?

Commonly high lipid in food waste confronts anaerobic digestion with improved energy production and also inhibition risk from the intermediate long chain fatty acids (LCFAs). Combined with operation challenges from anaerobic digestion of food waste itself, coping strategies are necessitated to ensure stable operation for oily food waste (OFW). A parallel thermophilic (TD) and mesophilic digestion (MD) of high-solid OFW was conducted and operated continuously for a long term. It was clarified that challenges were mainly from acidification, trace metal deficiency and LCFA inhibition. Acidification resulted in an abrupt pH decline to even below 6.00, and over 75% drop of biogas production rate. In addition to the requirements of saturated strong alkali to maintain an appropriate range, supplementation of trace metals were proven effective in counteracting the sharp decrease of biogas production rate. The TD was observed more competent in coping with the acidification than the MD, while the TD needed more supplementation of trace metals at approximately 0.10 mg Fe/g chemical oxygen demand (COD)added, 0.01 mg Co/g CODadded and 0.01 mg Ni/g CODadded. The TD was more adaptable in LCFA conversion due to the stronger ability of overcoming the palmitic acid (C16:0) accumulation. The MD experienced a prolonged recovery period owing to LCFA inhibition shortly after acidification. Similar operation performance was ultimately achieved for the TD and MD by the counteractions, with a methane yield and volatile solids (VS) removal efficiency at about 0.60 L/g VSadded and 75.0%, respectively. In summary, combined pH control and trace metal supplementation, and prevention and recovery of LCFA inhibition were necessary for the stability insurance of a long-term continuous digestion of oily food waste.

Synergistic effects of zinc and cadmium on phytoremediation potential of Christmas moss (Vesicularia montagnei)

The hyperaccumulation potential of zinc (Zn) and cadmium (Cd) and their synergistic effects were examined in relation to Christmas moss (Vesicularia montagnei (Bél) Broth., Hypnaceae), an aquatic and terrestrial moss, dosed with Cd (Cd1 and Cd2), Zn (Zn1 and Zn2) and combined Zn and Cd (Cd1Zn1 and Cd2Zn2). Zinc promoted plant growth and development, particularly in the highest Zn and combined Zn/Cd treatments (Zn2 and Cd2Zn2). The Zn treatment resulted in substantial moss chlorophyll content and highest percentage relative growth rate in biomass value (0.23 mg L-1 and 106.8%, respectively); however, the Cd2Zn2 treatment achieved maximal production of chlorophyll a and total chlorophyll (0.29 and 0.51 mg L-1, respectively) due to synergistic effects. These findings suggest that Christmas moss is a highly metal-tolerant and adaptable bryophyte species. Zinc was essential for reducing the detrimental effects of Cd while simultaneously promoting moss growth and biomass development. Furthermore, Christmas moss exhibited hyperaccumulation potential for Cd and Zn in the Cd2Zn2 and Zn alone treatments, as evidenced by highest Cd and Zn values in gametophores (1002 and 18,596 mg per colony volume, respectively). Using energy dispersive X-ray fluorescence (EDXRF) spectrometry, atomic percentages of element concentrations in moss gametophores in the Zn2, Cd2 and combined Zn/Cd treatments were generally in the order: K > Ca > P > Zn > Cd. When comparing the atomic percentages of Zn and Cd in gametophores, it is likely that the higher atomic percentage of Zn was because this element is essential for plant growth and development.

Anaerobic co-digestion of human excreta, food leftovers and kitchen residue: 1 ternary mixture design, synergistic effects and RSM approach

Anaerobic digestion of multiple substrates can generate more biogas while remaining stable, if positive synergistic effects are achieved. The type of co-digested substrates and the mixing ratio used, are the most important variables as each substrate has unique set of characteristics. Optimizing the volume ratios by testing various substrate mixing ratios is a popular method for determining the best-performing ratio of substrate mixture. The ternary mixture design has reportedly been found to quicken the process of testing different mixing ratios with high accuracy without running several experiments. Therefore, a ternary mixture design and a response surface approach are used in this work to ascertain the relationship between substrate mix and responses (biogas yield, methane yield, and synergy). The findings of the experiment revealed that R9 comprising 78.8 % human excreta, 11.8 % food leftovers and 9.4 % kitchen residue, had the highest methane production of 764.79 mLCH4/gVS and a synergistic index of 3.26. Additionally, the 3D response surface plots from the response surface model showed important and shared interactions between Human Excreta, (HE), Food Leftovers (FLO), and Kitchen Residue (KR). HE and KR had a similar positive synergistic effect on biogas yield, methane yield, and synergy, which was not the case for FLO. The response surface plots showed that the predicted responses (methane yield, biogas yield and synergy) increased with increasing HE and KR fractions and decreased with increasing FLO fractions in the substrate mixtures.

Genome-centric metagenomics revealed functional traits in high-solids anaerobic co-digestion of restaurant food waste, household food waste and rice straw

High-solids anaerobic co-digestion (HS-AcoD) of food waste (FW) and other organic wastes is an effective option to improve the biogas production and system stability compared to mono-digestion. However, the clean and sustainable HS-AcoD strategy for FW and associated microbial functional traits have not been well explored. Here, HS-AcoD of restaurant food waste (RFW), household food waste (HFW) and rice straw (RS) were performed. Results showed that the maximum synergy index (SI) of 1.28 were achieved when the volatile solids ratio of RFW, HFW and RS was 0.45:0.45:0.1. HS-AcoD alleviated the acidification process by regulating metabolism associated with hydrolysis and volatile fatty acids formation. The synergistic relationship between syntrophic bacteria and Methanothrix sp., and the enhanced metabolic capacity associated with the acetotrophic and hydrogenotrophic pathways dominated by Methanothrix sp., provided a further explanation of the synergistic mechanism. These findings advance the knowledge about microbial mechanisms underlying the synergistic effect of HS-AcoD.

Valorization of food waste by anaerobic digestion: A bibliometric and systematic review focusing on optimization

As food waste gets acknowledged as a global potential source of biomass, its valorization through anaerobic digestion becomes an attractive strategy. This work describes the state-of-the-art on the valorization of food waste by anaerobic digestion and the optimization of the process. The methodology used was a bibliometric and systematic review of the optimization of the process from 66 articles selected. Bibliometric mapping allowed us to identify that, until now, most studies have been focused on the: i) anaerobic co-digestion strategy in order to stabilize the process, ii) interest in the generation of biofuels to replace non-renewable fuels, iii) study of metabolic processes for a better understanding of the system iv) reactor design optimization and others facilities to increase process efficiency. The systematic analysis showed that the operational parameters has been extensively studied to optimize the process. Therefore, co-digestion has been the main strategy to improve the process. In this sense, knowledge of the substrate and co-substrate is extremely important to operate the reactors. For methane production, the ideal operating conditions indicated were: pH of 7, solids content between 4.0 and 15%, C/N ratio of 25, hydraulic retention time from 25 to 40 days and alkalinity from 2850 to 2970.5mgCaCO₃/L. In addition, the ideal OLR will vary mainly according to operating temperature, number of reactor stages, and raw material characteristics. This review indicates trends and knowledge gaps that are important to guide new research on the anaerobic digestion of food waste, pointing out the potential advantages, optimization strategies, by-products of interest and challenges of the process. The results were used for the development of references of ideal operating conditions for energy production, being able to guide the design and operation of reactors.

Anaerobic Co-Digestion of Sugarcane Leaves, Cow Dung and Food Waste: Focus on Methane Yield and Synergistic Effects

Anaerobic co-digestion (AcoD) of food waste (FW) and lignocellulose waste is a promising technology for methane production. This work investigated the methane generation from AcoD of FW, sugarcane leaves (SLs), and cow dung (CD) under mesophilic conditions in a batch test. As for AcoD of two feedstocks (SL and FW or CD and FW), introduction of SL and CD (25%, volatile solid (VS) basis) showed slight improvement in methane production from FW. In contrast, positive synergistic effect (synergy index = 1.03–1.14 > 1) was observed in all the AcoD reactors of the three feedstocks (SL, CD, and FW). The optimum mixing ratio of FW:SL:CD (VS basis) was 85:11.25:3.75 with a synergy index of 1.07, achieving a methane yield rate and methane content of 297.16 mL/g VS and 73.26%, respectively. This group cumulative methane production was an improvement of 110.45 and 444.72% higher than mono-digestion of SL and CD. The biodegradability, soluble chemical oxygen demand (SCOD), and VS removal rate were 56.44, 44.55 and 55.38%, respectively. The optimum results indicated that AcoD of FW, SL, and CD have higher potentials for energy recovery and provided forceful scientific evidence for their energy utilization.

Insight into enhanced acetic acid production from food waste in anaerobic hydrolysis/acidification with Fe3O4 supplementation

Fe₃O₄ supplementation has been reported as a high-efficient approach to enhance biogas production in anaerobic digestion (AD). Volatile fatty acids (VFAs), especially acetic acid (HAc), are considered as important products in acidification process of AD. However, the possible mechanisms involved in promotion effect of Fe₃O₄ on HAc production in hydrolysis and acidification processes of AD have not been comprehensively studied. This study first investigated the promotion effect of Fe₃O₄ on hydrolysis, acidogenesis and acetogenesis stages of AD and proposed the underlying mechanisms, using food waste (FW) as the feedstock, which is considered as the most suitable substrate for VFAs production. Results indicated that the HAc production (77.38 g-C/kg-VS) was enhanced by 79 % in AD of FW with addition of 10 g/L Fe₃O₄. The duration to reach the maximum HAc production was also shortened from 14 days to 10 days. The AD tests using model substrates revealed that Fe₃O₄ enhanced hydrolysis, acidogenesis, and degradation of propionic acid, thus resulting in enhanced HAc production. The enhanced activities of hydrolytic and acid-forming enzymes, and electron transport system (ETS) with Fe₃O₄ addition further demonstrated its function as an electron acceptor to stimulate electron transfer and accelerate microbial metabolisms in AD, which contributed to the enhanced HAc production from FW.

Current development and perspectives of anaerobic bioconversion of crop stalks to Biogas: A review

As one of the most abundant biomass resources, crop stalks are great potential feedstock available for anaerobic digestion (AD) to produce biogas. However, the specific physical properties and complex chemical structures of crop stalks form strong barriers to efficient AD bioconversion. To overcome these problems, many efforts have been made over the past few years. This paper reviewed recent research in the evolving field of anaerobic bioconversion of crop stalks and was focused on three critical aspects affecting AD performance: various pretreatment methods and their effects on the improvement of crop stalk biodegradability, determination of specific AD operation parameters for crop stalks, and development of AD technologies. Finally, recommendations on the future development of crop stalk AD were proposed.

Supplementation of KOH to improve salt tolerance of methanogenesis in the two-stage anaerobic digestion of food waste using pre-acclimated anaerobically digested sludge by air-nanobubble water

Air-nanobubble water (NBW) was applied to pre-acclimate anaerobically digested sludge that was then used as the inoculum in the two-stage anaerobic digestion (AD) of high saline (20 g NaCl/L) food waste (FW) to optimize NBW application in the AD of high saline FW. K⁺ was simultaneously supplemented during the methanogenic stage to resist the inhibition of salt on methanogens. Results showed that after the second pre-acclimation cycle, the inoculum activity was increased 27% in the Air-NBW supplemented reactor in comparison to the deionized water (DW) supplemented one. In the first-stage AD, H₂ yield was enhanced by 46% in the Air-NBW pre-acclimated sludge reactor compared with the DW pre-acclimated sludge reactor. Besides, supplementation of KOH in the methanogenic stage could enhance methane production by 17-25% in the DW reactors at initial pH 7.5, 8.0, and 9.0 when compared to the control reactor (using NaOH adjusted initial pH to 7.5), respectively.

Improving the efficiency of anaerobic digestion of Molasses alcohol wastewater using Cassava alcohol wastewater as a mixed feedstock

Cassava alcohol wastewater (CAW) was utilized as a mixed feedstock to explore whether the addition of CAW could improve the anaerobic digestion of Molasses alcohol wastewater (MAW). The result showed that the rate of removal of the soluble chemical oxygen demand in the M treatment mixed with CAW was 70.13 ± 0.16%, which was significantly higher than that of the C treatment (only MAW), which was 61.23 ± 0.36%. Co-digestion in the M treatment resulted in higher methane production, achieving 23.89% increase in methane yield compared to C treatment. The addition of CAW helps to alleviate the accumulation of volatile fatty acids (397.06 ± 141.82 mg·L^-1), enhance the stability of system and promote the establishment of stable and active microbial communities. Microbial community structure analysis indicated that hydrolytic bacteria such as Bacteroidetes, Firmicutes, and Proteobacteria, and acetoclastic methanogens, including Methanosaeta and Methanosarcina were more abundant in the co-digests.

Timing of biochar dosage for anaerobic digestion treating municipal leachate: Altered conversion pathways of volatile fatty acids

In this study, the anaerobic digestion (AD) applications of early & late biochar dosage were compared for municipal leachate treatment, with the objective of studying the flexible use of biochar as a mitigation measure for biomethane recovery. In two experimental phases, biochar was favourable for the immediate promotion of AD performances, as revealed by Gompertz's model of reduced lag phases, higher biomethane generation rates, and increased biomethane yields. Irrespective of late biochar dosage, it could still retrieve 89% of the ultimate biomethane potential. Comparing the residual VFAs (volatile fatty acids) compositions, it was found that the fraction of long-chain VFAs accounted for 81% of total VFAs in reactor set of early biochar dosage, while it was only 38% in the reactor of late one. Parallel evidence suggested that the schedule of biochar dosage not only could affect methanogenic responses but also the VFAs conversion pathways.

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.

Enhanced energy recovery via separate hydrogen and methane production from two-stage anaerobic digestion of food waste with nanobubble water supplementation

This study investigated the enhancement effect of N₂- and Air-nanobubble water (NBW) supplementation on two-stage anaerobic digestion (AD) of food waste (FW) for separate production of hydrogen and methane. In the first stage for hydrogen production, the highest cumulative H₂ yield (27.31 ± 1.21 mL/g-VS_added) was obtained from FW + Air-NBW, increasing by 38% compared to the control (FW + deionized water (DW)). In the second stage for methane production, the cumulative CH₄ yield followed a descending order of FW + Air-NBW (373.63 ± 3.58 mL/g-VS_added) > FW + N₂-NBW (347.63 ± 7.05 mL/g-VS_added) > FW + DW (300.93 ± 3.24 mL/g-VS_added, control), increasing by 24% in FW + Air-NBW and 16% in FW + N₂-NBW compared to the control, respectively. Further investigations indicate that different gas-NBW may positively impact the different stages of AD process. Addition of N₂-NBW only enhanced the hydrolysis/acidification of FW with no significant effect on methanogenesis. By comparison, addition of Air-NBW promoted both hydrolysis/acidification stage and methanogenesis stage, reflecting by the enhanced activities of four extracellular hydrolases at the end of hydrolysis/acidification and coenzyme F₄₂₀ at the end of methanogenesis, respectively. Results from this work suggest the potential application of Air-NBW in the two-stage AD for efficient renewable energy recovery from FW.

Composition-oriented estimation of biogas production from major culinary wastes in an anaerobic bioreactor and its associated CO2 reduction potential

Despite the wide applications of dry anaerobic digestion (AD), a number of fundamental issues, such as composition-oriented estimation of biogas production and CO₂ reduction potential, were not well understood yet. The objective of this study was to establish composition-oriented models for prediction of biogas production and the associated shift of microbial communities. Three important factors regarding feedstock, including loading, carbon-to-nitrogen ratio, and solid-to-liquid ratio, were found to significantly affect the biogas production. The biogas production and digestion kinetics were evaluated with the response surface methodology. The major contribution to biogas production was found to be hydrogenotrophic methanogens (82.6 ± 0.4%). The net CO₂ reduction potential was assessed from the life-cycle approach, and a substantial amount of CO₂ generation (i.e., 2.8-6.7 tonne/tonne-VS) could be reduced by AD, compared to incineration, revealing that dry AD for food waste treatment should be one of the essential practices in the portfolio of global CO₂ mitigation.