Anaerobic Digestion of Vegetable Wastes Using Biochemical Methane Potential Assays

This study aims to compare the performance and kinetics between the single-stage anaerobic digestion (SAD) and the two-stage anaerobic digestion (TAD) of vegetable waste (VW). The SAD was performed using continuously stirred tank reactors. Meanwhile, the TAD experiment was set up using a combined system involving a continuously stirred tank for hydrolysis/acidogenesis and an upflow reactor for methanogenesis. The hydrolytic reactor operated as a batch process with a retention time (RT) of 9 days, while the methane reactor was a continuous process operation with RT of 20 days. Both TAD and SAD were controlled at a temperature of 36 °C. The SAD experiments lasted for 143 days, and were characterised by the kinetic rate constant k = 0.02 day−1 which was much lower than that for the TAD (k = 0.66 − 2.16 day−1). The SAD seemed to be inhibited by high concentration of free ammonia and low inoculum to substrate ratio; herein, only 17.8–22.3% of the initial carbon could be converted into biogas (equivalent to 91–110 Nml/g-VSadded) with low methane content (44.1–48.7%). Meanwhile, TAD converted 41.67% initial carbon to biogas (equivalent to 299.0–374.6 Nml/g-VSadded) with high methane content (71.68–81.0%). Moreover, methanogenesis in the TAD was highly stable which enabled the digestion process to return to normal state within a few days, even though the concentrations of the influent increased to double (6.5–24.5 g-COD/l). As per these results, the TAD was much more stable, faster, and stronger than the SAD.

Optimization of semi-continuous dry anaerobic digestion process and biogas yield of dry yellow corn straw: Based on “gradient anaerobic digestion reactor”

Anaerobic digestion (AD) is a promising approach for vegetable waste (VW) recycling and energy recovery but methanogenesis is always inhibited by acid accumulation. The addition of a mixture of iron and other elements has proved effective on reducing acid inhibition during AD of VW, but the effect of use of iron alone has rarely been assessed. In the present study, we compared the effects of Fe0 and Fe2+ on methane fermentation from VW at an organic loading rate of 1.5 gVS/L/d. The results indicated that Fe0 maintained a pH > 7.7, oxidation reduction potential <−520, and methane production rate (MPR) at 250–300 mL/gVS/d. Partial least squares path modeling and correlation analysis revealed Fe0 maintained pH for high MPR mainly through enhancing the conversion of propionic and butyric acids to acetic acid, causing the total acid decreasing to 1500 mg/L. In contrast, Fe2+ caused an accumulation of all volatile fatty acids up to 4000 mg/L and aggravated acid inhibition, resulting in a significant reduction in MPR to 148 mL/gVS/d. In all treatments, the influence of ammonia nitrogen was not significant. This study provided a direction for the determination of engineering monitoring indicators and regulation measures in AD of VW.

With the development of agriculture, a huge amount of vegetable waste (VW) is produced every year, posing a large considerable environmental problem that cannot be ignored. Anaerobic digestion (AD), as an eco-friendly, efficient, and sustainable biomass conversion technology, may be used to address the pollution caused by VW. The compositional components of various VWs are different, which will affect their biomethane potential and directly determine whether they are suitable substrates for AD. Thus, this study involved a systematic analysis of the composition and biomethane potential of 20 typical VWs. The results showed that the methane yields of the VWs were different (207.5-346.3 mL/g VS) owing to the differences in composition. More importantly, a correlation between the contents of organic components and methane production was established, and then used to predict methane production by VW rapidly. In addition, first-order model, modified Gompertz, and Cone models were used to describe the biochemical methanogenesis mechanism of these VWs. The results of this study can provide a reference for fundamental research on the AD of VW as well as serve a convenient and precise method to predict methane production by different VWs through analyzing compositional components, which will be beneficial for pollution prevention and the comprehensive utilization of VW in the future.

International trade and the market demand for pre-prepared agricultural produce is not only increasing the total quantity of waste agricultural biomass but also centralising its availability, making it potentially useful for energy production. The current work considers the suitability of vegetable trimmings and rejects from high-value produce air-freighted between Africa and Europe as a feedstock for anaerobic digestion. The physical and chemical characteristics of a typical mixed vegetable waste of this type were determined and the theoretical energy yield predicted and compared to experimentally-determined calorific values, and to the energy recovered through a batch biochemical methane potential test. A semi-continuous digestion trial was then carried out with daily feed additions at different organic loading rates (OLR). At an OLR of 2 g VS L−1 day−1 the substrate gave a methane yield of 0.345 L g-1 VS added with VS destruction 81.3%, and showed that 76.2% of the measured calorific value of the waste could be reclaimed as methane. This was in good agreement with the estimated energy recovery of 68.6% based on reaction stoichiometry, and was 99% of the biochemical methane potential (BMP). Higher loading rates reduced the specific methane yield and energy conversion efficiency, and led to a drop in digester pH which could not be effectively controlled by alkali additions. To maintain digester stability it was necessary to supplement with additional trace elements including tungsten, which allowed loading rates up to 4 g VS L−1 day−1 to be achieved. Stability was also improved by addition of yeast extract (YE), but the higher gas yield obtained was as a result of the contribution made by the YE and no synergy was shown. Co-digestion using card packaging and cattle slurry as co-substrates also proved to be an effective means of restoring and maintaining stable operating conditions.

When vegetable waste (VW) is used as a sole substrate for anaerobic digestion (AD), the rapid accumulation of volatile fatty acids (VFAs) can impede interspecies electron transfer (IET), resulting in a relatively low biogas production rate. In this study, Chinese cabbage and cabbage were selected as the VW substrates, and four continuous stirred tank reactors (CSTRs) were employed. Different concentrations of biochar-loaded nano-Fe3O4(Fe3O4@BC) (100 mg/L, 200 mg/L, 300 mg/L) were added, and the organic loading rate (OLR) was gradually increased during the AD process. The changes in biogas production rate, VFAs, and microbial community structure in the fermentation tanks were analyzed to identify the optimal dosage of Fe3O4@BC and the maximum OLR. The results indicated that at the maximum OLR of 3.715 g (VS)/L·d, the addition of 200 mg/L of Fe3O4@BC most effectively promoted an increase in the biogas production rate and reduced the accumulation of VFAs compared to the other treatments. Under these conditions, the biogas production rate reached 0.658 L/g (VS). Furthermore, the addition of Fe3O4@BC enhanced both the diversity and abundance of bacteria and archaea. At the genus level, the abundance of Christensenellaceae_R-7_group, Sphaerochaeta, and the archaeal genus Thermovirga was notably increased.

Potentiality of municipal sludge for biological gas production at Soba Station South of Khartoum (Sudan)

This study aims to investigate the fate of carbon in two-stage anaerobic digestion (TAD) of vegetable waste. The TAD including a hydrolytic reactor and a methane reactor were controlled at mesophilic temperature (36 ± 1°C) with a retention time (RT) of 9 days and 20 days, respectively. Carbon tracking was conducted step by step throughout the system. Non-hydrolysable carbon accounted for a significant proportion of total initial carbon (25%). Meanwhile, a large amount of carbon in the feedstock (23.5%) was hydrolysed but remained in the effluent including water and suspended solids. It was mostly inorganic carbon which is not harmful to the environment. The only 41.3% initial carbon was converted to biogas in both reactors. In the hydrolytic reactor, biogas was mainly carbon dioxide (99%), accounted for 11.3% of total biogas and 35.8% of total CO2 product. In the methane reactor, biogas was 373.9 Nml/g-VS including 73.3% CH4, 21.9% CO2 and 4.8% others. Non-hydrolysable materials can be a source of thermal energy. Meanwhile, a large amount of hydrolysed carbon was not converted into biogas, was still in the effluent, it was a significant energy loss. Therefore, how to further increase the effectiveness of TAD is an issue that needs to study.

Integrating microbial electrochemical cell in anaerobic digestion of vegetable wastes to enhance biogas production

The rapid development of the economy has led to rapid consumption of fossil fuels, which results in extremely serious environmental problems. Biomass energy has been accepted as a way to reduce the usage of fossil fuels due to its cleanliness and renewability. In this study, vegetable wastes (VWs), an abundant kind of biomass resource, were treated by anaerobic digestion (AD) to be converted into methane. The total solids (TS), volatile solids (VS), elemental contents, and organic components of 17 kinds of typical VWs were systematically determined. The methane production performances were then measured and ranged from 120.1 mL/g VS (for pepper stem) to 377.7 mL/g VS (for bok choy). To easily and quickly predict the methane yields of VWs, a curvilinear relationship between different organic compositions (e.g., cellulose, hemicellulose, lignin, non-structural carbohydrate, protein, and VFA contents) and methane production was established and proved to be a useful tool for methane prediction. Four kinetic models (first-order model, Fitzhugh model, Cone model, modified Gompertz model) were applied to simulate the process of AD, and Cone and modified Gompertz models were shown to describe the AD process well. This study will not only provide basic data about the characteristics and methane production of 17 kinds of VWs but also contribute a method for predicting the methane yields of vegetable wastes, which is also valuable in future agro-industrial applications. Keywords: naerobic digestion, organic components, methane production, vegetable waste, kinetic models DOI: 10.25165/j.ijabe.20191203.4705 Citation: Cai F F, YanH, Zhang R H, Liu G Q, Chen C. Prediction of methane production performances based on the determination of organic components for different vegetable wastes. Int J Agric & Biol Eng, 2019; 12(3): 154–159.

The present study investigates the influence of individual and combined hydrogen peroxide and thermal pre-treatment of waste activated sludge on anaerobic digestion. For so doing, it employs anaerobic batch reactors in the mesophilic conditions. For comparison, soluble fractions of organic matter, biogas production, biochemical methane potential, removal of chemical oxygen demand (COD), and volatile solids (VS) have been measured during the anaerobic digestion process in systems with and without pre-treatment. Hydrogen peroxide pre-treatment has been tested in two concentrations of 30 g H2O2/kg VS and 60 g H2O2/kg VS and thermal pre-treatment has been performed at two temperatures of 75℃ and 90℃. According to the results, the solubalisation of organic matter considerably improves, when combined hydrogen peroxide and thermal pre-treatment is employed. As a result, in comparison to the control reactor, higher amounts of biogas (71%) and methane (81%) have been produced in the bioreactor, pre-treated with combined hydrogen peroxide (30 g H2O2/kg VS) and heat (90 ℃). In addition, the removal efficiency of COD and VS from the digested sludge has been enhanced in the pre-treated reactors (up to 39% and 92%, respectively) in comparison to the control reactor. The improved methane yield, COD, and VS are of paramount importance, not only because higher amounts of renewable energy are obtained from the anaerobic digestion process, but because sludge transport costs are reduced and the digested sludge obtains a higher potential application to agricultural lands.

The anaerobic digestion of vegetable waste (VW) often shows the accumulation of fatty acids and low buffering capacity that promotes instability and low methane productivity. This work evaluated the anaerobic co-digestion of VW with cow manure (CM) as a strategy to improve the process stability. As a reaction system, a 4 L semi-continuous stirred tank reactor with an HRT of 20 days and fed with a substrate formulation of 40 g of VS was used in two periods: 34 days of VW mono-digestion and 26 days of VW:CM co-digestion. The mono and co-digestion processes were numerically evaluated through three analysis tools: a proposed co-digestion model embedded in the Anaerobic Digestion Model No. 1 structure, statistical process control theory, and modeling the pH dynamics as the response of a first-order linear system to an impulse manipulation. The mono-digestion process showed productivity of 0.381 L CH4 L digester-1 d-1, which increased by 14 % during co-digestion. The results also indicated that in VW:CM co-digestion, the pH dynamics presents a slower response to the daily feed induced by pulses, keeping the values of this parameter within the statistical stability range; as well as the early warning indicator IA/BA (ratio between intermediate and bicarbonate alkalinity) outside the failure thresholds. It was shown that the addition of CM to a mono-digestion of VW increases the buffer capacity of the system and the production of CH4, promoting a stable and efficient process.

BACKGROUNDThe accumulative municipal solid waste (MSW) production calls for emerging efficient technology for its handling. Anaerobic digestion (AD) of MSW provides effective waste volatilization. The high C/N ratio of MSW and increasing organic loading rates (OLRs) are central AD restrictions that affect the AD process performance, inhibition due to aggregation of volatile fatty acids (VFAs), and rapid fall in pH.RESULTSThis study examines the consequence of OLRs on AD of MSW with a high C/N ratio of 406 in food waste (FW) to have a balanced C/N ratio of 30. Three batch scale digesters investigate under mesophilic conditions (35 °C) with OLRs of 10, 15, and 20 gVS L−1to assess the digester performance and stability in biogas yield, methane yield, and volatile solids (VS) reduction. The cumulative biogas and methane yield are observed to be 1336 and 776 mL gVS−1, respectively, with aVSreduction rate of 78%, at 10 gVS L−1. VFA/alkalinity ratio ranges from 0.02 to 0.01 at OLR of 10 and 15 gVS L−1, which designates a higher buffering capability of the digester. While VFA/alkalinity ratio of 0.48 observes at OLR of 20 gVS L−1. A rapid deprivation in digester performance and stability finds at ORL of 15 and 20 gVS L−1. The cumulative biogas yield and methane yield decrease with the increase in OLR from 10 to 20 gVS L−1.CONCLUSIONThis study provided sufficient information for better AD processes and operational circumstances that are an optimum and effective method to convert organic matter to biogas fuel. © 2021 Society of Chemical Industry (SCI).

A large quantity of food waste (FW) is generated annually across the world and results in environmental pollution and degradation. This study investigated the performance of a 160 L anaerobic biofilm single-stage reactor in treating FW. The reactor was operated at different hydraulic retention times (HRTs) of 124, 62, and 35 days under mesophilic conditions. The maximum biogas and methane yield achieved was 0.934 L/g VSadded and 0.607 L CH4/g VSadded, respectively, at an HRT of 124 days. When HRT decreased to 62 days, the volatile fatty acid (VFA) and ammonia accumulation increased rapidly whereas pH, methane yield, and biogas yield decreased continuously. The decline in biogas production was likely due to shock loading, which resulted in scum accumulation in the reactor. A negative correlation between biogas yield and volatile solid (VS) removal efficiency was also observed, owing to the floating scum carrying and urging the sludge toward the upper portion of the reactor. The highest VS (79%) and chemical oxygen demand (COD) removal efficiency (80%) were achieved at an HRT of 35 days. Three kinetic models—the first-order kinetic model, the modified Gompertz model, and the logistic function model—were used to fit the cumulative biogas production experimental data. The kinetic study showed that the modified Gompertz model had the best fit with the experimental data out of the three models. This study demonstrates that the stability and performance of the anaerobic digestion (AD) process, namely biogas production rate, methane yield, intermediate metabolism, and removal efficiency, were significantly affected by HRTs.

In anaerobic digestion (AD) processes, hydrolysis is considered the limiting stage in the degradation of solid wastes. Such is the case of swine manure digestion, which due to the complex physical and chemical structure of the lignocellulosic material that composes it, an energy wastage has been observed in terms of methane production. Among the strategies used to improve the hydrolysis stage, it is possible to mention the thermal pretreatment of the substrate, which can significantly improve the biodegradability of the material used as raw material in AD. In this study, the effect of temperature (60–177 °C) and exposure time (30–60 min) on the physicochemical properties of the substrate such as pH, volatile organic acids (VOAs), total inorganic carbonates (TIC), volatile solids (SV), total solids (ST), soluble chemical oxygen demand (CODs) and biochemical methane potential (BMP) were determined. The temperature factor had a higher level of significance compared to the exposure time on the parameters of pH, VOAs, and TIC before and after the biogas generation process. However, the effect was different for CODs, where time was more significant than treatment temperature. As for the parameters of total and volatile solids content (ST and SV), the factors evaluated (temperature and exposure time) did not show significant effect. Also, the pretreatments showed an increase in biochemical methane potential, outperforming the untreated substrate by up to 70.4 % (121.74vs.71.44 mLCH4 gVS-1). The best combination of heating temperature and operating time was 120 °C and 45 min, which promoted the hydrolysis step that was reflected in an increase in CODs and improvement in methane production by 42 % over the untreated substrate