Anaerobic co-digestion of forage radish and dairy manure in complete mix digesters

Anaerobic Digestion of Cereal Rye Cover Crop

The rapid growth of cover crop planting area in the U.S. helps with erosion control, soil health, control of greenhouse gases, and also provides abundant biomass for the production of bioenergy and bioproducts. Given the cover crops’ compositional heterogeneity and variability, a tolerate platform technology such as anaerobic digestion (AD) is preferred but has not been widely used for cover crop biorefining. This study evaluated the biogas and methane yields from six cereal rye (Secale cereale L.) cover crops grown in the Midwest, using both bench- and pilot-scale anaerobic digesters. The effects of two critical factors, the total solids (TS) content and ensiling, on digester performance were also investigated. Methane yields of 174.79–225.23 L/kg-VS were obtained from the bench-scale tests using cereal rye as the mono feedstock. The pilot-scale test with no pH adjustment showed a slightly higher methane yield. Ensiling increased the methane yield by 23.08% at 6% TS, but disturbed AD at 8% TS, and failed AD at 10% and 15% TS. Findings from this study would help farmers and the biorefining industry to determine the baseline performance and revenue of cereal rye AD and to develop strategies for process control and optimization.

Strategies to boost anaerobic digestion performance of cow manure: Laboratory achievements and their full-scale application potential

Cow manure represents a surplus manure waste in agricultural food sectors, which requires proper disposal. Anaerobic digestion, in this regard, has raised global interest owing to its apparent environmental benefits, including simultaneous waste diminishment and renewable energy generation. However, dedicated intensifications are necessary to promote the degradation of recalcitrant lignocellulosic components of cow manure. Hence, this manuscript presents a review of how to exploit cow manure in anaerobic digestion through different incentives extensively at lab-scale and full-scale. These strategies comprise 1) co-digestion; 2) pretreatment; 3) introduction of additives (trace metals, carbon-based materials, low-cost composites, nanomaterials, and microbial cultures); 4) innovative systems (bio-electrochemical fields and laser irradiation). Results imply that co-digestion and pretreatment approaches gain the predominance on promoting the digestion performance of cow manure. Particularly, for the co-digestion scenario, the selection of lignin-poor co-substrate is highlighted to produce maximum synergy and pronounced removal of lignocellulosic compounds of cow manure. Mechanical, thermal, and biological (composting) pretreatments generate mild improvement at laboratory-scale and are proved applicable in full-scale facilities. It is noteworthy that the introduction of additives (Fe-based nanomaterials, carbon-based materials, and composites) is acquiring more attention and shows promising full-scale application potential. Finally, bio-electrochemical fields stand out in laboratory trials and may serve as future reactor modules in agricultural anaerobic digestion installations treating cow manure.

Effect of nitrogen fertilization on the production of biogas from sweet sorghum and maize biomass

The aim of the study was to determine the biogas productivity from selected cultivars of sorghum (Sorghum bicolor Moench) and maize (Zea mays L.) depending on the dose of mineral fertilization with nitrogen. The topic is novelty in the northern Poland area due to the fact that this crop is not very widespread here. The silage samples were derived from two experiments: (1) two factors experiment with sorghum varieties (Arbatax, KWS Maja, Herkules) and two doses of mineral nitrogen fertilization (0 and 150 kg ha^-1N) in split-plot design. (2) one-factor experiment with fodder maize, variety NK Magitop, and two doses of mineral nitrogen fertilization (0 and 150 kg ha^-1N) in a randomized complete block design. The experiment was performed in four replications in the split-plot design. Methane fermentation was carried out under mesophilic conditions. The temperature of the process was 37°C ± 1°C, while pH 7 ± 0.1. The content of total solids in the bioreactor was 7.0%. The composition of the gas produced was measured once a day with the use of an automatic biogas analyser (GFM 416, GasData). The trial was run in triplicate until the daily yield was less than 1% of the cumulative biogas yield [DIN 38 414-S8. Sediments and sediments. Determination of fermentation characteristics; 1985]. Sorghum was characterized by higher average biogas productivity (about 12%), higher methane content in biogas (about 10%), and higher methane productivity (about 43%). It can, therefore, be stated that sorghum represents as an alternative plant to maize for the purpose of biogas production.

An optimum strategy for substrate mixture and pretreatment in biogas plants: Potential application for high-pH waste management

Optimization of anaerobic digestion poses many practical constraints. Therefore, a new approach was developed by integrating mathematical modeling and desirability analysis to determine optimum amounts of input factors. For this purpose, different proportions of co-digestion (grapefruit waste:cow manure) and pretreatments (NaOH and H₂O₂) were tested on the basis of a combined D-optimal experimental design. Different models were developed for certain slurry properties and CH₄ production (responses) depending on the input factors. To improve the models, Box-Cox transformation was used to transform the models into more accurate formats. The improved models were then used in desirability analysis, and optimum ranges were determined instead of optimum absolute values in three cases of given constraints, (i) constraints of the slurry properties, (ii) increase in CH₄ production, and (iii) constraints on all the responses (more important for plant efficiency). The optimum ranges were extracted for the desirability levels with values greater than 0.9 × (maximum desirability value). For the constraints given in all the responses, the optimum range of grapefruit waste:cow manure proportion was 2.5%:97.5% to 25%:75%, the optimum range of NaOH pretreatment was 0.3-2.64%, and the optimum range of H₂O₂ was range experimented except for 1.32-1.68%. Similar trends were determined for the other cases of constraints. To clarify the method of desirability analysis, the overlaying method was used to determine regions of interest according to some predefined constraints. Simultaneous consideration of range optimization and region of interest showed that (i) an amount greater than 25% grapefruit waste in the digestion decreased the CH₄ production significantly, and (ii) plant efficiency was improved with all amounts of H₂O₂ except 1.32-1.68%, and with amounts of 0.3-1.2% and 1.8-2.64% of NaOH. As shown the optimum ranges are more common for practical use in plants.

Biochar addition with Fe impregnation to reduce H2S production from anaerobic digestion

Corn stover biochar (CSB) and maple biochar (MB) were added into anaerobic digesters and evaluated for hydrogen sulfide (H2S) reductions. This was the first study to show Fe-impregnated biochar can eliminate H2S production. The novel study evaluated biochar addition on H2S reduction and nutrient concentrations using three experiments to test the effect of: 1) biochar concentration, 2) biochar particle size, and 3) Fe-impregnated biochar using triplicate lab-scale reactors. At the highest biochar dose (1.82 g biochar/g manure TS), H2S production was 90.5% less than the control treatment (351 mL H2S/kg VS). Biochar particle size did not significantly affect H2S concentration. The Fe-impregnated biochar (0.5 g biochar/g manure TS) reactors had no H2S detected in the CSB-Fe system. Methane (CH4) in the biochar and control treatments were not significantly different in all three experiments. The results show that biochar added to digesters can significantly reduce H2S production without affecting CH4 production.

Assessment of the biochemical methane potential of mono- and co-digested dairy farm wastes

This study aimed to evaluate the methane potential of mono- and co-digested dairy farm wastes. The tested substrates included manure from lactating, dry, and young cows, as well as waste milk and feed waste. The highest methane yield was achieved from the lactating cow manure, which produced an average of 412 L of CH₄ kg^-1 volatile solids, followed by young and dry cow manures (332 and 273 L of CH₄ kg^-1 volatile solids, respectively). Feed and milk yielded an average of 325 and 212 L of CH₄ kg^-1 volatile solids, respectively. Co-digesting the manures from lactating and young cows with feed improved methane production by 7%. However, co-digesting the dry cow manure with feed achieved only 85% of the calculated methane yield. Co-digesting manure and milk at a ratio of 70:30 enhanced the methane potential from lactating, dry, and young cow manures by 19, 30, and 37%, respectively. Moreover, co-digesting lactating, dry, and young cow manures with milk at a ratio of 30:70 enhanced the methane yield by 60, 30, and 88%, respectively. The cumulative methane production of all samples was accurately described using the Gompertz model with a maximum error of 10%. Carbohydrates contributed the most to methane potential, while proteins and lipids were limiting.

Effects of mixing time on methane production from anaerobic co-digestion of food waste and chicken manure: Experimental studies and CFD analysis

The relationship between mixing time and methane production was investigated by anaerobically co-digesting food waste (FW) and chicken manure (CM) at four different organic loading rates. The mixing pattern and turbulence intensity obtained from CFD were adopted to evaluate the mixing performance in digesters with different viscosities. The simulated mixing time from CFD was selected as a reference for the first time to analyze the methane yield. The results showed that if the digester was well mixed under intermittent mixing mode with relatively short mixing time, then extending mixing time or changing intermittent mixing to continuous mixing would have no substantial effects on methane production. By contrast, continuously mixed digesters performed better when the intermittent mixing modes cannot make the digester get to a high degree of uniformity. Hence, the simulated mixing time from CFD can be used as a reference to determine the experimental mixing time in different cases.

Non-airtight Fermentation of Dairy Manure with Waste Potato Peels and Subsequent Phosphorus Recovery via Struvite Precipitation

Two-phase anaerobic co-digestion of lignocellulosic crop residues with animal wastes can efficiently generate more biogas compared with the digestion of animal waste alone. Non-airtight fermentation of the mixed substrates is the primary step to hydrolyze complex organics and achieve simultaneous phosphorus release. Recycling phosphorus from tremendous animal wastes is remarkably meaningful regarding non-renewable resource recovery. In this study, the feasibility of a two-step process combining non-airtight fermentation of potato peels with dairy manure and the following struvite precipitation was explored. The hydrolysis and acidification process of the 6-day non-airtight mesophilic fermentation lowered pH to 6.4 under the highest mixed solid content of 4.8%; meanwhile, the ratio of reactive phosphorus to total phosphorus increased from 49.6 to 93.7% accordingly. Struvite formation was successfully induced by adjusting pH to 8.0 and 9.5. Under these two pHs, the precipitates were dominated by struvite as characterized by X-ray diffraction (XRD). Scanning electron microscopy and energy-dispersive spectrometry (SEM-EDS) results indicated that there should exist both struvite and calcium phosphate in the precipitates obtained under the two pHs. pH 8.0 precipitate should contain around 75% struvite, while the proportion rose to about 90% for pH 9.5 precipitate, based on the calculation of respective Mg/P and Ca/P molar ratios.

Mesophilic anaerobic co-digestion of acorn slag waste with dairy manure in a batch digester: Focusing on mixing ratios and bio-based carbon accelerants

Co-digestion of acorn slag waste (ASW) and dairy manure (DM) with two bio-based carbon (BC) accelerants are investigated via batch experiments under mesophilic condition. With the favorable synergistic effect of the mixed substrate and BC accelerant, the anaerobic digestion (AD) systems assembled with aloe peel-derived BC (2.16 g/L) show significantly improved methanogenesis on the basis of the optimum wet weight ratio of ASW to DM (1:3). The cumulative biogas yield is 580.9 mL/g VS, and the total chemical oxygen demand reduction is 79.37%. These results are higher than those of the AD systems without carbon-based accelerants. The feasibility of digestate utilization is evaluated by thermal and fertilizer analyses, which manifest outstanding stability and excellent fertility (6.93%-7.40%) of digestate in co-digestion systems. A general strategy for understanding the enhanced methanogenesis pathways, induced by BC in AD systems, is demonstrated. These important findings open an innovative opportunity for developing carbon-based accelerants in anaerobic co-digestion.

Multivariate analysis and biodegradability test to evaluate different organic wastes for biological treatments: Anaerobic co-digestion and co-composting

This study proposes the combination of statistical analysis and a biodegradability test to complement the composition of different wastes in order to find the optimal balance of nutrients for their joint bioconversion. Due to the need to determine the adequate balance of nutrients, the use of alternative techniques to experimental procedures could significantly reduce the cost and time of the process. With this aim, fifteen organic wastes (nine solid and six liquid wastes) were selected and different statistical analyses were performed on the physico-chemical characterization and respirometric variables. Liquid and solid wastes were analyzed separately using principal components analysis (PCA) (PC1 + PC2: 67% of total variance explained for solid substrates and PC1 + PC2: 85% of total variance explained for liquid substrates). The analysis provided considerable information about the predominant chemical composition of each substrate as well as their similarities and deficiencies to identify possible mixtures. In addition to PCA, cluster analyses (CA) were performed to group the substrates and identify the most significant differences between them. The joint evaluation of PCA and CA permitted identifying the optimal waste mixtures (i.e., glycerol-strawberry-fish waste) by correlating the loadings and scores plot, the cluster analysis dendograms and the COD/TKN ratio from the physico-chemical characterization. Moreover, multivariate regression was found to be an appropriate tool for predicting microbiological activity, as well as the soluble available biodegradable organic matter of each substrate. Inorganic carbon (C_IC) and total organic carbon (C_TOC) were found to be the most influential parameters in the prediction correlation of oxygen consumption and oxygen uptake rate.

Evaluating analytical methods for the characterization of lipids, proteins and carbohydrates in organic substrates for anaerobic co-digestion

This study provides insights into the characterization of lipids, proteins and carbohydrate content in substrates for codigestion, and evaluates their effects on biogas yield. Among the analytical methods evaluated, the Bligh and Dyer, Hach Total Nitrogen and the Anthrone method were found to be most suitable for lipids, proteins and carbohydrates analysis, respectively. The co-digestibility of ten co-substrate mixes prepared using various volume-to-volume ratios of foodwaste (FW), fats, oils and grease (FOG), and waste activated sludge (WAS) were tested using biomethane potential assays. The three main substrates were mono-digested as well. WAS mono-digestion yielded the lowest methane yield of 118mL CH₄/g VS, while a 50:50 mix of WAS and FOG, containing 85% lipid and 15% protein produced the highest methane yield of 1040mL CH₄/g VS. In general, lipid-rich samples yielded more biogas than samples rich in proteins and carbohydrates. However, samples rich in proteins and carbohydrates had faster biogas production rates.

Mesophilic anaerobic co-digestion of aloe peel waste with dairy manure in the batch digester: Focusing on mixing ratios and digestate stability

Anaerobic co-digestion of aloe peel waste (APW) with dairy manure (DM) was evaluated in terms of biogas and methane yield, volatile solids (VS) removal rate, and the stability of digestate. Batch experiments were performed under mesophilic condition (36±1°C) at five different APW/DM wet weight ratios (1:0, 3:1, 1:1, 1:3, and 0:1). Experimental methane yield from the mixtures was higher than the yield from APW or DM alone, indicating the synergistic effect and benefits of co-digestion of APW with DM. The optimal mixing ratio of APW/DM was found to be 3:1. The cumulative methane yield was 195.1mL/g VS and the VS removal rate was 59.91%. The characteristics of the digestate were investigated by the thermal analysis which indicated the high stability in the samples of the co-digestion. The co-digestion can be an efficient way to improve the degradation efficiency of the bio-wastes and increase the energy output.

Computational fluid dynamics study on mixing mode and power consumption in anaerobic mono- and co-digestion

Computational fluid dynamics (CFD) was applied to investigate mixing mode and power consumption in anaerobic mono- and co-digestion. Cattle manure (CM) and corn stover (CS) were used as feedstock and stirred tank reactor (STR) was used as digester. Power numbers obtained by the CFD simulation were compared with those from the experimental correlation. Results showed that the standard k-ε model was more appropriate than other turbulence models. A new index, net power production instead of gas production, was proposed to optimize feedstock ratio for anaerobic co-digestion. Results showed that flow field and power consumption were significantly changed in co-digestion of CM and CS compared with those in mono-digestion of either CM or CS. For different mixing modes, the optimum feedstock ratio for co-digestion changed with net power production. The best option of CM/CS ratio for continuous mixing, intermittent mixing I, and intermittent mixing II were 1:1, 1:1 and 1:3, respectively.