Effect of organic loading rate on anaerobic co-digestion of rice straw and pig manure with or without biological pretreatment

Bio-CH4 yield of swine manure and food waste optimized by co-substrate proportions diluted in domestic sewage and pH interactions using the response surface approach

This research aimed at evaluating optimal conditions to obtain value-added metabolites, such as bio-CH4, by co-digesting swine manure and food waste diluted in domestic sewage. The assays were carried out in batches using the statistical methods of Rotational Central Composite Design (RCCD) and Surface Response to evaluate the ranges of food waste (1.30-9.70 gTS.L-1), pH (6.16-7.84) and granular Upflow Anaerobic Sludge Blanket sludge as inoculum (2.32-5.68 gTS.L-1), besides about 250 mL of swine manure in 500 mL Duran flasks. According to the RCCD matrix, bio-CH4 yields among 600.6 ± 60.1 and 2790.0 ± 112.0 mL CH4 gTS.L-1 were observed, besides the maximum CH4 production rate between 0.4 ± 0.5 and 49.7 ± 2.0 mL CH4 h-1 and λ between ≤0.0 and 299.3 ± 4.5 h. In the validation assay, the optimal conditions of 9.98 gTS.L-1 of food waste, pH adjusted to 8.0 and 2.20 gTS.L-1 of inoculum were considered, and the bio-CH4 yield obtained (5640.79 ± 242.98 mL CH4 gTS.L-1 or also 5201.83 ± 224.07 mL CH4 gTVS.L-1) was 11.3 times higher than in assays before optimization (499.3 ± 16.0 mL CH4 gTS.L-1) with 5 gTS.L-1 of food waste, 3 gTS.L-1 of inoculum and pH 7.0. Besides, the results observed about the energetic balance of the control and validation assays highlight the importance of process optimization, as this condition was the only one with energy supply higher than the energy required for its operation, exceeding max consumption sevenfold. Based on the most dominant microorganisms (Methanosaeta, 31.06%) and the metabolic inference of the validation assay, it could be inferred that the acetoclastic methanogenesis was the predominant pathway to CH4 production.

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

In China, the problem of low biogas yield of traditional biogas projects has become increasingly prominent. This study investigated the effects of different hydraulic retention times (HRTs) on the biogas production efficiency and microbial community under pilot conditions. The results show that the "Gradient anaerobic digestion reactor" can stably carry out semi-continuous dry anaerobic digestion and improve biogas yield. The highest volatile solids (VS) biogas yield (413.73 L/kg VS and 221.61 L CH4/kg VS) and VS degradation rate (48.41%) were observed at an HRT of 25 days. When the HRT was 15 days, the volumetric biogas yield was the highest (2.73 L/L/d, 1.43 L CH4/L/d), but the VS biogas yield and degradation rate were significantly decreased. Microbial analysis showed that HRT significantly affected microbial community. It provides basic data support for the development of a new anaerobic digestion process and the practical application of the straw biogas project in China.

Rice straw for energy and value-added products in China: a review

The rise of global waste and the decline of fossil fuels are calling for recycling waste into energy and materials. For example, rice straw, a by-product of rice cultivation, can be converted into biogas and by-products with added value, e.g., biofertilizer, yet processing rice straw is limited by the low energy content, high ash and silica, low nitrogen, high moisture, and high-quality variability. Here, we review the recycling of rice straw with focus on the global and Chinese energy situations, conversion of rice straw into energy and gas, biogas digestate management, cogeneration, biogas upgrading, bioeconomy, and life cycle assessment. The quality of rice straw can be improved by pretreatments, such as baling, ensiling, and co-digestion of rice straw with other feedstocks. The biogas digestate can be used to fertilize soils. The average annual potential energy of collectable rice straw, with a lower heating value of 15.35 megajoule/kilogram, over the past ten years (2013-2022) could reach 2.41 × 109 megajoule.

Co-production of biogas and humic acid using rice straw and pig manure as substrates through solid-state anaerobic fermentation and subsequent aerobic composting

Compared with wet anaerobic digestion, solid-state fermentation possesses many merits such as low water consumption, high biogas yield and low processing cost. In this work, co-producing biogas and humic acid (HA) by two-step solid-state fermentation was innovatively investigated using rice straw and pig manure as materials. The result indicates that C/N ratio, straw particle size, and total solid content (TS%) caused significant effects on the solid-state fermentation process. At the first step for anaerobic biogas fermentation, the optimal fermentation conditions included C/N ratio of 27.5, straw particle size of 0.85 mm and TS% of 25%. The maximal biogas productivity and methane content were up to 0.43 m³/(m³·d) and 64.88%, respectively. This means that biogas production was significantly improved by adjusting C/N ratio during the co-fermentation of rice straw and pig manure. Following, the digested residue was aerobically composted for HA biosynthesis to improve the fertilizer efficiency of the fermented substrate. The optimal aeration rate of 0.75 L/min was obtained, and the volatile solid (VS) degradation rate, HA content, and the germination index (GI) value were up to 19.16%, 100.89 mg/g, and 103.07%, respectively, which indicates that HA biosynthesis and compost maturity were significantly enhanced. Therefore, the co-production of biogas and HA using rice straw and pig manure as fermentation materials was achieved by adopting the two-step solid-state fermentation, and the bioconversion efficiencies of livestock manure and straw were significantly improved.

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.

Pilot-Scale Anaerobic Digestion of Pig Manure with Thermal Pretreatment: Stability Monitoring to Improve the Potential for Obtaining Methane

Monitoring and controlling stability in anaerobic digestion (AD) systems are essential, since it allows to obtain information that helps to take corrective actions in case of deviations in the system and to guarantee a stable performance in the biogas production. In this work, a pilot-scale CSRT reactor (1 m3) was monitored during the anaerobic digestion of pig manure with thermal pretreatment (80 °C) operated at thermophilic temperature (45 °C). The ratio of the volatile organic acids (FOS) to the total inorganic carbonate (TAC) and the pH were the indicators used during the monitoring process to identify deviations in the AD system. Additionally, alkaline solution NaOH (98%) was applied to counteract pH deviations and maintain stability. Chemical oxygen demand (COD) and biogas composition were measured during the AD process. It was found that during the AD process, the FOS/TAC was between the range of 0.5 and 1. The results revealed that, in the anaerobic digestion of pig manure with thermal pretreatment, the pH was kept stable in the range of 6.7–7.4 since no medium acidification occurred. Additionally, the tendency of the chemical oxygen demand decreased from the 10th day of operation, product of the favorable enzymatic activity of the microorganisms, reflected in the stable production of biogas (69% CH4). Finally, it is concluded that thermophilic AD of pig manure with thermal pretreatment is a good option when it is carried out efficiently by employing an adequate energetic integration.

Enrichment and balancing of nutrients for improved methane production using three compositionally different agro-livestock wastes: Process performance and microbial community analysis

Balanced nutrition is important for maximizing anaerobic digestion (AD) performance. Herein, the strategy of balancing sugar-fiber-nitrogen nutrients was first established for improved methane production by co-digesting two agricultural and one livestock wastes with complementary compositional properties, such as banana pseudo-stem (BPS), sugarcane baggage (SCB), and chicken manure (CM) having high sugar, fiber and nitrogen contents, respectively. The maximum methane yield was 186.5 mL/g VS_added with a mixture of 45.7% BPS, 26.2% SCB and 28.1% CM (with 1: 11.3: 0.3 of sugar to fiber to nitrogen ratio), increasing by 16.1%, 53.3%, 122.6% than those of mono- BPS, SCB, and CM, respectively. The co-digestion process remained stable under an organic load of 4 g VS/(L·day), which was attributed to the predominant presence of Bacteroidetes, Proteobacteria, Thauera, uncultured_bacterium_p_Aegiribacteria, and hydrogenotrophic methanogens. This study provides a deeper understanding of the co-digestion with agricultural and livestock wastes from the perspective of nutrient balance.

Effects of organic loading rate and pretreatments on digestion performance of corn stover and chicken manure in completely stirred tank reactor (CSTR)

The performance, system stability, and microbial community response in anaerobic co-digestion (AcoD) of corn stover (CS) and chicken manure (CM) were investigated by running completely stirred tank reactor (CSTR) under controlled organic loading rate (OLR). Prior to anaerobic digestion (AD), potassium hydroxide (KOH) or liquid fraction of digestate (LFD) was applied to pretreat CS, respectively. The results showed that the daily biogas production (DBP) in co-digestion showed a gradual increasing trend with an increase in the OLR from 65 g TS·L^-1 to 100 g TS·L^-1. The daily methane production per g volatile solids (DMP-VS) in co-digestion increased by 23.0%-27.1%, 18.7%-18.8%, and 17.5%-18.0% at the OLRs of 65, 80, and 100 g TS·L^-1, respectively, upon pretreatment with KOH or LFD, as compared to that in co-digestion CSTR without any pretreatment. In addition, all co-digestion CSTRs were operated in stable state. Approximately half of the total carbon in the substrates was recovered in the form of a biogas product, with the carbon mass balance being impacted by the OLR as well as pretreatment. The diversity as well as function of the microbial community varied in response to different OLRs and pretreatment methods. The majority of bacterial genera were strongly correlated with operational parameters. The study indicates that management of OLR and selection of proper pretreatment method could enhance the efficiency and productivity of CS and CM co-digestion in CSTR.

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.

Modified biochar promotes the direct interspecies electron transfer between iron-reducing bacteria and methanogens in high organic loading co-digestion

High organic loading (HOL) could reduce substrate degradation and methane production. The objective of this study was to investigate the promotion mechanism of iron-modified biochar in HOL co-digestion. The results showed that the specific surface area of iron-modified biochar prepared at 500 ⁰C (500Fe@BC) was 131.7 m²/g. In 12% (w/w) of HOL co-digestion, 500Fe@BC addition enhanced methanogenesis by both aceticlastic and hydrogenotrophic pathways and showed the best methane yield performance. Compared with the non-biochar addition group, an increase of 56.6% and 11% in average methane content and cumulative methane yield was observed in the presence of 500Fe@BC during 25 days of hydraulic retention time. Furthermore, the buffer capacity of HOL co-digestion has been intensified, which attributed to the 500Fe@BC accelerated the hydrolysis of substrates and promoted the consumption of the volatile fatty acids. Moreover, 500Fe@BC promoted the enrichment of iron-reducing bacteria (Clostridium_sensu_stricto_1, Romboutsia) and methanogens (Methanosarcina, Methanobacterium).

Methods of pretreatment and their impacts on anaerobic codigestion of multifeedstocks: A review

Anaerobic codigestion (AnCoD) has attracted attention owing to its advantages over conventional anaerobic digestion, and attempts are still going on to develop methods for improving the efficiency of this technology. Mostly, addition of cosubstrates without applying a proper pretreatment cannot adequately enhance the performance of the digestion. However, there is a lack of a comprehensive study on different pretreatment methods specific to the wide range of cosubstrates. This review aimed to (i) categorize pretreatment techniques that have been developed for improving AnCoD, (ii) present the results of the studies on the effect of pretreatment on improving AnCoD, and (iii) provide a comparison between pretreatment methods and their application for different types of cosubstrates. The findings primarily validated the influence of pretreatment to enhance the process by increasing biodegradability, improved hydrolysis, reduced hydraulic retention time (HRT), and improved methane production. The five main categories of pretreatment employed in codigestion included the following: mechanical, thermal, chemical, biological, and hybrid pretreatment. Among them, mechanical and biological pretreatment have the most and least application in codigestion, respectively. Greater efforts are required on the application of biological pretreatment and cost-benefit analysis of different pretreatment options on the variety of the cosubstrates. PRACTITIONER POINTS: Pretreatment can significantly enhance biomethane production in anaerobic digestion Anaerobic codigestion along with pretreatment can further enhance the conventional anaerobic digestion of single feedstock Mechanical and biological methods have been the most and least practiced pretreatment options Selection of applicable pretreatment option to enhance methane production is subject to the type of cosubstrates in the system There is a research gap in evaluating the application of biological pretreatment for various types of cosubstrates.

Microbial and functional characterization of an allochthonous consortium applied to hydrogen production from Citrus Peel Waste in batch reactor in optimized conditions

Energy recovery from lignocellulosic waste has been studied as an alternative to the problem of inappropriate waste disposal. The present study aimed at characterizing the microbial community and the functional activity of reactors applied to H₂ production through lignocellulosic waste fermentation in optimized conditions. The latter were identified by means of Rotational Central Composite Design (RCCD), applied to optimize allochthonous inoculum concentration (2.32-5.68 gTVS/L of granular anaerobic sludge), pH (4.32-7.68) and Citrus Peel Waste (CPW) concentration (1.55-28.45 g/L). After validation, the conditions identified for optimal H₂ production were 4 gSTV/L of allochthonous inoculum, 29.8 g/L of CPW (substrate) and initial pH of 8.98. In these conditions, 48.47 mmol/L of H₂ was obtained, which is 3.64 times higher than the concentration in unoptimized conditions (13.31 mmol H₂/L using 15 g/L of CPW, 2 gTVS/L of allochthonous inoculum, pH 7.0). Acetogenesis was the predominant pathway, and maximal concentrations of 3,731 mg/L of butyric acid and 3,516 mg/L of acetic acid were observed. Regarding the metataxonomic profile, Clostridium genus was dramatically favored in the optimized condition (79.78%) when compared to the allochthonous inoculum (0.43%). It was possible to identify several genes related to H₂ (i.e dehydrogenases) and volatile fatty acids (VFA) production and with cellulose degradation, especially some CAZymes from the classes Auxiliary Activities, Glycoside Hydrolases and Glycosyl Transferase. By means of differential gene expression it was observed that cellulose degradation and acetic acid production pathways were overabundant in samples from the optimized reactors, highlighting endo-β-1,4-glucanase/cellulose, endo-β-1,4-xylanase, β-glucosidase, β-mannosidase, cellulose β-1,4-cellobiosidase, cellobiohydrolase, and others, as main the functions.

Effects of green waste addition on waste activated sludge and fat, oil and grease co-digestion in mesophilic batch digester

Anaerobic digestion (AD) is regarded as an effective method to treat waste activated sludge (WAS) and fat, oil and grease (FOG). Co-digestion of WAS/FOG could promote the methane yield but it will cause acid and salinity inhibition. Green waste (GW) was added into the digesters, and its effects on co-digestion of WAS and FOG in the mesophilic batch digester were investigated. Digestive performances (such as hydrolysis, acidogenesis and methanogenesis) were studied emphatically. The results showed that digester L6 (WAS:FOG:GW = 1:2:1, VS basis) presented the highest specific methane yield (SMY, 341.5 mL/g VS). The results of kinetics study verified that there was a slower hydrolysis rate when GW was applied as a co-substrate, which could reduce the potential of acid inhibition. Volatile fatty acid (VFA) and electrical conductivity analysis showed that GW addition could keep moderate VFA concentrations and alleviate the negative effects of high-salinity substrates on the digestive systems. The microbial community and diversity analysis proved that GW addition was beneficial to keep the balance of hydrolytic bacteria, acidogens and acetogens. The results of this study indicated that GW addition could enhance the energy recovery and system stability in the WAS/FOG co-digestive system.

Predicting the performance of anaerobic digestion using machine learning algorithms and genomic data

Modeling of anaerobic digestion (AD) is crucial to better understand the process dynamics and to improve the digester performance. This is an essential yet difficult task due to the complex and unknown interactions within the system. The application of well-developed data mining technologies, such as machine learning (ML) and microbial gene sequencing techniques are promising in overcoming these challenges. In this study, we investigated the feasibility of 6 ML algorithms using genomic data and their corresponding operational parameters from 8 research groups to predict methane yield. For classification models, random forest (RF) achieved accuracies of 0.77 using operational parameters alone and 0.78 using genomic data at the bacterial phylum level alone. The combination of operational parameters and genomic data improved the prediction accuracy to 0.82 (p<0.05). For regression models, a low root mean square error of 0.04 (relative root mean square error =8.6%) was acquired by neural network using genomic data at the bacterial phylum level alone. Feature importance analysis by RF suggested that Chloroflexi, Actinobacteria, Proteobacteria, Fibrobacteres, and Spirochaeta were the top 5 most important phyla although their relative abundances were ranging only from 0.1% to 3.1%. The important features identified could provide guidance for early warning and proactive management of microbial communities. This study demonstrated the promising application of ML techniques for predicting and controlling AD performance.

Improving two-stage thermophilic-mesophilic anaerobic co-digestion of swine manure and rice straw by digestate recirculation

The anaerobic co-digestion (coAD) of swine manure (SM) and rice straw (RS) is appealing for renewable energy recovery and waste treatment worldwidely. Improving its performance is very important for its application. In this study, long-term semi-continuous experiments were conducted to evaluate the improving effects of digestate recirculation on the performance, energy recovery, and microbial community of two-stage thermophilic-mesophilic coAD of swine manure (SM) and rice straw (RS). The experimental results indicated that the coAD systems of SM and RS (mixing ratio of 3:1) with or without digestate recirculation could not realize phase separation. The reactors of both coAD systems were characterized by pH values ranging from 7.74 to 7.85, methane production as 0.41 ± 0.02 and 0.44 ± 0.03 L/L/d, and stable operation. Notably, digestate recirculation increased total methane production, organic matter removal, and reaction rate of the coAD system by 9.92 ± 5.08, 5.22 ± 1.94, and 9.73-12.60%, respectively. Digestate recirculation improved the performance of the coAD by significantly increasing the abundance of Methanosarcina (from 4.1% to 7.5%-10.7% and 35.7%) and decreasing that of Methanothermobacter (from 94.2% to 87.3%-83.6% and 56.8%). Thus, the main methanogenesis pathway of the coAD system was changed by digestate recirculation and the methane production was effectively improved. Although the energy input of the coAD system increased by 30.26%, digestate recirculation improved the energy balance of the total system by 6.83%.

Review on anaerobic digestion of rice straw for biogas production

India is an agrarian country producing a large amount of rice straw as an agricultural residue. These residues are burnt openly leading to severe environmental pollution and health hazards. Among several options available, anaerobic digestion of rice straw into biomethane gas and digestate is a promising technology. The current paper reviews the characteristics, principles of rice straw and the process variables (temperature, volatile fatty acids, and pH, carbon to nitrogen ratio, metal elements and organic loading rate) that affect the performance of the rice straw digestion and process strategies which may alleviate the barriers and may improve the biomethane yield. Co-digestion of rice straw with nitrogen-rich substrates is proven to be an effective way to balance the carbon to nitrogen ratio, in turn, leads to nutrient balance and enhance the biomethane yields of anaerobic co-digestion system. Moreover, pretreatment is another effective strategy; physical, chemical and biological pretreatments are reviewed in the article which improved the performance of digester. The utilisation of rice straw along with other co-substrates and appropriate pretreatment may be a recommended sustainable solution for preventing environmental and health hazards.

Effects of organic loading rate and temperature fluctuation on the microbial community and performance of anaerobic digestion of food waste

Semi-continuous anaerobic fermentation of food waste was carried out using a solar-assisted heat reactor to explore effects of temperature fluctuation and organic loading rate (OLR: 2.0, 4.0, 6.0, 7.0 kg/(m³ day)VS on the reactor performance and microbial community structure. The results showed that the best methane production was achieved when OLR was 6.0 kg/(m³ day)VS because the reactors did not operate stably at 7.0 kg/(m³ day)VS. Compared with fluctuation of fermentation temperature, methane production at stable fermentation temperature increased by 21.72%, but higher power consumption occured. The results of high-throughput sequencing showed that OLR played a decisive role in succession of microbial community structure, while temperature fluctuation was more likely to affect microbial activity. When OLR was lower than 4.0 kg/(m³ day)VS, aceticlastic methanogens Methanosaeta were the dominant bacteria, while at 6.0 kg/(m³ day)VS, relative abundance of hydrogenotrophic methanogens Methanoregula and Methanospirillum increased.

Anaerobic Co-digestion of Rice Straw and Pig Manure Pretreated With a Cellulolytic Microflora: Methane Yield Evaluation and Kinetics Analysis

Agricultural wastes, such as rice straw (RS) and pig manure (PM), cause serious environmental pollution due to the non-existence of effective disposal methods. Urgent investigations are needed to explore how such wastes can be transformed into resources. In this study, we comprehensively assessed methane yield and kinetics of RS and PM anaerobic co-digestion, with or without pretreatment of a previously developed cellulolytic microflora, under conditions of their maximum organic loading rate. The anaerobic co-digestion results revealed that the cumulative methane production of RS and PM after bio-pretreatment was 342.35 ml (g-VS)⁻¹, which is 45% higher than that of the control group [236.03 ml·(g-VS)⁻¹]. Moreover, the kinetic analysis showed the first-order kinetic, while the modified Gompertz models revealed higher fitting properties (R² ≥ 0.966). After bio-pretreatment, the hydrolytic constant, maximum accumulative methane production, and maximum methane production rates of RS and PM reached 0.46 day⁻¹, 350.79 ml·(g-VS)⁻¹, and 45.36 ml·(g-VS)⁻¹·day⁻¹, respectively, which were 77, 45.1, and 84.3% higher than those without pretreatment. Also, we found that the lag phase and effective methane production time after bio-pretreatment decreased from 2.43 to 1.79 days and 10.7 to 8.92 days, respectively. Upon energy balance evaluation, we reported a net energy output of 5133.02 kWh·ton⁻¹ after bio-pretreatment. Findings from this present study demonstrated that bio-pretreatment of RS and PM mixtures with cellulolytic microflora could greatly enhance methane production and anaerobic digestion efficiency.

Biogas Production from Anaerobic Co-Digestion of Spent Mushroom Substrate with Different Livestock Manure

Spent mushroom substrate (SMS) is defined as the biomass waste generated during industrial mushroom cultivation. Utilization of SMS has been extensively researched and has immense potential as a sustainable substrate for generating biogas that can offset fossil fuel use. This closed loop energy generation process that can be set up in mushroom plants will reduce the dependence on fossil fuels and has the potential to reduce greenhouse gas emissions, which will benefit the environment. Anaerobic co-digestion of SMS with different agricultural wastes such as livestock manure would result in enhanced biogas production. In this study, the anaerobic co-digestion of SMS was carried out by combing yellow back fungus SMS along with chicken, dairy and pig manure. SMS combined with chicken manure yielded a slightly higher cumulative methane yield when compared with the combination of dairy manure and pig manure. Factors such as the total solids (TS) and the relative ratio of manure to SMS loading had a significant impact on the cumulative methane yield, volatile solids removal, with a particularly prominent synergistic effect. The synergistic effect was also closely related to the C/N ratio, and under experimental conditions (TS = 15%, SMS relative ratio of 50% and C/N ratio = 25.6), the cumulative methane yield of SMS with chicken manure (CM) was increased by 414% compared with that obtained using SMS or CM separately. We carried out a multiple linear regression (MLR) analysis, a statistical technique that uses several explanatory variables to predict the outcome of a response variable. Our analysis concluded that by using operating conditions (TS = 15%, and SMS ratio = 38.9), we were able to achieve the maximum cumulative methane yield (CMY).

Biohythane production and microbial characteristics of two alternating mesophilic and thermophilic two-stage anaerobic co-digesters fed with rice straw and pig manure

To investigate biohythane production and microbial behavior during temperature-phased (TP) anaerobic co-digestion (AcD) of rice straw (RS) and pig manure (PM), a mesophilic-thermophilic (M1-T1) AcD system and a thermophilic-mesophilic (T2-M2) AcD system were continuously operated for 95 days in parallel. The maximal ratio (8.44%v/v) of produced hydrogen to methane demonstrated the feasibility of biohythane production by co-digestion of RS and PM. T2-M2 exhibited higher hydrogen (16.68 ± 1.88 mL/gVS) and methane (197.73 ± 11.77 mL/gVS) yields than M1-T1 (3.08 ± 0.39 and 109.03 ± 4.97 mL/gVS, respectively). Methanobrevibacter (75.62%, a hydrogenotrophic methanogen) dominated in the M1 reactor, resulting in low hydrogen production. Hydrogen-producing bacteria (Thermoanaerobacterium 32.06% and Clostridium_sensu_stricto_1 27.33%) dominated in T2, but the abundance of hydrolytic bacteria was low, indicating that hydrolysis could be a rate-limiting step. The thermophilic acid-producing phase provided effective selective pressure for hydrogen-consuming microbes, and the high diversity of microbes in M2 implied a more efficient pathway of methane production.

Influence of Aerobic Pretreatment of Poultry Manure on the Biogas Production Process

Anaerobic digestion of poultry manure is a potentially-sustainable means of stabilizing this waste while generating biogas. However, technical, and environmental protection challenges remain, including high concentrations of ammonia, low C/N ratios, limited digestibility of bedding, and questions about transformation of nutrients during digestion. This study evaluated the effect of primary biological treatment of poultry manure on the biogas production process and reduction of ammonia emissions. Biogas yield from organic matter content in the aerobic pretreatment groups was 13.96% higher than that of the control group. Biogas production analysis showed that aerobic pretreatment of poultry manure has a positive effect on biogas composition; methane concentration increases by 6.94–7.97% after pretreatment. In comparison with the control group, NH3 emissions after aerobic pretreatment decreased from 3.37% (aerobic pretreatment without biological additives) to 33.89% (aerobic pretreatment with biological additives), depending on treatment method.

Impacts of Cu and Zn on the performance, microbial community dynamics and resistance genes variations during mesophilic and thermophilic anaerobic digestion of swine manure

In this work, fate of antibiotic resistance genes (ARGs), heavy metal resistance genes (MRGs) and intI1 were investigated during mesophilic (mAD) and thermophilic anaerobic digestion (tAD) of swine manure with presence of Cu and Zn. Results showed that metal reduced the lag phase time. Cu showed stronger inhibition than Zn on archaea community and metals inhibited the growth of acetoclastic methanogens during mAD. Although total concentration of metals increased after AD, they were transformed into stable state. The abundance of qnrS, sul1, sul2 and drfA7 increased 1.2-5.7 times after mAD, while reduced after tAD, showed that tAD was effective in ARGs removal. Structural equation model analysis suggested that intI1 had the most standardized direct effects on ARGs variation in mAD (R = 0.85, p < 0.01), while the co-occurrence of MRGs with ARGs showed significantly positive influences on ARGs variation in tAD (R = 0.82, p < 0.01).

A Review on the Fate of Nutrients and Enhancement of Energy Recovery from Rice Straw through Anaerobic Digestion

Open field burning and tilling the rice straw (RS) back into the fields causes environmental threats by contributing to the increased greenhouse gas emissions. Energy and nutrient recovery from RS through anaerobic digestion (AD) is an effective solution for its utilization. Although RS has good methane potential, its characteristics make it a difficult substrate for AD. This paper reviews the characteristics of RS, mass balance, and distribution of nutrients into liquid and solid digestate in the AD. The present review also discusses the effect of temperature, co-digestion, mixing, inoculum, organic loading rate, recycling liquid digestate, the addition of trace elements, and their bioavailability on the enhancement of biogas/methane yield in the AD of RS. In addition, the digestion of RS at various scales is also covered in the review.

Digestive performance of sludge with different crop straws in mesophilic anaerobic digestion

This study evaluated the digestive performance of excess sludge and different crop straws. Methane production, hydrolytic and acidogenic performance of different substrates were specially focused. Results showed that wheat straw presented the highest experimental biochemical methane potential (BMP, 462.5 mL/g VS_removal). Corn cob-added digester obtained the highest VS removal (68.8%) and the lowest methane content due to its high hemicellulose content. Kinetic studies showed that addition of crop straws could shorten the lag phase and accelerate the hydrolysis of substrates. 3DEEM results indicated that lignocellulosic compositions play important roles in sludge hydrolysis, while FE-SEM analysis showed that the lignocellulosic textures of different crop straws determine their hydrolytic performances. Microbial community and diversity results showed that the crop straws in sludge provided abundant nutrients and their unique structures supplied attachment points for microorganisms, which ensured the efficient operation of digestive process.

A Review of the Role of Critical Parameters in the Design and Operation of Biogas Production Plants

Many operating parameters, individually or together, may influence the performance of anaerobic digestion towards biogas or digestate yield and quality maximization. The most preferred method of optimizing an anaerobic digestion plant often relies on how carefully the crucial parameters, such as pH, temperature, organic loading rate, hydraulic retention time, and pressure, are chosen. There is a large amount of literature available on optimization of anaerobic digestion; however, given the continued development and implementation of innovative technologies, together with the introduction of increasingly complex systems, it is necessary to update present knowledge on process parameters and their role on operational ranges and flexibilities in real-life anaerobic digestion system. Accordingly, the present review discusses the importance of the selection of operational parameters in existing technologies and their impact on biogas yield. Notably, the four broad areas of feedstock utilization (substrate, inoculum, codigestion and pretreatment), process condition (pH, temperature, pressure, and reactor design), reactor control (HRT and OLR) and inhibition (Ammonia and VFAs) are covered in this review. In addition, particular emphasis is placed on the most recent innovations that have been or may be implemented in current or future biogas plants.

Effects of various feedstocks on isotope fractionation of biogas and microbial community structure during anaerobic digestion

Feedstock type influences bacterial and methanogenic communities in anaerobic digestion. These two communities work tightly to maintain the stability of anaerobic digestion. How to quick report the changes of microbial community structure especially methanogenesis is the key issue for optimizing anaerobic digestion process. In this study, ¹³C isotope fractionations of CH₄ and CO₂ in biogas and microbial community composition were analyzed in 5 different feedstocks. Our results showed that grass silage, maize silage and swine manure fed reactors had similar δ ¹³C values and methanogenic community composition, dominated by Methanosarcinaceae. The lowest δ ¹³CH₄ values were detected in straw and chicken manure fed reactors, reflecting reduced microbial degradation of material or the presence of toxic components in these feedstocks. The straw fed bioreactor lead to low δ ¹³CH₄ values, probably reflecting relatively high levels of the syntrophic acetate oxidizing bacteria, Synergistaceae and Syntrophaceae, which might work collectively with hydrogenotrophic methanogens, resulting in the low δ ¹³CH₄ values in this bioreactor. Significantly, all core microbes in the 5 different feedstock fed bioreactors were either Clostridia species or related to the Synergistaceae (syntrophic acetate oxidizing bacteria).

Optimization of hydraulic retention time and organic loading rate for volatile fatty acid production from low strength wastewater in an anaerobic membrane bioreactor

This study aims to investigate the production of volatile fatty acids (VFAs) from low strength wastewater at various hydraulic retention time (HRT) and organic loading rate (OLR) in a continuous anaerobic membrane bioreactor (AnMBR) using glucose as carbon source. This experiment was performed without any selective inhibition of methanogens and the reactor pH was maintained at 7.0 ± 0.1. 48, 24, 18, 12, 8 and 6 h-HRTs were applied and the highest VFA concentration was recorded at 8 h with an overall VFA yield of 48.20 ± 1.21 mg VFA/100 mg COD_feed. Three different ORLs were applied (350, 550 and 715 mg COD_feed) at the optimum 8 h-HRT. The acetic and propanoic acid concentration maximums were (1.1845 ± 0.0165 and 0.5160 ± 0.0141 mili-mole/l respectively) at 550 mg COD_feed. The isobutyric acid concentration was highest (0.3580 ± 0.0407 mili-mole/l) at 715 mg COD_feed indicating butyric-type fermentation at higher organic loading rate.

Dry Anaerobic Digestion Technologies for Agricultural Straw and Acceptability in China

Dry anaerobic digestion technology (DADT) is considered a highly feasible way to treat agricultural straw waste; however, most practical operations are always in low efficiency, due to the poor fluidity behavior and complex lignocellulosic structure of straw, which is not easily decomposed by anaerobic bacteria. Hence, it is necessary to further investigate the operation boundary, in order to increase biogas production efficiency for effective applications. In this paper, typical DADTs are reviewed and their suitability for application in China is analyzed. The advantages and disadvantages of different anaerobic digestion processes are evaluated considering pretreatment, organic loading rate, anaerobic digestion temperature, and homogenization of the feedstock and inoculate. The suitability of the DADTs is evaluated considering the accessibility of straw resources and the convenience of biogas use. It is concluded that batch anaerobic digestion processes would be more suitable for the development of southern China due to the prevalence of small-scale agriculture, while continuous anaerobic digestion would be preferable in the north where large-scale agriculture is common. However, the DADTs discussed here need to broad application in China.

Semi-continuous anaerobic co-digestion of chicken litter with agricultural and food wastes: A case study on the effect of carbon/nitrogen ratio, substrates mixing ratio and organic loading

In this study, four agro-industrial substrates, chicken litter (CL), food waste (FW), wheat straw (WS) and hay grass (HG) were assessed as feedstock for anaerobic digestion (AD) under semi-continuous conditions at organic loading rates (OLRs) of 2.0-3.0 g TS/L.d and hydraulic retention time (HRT) of 20 days. Six different substrate mixtures were prepared such that the C/N ratio of each was 20 or more. Using principal component analysis 68.1% of data variability was explained. Biogas production from CL, as a single substrate, was 181.3 ± 9.8mL_N biogas/g VS_added at OLR of 2.0gTS/L.d. The optimum substrates mixture was CL:FW:WS 60:20:20, where 73.0%, 167.2% and 116.9% increase in total biogas production at OLR of 2.0, 2.5, 3.0gTS/L.d, respectively, compared to that from CL, was obtained. Digestate sequential fractionation revealed carbohydrate degradation is an important factor that can explain the variation in performance and production of biogas for feedstocks of balanced C/N ratio.

Mesophilic anaerobic co-digestion of residual sludge with different lignocellulosic wastes in the batch digester

Co-digestion of residual sludge (RS) and different lignocellulosic wastes (LWs) including greening waste (GW), decocted Chinese herbs waste (DCHW) and sugarcane bagasse waste (SCBW) was investigated in batch digester. Results show that the application of GW presented the highest specific methane yield (161 mL CH₄/g VS_added) due to its high carbohydrate fraction and more balanced C/N ratio in co-substrate mixture. Buswell equation was applied and it is found that biodegradability index (BI) for co-digestion varied from 68.1% to 74.2% (53.0% for RS mono-digestion) depending on the lignin fractions of the LWs. Variation of pH, VFAs, alkalinity and ammonia throughout the digestion were also examined. The addition of LWs induced VFAs formation, as well as their conversion to methane. The higher microbial diversity in RS/LWs co-digestion further confirmed the positive effect of LWs addition in co-digestion.