Effects of feedstock ratio and organic loading rate on the anaerobic mesophilic co-digestion of rice straw and cow manure

Advanced anaerobic co-digestion of hydrothermally pretreated wheat straw: Process performance, techno-economic and life cycle assessment

Hydrothermal and thermal-alkali pretreatment potential was investigated to enhance agro-wastes' anaerobic co-digestion (AcoD). The techno-economic (TEA) and life cycle assessment (LCA) of biogas upgrading (BioCNG) and energy generation via combined heat and power (CHP) processes for energy utilization were carried out to realize the environmental impacts and cost-effectiveness of the studied processes. Three AcoD conditions of untreated, hydrothermally (150 °C, 60 min) and thermal-alkali pretreated (1% NaOH, 150 °C- 60 min) wheat straw (WS) with food waste and cow manure were studied in semi-continuous mode for 340 days under variable organic loading rates (OLR, 1.25-3.75 gVS/L.d). At an OLR of 1.67 gVS/L.d and 45 days HRT, the methane yield and volatile solids removal in control digester were 231 mL/gVS and 39.7%, and in thermal-alkali pretreated digester were 302 mL/gVS and 53.3%, presenting an increase of 30% and 34% over control, respectively. The LCA and TEA of all three processes under 45 days of HRT operations were carried out. The environmental performance evaluation across various scenarios indicated that integrating pretreatment with the combined heat and power (CHP) process significantly lowered the global warming potential (GWP), outperforming the BioCNG process. In particular, hydrothermal pretreatment made the largest contribution to the reduction in GWP, achieving a value of -0.1935 kg CO2-equivalent, primarily due to energy generation from the CHP process. In comparison, the BioCNG process recorded a GWP of -0.0377 kg CO2-equivalent. This result demonstrates that the CHP process led to an 81% greater reduction in GWP than the BioCNG process. While the BioCNG process surpassed CHP in terms of economic performance, the hydrothermal pretreatment scenario proved to be financially the most advantageous, with a net present value (NPV) of $10.16 million, an internal rate of return (IRR) of 13%, and a shorter payback period (PBP) of 14 years.

Modification of Anaerobic Digestion Model No. 1 for modeling anaerobic digestion of cattle manure with changing solids retention time

In this study, Anaerobic Digestion Model No.1 (ADM1) was modified to incorporate changes in biochemical parameters due to solids retention time (SRT) variations. Cattle manure (CM) and thermally hydrolyzed CM were selected for testing. Continuous anaerobic digestion reactors were operated under different SRT conditions ranging from 6.6 to 36.0 days for both samples. The biochemical parameters (kch, kli, kpr, um,ac, um,bu, um,pro, um,va, Kac, Kbu, Kpro, and Kva) for each SRT condition were determined. To modify ADM1, the equations obtained through linear regression were substituted into biochemical parameters as a function of SRT. The modified ADM1 demonstrated superior accuracy compared with conventional ADM1. This study implies the feasibility of optimizing biochemical parameters for modeling in response to changes in environmental variables.

Microbial contributions to sustainable paddy straw utilization for economic gain and environmental conservation

Paddy straw is a versatile and valuable resource with multifaceted benefits for nutrient cycling, soil health, and climate mitigation. Its role as a rich nutrient source and organic matter significantly enhances soil vitality while improving soil structure and moisture retention. The impact of paddy straw extends beyond traditional agricultural benefits, encompassing the promotion of microbial activity, erosion control, and carbon sequestration, highlighting its crucial role in maintaining ecological balance. Furthermore, the potential of paddy straw in bioenergy is explored, encompassing its conversion into biogas, biofuels, and thermal energy. The inherent characteristics of paddy straw, including its high cellulose, hemicellulose, and lignin content, position it as a viable candidate for bioenergy production through innovative processes like pyrolysis, gasification, anaerobic digestion, and combustion. Recent research has uncovered state-of-the-art techniques and innovative technologies capable of converting paddy straw into valuable products, including sugar, ethanol, paper, and fiber, broadening its potential applications. This paper aims to underscore the possibilities for value creation through paddy straw, emphasizing its potential use in bioenergy, bio-products, and other environmental applications. Therefore, by recognizing and harnessing the value of paddy straw, we can advocate for sustainable farming practices, reduce waste, and pave the way for a resource-efficient circular economy. Incorporating paddy straw utilization into agricultural systems can pave the way for enhanced resource efficiency and a more sustainable circular economy.

Engineering Escherichia coli for cost-effective production of medium-chain fatty acids from soy whey using an optimized galactose-based autoinduction system

Medium-chain fatty acids (MCFAs) are essential chemical feedstocks. Microbial production of MCFAs offers an attractive alternative to conventional methods, but the costly media and external inducers limit its practical application. To address this issue and make MCFA production more cost-effective, an E.coli platform was developed using soy whey as a medium and galactose as an autoinducer. We first designed an efficient, stringent, homogeneous, and robust galactose-based autoinduction system for the expression of pathway enzymes by rationally engineering the promoter of the galactose-proton symporter (GalP). Subsequently, the intracellular acetyl-CoA availability and NADH regeneration were enhanced to improve the reversal of the β-oxidation cycle. The resulting strain yielded 8.20 g/L and 16.42 g/L MCFA in pH-controlled batch fermentation and fed-batch fermentation with glucose added using soy whey as medium, respectively. This study provided a cost-effective and promising platform for MCFA production, as well as future strain development for other value-added chemicals production.

Machine Learning Approach for Determining and Optimizing Influential Factors of Biogas Production from Lignocellulosic Biomass

Machine learning (ML) was used to predict specific methane yields (SMY) with a dataset of 14 features from lignocellulosic biomass (LB) characteristics and operating conditions of completely mixed reactors under continuous feeding mode. The random forest (RF) model was best suited for predicting SMY with a coefficient of determination (R²) of 0.85 and root mean square error (RMSE) of 0.06. Biomass compositions greatly influenced SMYs from LB, and cellulose prevailed over lignin and biomass ratio as the most important feature. Impact of LB to manure ratio was assessed to optimize biogas production with the RF model. Under typical organic loading rates (OLR), optimum LB to manure ratio of 1:1 was identified. Experimental results confirmed influential factors revealed by the RF model and provided the highest SMY of 79.2% of the predicted value. Successful applications of ML for anaerobic digestion modelling and optimization specifically for LB were revealed in this work.

The effect of heat pre-treatment on the anaerobic digestion of high-solid pig manure under high organic loading level

Since more and more large-scale farms appear in China and changes in fecal sewage source disposal, the production of high-concentration solid manure waste is also increasing, and its conversion and utilization are gaining attention. This study investigated the effect of heat pre-treatment (HPT) on the thermophilic anaerobic digestion (AD) of high-solid manure (HSM). Pig manure (PM) feed with a total solids of 13% was used for the HPT and subsequent anaerobic digestion (AD) test. The HPT was carried out at 60°C, 80°C, and 100°C, respectively, for 15 min after the heating reached the set temperature. The results show that HPT led to PM feed COD solubilization, observing a maximum increase of 24.57% after pretreated at 100°C, and the treated PM feed under this condition received the maximum methane production potential of 264.64 mL·g−1 VS in batch AD test, which was 28.76% higher than that of the untreated group. Another semi-continuous AD test explored the maximum volume biogas production rate (VBPR). It involves two organic loading rates (OLR) of 13.4 and 17.8 g VSadded·L−1·d−1. The continuous test exhibited that all the HPT groups could produce biogas normally when the OLR increased to the high level, while the digester fed with untreated PM showed failure. The maximum VBPR of 4.71 L L−1·d−1 was observed from PM feed after pre-treated at 100°C and running at the high OLR. This reveals that thermal treatment can weaken the impact of a larger volume of feed on the AD system. Energy balance analysis demonstrates that it is necessary to use a heat exchanger to reuse energy in the HPT process to reduce the amount of energy input. In this case, the energy input to energy output (Ei/Eo) ranged from 0.34 to 0.55, which was much less than one, suggesting that biogas increment due to heat treatment can reasonably cover the energy consumption of the pre-treatment itself. Thus combining HPT and high-load anaerobic digestion of PM was suitable.

Performance evaluation of a full-scale upflow anaerobic sludge blanket reactor coupled with trickling filters for municipal wastewater treatment in a developing country

Poor wastewater management remains a critical health and environmental challenge in most developing countries in Sub-Saharan Africa due to the lack of adequate infrastructure for collection and treatment. This study evaluated the performance and methane production of a full-scale upflow anaerobic sludge blanket (UASB) reactor of capacity 18000 m3/d, with post-treatment unit: trickling filters followed by final settling tanks for municipal wastewater treatment in Ghana. Data was collected on operational conditions and physicochemical parameters of wastewater (influent and effluent) over a period of 35 weeks in 2021 (from January to August). The influent biochemical oxygen demand to chemical oxygen demand (BOD:COD) ratio was 0.58 ± 0.16, indicating the presence of highly biodegradable compounds in the sewage. Operational conditions for the UASB reactors were observed to be within the optimal range for anaerobic systems, with an applied organic loading rate of 1.30 ± 0.79 kgCOD/m3/d. Generally, Plant performance was satisfactory with carbon removal at 93% for COD and 98% for BOD. Biogas yield was 0.2 m3/kgCOD removed, culminating in an average biogas production rate of 831.6 ± 292.7 m3/d. Average methane composition was 64.7 ± 11.9% of the biogas output, whilst an estimated 35% of the methane generated remained dissolved in the UASB effluent. The UASB reactor presents an efficient technology that can be implemented in developing countries for effective and sustainable wastewater management.

Optimization of subcritical water pre-treatment for biogas enhancement on co-digestion of pineapple waste and cow dung using the response surface methodology

The optimal pre-treatment method and conditions depend on the types of lignocellulose present due to the complexity and the variability of biomass chemical structures. This study optimized subcritical water pre-treatment to ensure maximum methane production from pineapple waste prior to anaerobic co-digestion with cow dung using the response surface methodology. A central composite design was achieved with three different factors and one response. A total of 20 pre-treatment runs were performed at different temperatures, reaction times and water to solid ratios suggesting optimum values for subcritical water pre-treatment at 128.52℃ for 5 min with 5.67 to 1 water to solid ratio. Under these conditions, methane yield increased from 59.09 to 85.05 mL CH₄/g VS with an increase of 23% biogas yield and 44% methane yield from the untreated. All pre-treatments above 200℃ showed reductions in biogas yield. Compositional analysis showed slight reduction of lignin and increase in α-cellulose content after the pre-treatment. Analysis using Fourier transform infrared spectroscopy and thermogravimetric analysis verified the presence of cellulosic material in pre-treated pineapple waste. Most of the hemicellulose was solubilized in the liquid samples after SCW pre-treatment. The crystallinity index of pineapple waste was reduced from 57.58% (untreated) to 54.29% (pre-treated). Scanning electron microscopy confirmed the structural modification of pre-treated pineapple waste for better microbial attack. Subcritical water pre-treatment is feasible as a promising method to enhance the anaerobic co-digestion process. Further study should be conducted to assess the scale-up of the process from pre-treatment to anaerobic digestion at the pilot plant level.

A Review of the Sustainable Utilization of Rice Residues for Bioenergy Conversion Using Different Valorization Techniques, Their Challenges, and Techno-Economic Assessment

The impetus to predicting future biomass consumption focuses on sustainable energy, which concerns the non-renewable nature of fossil fuels and the environmental challenges associated with fossil fuel burning. However, the production of rice residue in the form of rice husk (RH) and rice straw (RS) has brought an array of benefits, including its utilization as biofuel to augment or replace fossil fuel. Rice residue characterization, valorization, and techno-economic analysis require a comprehensive review to maximize its inherent energy conversion potential. Therefore, the focus of this review is on the assessment of rice residue characterization, valorization approaches, pre-treatment limitations, and techno-economic analyses that yield a better biofuel to adapt to current and future energy demand. The pre-treatment methods are also discussed through torrefaction, briquetting, pelletization and hydrothermal carbonization. The review also covers the limitations of rice residue utilization, as well as the phase structure of thermochemical and biochemical processes. The paper concludes that rice residue is a preferable sustainable biomass option for both economic and environmental growth.

Improving the efficiency of anaerobic digestion: Domesticated paddy soil microbes enhance the hydrolytic acidification of rice straw and pig manure

Improving the efficiency of hydrolytic acidification is critical for methane production from agricultural waste. This study is the first to apply domesticated paddy soil microbes to (DPSM) enhance the hydrolytic acidification of rice straw (RS) and pig manure (PM) to obtain acidizing fluid for anaerobic digestion (AD). At a substrate concentration of 20%, the inoculation of an RS-PM mixture (1:3) with 35% DPSM degraded the volatile solids by 48.1% and yielded 6.8 g/L of volatile fatty acids and 4.7 g/L of acetic acid after seven days of hydrolytic acidification. After 10 days of subsequent AD, the cumulative methane production of the acidizing fluid was 304.96 mL/g COD, similar (P ＞ 0.05) to the control (318.27 mL/g COD). However, the methane production time decreased by 43.4% (from 30 to 17 days), thereby improving the AD efficiency. Inoculation with DPSM is therefore an effective pre-treatment for agricultural waste for methane production.

Rice recycling: a simple strategy to improve conidia production in solid-state cultures

Here we studied at a laboratory scale a potential strategy to revalorize the residual rice remaining at the end of a conventional conidia production process in solid-state culture. The conidia production of Trichoderma asperellum Th-T4 (3) and Metarhizium robertsii Xoch-8.1 started with the use of fresh rice (unrecycled rice) as the substrate (cycle one), and continued with the use of recycled rice in successive cycles of conidia production. The rice remaining at the end of the first cycle was reused without any further sterilization or reinoculation. As a result, it was observed that the conidia production and productivity significantly increased in both fungi. Conidia production in T. asperellum Th-T4 (3) increased from 1 × 10⁹ (first cycle) to 2·9 × 10⁹ conidia per gram of initial dry substrate (con⋅gds^-1 ) (second cycle using recycled rice), while in M. robertsii Xoch-8.1, this parameter increased form 5·7 × 10⁸ to 1·4 × 10⁹ con⋅gds^-1 . Both fungi grew faster and conidiated earlier when recycled rice was used as the substrate, therefore, conidia productivity was also significantly improved. Furthermore, the use of recycled rice did not affect conidia viability. This is the first report about a recycling methodology completely free of extra-processing steps, and useful to increase conidia production and productivity.

Enhancement of Bioreactor Performance Using Acclimatised Seed Sludge in Anaerobic Treatment of Chicken Slaughterhouse Wastewater: Laboratory Achievement, Energy Recovery, and Its Commercial-Scale Potential

Lack of good management practice of chicken slaughterhouse wastewater (CSWW) has caused pollution into water bodies. In this study, the potential of seed sludge acclimatised modified synthetic wastewater (MSWW) on bioreactor performance and energy recovery of CSWW treatment was investigated. Two sets of upflow anaerobic sludge blanket (UASB) reactors were employed. The seed sludge in UASB 2 was acclimatised with MSWW for 30 days. In UASB 1, no acclimatisation process was undertaken on seed sludge for control purposes. After the acclimatisation process of UASB 2, both reactors were supplied with CSWW under the same condition of organic loading rate (OLR = 0.5 to 6 gCOD/L/d) and mesophilic condition (37 °C). COD removal efficiencies of UASB 2 were >80% all through the steady-state of the OLR applied. Meanwhile, a drastic decrease in overall performance was observed in UASB 1 when the OLR was increased to 3, 4, 5, and 6 gCOD/L/d. Energy recovery from laboratory scale and projected value from commercial-scale bioreactor were 0.056 kWh and 790.49 kWh per day, respectively. Preliminary design of an on-site commercial-scale anaerobic reactor was proposed at a capacity of 60 m3.

Review and perspectives of enhanced volatile fatty acids production from acidogenic fermentation of lignocellulosic biomass wastes

Lignocellulosic biomass wastes are abundant resources that are usually valorized for methane-rich biogas via anaerobic digestion. Conversion of lignocellulose into volatile fatty acids (VFA) rather than biogas is attracting attention due to the higher value-added products that come with VFA utilization. This review consolidated the latest studies associated with characteristics of lignocellulosic biomass, the effects of process parameters during acidogenic fermentation, and the intensification strategies to accumulate more VFA. The differences between anaerobic digestion technology and acidogenic fermentation technology were discussed. Performance-enhancing strategies surveyed included (1) alkaline fermentation; (2) co-digestion and high solid-state fermentation; (3) pretreatments; (4) use of high loading rate and short retention time; (5) integration with electrochemical technology, and (6) adoption of membrane bioreactors. The recommended operations include: mesophilic temperature (thermophilic for high loading rate fermentation), C/N ratio (20-40), OLR (< 12 g volatile solids (VS)/(L·d)), and the maximum HRT (8-12 days), alkaline fermentation, membrane technology or electrodialysis recovery. Lastly, perspectives were put into place based on critical analysis on status of acidogenic fermentation of lignocellulosic biomass wastes for VFA production.

Foaming mechanisms and control strategies during the anaerobic digestion of organic waste: A critical review

Foaming is a problem that affects the efficient and stable operation of the anaerobic digestion process. Characterizing foaming mechanisms and developing early warning and foaming control methods is thus critically important. This review summarizes the correlation of process parameters, state parameters, and microbial communities with foaming in anaerobic digesters; discusses the applicability of the above-mentioned multi-scale parameters and foaming potential evaluation methods for the prediction of foaming risk; and introduces the principles and practical applications of antifoaming and defoaming methods. Multiple causes of foaming in anaerobic digestion systems have been identified, but a generalizable foaming mechanism has yet to be described. Further study of the correlation between extracellular polymeric substances and soluble microbial products and foaming may provide new insights into foaming mechanisms. Monitoring the foaming potential (including the volume expansion potential) is an effective approach for estimating the risk of foaming. An in-situ monitoring system for determining the foaming potential in anaerobic digestion sites could provide an early warning of foaming risk. Antifoaming methods based on operating parameter management and process regulation help prevent foaming from the source, and biological defoaming methods are highly targeted and efficient, which is a promising research direction. Clarifying foaming mechanisms will aid the development of active antifoaming methods and efficient biological defoaming methods.

Enhanced Methane Production from Anaerobic Co-Digestion of Wheat Straw Rice Straw and Sugarcane Bagasse: A Kinetic Analysis

Future energy and environmental issues are the major driving force towards increased global utilization of biomass, especially in developing countries like Pakistan. Lignocellulosic residues are abundant in Pakistan. The present study investigated the best-mixed proportion of mechanically pretreated lignocellulosic residues i.e., wheat straw and rice straw (WSRS), bagasse and wheat straw (BAWS), bagasse, and rice straw (BARS), bagasse, wheat straw, and rice straw (BAWSRS) through anaerobic co-digestion. Anaerobic batch mode bioreactors comprising of lignocellulosic proportions and control bioreactors were run in parallel at mesophilic temperature (35 °C) for the substrate to inoculum (S/I) ratio of 1.5 and 2.5. Maximum and stable biomethane production was observed at the substrate to inoculum (S/I) ratio of 1.5, and the highest biomethane yield 339.0089123 NmLCH4/gVS was achieved by co-digestion of wheat straw and rice straw (WSRS) and lowest 15.74 NmLCH4/gVS from bagasse and rice straw (BARS) at 2.5 substrates to inoculum ratio. Furthermore, anaerobic reactor performance was determined by using bio-kinetic parameters i.e., production rate (Rm), lag phase (λ), and coefficient of determination (R2). The bio-kinetic parameters were evaluated by using kinetic models; first-order kinetics, Logistic function model, Modified Gompertz Model, and Transference function model. Among all kinetic models, the Logistic function model provided the best fit with experimental data followed by Modified Gompertz Model. The study suggests that a decrease in methane production was due to lower hydrolysis rate and higher lignin content of the co-digested substrates, and mechanical pretreatment leads to the breakage of complex lignocellulosic structure. The organic matter degradation evidence will be utilized by the biogas digesters developed in rural areas of Pakistan, where these agricultural residues are ample waste and need a technological solution to manage and produce renewable energy.

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.

Effect of Biochar Addition on the Microbial Community and Methane Production in the Rapid Degradation Process of Corn Straw

Anaerobic digestion with corn straw faces the problems of difficult degradation, long fermentation time and acid accumulation in the high concentration of feedstocks. In order to speed up the process of methane production, corn straw treated with sodium hydroxide was used in thermophilic (50 °C) anaerobic digestion, and the effects of biochar addition on the performance of methane production and the microbial community were analyzed. The results showed that the cumulative methane production of all treatment groups reached over 75% of the theoretical methane yield in 7 days and the addition of 4% biochar increased the cumulative methane production by 6.75% compared to the control group. The addition of biochar also decreased the number of biogas and methane production peaks from 2 to 1, and had a positive effect on shortening the digestion start-up period and reducing the fluctuation of biogas production during the digestion process. The addition of 4% biochar increased the abundance of the bacterial family Peptococcaceae throughout the digestion period, promoting the hydrolysis rate of corn straw. The dominant archaeal genus Methanosarcina was significantly more abundant at the peak stage and the end of methane production with 4% biochar added compared to the control group.

Novel insight into enhanced recoverability of acidic inhibition to anaerobic digestion with nano-bubble water supplementation

Nano-bubble water (NBW) has been proven to be effective in promoting organics utilization and CH₄ production during anaerobic digestion (AD) process, suggesting its potential in improving the stability of the AD process and thereby alleviating acidic inhibition. In this work, the effect of NBW on digestion stability and CH₄ production was investigated to evaluate the ability of NBW on AD recovery from acidic inhibition. Results showed that NBW supplementation increased the total alkalinity (TA) and partial alkalinity (PA), and reduced the ratio of VFA/TA, thus maintained the stability of the AD process. Generation/consumption of VFAs was also enhanced with NBW supplementation under acidic inhibition with pH values of 5.5, 6.0 and 6.5. The cumulative CH₄ production was 246-257 mL/g-VS in NBW groups, which was 12.1-17.2% higher than the control. Moreover, with NBW supplementation, the maximum CH₄ production rate was raised according to the modeling results.

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.

The effects of C/N (10-25) on the relationship of substrates, metabolites, and microorganisms in "inhibited steady-state" of anaerobic digestion

The carbon/nitrogen ratio (C/N) is a key parameter that affects the performance of anaerobic digestion (AD). Recent AD research has focused on optimizing the C/N of feedstock. The so-called "inhibited steady-state" refers to a special state of ammonia inhibition of AD that often occurs at low-C/N (below 25) when degradable nitrogen-rich substrates, such as livestock manure, are used as feedstock. However, the mechanism behind the "inhibited steady-state" is still unknown. In the current study, co-digestion and recirculation were used to create a C/N gradient in the influent to explore the relationship between substrates, metabolites, and microorganisms in the "inhibited steady-state." Data were collected at the macro, microbial, and genetic levels. Three CSTRs were successfully made run into the "inhibited steady-state" using influent C/Ns of 10-12. Digestion performance levels of R10-R12 were low and stable, transitioning from an aceticlastic methane-producing pathway to a hydrogenotrophic pathway as the C/N gradually decreased. As the abundance of the hydrogenophilic methanogens increased, the abundance of syntrophic acetate-oxidizing bacteria (SAOB) also increased. The succession between populations of Methanosaeta and Methanosarcina may be used as a microbiological indicator of ammonia inhibition. Under high-C/Ns, cooperation among bacteria was high, while under low-C/Ns, competition among bacteria was high. These results clarify the processes underlying the "inhibited steady-state," which is a condition often faced in actual large-scale biogas facilities that use degradable nitrogen-rich substrates. Moreover, practical guidelines for evaluating ammonia inhibition are provided, and strategies to alleviate ammonia suppression are developed.

Thermophilic Anaerobic Co-Digestion of Exhausted Sugar Beet Pulp with Cow Manure to Boost the Performance of the Process: The Effect of Manure Proportion

Sugar beet by-products are a lignocellulosic waste generated from sugar beet industry during the sugar production process and stand out for their high carbon content. Moreover, cow manure (CM) is hugely produced in rural areas and livestock industry, which requires proper disposal. Anaerobic digestion of such organic wastes has shown to be a suitable technology for these wastes valorization and bioenergy production. In this context, the biomethane production from the anaerobic co-digestion of exhausted sugar beet pulp (ESBP) and CM was investigated in this study. Four mixtures (0:100, 50:50, 75:25, and 90:10) of cow manure and sugar beet by-products were evaluated for methane generation by thermophilic batch anaerobic co-digestion assays. The results showed the highest methane production was observed in mixtures with 75% of CM (159.5 mL CH4/g VolatileSolids added). Nevertheless, the hydrolysis was inhibited by volatile fatty acids accumulation in the 0:100 mixture, which refers to the assay without CM addition. The modified Gompertz model was used to fit the experimental results of methane productions and the results of the modeling show a good fit between the estimated and the observed data.

Anaerobic digestion of bean straw applying a fungal pre-treatment and using cow manure as co-substrate

The significant amounts of agriculture residues such as bean straw (BS) in rural areas, advises its valorisation for energy recovery. The feasibility of using BS for biogas production through anaerobic digestion was assessed. Prior to this, a fungal pre-treatment to hydrolyse BS with Pleutorus ostreatus was studied at 30°C and 100 rpm in orbital incubators with 1, 10 and 30 mg fungus/g straw for 14, 21 and 28 days. Then, anaerobic digestion experiments were performed in batch with cow manure (CM) as co-substrate and pre-treated BS at ratios (g/g total solids) of 1/2, 1/3, 1/5 and 0/1. Maximum lignin (18%) and hemicellulose (44%) degradation occurred at 30 mg fungus/g straw and 28 days, along with the highest total methane yield (38 mL CH₄/g VS loaded). The total amount of methane decreased when increasing CM in the experiments (701.4-191.5 mL CH₄), suggesting inhibition owed to a component of CM. Self-sustained biogas production of BS occurred due to the presence of bacteria (i.e. Bacilli and Bacteroidia) and archea (i.e. Methanobacteria and Methanomicrobia). However, the usage of a full-active inoculum should be studied for higher biogas production rates.

Enhancing the production of biogas through anaerobic co-digestion of agricultural waste and chemical pre-treatments

Large amounts of agricultural solid wastes (ASWs) and animal dung are produced annually causing serious environmental problem that requires proper treatment. The present study proposes a strategy for optimizing the anaerobic co-digestion of ASWs and cow dung (CD), identifies the key factors governing the co-digestion performance and evaluates the effect of NaHCO₃ alkalinity treatment on improving the economy and performance of anaerobic digestion (AD). The results revealed that the highest cumulative methane production (CMP) of 297.99 NL/kgVS can be generated by co-digestion of ASWs and CD at a ratio of 60:40. Further improvement was achieved via alkalinity treatment with 1.0 g of NaHCO₃/gVS leading to decrease in lignin, cellulose, and hemicellulose contents of feedstock by 3.5%, 10.5% and 15.9%, respectively, converting them to soluble fractions and improving the CMP by 11.2-29.7% based on substrate quality. The improved CMP in the chemically treated substrates reflects a 19% increase in the generated revenue. The kinetics of the AD process was successfully fitted to modified Gompertz model with very low standard deviation residuals (SDR) ≤ 5.21 and R² ≥ 0.979. Results confirm that the proposed strategy is an effective method for producing biogas from co-digestion of ASWs and CD.

Development of sustainable approaches for converting the organic waste to bioenergy

Dependence on fossil fuels such as oil, coal and natural gas are on alarming increase, thereby causing such resources to be in a depletion mode and a novel sustainable approach for bioenergy production are in demand. Successful implementation of zero waste discharge policy is one such way to attain a sustainable development of bioenergy. Zero waste discharge can be induced only through the conversion of organic wastes into bioenergy. Waste management is pivotal and considering its importance of minimizing the issue and menace of wastes, conversion strategy of organic waste is effectively recommended. Present review is concentrated on providing a keen view on the potential organic waste sources and the way in which the bioenergy is produced through efficient conversion processes. Biogas, bioethanol, biocoal, biohydrogen and biodiesel are the principal renewable energy sources. Different types of organic wastes used for bioenergy generation and its sources, anaerobic digestion-biogas production and its related process affecting parameters including fermentation, photosynthetic process and novel nano-inspired techniques are discussed. Bioenergy production from organic waste is associated with mitigation of lump waste generation and its dumping into land, specifically reducing all hazards and negativities in all sectors during waste disposal. A sustainable bioenergy sector with upgraded security for fuels, tackles the challenging climatic change problem also. Thus, intensification of organic waste conversion strategies to bioenergy, specially, biogas and biohydrogen production is elaborated and analyzed in the present article. Predominantly, persistent drawbacks of the existing organic waste conversion methods have been noted, providing consideration to economic, environmental and social development.

Improving performance and phosphorus content of anaerobic co-digestion of dairy manure with aloe peel waste using vermiculite

Phosphorus content of the digestate is crucial for evaluating its fertilizer utilization in anaerobic digestion system. The vermiculite containing rich-phosphorus is firstly used as an accelerant in anaerobic batch co-digestion system of aloe peel waste and dairy manure. After introducing vermiculite, the cumulative biogas production (295.14-353.96 mL/g VS), chemical oxygen demand removal rate (45.53%-71.03%), and volatile solid removal rate (50.70%-52.76%) are remarkably higher than those of reference reactor (234.08 mL/g VS, 39.38%, 45.10%). The thermal and fertility analyses manifest the digestates with vermiculite possess superior stability, admirable fertilizer values (5.97%-6.81%), and excellent total phosphorus content (11.44-13.29 g/kg). The improved co-digestion performance can be attributed to the addition of vermiculite. This work introduces a novel approach for improving the performance of anaerobic co-digestion and the fertilizer utilization of digestate in the co-digestion systems.

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.

Biomethanation at 45 °C offers high process efficiency and supports hygienisation

The aim of this work was to prove a process temperature of 45 °C as a practical alternative to commonly applied mesophilic (37 °C) and thermophilic (55 °C) anaerobic digestion (AD). Regarding methane production, no differences were found between the three temperature regimes. However, the maximum possible loading rate at 45 °C exceeded that at 37 °C and 55 °C. Pathogen inactivation at 45 °C was higher than at 37 °C and similarly efficient as at 55 °C. At each process temperature, a unique microbial community established. In addition, the archaeome at 55 °C was dominated by hydrogenotrophs, while at 37 °C and 45 °C it was dominated by acetotrophs. For the investigated substrate mixture, liquid cattle manure with wheat straw as co-substrate, 45 °C turned out to be preferable for AD. For other substrates, these findings still need to be confirmed.

Enhanced solubilization of solid organics and methane production by anaerobic digestion of swine manure under nano-bubble water addition

Nano-bubble water (NBW) refers to water with a large number of nanoscale particle bubbles. The aim of this work was to study the mechanism of NBW addition into the anaerobic digestion (AD) of swine manure (SM). The results showed that the cumulative methane production from the NBW added reactor was 192-225 mL/g-VS and 19-39% higher than the control group (without NBW addition). Based on the analysis of soluble organics, NBW addition not only accelerated hydrolysis rates of proteins and carbohydrates, but also enhanced the production of VFAs. Moreover, mechanism analysis reveals that NBW with higher spin-spin relaxation time and absolute value of zeta potential might promote enzyme activity and the hydrolysis of organic solids. Simultaneously, the electron transport system activity of the methanogenic communities and electric conductivity were enhanced by NBW addition. This work implies that NBW addition is promising for enhancing AD for enhancement of methane production.

Effect of viscosity on process stability and microbial community composition during anaerobic mesophilic digestion of Maotai-flavored distiller's grains

To investigate the effects of viscosity on the mesophilic digestion of Maotai-flavored distiller's grains, a continuous experiment was conducted in a 70 L reactor at organic loading rates of 3, 4, 5, and 6 g VS/(L·d) with and without effluent recirculation. High organic loading rates and continuous effluent recirculation increased the digestate viscosity, and high viscosity caused severe foaming, which blocked the biogas outlet pipe. Moreover, a viscosity above 782 mPa·s was proposed as an early warning indicator for foaming. A maximum volumetric biogas production rate of 1.72 L/(L·d) was accomplished by diluting the feed without effluent recirculation at a recommended organic loading rate of 5 g VS/(L·d). Proteiniphilum, Ruminococcus_2, norank_f_Synergistaceae, norank_o__DTU014, Syntrophomonas, Methanosarcina, Methanobacterium, and Methanosaeta were the dominant acidogens, syntrophic bacteria, and methanogens existed in both low and high viscosity groups. Candidatus_Methanofastidiosum capable of employing the methylated thiol reduction pathway was found only in the high viscosity system.

A Comparative Study of Biogas Production from Cattle Slaughterhouse Wastewater Using Conventional and Modified Upflow Anaerobic Sludge Blanket (UASB) Reactors

Cattle slaughterhouses generate wastewater that is rich in organic contaminant and nutrients, which is considered as high strength wastewater with a high potential for energy recovery. Work was undertaken to evaluate the efficiency of the 12 L laboratory scale conventional and a modified upflow anaerobic sludge blanket (UASB) reactors (conventional, R1 and modified, R2), for treatment of cattle slaughterhouse wastewater (CSWW) under mesophilic condition (35 ± 1 °C). Both reactors were acclimated with synthetic wastewater for 30 days, then continuous study with real CSWW proceeds. The reactors were subjected to the same loading condition of OLR, starting from 1.75, 3, 5 10, 14, and 16 g L-1d-1, corresponding to 3.5, 6, 10, 20, 28, and 32 g COD/L at constant hydraulic retention time (HRT) of 24 h. The performance of the R1 reactor drastically dropped at OLR 10 g L-1d-1, and this significantly affected the subsequent stages. The steady-state performance of the R2 reactor under the same loading condition as the R1 reactor revealed a high COD removal efficiency of 94% and biogas and methane productions were 27 L/d and 89%. The SMP was 0.21 LCH4/gCOD added, whereas the NH3-N alkalinity ratio stood at 651 mg/L and 0.2. SEM showed that the R2 reactor was dominated by Methanosarcina bacterial species, while the R1 reactor revealed a disturb sludge with insufficient microbial biomass.

A Review on Anaerobic Digestion of Lignocellulosic Wastes: Pretreatments and Operational Conditions

Anaerobic digestion (AD) has become extremely popular in the last years to treat and valorize organic wastes both at laboratory and industrial scales, for a wide range of highly produced organic wastes: municipal wastes, wastewater sludge, manure, agrowastes, food industry residuals, etc. Although the principles of AD are well known, it is very important to highlight that knowing the biochemical composition of waste is crucial in order to know its anaerobic biodegradability, which makes an AD process economically feasible. In this paper, we review the main principles of AD, moving to the specific features of lignocellulosic wastes, especially regarding the pretreatments that can enhance the biogas production of such wastes. The main point to consider is that lignocellulosic wastes are present in any organic wastes, and sometimes are the major fraction. Therefore, improving their AD could cause a boost in the development in this technology. The conclusions are that there is no unique strategy to improve the anaerobic biodegradability of lignocellulosic wastes, but pretreatments and codigestion both have an important role on this issue.

Biomethane production through anaerobic co-digestion with Maize Cob Waste based on a biorefinery concept: A review

Maize Cob Waste (MCW) is available worldwide in high amounts, as maize is the most produced cereal in the world. MCW is generally left in the crop fields, but due to its low biodegradability it has a negligible impact in soil fertility. Moreover, MCW can be used as substrate to balance the C/N ratio during the Anaerobic co-Digestion (AcoD) with other biodegradable substrates, and is an excellent precursor for the production of Activated Carbons (ACs). In this context, a biorefinery is theoretically discussed in the present review, based on the idea that MCW, after proper pre-treatment is valorised as precursor of ACs and as co-substrate in AcoD for biomethane generation. This paper provides an overview on different scientific and technological aspects that can be involved in the development of the proposed biorefinery; the major topics considered in this work are the following ones: (i) the most suitable pre-treatments of MCW prior to AcoD; (ii) AcoD process with regard to the critical parameters resulting from MCW pre-treatments; (iii) production of ACs using MCW as precursor, with the aim to use these ACs in biogas conditioning (H₂S removal) and upgrading (biomethane production), and (iv) an overview on biogas upgrading technologies.

Illustration of the microbial community selected by optimized process and nutritional parameters resulting in enhanced biomethanation of rice straw without thermo-chemical pretreatment

Effects of different process and nutritional parameters on microbial community structure and function were investigated to enhance the biomethanation of rice straw without any thermochemical pre-treatment. The study was performed in a mesophilic anaerobic digester with cattle dung slurry as inoculum. The highest methane yield of 274 ml g^-1 volatile solids was obtained from particulate rice straw (1 mm size, 7.5% solids loading rate) at 37 °C, pH-7, when supplemented with urea (carbon: nitrogen ratio, 25:1) and zinc as trace element (100 µM) at 21 days hydraulic retention time. The optimization of conditions selected Clostridium, Bacteroides, and Ruminococcus as dominant hydrolytic bacteria and Methanosarcina as the methanogen. Analysis of metagenome and metatranscriptome revealed wide array of bacterial lignocellulolytic enzymes that efficiently hydrolyzed the rice straw. The methane yield was >80% of the theoretical yield, making this green process a sustainable choice for efficient extraction of energy from rice straw.

Simultaneous biogas and biogas slurry production from co-digestion of pig manure and corn straw: Performance optimization and microbial community shift

Anaerobic co-digestion (AcoD) is proved as an effective approach to solving a bottleneck problem of the low biogas yield in agricultural biomass waste treatment with anaerobic digestion (AD) technology. The present study investigated the effect of C/N radio, organic loading rate (OLR) and total solids (TS) contents on reactor performance in AcoD of pig manure and corn straw for simultaneous biogas and biogas slurry production. It was found that the highest biogas production was obtained at C/N ratio of 25, while the best biogas slurry performance was achieved at C/N ratio of 35. And high OLR and TS resulted in good performances in both biogas production and biogas slurry. At last, the microbial community analysis suggested that Bacteroidetes played a significant role in AcoD process. Acetoclastic methanogenesis was the main pathway for methane production in the stable system. And changing operational parameters could transform and shift the microbial community.

Early warning indicators for mesophilic anaerobic digestion of corn stalk: a combined experimental and simulation approach

BACKGROUND

Monitoring and providing early warning are essential operations in the anaerobic digestion (AD) process. However, there are still several challenges for identifying the early warning indicators and their thresholds. One particular challenge is that proposed strategies are only valid under certain conditions. Another is the feasibility and universality of the detailed threshold values obtained from different AD systems. In this article, we report a novel strategy for identifying early warning indicators and defining threshold values via a combined experimental and simulation approach.

RESULTS

The AD of corn stalk (CS) was conducted using mesophilic, completely stirred anaerobic reactors. Two overload modes (organic and hydraulic) and overload types (sudden and gradual) were applied in order to identify early warning indicators of the process and determine their threshold values. To verify the selection of experimental indicators, a combined experimental and simulation approach was adopted, using a modified anaerobic bioconversion mathematical model (BioModel). Results revealed that the model simulations agreed well with the experimental data. Furthermore, the ratio of intermediate alkalinity to bicarbonate alkalinity (IA/BA) and volatile fatty acids (VFAs) were selected as the most potent early warning indicators, with warning times of 7 days and 5-8 days, respectively. In addition, IA, BA, and VFA/BA were identified as potential auxiliary indicators for diagnosing imbalances in the AD system. The relative variations for indicators based on that of steady state were observed instead of the absolute threshold values, which make the early warning more feasible and universal.

CONCLUSION

The strategy of a combined approach presented that the model is promising tool for selecting and monitoring early warning indicators in various corn stalk AD scenarios. This study may offer insight into industrial application of early warning in AD system with mathematical model.

Methane Production from Alginate-Extracted and Non-Extracted Waste of Laminaria japonica: Anaerobic Mono- and Synergetic Co-Digestion Effects on Yield

This study investigated the potentiality of methane production from alginate-extracted (AEWLJ) and non-extracted (NAEWLJ) waste of Laminaria japonica through batch anaerobic fermentation in mono- and co-digestion with rice straw (RS) at different mixing ratios. Optimal C/N ratio was demonstrated, and system stability was monitored in terms of the total ammonia nitrogen, total volatile fatty acids, and pH throughout the digestion period. The results show that the combination of AEWLJ/RS at 67% mixing ratio generated the highest biogas yield of 247 NmL/gVS, which was 36% higher than the AEWLJ alone. The synergetic effect was clearly observed leading to an increase in the total methane yield up to 78% and 88%, respectively, for arrays of AEWLJ/RS and NAEWLJ/RS. The kinetic model showed a high coefficient of determination (R2 ≥ 0.9803) when the modified Gompertz model was applied to predict methane production. These outcomes support the possibility of an integrated biorefinery approach to attain value-added products in order to achieve circular economies.

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.

Microbial community composition and diversity in rice straw digestion bioreactors with and without dairy manure

Anaerobic digestion (AD) uses a range of substrates to generate biogas, including energy crops such as globally abundant rice straw (RS). Unfortunately, RS is high in lignocellulosic material and has high to C:N ratios (~80:1), which makes it (alone) a comparatively poor substrate for AD. Co-digestion with dairy manure (DM) has been promoted as a method for balancing C:N ratios to improve RS AD whilst also treating another farm waste and co-producing a potentially useful fertiliser. However, past co-digestion studies have not directly compared RS AD microbial communities with and without DM additions, which has made it hard to assess all impacts of DM addition to RS AD processes. Here, four RS:DM ratios were contrasted in identical semi-continuous-fed AD bioreactors, and 100% RS was found to produce the highest specific methane yields (112 mL CH4/g VS/day; VS, volatile solids), which is over double yields achieved in the reactor with the highest DM content (30:70 RS:DM by mass; 48 mL CH4/g VS/day). To underpin these data, microbial communities were sequenced and characterised across the four reactors. Dominant operational taxonomic units (OTUs) in the 100% RS unit were Bacteroidetes/Firmicutes, whereas the 30:70 RS:DM unit was dominated by Proteobacteria/Spirochaetes, suggesting major microbial community shifts occur with DM additions. However, community richness was lowest with 100% RS (despite higher specific yields), suggesting particular OTUs may be more important to yields than microbial diversity. Further, ambient VFA and VS levels were significantly higher when no DM was added, suggesting DM-amended reactors may cope better with higher organic loading rates (OLR). Results show that RS AD without DM addition is feasible, although co-digestion with DM will probably allow higher OLRs, resulting in great RS throughput in farm AD units.

Biomethanation potential for co-digestion of municipal solid waste and rice straw: A batch study

Rice straw (RS) contains a high amount of lignocellulosic materials which are difficult to degrade without thermal pretreatment. In the present study, co-digestion of municipal solid waste (MSW) and RS was carried out in three different ratios i.e., 1:1, 2:1, and 3:1 to get the maximum biomethanation potential and methane generation rate constant (k). The biogas and methane (CH₄) potential increased by 60% and 57%, respectively for MSW and RS in the ratio 2:1 as compared to other combination. The values of k, biochemical methane potential (µ_b) and sludge activity were measured as 0.1 d^-1, 0.99 CH₄-COD/COD_fed and 0.50 g CH₄-COD/g VSS, respectively. The sludge activity was found to be 100% for 2:1 ratio. Co-digestion of RS with MSW can also optimize the C/N ratio which is an essential parameter in the anaerobic digestion process.

Microbial and nutritional regulation of high-solids anaerobic mono-digestion of fruit and vegetable wastes

The anaerobic digestion of single fruit and vegetable wastes (FVW) can be easily interrupted by rapid acidogenesis and inhibition of methanogen, and the digestion system tends to be particularly unstable at high solid content. In this study, the anaerobic digestion of FVW in batch experiments under mesophilic condition at a high solid concentration of 10% was successfully conducted to overcome the acidogenesis problem through several modifications. Firstly, compared with the conventional anaerobic sludge (CAS), the acclimated anaerobic granular sludge (AGS) was found to be a better inoculum due to its higher Archaea abundance. Secondly, waste activated sludge (WAS) was chosen to co-digest with FVW, because WAS had abundant proteins that could generate intermediate ammonium. The ammonium could neutralize the accumulated volatile fatty acids (VFAs) and prevent the pH value of the digestion system from rapidly decreasing. Co-digestion of FVW and WAS with TS ratio of 60:40 gave the highest biogas yield of 562 mL/g-VS and the highest methane yield of 362 mL/g-VS. Key parameters in the digestion process, including VFAs concentration, pH, enzyme activity, and microbial activity, were also examined.

Co-digestion of rice straw and cow dung to supply cooking fuel and fertilizers in rural India: Impact on human health, resource flows and climate change

Anaerobic digestion of cow dung with new feedstock such as crop residues to increase the biogas potential is an option to help overcoming several issues faced by India. Anaerobic digestion provides biogas that can replace biomass cooking fuels and reduce indoor air pollution. It also provides digestate, a fertilizer that can contribute to compensate nutrient shortage on agricultural land. Moreover, it avoids the burning of rice straw in the fields which contributes to air pollution in India and climate change globally. Not only the technical and economical feasibility but also the environmental sustainability of such systems needs to be assessed. The potential effects of implementing community digesters co-digesting cow dung and rice straw on carbon and nutrients flows, human health, resource efficiency and climate change are analyzed by conducting a Substance Flow Analysis and a Life Cycle Assessment. The implementation of the technology is considered at the level of the state of Chhattisgarh. Implementing this scenario reduces the dependency of the rural community to nitrogen and phosphorus from synthetic fertilizers only by 0.1 and 1.6%, respectively, but the dependency of farmers to potassium from synthetic fertilizers by 31%. The prospective scenario returns more organic carbon to agricultural land and thus has a potential positive effect on soil quality. The implementation of the prospective scenario can reduce the health impact of the local population by 48%, increase the resource efficiency of the system by 60% and lower the impact on climate change by 13%. This study highlights the large potential of anaerobic digestion to overcome the aforementioned issues faced by India. It demonstrates the need to couple local and global assessments and to conduct analyses at the substance level to assess the sustainability of such systems.

The influence of decreased hydraulic retention time on the performance and stability of co-digestion of sewage sludge with grease trap sludge and organic fraction of municipal waste

The effect of hydraulic retention time ranging from 12 to 20 d on process performance and stability was investigated in two anaerobic completely stirred tank reactors with a working liquid volume equal to 6 litres. The reactors were fed with mixtures containing (on volatile solids basis): 40% of sewage sludge, 30% of organic fraction of municipal waste and 30% of grease trap sludge. The change of hydraulic retention time did not significantly affect process stability. However, methane yields as well as volatile solids removal decreased from 0.54 to 0.47 l per kg of added volatile solids and 65% to 60% respectively, with the decrease of hydraulic retention time. Despite the fact that the best process performance was achieved for hydraulic retention time of 20 days, the obtained results showed that it is also possible to carry out the co-digestion process at shorter hydraulic retention times with good results. Furthermore, gas production rate as well as biogas production at the shortest hydraulic retention time were approximately 46% higher in comparison to results obtained at the longest hydraulic retention time. In this context, the proposed solution seems to be an interesting option, because it provides an unique opportunity for wastewater treatment plants to improve their profitability by enhancing energy recovery from sludge as well as full utilisation of the existing infrastructure and hence creates a new potential place for alternative treatment of organic industrial waste such as: fat-rich materials or food waste. However, implementation of the solution at wastewater treatment plants is still a big challenge and needs studies including identification of optimal digesting conditions, information about substrate pumping, inhibition thresholds and processing properties. Additionally, due to the characteristics of both co-substrates their introduction to the full-scale digester should be carefully planned due to a potential risk of overloading of the digester. For this reason, a gradual increase of the share of these wastes in the co-digestion mixture is highly recommended, because it will allow for the acclimatization of bacteria as well as prevent overloading. The results of this study show the importance of gradual acclimatization of microorganisms to the changing environmental conditions. It was found that concentration of long chain fatty acids in effluents increased with the reduction of hydraulic retention time, but this phenomenon did not significantly influence the performance and stability of the process probably due to changes hydraulic retention time being gradual. Although for palmitic acid a moderate negative correlation with volatile solids removal was observed.

Effects of different swine manure to wheat straw ratios on antibiotic resistance genes and the microbial community structure during anaerobic digestion

This study explored the effects of different mass ratios of swine manure relative to wheat straw (3:7, 5:5, and 7:3, i.e., control reactors C1, C2, and C3, respectively) on variations in antibiotic resistance genes (ARGs) and the microbial community during anaerobic digestion (AD). The cumulative biogas production volumes were 1711, 3857, and 3226mL in C1, C2, and C3, respectively. After AD, the total relative abundance of ARGs decreased by 4.23 logs in C3, whereas the reductions were only 1.03 and 1.37 logs in C1 and C2, respectively. Network analysis showed that the genera Solibacillus, Enterococcus, Facklamia, Corynebacterium_1, and Acinetobacter were potential hosts of ermB, sul1, and dfrA7. Redundancy analysis showed that the bacterial communities and environmental factors played important roles in the variation in ARGs. Thus, reductions in ARGs should be considered before reusing animal manure treated by AD.

Batch and semi-continuous anaerobic co-digestion of goose manure with alkali solubilized wheat straw: A case of carbon to nitrogen ratio and organic loading rate regression optimization

The present study focused on carbon to nitrogen ratio (C/N) and organic loading rate (OLR) optimization of goose manure (GM) and wheat straw (WS). Dealing the anaerobic digestion of poultry manure on industrial scale; the question of optimum C/N (mixing ratio) and OLR (daily feeding concentration) have significant importance still lack in literature. Therefore, Batch and CSTR co-digestion experiments of the GM and WS were carried out at mesophilic condition. The alkali (NaOH) solubilization pretreatment for the WS had greatly enhanced its anaerobic digestibility. The highest methane production was evaluated between the C/N of 20-30 during Batch experimentation while for CSTRs; the second applied OLR of (3g.VS/L.d) was proved as the optimum with maximum methane production capability of 254.65ml/g.VS for reactor B at C/N of 25. The C/N and OLR regression optimization models were developed for their commercial scale usefulness.