Co-digestion of livestock effluents, energy crops and agro-waste: feeding and process optimization in mesophilic and thermophilic conditions

A Review on Performance Improvement of Anaerobic Digestion Using Co-Digestion of Food Waste and Sewage Sludge

Anaerobic co-digestion (AcoD) is a vital technology in the decarburization of the economy because of its ability to process organic waste, recover nutrients, and create biogas as a sustainable biofuel all at the same time. This attribute also makes this technology a viable partner in pursuing a circular economic model. However, the poor biogas output of typical substrates like sewage sludge and animal manure and the hefty installation costs limit its viability. This review paper with literature analysis provides a good grasp of the anaerobic co-digesting process with diverse food digestion methods. In this survey, we have analyzed the Anaerobic Digestion of water waste, food waste, and animal manure and the anaerobic co-digestion of animal waste with water waste and food waste with water waste. This analysis demonstrates that anaerobic co-digestion produces more methane biogas than anaerobic digestion. Also, it has been shown that by adjusting the ratio of food and animal waste to water waste, we can produce more methane. In the future, we would like to supplement anaerobic co-digestion by altering the proportion of different wastes that are mixed with water waste in order to increase methane production.

Co-digestion of food waste and cellulose-based bioplastic: From batch to semi-continuous scale investigation

Only few studies on the behaviour of bioplastics in anaerobic co-digestion could be found in literature and most of them are conducted in batch mode. Despite the fact that continuous experiments confirm or add new insight to the findings acquired from batch ones, there is still lack of such studies. This work aims to cover this gap, carrying out a semi-continuous anaerobic co-digestion of food waste and cellulose acetate (which its behaviour under anaerobic environment is also quite unexplored). After a first evaluation of the potential methane production from each substrate at batch scale, the semi-continuous co-digestion of food waste and cellulose acetate was carried out in three configurations. During the semi-continuous process, a methane yield of 331 NmlCH₄/gVS was generated from the co-digestion of food waste and cellulose acetate while bioplastics specimens achieved a weight loss of about 45 %. The results were both lower than the one obtained from batch co-digestion, although methane production rates were comparable regardless of being fed with or without bioplastics. An increase was registered after 65 days of semi-continuous process, due to the accumulation of CA specimens. This confirms the different degradation trends between bioplastics and food waste.

Hydrogen and Methane Production from Anaerobic Co-Digestion of Sorghum and Cow Manure: Effect of pH and Hydraulic Retention Time

The need for alternative energy sources is constantly growing worldwide, while the focus has shifted to the valorization of biomass. The aim of the present study was to determine the optimal pH and hydraulic retention time (HRT) values for treating a mixture of sorghum biomass solution with liquid cow manure (in a ratio 95:5 v/v) through anaerobic digestion, in a two-stage system. Batch tests were initially carried out for the investigation of the pH effect on bio-hydrogen and volatile fatty acids (VFA) production. The highest hydrogen yield of 0.92 mol H2/mol carbohydratesconsumed was obtained at pH 5.0, whereas the maximum degradation of carbohydrates and VFA productivity was observed at pH 6.0. Further investigation of the effect of HRT on hydrogen and methane production was carried out. The maximum yield of 1.68 mol H2/mol carbohydratesconsumed was observed at an HRT of 5 d, with H2 productivity of 0.13 L/LR·d. On the other hand, the highest CH4 production rate of 0.44 L/LR·d was achieved at an HRT of 25 d, with a methane yield of 295.3 mL/g VSadded, whereas at a reduced HRT of 20 d the process exhibited inhibition and/or overload, as indicated by an accumulation of VFAs and decline in CH4 productivity.

Evaluating the biomethane potential from the anaerobic co-digestion of palm oil mill effluent, food waste, and sewage sludge in Malaysia

The ever-increasing organic waste generation in Malaysia is a significant contributor to greenhouse gas (GHG) emissions. However, organic wastes can be utilized to produce biogas by anaerobic digestion, which is a promising option for both energy and material recovery from organic wastes with high moisture content. Therefore, this study was formulated to investigate the feasibility of anaerobic co-digestion of three types of organic wastes generated in significantly huge quantities in Malaysia, namely palm oil mill effluent (POME), food waste (FW), and sewage sludge (SWS). The biomethane potential (BMP) test was used to evaluate the biomethane potential from these organic wastes under mesophilic conditions to establish a stable and balanced microbial community, which may lack in mono-digestion, to improve biogas production. Comparative performance was made at different food to microorganism (F/M) ratios to investigate methane production in three groups of assays, namely A, B, and C. In groups A and B, the effect of F/M ratio variation on methane production was investigated, while in group C, the effect of varying the co-substrate mixture on methane yield was examined. The findings showed that the highest methane yields achieved for mono-digestion of POME and SWS in group A were 164.44 mL-CH₄/g-COD_added and 65.34 mL-CH₄/g-COD_added, respectively, at an F/M ratio of 0.8 and 197.90 mL-CH₄/g-COD_added for FW in group B at an F/M ratio of 0.5. In addition, the highest methane yield achieved from the anaerobic co-digestion was at 151.47 mL-CH₄/g-COD_added from the co-digestion of the POME and SWS (50:50) at an F/M ratio of 1.7 in group A. Both AD and AcoD were tested to fit into two kinetic models: the modified Gompertz and the transfer function models. The results showed that the modified Gompertz model had a better fit and was more adjusted to the experimental results for both AD and AcoD. The importance of this research lies in the economics of anaerobically co-digesting these abundance feedstocks and the variations in their characteristics which were found to increase their methane yield and process efficiency in anaerobic co-digestion.

Industrial symbiosis of anaerobic digestion and pyrolysis: Performances and agricultural interest of coupling biochar and liquid digestate

The sustainability of the anaerobic digestion industry is closely related to proper digestate disposal. In this study, an innovative cascading biorefinery concept coupling anaerobic digestion and subsequent pyrolysis of the digestate was investigated with the aim of enhancing the energy recovery and improving the fertilizers from organic wastes. Continuous anaerobic co-digestion of quinoa residues with wastewater sludge (45/55% VS) exhibited good stability and a methane production of 219 NL CH₄/kg VS. Subsequent pyrolysis of the solid digestate was carried out (at 500 °C, 1 h, and 10 °C/min), resulting in a products distribution of 40 wt% biochar, 36 wt% bio-oil, and 24 wt% syngas. The organic phase (OP) of bio-oil and syngas exhibited higher and lower heating values of 34 MJ/kg and 11.8 MJ/Nm³, respectively. The potential synergy of coupling biochar with liquid digestate (LD) for agronomic purposes was investigated. Interestingly, coupling LD (at 170 kg N/ha) with biochar (at 25 tons/ha) improved the growth of tomato plants up to 25% compared to LD application alone. In parallel, co-application of biochar with LD significantly increased the ammonia volatilization (by 64%) compared to LD application alone, although their simultaneous use did not impact the C and N mineralization rates.

High-Solid Anaerobic Digestion: Reviewing Strategies for Increasing Reactor Performance

High-solid and solid-state anaerobic digestion are technologies capable of achieving high reactor productivity. The high organic load admissible for this type of configuration makes these technologies an ideal ally in the conversion of waste into bioenergy. However, there are still several factors associated with these technologies that result in low performance. The economic model based on a linear approach is unsustainable, and changes leading to the development of a low-carbon model with a high degree of circularity are necessary. Digestion technology may represent a key driver leading these changes but it is undeniable that the profitability of these plants needs to be increased. In the present review, the digestion process under high-solid-content configurations is analyzed and the different strategies for increasing reactor productivity that have been studied in recent years are described. Percolating reactor configurations and the use of low-cost adsorbents, nanoparticles and micro-aeration seem the most suitable approaches to increase volumetric production and reduce initial capital investment costs.

Dark Fermentation of Sweet Sorghum Stalks, Cheese Whey and Cow Manure Mixture: Effect of pH, Pretreatment and Organic Load

The aim of this study was to determine the optimal conditions for dark fermentation using agro-industrial liquid wastewaters mixed with sweet sorghum stalks (i.e., 55% sorghum, 40% cheese whey, and 5% liquid cow manure). Batch experiments were performed to investigate the effect of controlled pH (5.0, 5.5, 6.0, 6.5) on the production of bio-hydrogen and volatile fatty acids. According to the obtained results, the maximum hydrogen yield of 0.52 mol H2/mol eq. glucose was measured at pH 5.5 accompanied by the highest volatile fatty acids production, whereas similar hydrogen productivity was also observed at pH 6.0 and 6.5. The use of heat-treated anaerobic sludge as inoculum had a positive impact on bio-hydrogen production, exhibiting an increased yield of 1.09 mol H2/mol eq. glucose. On the other hand, the pretreated (ensiled) sorghum, instead of a fresh one, led to a lower hydrogen production, while the organic load decrease did not affect the process performance. In all experiments, the main fermentation end-products were volatile fatty acids (i.e., acetic, propionic, butyric), ethanol and lactic acid.

Co-precipitation of Cu and Zn in precipitation of struvite

Struvite recovered from wastewater can be used as a slow-release fertilizer. Nevertheless, hazardous metals easily precipitated with struvite would increase the ecological risk for its agricultural use. In this study, the influence of individual and coexistence of Cu and Zn on the precipitation of struvite was investigated. The loading of Cu and/or Zn in precipitates increased with the increase of initial metal concentrations (0.1-100 mg/L). Quantitative X-ray diffraction (QXRD) analysis revealed that the increase of Cu and/or Zn level in reaction solution disturbed crystal growth of struvite and promoted the formation of amorphous phase(s). Scanning electron microscopy (SEM) revealed the pit formation on struvite crystal surfaces, combined with X-ray photoelectron spectroscopy (XPS) data, the results indicated a surface interaction for the formation of Cu-OH and Cu-NH₃ on struvite surface at Cu of 0.1-10 mg/L. With the increase of Cu to 25-100 mg/L, the precipitation of amorphous Cu phosphate(s) was confirmed by XPS and QXRD. At Zn of 0.1-10 mg/L, the enrichment of Zn-PO₄ and Zn-OH on struvite surface was observed, whereas, the precipitation of amorphous Zn hydroxide(s) was confirmed at Zn of 25-100 mg/L. At Cu and Zn co-existed solution, the decrease of Cu-PO₄ and increase of Zn-PO₄ suggested the competitive binding of PO₄ between Cu and Zn. In addition, the formation of amorphous Mg hydroxide(s) and phosphate(s) was detected regardless of the addition of Cu in solutions. The overall results revealed that the existence of Cu and Zn during struvite formation can greatly affect its content by formation of different metal-containing products.

Co-Digestion Biomethane Production and the Effect of Nanoparticle: Kinetics Modeling and Microcalorimetry Studies

To improve the production rate of methane, powder-activated carbon (PAC), granule activated carbon (GAC), titanium dioxide-anatase (TiO₂), and synthesized zeolite (permutit) were added in the co-digestion process. The co-substrates were corn stover (CS) and pig manure (PM) mixed in the ratio of 1:2 (w/w). The kinetic analysis model and ADM1da model were applied to obtain the kinetic parameters of the process. Besides, the heat flow analysis of the co-digestion process was determined using isothermal microcalorimetry. The addition of the PAC, GAC, TiO₂, and synthesized zeolite improved the methane cumulative yield by 40.12, 31.25, 31.17, and 43.74% respectively, as compared with the control reactor. The kinetic analysis and ADM1da model results indicated that the overall rate constant of the co-digestion process increased by 1.5 times averagely because of the effect of these materials. It was also observed that much higher heat energy released from the experimental sample compared with the control reactor, which indicated that the improvement of the metabolic process of the AcoD system. The addition of TiO₂-anatase improved methane production by 31.17%, which could be a promising method to improve the biomethane in a large-scale due to its availability and accessibility.

Modelization of anaerobic processes during co-digestion of slowly biodegradable substrates

The influence of the soluble substrates over the anaerobic processes has been extensively investigated, but little is known about the effects of particulate substrate. The biodegradation of these substrates starts with the hydrolytic step, this process is slower than the other ones involved in the biodegradation of particulate substrates and usually becomes the rate-limiting step. This study investigate the effect of the initial total solids (TS) concentration on the anaerobic co-digestion of two slowly biodegradable organic substrates. The wastes mixtures were prepared at different dilutions in the range from 10% to 28% TS. From these experiments it was observed that as TS concentration increased, the methane production decreased. These results were modelled and it was observed that neither hydrolysis nor fermentation stages controlled the methane production rate. Being a substrate inhibition event experienced at the methanogenic stage the responsible of the lower methane production when operating at high TS concentrations.

Chemical and Bioenergetic Characterization of Biofuels from Plant Biomass: Perspectives for Southern Europe

The global demand for and, therefore, the production of primary energy is continuously increasing. Consequently, the need to intervene with appropriate measures has arisen in order to achieve sustainable economic, social, and environmental objectives. The reduction of fuel and electricity consumption, the containment of atmospheric emissions of greenhouse gases (like carbon dioxide, methane, other hydrocarbons, and nitrous oxide), and the improvement of environmental quality in urban centers can be considered to be among these objectives. The search for efficient measures for the overall improvement of the environment is directed towards the replacement of traditional fossil fuels with the production of bioenergy (also known as green energy) from different materials and biomasses obtained from specific agricultural activities and/or plant residues. These materials have physico-chemical and biological characteristics of interest regarding their use as sources of renewable energy. The purpose of this review was to provide an overview of the chemical and bioenergetic characteristics of biofuels, the main techniques and processes employed for their production, and the characteristics of the different feedstock materials, especially potential energy crops.

The effects of Microalgae Biomass Co-Substrate on Biogas Production from the Common Agricultural Biogas Plants Feedstock

The aim of this study was to determine the effects on methane production of the addition of microalgae biomass of Arthrospira platensis and Platymonas subcordiformis to the common feedstock used in agricultural biogas plants (cattle manure, maize silage). Anaerobic biodegradability tests were carried out using respirometric reactors operated at an initial organic loading rate of 5.0 kg volatile solids (VS)/m3, temperature of 35°C, and a retention time of 20 days. A systematic increase in the biogas production efficiency was found, where the ratio of microalgae biomass in the feedstock increased from 0% to 40% (%VS). Higher microalgae biomass ratio did not have a significant impact on improving the efficiency of biogas production, and the biogas production remained at a level comparable with 40% share of microalgae biomass in the feedstock. This was probably related to the carbon to nitrogen (C/N) ratio decrease in the mixture of substrates. The use of Platymonas subcordiformis ensured higher biogas production, with the maximum value of 1058.8 ± 25.2 L/kg VS. The highest content of methane, at an average concentration of 65.6% in the biogas produced, was observed in setups with Arthrospira plantensis biomass added at a concentration of between 20%–40% to the feedstock mixture.

Biohythane production from food processing wastes - Challenges and perspectives

The food industry generates enormous quantity of food waste (FW) either directly or indirectly including the processing sector, which turned into biofuels for waste remediation. Six types of food processing wastes (FPW) such as oil, fruit and vegetable, dairy, brewery, livestock and finally agriculture based materials that get treated via dark fermentation/anaerobic digestion has been discussed. Production of both hydrogen and methane is daunting for oil, fruit and vegetable processing wastes because of the presence of polyphenols and essential oils. Moreover, acidic pH and high protein are the reasons for increased concentration of ammonia and accumulation of volatile fatty acids in FPW, especially in dairy, brewery, and livestock waste streams. Moreover, the review brought to forefront the enhancing methods, (pretreatment and co-digestion) operational, and environmental parameters that can influence the production of biohythane. Finally, the nature of feedstock's role in achieving successful circular bio economy is also highlighted.

Co-digestion of by-products and agricultural residues: A bioeconomy perspective for a Mediterranean feedstock mixture

This study focused on applying batch and continuous co-digestion approaches to investigate the effects of a feedstock mixture (FM) constituted by ten Mediterranean feedstocks highly available in the Mediterranean area (i.e., olive pomace, olive mill wastewater, citrus pulp, poultry litter, poultry and cattle manure, whey and cereal straw) on methane production for bioenergy generation. For the same feedstock mixture (FM), two different anaerobic digestion (AD) tests were carried out to evaluate the possible inhibitory effects of some biomasses on the biological process. The first AD test showed a methane yield equal to 229 Nm³CH₄/tVS (27% lower than that measured during the batch test). During the second AD test, the specific production was 272 m³CH₄/tVS. Both tests showed a similar methane content of methane in the biogas, equal to about 57%. The first AD test showed an inhibition effect of the process: total conversion of the organic matter into biogas was not ended. The second batch test demonstrated that the selected FM could be viable to carry out the co-digestion and could provide a flexible solution to generate advanced biofuels in biogas plants located in the Mediterranean area.

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.

Biological pretreatment enhances the activity of functional microorganisms and the ability of methanogenesis during anaerobic digestion

Biological pretreatment can increase the methane production of anaerobic digestion. In this study, stover was pretreated via microbial consortium prior to anaerobic digestion; through 16S rRNA gene and 16S rRNA amplicon sequencing and metatranscriptomic analysis, and the effects of the pretreatment on the microbial community and critical factors of the increased methane production were studied. Microbial community structure was less affected by the pretreatment, which ensures the stable performance of anaerobic digestion. The methane production increased by 62.85% at the peak phase compared to the untreated stover. The activity of Methanosaeta increased from 2.0% to 10.1%, significantly enhancing the ability of the community to capture acetic acid and reduce CO₂ to methane. The main contribution to the increase in methane production was a unique acetyl-CoA synthetase, which showed significant up-regulation (121.8%). This research demonstrated the importance of Methanosaeta and its unique metabolic pathways in anaerobic digestion utilizing a biological pretreatment.

Energy and Nutrients’ Recovery in Anaerobic Digestion of Agricultural Biomass: An Italian Perspective for Future Applications

Anaerobic digestion (AD) is the most adopted biotechnology for the valorization of agricultural biomass into valuable products like biogas and digestate, a renewable fertilizer. This paper illustrates in the first part the actual situation of the anaerobic digestion sector in Italy, including the number of plants, their geographical distribution, the installed power and the typical feedstock used. In the second part, a future perspective, independent of the actual incentive scheme, is presented. It emerged that Italy is the second European country for the number of anaerobic digestion plants with more than 1500 units for a total electricity production of about 1400 MWel. More than 60% of them are in the range of 200 kW–1 MW installed power. Almost 70% of the plants are located in the northern part of the Country where intensive agriculture and husbandry are applied. Most of the plants are now using energy crops in the feedstock. The future perspectives of the biogas sector in Italy will necessarily consider a shift from power generation to biomethane production, and an enlargement of the portfolio of possible feedstocks, the recovery of nutrients from digestate in a concentrated form, and the expansion of the AD sector to southern regions. Power to gas and biobased products will complete the future scenario.

Biomethane Production From Lignocellulose: Biomass Recalcitrance and Its Impacts on Anaerobic Digestion

Anaerobic digestion using lignocellulosic material as the substrate is a cost-effective strategy for biomethane production, which provides great potential to convert biomass into renewable energy. However, the recalcitrance of native lignocellulosic biomass makes it resistant to microbial hydrolysis, which reduces the bioconversion efficiency of organic matter into biogas. Therefore, it is necessary to critically investigate the correlation between lignocellulose characteristics and bioconversion efficiency. Accordingly, this review comprehensively summarizes the anaerobic digestion process and rate-limiting step, structural and compositional properties of lignocellulosic biomass, recalcitrance and inhibitors of lignocellulose and their major effects on anaerobic digestion for biomethane production. Moreover, various type of pretreatment strategies applied to lignocellulosic biomass was discussed in detail, which would contribution to cell wall degradation and improvement of biomethane yields. In the view of current knowledge, high energy input and cost requirements are the main limitations of these pretreatment methods. In addition to optimization of fermentation process, further studies should focus much more on key structural influence factors of biomass recalcitrance and anaerobic digestion efficiency, which will contribute to improvement of biomethane production from lignocellulose.

Evaluation of biomethane potential from by-products and agricultural residues co-digestion in southern Italy

The suitability of the co-digestion of feedstock-mixtures (by-products and agricultural residues) depends on their ability to produce biogas. In this study, the effects of mixing five feedstocks (citrus pulp, olive pomace, poultry manure, Italian sainfoin silage and opuntia fresh cladodes) on anaerobic digestion for biogas production have been investigated by carrying out biomethane potential (BMP) tests on six different mixing ratios of the selected five biomasses. The BMP test results demonstrated that all the six studied feedstock-mixtures could be potentially used for renewable energy generation by biogas plants. More in detail, two mixing ratios of the studied feedstock-mixtures showed the best biomethane potential of 249.9 and 260.1 Nm³CH₄/tVS, respectively. Since this research study made it possible to screen the suitability and technical feasibility of the feedstock-mixtures analysed, the results provide the basis for subsequent pilot scale evaluation of anaerobic digestion in Mediterranean area, where by-products and agricultural residues are profuse and necessary to produce advanced biofuels.

Anaerobic co-digestion of multiple agricultural residues to enhance biogas production in southern Italy

To valorize agricultural wastes and byproducts in southern Italy, anaerobic co-digestion of six feedstocks (citrus pulp, olive pomace, cattle manure, poultry litter, whey, and corn silage) was studied to produce biogas for renewable energy generation. Both batch and semi-continuous co-digestion approaches were adopted to carry out the investigation. The feedstocks were mixed at different percentages according to their availabilities in southern Italy. The batch anaerobic co-digestion demonstrated that six studied feedstock mixtures generated an average of 239 mL CH₄/g VS loading without significant difference between each other, which concluded that the feedstock mixtures can be used for biogas production. Considering the feedstock availability of citrus pulp and olive pomace in Sicily, three feedstock mixtures with the highest volatile solids concentration of citrus pulp (42% citrus pulp, 17% corn silage, 4% cattle manure, 8% poultry litter, and 18% whey; 34% citrus pulp, 8% olive pomace, 17% corn silage, 4% cattle manure, 8% poultry litter, and 18% whey; and 25% citrus pulp, 16% olive pomace, 17% corn silage, 4% cattle manure, 8% poultry litter, and 18% whey, respectively) were selected to run the semi-continuous anaerobic digestion. Under the stabilized culture condition, the feed mixture with 42% citrus pulp, 17% corn silage, 4% cattle manure, 8% poultry litter, and 18% whey presented the best biogas production (231 L methane/kg VS loading/day). The corresponding mass and energy balance concluded that all three tested feedstock mixtures have positive net energy outputs (1.5, 0.9, and 1.2 kWh-e/kg dry feedstock mixture, respectively).

Nutrients recovery from anaerobic digestate of agro-waste: Techno-economic assessment of full scale applications

The sustainable production of fertilizers, especially those based on phosphorus, will be one of the challenges of this century. Organic wastes produced by the agriculture, urban and industrial sectors are rich in nutrients which can be conveniently recovered and used as fertilizers. In this study five full scale systems for the recovery of nutrients from anaerobic digestate produced in farm-scale plants were studied. Monitored technologies were: drying with acidic recovery, stripping with acidic recovery and membrane separation. Results showed good performances in terms of nutrients recovery with average yields always over 50% for both nitrogen and phosphorus. The techno-economic assessment showed how the specificity of the monitored systems played a major role: in particular, membranes were able to produce a stream of virtually pure water (up to 50% of the treated digestate) reducing the digestate volume, while drying, because of the limitation on recoverable heat, could treat only a limited portion (lower than 50%) of produced digestate while stripping suffered some problems because of the presence of suspended solids in the liquid fraction treated. Specific capital and operational costs for the three systems were comparable ranging between 5.40 and 6.97 € per m³ of digestate treated and followed the order stripping > drying > membranes. Costs determined in this study were similar to those observed in other European experiences reported in literature.

Improvement of wheat straw anaerobic digestion through alkali pre-treatment: Carbohydrates bioavailability evaluation and economic feasibility

Lignocellulosic biomasses such as wheat straw are widely used as a feedstock for biogas production. However, these biomasses are mainly composed of a compact fibre structure and therefore, it is recommended to treat them prior to its usage for biogas production in order to improve their bioavailability. The aim of this work is to evaluate, in terms of performance stability, methane yield and economic feasibility, two different scenarios: a mesophilic codigestion of wheat straw and animal manure with or without a low-energy demand alkaline pre-treatment (0.08g_KOHg_TS^-1of wheat straw, for 24h and at 25°C). Besides this, said pre-treatment was also analysed based on the improvement of the bioavailable carbohydrate content in the untreated versus the pre-treated wheat straw. The results pointed out that pre-treated wheat straw prompted a more stable performance (in terms of pH and alkalinity) and an improved methane yield (128% increment) of the mesophilic codigestion process, in comparison to the "untreated" scenario. The pre-treatment increased the content of cellulose, hemicellulose and other compounds (waxes, pectin, oil, etc.) in the liquid fraction, from 5% to 60%, from 11.5% to 39.1% TS and from 57% to 79% of the TS in the liquid fraction for the untreated and pre-treated wheat straws, respectively. Finally, the pre-treated scenario gained an energy surplus of a factor 13.5 and achieved a positive net benefit of 90.4€t_VS-WS^-1d^-1, being a favourable case for an eventual scale-up of the combined process.

Investigation of food waste valorization through sequential lactic acid fermentative production and anaerobic digestion of fermentation residues

This work concerns the investigation of the sequential production of lactic acid (LA) and biogas from food waste (FW). LA was produced from FW using a Streptococcus sp. strain via simultaneous saccharification and fermentation (SSF) and separate enzymatic hydrolysis and fermentation (SHF). Via SHF a yield of 0.33g_LA/g_FW (productivity 3.38g_LA/L·h) and via SSF 0.29g_LA/g_FW (productivity 2.08g_LA/L·h) was obtained. Fermentation residues and FW underwent anaerobic digestion (3wt% TS). Biogas yields were 0.71, 0.74 and 0.90Nm³/kg_VS for FW and residues from SSF and SHF respectively. The innovation of the approach is considering the conversion of FW into two different products through a biorefinery concept, therefore making economically feasible LA production and valorising its fermentative residues. Finally, a mass balance of three different outlines with the aim to assess the amount of LA and biogas that may be generated within different scenarios is presented.

Biological H2 potential harvested from complex gelatinaceous wastewater via attached versus suspended growth culture anaerobes

The effect of cultural growth treating gelatinaceous wastewater on hydrogen fermentative was assessed using up-flow multi-stage anaerobic sponge reactor (UMASR) and anaerobic sequencing batch reactor (AnSBR). Both reactors were operated at five hydraulic retention times (HRTs). UMASR achieved the maximum COD removal efficiency of 60.2±4.4% at HRT of 48h. Moreover, UMASR exhibited superiority in the course of carbohydrates and proteins removal efficiencies' of 100 and 52.5±2.4% due to high amylase and protease activities' of 4.1±0.3 and 0.032±0.002U, respectively. Contrariwise, AnSBR assigned for the peak hydrogen production rate of 1.17±0.14L/L/day at HRT of 24-h. Lipase activity was quite high (0.307±0.023U) in AnSBR resulting in removal efficiency of 35.2±2.1% for lipids. Stover-Kincannon model emphasized that UMASR required lesser volume than AnSBR to sustain the same substrate degradation efficacy. Nevertheless, the net gain energy harvested from AnSBR surpassed UMASR by 4.0-folds at HRT of 24-h.

SIMSWASTE-AD - A modelling framework for the environmental assessment of agricultural waste management strategies: Anaerobic digestion

On-farm anaerobic digestion (AD) has been promoted due to its improved environmental performance, which is based on a number of life cycle assessments (LCA). However, the influence of site-specific conditions and practices on AD performance is rarely captured in LCA studies and the effects on C and N cycles are often overlooked. In this paper, a new model for AD (SIMS_WASTE-AD) is described in full and tested against a selection of available measured data. Good agreement between modelled and measured values was obtained, reflecting the model capability to predict biogas production (r²=0.84) and N mineralization (r²=0.85) under a range of substrate mixtures and operational conditions. SIMS_WASTE-AD was also used to simulate C and N flows and GHG emissions for a set of scenarios exploring different AD technology levels, feedstock mixtures and climate conditions. The importance of post-digestion emissions and its relationship with the AD performance have been stressed as crucial factors to reduce the net GHG emissions (-75%) but also to enhance digestate fertilizer potential (15%). Gas tight digestate storage with residual biogas collection is highly recommended (especially in temperate to warm climates), as well as those operational conditions that can improve the process efficiency on degrading VS (e.g. thermophilic range, longer hydraulic retention time). Beyond the effects on the manure management stage, SIMS_WASTE-AD also aims to help account for potential effects of AD on other stages by providing the C and nutrient flows. While primarily designed to be applied within the SIMS_DAIRY modelling framework, it can also interact with other models implemented in integrated approaches. Such system scope assessments are essential for stakeholders and policy makers in order to develop effective strategies for reducing GHG emissions and environmental issues in the agriculture sector.

Life-cycle assessment of a biogas power plant with application of different climate metrics and inclusion of near-term climate forcers

This study assesses the environmental sustainability of electricity production through anaerobic co-digestion of sewage sludge and organic wastes. The analysis relies on primary data from a biogas plant, supplemented with data from the literature. The climate impact assessment includes emissions of near-term climate forcers (NTCFs) like ozone precursors and aerosols, which are frequently overlooked in Life Cycle Assessment (LCA), and the application of a suite of different emission metrics, based on either the Global Warming Potential (GWP) or the Global Temperature change Potential (GTP) with a time horizon (TH) of 20 or 100 years. The environmental performances of the biogas system are benchmarked against a conventional fossil fuel system. We also investigate the sensitivity of the system to critical parameters and provide five different scenarios in a sensitivity analysis. Hotspots are the management of the digestate (mainly due to the open storage) and methane (CH₄) losses during the anaerobic co-digestion. Results are sensitive to the type of climate metric used. The impacts range from 52 up to 116 g CO₂-eq./MJ electricity when using GTP100 and GWP20, respectively. This difference is mostly due to the varying contribution from CH₄ emissions. The influence of NTCFs is about 6% for GWP100 (worst case), and grows up to 31% for GWP20 (best case). The biogas system has a lower performance than the fossil reference system for the acidification and particulate matter formation potentials. We argue for an active consideration of NTCFs in LCA and a critical reflection over the climate metrics to be used, as these aspects can significantly affect the final outcomes.

Comparative evaluation of biogas production from dairy manure and co-digestion with maize silage by CSTR and new anaerobic hybrid reactor

This study aimed to investigate potential methane production through anaerobic digestion of dairy manure and co-digestion with maize silage. Two different anaerobic reactor configurations (single-stage continuously stirred tank reactor [CSTR] and hybrid anaerobic digester) were used and biogas production performances for each reactor were compared. The HR was planned to enable phase separation in order to improve process stability and biogas production under higher total solids loadings (≥4%). The systems were tested under six different organic loading rates increased steadily from 1.1 to 5.4 g VS/L.d. The CSTR exhibited lower system stability and biomass conversion efficiency than the HR. The specific biogas production of the hybrid system was between 440 and 320 mL/gVS with 81-65% volatile solids (VS) destruction. The hybrid system provided 116% increase in specific biogas production and VS destruction improved by more than 14%. When MS was co-digested together with dairy manure, specific biogas production rates increased about 1.2-fold. Co-digestion was more beneficial than mono-material digestion. The hybrid system allowed for generating methane enriched biogas (>75% methane) by enabling phase separation in the reactor. It was observed that acidogenic conditions prevailed in the first two compartments and the following two segments as methanogenic conditions were observed. The pH of the acidogenic part ranged between 4.7 and 5.5 and the methanogenic part was between 6.8 and 7.2.

Microalgae Cultivation on Anaerobic Digestate of Municipal Wastewater, Sewage Sludge and Agro-Waste

Microalgae are fast-growing photosynthetic organisms which have the potential to be exploited as an alternative source of liquid fuels to meet growing global energy demand. The cultivation of microalgae, however, still needs to be improved in order to reduce the cost of the biomass produced. Among the major costs encountered for algal cultivation are the costs for nutrients such as CO₂, nitrogen and phosphorous. In this work, therefore, different microalgal strains were cultivated using as nutrient sources three different anaerobic digestates deriving from municipal wastewater, sewage sludge or agro-waste treatment plants. In particular, anaerobic digestates deriving from agro-waste or sewage sludge treatment induced a more than 300% increase in lipid production per volume in Chlorella vulgaris cultures grown in a closed photobioreactor, and a strong increase in carotenoid accumulation in different microalgae species. Conversely, a digestate originating from a pilot scale anaerobic upflow sludge blanket (UASB) was used to increase biomass production when added to an artificial nutrient-supplemented medium. The results herein demonstrate the possibility of improving biomass accumulation or lipid production using different anaerobic digestates.

Water-energy nexus: Anaerobic co-digestion with elephant grass hydrolyzate

The anaerobic co-digestion process in a continuous stirred-tank reactor (CSTR) was carried out under mesophilic conditions (37 ± 0.2 °C). All the trials were performed at a hydraulic retention time (HRT) of 15 days and the AD reactor was daily fed with a mixture of sewage sludge (SS) and elephant grass hydrolyzate (EGH). In this study, three different trials were assessed, with different mixture proportions of SSSS and EGH: F0 (100:0,v/v), F1 (75:25, v/v) and F2 (50:50, v/v), during 90 days each trial, keeping the organic loading rate (OLR) in a range of 0.94-1.16 g VS L(-1) day(-1). The experimental results obtained showed that the soluble chemical oxygen demand (SCOD) removal efficiency was around 77% and 86% for trials F1 and F2, respectively. SS co-digestion with EGH enhanced methane yield, leading to an increment between 23% and 38%, in comparison with the reference scenario (F0).

Improving biogas quality and methane yield via co-digestion of agricultural and urban biomass wastes

Impact of co-digestion versus mono-digestion on biogas and CH4 yield for a set of five biomass materials (vegetable food waste, cow dung, pig manure, grass clippings, and chicken manure) was investigated considering 95 different biomass mixes of the five materials under thermophilic conditions in bench-scale batch experiments over a period of 65days. Average biogas and CH4 yields were significantly higher during co-digestion than during mono-digestion of the same materials. This improvement was most significant for co-digestion experiments involving three biomass types, although it was independent of the specific biomasses being co-digested. Improvement in CH4 production was further more prominent early in the digestion process during co-digestion compared to mono-digestion. Co-digestion also appeared to increase the ultimate CH4/CO2 ratio of the gas produced compared to mono-digestion although this tendency was relatively weak and not statistically significant.

Effect of increased load of high-strength food wastewater in thermophilic and mesophilic anaerobic co-digestion of waste activated sludge on bacterial community structure

In recent years, anaerobic co-digestion (AcoD) has been widely used to improve reactor performance, especially methane production. In this study, we applied two different operating temperatures (thermophilic and mesophilic) and gradually increased the load of food wastewater (FWW) to investigate the bacterial communities during the AcoD of waste activated sludge (WAS) and FWW. As the load of FWW was increased, methane production rate (MPR; L CH4/L d) and methane content (%) in both Thermophilic AcoD (TAcoD) and Mesophilic AcoD (MAcoD) increased significantly; the highest MPR and methane content in TAcoD (1.423 L CH4/L d and 68.24%) and MAcoD (1.233 L CH4/L d and 65.21%) were observed when the FWW mixing ratio was 75%. However, MPR and methane yield in both reactors decreased markedly and methane production in TAcoD ceased completely when only FWW was fed into the reactor, resulting from acidification of the reactor caused by accumulation of organic acids. Pyrosequencing analysis revealed a decrease in bacterial diversity in TAcoD and a markedly different composition of bacterial communities between TAcoD and MAcoD with an increase in FWW load. For example, Bacterial members belonging to two genera Petrotoga (assigned to phylum Thermotogae) and Petrimonas (assigned to phylum Bacteroidetes) became dominant in TAcoD and MAcoD with an increase in FWW load, respectively. In addition, quantitative real-time PCR (qPCR) results showed higher bacterial and archaeal populations (expressed as 16S rRNA gene concentration) in TAcoD than MAcoD with an increase in FWW load and showed maximum population when the FWW mixing ratio was 75% in both reactors. Collectively, this study demonstrated the dynamics of key bacterial communities in TAcoD and MAcoD, which were highly affected by the load of FWW.

Co-digestion of pig slaughterhouse waste with sewage sludge

Slaughterhouse wastes (SHW) are potentially very attractive substrates for biogas production. However, mono-digestion of these wastes creates great technological problems associated with the inhibitory effects of ammonia and fatty acids on methanogens as well as with the foaming in the digesters. In the following study, the co-digestion of slaughterhouse wastes with sewage sludge (SS) was undertaken. Batch and semi-continuous experiments were performed at 35°C with municipal sewage sludge and pig SHW composed of meat tissue, intestines, bristles and post-flotation sludge. In batch assays, meat tissue and intestinal wastes gave the highest methane productions of 976 and 826 dm(3)/kg VS, respectively, whereas the methane yield from the sludge was only 370 dm(3)/kg VS. The co-digestion of sewage sludge with 50% SHW (weight basis) provided the methane yield exceeding 600 dm(3)/kg VS, which was more than twice as high as the methane production from sewage sludge alone. However, when the loading rate exceeded 4 kg VS/m(3) d, a slight inhibition of methanogenesis was observed, without affecting the digester stability. The experiments showed that the co-digestion of sewage sludge with large amount of slaughterhouse wastes is feasible, and the enhanced methane production does not affect the digester stability.

Scale effect of anaerobic digestion tests in fed-batch and semi-continuous mode for the technical and economic feasibility of a full scale digester

Methane production capacity in mesophilic conditions of waste from two food industry plants was assessed in a semi-pilot (6L, fed-batch) and pilot (300 L, semi-continuous) scale. This was carried out in order to evaluate the convenience of producing heat and electricity in a full scale anaerobic digester. The pilot test was performed in order to obtain more reliable results for the design of the digester. Methane yield, returned from the pilot scale test, was approximately 80% of that from the smaller scale test. This outcome was in line with those from other studies performed in different scales and modes and indicates the success of the pilot scale test. The net electricity produced from the digester accounted for 30-50% of the food industry plants' consumption. The available thermal energy could cover from 10% to 100% of the plant requirements, depending on the energy demand of the processes performed.

Anaerobic digestion of lignocellulosic biomass: challenges and opportunities

Anaerobic digestion (AD) of lignocellulosic biomass provides an excellent opportunity to convert abundant bioresources into renewable energy. Rumen microorganisms, in contrast to conventional microorganisms, are an effective inoculum for digesting lignocellulosic biomass due to their intrinsic ability to degrade substrate rich in cellulosic fiber. However, there are still several challenges that must be overcome for the efficient digestion of lignocellulosic biomass. Anaerobic biorefinery is an emerging concept that not only generates bioenergy, but also high-value biochemical/products from the same feedstock. This review paper highlights the current status of lignocellulosic biomass digestion and discusses its challenges. The paper also discusses the future research needs of lignocellulosic biomass digestion.

Anaerobic mesophilic co-digestion of ensiled sorghum, cheese whey and liquid cow manure in a two-stage CSTR system: Effect of hydraulic retention time

The aim of this study was to investigate the effect of hydraulic retention time (HRT) on hydrogen and methane production using a two-stage anaerobic process. Two continuously stirred tank reactors (CSTRs) were used under mesophilic conditions (37°C) in order to enhance acidogenesis and methanogenesis. A mixture of pretreated ensiled sorghum, cheese whey and liquid cow manure (55:40:5, v/v/v) was used. The acidogenic reactor was operated at six different HRTs of 5, 3, 2, 1, 0.75 and 0.5d, under controlled pH5.5, whereas the methanogenic reactor was operated at three HRTs of 24, 16 and 12d. The maximum H2 productivity (2.14L/LRd) and maximum H2 yield (0.70mol H2/mol carbohydrates consumed) were observed at 0.5d HRT. On the other hand, the maximum CH4 production rate of 0.90L/LRd was achieved at HRT of 16d, whereas at lower HRT the process appeared to be inhibited and/or overloaded.

Anaerobic co-digestion of cattle manure and alternative crops for the substitution of maize in South Europe

In this study alternative agricultural substrates are investigated as potential substitutes of maize for biogas production in the region of South Europe. Crop silages of cardoon, maize, milk thistle and sorghum as well as bedding straw from cattle farm were examined in the anaerobic co-digestion procedure with cattle manure. Milk thistle crop was further investigated in a naturally sun dried form and the effect of mechanical, thermal and thermo-chemical pretreatments on fiber composition and methane yield was evaluated. Pretreatment with NaOH increase the solubilization by 77.7%. The co-digestion experiment was carried out in 28 batch reactors at 37°C. The highest methane yields of 308, 271 and 267LCH4kg(-1) of volatile solids were obtained by co-digestion of cattle manure with cardoon silage, thermo-chemical pretreated milk thistle stalks with NaOH and maize silage, respectively. Furthermore, co-digestion of bedding straw and cattle manure had similar methane yield with maize silage.

Towards eliminating systematic errors caused by the experimental conditions in Biochemical Methane Potential (BMP) tests

The Biochemical Methane Potential (BMP) test is increasingly recognised as a tool for selecting and pricing biomass material for production of biogas. However, the results for the same substrate often differ between laboratories and much work to standardise such tests is still needed. In the current study, the effects from four environmental factors (i.e. ambient temperature and pressure, water vapour content and initial gas composition of the reactor headspace) on the degradation kinetics and the determined methane potential were evaluated with a 2(4) full factorial design. Four substrates, with different biodegradation profiles, were investigated and the ambient temperature was found to be the most significant contributor to errors in the methane potential. Concerning the kinetics of the process, the environmental factors' impact on the calculated rate constants was negligible. The impact of the environmental factors on the kinetic parameters and methane potential from performing a BMP test at different geographical locations around the world was simulated by adjusting the data according to the ambient temperature and pressure of some chosen model sites. The largest effect on the methane potential was registered from tests performed at high altitudes due to a low ambient pressure. The results from this study illustrate the importance of considering the environmental factors' influence on volumetric gas measurement in BMP tests. This is essential to achieve trustworthy and standardised results that can be used by researchers and end users from all over the world.

Anaerobic co-digestion of dairy cattle manure and pear waste

Anaerobic co-digestion of pre-treated dairy cattle manure (LCM) with pear waste after a storage period (PLF) was tested at four inclusion levels: 0%, 25%, 75% and 100%. Inclusion levels consisted in the replacement of the volatile solids (VS) from the LCM with the VS from PLF keeping the organic loading rate around 1.1 ± 0.4 g SVL(-1)d(-1). The introduction of the co-substrate clearly enhanced methane production rate (MPR) in comparison to single substrate (phase I) as phases II and III, respectively, achieving values 1.3 and 2.8 times higher than phase I. The overall performance was optimized for the mixture 25:75 (LCM:PLF; v:v). Moreover, storage of pear waste did not compromise its use in AD. This fact is important once it can improve waste management from pear production through its valorisation as co-substrate in AD process.

Neural and hybrid modeling: an alternative route to efficiently predict the behavior of biotechnological processes aimed at biofuels obtainment

The present paper was aimed at showing that advanced modeling techniques, based either on artificial neural networks or on hybrid systems, might efficiently predict the behavior of two biotechnological processes designed for the obtainment of second-generation biofuels from waste biomasses. In particular, the enzymatic transesterification of waste-oil glycerides, the key step for the obtainment of biodiesel, and the anaerobic digestion of agroindustry wastes to produce biogas were modeled. It was proved that the proposed modeling approaches provided very accurate predictions of systems behavior. Both neural network and hybrid modeling definitely represented a valid alternative to traditional theoretical models, especially when comprehensive knowledge of the metabolic pathways, of the true kinetic mechanisms, and of the transport phenomena involved in biotechnological processes was difficult to be achieved.

Anaerobic co-digestion of swine and poultry manure with municipal sewage sludge

The anaerobic digestion of municipal sewage sludge (SS) with swine manure (SM) and poultry manure (PM) was undertaken. It was found that a mixture of sewage sludge with a 30% addition of swine manure gave around 400 dm(3)/kg VS of biogas, whereas the maximal biogas yield from ternary mixture (SS:SM:PM=70:20:10 by weight) was only 336 dm(3)/kg VS. An inhibition of methanogenesis by free ammonia was observed in poultry manure experiments. The anaerobic digestion was inefficient in pathogen inactivation as the reduction in the number of E. coli an Enterobacteriaceae was only by one logarithmic unit. A substantial portion of pathogens was also released into the supernatant.

Biogas production from wheat straw and manure--impact of pretreatment and process operating parameters

Non-treated or steam-exploded straw in co-digestion with cattle manure was evaluated as a substrate for biogas production compared with manure as the sole substrate. All digestions were performed in laboratory-scale CSTR reactors (5L) operating with an organic loading late of approximately 2.8 g VS/L/day, independent of substrate mixture. The hydraulic retention was 25 days and an operating temperature of 37, 44 or 52°C. The co-digestion with steam exploded straw and manure was evaluated with two different mixtures, with different proportion. The results showed stable performance but low methane yields (0.13-0.21 N L CH4/kg VS) for both manure alone and in co-digestion with the straw. Straw appeared to give similar yield as manure and steam-explosion treatment of the straw did not increase gas yields. Furthermore, there were only slight differences at the different operating temperatures.

Anaerobic co-digestion of the marine microalga Nannochloropsis salina with energy crops

Anaerobic co-digestion of corn silage with the marine microalga Nannochloropsis salina was investigated under batch and semi-continuous conditions. Under batch conditions process stability and biogas yields significantly increased by microalgae addition. During semi-continuous long-term experiments anaerobic digestion was stable in corn silage mono- and co-digestion with the algal biomass for more than 200 days. At higher organic loading rates (4.7 kg volatile solids m(-3)d(-1)) inhibition and finally process failure occurred in corn silage mono-digestion, whereas acid and methane formation remained balanced in co-digestion. The positive influences in co-digestion can be attributed to an adjusted carbon to nitrogen ratio, enhanced alkalinity, essential trace elements and a balanced nutrient composition. The results suggest that N. salina biomass is a suitable feedstock for anaerobic co-digestion of energy crops, especially for regions with manure scarcity. Enhanced process stability may result in higher organic loading rates or lower digester volumes.

Effects of thermal pretreatment on anaerobic digestion of Nannochloropsis salina biomass

The marine microalga Nannochloropsis salina was investigated as feedstock for anaerobic digestion under batch and semi-continuous conditions for the first time. Biodegradability and methane yield were low under both digestion conditions. Thermal pretreatment prior to anaerobic digestion significantly increased the methane yield from 0.2 to 0.57 m(3) kg VS(-1) under batch conditions and from 0.13 to 0.27 m(3) kg VS(-1) in semi-continuous digestion. Still, the methane yield was limited with semi-continuous feeding due to volatile fatty acid (VFA) accumulation in the digester caused by high ammonium and salt concentrations in the feedstock. Despite VFA accumulation adaption of the microorganisms to the changing conditions and high buffer capacity resulted in steady methane production. A first energy balance considering the required heat for thermal pretreatment revealed significant benefit from the pretreatment. Conversely, the high energy demand for dewatering algal cultures is one major bottleneck for industrial-scale processing of microalgae.

Effects of total ammonia nitrogen concentration on solid-state anaerobic digestion of corn stover

The inhibitive effect of total ammonia nitrogen (TAN) (including NH3 and NH4(+)) on solid-state anaerobic digestion of corn stover was investigated in batch reactors at 37°C. The highest methane yield of 107.0 L/kg VS(feed) was obtained at a TAN concentration of 2.5 g/kg (based on total weight). TAN concentrations greater than 2.5 g/kg resulted in decreased methane yields, with a 50% reduction observed at a concentration of 6.0 g/kg. Reduced reaction rates and microbial activities for hydrolysis of cellulose and methanogenesis from acetate were observed at TAN concentrations higher than 4.3 g/kg. Strong ammonia stress was indicated at butyrate concentrations higher than 300 mg/kg. Result showed that the effluent of liquid anaerobic digestion can provide enough nitrogen for solid-state anaerobic digestion of corn stover.