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The depletion of fossil fuels coupled with stringent environmental laws has encouraged us to develop sustainable renewable energy. Due to its numerous benefits, anaerobic digestion (AD) has emerged as an environment-friendly technology. Biogas generated during AD is primarily a mixture of CH4 (65–70%) and CO2 (20–25%) and a potent energy source that can combat the energy crisis in today's world. Here, an attempt has been made to provide a broad understanding of AD and delineate the effect of various operational parameters influencing AD. The characteristics of fruit and vegetable waste (FVW) and its feasibility as a potent substrate for AD have been studied. This review also covers traditional challenges in managing FVW via AD, the implementation of various bioreactor systems to manage large amounts of organic waste and their operational boundaries, microbial consortia involved in each phase of digestion, and various strategies to increase biogas production. [ABSTRACT FROM AUTHOR]

Kinetic information extracted from biochemical methane potential (BMP) tests is often reported but its value is unclear. Inter-laboratory reproducibility provides a useful indication of its value. Here we extracted estimates of the first-order rate constant k from 1259 methane production curves collected in a large inter-laboratory study on BMP in order to quantify reproducibility. Reproducibility in k was poor; relative standard deviation was 50–140%. Substrate comparisons (k for one substrate compared to another) also had low reproducibility, regardless of low p values from inferential statistical tests. The use of a shared inoculum did not improve reproducibility in k. We conclude that k estimates from BMP tests only partially reflect intrinsic substrate properties. Therefore, interpretation and application of batch kinetic results should be done cautiously.

This research study investigates the effect of feedstock chemical composition and voltage supply on methane production from bio-electrochemical anaerobic digestion (BEAD). The experimental work was conducted on batch biomethane potential (BMP) bottles equipped with electrodes and connected to external power. Three substrates with distinct elemental compositions, waste activated sludge (WAS), and cow manure (CM), organic fraction of municipal solid waste (OFMSW), were tested under different voltages (0.7–1.5V). The BMP tests showed that OFMSW had the highest methane yield in the AD control reactors, followed by WAS and CM, which concurred with the estimated theoretical yields. Supplying voltage enhanced the performance of all BEAD reactors, but with different levels of improvement; methane yield, organics removal, and net energy efficiency increased by 7–46%, 16–54%, and 5–37%, respectively, compared to the equivalent controls. The strong relationship between the voltage and substrates with methane production was confirmed statistically through analysis of variance and t -test. Moreover, a regression model was established between the feedstock elemental fractions, power supply, and methane yield, with an adjusted correlation coefficient of 0.9 and a maximum prediction error of 5.4%. The overall findings of this research provide valuable operation and performance insights on the potential bio-electrochemical upgrades to AD systems. [ABSTRACT FROM AUTHOR]

Lignocellulosic feedstocks are year-round, available bio-residues that are the right candidates for counteracting the energy crises and global warming facing the world today. However, lignin leads to a slow hydrolysis rate and is a major bottleneck for biogas production via anaerobic digestion. Anaerobic co-digestion (AcoD) is an economical method available, which overcomes the limitation of a single feedstock's properties in an anaerobic digestion process. This paper critically reviews the impacts of co-digestion on lignocellulosic biomass degradation, process stability, various working parameters, and microbial activities that improve methane yields. A combination of compatible substrates is chosen to improve the biomethane yield and conversion rate of organic matter. AcoD is a promising method in the delignification of lignocellulosic biomass as an acid pretreatment. Ultimate practices to control the impact of co-digestion on system performances include co-feed selection, in terms of both carbon-to-nitrogen (C/N) and mixing ratios, and other operating conditions. A detailed analysis is performed using data reported in the recent past to assess the sensitivity of influencing parameters on the resultant biogas yield. For the investigators motivated by the basic principles of AcoD technology, this review paper generates baseline data for further research work around co-digestion. [ABSTRACT FROM AUTHOR]

Owing to the continued global environmental crisis, wastewater treatment is seen with great potential to limit the demand on freshwater usage while contributing to depletion of global warming with alternative source of renewable energy. Thus, in the wastewater settings, anaerobic digestion (AD) reduces organic pollutants while generating green energy in the form of biogas. However, AD is a relatively slow microbial–based process, which has become a major challenge to produced methane–enriched biogas. Therefore, augmentation of AD was investigated via the addition of metallic–based nanoparticles (NPs) (Fe, Cu, and Ni) at a concentration of 1 g and 2 g. Scanning electron microscopy and energy dispersive X-ray (SEM/EDX) were used to track the NPs distribution and utilisation in the post-AD digestate. A biomethane potential (BMP) technique was employed with 1 L Duran schott bottles. This was operated at a working volume of 0.8 L (0.3 L inoculum and 0.5 L wastewater), hydraulic retention time of 10 days and mesophilic temperature 40 °C. The wastewater treatability performance showed over 65% removal of chemical oxygen demand (COD), turbidity and colour. The NP additives showed a synergetic effect on biogas production as it hinged on the NPs type and concentration (1 g Fe-NPs > 2 g FeNiCu-NPs > 1 g Cu-NPs > 2 g Cu-NPs > 1 g Ni-NPs > 2 g Ni-NPs > 2 g Fe-NPs). Usage of 1 g of Fe NPs balanced the nutrient supply of the microbes, which increased the biogas production (by 75% >3 mL/d of the control) with 100% methane composition. The prospects of Fe-based NPs seems very promising and industrially worthy for wastewater treatment and bioenergy production. This warrants future research into NPs recovery and reuse as a way of mitigating its economic viability and environmental risk.

Processing of the produced primary and secondary sludge during sewage treatment is demanding and requires considerable resources. Most common practices suggest the cotreatment of primary and secondary sludge starting with thickening and anaerobic digestion. The aim of this study is to investigate the anaerobic digestion of the primary sludge only and estimate its impact on sludge treatment and energy recovery. Within this context, the performance of the anaerobic digestion of primary sludge is explored and focused on practices to further enhance the methane production by using additives, e.g., a cationic polyelectrolyte and attapulgite. The results showed that the overall yield in methane production during anaerobic digestion of primary sludge alone was higher than that obtained by the anaerobic digestion of mixed primary and secondary sludge (up to 40%), while the addition of both organic polyelectrolyte and attapulgite enhanced further the production of methane (up to 170%). Attapulgite increased the hydrolysis rate of biosolids and produced relatively stabilized digestate, though of lower dewaterability. Moreover, the results suggest that single digestion of primary sludge may accomplish higher methane production capacities at lower digestors' volume increasing their overall efficiency and productivity, while the produced digestates are of adequate quality for further utilization mainly in agricultural or energy sectors. [ABSTRACT FROM AUTHOR]

The objective of this study was to determine the efficiency and theoretical stability of the anaerobic digestion of the wastewater sludge obtained from a municipal wastewater treatment plant, using co-digestion with cocoa pod husks. The chemical structure of sewage sludge, including its high carbon and nitrogen content in carbohydrates and fats, gives it vast potential for biomethane generation. In this study, the main agri-food residues in the geographical area where the study was conducted that could be used for co-digestion were determined, and cocoa pod husks were found to be the best option based on elemental chemical analysis. The results demonstrate that the co- digestion of sewage sludge with cocoa pod husks can produce up to 555.7 L CH4/KG SV. In this article, we also propose a method for estimating biochemical methane potential (theoretical BMP) based on chemical equations and a systematic review of the most relevant research in BMP. [ABSTRACT FROM AUTHOR]

Processing of the produced primary and secondary sludge during sewage treatment is demanding and requires considerable resources. Most common practices suggest the cotreatment of primary and secondary sludge starting with thickening and anaerobic digestion. The aim of this study is to investigate the anaerobic digestion of the primary sludge only and estimate its impact on sludge treatment and energy recovery. Within this context, the performance of the anaerobic digestion of primary sludge is explored and focused on practices to further enhance the methane production by using additives, e.g., a cationic polyelectrolyte and attapulgite. The results showed that the overall yield in methane production during anaerobic digestion of primary sludge alone was higher than that obtained by the anaerobic digestion of mixed primary and secondary sludge (up to 40%), while the addition of both organic polyelectrolyte and attapulgite enhanced further the production of methane (up to 170%). Attapulgite increased the hydrolysis rate of biosolids and produced relatively stabilized digestate, though of lower dewaterability. Moreover, the results suggest that single digestion of primary sludge may accomplish higher methane production capacities at lower digestors’ volume increasing their overall efficiency and productivity, while the produced digestates are of adequate quality for further utilization mainly in agricultural or energy sectors.

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 Nm 3 CH 4 /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. [ABSTRACT FROM AUTHOR]

Production of biogas from different organic materials is a most interesting source of renewable energy. The biomethane potential (BMP) of these materials has to be determined to get insight in design parameters for anaerobic digesters. Although several norms and guidelines for BMP tests exist, inter-laboratory tests regularly show high variability of BMPs for the same substrate. A workshop was held in June 2015, in Leysin, Switzerland, with over 40 attendees from30 laboratories around the world, to agree on common solutions to the conundrum of inconsistent BMP test results. This paper presents the consensus of the intense roundtable discussions and cross-comparison of methodologies used in respective laboratories. Compulsory elements for the validation of BMP results were defined. They include the minimal number of replicates, the request to carry out blank and positive control assays, a criterion for the test duration, details on BMP calculation, and last but not least criteria for rejection of the BMP tests. Finally, recommendations on items that strongly influence the outcome of BMP tests such as inoculum characteristics, substrate preparation, test setup, and data analysis are presented to increase the probability of obtaining validated and reproducible results. [ABSTRACT FROM AUTHOR]

Co-digestion of organic rich wastes and wastewater sludge to enhance biogas production has become an attractive economic possibility for water utilities. The suitability of the organic rich waste depends on its ability to produce biogas as well as its influence on the overall anaerobic digestion process. Biomethane potential evaluation was conducted to screen seven organic wastes and dehydrated algae. All co-substrates increased the bio-methane yield by three to six times compared with conventional anaerobic digestion of sewage sludge. Maximum co-digestion ratios were identifiable for most solid co-substrates including algae (6% wt/wt), undiluted food waste (5% wt/wt), bakery waste (5% wt/wt), and diluted commercial food waste (10% wt/wt). On the other hand, the maximum co-digestions ratio of beverage reject and sewage sludge was 10% (wt/wt). With the exception of fat-oil-grease, all solids free liquid co-substrates evaluated in this study showed a notable synergistic effect, to enhanced removals of total solids, volatile solids (VS) and chemical oxygen demand (COD) during anaerobic digestion. The increase in COD removal when co-digesting wastewater sludge and liquid waste was from 2 to 41%. Conversely, the co-digestion of most solid co-substrates resulted in additional VS and COD residuals in the final biosolids. Elevated concentrations of sulphur and phosphorous in all food waste co-substrates suggest that control measures to address H 2 S in biogas and the accumulation of phosphorus in sludge centrate may be necessary during full scale operation. Data presented here provide the basis for subsequent pilot scale evaluation of anaerobic digestion of these organic rich wastes and wastewater sludge. [ABSTRACT FROM AUTHOR]

Isochrysis galbana and Selenastrum capricornutum , marine and freshwater microalgae species respectively, were co-digested with sewage sludge under mesophilic and thermophilic conditions. The substrates and the temperatures significantly influenced biogas production. Under mesophilic conditions, the sewage sludge digestion produced 451 ± 12 mL Biogas /g SV . Furthermore, all digesters were fed with I. galbana , or mixed with sludge, resulting in an average of 440 ± 25 mL Biogas /g SV . On the contrary, S. capricornutum produced 271 ± 6 mL Biogas /g SV and in the mixtures containing sludge produced intermediate values between sludge and microalgae production. Under thermophilic conditions, the sewage sludge digestion achieved yet the highest biogas yield, 566 ± 5 mL Biogas /g SV . During co-digestion, biogas production decreased when the microalgae content increased, and for I. galbana and for S. capricornutum it reached minimum values, 261 ± 11 and 185 ± 7 mL Biogas /g SV, respectively. However, no evidence of inhibition was found and the low yields were attributed to microalgae species characteristics. The methane content in biogas showed similar values, independently from the digested substrate, although this increased by approximately 5% under thermophilic condition. [ABSTRACT FROM AUTHOR]

Highlights: [•] A statistical model for predicting BMP from lignocellulosic material is developed. [•] The true effect of lignin and carbohydrates on BMP is described. [•] The best prediction is proposed using a canonical linear mixture model. [•] An expression for prediction founded on the largest dataset to date, is presented. [Copyright &y& Elsevier]

Abstract: Two types of non-axenic algal cultures, one dominated by the freshwater microalgae Scenedesmus obliquus and the other by the marine microalgae Phaeodactylum tricornutum, were cultivated in two types of simple photobioreactor systems. The production rates, expressed on dry matter (DM) basis, were in the order of 0.12 and 0.18g DML−1 d−1 for S. obliquus and P. tricornutum respectively. The biogas potential of algal biomass was assessed by performing standardized batch digestion as well as digestion in a hybrid flow-through reactor (combining a sludge blanket and a carrier bed), the latter under mesophilic and thermophilic conditions. Biomethane potential assays revealed the ultimate methane yield (B 0) of P. tricornutum biomass to be about a factor of 1.5 higher than that of S. obliquus biomass, i.e. 0.36 and 0.24LCH4 g−1 volatile solids (VS) added respectively. For S. obliquus biomass, the hybrid flow-through reactor tests operated at volumetric organic loading rate (Bv) of 2.8gVSL−1 d−1 indicated low conversion efficiencies ranging between 26–31% at a hydraulic retention time (HRT) of 2.2days for mesophilic and thermophilic conditions respectively. When digesting P. tricornutum at a Bv of 1.9gVSL−1 d−1 at either mesophilic or thermophilic conditions and at an HRT of 2.2days, an overall conversion efficiency of about 50% was obtained. This work indicated that the hydrolysis of the algae cells is limiting the anaerobic processing of intensively grown S. obliquus and P. tricornutum biomass. [Copyright &y& Elsevier]

The application of anaerobic digestion technology is growing worldwide because of its economic and environmental benefits. As a consequence, a number of studies and research activities dealing with the determination of the biogas potential of solid organic substrates have been carrying out in the recent years. Therefore, it is of particular importance to define a protocol for the determination of the ultimate methane potential for a given solid substrates. In fact, this parameter determines, to a certain extent, both design and economic details of a biogas plant. Furthermore, the definition of common units to be used in anaerobic assays is increasingly requested from the scientific and engineering community. This paper presents some guidelines for biomethane potential assays prepared by the Task Group for the Anaerobic Biodegradation, Activity and Inhibition Assays of the Anaerobic Digestion Specialist Group of the International Water Association. This is the first step for the definition of a standard protocol. [ABSTRACT FROM AUTHOR]