Ultimate Methane Yield Research Articles

Acidogenic fermentation of biowaste coupled with nitrogen recovery using selective membranes to produce a VFA-rich liquid with a high C/N ratio

This research focuses on the production of a liquid stream rich in volatile fatty acids (VFAs) and low ammoniacal nitrogen content (<0.1 g N/L) from biowaste. The liquid stream was obtained by combining (i) mixed culture acidogenic fermentation to maximise VFA production and (ii) gas-permeable membrane (GPM) contactor to recover ammoniacal nitrogen. Three batch fermentation tests of biowaste collected in a full-scale mechanical-biological treatment plant provided high and stable VFA concentrations (37–39 g CODVFA/L). VFAs represented 73–81 % of the soluble chemical oxygen demand (sCOD) concentration, with a predominance of acetic, propionic and butyric acids. A highly specialized microbial community was observed in all batch tests, with Bacteroidota and Firmicutes as predominant phyla (>90 % of relative abundance). The GPM contactor recovered more than 99 % of the ammoniacal nitrogen in the fermentation liquid without VFA losses. The suitability of the produced fermentation liquid with a high C/N ratio for downstream applications was evaluated using biomethane potential tests (BMP) at different total ammonium nitrogen (TAN) concentrations (0.76–3.15 g N/L) and circumneutral pH. Despite achieving similar ultimate methane yields (279–314 NmL CH4/g CODfeed), lower TAN concentrations in the biowaste fermentation liquid improved anaerobic biodegradation kinetics, enhancing its potential applicability for methane production.

Evaluation of batch mesophilic anaerobic digestion of waste Euro banknotes for methane Production: Preliminary studies and kinetic approach

The European Central Bank is striving to find environmentally friendly alternative methods of waste disposal. In 2020, it decided to end the disposal of Waste Euro Banknotes (W€B) in landfills and planned to use them for recycling and/or energy recovery. Despite being recognized as an effective tool in a circular economy model, there are no reported studies on the use of W€B as a substrate in anaerobic digestion (AD). Thus, the aim of this research was to assess the viability of W€B to be converted into high-value-added energy products (mainly methane) through AD. W€B (€10 and €20 denominations) provided by the Bank of Spain were used. Biochemical methane potential (BMP) tests of pre-treated (grinded) and untreated W€B were assessed at mesophilic temperature. The ultimate methane yield was considerably higher for pre-treated W€B (334 ± 23 NmL CH4 g VS-1added) than for untreated W€B (297 ± 14 NmL CH4 g VS-1added). The Logistic or Sigmoidal kinetic model adequately fit the experimental data and allowed for obtaining the kinetic parameters of each case studied. In this sense, an increase of 22.4 % in the maximum methane production rate (Rmax) was observed for the pre-treated W€B (52.5 ± 0.9 mL CH4 (g VS·d)-1 compared to the untreated W€B (16.2 ± 1.8 mL CH4 (g VS·d)-1). According to the obtained results, AD may be a good alternative for the energetic valorization and recycling of W€B.

Comparison of Thermophilic-Mesophilic and Mesophilic-Thermophilic Two-Phase High-Solid Sludge Anaerobic Digestion at Different Inoculation Proportions: Digestion Performance and Microbial Diversity.

This study investigated the performance of thermophilic-mesophilic (T-M) and mesophilic-thermophilic (M-T) two-phase sludge anaerobic digestion at different inoculation proportions after a change in digestion temperature. After temperature change, the pH, total ammonia nitrogen (TAN), free ammonia nitrogen (FAN), solubility chemical oxygen demand (SCOD), and total alkalinity (TA) levels of two-phase digesters were between thermophilic control digesters and mesophilic control digesters. However, the volatile fatty acid (VFA) levels of two-phase digesters were higher than those of thermophilic or mesophilic control digesters. The bacteria communities of M-T two-phase digesters were more diverse than those of T-M. After a change in digestion temperature, the bacterial community was dominated by Coprothermobacter. After a change of digestion temperature, the relative abundance (RA) of Methanobacterium, Methanosaeta, and Methanospirillum of M-T two-phase digesters was higher than that of T-M two-phase digesters. In comparison, the RA of Methanosarcina of T-M two-phase digesters was higher than that of M-T two-phase digesters. The ultimate methane yields of thermophilic control digesters were greater than those of mesophilic control digesters. Nevertheless, the ultimate methane yield levels of M-T two-phase digesters were greater than those of T-M two-phase digesters. The ultimate methane yields of all two-phase digesters presented an earlier increase and later decrease trend with the increasing inoculation proportion. Optimal methane production condition was achieved when 15% of sludge (T-M15) was inoculated under mesophilic-thermophilic conditions, which promoted 123.6% (based on mesophilic control) or 27.4% (based on thermophilic control). An optimal inoculation proportion (about 15%) balanced the number and activity of methanogens of high-solid sludge anaerobic digestion.

Active microbial communities during biodegradation of biodegradable plastics by mesophilic and thermophilic anaerobic digestion.

Biodegradable plastics, if they are not properly managed at their end-of-life, can have the same hazardous environmental consequences as conventional plastics. This study investigates the treatment of the main biodegradable plastics under mesophilic and thermophilic anaerobic digestion using biochemical methane potential test and the microorganisms involved in the process using amplicon sequencing of the 16S rRNA. Here we showed that, only PHB and TPS undergone important and rapid biodegradation under mesophilic condition (38°C). By contrast, PCL and PLA exhibited very low biodegradation rate as 500 days were required to reach the ultimate methane yield. Little or no degradation occurred for PBAT and PBS at 38°C. Under thermophilic conditions (58°C), TPS, PHB, and PLA reached high levels of biodegradation in a relatively short period (&lt; 100 d). While PBS, PBAT, and PCL could not be converted into methane at 58°C. PHB degraders (Enterobacter and Cupriavidus) and lactate-utilizing bacteria (Moorella and Tepidimicrobium) appeared to play an important role in the PHB and PLA degradation, respectively. This work not only provides crucial data on the anaerobic digestion of the main biodegradable plastics but also enriches the understanding of the microorganisms involved in this process, which are of great importance for future development of the treatment of biodegradable plastics in anaerobic digestion systems.

Prediction of methane yield and pretreatment efficiency of lignocellulosic biomass based on composition

Lignocellulosic biomass is considered a key resource for the future expansion of biogas production through anaerobic digestion (AD), and research on the development of pretreatment technologies for improving biomass conversion is an intensive and fast-growing field. Consequently, there is a need for creating tools able to predict the efficiency of a certain pretreatment on different biomass types, fast and accurately, and to assist in selecting a pretreatment technology for a specific biomass. In this study, seven different types of raw lignocellulosic biomass of industrial relevance were systematically analyzed regarding their composition (carbohydrates, lignin, lipids, ash, extractives, etc.) and subjected to a common pretreatment. The aim of the study was to identify the most important characteristics that make a biomass good receptor of the specific pretreatment prior to AD. A simple ammonia pretreatment was chosen as a case study and partial least squares regression (PLS-R) was used for modeling initially the ultimate methane yield of raw and pretreated biomass. In the sequel, PLS-R was used for modeling the efficiency of the pretreatment on increasing the ultimate methane yield and hydrolysis rate as a function of the biomass composition. The fit of the models was satisfactory, ranging from R2 = 0.89 to R2 = 0.97. The results showed that the most decisive characteristics for predicting the efficiency of the pretreatment were the lipid (r = −0.88), ash (r = +0.79), protein (r = −0.61), and hemicellulose/lignin (r = −0.53) content of raw biomass. Finally, the approach followed in this study facilitated an improved understanding of the mechanism of the pretreatment and presented a methodology to be followed for developing tools for the prediction of pretreatment efficiency in the field of lignocellulosic biomass valorization.

Integrated two-phase acidogenic-methanogenic treatment of municipal sludge with thermal hydrolysis

The purpose of this research was to investigate the impact of two process configurations integrating two-phase anaerobic digestion (AD) of municipal sludge with thermal hydrolysis (TH). The TH was positioned either before or after the acidogenic fermentation phase. The fermentation process was carried out under the semi-continuous flow regime with a retention time of three days. The TH was done at a temperature of 170 °C and for 30 min. Among all the tested scenarios, the TH of sludge followed by the acidogenic fermentation resulted in the highest COD solubilization ratio (39.5%) and volatile fatty acids production (6,420 ± 400 mg/L), which was 630% and 500% more than that of the raw sludge, respectively. The sequential TH/fermentation process achieved 40% higher ultimate methane yield (240 mL/g COD) than the non-pretreated (raw) sludge. Positioning TH after the fermentation process reduced the ultimate methane yield to 231 240 mL/g COD, although it was still 32% higher than that of the raw sludge. The analysis of methane production rate and biodegradation kinetics data suggested the formation of refractory intermediates during the thermal process of sludge, which reduced the overall performance rate during the first week of the AD process. It was also revealed that acidogenic fermentation of thermally-processed sludge could diminish the adverse effect of the recalcitrant compounds formed during the thermal hydrolysis on the subsequent AD process.

Influence of lignocellulosic components on the anaerobic digestibility of aquatic weeds: Comparison with terrestrial crops

Anaerobic digestion treatment of excessively growing aquatic weeds has been paid attention due to the energy recovery with low energy requirement. The effects of lignocellulosic components on anaerobic digestibility of terrestrial herbaceous plants have been extensively researched, not much work has been done on aquatic weeds. Because of the flexible structure, aquatic weeds' chemical compositions and anaerobic digestibility may differ from terrestrial plants. This study investigated the relationships between the amount and ratio of lignocellulosic components (cellulose, hemicellulose, and lignin) and the kinetic parameters (ultimate methane yield, maximum methane production rate, lag phase, and T80, which refers to the time required to produce 80% of the total methane) calculated using the modified Gompertz model. Negative correlations were found out not only between lignin contents and the ultimate methane yield but also between hemicellulose content and the maximum methane production rate. The correlations between cellulose/lignin and ultimate methane yield and between cellulose/hemicellulose and maximum methane production rate, suggesting these parameters were primarily regulated by the lignocellulosic ratios. The correlation between cellulose/lignin and ultimate methane yield including aquatic weeds and terrestrial plants was approximately fitted using the same equation. While the maximum methane production rate of aquatic weeds and terrestrial herbaceous plants was not fitted to one equation with cellulose/hemicellulose, correlated with cellulose/estimated xylose on the same equation. This might be because cellulose degradation was hindered by xylose owing to the bonding of cellulose to lignin, thus determining the degradation rate of cellulose. Therefore, while cellulose can be degraded and converted to methane, coverage by lignin and xylose limits cellulose degradability and thus regulated methane production from aquatic weed.

The Performance Evaluation on Co-Digestion of Domestic Sewage Sludge and Food Waste for Methane Yield and Kinetics Analysis

Sewage sludge and food waste; are organic wastes suitable for the anaerobic digestion. However, the digestion of sewage sludge and food waste as solely substrate is having a drawback in term of methane yield. Therefore, many researchers combined these two wastes as a co-substrate and used in co-digestion. This study focused to evaluate the anaerobic co-digestion of domestic sewage sludge (in form of primary and secondary sewage sludge) with food waste under mesophilic temperature in a batch assay. Two series of batch biochemical methane potential (BMP) test were conducted using the Automatic Methane Potential Test System (AMPTS II). Each set are labelled with BMP 1(PSS:FW) and BMP 2 (SSS:FW). The BMP tests were monitored automatically until the methane production is insignificant. Using the data observed in the laboratory, the kinetic paremeters were calculated. Also, the First-order and Modified Gompertz modeling were included to predict the anaerobic digestion performance. Finding showed that BMP 1(PSS:FW) have better performance with respect to the higher ultimate methane yield and methane production rate as compared to BMP 2 (SSS:FW). Besides, the kinetic parameters from laboratory work and modeling were slightly different. In which the kinetic paremetes from modelling is lesser. However, both modelling are well fitted to the experimental data with high correlation coefficient, R2 ranged from 0.993 to 0.997.

Improving methane yield of oil palm empty fruit bunches by wet oxidation pretreatment: Mesophilic and thermophilic anaerobic digestion conditions and the associated global warming potential effects

Palm oil production is a significant agricultural industry in the South East Asia region and the empty fruit bunch by-product OPEFB is a lignocellulosic biomass left after palm oil recovery, which has shown to be recalcitrant to bioconversion. Pretreatment experiments were carried out in this study to mitigate the recalcitrance of this material using oxygen assisted hydrothermal pretreatment (with 3–9% oxygen loading in the form of H2O2) followed by batch anaerobic digestion at both mesophilic and thermophilic conditions. The optimal pretreatment for OPEFB was found to be with 6% oxygen at mesophilic condition resulting in 43% increase in methane yield over the control while at thermophilic conditions, the ultimate methane yield was ultimately lower, but received within a shorter time period. The gate-to-gate process was analyzed for global warming potential impacts and it was concluded that for the production of methane at the optimal condition resulted in the lowest greenhouse gas emissions (30% less than the control).

Establishment and differential performance of hyperthermophilic microbial community during anaerobic self-degradation of waste activated sludge

Hyperthermophilic anaerobic digestion, especially at 70 °C, has drawn wide attention. In order to acquire the inoculum and digestion characteristics, batch acclimation and continuous operation experiments were conducted under hyperthermophilic (70 °C), thermophilic (55 °C) and mesophilic (35 °C) conditions, respectively. Archaea at each temperature was successfully enriched from the sole-source waste activated sludge (WAS). Hyperthermophilic digestion achieved higher archaea diversity, close to the Shannon index 2.23 for the thermophilic digestion, but the population were not improved, at a 16S rRNA genes 5.99 × 105 copies mL−1. Hydrogenotrophic methanogens, Methanospirillum and Methanothermobacter, dominated in the hyperthermophilic digester, accounting for 27.15%, while the primary phylum Firmicutes was promoted to 36.31%, with the proteolytic genus Coprothermobacter in Firmicutes at 19.50%. Refractory organic fractions were converted more with a higher digestion temperature, which was demonstrated by the fact that the COD/VS increased to 5.8, 5.2 and 4.2 at 70 °C, 55 °C and 35 °C, respectively, at the end of batch acclimation. In addition, the most solubilization for the dominant fraction protein in the WAS occurred at 70 °C as well. Similar hydrolysis ratio, over 10%, and specific hydrolysis rate, around 0.025 g COD (g VSS·d)−1, were achieved at 70 °C and 55 °C. The higher hydrolysis for hyperthermophilic digestion even resulted in a higher methane yield than that for the mesophilic digestion. Nevertheless, contrary to higher hydrolysis, methanogenesis limited hyperthermophilic digestion in WAS degradation, with an ultimate methane yield 71.2 mL g−1 VSadded, despite an almost complete VFA conversion through the continuous operation.

Influence of electrostatic field and conductive material on the direct interspecies electron transfer for methane production

The influence of electrostatic field on the direct interspecies electron transfer (DIET) pathways for methane production was investigated in a batch bioelectrochemical anaerobic digester (BEAD). The ultimate methane production and methane yield in the BEAD reactor saturated to 925 ± 29 mL/L and 309.9 ± 9.6 mL CH4/g COD, respectively, which were much higher than 616 ± 3 mL/L and 205.4 ± 205.4 mL CH4/g COD in the anaerobic digester (AD). In the cyclic voltammogram (CV) for bulk solution, the oxidation peak current was 0.52 mA in the BEAD reactor, which was higher than 0.24 mA of AD reactor. This shows that the oxidizing ability of microorganisms was greatly improved in the BEAD reactor. Anaerolineaceae, a well-known electroactive bacterial family, was well enriched in the BEAD reactor. It indicates that the electrostatic field can enrich the electroactive bacteria and activate the DIET pathways for methane production. Moreover, the conductive material (activated carbon) further improved the performance of BEAD reactor, implies that the conductivities of bulk solution is one of the important parameters for the DIET pathways.

Improving Inter-Laboratory Reproducibility in Measurement of Biochemical Methane Potential (BMP)

Biochemical methane potential (BMP) tests used to determine the ultimate methane yield of organic substrates are not sufficiently standardized to ensure reproducibility among laboratories. In this contribution, a standardized BMP protocol was tested in a large inter-laboratory project, and results were used to quantify sources of variability and to refine validation criteria designed to improve BMP reproducibility. Three sets of BMP tests were carried out by more than thirty laboratories from fourteen countries, using multiple measurement methods, resulting in more than 400 BMP values. Four complex but homogenous substrates were tested, and additionally, microcrystalline cellulose was used as a positive control. Inter-laboratory variability in reported BMP values was moderate. Relative standard deviation among laboratories (RSDR) was 7.5 to 24%, but relative range (RR) was 31 to 130%. Systematic biases were associated with both laboratories and tests within laboratories. Substrate volatile solids (VS) measurement and inoculum origin did not make major contributions to variability, but errors in data processing or data entry were important. There was evidence of negative biases in manual manometric and manual volumetric measurement methods. Still, much of the observed variation in BMP values was not clearly related to any of these factors and is probably the result of particular practices that vary among laboratories or even technicians. Based on analysis of calculated BMP values, a set of recommendations was developed, considering measurement, data processing, validation, and reporting. Recommended validation criteria are: (i) test duration at least 1% net 3 d, (ii) relative standard deviation for cellulose BMP not higher than 6%, and (iii) mean cellulose BMP between 340 and 395 NmLCH4 gVS−1. Evidence from this large dataset shows that following the recommendations—in particular, application of validation criteria—can substantially improve reproducibility, with RSDR &lt; 8% and RR &lt; 25% for all substrates. The cellulose BMP criterion was particularly important. Results show that is possible to measure very similar BMP values with different measurement methods, but to meet the recommended validation criteria, some laboratories must make changes to their BMP methods. To help improve the practice of BMP measurement, a new website with detailed, up-to-date guidance on BMP measurement and data processing was established.

Optimization of substrate composition in anaerobic co-digestion of agricultural waste using central composite design

Biochemical methane potential (BMP) tests and modelling of anaerobic co-digestion (AcoD) of ternary mixtures of agricultural waste, i.e., sugar beet root waste (BW), cow dung (CD), and poultry manure (PM), are presented. According to a two-factor central composite design (CCD), 12 BMP tests were conducted under mesophilic conditions for 30 days (d). CD mass fraction (0.146–0.854) in the animal manure mixture and C/N ratio (13.515–30.485) of ternary waste mixture were selected as process factors. Experimental values of ultimate cumulative methane yield as process response were in the range of 105.32–356.10 mL/g VS. A first-order rate model was applied to predict the process kinetics. Enhanced values of methane production rate, evaluated as rate constant (0.044–0.123 d−1), were obtained for lower levels of CD mass fraction and higher levels of C/N ratio. Response surface methodology (RSM) was applied to assess the effects of process factors on the response and to find the optimum factor levels leading to a maximum performance. A maximum value of response variable of 347.48 mL/g VS was predicted for a CD fraction of 0.323 and a C/N ratio of 26.24. Values of experimental ultimate methane yield of 358.45 ± 33.40 mL/g VS, obtained in the BMP tests at optimum levels of process factors, were close to that of the maximum response predicted by the statistical model, demonstrating the model validity. Moreover, a significant synergistic effect (an increase in methane yield by 41.2%) related to the weighed experimental BMP of substrate mixture was obtained under optimum conditions.

Energy Recovery from Anaerobic Digestion of Banana Peels

Bananas are tropical fruits mostly eaten in Malaysia. The banana peels are high in organic, and putrescible caused the odour and leachate problem where it has been a dump. In practice, banana peels considered as a waste product that has been combined with municipal solid waste and dumped into the landfills. However, banana peels are bountiful in organic matter and high with moisture content. Thus, it could be a convincing substrate for biogas production through anaerobic digestion so that the major concerns of environmental protection is achieved aside from producing energy in a sustainable way. Therefore, this study was initiated to estimate the ultimate methane yield from the unripe banana peel (UBP) and ripe banana peel (RBP). Besides that, the assessment on the kinetics of the methane production from UBP and RBP is conducted using Modified Gompertz and first-order kinetic modelling. In this study, the anaerobic digestibility of banana peels measured in a batch reactor for 25 days each fed by UBP and RBP. The batch reactors operated at an inoculum to substrate ratio (I/S) of 1.0 and at a mesophilic temperature (37°C). The ultimate methane yields from UBP and RBP digestion were 847.57mLCH4/gVS and 1405.31mLCH4/gVS, respectively. The higher bioavailability (in term of COD, and solid) in RBP resulted in the higher methane production rate. Two first-order and modified Gompertz kinetic models were compared for the prediction of organic degradation, and the results indicated that the first-order kinetic model of the RBP fitted the experiment best. It concluded that ripe banana peels are the most preferable feedstock for the anaerobic digestion.

Mesophilic Anaerobic Co-Digestion of Fruit and Vegetable Wasteand Domestic Primary Sewage Sludge: Performance and Kinetic

Wastes such as fruit and vegetable waste and sewage sludge are not easy to be manage because of their chemical and physical properties. These waste also high putrescible. Recent years, landfill disposal is no longer a sustainable way for waste management as the area of landfill has become more limited. Therefore, new approach such anaerobic digestion should be considered for these wastes. However, the mono-digestion has a drawback, including lesser methane yield due to the substrate characteristics. To overcome this problem, the co-digestion is introduced, in which two or more substrate feed to the digester concurrently. This study aims to evaluate the performance and the kinetics from the co-digestion of fruit and vegetable waste with domestic primary sewage sludge. In this study, the bio-methane potential assay (BMP) at batch mode is conducted. The BMP assay is carried out at the inoculum to substrate (I/S) ratio of 2.0 at 37oC. The kinetics analysis was included using Modified Gompertz Modelling. The characteristics study showed that the complex organic compounds existed in form of particulate mostly. The methane production was stopped at day 12 resulted in the ultimate methane yield of 1149.50 mLCH4/gVS. In addition, the methane kinetics parameter observed from laboratory work slightly different from what was observed from the modelling.

Two-phase anaerobic digestion of lignocellulosic hydrolysate: Focusing on the acidification with different inoculum to substrate ratios and inoculum sources

Biogas production from lignocellulosic hydrolysate is of great potential for lignocellulosic materials. Two-phase anaerobic digestion was proposed in this study. Acidogenic fermentation was carried out with corn straw hydrolysate as feedstock for volatile fatty acids (VFAs) production. Using anaerobic sludge (AnS), different inoculum to substrate ratios (ISRs) of 0.5:1, 1:1 and 2:1 were investigated. The highest VFAs yield was obtained at ISR of 0.5:1.VFAs composition analysis showed that butyric acid was the predominant acid, followed by acetic acid and propionic acid. The effects of AnS and aerobic sludge (AeS) on the acidogenic performance of hydrolysate were compared. The optimum VFA yields were 0.38 g/g COD-added for AnS and 0.32 g/g COD-added for AeS with HRT of 5 d, respectively. The bacterial diversities of inocula and digestates were analyzed by high-throughput sequencing. Two origins of inocula had distinct bacterial structures, but they did share core communities that included Firmicutes, Chloroflexi, Proteobacteria and Bacteroidetes at phylum level. The bacterial communities of both digestates changed significantly as compared with those in inoculum. Firmicutes was absolutely dominant in all the bacterial species. Therefore, the AeS could be an option as the acidogenic inoculum. The microbial information will be beneficial for the enrichment and acclimatization of microbes. In methanogenic process, VFAs obtained in acidogenic stage could be efficiently converted into methane. The ultimate methane yield at organic loading rate (OLR) of 8 g/L·d could reach 290 mL CH4/g COD-added and 279 mL CH4/g COD-added for AnS and AeS acidified digestate. Two-phase anaerobic digestion was proved to be suitable for bioconversion of lignocellulosic hydrolysate into biogas.

The effect of temperature on the anaerobic co-digestion of poultry droppings and sugar mill press mud

Press mud (PM), generated from sugar mills, and poultry droppings (PD) are treated as waste materials and may be a solution for a country’s energy deficit in that their anaerobic co-digestion can produce high-value bio-energy in the form of biogas. Both a batch experiment and continuous experiments in continuous stirred tank reactor (CSTR) were conducted to determine the optimal temperature from the mesophilic temperature regimes in terms of methane yield. Also studied was the variability of other major parameters at varying temperatures with co-digestion of PM and PD in microbial consortia. The average highest ultimate methane yield and specific methane yields were 325 and 260 normalized litre per kg volatile solids (NL kg−1 VS) for the batch and continuous experiments, respectively. The maximum average specific methane yields were obtained at 35–45 °C, as expected, but yields did not differ significantly. The rate of methane production was relatively lower at temperatures of less than 35 °C and significant yield differences were found among temperatures of 35, 30, 25 and 20 °C. The results show that the temperature influenced the anaerobic digestion performance significantly. The optimum temperature was predicted using MINITAB-18 software according to the best-fit regression models, and modeling showed the optimal temperature was 44 °C.

Coupling energy-production processes: The use of residues from bioethanol production to improve the anaerobic digestion of corn stover

Currently, industry is migrating towards sustainable practises, where the wastes produced in one process are used as feedstock for other productive purposes. In the present study, we show that a by-product from the fabrication of second-generation bioethanol can be used to improve the production of biogas from corn stover. The washing water generated during the pretreatment stage of bioethanol production was recovered and reused as a co-substrate for the anaerobic digestion of corn stover. This water contained high concentrations of lignocellulolytic enzymes that once added to the digester improved the methane production kinetics. In comparison with digestion in the absence of these extra enzymes, the ultimate methane yield (P) increased 42%, with a reduction in the lag-phase time (λ) of 67% and an increase in the methane production rate (Rm) of almost 30%. These results were compared with those obtained during the co-digestion of corn stover and cattle manure, which is a well-known strategy to improve methane yields from lignocellulosic residues. However, despite the fact that the use of cattle manure improved methane yields, the results obtained using the enzyme-rich washing water were significantly better. The present study describes a simple strategy for the revalorisation of a by-product from the production of second-generation bioethanol.

Kinetics Modelling of Batch Anaerobic Co-digestion of Domestic Primary Sewage Sludge and Food Waste in a Stirred Reactor

Anaerobic digestion was conventionally applied for treating sewage sludge. However, the accumulation of solid waste particularly food waste has reach the critical levels worldwide. In practice, the food waste was dumped into the landfill for ultimate disposal. However, the greenhouse gases produced in the landfill makes this is no longer a preferable option. Anaerobic digestion was seen as an alternative for managing the food waste in a sustainable way. Methane, a renewable energy is potentially in replacing fossil fuel. However, the methane yield from the digestion of food waste inefficient. Therefore, a study of the co-digestion of sewage sludge and food waste was conducted to investigate the improvement of the methane yield. This study was conducted by using a mixture of domestic primary sewage sludge and food waste as a co-substrate for the anaerobic digester. The kinetics modified Gompertz modelling was applied to describe the anaerobic digestion process. A series of batch biochemical methane potential (BMP) assay was prepared using Automatic Methane Potential Test System (AMPTS II) to investigate the anaerobic digestibility of the mixture of domestic primary sewage sludge and food waste. The BMP assay showed that the co-digestion improved the ultimate methane yield by 32.6% higher than domestic primary sewage sludge alone, indicated that the co-substrate characteristics influencing the methane yield. Besides that, the greater VS/TS ratio of the substrate also resulted in the greater methane yield. The kinetics parameter from the modelling analysis were slightly lower as compared to the laboratory data.

Co-digestion of Wastewater-Grown Filamentous Algae With Sewage Sludge Improves Biomethane Production and Energy Balance Compared to Thermal, Chemical, or Thermochemical Pretreatments

Wastewater algal treatment systems show improved economic viability and enhanced energy return on investment if integrated with biofuel production. One option is to anaerobically digest the algae to generate bio-methane. This method is appropriate for relatively low lipid filamentous algae typical of turf scrubbers®. But unbalanced carbon-to-nitrogen (C/N) ratio and the resistance of algae to biodegradation can limit biomass conversion into bio-methane. To evaluate options to enhance bio-methane production, an indigenous assembly of macro-algae was established and cultivated in CO2-infused secondary wastewater effluent, then harvested and either anaerobically digested using pretreatments or co-digested with sewage sludge. Results were used to develop methane production kinetic models and perform an AD system energy balance analysis to assess the feasibility of pretreatment and co-digestion for a scaled process. Floways were dominated by Ulothrix and Oedogonium algae and had periphyton biomass production rates that averaged 3.7±0.4 g VS m-2 d-1 (±1SD) during the initial 7-day colonization period. Biomass increased by 62% in the second half of the 14-day experiment. Ultimate methane yield from harvested biomass was improved relative to controls (30613 mL gVS-1) through thermal pretreatment by 15%, dilute acid by 5%, dilute alkali by 17%, acid- and alkali-assisted thermochemical pretreatments by 23 and 27%, respectively. However, all pretreatment methods undermined the energy balance parameters including Net Energy Ratio (NER) and Net Energy Efficiency (NEE) due to the heat required for thermal pretreatments and electricity needed to produce chemical reagents. In contrast, co-digestion of algal biomass with sewage sludge enhanced methane generation yielding up to 4013 mL gVS-1 at algae to sludge ratio of 20% to 80%. Co-digestion with sludge also strongly improved AD system energy balance. NER and NEE increased from 2.8 and 73% for algae alone to 4.3 and 81% for a 20%:80% algae to sludge mix respectively. Moreover, the Net Energy Recovery during co-digestion reached 39% compared to 26% and 33% when algae or sewage sludge were processed as single-substrates. Thus, co-digestion of algae with sewage sludge serves as an attractive option for maximizing energy gain from AD of biomass harvested from filamentous algal treatment systems.