Semi-continuous anaerobic digestion of solid poultry slaughterhouse waste: effect of hydraulic retention time and loading

Influence of substrate/inoculum ratio, inoculum source and ammonia inhibition on anaerobic digestion of poultry waste

Poultry wastes are rich in organic matter, allowing their use as substrates for biogas production by anaerobic digestion (AD). The major difficulty in the anaerobic digestion of this protein-rich waste is ammonia inhibition. Different results of biochemical methane potential (BMP) were obtained after the mesophilic anaerobic digestion of different avian waste in batch mode. It was shown that using two different inoculum (Liger and Saint-Brieuc) sources and different substrate-to-inoculum (S/I) ratios does not have a significant effect on the biochemical methane potential of organic laying hen droppings (OLHD); an average of 0.272 Nm3 CH4·kg-1·VS was obtained with both inocula. Otherwise, it affects the hydrolysis constant KH, and it decreases when the substrate-to-inoculum ratio increases. Furthermore, Liger is the most suitable inoculum for our substrate because it shows stability during the process even with different organic loads. Comparing the biochemical methane potential of multiple avian wastes such as organic laying hen droppings and different slaughterhouse waste highlights the importance of slaughterhouse waste in the anaerobic digestion process because of the high methane yield observed especially with the viscera (0.779 Nm3 CH4·kg-1 VS, SD = 0.027 Nm3 CH4·kg-1 VS). Moreover, methane production was affected by increasing the ammonia concentrations; when [N-NH3] > 9.8 g·N-NH3·L-1, the biochemical methane potential decreases and the lag phase increases (λ > 30 days); a total inhibition of the process was observed when ammonia concentration is above 21.8 g·L-1.

Complex network analysis of slaughterhouse waste anaerobic digestion: From failure to success of long-term operation

The study explored slaughterhouse waste (SHW) as prime feedstock associated with and without supplement of an external slowly degradable lignocellulosic carbon source to overcome the synergistic co-inhibitions of ammonia and fatty acids. Long-term solid-state digestion (SSD) and liquid-state digestion (LSD) were investigated using a mixture of pork liver and fat. At 2.0 g volatile solids (VS) L^-1 d^-1 of organic loading rate (OLR), the two reactors of SSD experienced operational instability due to ammonia inhibition and volatile fatty acid (VFA) accumulation while LSD successfully produced 0.725 CH₄ L CH₄ g^-1VS during 197 d of working days under unfavorable condition with high total ammonia nitrogen (>4.7 g/L) and VFAs concentration (>1.9 g/L). The network analysis between complex microflora and operational parameters provided an insight for sustainable biogas production using SHW. Among all, hydrogenotrophic methanogens have shown better resistance than acetoclastic methanogens.

Slaughterhouse and poultry wastes: management practices, feedstocks for renewable energy production, and recovery of value added products

The slaughterhouse and poultry industry is possibly one of the fastest-growing sectors driven by the increasing demand in food availability. Subsequently, the wastes produced from the slaughterhouse and poultry industry are in huge quantities, which could be a promising resource for the recovery of value added products, and bioenergy production to minimize the dependence on fossil fuels. Furthermore, the wastes from slaughterhouses and poultry are a hub of pathogens that is capable of infecting humans and animals. This demands the emerging need for an effective and safe disposal method to reduce the spread of diseases following animal slaughtering. In light of that, the state of the production of slaughterhouse and poultry wastes was presented at first. Following this, the impact of solid waste exposure in terms of air, water, and soil pollution and the associated health challenges due to improper solid waste management practices were presented to highlight the importance of the topic. Secondly, the potency of these solid wastes and the various waste-to-energy technologies that have been employed for effective management and resource utilization of wastes generated from slaughterhouses and poultry were reviewed in detail. Finally, this review also highlights the opportunities and challenges associated with effective solid waste management, future requirements for the development of effective technologies for the recovery of value added products (like keratin, fibreboards), and biofuel production.

Experimental Investigation of Methane Generation in the Presence of Surface and Un-Surface Nanoparticles of Iron Oxide

The exploitation and harnessing of renewable energies are becoming increasingly important throughout the world. This study presents a method of methane (CH4) generation using biological disintegration of food waste (FW) by anaerobic digestion (AD). The CH4 production was enhanced by the addition of three different types of iron oxide (Fe3O4) nanoparticles (NPs) (Cetyletrimethlebromide (CTAB), urea-capped Fe3O4 NPs and Fe3O4 NPs without capping). The bio generation of CH4 and biodegradation of volatile solids (VS) were carried out in an AD treatment at mesophilic conditions (35–37 °C) for more than 50 days in batch mode. The concentration of all three types of NPs was kept constant at 75 mg/L. It was noticed that urea-capped NPs produced the maximum CH4 (5.386 L), followed by Fe3O4 NPs (5.212 L). Methane production in the control bioreactor was 2.143 L. The experimental results of CH4 generation (a dependent variable) were analyzed against the concentrations of NPs used (as independent variables) in multiple regression analysis (MRA). The overall model for the experiments resulted in R2 and R-adjusted values of 0.995 and 0.993, respectively.

Use of membranes for the treatment and reuse of water from the pre-cooling system of chicken carcasses

Poultry processing plants generate large amounts of wastewater in the many steps necessary to provide high quality and safe products. Carcass chilling is one of these steps, where the temperature of the carcass is reduced from 40°C to 4°C, for reducing the growth rate of microorganisms and affecting flavour, texture and appearance. In this operation, carcasses are continually displaced through a series of two tanks (called pre-chiller and chiller) filled with cold water, thus being responsible for a considerable amount of wastewater generation. This work aimed to regenerate the wastewater of the pre-chiller tank employing microfiltration (pore size 0.10 and 0.20 µm) and ultrafiltration (UF; MWCO 10 and 50 kDa) polymeric membranes in bench and pilot scales, with the final purpose of reuse. Membrane performance was evaluated in terms of the capacity of removing the contaminants and producing sufficient permeate flux in different working pressures. Bench-scale UF membrane presented the highest initial permeate flux of 112.1 L/m²h at 200 kPa. The four membranes tested presented good retention of microorganisms, with apparent rejection of up to 100%. Pilot-scale membranes presented better apparent rejection, with retentions above 99% for turbidity, apparent colour and fat content. Moreover, organic matter retention was also very high, up to 94% for chemical oxygen demand and 92% for total organic carbon. The use of membranes seems to be a promising approach for recycling and reuse of poultry pre-chiller wastewater.

Identification, overexpression, purification, and biochemical characterization of a novel hyperthermostable keratinase from Geoglobus acetivorans

The goal of this study was to identify and biochemically characterize a novel hyperthermostable keratinase from microorganisms for feather waste degradation. Here, a hyperthermophilic Geoglobus acetivorans keratinase (GacK) gene was chosen based on a search of a sequence database. The selected GacK gene was synthesized, cloned, and successfully expressed without a signal peptide in the E. coli system. A monomer of approximately 58 kDa was obtained in a soluble form and purified. The recombinant GacK displayed the highest activity at an optimum temperature of 100 °C and a pH of 10. The hyperthermostable GacK enzymatic performance remained high even after incubation in nonionic surfactants and the chelating agent EDTA. The residual and keratinolytic activities of GacK, as determined with azocasein and keratin azure used as substrates, remained significantly greater than 80% at 130 °C for 7 h. The kinetic parameters Km and Vmax for azure keratin were 0.41 mg/ml and 875.14 unit/mg, respectively, while those for azocasein were 1.51 mg/ml and 505.32 unit/mg, respectively. The results suggest that the enzyme is among the most hyperthermostable keratinases. Because of its enzymatic characteristics to degrade keratin azure at high temperatures, GacK may potentially be utilized in future industrial applications.

Genome-centric microbiome analysis reveals solid retention time (SRT)-shaped species interactions and niche differentiation in food waste and sludge co-digesters

Co-digestion of food waste with sewage sludge is widely applied for waste stabilization and energy recovery around the world. However, the effect of solid retention time (SRT) on the microbial population dynamics, metabolism and interspecies interaction have not been fully elucidated. Here, the influence of SRTs (5-25 days) on the performance of the co-digestion system was investigated and state-of-the-art genome-centric metagenomic analysis was employed to uncover the dynamics and metabolic network of the key players underlying the well-functioned and poorly-functioned co-digestion microbial communities. The results of the microbial analyses indicated that SRT largely shaped microbial community structure by enriching the syntrophic specialist Syntrophomonas and CO₂/H₂ ( formate)-using methanogen Methanocorpusculum in the well-functioned co-digester operated at SRT of 25 days, while selecting acid-tolerant populations Lactobacillus at SRT of 5 days. The metagenome assembled genomes (MAGs) of key players, such as Syntrophomonadaceae, Methanocorpusculum, and Mesotoga, were retrieved, additionally, the syntrophic acetate oxidation plus hydrogenotrophic methanogenesis (SAO-HM) were proposed as the dominant pathway for methane production. The metabolic interaction in the co-digestion microbial consortia was profiled by assigning MAGs into functional guilds. Functional redundancy was found in the bacterial groups in hydrolysis step, and the members in these groups reduced the direct competition by niche differentiation.

Recuperative thickening for sludge retention time and throughput management in anaerobic digestion with thermal hydrolysis pretreatment

This study evaluated the application of recuperative thickening (RT) to enhance anaerobic digestion (AD) performance for AD systems with thermal hydrolysis pretreatment (THP). RT was applied for two different reasons: (a) for increasing the sludge retention time (SRT) to degrade slowly hydrolyzable materials more efficiently and (b) for maintaining SRT at decreased hydraulic retention time (HRT) thus showing potential for increased AD throughput rates. A SRT increase from 15 to 30 days by RT application did not improve AD performance or hydrolysis rates significantly as 15-day SRT was already a factor 2 higher than the estimated washout SRT. When applying RT to increase throughput rates (HRT of 7 days) while maintaining 15-day SRT, no negative impact on biogas production or hydrolysis kinetics was observed. It was estimated that RT application on THP digesters can increase digester throughput by 100% and thus show clear potential for further AD intensification. PRACTITIONER POINTS: Increased SRT from 15 to 30 days through recuperative thickening application did not improve biogas production. A lower required minimum SRT (6-7 days) was estimated in THP-AD systems compared to conventional AD. Operation at decreased HRT by RT application resulted in similar AD performance under constant organic loading rates. A 100% increase in throughput rates can be applied using RT without decreasing AD performance.

Evaluation of recycled paper mill effluent digestion in a modified anaerobic hybrid baffled (MAHB) reactor: reactor performance & kinetic studies

Recycled paper mill effluent (RPME) consists of various organic and inorganic compounds. In this study, modified anaerobic hybrid baffled (MAHB) bioreactor has been successfully used to anaerobically digest RPME. The anaerobic digestion was investigated in relation to methane production rate, lignin removal, and chemical oxygen demand (COD) removal, with respect to organic loading rate (OLR) and hydraulic retention time (HRT). The analysis using kinetic study was carried out under mesophilic conditions (37 ± 2 °C) and influent COD concentrations (1000-4000 mg L^-1), to prove its practicability towards RPME treatment. First-order kinetic model was used to clarify the behavior of RPME anaerobic digestion under different OLRs (0.14-4.00 g COD L^-1 d^-1) and HRT (1-7 d). The result shows that the highest COD removal efficiency and methane production rate were recorded to be 98.07% and 2.2223 L CH₄ d^-1, respectively. This result was further validated by evaluating the biokinetic coefficients (reaction rate constant (k) and maximum biogas production (y_m)), which gave values of k = 0.57 d^-1 and y_m = 0.331 L d^-1. This kinetic data concludes that MAHB presented satisfactory performance towards COD removal with relatively high methane production, which can be further utilized as on-site energy supply.

Anaerobic digestion of lipid-rich swine slaughterhouse waste: Methane production performance, long-chain fatty acids profile and predominant microorganisms

This study investigated methane production, long-chain fatty acids (LCFAs) profile, and predominant microorganisms in anaerobic digestion (AD) of lipid-rich swine slaughterhouse waste (SSW). The maximum methane yield was 999.2 mL/g VS. LCFAs, as inhibitory hydrolysis products, accumulated first to 1165 mg/L on day 3, and then decreased sharply to 125.7 mg/L on day 9, and finally were degraded to 20 mg/L on day 27. Linoleic acid (C18:2), oleic acid (C18:1) and palmitic acid (C16:0) were the dominant LCFAs. The easy conversion of C18:1 to C16:0 compared with difficult degradation of C16:0 resulted in an increase of C16:0 on day 4-6. Predominant microorganisms were Clostridium, Syntrophomonas and Methanospirillum. This study proved the high methane potential of lipid-rich SSW and gained insights into the degradation process by analysis of intermediates of LCFAs and predominant microorganisms. The results can provide valuable guidance for efficient utilization of this waste to produce methane in future.

Modeling of anaerobic digestion of slaughterhouse wastes after thermal treatment using ADM1

According to the European legislation, thermal treatment of category 2 slaughterhouse by-products at 140 °C, 4-5 bar for 20 min is obligatory for their hygienization prior to disposal. This process is known as "rendering". The product of the rendering process is rich in lipids and proteins making it an appropriate feedstock for biogas plants. The mathematical modeling of biogas production from slaughterhouse wastes after the rendering process has been studied adjusting the anaerobic digestion model (ADM1). For this purpose, two mesophilic (38-39 °C) continuous stirred tank reactors (CSTRs) have been operated in parallel under a hydraulic retention time of 21.5 ± 2.14 d, while the organic load was increased from 50 to 149.6 g COD L^-1. Recirculation of the mixed liquor suspended solids (MLSS) took place in one of the CSTRs, resulting in a different solids' concentration in it. The ADM1 was calibrated by estimating key kinetic parameters, such as the maximum specific consumption rate constant and the half-saturation constants of volatile fatty acids and verified. The degradation kinetics of this type of waste seemed to be faster, as a result of its emulsification through rendering, while the coefficient yields of the acidogens were lower than the default values of ADM1. The structure of the model was proven suitable for predicting the response of both bioreactors under small or medium step transitions, but not for abrupt impulse disturbances in the organic loading rate.

Effect of Organic Loading Rate on Anaerobic Digestion Performance of Mesophilic (UASB) Reactor Using Cattle Slaughterhouse Wastewater as Substrate

In this study, the performance of a laboratory scale upflow anaerobic sludge blanket (UASB) reactor operating at mesophilic temperature (35 °C) was examined. Cattle slaughterhouse wastewater (CSWW) was used as the main substrate. The total and effective volumes of the reactor were 8 L and 6 L, respectively. Twelve different organic loading rates (OLR) were applied and the performance was evaluated. The chemical oxygen demand (COD) removal efficiency was more than 90% during batch study. In the continuous study, COD removal was also approximately 90% at OLR 0.4 g/L d⁻¹ which subsequently dropped to below 50% when the loading rate increased to 15 g/L d⁻¹. Approximately 5 L/d of biogas was obtained with high methane concentration at stages VI and XI corresponding to OLR of 2 and 10 g/L d⁻¹, respectively. It was observed that the concentration of volatile fatty acids was low and that the alkalinity of the wastewater was sufficient to avoid acidification. Specific methane yields of 0.36 and 0.38 LCH₄/g COD added were achieved at OLR 7 and 10 g/L d⁻¹. A hydraulic retention time (HRT) of 1 day was sufficient to remove greater than 70% of COD which correspond to 89% methane concentration. Parameters like soluble COD, NH₃-N, pH, alkalinity, total suspended solid (TSS), fats, oil, and grease were also investigated. The results show that the UASB reactor could serve as a good alternative for anaerobic treatment of CSWW and methane production.

Anaerobic digestion of thermal pre-treated emulsified slaughterhouse wastes (TESW): Effect of trace element limitation on process efficiency and sludge metabolic properties

Slaughterhouse solid wastes, characterized by a high lipid content, are considered a valuable resource for energy production by means of anaerobic digestion technologies. Aim of this study was to examine the effect of trace element limitation on the mesophilic anaerobic digestion of thermally pre-treated emulsified slaughterhouse wastes (TESW). Under two distinct experimental periods (Period I - low and Period II - high trace element dosage respectively) a CSTR with sludge recirculation was operated at increasing organic loading rate (OLR) from 1.5 to 10 g L^-1 d^-1. Under optimum conditions, COD removal was higher than 96%, biogas yield equal to 0.53 L g^-1 COD feed and the biogas methane content 77%. Trace element limitation however, resulted in a dramatic decline in process efficiency, with VFA accumulation and events of extreme sludge flotation, despite that the soluble concentration of Ni, Co and Mo were between 12 and 28 μg L^-1. This is indicative of mass transfer limitations caused by lipids adsorption onto the anaerobic biomass.

The influence of the total solid content on the stability of dry-thermophilic anaerobic digestion of rice straw and pig manure

Dry anaerobic digestion is a promising technology for the recycling of agricultural waste to produce energy and fertilizer. Adding water to the substrate enables better handling and avoid inhibition caused by high total solid (TS) content in the reactor; however, it also increases leachate and operational costs. To assess the extent to which the amount of water added can be reduced, it was examined how the TS content in the reactor influenced the production of biogas. A semi-batch dry thermophilic anaerobic digester was fed with substrate (rice straw and pig manure) at a constant organic loading rate, and varied the TS contents (27%, 32%, 37%, and 42%) of the substrate by adding different amounts of water (representing 0-36% of the total substrate). During incubation, the TS content in the reactor gradually increased from 18% to 31%. Biogas production was stable and high (564 ± 13-580 ± 36 N m³ t^-1 VS), and there was no accumulation of volatile fatty acids when the TS content of the reactor was between 18% and 27%. However, the rate decreased sharply and propionate and acetate were also produced when the TS content of the reactor exceeded 28%. By applying a simple TS balance model, it was found that stable biogas production could be achieved at a substrate TS content of 32%, at which reactor TS content reached 23% at steady-state condition.

Microbial Keratinases: Enzymes with Promising Biotechnological Applications

Keratin is a complex and structurally stable protein found in human and animal hard tissues, such as feathers, wool, hair, hoof and nails. Some of these, like feathers and wool, represent one of the main sources of protein-rich waste with significant potential to be transformed into value-added products such as feed, fertilizers or bioenergy. A major limitation impeding valorization of keratinous substrates is their recalcitrant structure and resistance to hydrolysis by common proteases. However, specialized keratinolytic enzymes produced by some microorganisms can efficiently degrade these substrates. Keratinases have already found a purpose in pharmaceutical, textile and leather industries. However, their wider implementation in other processes, such as cost-effective (pre)treatment of poultry waste, still requires optimization of production and performance of the available enzymes. Here we present a comprehensive review covering molecular properties and characteristics of keratinases, their classification, traditional and novel approaches in discovery of novel enzymes, production, characterization, improvement and biotechnological applications.

Effect of hydraulic retention time on deterioration/restarting of sludge anaerobic digestion: Extracellular polymeric substances and microbial response

In this study, the transformation of the sludge-related extracellular polymeric substances (EPS) during mesophilic anaerobic digestion was characterized to assess the effect of hydraulic retention time (HRT) on reactor deterioration/restarting. Experimental HRT variations from 20 to 15 and 10d was implemented for deterioration, and from 10 to 20d for restarting. Long-term digestion at the lowest HRT (10d) resulted in significant accumulation of hydrolyzed hydrophobic materials and volatile fatty acids in the supernatants. Moreover, less efficient hydrolysis of sludge EPS, especially of proteins related substances which contributed to the deterioration of digester. Aceticlastic species of Methanosaetaceae decreased from 36.3% to 27.6% with decreasing HRT (20-10d), while hydrogenotrophic methanogens (Methanomicrobiales and Methanobacteriales) increased from 30.4% to 38.3%. Proteins and soluble microbial byproducts related fluorophores in feed sludge for the anaerobic digester changed insignificantly at high HRT, whereas the fluorescent intensity of fulvic acid-like components declined sharply once the digestion deteriorated.

Long chain fatty acids (LCFA) evolution for inhibition forecasting during anaerobic treatment of lipid-rich wastes: Case of milk-fed veal slaughterhouse waste

A detailed study of a solid slaughterhouse waste (SHW) anaerobic treatment is presented. The waste used in this study is rich in lipids and proteins residue. Long chain fatty acids (LCFA), coming from the hydrolysis of lipids were inhibitory to anaerobic processes at different degrees. Acetogenesis and methanogenesis processes were mainly affected by inhibition whereas disintegration and hydrolysis processes did not seem to be affected by high LCFA concentrations. Nevertheless, because of the high energy content, this kind of waste is very suitable for anaerobic digestion but strict control of operating conditions is required to prevent inhibition. For that, two inhibition indicators were identified in this study. Those two indicators, LCFA dynamics and LCFA/VS_biomass ratio proved to be useful to predict and to estimate the process inhibition degree.

Optimization of the anaerobic co-digestion of pasteurized slaughterhouse waste, pig slurry and glycerine

The feasibility of co-digestion of blends of two different animal by-products (pig manure and pasteurized slaughterhouse waste) and recovered glycerine was studied in mesophilic conditions. Experiments were performed in a lab-scale CSTR along 490days, with a hydraulic retention time of 21-33days and with a step-wise increased organic loading rate, by adding and/or changing the wastes ratio, from 0.8 to 3.2kg_CODm^-3d^-1. The best methane production rate (0.64Nm³_CH4m^-3d^-1) represented an increment of 2.9-fold the initial one (0.22Nm³_CH4m^-3d^-1 with pig manure solely). It was attained with a ternary mixture composed, in terms of inlet volatile solids, by 35% pig slurry, 47% pasteurized slaughterhouse waste and 18% glycerine. This blend was obtained through a stepwise C/N adjustment: this strategy led to a more balanced biodegradation due to unstressed bacterial populations through the performance, showed by the VFA-related indicators. Besides this, an improved methane yield (+153%) and an organic matter removal efficiency (+83%), regarding the digestion of solely pig slurry, were attained when the C/N ratio was adjusted to 10.3.

Identification of two new keratinolytic proteases from a Bacillus pumilus strain using protein analysis and gene sequencing

The Bacillus strain (CCUG 66887) has a high capacity to excrete keratinase with the ability to degrade both alpha- and beta keratin. In this study we aimed to show the characteristics of the keratinolytic protease and to identify its gene by using liquid chromatography–electrospray ionization tandem mass spectrometry methods (nanoHPLC–ESI–MS/MS) followed by Mascot data base search. The results showed that the enzyme in fact consists of two different keratinases, both with a molecular mass of 38 kDa. Further, DNA sequencing generated the open reading frame (ORF) of one of the genes (Ker1), and de novo genome sequencing identified the ORF of the second gene (Ker2). The two keratinase genes contain 1153 base pairs each and have a gene similarity of 67 %. In addition, the Bacillus strain was classified as Bacillus pumilus and its genes were annotated in the GeneBank at NCBI (accession: CP011109.1). Amino acid sequences alignment with known B. pumilus proteases indicated that the two keratinases of B. pumilus strain C₄ are subtilisin-like serine proteases belonging to the Protease S8 family. Taken together, these result suggest the two keratinases as promising candidates for enzymatic processing of keratinous wastes in waste refinery.

Effect of temperature on continuous dry fermentation of swine manure

Laboratory-scale experiments were performed on the dry digestion of solid swine manure in a semi-continuous mode using 4.5 L down plug-flow anaerobic reactors with an organic loading rate of 3.46 kg volatile solids (VS) m(-3) d(-1) to evaluate the effects of temperature (15, 25 and 35 °C). At 15 °C, biogas production was the poorest due to organic overload and acidification, with a methane yield of 0.036 L CH4 g(-1) VS added and a volumetric methane production rate of 0.125 L CH4 L(-1) d(-1). The methane yield and volumetric methane production rate at 25 °C (0.226 L CH4 g(-1) VS added and 0.783 L CH4 L(-1) d(-1), respectively) were 6.24 times higher than those at 15 °C. However, the methane yield (0.237 L CH4 g(-1) VS added) and the volumetric methane production rate (0.821 L CH4 L(-1) d(-1)) at 35 °C were only 4.86% higher than those at 25 °C, which indicated similar results were obtained at 25 °C and 35 °C. The lower biogas production at 35 °C in dry digestion compared with that in wet digestion could be attributed to ammonia inhibition. For a single pig farm, digestion of solid manure is accomplished in small-scale domestic or small-farm bioreactors, for which operating temperatures of 35 °C are sometimes difficult to achieve. Considering biogas production, ammonia inhibition and net energy recovery, an optimum temperature for dry digestion of solid swine manure is 25 °C.

Metagenome from a Spirulina digesting biogas reactor: analysis via binning of contigs and classification of short reads

BACKGROUND

Anaerobic digestion is a biological process in which a consortium of microorganisms transforms a complex substrate into methane and carbon dioxide. A good understanding of the interactions between the populations that form this consortium can contribute to a successful anaerobic digestion of the substrate. In this study we combine the analysis of the biogas production in a laboratory anaerobic digester fed with the microalgae Spirulina, a protein rich substrate, with the analysis of the metagenome of the consortium responsible for digestion, obtained by high-throughput DNA sequencing. The obtained metagenome was also compared with a metagenome from a full scale biogas plant fed with cellulose rich material.

RESULTS

The optimal organic loading rate for the anaerobic digestion of Spirulina was determined to be 4.0 g Spirulina L(-1) day(-1) with a specific biogas production of 350 mL biogas g Spirulina (-1) with a methane content of 68 %. Firmicutes dominated the microbial consortium at 38 % abundance followed by Bacteroidetes, Chloroflexi and Thermotogae. Euryarchaeota represented 3.5 % of the total abundance. The most abundant organism (14.9 %) was related to Tissierella, a bacterium known to use proteinaceous substrates for growth. Methanomicrobiales and Methanosarcinales dominated the archaeal community. Compared to the full scale cellulose-fed digesters, Pfam domains related to protein degradation were more frequently detected and Pfam domains related to cellulose degradation were less frequent in our sample.

CONCLUSIONS

The results presented in this study suggest that Spirulina is a suitable substrate for the production of biogas. The proteinaceous substrate appeared to have a selective impact on the bacterial community that performed anaerobic digestion. A direct influence of the substrate on the selection of specific methanogenic populations was not observed.

Anaerobic digestion of the microalga Spirulina at extreme alkaline conditions: biogas production, metagenome, and metatranscriptome

A haloalkaline anaerobic microbial community obtained from soda lake sediments was used to inoculate anaerobic reactors for the production of methane rich biogas. The microalga Spirulina was successfully digested by the haloalkaline microbial consortium at alkaline conditions (pH 10, 2.0 M Na⁺). Continuous biogas production was observed and the obtained biogas was rich in methane, up to 96%. Alkaline medium acted as a CO₂ scrubber which resulted in low amounts of CO₂ and no traces of H₂S in the produced biogas. A hydraulic retention time (HRT) of 15 days and 0.25 g Spirulina L⁻¹ day⁻¹ organic loading rate (OLR) were identified as the optimal operational parameters. Metagenomic and metatranscriptomic analysis showed that the hydrolysis of the supplied substrate was mainly carried out by Bacteroidetes of the “ML635J-40 aquatic group” while the hydrogenotrophic pathway was the main producer of methane in a methanogenic community dominated by Methanocalculus.

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.

Comprehensive microbial analysis of combined mesophilic anaerobic-thermophilic aerobic process treating high-strength food wastewater

A combined mesophilic anaerobic-thermophilic aerobic process was used to treat high-strength food wastewater in this study. During the experimental period, most of solid residue from the mesophilic anaerobic reactor (R1) was separated by centrifugation and introduced into the thermophilic aerobic reactor (R2) for further digestion. Then, thermophilic aerobically-digested sludge was reintroduced into R1 to enhance reactor performance. The combined process was operated with two different Runs: Run I with hydraulic retention time (HRT) = 40 d (corresponding OLR = 3.5 kg COD/m(3) d) and Run II with HRT = 20 d (corresponding OLR = 7 kg COD/m(3)). For a comparison, a single-stage mesophilic anaerobic reactor (R3) was operated concurrently with same OLRs and HRTs as the combined process. During the overall digestion, all reactors showed high stability without pH control. The combined process demonstrated significantly higher organic matter removal efficiencies (over 90%) of TS, VS and COD and methane production than did R3. Quantitative real-time PCR (qPCR) results indicated that higher populations of both bacteria and archaea were maintained in R1 than in R3. Pyrosequencing analysis revealed relatively high abundance of phylum Actinobacteria in both R1 and R2, and a predominance of phyla Synergistetes and Firmicutes in R3 during Run II. Furthermore, R1 and R2 shared genera (Prevotella, Aminobacterium, Geobacillus and Unclassified Actinobacteria), which suggests synergy between mesophilic anaerobic digestion and thermophilic aerobic digestion. For archaea, in R1 methanogenic archaea shifted from genus Methanosaeta to Methanosarcina, whereas genera Methanosaeta, Methanobacterium and Methanoculleus were predominant in R3. The results demonstrated dynamics of key microbial populations that were highly consistent with an enhanced reactor performance of the combined process.

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.

Effect of hydraulic retention time (HRT) on the anaerobic co-digestion of agro-industrial wastes in a two-stage CSTR system

A two-stage anaerobic digestion system consisting of two continuously stirred tank reactors (CSTRs) operating at mesophilic conditions (37°C) were used to investigate the effect of hydraulic retention time (HRT) on hydrogen and methane production. The acidogenic reactor was fed with a mixture consisting of olive mill wastewater, cheese whey and liquid cow manure (in a ratio 55:40:5, v/v/v) and operated at five different HRTs (5, 3, 2, 1 and 0.75 d) aiming to evaluate hydrogen productivity and operational stability. The highest system efficiency was achieved at HRT 0.75 d with a maximum hydrogen production rate of 1.72 L/LRd and hydrogen yield of 0.54 mol H2/mol carbohydrates consumed. The methanogenic reactor was operated at HRTs 20 and 25 d with better stability observed at HRT 25 d, whereas accumulation of volatile fatty acids took place at HRT 20 d. The methane production rate at the steady state of HRT 25 d reached 0.33 L CH4/LRd.

Bacterial and methanogenic archaeal communities during the single-stage anaerobic digestion of high-strength food wastewater

Single-stage anaerobic digestion (AD) was operated to treat high-strength food wastewater (FWW) derived from food waste recycling facilities at two different organic loading rates (OLRs) of 3.5 (Phase I) and 7 (Phase II) kgCOD/m(3)d. Changes in composition of microbial communities were investigated using quantitative real-time PCR (qPCR) and barcoded-pyrosequencing. At the high FWW loading rate, AD showed efficient performance (i.e., organic matter removal and methane production). Bacterial communities were represented by the phyla Bacteroidetes, Firmicutes, Synergistetes and Actinobacteria. During the entire digestion process, the relative abundance phylum Chloroflexi decreased significantly. The qPCR analysis demonstrated that the methanogenic communities shifted from aceticlastic (Methanosarcinales) to hydrogenotrophic methanogens (Methanobacteriales and Methanomicrobiales) with high increase in the proportion of syntrophic bacterial communities. Canonical correspondence analysis revealed a strong relationship between reactor performance and microbial community shifts.

Effluents quality during the grow-out phase of the amazon shrimp macrobrachium amazonicum

In recent years shrimp culture farms have been one of the most growing sectors in aquaculture. Research has been carried out in order to establish a sustainable production maintaining profit and low environmental impact. Current investigation analyzed source and effluent water produced during the final grow-out phase of the Amazon shrimp (Macrobrachium amazonicum). Twelve natural-bottom ponds, with continuous water flow and stock density comprising 40, 60, 80 and 100 young shrimps/m² were analyzed. The experiment design comprised totally randomized blocks with four treatments and three replications. Microbiological analyses for Escherichia coli was attempted, coupled to physical and chemical analyses for pH, temperature, total suspended solids, total nitrogen, nitrite, nitrate, biochemical oxygen demand and chemical oxygen demand of water supply and pond effluents. Results show that whereas effluent quality complied with current legal rules, there was no significant difference between supply and effluent water for the analyzed variables and between stock densities. Under the conditions investigated and the intensification of culture in the final grow-out phase up to a density of 100 young shrimps/m², the production of M. amazonicum reveals low potential for environmental impact for the variables analyzed.

Influence of organic loading rate on methane production in a CSTR from physicochemical sludge generated in a poultry slaughterhouse

The influence of the increase of the organic loading rate (OLR) on methane production in a continuous stirred-tank reactor (CSTR) from physicochemical sludge generated in a poultry slaughterhouse was evaluated. Total solid (TS) to obtain OLR of 1, 5, 10 and 15 g VS L(-1) day(-1), with hydraulic retention times of 29, 6, 6 and 4, respectively, were conditioned. The results showed a decrease in pH levels and an increase in the theoretical volatile fatty acids (VFA). While the yield of methane production decreased from 0.48 to 0.10 LCH4/g VSremoved, respectively, the OLR-10 managed on average 38% removal of volatile solids (VS) and a yield biogas production of 0.81 Lbiogas g(-1) VSremoved and 1.35 L day(-1). This suggests that the OLR increases in an anaerobic system from physicochemical sludge only inhibits the methanogenic metabolism, because there is still substrate consumption and biogas production.

Decentralized application of anaerobic digesters in small poultry farms: performance analysis of high rate self mixed anaerobic digester and conventional fixed dome anaerobic digester

Biomethanation of poultry litter was studied in conventional fixed dome anaerobic digester (CFDAD) and high rate self mixed anaerobic digester (SMAD) for possible decentralized application in poultry farms generating litter in the range of 500 kg/day. The performance of CFDAD and SMAD was compared. The study revealed that optimized hydraulic residence time (HRT), volatile solids (VS) loading rate, VS reduction, methane yield was 24 days, 4.0 kg VS/m(3)/day, 64%, 0.15 m(3)/(kg VS fed) and 40 days, 2.15 kg/m(3)/day, 42%, 0.083 m(3)/(kg VS fed) for SMAD and CFDAD, respectively. Better results with SMAD could be attributed to specific design features and intermittent mixing of the digester contents due to self-mixing mechanism. Preliminary cost estimates revealed that installation of SMAD would be remunerative for the farmer in terms of biogas and bio-manure.

Anaerobic slurry co-digestion of poultry manure and straw: effect of organic loading and temperature

In order to obtain basic design criteria for anaerobic digestion of a mixture of poultry manure and wheat straw, the effects of different temperatures and organic loading rates on the biogas yield and methane contents were evaluated. Since poultry manure is a poor substrate, in term of the availability of the nutrients, external supplementation of carbon has to be regularly performed, in order to achieve a stable and efficient process. The complete-mix, pilot-scale digester with working volume of 70 L was used. The digestion operated at 25°C, 30°C and 35°C with organic loading rates of 1.0, 2.0, 2.5, 3.0, 3.5 and 4.0 kg Volatile solid/m3d and a HRT of 15 days. At a temperature of 35°C, the methane yield was increased by 43% compared to 25°C. Anaerobic co-digestion appeared feasible with a loading rate of 3.0 kg VS/m3d at 35°C. At this state, the specific methane yield was calculated about 0.12 m3/kg VS with a methane content of 53-70.2% in the biogas. The volatile solid (VS) removal was 72%. As a result of volatile fatty acid accumulation and decrease in pH, when the loading rate was less than 1 or greater than 4 kg VS/m3d, the process was inhibited or overloaded, respectively. Both the lower and higher loading rates resulted in a decline in the methane yield.

Biogas production by encased bacteria in synthetic membranes: protective effects in toxic media and high loading rates

A bioreactor including encased digesting bacteria for biogas production was developed, and its performance in toxic media and under high organic loading rates (OLRs) was examined and compared with traditional digestion reactors. The bacteria (3 g) were encased and sealed in 3 x 6 cm2 PVDF (polyvinylidene fluoride) membranes with a pore size of 0.1 microm, and then several sachets were placed in the reactors. They were then examined in toxic medium containing up to 3% limonene as a model inhibitor in batch reactors, and OLRs of up to 20 g COD/L.day in semi-continuous digestions. The free and encased cells with an identical total bacterial concentration of 9 g in a medium containing 2% limonene produced at most 6.56 and 23.06 mL biogas per day, respectively. In addition, the digestion with free cells completely failed at an OLR of 7.5 gCOD/L.day, while the encased cells were still fully active with a loading of 15 g COD/L x day.

454 pyrosequencing analyses of bacterial and archaeal richness in 21 full-scale biogas digesters

The microbial community of 21 full-scale biogas reactors was examined using 454 pyrosequencing of 16S rRNA gene sequences. These reactors included seven (six mesophilic and one thermophilic) digesting sewage sludge (SS) and 14 (ten mesophilic and four thermophilic) codigesting (CD) various combinations of wastes from slaughterhouses, restaurants, households, etc. The pyrosequencing generated more than 160,000 sequences representing 11 phyla, 23 classes, and 95 genera of Bacteria and Archaea. The bacterial community was always both more abundant and more diverse than the archaeal community. At the phylum level, the foremost populations in the SS reactors included Actinobacteria, Proteobacteria, Chloroflexi, Spirochetes, and Euryarchaeota, while Firmicutes was the most prevalent in the CD reactors. The main bacterial class in all reactors was Clostridia. Acetoclastic methanogens were detected in the SS, but not in the CD reactors. Their absence suggests that methane formation from acetate takes place mainly via syntrophic acetate oxidation in the CD reactors. A principal component analysis of the communities at genus level revealed three clusters: SS reactors, mesophilic CD reactors (including one thermophilic CD and one SS), and thermophilic CD reactors. Thus, the microbial composition was mainly governed by the substrate differences and the process temperature.

Thermophilic co-digestion of pig manure and crude glycerol: process performance and digestate stability

Anaerobic co-digestion has been widely used to enhance biogas production of digesters and, therefore, to improve the anaerobic plants economic feasibility. In the present study, glycerol, a by-product of the biodiesel industry, was used as a co-substrate for pig manure. The results showed that the thermophilic anaerobic co-digestion of pig manure supplemented with 3% of glycerol, on weight basis, was satisfactory. The specific biogas production of the co-digester was 180% higher than the one obtained by the reference digester, which was only fed with pig manure. The improvement was related to the double of the organic loading rate, the high biodegradability of the crude glycerol, the slight reduction of the free ammonia concentration and the optimisation of the carbon-to-nitrogen ratio. Moreover, the analysis of the organic matter (protein, lipids, carbohydrates and fibers) of the influent and the effluent of both digesters together with their biogas flow rates indicated that the microorganisms in the co-digester obtained large amounts of nutrients from the glycerol, whereas the microorganisms of the reference digester mainly produced biogas from the particulate matter. However, the digestate obtained from the co-digester cannot be directly applied as soil fertiliser or conditioner due to the presence of high levels of biodegradable matter, which may exert negative impacts on the plant-soil system. Thus, a longer hydraulic retention time, a reduction of the glycerol concentration and/or a post-treatment is required if the digestate is to be used as soil fertiliser or conditioner. In contrast, pig manure digestate can be directly applied on land.

A simple methodology for rate-limiting step determination for anaerobic digestion of complex substrates and effect of microbial community ratio

Anaerobic digestion (AD) of complex substrates is a multi-step process, which is kinetically controlled by an individual rate-limiting step. A methodology for determining the rate-limiting step during AD of complex substrates was developed by supplementation of metabolic intermediates from each digestion step with dairy manure as an emblematic complex substrate. This method elucidated that hydrolysis of dairy manure was the rate-limiting step when normal anaerobic sludge was used as inoculum. Furthermore, the concept and effect of microbial community ratio was introduced by manipulating two different inocula, i.e. normal anaerobic sludge and heated anaerobic sludge, so that varying ratios (r) of hydrolytic and methanogenic bacteria could be studied. Results revealed that the rate-limiting step changed with the variation of r. For dairy manure, results indicated a critical ratio r∗=24 between hydrolytic bacteria and methanogens, whereby as r decreased or exceeded from this value, hydrolysis or methanogenesis limited the AD process, respectively.

Effect of additives on process stability of mesophilic anaerobic monodigestion of pig slaughterhouse waste

The effect of two additives on anaerobic monodigestion of pig slaughterhouse waste was studied in three laboratory scale continuously stirred tank reactors (CSTRs) operated for 242-284 days at 35°C. A higher organic loading rate (OLR), namely 2.25 kg volatile solids (VS)/m(3)d with a hydraulic retention time (HRT) of 30 d was feasible with an additive containing Fe, HCl and trace elements while an OLR of 1.5 kg VS/m(3)d (HRT 30 d) led to instabilities in CSTR with an additive consisting of Fe and HCl and in CSTR without additives. Methane yields of ca. 700 dm(3) CH(4)/kg VS(fed) were obtained in all reactors under stable conditions. Both additives decreased H(2)S concentration of the biogas below detection limit 0.5 ppm while in a control reactor H(2)S concentration was up to 1900 ppm. In conclusion, the results suggest the possibility that additives may facilitate higher OLRs in monodigestion of pig slaughterhouse waste.

Anaerobic co-digestion of pig manure and crude glycerol at mesophilic conditions: biogas and digestate

Crude glycerol derived from biodiesel production is characterized by its high concentration of organic carbon and its solubility in water; properties that make it a suitable co-substrate to improve the efficiency of a manure digester. An increase of about 400% in biogas production was obtained under mesophilic conditions when pig manure was co-digested with 4% of glycerol, on a wet-basis, compared to mono-digestion. The increase in biogas production was mainly a consequence of the increase in organic loading rate. However, the differences could also be related to the synergy between both substrates and the carbon-to-nitrogen ratio. Moreover, the analysis of the macro-compounds, protein, lipids, carbohydrates and fibers, showed lower removal efficiencies in the co-digester as the microorganisms obtained nutrients from the soluble carbohydrates provided by the glycerol. The digestate stability, evaluated through a respirometric assay, showed that co-substrate addition does not exert a negative impact on digestate quality.

Anaerobic digestion of chicken feather with swine manure or slaughterhouse sludge for biogas production

Biogas production from anaerobic digestion of chicken feathers with swine manure or slaughterhouse sludge was assessed in two separate experiments. Ground feathers without any pre-treatment were added to 42-L digesters inoculated with swine manure or slaughterhouse sludge, representing 37% and 23% of total solids, respectively and incubated at 25°C in batch mode. Compared to the control without feather addition, total CH(4) production increased by 130% (P<0.001) and 110% (P=0.09) in the swine manure and the slaughterhouse sludge digesters, respectively. Mixed liquor NH(4)N concentration increased (P<0.001) from 4.8 and 3.1g/L at the beginning of the digestion to 6.9 and 3.5 g/L at the end of digestion in the swine manure and the slaughterhouse sludge digesters, respectively. The fraction of proteolytic microorganisms increased (P<0.001) during the digestion from 12.5% to 14.5% and 11.3% to 13.0% in the swine manure and the slaughterhouse sludge digesters with feather addition, respectively, but decreased in the controls. These results are reflective of feather digestion. Feather addition did not affect CH(4) yields of the swine manure digesters (P=0.082) and the slaughterhouse sludge digesters (P=0.21), indicating that feathers can be digested together with swine manure or slaughterhouse sludge without negatively affecting the digestion of swine manure and slaughterhouse sludge.

Mesophilic and thermophilic anaerobic co-digestion of rendering plant and slaughterhouse wastes

Co-digestion of rendering and slaughterhouse wastes was studied in laboratory scale semi-continuously fed continuously stirred tank reactors (CSTRs) at 35 and 55 °C. All in all, 10 different rendering plant and slaughterhouse waste fractions were characterised showing high contents of lipids and proteins, and methane potentials of 262-572 dm(3)CH(4)/kg volatile solids(VS)(added). In mesophilic CSTR methane yields of ca 720 dm(3) CH(4)/kg VS(fed) were obtained with organic loading rates (OLR) of 1.0 and 1.5 kg VS/m(3) d, and hydraulic retention time (HRT) of 50 d. For thermophilic process, the lowest studied OLR of 1.5 kg VS/m(3) d, turned to be unstable after operation of 1.5 HRT, due to accumulating ammonia, volatile fatty acids (VFAs) and probably also long chain fatty acids (LCFAs). In conclusion, mesophilic process was found to be more feasible for co-digestion than thermophilic process, methane yields being higher and process more stable in mesophilic conditions.

From wastewater to bioenergy and biochemicals via two-stage bioconversion processes: a future paradigm

Recovery of bioenergy and biochemicals from wastewater has attracted growing and widespread interests. In this respect, two-stage bioconversion process (TSBP) offers an appealing avenue to achieve stepwise and directional substrate conversion in separated stages. Such a biosystem not only enables enhanced degradation of organics, but also favors a high product yield and quality. Various TSBRs have been developed for the production of methane, hydrogen, electricity, bioplastics, bioflocculants, biopesticides, biosurfactants and other value-added products, demonstrating marked advantages over the conventional one-stage processes. It represents a promising, and likely the sole viable, paradigm for future application. However, there are also many remaining challenges. This paper provides an overview of the various TSBPs, introduces the recent advances, and discusses the major challenges and the future perspectives for practical application.

Optimization of food waste hydrolysis in leach bed coupled with methanogenic reactor: effect of pH and bulking agent

The effects of pH and bulking agents on hydrolysis/acidogenesis of food waste were studied using leach bed reactor (LBR) coupled with methanogenic up-flow anaerobic sludge blanket (UASB) reactor. The hydrolysis rate under regulated pH (6.0) was studied and compared with unregulated one during initial experiment. Then, the efficacies of five different bulking agents, i.e. plastic full particles, plastic hollow sphere, bottom ash, wood chip and saw dust were experimented under the regulated pH condition. Leachate recirculation with 50% water replacement was practiced throughout the experiment. Results proved that the daily leachate recirculation with pH control (6.0) accelerated the hydrolysis rate (59% higher volatile fatty acids) and methane production (up to 88%) compared to that of control without pH control. Furthermore, bottom ash improved the reactor alkalinity, which internally buffered the system that improved the methane production rate (0.182 l CH(4)/g VS(added)) than other bulking agents.

Multi stage high rate biomethanation of poultry litter with self mixed anaerobic digester

A multi stage high rate biomethanation process with novel self mixed anaerobic digester (SMAD) was developed in the present study to reduce the hydraulic residence time (HRT), increase the volatile solids (VS) loading rate, improve the VS destruction efficiency and enhance the methane yield. Specific design features of SMAD were useful in mixing the digester contents without consuming power and de-alienated the problem of scum formation. In the first phase, poultry litter having 10% total solids (TS) was subjected to high rate biomethanation in multi stage configuration (SMAD-I and II in series with UASB reactor). It was observed that gross VS reduction of 58%, gross methane yield of 0.16 m3 kg(-1) (VS reduced) and VS loading rate of 3.5 kg VS m(-3) day(-1) at HRT of 13 days was obtained. In the second phase SMAD-II was bypassed from the process scheme keeping the other parameters same as in the first phase. The results obtained were not as encouraging as in the first phase. The study showed that multi stage configuration with SMAD design improved the anaerobic digestion process efficiency of poultry litter.

Biological treatment of chicken feather waste for improved biogas production

A two-stage system was developed which combines the biological degradation of keratin-rich waste with the production of biogas. Chicken feather waste was treated biologically with a recombinant Bacillus megaterium strain showing keratinase activity prior to biogas production. Chopped, autoclaved chicken feathers (4%, W/V) were completely degraded, resulting in a yellowish fermentation broth with a level of 0.51 mg/mL soluble proteins after 8 days of cultivation of the recombinant strain. During the subsequent anaerobic batch digestion experiments, methane production of 0.35 Nm3/kg dry feathers (i.e., 0.4 Nm3/kg volatile solids of feathers), corresponding to 80% of the theoretical value on proteins, was achieved from the feather hydrolyzates, independently of the pre-hydrolysis time period of 1, 2 or 8 days. Cultivation with a native keratinase producing strain, Bacillus licheniformis resulted in only 0.25 mg/mL soluble proteins in the feather hydrolyzate, which then was digested achieving a maximum accumulated methane production of 0.31 Nm3/kg dry feathers. Feather hydrolyzates treated with the wild type B. megaterium produced 0.21 Nm3 CH4/kg dry feathers as maximum yield.