Demographic and economic growth, rapid urbanization and rising living standards lead to an increase in quantity and quality of solid waste production    . This phenomenon creates enormous risks to the environment and consequently to population health   . The management of solid waste has become a major problem today, especially in developing countries (DCs), due to the lack of resources and difficulty to develop an approach adapted to their context. Despite recommendations and regulatory measures to reduce waste generation at source and ultimate waste, promoting eco-design, sorting, recycling and incineration, landfill remains a significant part of waste. In order to protect environment, the controlled methanization process in sealed enclosed spaces has been developed as opposed to landfill centers  . Indeed, the advantages of this technique are multiple. Methanization requires less space and considerably reduces volume and weight of waste to be buried. It permits significant reductions in greenhouse gas emissions (CO2 and CH4), and odors elimination. The digestate issued can be used to reinforce agriculture. The biogas (renewable fuel) generated can be used in several final applications. Therefore, methanization is more profitable than all other forms of waste treatment      . Biogas containing methane was recognized as a new renewable energy source according European Directive 2001/77/EC. Also, it added that 20% of energy consumed in EU in 2020 will coming from renewable sources. Thus, studies have been carried out in a context of anaerobic digestion process optimization. The different stages of anaerobic digestion process were dissected  -  . Many studies are interested in substrates diversification and their biomethanogenic potential     . Co-digestion systems that treat a mixture of different king of waste, including animal, food and organic household waste have been developed    . Nations have embarked on energy crops to increase renewable energy production more rapidly   . Amarante  reported several commercialized technologies worldwide for municipal organic residues treatment. The use of adequate inoculums is an important parameter for anaerobic digestion. It must be adapted to the substrate. Most digesters are inoculated with old sludges from other mesophilic or thermophiles anaerobic digesters    . In other studies a commercial solution containing some microorganisms was involved in anaerobic degradation  . Nikièma et al.  and Traoré et al.  showed the possibility of using wastewater and bovine dung from slaughterhouse as a source of inoculum for biogas production. Nature and proportion of inoculums used have potentially a significant impact on methane production      . The aim of this study was to develop activated sludge to inoculate municipal organic solid waste digesters.
2. Material and Methods
2.1. Inoculum Sampling
Wastewater (WW) and bovine dung (BD) were sampled at slaughterhouse in Ouagadougou, Burkina Faso (12˚25'5.87"N, 1˚28'29.23"O). The cow dung was freshly removed, placed in a sterile stomacher plastic bag and transferred to the Laboratory. The wastewater was removed at depth (1 meter) and all samples were stored at 4˚C.
2.2. Experimental Conditions
The experimental conditions for the production of activated sludge described was adapted from the process of Angelidaki  . The experimental conditions are presented in Table 1.
Experiments were carried out in 300 mL flask filled with 20 ml of mineral solution supplemented with 29 mL cellulose 5% (w/v). Combinations of different proportions of bovine dung and wastewater were used as inoculum: 100% WW, 100% CD, 90% CD + 10% WW, 70% CD + 30% WW, 50% CD + 50% WW, 30% CD + 70% WW and 10% CD + 90% WW. 0.4 ml of a buffer solution (NaHCO3, 10%) and 0.6 ml of a reducing solution (Na2S9, H2O, 2.4%) were added in a sterile manner. The tests were carried out in duplicate. The cow dung was washed in distilled water (2% w/v) prior to combinations.
The mineral solution contained 10 mL of phosphate buffer 10 mL (2 g/L K2HPO4 and 2 g/L NH4Cl), 8 mL of Balch’s trace mineral solution  ; 1 mL of Widdel trace element solution  and 1 mL of vitamin solution.
2.3. Evaluation of Methanogenic Activities
The evaluation of activity of the different microbial groups was followed by the measurement of methane production  . Estimation of biogas production was carried out using liquid raising method. The gaseous products (CO2 and CH4) were analyzed using a gas chromatograph (Girdel series 30 catharometer equipped with a thermal conductivity detector [TCD] associated to SERVOTRACE potentiometer recorder type Sefram Paris of 1 mV). CH4 and CO2 were analyzed under following: Injector temperature 90˚C, column temperature 60˚C, detector temperature 100˚C, filament current 150 mA, carrier gas pressure 1 bar, and attenuation 32, Paper flow rate 10 mm/min. 0.5 ml of the flask gas phase was taken
Table 1. Experimental design matrix.
MS: Mineral solution, WW: Wastewater, CD: Cow dung.
and then injected into the chromatograph using a 1 ml waterproof syringe. The CH4 and CO2 content were determined using a standard curve based on CH4 and CO2.
2.4. Monitoring for the Selection of Microbial Complex
The microbial complex that gave more production of biogas was chosen as the best activated sludge. For this complex, the physicochemical parameters were determined. PH, salinity, Total dissolved solutes (TDS), electrical conductivity (EC) and resistivity were measured with a multi-parameter analyzer (9420 WTW). Dry matter (DM), total ash (Ct), organic matter (MO), total organic carbon (TOC) in the sample was obtained according to the protocol described by Sakaki  . Total alkalinity (TAC) and volatile fatty acids (VFAs) content were determined using the method described by Dilallo and Albertson  . An amount of 25 mL of the sample was titrated with H2SO4 (0.1 M) at pH 4.0 and the amount of titration was noted. The sample was then boiled lightly for 3 minutes, cooled, titrated with Na2CO3 (0.05 M), and the titration amount was noted from pH 4.0 to 7.0. Viable methanogenic bacteria were enumerated by the three-tube most probable number (MPN) technique (8-fold dilutions) using a medium adapted from Dianou  and a modified medium of Zhilina  . The composition of medium for serial dilution was respectively: NH4Cl 2 g/L, K2HPO4 0.33 g/L, KH2PO4 0.33 g/L, methanol 10 mM, sodium acetate 20 mM, sodium formate 20 mM, yeast extract 1 g/L, oligoelements solution 1 mL, resazurin solution (0.1%: v/v) 1 mL. Also 1 mL reducing solution was added to the medium and the headspace was filled with N2 gas.
2.5. Statistical Analysis
The XLSAT software was used for statistical analysis of the data. The analysis of variance (ANOVA) was carried out to compare the mean values of biogas production obtained from different combinations using Fisher's tests at the probability threshold p = 5%. Principal Component Analysis (PCA) was performed to reduce geometric space and visualize data, using a linear combination of variables that maximizes variance. This method allowed to visualize the typology of the different combinations and to avoid the redundancy of the variables by considering the study in the reduced space of uncorrelated.
3. Results and Discussion
3.1. Evolution of pH
Figure 1 shows the evolution of pH according to the combinations of inoculums during the anaerobic digestion. The optimum pH during the anaerobic digestion is around neutrality and varied from, 6.5 to 8. pH value is less than 6.5 or greater than 8, a malfunction of the process   . The analytical follow-up concerned the control of pH by adding NaOH (5N), in order to correct it. The pH of mixture C1 (100% WW) was increased to around 9. Parawina et al.  have
Figure 1. Evolution of the pH as a function of time of different inoculums.
found similar results with pH evolving from 6 to 9 until the 20th of anaerobic digestion. These results are in agreement with those previously reported by Cuetos et al.  and  . According to Bechir  , wastewaters from the slaughterhouse of Ouagadougou are loaded with solid waste, organic material (residue of rumen) and concentrated by blood, flesh, fat, haires, excrement and urine of slaughtered animals. These waters have COD and BOD in order 2827, 1620 mg O2/L, respectively  . This is confirmed by   . Study of Cuetos et al.  was showed that the first 10 days, nitrogen compounds are degraded and ammonia nitrogen is released into the environment. Indeed, anaerobic bacteria mainly needed in organic form, nitrogen source (ammonia or urea nitrogen) and a source of phosphorus (orthophosphates). These nutrients are used for synthesis of molecule and energy during biological reactions. In the case of bovine dung, an increase of proportion causes acidification of environment. As also underlined some studies, the anaerobic digestion of bovine dung causes acidification of environment   . Acidification is due to the presence of macromolecules in rapidly decomposing bovine dung   . A weak variation in pH around neutrality (pH 7) was observed with the combinations C7 and C6.
3.2. Microbial Activities
Microbial activities were correlated by gas production during anaerobic digestion  . Studies showed that animal manure as co-substrate allows optimization of biomethane production, thus improving the anaerobic digestion system   . These conditions permit a good development of microbial communities (bacteria and archaea) in anaerobic digestion process. Microbial community plays a role in process performance and stability  .
3.3. Assessment of Biogas Production
Combinations C5, C6 and C7 were presented in Table 2 and demonstrated no significant difference about biogas production. The difference in production was found significant (p < 0.05) between two combination groups as C5, C6, C7 and C1, C4, C3, C2. Biogas production was depleted with the increasing of cow dung proportion in the mixture. In fact, cow dung contains macromolecules, the disintegration of which could produce acids components implicated in instability of process     . They argument that some toxic particles or molecules could limit anaerobic digestion and consequently block biogas synthesis. This report was supported by Mekonnen  who worked on anaerobic co-treatment of tannery wastewater and bovine dung. It revealed that tanneries rich in chromium and sulphide, were highly toxic to the anaerobic digestion process causing fall of biogas production   .
3.4. Comparison of Combinations Activities
The influence of matrices characteristics on the selection of combination was based on principal components analysis (PCA). This analysis was used for description and visualization of combinations with three parameters studied: Biogas, Methane (CH4) and Carbon dioxide (CO2). The main components are shown in Figure 2. The matrix of linear correlations is shown in Table 3. The Kaiser criterion indicated that two (02) principal components (PC) should be considered. Indeed, the three (02) first main components have an Eigen value greater than 1 (Figure 2). However, PC3 accounts for only 0.75% of data variability (Figure 2). Therefore, only the first two (02) components describing 99%
Table 2. Biogas production after 25 days of anaerobic digestion (mean of 3 replicates).
In a column, values that have a different letter are not significantly different according to the Fisher test (LSD) at 5% threshold.
Table 3. Coordinates of variables and their contribution to identification.
Figure 2. Results of principal component analysis.
Figure 3. Principal component analysis (a) plot of variables biogas production, CO2 and CH4 proportions and (a) distribution of combinations on 1 × 2 axis of principal components.
of the data variability were retained for data description.
The typologies of variables (Biogas, CH4 and CO2) and samples on factorial planes constituted by axes 1 and 2 were presented in Figure 3(a) and Figure 3(b), respectively. In these two figures, only variables close to correlation circle should be taken into account. In Figure 3(a) there are clearly three groups of variables close to the circle (Biogas, CH4 and CO2, so that the projections on the F1 and F2 axes are 99.24%). The first group consisting of the Biogas variable was positively correlated with the two axes F1 and F2. The second group (CH4) was positively correlated with axis 1 and negatively with axis 2. The third group (CO2) was negatively correlated with axis 1 and positively with axis 2. It is interesting to note that there is no particular affinity or opposition with the production of Biogas and the proportions of CH4 and CO2 (Table 4). Since the biogas consists of CH4 and CO2, the correlation exists between the productions of different gas. The representation of the combinations on the two factorial planes described by the axes F1 × F2 (Figure 3(a)) makes it possible to compare them according to the production of biogas and the proportions in CH4 and CO2. Combinations C5, C7 and C6 have a significant production of biogas. Combina-
Table 4. Pearson correlation matrix of variables distributions.
Table 5. Physicochemical characteristics of the best microbial complex.
CD = cow dung; WW = wastewater; C7 = 10CD + 90WW; C6 = 30CD + 70WW; TDS = Total dissolved solutes; EC = Electrical conductivity; R = Resistivity.
tions C1, C7 and C6 have high proportions of CH4. And the combination C5 has a high proportion of CO2.
3.5. Characteristics of the Sludge Produced
The physicochemical characteristics of the C7 and C6 sludge were presented in Table 5. Organic matter was respectively 41.06%, 47.02% for C7 and C6. VFAs and TAC concentration were 1320 mg acetic acid/L, 520 mg CaCO3/L for C7 and 3036 mg acetic acid/L, 1310 mg CaCO3 /L for C6. These characteristics are comparable to those of      . Ali Shah  showed that physicochemical parameters influence the densities of archaea and bacteria in the medium. In died, according to Aguilar  significant inhibitory effect was observed in the process at 10,000 mg/L of VFAs. Vedrenne  was found 2000 - 3000 mg/L with the same substrate. The alkalinity of the digesters should not exceed 1000 mg of CaCO3/l of alkalinity according to Hawkes et al.  . The VFAs values in the case of C6 allow adaptation to microorganisms with high values of acid production in the digesters. Methanogenic bacteria were estimated to 3.5 × 105 CFU/mL for C7 and C6. Ueki et al.  obtained respectively 7.17 × 104/mL or 1.6 × 106/mL with H2 or acetate as the substrate, in anaerobic digestion of municipal sewage sludge.
The evolution of the pH showed that the formulation with the low proportion in cow dung had the best production profile. C7 and C6 gave best production of CH4 and CO2 then also exhibited the growth of methanogenic bacteria. Their physico-chemical characteristics showed an interesting media rich in organic matter, VFAs and TAC which could maintain microbial flora stability. The combination C6 was presented good conditions of growth and maintenance associated to anaerobic digestion of bacterial consortia. It buffering properties can use to prevent medium acidification during VFAs production. To conclude C6 could be considered as best inoculum for organic waste digesters.
We thank the Cooperation and Cultural Action Service (SCAC) of the French Embassy in Burkina Faso and UEMOA through the CRSBAN/University Ouaga I Prof. Joseph KI-ZERBO for their financial support.
Conflict of Interest Statement
We declare that we have no conflict of interest.
 Alouemine, S.O. (2006) Méthodologie de caractérisation des déchets ménagers à Nouakchott (Mauritanie): Contribution à la gestion des déchets et outils d’aide à la décision. Thèse, Université de Limoges, 192 p.
 Minghua, Z., Xiumin, F., Rovetta, A., Qichang, H., Vicentini, F., Bingkai, L., Giusti, A. and Yi, L. (2009) Municipal Solid Waste Management in Pudong New Area, China. Waste Management, 29, 1227-1233. https://doi.org/10.1016/j.wasman.2008.07.016
 Sharholy, M., Ahmad, Vaishya, K.R.C. and Gupta, R.D. (2007) Municipal Solid Waste Characteristics and Management in Allahabad, India. Waste Management, 27, 490-496. https://doi.org/10.1016/j.wasman.2006.03.001
 Seng, B., Kaneko, H., Hirayama, K. and Katayama-hirayama, K. (2011) Municipal solid Waste Management in Phnom Penh, Capital City of Cambodia. Waste Management & Resource, 29, 5. https://doi.org/10.1177/0734242X10380994
 Romano, P.V., Krogmann, U., Westendorf, M.L. and Strom, P.F. (2006) Small-Scale Composting of Horse Manure Mixed with Wood Shavings Small-Scale Composting of Horse Manure Mixed with Wood Shavings. Compost Science & Utilization, 14, 132-141.
 Sakar, S., Yetilmezsoy, K. and Kocak, E. (2009) Anaerobic Digestion Technology in Poultry and Livestock Waste Treatment—A Literature Review. Waste Management and Research, 27, 3-18.
 M’Sadak, Y., Ben M’Barek, A. and Baraket, S. (2011) Suivis physico-chimique et énergétique de la biométhanisation expérimentale appliquée à la biomasse bovine. Revue “Nature et Technology”, n° 07, 81-86.
 Nelson, M.C., Morrison, M., Schanbacher, F. and Yu, Z. (2012) Shifts in Microbial Community Structure of Granular and Liquid Biomass in Response to Changes to Infeed and Digester Design in Anaerobic Digesters Receiving Food-Processing Wastes. Bioresource Technology, 107, 135-1343. https://doi.org/10.1016/j.biortech.2011.12.070
 Appels, L., Houtmeyers, S., Ruyters, S., Busschaert, P., Lievens, B., Impe, J.V. and Dewi, R. (2013) Influence of Sludge Pre-Treatment on the Microbial Community Structure in Anaerobic Digesters. 13th World Congr Anaerob Dig.
 Ostrem, K. (2004) Greening Waste: Anaerobic Digestion for Treating the Organic Fraction of Municipal Solid Waste. Columbia University (The Earth Engineering Center and the Henry Krumb School of Mines), 1-59.
 Amani, T., Nosrati, M. and Sreekrishnan, T.R. (2010) Anaerobic Digestion from the Viewpoint of Microbiological, Chemical, and Operational Aspects—A Review. Environmental Reviews, 18, 255-278. https://doi.org/10.1139/A10-011
 Girault, R., Peu, P., Béline, F., Lendormi, T. and Guillaume, S. (2013) Caractéristiques des substrats et interactions dans les filières de co-digestion: Cas particulier des co-substrats d’origine agro-industrielle. Sciences Eaux & Territoires, 3, 44-53.
 Barrantes, L.M., Hosseini, K.E. and Eskicioglu, C. (2014) Anaerobic Co-Digestion of Wine/Fruit-Juice Production Waste with Landfill Leachate Diluted Municipal Sludge Cake under Semi-Continuous Flow Operation. Waste Management, 34, 1860-1870.
 Hosseini, K.E., Barrantes, L.M., Eskicioglu, C. and Dutil, C. (2014) Mesophilic Batch Anaerobic Co-Digestion of Fruit-Juice Industrial Waste and Municipal Waste Sludge: Process and Cost-Benefit Analysis. Bioresource Technology, 152, 66-73.
 Macias-Corral, M., Samani, Z., Hanson, A., Smith, G., Funk, P., Yu, H. and Longworth, J. (2008) Anaerobic Digestion of Municipal Solid Waste and Agricultural Waste and the Effect of Co-Digestion with Dairy Cow Manure. Bioresource Technology, 99, 8288-8293.
 Nielsen, H.B. and Angelidaki, I. (2008) Congestion of Manure and Industrial Organic Waste at Centralized Biogas Plants: Process Imbalances and Limitations. Water Science Technology, 58, 1521-1528. https://doi.org/10.2166/wst.2008.507
 Risoud, B. and Chopinet B. (1999) Efficacité énergétique et diversité des systèmes de production agricole—Application à des exploitations bourguignonnes. Ingénieries-EAT, IRSTEA Edition 1999, 17-25.
 Shelton, D.R. and Tiedjei, J.M. (1984) Isolation and Partial Characterization of Bacteria in an Anaerobic Consortium That Mineralizes 3-Chlorobenzoic Acid. Applied Environmental Microbiology, 48, 840-848.
 Hansen, T.L., Ejbye, J., Angelidaki, I., Marca, E., Jansen, C., Mosbæk, H. and Christensen, T.H. (2004) Method for Determination of Methane Potentials of Solid Organic Waste. Waste Management, 24, 393-400. https://doi.org/10.1016/j.wasman.2003.09.009
 Lee, H. and Shoda, M. (2008) Stimulation of Anaerobic Digestion of Thickened Sewage Sludge by Iron-Rich Sludge Produced by the Fenton Method. Journal of Bioscience and Bioengineering, 106, 107-110. https://doi.org/10.1263/jbb.106.107
 Nikiema, M., Sawadogo, J.B., Somda, M.K., Traore, D., Dianou, D. and Traore, A. (2015) Optimisation de la production de biométhane à partir des déchets organiques municipaux [Optimization of Biomethane Production from Municipal Solid Organic Wastes]. International Journal of Biological and Chemical Sciences, 9, 2743-2756. https://doi.org/10.4314/ijbcs.v9i5.43
 Lopes, S.W., Leite, D.V. and Prasad, S. (2004) Influence of Inoculum on Performance of Anaerobic Reactors for Treating Municipal Solid Waste. Bioresource Technology, 94, 261-266. https://doi.org/10.1016/j.biortech.2004.01.006
 Park, H., Rosenthal, A., Jezek, R., Ramalingam, K., Fillos. J. and Chandran, K. (2010) Impact of Inocula and Growth Mode on the Molecular Microbial Ecology of Anaerobic Ammonia Oxidation (Anammox) Bioreactor Communities. Water Resource, 44, 5005-5013.
 Angelidaki, I., Alves, M., Bolzonella, D., Borzacconi, L., Campos, J.L., Guwy, A.J., Kalyuzhnyi, S., Jenicek, P. and van Lier, J.B. (2009) Defining the Biomethane Potential (BMP) of Solid Organic Wastes and Energy Crops: A Proposed Protocol for Batch Assays. Water Science & Technology, 59, 927-934. https://doi.org/10.2166/wst.2009.040
 Balch, W.E., Fox, G.E., Magrum, L.J., Woese, C.R. and Wolfe, R.S. (1979) Methanogens: Reevaluation of a Unique Biological Group. Microbiology Reviews, 43, 260-296.
 Nopharatana, A., Clarke, W.P., Pullammanapallil, P.C., Silvey, P. and Chynoweth, D.P. (1998) Evaluation of Methanogenic Activities during Anaerobic Digestion of Municipale Solid Waste. Bioresource Technology, 64, 169-174. https://doi.org/10.1016/S0960-8524(97)00190-9
 Sakaki, S. (2014) Etude de la production des acides gras volatils à partir de la fermentation acidogène des boues d’épuration des effluents issus d’un e usine de pates et papiers de la fermentation acidogène des boues d’épuration des effluents issus d’une usine de pates et papiers. Thèse, Université du Québec, Québec, 84.
 Dianou, D. (1993) Etude de l’influence des interactions entre les bactéries réductrices du cycle du soufre et les bactéries méthanogéniques sur la production du riz des bas-fonds. Thèse, Université de Ouagadougou, Ouagadougou, 155.
 Lee, D.H., Behera, S.K., Kim, J.W. and Park, H. (2009) Methane Production Potential of Leachate Generated from Korean Food Waste Recycling Facilities: A Lab-Scale Study. Waste Management, 29, 876-882. https://doi.org/10.1016/j.wasman.2008.06.033
 Parawira, W., Murto, M., Zvauya, R. and Mattiasson, B. (2006) Comparative Performance of a UASB Reactor and an Anaerobic Packed-Bed Reactor When Treating Potato Waste Leachate. Renew Energy, 31, 893-903. https://doi.org/10.1016/j.renene.2005.05.013
 Cuetos, M.J., Gomez, X., Otero, M. and Moran A. (2008) Anaerobic Digestion of Solid Slaughterhouse Waste (SHW) at Laboratory Scale: Influence of Co-Digestion with the Organic Fraction of Municipal Solid Waste (OFMSW). Biochemical Engineering Journal, 40, 99-106. https://doi.org/10.1016/j.bej.2007.11.019
 Bouallagui, H., Rachdi, B., Gannoun, H. and Hamdi, M. (2009) Mesophilic and Thermophilic Anaerobic Co-Digestion of Abattoir Wastewater and Fruit and Vegetable Waste in Anaerobic Sequencing Batch Reactors. Biodegradation, 20, 401-409.
 Zongo, I., Diallo-Kone, M., Palm, K., Tiemtore, A., Sanogo, O., Guiguemde, E., Lapicque, F. and Leclerc, J.P. (2012) Review of Wastewater from the City of Ouagadougou: Self-Purification Capacity for the Production of Biogas. Study & Research: Chemistry & Chemical Engineering, Biotechnology, Food Industry, 13, 153-167.
 Khennoussi, A., Chaouch, M. and Chahlaoui, A. (2013) Traitement des effluents d’abattoir de viande rouge par électrocoagulation flottation avec des électrodes en fer. Revue des Sciences de l’Eau, 26, 81-171. http://id.erudit.org/iderudit/1016064ar
 Zhu, G. and Jha, A.K. (2013) Psychrophilic Dry Anaerobic Digestion of Cow Dung for Methane Production: Effect of Inoculum. Science Asia, 39, 500-510.
 Mekonnen, A., Leta, S. and Njau, N.K. (2017) Kinetic Analysis of Anaerobic Sequencing Batch Reactor for the Treatment of Tannery Wastewater. African Journal of Environment Sciences and Technology, 11, 339-348.
 Chomini, M.S., Ogbonna, C.I.C., Falemara, B.C. and Micah, P. (2015) Effect of Co-Digestion of Cow Dung and Poultry Manure on Biogas Yield, Proximate and Amino Acid Contents of Their Effluents. Journal of Agriculture and Veterinary Science, 8, 2319-2372.
 Wang, S., Hou, X. and Su, H. (2017) Exploration of the Relationship between Biogas Production and Microbial Community under High Salinity Conditions. Scientific Reports, 7, Article No. 1149.
 Mondal, N.C., Saxena, V.K. and Singh, V.S. (2005) Assessment of Groundwater Pollution Due to Tannery Industries in and around Dindigul, Tamilnadu, India. Environmental Geology, 48, 149-157. https://doi.org/10.1007/s00254-005-1244-z
 Yi, J., Dong, B., Jin, J. and Dai, X. (2014) Effect of Increasing Total Solids Contents on Anaerobic Digestion of Food Waste Under Mesophilic Conditions: Performance and Microbial Characteristics Analysis. PLoS ONE, 9, e102548. https://doi.org/10.1371/journal.pone.0102548
 Dhamodharan, K., Kumar, V. and Kalamdhad, A.S. (2015) Effect of Different Livestock Dungs as Inoculum on Food Waste Anaerobic Digestion and Its Kinetics. Bioresource Technology, 180, 237-241. https://doi.org/10.1016/j.biortech.2014.12.066
 Rajagopal, R. and Massé, D.I. (2016) Start-Up of Dry Anaerobic Digestion System for Processing Solid Poultry Litter Using Adapted Liquid Inoculum. Process Safety and Environmental Protection, 102, 495-502. https://doi.org/10.1016/j.psep.2016.05.003
 Ali Shah, F., Mahmood, Q., Maroof Shah, M., Pervez, A. and Ahmad Asad, S. (2014) Microbial Ecology of Anaerobic Digesters: The Key Players of Anaerobiosis. The Scientific World Journal, 2014, Article ID: 183752.
 Aguilar, A., Casas, C. and Lema, J.M. (1995) Degradation of Volatile Fatty Acids by Differently Enriched Methanogenic Cultures: Kinetics and Inhibition. Water Research, 29, 505-509. https://doi.org/10.1016/0043-1354(94)00179-B
 Hawkes, F.R., Guwy, A.J., Rozzi, A.G. and Hawkes, D.L. (1993) A New Instrument for On-Line Measurement of Bicarbonate Alkalinity. Water Research, 27, 167-170.
 Ueki, K., Ueki, K., Takahashi, K. and Iwatsu, M. (1992) The Role of Sulfate Reducing in Methanogenic Digestion of Municipal Sewage Sludge. Journal of General and Applied Microbiology, 38, 195-207. https://doi.org/10.2323/jgam.38.195