Research Article

Energy Generation from Urban Organic Waste by Anaerobic Digestion Method

Eisa Jahed1, Amin Mousavi Khaneghah2,*, Mahdi Jalali3, Akbar Sanaei-Moghadam4,5 , Ismail Es6

1 Department of Food Science and Technology, Urmia University, Urmia, Iran
2 Department of Food Science, Faculty of Food Engineering, University of Campinas (UNICAMP), Campinas, São Paulo, Brazil
3 Department of Food Science and Technology, Sabzevar Branch, Islamic Azad University, Sabzevar, Iran
4 Bio-System Engineering Department, Ferdowsi University of Mashhad, Mashhad, Iran
5Laboratory of Waste Management Organization of Mashhad Municipality, Mashhad, Iran
6Department of Material and Bioprocess Engineering, Faculty of Chemical Engineering, University of Campinas (UNICAMP), Campinas, São Paulo, Brazil
*Corresponding author:

Amin Mousavi Khaneghah, Department of Food Science, Faculty of Food Engineering, University of Campinas (UNICAMP), Campinas, São Paulo, Brazil, Email:


Urban organic waste, Cow manure, Anaerobic digestion, Biogas production, Methane, Recycle

In order to perform the evaluation of biogas production using one-step anaerobic digestion at pilot-scale was established using three different treatments that were defined as organic waste without cow manure (OWWM), cow manure without organic waste (MWM) and a mixture of cow manure-organic waste (OWAM) with the ratio of 50:50 w/w. The gas production was not observed after 45 days of storage in OWWM, whereas in treatments of OWAM and MWM, an average of 300 and 147 liters of methane per each kilogram of dry matter were produced, respectively. The amount of methane production increased up to 100% in comparison with the treatment of 50:50 and the equivalent amount of energy of produced methane were 3.15 kWh/kgTS. In consideration of the high amount of urban organic waste accumulation in the proposed region (Mashhad, Iran), there is a significant potential for energy generation. Therefore, more accurate economic analyses, as well as technical assessments, were recommended.

CHP: Combined heat and power; C: Carbon; C:N: Ratio of carbon to nitrogen; CH4: Methane; CO2: Dioxide Carbon; EC: Electrical conductivity; kg: Kilogram; kw: kilowatt; L: Liter; N: Nitrogen; pH: Acidity; TS: Total solid; VS: Volatile solid; wet: Humidity

Considering the current increase rate of population, limitations of fossil resources as well as growing environmental issues; novel techniques for urban waste management must be taken into account to minimize the transition of organic matters on landfills [1]. Maximum recycling of waste can be proposed as possible solutions in order to prevent waste output [2,3]. The major concerns of transferring materials to landfills are pollution of surface and groundwaters, emission of greenhouse gasses and causing inappropriate consequences for nearby inhabitants due to odor, dust, leachate and transportation noises [4]. Application of energy generation technologies (incineration, pyrolysis and anaerobic digestion) to process waste can be considered one of the applicable methods to overcome the mentioned complications. The anaerobic digestion process is a promising technology for generating energy from organic wastes in both environmental and economic perspectives. During this process, urban waste containing valuable organic matters is converted into biogas, which commonly comprises of methane (CH4) gas (55-60%). Anaerobic digestion of organic waste in landfills releases the gases methane and carbon dioxide that escape into the atmosphere and pollute the environment [5]. Under controlled conditions the same process has the potential to provide useful products such as biofuel and organic amendment (soil conditioner) and the treatment system does not require an oxygen supply [6]. Further, methane and hydrogen as potential fuels are considered comparatively cleaner than fossil fuel. In addition, this has the benefit of not depending on fossil fuel for energy consumption [7]. The energy obtained from CH4 gas could be utilized for production of heat and consequently generation of electricity [8,9]. Thus, anaerobic digestion represents an opportunity to decrease environmental pollution and at the same time, providing biogas and organic fertilizer or carrier material for biofertilizers.

Biogas can be obtained in different reactor systems via anaerobic digestion using urban wastes. Biogas production from existing vegetables that are present in urban waste has been investigated by Ojalo et al. [10]. In this study, the amount of dry matter processed inside the digester (inside temperature between 29 and 33°C and volume of 200 L) was varied between 8-10% with a total process time of 40 days. Special volume of biogas production was reported between 5.15 and 5.83 L/kg of dry matter. Also, it has been indicated that there is a polynomial regression relationship with a high correlation coefficient between storage time and biogas production [10]. In a different study, the production of biogas was carried out by a combination of urban organic waste and domestic sewage via anaerobic digestion [11]. In this process, the temperature was set between 25-36°C with a total production time of 25 days. The amount of volatile organic matter introduced into the system as an independent variable was varied between 0.5 and 4.3 kg/m3. The highest amount of biogas reported in this process was 360 L/kg for 2.9 kg volatile organic matter. The effect of domestic sewage addition into urban organic waste for biogas production was reported as a viable process [12]. According to this study, 800 L biogas per each kilogram of organic matter was produced by anaerobic fermentation. According to previous studies, it is observed that different organic matters have varied functions in biogas production. In addition, urban organic waste could be assessed as a significant resource for generating energy [2,13–15].

Mashhad is the second largest city in Iran and one of the most popular touristic destinations in the world with a daily urban waste production of 1700 tons. The major part of this waste is transmitted to landfills and rest of them is transferred to industrial recycling constructions for the further process [16]. Based on reports about the physical analysis of urban waste in Mashhad, 70% of waste consists of biodegradable organic matters. Only approximately 20% of total matters are processed [17]. Therefore, a significant potential is accessible in generating energy from this massive amount of resource. In this study, with consideration of the huge amount of waste production in Mashhad, the potential of biogas production from organic waste by anaerobic digestion using an experimental laboratory-scale production was evaluated.

Sample preparation

Raw materials were collected from Mashhad, Iran and transferred to industrial recycling constructions then delivered to separate hall of compost manufacture. After separation steps of inorganic materials, the residual waste including organic matters was directed to aerobic fermentation room for compost manufacture. Samples were taken from the entrance of the hall and transferred to the laboratory of Mashhad Waste Management Organization (MWMO) to perform necessary examinations. Input materials consisted of biodegradable and non-organic materials. Before performing an examination, inorganic materials were separated. Afterwards, some of the physical and chemical properties such as carbon (C), nitrogen (N), humidity content, dry matter and volatile solid material of raw materials were analyzed according to American Public Health Association (APHA) Standard (APHA, 1998) in waste management organization of Mashhad municipality (Table 1). Fresh cow manure was obtained from one of the cattle farms of Faculty of Agriculture at Ferdowsi University in Mashhad. Cow manure was utilized as starter bacterial culture for the anaerobic digestion process.

Measuring factors

Type of substance

Cow manure

Urban organic waste

TS (%)



VS* (%)



Wet (%)



%C (dry basis)



%N (dry basis)









EC (ms/cm)



*VS is a percentage of TS

Table 1. Physical and chemical characteristics of primary substances for treatments.


To perform anaerobic digestion, three digesters were employed with similar features (Figure 1). The body of digesters was made of stainless steel with the volume of 6 L, and equipped with an automatic control system for measurement of temperature and mixing time. In order to provide indoor heat of digester; copper coils, which twist around the body of digester and insulated-toward outside, were used. Warm water is pumped from a resource (Ben Murray) to coils. To calculate the volume of produced gas, the method previously described by Sanaei-Moghadam et al. (2014) was used. A gas analyzer system was utilized in order to define the percentage of each gas in biogas composition including methane (CH44), carbon dioxide (CO2) and oxygen (O2). pH and electrical conductivity were measured by digital pH meter (Metrohm AG, model 744, with an accuracy of 0.01, Swiss) and digital EC meter (WTM, model 1170 with an accuracy of 0.01), respectively.

Experimental method

Before charging digester, materials were chopped into smaller pieces with size varying between 6 and 8 mm. For each treatment, they were diluted to the volume of dry matter in digester reaches to 10%. In Table 2, the amount of wet weight of each treatment, the ratio of carbon to nitrogen (C:N) and volume of water added to reach considerable concentration were shown. The prepared materials were strongly linked to digester, charge, and covers. Before closing cover, in order to provide anaerobic condition, the contact surface of the cover, the body of digester and gaps of all connections were glued.

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Figure 1: Schematic picture of used digesters in laboratory of Mashhad waste management organization. 1. Insulated digester, 2. Scaled gas tanks, 3. Temperature control and mixing box.

The mixing ratio was based on dry matter weight of substances. All examinations were conducted for each treatment in two replicates. In all examinations, the temperature of digester was adjusted by digital thermostat close to mesophilic condition (35-37 ºC). Materials inside the digester were mixed for 5 minutes every 30 minutes. For each treatment, volume and percentage of different gasses were measured daily while pH was measured every three days. Finally, the results were analyzed using Excel software (MS Excel, version 2007, Microsoft, USA).

Measurement factors

Type of treatment




TS (%)




VS (%)




Wet matter content (g)




Amount of added water (L)












EC (ms/cm)




Ratio of Manure: Urban Organic Waste

Table 2. The values of measured factors in treatments.

The amount of biogas production

The process of Biogas generation from solid organic waste is often carried out by several different anaerobic bacteria [7]. Biogas is generally composed of 48-65% methane, 36-41% carbon dioxide, up to 17% nitrogen, <1% oxygen, 32-169 ppm hydrogen sulphide and traces of other gases [19]. Unlike fossil fuel, biogas does not contribute much to the greenhouse effect, ozone depletion or acid rain [20]. This is one of the main reasons that anaerobic digestion may play a very crucial role in meeting energy challenges of the future generation. Figure 2 shows the diagram of a special volume of Biogas in all treatments. According to the results, Biogas production was not observed for the treatment of Organic Waste without Mixing (OWWM). The possible reason could be attributed to the lack of sufficient concentration of anaerobic bacteria to start the anaerobic digestion process. However, due to the presence of anaerobic bacteria in cow manure, in MWM and OWAM treatments, the production of Biogas was detected. With consideration of the presented diagrams, the speed of Biogas production at the initial stage of the experiment (first ten days) was considerably low. Afterward, the rate of Biogas production and the content of methane increased dramatically. The maximum Biogas production rate for MWM and OWAM treatments was observed between 10th and 15th days and 25th to 35th days, respectively. After these periods, consumption of organic material and consequently Biogas production were drastically decreased, until the rate of Biogas production became zero. These processes were concluded at 30th and 45th days for MWM and OWAM, respectively.

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Figure 2: Diagrams of special volume changes of produced biogas for WMW (■) and OWAM (▲). Each point is an average of two repetitions.

Several reports indicate that anaerobic digestion of the organic fraction of solid waste yields promising amounts of Biogas. Sanaei-Moghadam et al. [17] evaluated the efficiency of methane production as Biogas from a combination of potato pulp and cow manure with different mixing ratio using Continuously Stirred Tank Reactor (CSTR) at pilot-scale. During 50 days, it was possible to produce methane gas between 149 and 348 L/kg VS. for four different mixing ratio. Methane production increased significantly by using a mixture of potato skin and cow manure comparing with just cow manure. The system showed a significant potential for energy generation with total methane production of 2.8 kWh/kg VS. Kryvoruchko et al. [21] investigated anaerobic digestion of sugar beet and potato by-products for methane production. During 28-38 days of the process, it was possible to produce methane with the specific yield from 323 to 481 L/kgTS of methane per volatile solids. As shown in Table 3, Biogas production was 210 and 526 (L/kgTS) in MWM and OWAM treatments, respectively. It was observed that mixing organic waste and cow manure had a positive effect on Biogas production, with a total increase of 250% in biogas volume. In‏ a similar study, Davidsson et al. [22] reported that applying co-digestion system by adding sludge from grease traps, fruit, and vegetable into municipal waste, the potential of energy and Biogas production could be increased more than energy production obtained using single-substrate digestion. Similar results have been achieved by Panichnumsin et al. [23] using cassava pulp in a mixture of pig manure. In this study, co-digestion also resulted in a higher methane production and reduced volatile solids.

Type of treatment

Biogas (L/kgTS)


Energy (KWh/kgTS)


% CH4
















*Numbers are related to the average of two repetitions. Numbers are average of total staying days for each treatment

Table 3.The comparison of biogas, methane and equal energy in experimental treatments.

The variations of methane and carbon dioxide (CO2) content in biogas formation were shown in Figure 3 for each treatment. With consideration of recorded results, the contribution of CO2 in hydrolysis and acidification steps of anaerobic digestion process could be explained. Nevertheless, after a period of time of suitable environmental condition, proliferation and activity of methanogenic bacteria were increased during biogas production, and consequently, the production of CH4 was stimulated until reaching to the maximum level in OWAM treatment (72%).

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Figure 3:Diagram of CH4 (▲) and CO2 (●) contents during digestion of A) OWAM and B) MWM treatments.

Cumulative curves of methane for each treatment were shown in Figure 4. It is concluded that approximation of slope of the curve to the zero point could indicate relatively complete digestion of the material in the digester. In Table 3, the average of methane percentage during the process and a special volume of produced methane were presented. Comparing with total methane production in two treatments, it was possible to increase the production up to 100% with the use of organic waste. Considering the heating value of methane, which is 10.5 kWh/m3, the equivalent energy of produced methane in MWM and OWAM is calculated as 1.54 and 3.15 kWh per each kilogram of dry matter, respectively. Obtained results are in good agreement with the findings of previous investigations about positive synergism of digestion of more than one substrate (Sanaei-Moghadam et al. 2014; Callaghan et al., 2002; Misi and Forster, 2001).

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Figure 4:Cumulative diagram of biogas production in WMW (▲) and OWAM (■).

It has been observed that co-digestion (is a waste treatment method in which different wastes are mixed and treated together) of mixtures stabilizes the feed to the bioreactor, thereby improving the C/N ratio and decreasing the concentration of nitrogen (Castillo et al., 2007). The use of a co-substrate with a low nitrogen and lipid content waste increases the production of biogas due to complementary characteristics of both types of waste, thus reducing problems associated with the accumulation of intermediate volatile compounds and high ammonia concentrations (Cuetos et al., 2008).

Co-digestion is preferably used for improving yields of anaerobic digestion of solid organic wastes due to its numeral benefits. For example, dilution of toxic compounds, increased load of biodegradable organic matter, improved balance of nutrients, synergistic effect of microorganisms and better biogas yield are the potential benefits that are achieved in a co-digestion process. Co-digestion of an organic waste also provides nutrients in excess (Sosnowski et al., 2003), which accelerates biodegradation of solid organic waste through biostimulation. Additionally, digestion rate and stabilization are increased (Lo et al., 2010). Several studies have shown that mixtures of agricultural, municipal and industrial wastes can be digested successfully and efficiently together. A stimulatory effect on synthesis of methane gas has been observed when industrial sludge was co-digested with municipal solid waste (Ağdağ and Sponza, 2007). The co-digestion of municipal solid waste with an industrial sludge ratio of 1:2 yielded the highest amount methane gas, compared to municipal solid waste alone. Similarly, in a two-phase anaerobic digestion system, Fezzani and Cheikh (2010) recorded the highest methane productivity when a mixture of olive mill wastewater and olive mill solid waste was co-digested. The process has also been useful in obtaining a valuable sludge which can eventually be used as a soil amendment after minor treatments (Gómez et al., 2006).

Jingura and Matengaifa (2009) described the following multiple benefits of co-digestion: the facilitation of a stable and reliable digestion performance and production of a digested product of good quality, and an increase in biogas yield.

The anaerobic digestion of organic material is a complex process, involving a number of different degradation steps. The microorganisms that participate in the process may be specific for each degradation step and thus could have different environmental requirements (Misi and Forster, 2001).

One of the important factors in the process of biogas production is acidity (pH) (Callaghan et al. 2002). The type and activity of anaerobic bacteria could be determined by analysis of pH changes. As shown in Figure 5, different pH changes were recorded in each treatment. It has been observed that pH was out of optimal range (6.8-7.5) in OWWM and did not reach to the expected range during 30 days. However, in MWM, due to a rich environment of methane producer bacteria, pH was varied between 7.41-7.51. In OWAM treatment, pH exhibited decreasing trend during first ten days (from 7.45 to 6.75), and again it increased up to 7.4, as the maximum recorded value (Moen et al., 2003). According to anaerobic digestion process theory, the observed decrease in pH could be attributed to increased concentration of volatile fatty acids in acidification step of the process. In the next days, with growing population of methane bacteria, besides the pH, the speed of biogas production and existing methane in biogas were increased significantly. At the first stage of a two-stages biogas production from potato waste system which was conducted by Parawira et al. (2005), similar trend in pH changes of digester was observed; however, in the second stage, because it of presence of high concentration of methane producer microorganisms, fewer changes in pH were reported.

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Figure 5:Diagram of pH changes during anaerobic digestion process for OWWM (▲), OWAM (■) and MWM (●).

The biogas yield is affected by many factors including type and composition of substrate, microbial composition, temperature, moisture and bioreactor design,. Hernandez-Berriel et al. (2008) studied the methane production from biodegradation of municipal solid waste. They found that the process reached the onset of the methanogenic phase at day 63 and the methane production rate was greater at a moisture level of 70%. However, a decrease in biogas production was observed in the case of fruit and vegetable waste due to rapid acidification of these wastes, resulting in a lowering of the pH in the bioreactor. Moreover, production of larger volatile fatty acids from such waste under anaerobic conditions inhibits the activity of methanogenic bacteria.

A range of pH values suitable for anaerobic digestion has been reported by various researchers, but the optimal pH for methanogenesis has been found to be around 7.0 (Yang and Okos, 1987). Ağdağ and Sponza (2007) reported a very narrow range of suitable pH (7.0–7.2) in the industrial sludge added bioreactors during the last 50 days of the anaerobic incubation. Similarly, Ward et al. (2008) found that a pH range of 6.8–7.2 was ideal for anaerobic digestion. Lee et al. (2009) reported that methanogenesis in an anaerobic digester occurs efficiently at pH 6.5– 8.2, while hydrolysis and acidogenesis occurs at pH 5.5 and 6.5, respectively. From the batch experiments, it was shown that the appropriate pH range for thermophilic acidogens was 6–7 (Park et al., 2008). Dong et al. (2009) suggested that the hydrogen production will be at a maximum if the initial pH of a biosystem is maintained at 9. However, similar results can also be achieved at pH 5–6 (Kapdan and Kargi, 2006). Liu et al. (2008) showed that the most favorable range of pH to attain maximal biogas yield in anaerobic digestion is 6.5–7.5.

Our results are in agreement with the results of Parawira et al. (2008), and Pind et al. (2003).

Due to the energy crisis and climate changes, the demand for a green, efficient, carbon-neutral energy sources in order to replace fossil fuels increased. Biogas, formed by anaerobic digestion of organic materials, could provide sustainable, reliable, renewable sources of energy. In this study, the attempt was to assess the energy generation from urban organic waste with anaerobic digestion method. Based on the experimental results, it can be concluded that increasing the percent of cow manure in urban organic waste improves the efficiency of the biogas production. Incorporating cow manure to urban waste provides a positive synergism system for methane production. Incorporation of cow manure aimed to increase the buffering capacity and provide a nitrogen source for microbial synthesis, which could lead to a stable anaerobic digestion process. So, according to the high volume of produced waste in Metropolis, production of energy from urban waste also of a great source of energy could provide benefits for both socially and environmentally.

A. Mousavi Khaneghah likes to thank the support of CNPq-TWAS Postgraduate Fellowship (Grant #3240274290).
I. Es gratefully acknowledges the financial support of the Fundacão de Amparo à Pesquisa do Estado de São Paulo (FAPESP) (Grant #2015/14468-0).

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Published: 18 May 2017

Reviewed By : Dr. Francesco Di Maria, Dr. Almendro Candel,


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