Next Article in Journal
Integration of Membrane Bioreactors with Edible Filamentous Fungi for Valorization of Expired Milk
Next Article in Special Issue
Assessment of Air Pollution from Household Solid Waste Open Burning in Thailand
Previous Article in Journal
Bayesian Approach for Estimating the Probability of Cartel Penalization under the Leniency Program
Previous Article in Special Issue
An Exploration of the Impacts of Compulsory Source-Separated Policy in Improving Household Solid Waste-Sorting in Pilot Megacities, China: A Case Study of Nanjing
Article Menu
Issue 6 (June) cover image

Export Article

Sustainability 2018, 10(6), 1939; doi:10.3390/su10061939

Pilot-Scale Anaerobic Co-Digestion of the OFMSW: Improving Biogas Production and Startup
Department of Energy Production and Use, Faculty of Power Engineering, University POLITEHNICA of Bucharest, Bucharest 060042, Romania
Faculty of Engineering in Applied Sciences, Technical University of the North, Ibarra 100150, Ecuador
Authors to whom correspondence should be addressed.
Received: 26 April 2018 / Accepted: 8 June 2018 / Published: 10 June 2018


This paper presents experimental results regarding anaerobic co-digestion of the organic fraction of municipal solid waste and fruit and vegetable waste in order to establish the efficiency of a 2 m3 volume pilot plant in terms of biogas and methane yield and stability of the process. The research study presents the feasibility of developing anaerobic digestion as an effective method for municipal solid waste management. The experiments were conducted in mesophilic conditions (35 °C). Domestic waste water was used as inoculum. The results showed that the inoculum presence, temperature, and pH control, were essential in order to improve biogas production and its composition. Using liquid inoculum, the CH4 percentage in the biogas oscillated between 44% and 51%, and the biogas production from 0.504 and 0.6 m3/day. Compared to domestic waste water, animal manure increased the CH4 concentration in biogas (up to 63%), while the daily biogas production increased by 26% and varied from 0.693 to 0.786 m3. The cumulative biogas production at the end of the experiments were 11.7 m3 and 15.89 m3, respectively. Using inoculum and co-digestion, the plant startup time was significantly reduced, the total solids content decreased from 22.7% to 19.8%, while the volatile solids decreased from 37.6% to 31.2%.
biogas; OFMSW; anaerobic digestion; co-digestion; pilot-scale biogas plant

1. Introduction

Global warming and climate change represent one of the major environmental problems facing society today, mainly because of the dependency on the fossil fuel energy (88.4% out of the total energy consumed in 2015) [1,2]. The world urban population has grown approximately 5.6 times between 1950 and 2017 [3], having as a consequence the increase in quantities of waste generated. In 2017, it is estimated that 1.3 billion tons of municipal solid waste (MSW) are generated at the global level [4,5], contributing to the current environmental problems. Therefore, this requires both the diversification of renewable energy resources and proper management of MSW so as to reduce their negative impact on the environment. This is the answer to the current policies and regulations regarding the targets for greenhouse gas emissions reduction, alternative solutions for conventional energy, waste generation reduction, and energy recovery from the municipal waste [6,7,8,9].
Improper MSW management contributes to negative effects on the environment and human health [10], and hence policies and stringent national and international regulations are provided [8,10,11] that require organic fraction MSW (OFMSW) reduction in landfilling and energy recovery [9].
Due to the heterogeneous and different sizes of MSW, it is very difficult to do/deal with the physico-chemical characterization [4,5].
Generally, the biodegradable fraction of the MSW (i.e., food and biomass residues, paper, and cardboard) varies between 30% and 65% [4,5,11]. In this respect, different research [12,13,14,15,16,17,18,19] shows that MSWs are a potential primary energy source, especially the organic fraction of the municipal solid waste (i.e., OFMSW) by the amount of biogas that can be produced through anaerobic digestion (AD). Therefore, AD is seen as a proper waste management solution in terms of solving the problem of waste generation and renewable energy production [20,21,22].
Anaerobic digestion is a microbiological process of organic matter decomposition in the absence of air. The key process phases of AD are as follows: (1) hydrolysis, in which the complex organic molecules are converted into simple sugars, fatty acids, amino acids, and water; (2) acidogenesis, where the molecules from the previous stage are broken into organic acids, ammonia, hydrogen sulfide, and carbon dioxide; (3) acetogenesis, in which hydrogen and carbon dioxide are converted into acetic acid; and (4) methanogenesis, the final stage of AD, when the intermediate products of the preceding stages are converted into methane, carbon dioxide, and water [23]. Different research studies [24,25,26,27,28,29,30] showed that both anaerobic digestion and biogas production are influenced by some physical and chemical parameters such as the total solids content (TS), the volatile solids content (VS), the carbon and nitrogen ratio (C/N), the particle size, the temperature and pH, the inoculum type, and the co-digestion with other organic wastes (i.e., animal manure, sewage sludge, waste from food industry).
Anaerobic digestion has been generally used for the anaerobic treatment of one type of substrate, but in recent years, researchers have demonstrated that AD becomes more stable when using multiple types of organic waste [15,31,32,33,34,35,36,37,38,39]. This has been presented in several studies on the anaerobic co-digestion of the OFMSW mixed with other wastes, for example OFMSW with sewage sludge; OFMSW with agricultural residues; and OFMSW with animal manure [12,13,14,15,25,37,40,41,42,43,44,45]. An important aspect to take into account concerning the anaerobic co-digestion is the total C/N ratio from the substrate composition. Anaerobic digestion is developing under favorable conditions when C/N is between 20:1–30:1 [18,20,37]. Fruits and vegetable wastes (FVWs) represent an important residue because they are produced in very big amounts in agricultural activities and supermarkets. Their generation increases the final operational costs of markets due to sales losses and disposal costs [46]. The co-digestion of the OFMSW and FVW with animal manure represents an option for controlling the AD stability process and maximizing biogas production [46,47]. Both residues can be used as an energy source meeting the specification of European Parliament legislation (Directive 2008/98/CE), which establishes that disposal in ecological landfills must be the direction only for the residues which cannot be recycled, reused or recovered [48].
This paper’s main objective is to evaluate the performance of an anaerobic digestion pilot plant for biogas production, based on the OFMSW and FVW in co-digestion with other organic substances as an activator of methanogenesis processes. In this study, the following characteristics were assessed: the reactor start-up time, the optimization of the biogas production, and composition and the OFMSW biodegradability based on the TS and VS reduction, since the VS is the organic part of the TS that is transformed into biogas [21]. Biogas production is mainly influenced by the distribution of the TS and VS [22,23,24]. The experimental setup is placed in the Laboratory of Environmental Monitoring and Certification of the Energy Potential of Waste from the Faculty of Power Engineering, from the University POLITEHNICA of Bucharest.
Additionally, the paper presents statistical data regarding the waste management in Romania and Ecuador in order to estimate the potential of biogas production and to establish the feasibility of developing AD as an effective method for municipal solid waste management in these countries and also other countries that have similar waste composition. Thus, one preliminary test and two different experiments were conducted: mono and co-digestion of OFMSW and FVW with other organic waste and animal manure. The process was assessed with and without inoculum as domestic wastewater.

2. Overview of Municipal Solid Waste Management in Romania and Ecuador

An efficient integrated waste management solution involves different steps such as separate collection, transport, recycling, and landfilling with monitoring of the landfill sites after closure. The waste management responsibility goes to local public administrations. Even though there are differences in the social and economic climate between Romania and Ecuador, they present a similarity in terms of MSW generation, composition, and management [11,13,14,37], with some exceptions corresponding to the Romanian economic crisis between 2007 and 2010 (Figure 1).
In terms of MSW production, Romanian statistics show that each person generated, on average, 0.9 kg/day of waste in urban areas and 0.5 kg/day of waste in rural areas. In 2015, in Romania, the MSW annual generation rate was 4.38 million tons of waste. About 73.5% from the total waste generation was landfilled, 13.11% was recycled, and 13.39% was recovered and treated by other methods [11,12]. In Ecuador, 4.06 million tons of municipal solid waste was generated in 2015, with a waste generation rate of 0.7 kg/per capita/day. From the total waste generation, 68% was landfilled, 18% was composed, 12% was recycled, and 2% was submitted to other treatments [37]. Accordingly, in both countries the most used treatment solution for MSW remains landfilling, which is no longer an actual and sustainable solution. Figure 2 presents the similarities between the MSW medium composition in Romania and Ecuador [11,12,37].
The difference between the organic fraction of the MSW from Romania and Ecuador is almost 10%, which can be explained by the different lifestyles of the populations from these countries. The high content of the biodegradable fraction contributes to a good potential for biogas production. Knowing that biogas production is between 80–160 m3 biogas/t of waste [21], the anaerobic digestion of the OFMSW as a method of MSW treatment is an attractive choice for both countries.

3. Materials and Methods

3.1. Substrates and Inoculum

The quantity and quality of the substrate is the main factor influencing the production and biogas composition [20,33].
In the framework of the experimental study, OFMSW and FVW were used as substrates. The OFMSW was collected from a local food market. It was basically composed of raw food waste: cabbage, potatoes, tomatoes, and cucumber. Fruit and vegetable waste was collected from the fresh product department of a supermarket. The mixture of fruits that was used is composed of apples (25%), pears (25%), bananas (10%), plums (15%), apricots (15%), and peaches (10%). Additionally, dry grass and green corn stalks were used. After the collection process, the OFMSW and FVW were ground in order to reduce particles size.
The considered liquid inoculum is made up of domestic wastewater, animal manure, and organic waste. The used inoculum had a high pH indicating high buffering capacity. The buffering contributed to maintaining pH in an adequate range, refraining the accumulation of volatile fatty acids, which occurs often during the digestion of OFMSW that contains high amounts of food waste [49]. The characteristics of substrate and inoculum are presented in Table 1.
Because of the high moisture content (83.2%), no added water was necessary for the AD process. As inoculum for the AD process, an optimized amount of 30 liters of domestic wastewater from a septic tank were used.
The presented research approach was taken as different studies [35,36,37,38] have shown that anaerobic co-digestion with different organic substrates (e.g., municipal wastewater treatment sludge and fat, oil, and grease; food waste with tall fescue; pig slurry with maize cob, etc.) has improved the AD process in terms of biogas production and composition.

3.2. Analytical Methods

The characteristics of the substrate and inoculum contribute to the determination of parameters such as the humidity, TS content, VS content, pH, and C/N ratio. To identify the humidity, TS, and VS, the thermogravimetric method was used. The pH was measured using a digital measuring device with combine electrode, type IQ SENSOR NET System 2020 XT. The humidity was measured with specific equipment, an electric heated oven, type Nitech SLW53ECO. TS and VS were measured with a calcination oven Nabertherm L9/11/SW type. By an elemental analyzer, Euro EA3000 type, the C/N ratio was measured.
The biogas composition was determined by means of a Dräger X-am 5600 gas analyzer type, which uses infrared technology to determine CH4 and CO2 and electrochemical sensors to determine O2 and H2S.
For the experimental study, a quantity of 80 kg of inoculum (i.e., animal livestock manure from pigs and cows) was used. The ratio of inoculum TS/substrate was 36.1%, which is in accordance with data from the literature specifying 30–50% [14,39]. The OFMSW/other organic wastes ratio is 1:1. In this way, we were able to evaluate the influence of the different parameters on the pilot plant performance. The total load of the reactor was 293 kg.
As shown in Table 1, the organic substrate had a high content of VS (79.17% of total solids), and consequently it had a high biogas production potential. Additionally, the C/N ratio was about 23 and a high moisture content of 79.5% increases the AD process efficiency.
On the other hand, the pretreatment of the OFMSW in co-digestion with other organic wastes consisted in particle size reduction by shredding at 4–12 mm, so that the degradation of the organic matter could be conducted under proper conditions [14,21,25].

3.3. Pilot-Scale Startup and Operation

The experiments were conducted using an anaerobic digestion pilot plant that was built according to a proper design. The vertical pilot plant was a dry anaerobic digester in one stage with a volume of 2 m3 with mixing and recirculation of the leachate (Figure 3). The dry process was used based on the literature data regarding the advantages of the wet AD, such as simple systems, which are easier to maintain because they do not require complex equipment (e.g., pumps, stirring systems, substrate feed in systems) [18,19,20].
The pilot plant is flexible in use in terms of the process temperature control, and it can operate at mesophilic (35 °C) or thermophilic (55 °C) conditions due to the thermal insulation with mineral wood and the electrical heating thermostatic system arranged on the entire surface of the digester for operation in cold seasons.
The reactor was operated under mesophilic conditions at 35 °C ± 2 °C. The process temperature can be automatically controlled, and the inside temperature is continuously monitored by means of temperature sensors connected to a Thermocouple Data Logger with LCD Display and USB Interface.
The main components of the pilot plant were: (1) mechanical resistance system of the plant (pilot plant support); (2) AD reactor; (3) mobile biogas capture and measuring system; (4) guidance structure of biogas capture and measurement system; (5) feeding system; (6) temperature sensors; (7) leachate collection and recirculation system; (8) biogas sampling; (9) digestate discharge system (crank driving a screw conveyor); and (10) discharge flange.
The pilot plant could be loaded with size particles in the range of 30–50 mm, and on the bottom of the plant was the leachate collection and recycling system. Due to this system, the proper conditions for the homogeneous distribution of the microorganisms responsible for the decomposition of organic matter were maintained. The biogas composition and quantity were periodically monitored (3 to 5 times/day).
The daily biogas volume, in m3, was determined by measuring the vertical movement of the reactor top (Figure 4) based on the biogas pressure inside the reactor.
The biogas production was estimated using Equation (1):
VBG = πr2h

3.4. Experimental Conditions

3.4.1. Experiment No. 1

This preliminary experiment was aimed at highlighting the importance of the inoculum in biogas production and to test, according to the time parameter, and whether the absence of inoculum may lead to biogas formation mainly due to the biodegradability of the substrate used (high content of VS). Therefore, the first material used in this experiment was 150 kg of OFMSW composed of raw food waste: cabbage, potatoes, tomatoes, and cucumbers. The batch experiments were conducted for a hydraulic retention time (HRT) of 21 days, and the temperature varied between 26.5 °C and 39.5 °C; the average temperature was of 34 °C to maintain the mesophilic conditions [13,21,33].

3.4.2. Experiment No. 2

In this experimental set-up inoculum was used, and an extra 48 kg of OFMSW sorted at the source was added to the reactor filled with the previous substrate experimental test. For inoculum, we used 30 liters of domestic wastewater from a septic tank. The batch experiment was conducted for an HRT of 30 days at mesophilic temperature with continuous parameter monitoring to observe the influence of the temperature and pH on biogas production [15,17,20,21,40,45,46,50,51,52,53]. The reactor operated under mesophilic conditions with leachate recirculation. The biogas production was measured from 3 to 5 times per day based on the quantity of the resulting biogas.

3.4.3. Experiment No. 3

For this batch experimental test, the digester was fed with fresh 293 kg of organic substrate, comprised of the following: OFMSW and FVW, dry grass, green corn stalks, and animal manure. The composition and physico-chemical characterization is presented in Table 1.
The retention time was of 31 days, and the reactor operated under mesophilic conditions and leachate recirculation.

4. Results and Discussion

4.1. Biogas Production

In the first experiment test, due to the absence of inoculum, the pH value varied around 6.3 and required daily value correction by adding leachate calcium hydroxide (lime). The pH was stabilized at 6.7 after a period of 16 days, indicating the possibility of the beginning of the methanogenesis phase [13,34].
After seven days from the plant startup, a gaseous emission was measured, being composed of CO2, H2S, and O2. On day 14, the CO2 was 92%, while the concentration of O2 was 8%, which was inhibitive for microorganism formation. On the 18th day, the CH4 measured in the gas composition was only 8%. After this, the gas production mainly consisted of CO2 and rapidly decreased in time.
Therefore, in the first experimental test of the AD of the OFMSW without inoculum, the production of biogas was very low. Only gas containing CO2 and other traces (i.e., H2S and O2) was obtained with low CH4 content and noncombustible. The methanogenesis phase did not develop in appropriate conditions, due to the absence of the inoculum, and a low pH (up to 6.7) was obtained. This generated an acid environment that inhibited the formation and growth of methanogenic bacteria. This fact has also been noticed in other scientific research [17,36,38,39,41,42,43,44,54]. Under these conditions, on day 21 we decided to stop the experiment.
Using the substrate accumulated in the first experiment, liquid inoculum was added to the installation. The temperature variation throughout the second experimental test varied from 29.5 °C to 37 °C, with a medium temperature of 33.7 °C (Figure 5), while the pH varied from 6.1 to 7.4 (Figure 6).
In the first four days of the test, the pH encountered a decrease from 6.9 to 6.1, indicating that the process was under acid phase. The methanogenesis phase began after five days from the reactor loading. The pH in this phase was constant, around the value of 7.04. For the process stabilization, the pH value was maintained at 7.7 by adding 3–5 g of calcium hydroxide (lime) per liter of leachate. To maintain the conditions necessary for a homogeneous distribution of the micro-organisms responsible for organic matter decomposition, the leachate was recirculated periodically: every three hours from the top of the plant by spraying.
The quality of the biogas was quantified in terms of CH4 percentage. The cumulative biogas and CH4 production, in m3, was determined by summing the daily values. The obtained results are illustrated in Figure 7.
Across the experimental activities it was observed that the daily production of biogas increased rapidly in the first 10 days of the digester loading, reaching a value of 0.58 m3. Between days 7–19, the biogas production varied between 0.5 and 0.6 m3/day. The average biogas production during this period was of 0.58 m3/day (0.261 m3 of CH4 per day). The maximum production of biogas (0.6 m3/day) was recorded on day 14. After day 19, until the end of the experiment, the biogas production started to drop, reaching a minimum of 0.087 m3/day on day 30.
The cumulative production of CH4 after the 30 days of the experimental study was of approximately 5.02 m3 CH4. The quantity of biogas that can be produced per ton of waste is equivalent to about 59.1 m3 biogas (25.35 m3 CH4). The specific CH4 production in relation to the VS, considering the information from Table 1, is about 0.172 m3 CH4/kg_VS (0.40 m3 biogas/kg_VS). The experimental results are comparable to those reported in the scientific literature. Thus, Pavi S. et al. [21], K. D. Monson et al. [26], J. Rapport et al. [27] reported that at an industrial level the specific biogas production is between 80–120 m3/tons of waste.
Other studies such as Mao et al. [55] and Velmurugan et al. [56] showed that the specific production of methane at a mesophilic and retention time (HRT) of 28 to 60 days, is between 0.2 and 0.477 m3 CH4/kg_VS for the OFMSW sorted at the source. On the other hand, Li et al. [46] and Velmurugan and Ramanujan [56] demonstrated that the specific production of biogas from vegetable waste is 0.387 m3 biogas/kg_VS at mesophilic temperature and an HRT of 30 days. The differences can be explained by the fact that the experiments with the pilot plant took place outside. Consequently, there are high temperature variations, in particular during the night. Additionally, the data reported by the scientific research mentioned above refers to longer periods of study (30–60 days).
The digestate resulting from the test was analyzed in terms of TS and VS. The obtained results showed a decrease of 19.8% and 31.2%, respectively, compared to the preliminary experiment when inoculum was not used. Other studies, such as Forster-Carneiro et al. [57], showed that on thermophilic temperature using OFMSW sorted at the source, the TS and VS was reduced by 34.7% and 44.2%, respectively, for an HRT of 60 days. Also, Soavcool et al. [44] and Li et al. [54] reported a decrease of 41.8% of the VS at 30 °C mesophilic conditions and an HRT of 60 days. The difference that exists between the degradability is due to the temperatures and the higher HRT used in the studies mentioned.
From the experiments, we observed that the use of inoculum improved the AD process by reducing the starting time for the acidogenesis phase and that biogas production was accelerated.
In the third experiment, the temperature varied from 28.5 °C to 38 °C with an average temperature of 33.8 °C (Figure 8), while the pH varied between 5.9 and 6.9, after correction by adding calcium hydroxide (lime) (Figure 9).
We can appreciate that the acid phase lasted eight days after the loading of the installation. Once the pH was 7.1, the biogas production increased because of the methanogenic phase (Figure 10). The pH was stabilized at around 7.4 after 10 days.
In the first 12 days, the CH4 and biogas production reached 0.36 m3 per day and 0.693 m3 per day, respectively. The highest rate of CH4 and biogas production was achieved between days 12–25, oscillating from 0.693 to 0.786 m3 per day. In this period of time, the average daily production of CH4 and biogas was 0.405 m3 and 0.732 m3, respectively. The maximum CH4 production of 0.463 m3 was reached by day 20. From day 24 until the end of the experiment period, the CH4 production began to decline gradually. This can be explained by the fact that the vegetable and fruit waste from the substrate had a high degree of degradability and the animal manure contributed to the degradation of the OFMSW.
The cumulative production of CH4 by day 31 of the experiment was of about 8.21 m3 CH4. The specific biogas production related to the VS, based on Table 1, was of about 0.334 m3 biogas/kg_VS (0.173 m3 CH4/kg_VS).
These values are comparable to the data from different past scientific research. Thus, the studies of Pavi et al. [21] showed that the specific production of biogas from the anaerobic co-digestion of the OFMSW with vegetables and fruit wastes was between 0.20–0.42 m3/kg_VS at mesophilic conditions. Also, Gashaw [19] demonstrated that the specific biogas production of the OFMSW in co-digestion with animal manure was 0.34 m3/kg_VS at a ratio of OFMSW: animal manure of 50% at 55 °C and HRT of six weeks.
We can observe that the use of the animal manure contributed to a more stable methanogenesis phase with a lower variation of the pH value. Also, the degradability degree of the substrate was substantially improved.
In Figure 11, the average cumulative biogas yields measured from mono-digestion of OFMSW with inoculum and co-digestion of OFMSW with FVW and animal manure are displayed.
The cumulative biogas yield by day 31 of the experiment was of about 15.89 m3 biogas in the case of co-digestion, while for mono-digestion of OFMSW it was 11.7 m3 biogas, which is an increase of 26%. In the case of co-digestion, the biogas cumulative production was higher, which had been expected seeing that the hydrolysis and the alcoholic fermentation of fruits and vegetables takes place at a much higher speed than the other organic substrates [30,34,35]. The mono-digestion resulted in an average cumulative biogas production lower than the co-digestion, but in every condition, the biogas production started relatively fast after feeding the reactor. This is due to the high concentration of simple carbohydrates in fruits and vegetables, which are rapidly biodegraded under anaerobic conditions [19,30,31].
The digestate characterization showed that TS and VS decreased by 22.7% and 37.6%, respectively, compared to initial ones.

4.2. Biogas Composition

In this study, the biogas composition in terms of CH4, CO2, and O2 was analyzed. In Figure 11, the biogas composition variation (% in volume) over time is presented. After two days, CH4 was identified in the gas composition, reaching up to 6%. The CH4 concentration in the biogas increased rapidly over the next eight days up to 44%, while the pH increased (Figure 12), and the CO2 concentrations decreased. The maximum CH4 concentration of 51% was recorded by day 17. Between days 10 to 25, the CH4 concentration oscillated between 44% and 51%. This indicated that the methanogenesis phase of the AD process was stable. In this period, the average concentration of the CH4 in the biogas was 45.6%. At the end of the experiment (days 26–30), the CH4 concentration began to decrease from 47% to 44%. The concentration of the CH4 identified in the biogas composition was according to the values given in the specific literature (45–65%) [17,35,39,42].
Throughout the experimental work, the biogas composition varied, and the H2S was between 150–190 ppm (parts per million). During the entire methanogenesis phase, the O2 concentration was on average of 0.25%.
In the experiment with co-digestion of the OFMSW with FVW and animal manure, we observed that the CH4 concentration increased rapidly during the first 12 days reaching 52% (Figure 13). Afterwards, from day 13 to day 31, the CH4 concentration varied from 44% to 63%. This shows that the methanogenesis phase was stable. During this period, the average CH4 content was 55.73%.
Also, the CO2 concentration in the methanogenesis phase (days 13–31) varied from 35% to 54%, with an average of 42.3%. In the same period, the average O2 concentration was 0.3%. Comparing the results with those from the previous test, we can observe that the use of manure led to an increased CH4 concentration in the biogas composition of 10%, going from about 46% to 56%.
As presented in Figure 11 and Figure 12, the average methane content of biogas produced in the pilot plant ranged between 51% and 63%. The co-digestion of OFMSW and FVW resulted in higher methane content, compared with mono-digestion. This is because of the balanced supply of nutrients that contributed to an optimal living environment for anaerobic microorganisms [46].

5. Conclusions

This experimental study demonstrated that using co-digestion of OFMSW and FVW increased the cumulative biogas yield and the cumulative methane production compared with mono-digestion. In every experimental condition, biogas production started relatively fast after feeding, 8–10 days. This is due to the high concentration of simple carbohydrates in fruits and vegetables, which are rapidly biodegraded under anaerobic conditions. On the other hand, the use of manure as well as inoculum allowed a better methanogenesis phase stability with a lower variation of the pH value which contributed to a higher CH4 concentration in the biogas (from 45.6% to 55.73%). Also, biogas production increased by about 26% compared to the AD of the OFMSW with liquid inoculum (domestic wastewater).
The anaerobic co-digestion of the OFMSW with other types of organic substrates led to the AD process stability and improved the biogas composition by increasing the methane concentration and biogas generation. The average daily methane production during the methanogenesis phase was 0.405 m3 compared to 0.261 m3 in the case of the OFMSW AD. At the end of the experiments, the cumulative methane production was higher by about 39%, and in the co-digestion case reaching values of 8.21 m3. In this way, we illustrated how the AD practice could be improved, enhancing at the same time the biogas productivities. Additionally, from the results of the digestate analysis, we pointed out that the degradability was enhanced. Thus, the TS decreased from 19.8% to 22.7% and VS from 31.2% to 37.6%.
The results obtained in this work could potentially increase the renewable energy sources in Romania and worldwide, by offering practical and appropriate options for a proper management of the OFMSW and FVW. Future developments will be focused on improving the anaerobic digestion process of OFMSW and the economic feasibility of a biogas plant. Therefore, future research will be extended in terms of: reducing HRT with environmental benefits and the optimization of the volume of the digesters; use of more efficient mechanical mixing processes; use of alkaline inoculants; improving the inoculants concentrations and the environment development for methanogenic bacteria (development of biocatalysts); and AD reactor automated control systems improvement so that this can respond to changes in process parameters (pH, temperature, and others). Also, future works would study the possibility of establishing the market for AD-derived products and the decrease of AD projects in terms of costs (i.e., construction and operation of AD plants) based on regulation in force, public policies, and tax incentives.

Author Contributions

C.S. (Constantin Stan) conceived and contributed to the writing of the paper, together with G.C. and C.S. (Constantin Streche) designed and performed the experiments. C.S. (Constantin Stan), C.S. (Constantin Streche), and T.A. contributed to the final pilot plant design. G.C. analyzed the data and optimized the process parameters. T.A. performed analysis of the data and validation of the results. D.M.C. contributed to the writing and the final design of the paper.


This work was supported by the University POLITEHNICA of Bucharest, through the “Excellence Research Grants” Program, UPB-GEX 2017. Identifier: UPB-GEX 2017, Ctr. No. 12/25.09.2017 (MODEM) and under FSE—European Structural Funds POC-A1-A1.2.3-G-2015, PROVED, ID P_40_301, My SMIS: 105707, Nb. 78/08.09.2016 (2016–2021).


The financial support by the University POLITEHNICA of Bucharest, Romania is highly acknowledged.

Conflicts of Interest

The authors declare no conflict of interest.


  1. International Energy Agency (IEA). Key World Energy Statistics 2017. Available online: (accessed on 2 February 2018).
  2. World Energy Council. World Energy Resources 2016. Available online: (accessed on 12 December 2017).
  3. Population Division, Department of Economic and Social Affairs, United Nations. World Urbanization Prospects: The 2017 Revision. Available online: (accessed on 13 December 2017).
  4. Hoornweg, D.; Bhada-Tata, P. What a Waste: A Global Review of Solid Waste Management; World Bank: Washington, DC, USA, 2012. [Google Scholar]
  5. United Nations Environment Programme (UNEP). Guidelines for National Waste Management Strategies: Moving from Challenges from Opportunities. Available online: (accessed on 10 November 2017).
  6. Protocol on Pollutants Release and Transfer Registers. Available online: (accessed on 10 November 2017).
  7. Decision 406/2009/EC of the European Parliament and of the Council of 23 April 2009, on the Effort Members States to Reduce Their Greenhouse Gas Emissions to Meet the Community’s Greenhouse Gas Emission Reduction Commitments up to 2020. Available online: (accessed on 12 November 2017).
  8. Directive 1999/31/EC of the European Parliament and of the Council of 26 April 1999, on the Landfill of Waste. Available online: (accessed on 2 December 2017).
  9. Directive 2009/28/CE of the European Parliament and of the Council of 23 April 2009, on the Promotion of the Use of Energy from Renewable Sources. Available online: (accessed on 12 January 2018).
  10. Kangle, K.M.; Kore, S.V.; Kore, V.S.; Kulkarni, G.S. Recent Trends in Anaerobic Co-digestion: A Review. Univers. J. Environ. Res. Technol. 2012, 2, 210–219. [Google Scholar]
  11. National Environment Agency (ANPM). Annual Report on the State of the Environment 2014, Bucharest. 2015. Available online: (accessed on 18 January 2018).
  12. National Institute of Statistics and Census (INEC). Integrated System of Environmental Statistics. Available online: (accessed on 24 February 2018).
  13. Programa Nacional Para la Gestión Integral de Desechos Sólidos—PNGIDS ECUADOR. Available online: (accessed on 2 June 2018).
  14. National Waste Management Plan—PNGD ROMANIA. Available online: (accessed on 2 June 2018).
  15. Kumar, S.; Smith, S.R.; Fowler, G.; Velis, C.; Kumar, S.J.; Arya, S.; Rena; Kumar, R.; Cheeseman, C. Challenges and opportunities associated with waste management in India. R. Soc. Open Sci. 2017, 4, 1–11. [Google Scholar] [CrossRef] [PubMed]
  16. Zakir, H.M.; Hasna, H.Q.; Uddin, M.; Ahmed, T. Municipal solid waste (MSW) as a source of renewable energy in Bangladesh: Revisited. Renew. Sustain. Energy Rev. 2014, 39, 35–41. [Google Scholar] [CrossRef]
  17. Sathish, S.; Chandrasekaran, M.; Parthiban, A. Effect of co-digestion agricultural-industrial residues: Various slurry temperatures. Int. J. Ambient Energy 2017. [Google Scholar] [CrossRef]
  18. Das, A.; Mondal, C. Biogas production from Co-digestion of substrates: A Review. Int. Res. J. Environ. Sci. 2016, 5, 49–57. [Google Scholar]
  19. Gashaw, A. Co-digestion of municipal organic wastes with night soil and cow dung for biogas production: A Review. Afr. J. Biotechnol. 2016, 15, 32–44. [Google Scholar] [CrossRef]
  20. Fernandez, J.; Perez, M.; Romero, L. Comparison of mesophifilic and thermophifilic dry anaerobic digestion of OFMSW: Kinetics Analysis. Chem. Eng. J. 2013, 232, 59–64. [Google Scholar] [CrossRef]
  21. Pavi, S.; Kramer, L.E.; Gomes, L.P.; Miranda, S. Biogas production from co-digestion of organic fraction of municipal solid waste and fruit and vegetable waste. Bioresour. Technol. 2017, 228, 362–367. [Google Scholar] [CrossRef] [PubMed]
  22. Ragazzi, M.; Maniscalco, M.; Toretta, V.; Ferronato, N.; Rada, E.C. Anaerobic digestion as sustainable source of energy: A dynamic approach for improving the recovery of organic waste. Energy Procedia 2017, 119, 602–614. [Google Scholar] [CrossRef]
  23. Chatterjee, B.; Mazumder, D. Anaerobic digestion for the stabilization of the organic fraction of municipal solid waste: A review. Environ. Rev. 2016, 24, 426–459. [Google Scholar] [CrossRef]
  24. Ciuta, S.; Antognoni, S.; Rada, E.C.; Ragazzi, M.; Badea, A.; Cioca, L.I. Respirometric index and biogas potential for different food and agricultural discarded biomass. Sustainability 2016, 8, 1311. [Google Scholar] [CrossRef]
  25. Andreottola, G.; Ragazzi, M.; Foladori, P.; Villa, R.; Langone, M.; Rada, E.C. The UNITN integrated approach for OFMSW treatment. UPB Sci. Bull. Ser. C 2012, 74, 19–26. [Google Scholar]
  26. Monson, K.D.; Esteves, S.R.; Guwy, A.J.; Dinsdale, R.M. Anaerobic Digestion of Biodegradable Municipal Wastes–A Review; SERC, University of Glamorgan: Wales, UK, 2007. [Google Scholar]
  27. Rapport, J.; Zhang, R.; Jenkins, B.M.; Williams, R.B. Current Anaerobic Digestion Technologies Used for Treatment of Municipal Organic Solid Waste; California Environmental Protection Agency: Sacramento, CA, USA, 2008. Available online: (accessed on 2 February 2018).
  28. Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ). Guide to Biogas. From Production to Use, Federal Ministry for Economic Cooperation and Development, Germany. 2010. Available online: (accessed on 5 January 2018).
  29. Liotta, F.; D’Antonio, G.; Esposito, G.; Fabbricino, F.; van Hullebusch, E.D.; Lens, P.; Pirozzi, F.; Pontoni, L. Effect of total solids content on methane and volatile fatty acid production in anaerobic digestion of food waste. Waste Manag. Res. 2014, 32, 947–953. [Google Scholar] [CrossRef] [PubMed]
  30. Yi, J.; Dong, B.; Jin, J.; Dai, X. Effect of Increasing Total Solids Contents on Anaerobic Digestion of Food Waste under Mesophilic Conditions: Performance and Microbial Characteristics Analysis. PLoS ONE 2014, 9, e102548. [Google Scholar] [CrossRef] [PubMed]
  31. Saajena, B.B.; Jose, P.P.; Mandu, G. Effect of Total Solid Concentration of Anaerobic Digestion of the Organic Fraction of Municipal Solid Waste. Int. J. Sci. Res. Publ. 2013, 3, 1–5. Available online: (accessed on 15 February 2018). [Google Scholar]
  32. Haji, A.; Rhachi, M. The influence of particule size on the performance anaerobic digestion of municipal solid waste. Energy Procedia 2013, 36, 515–520. [Google Scholar] [CrossRef]
  33. Hassan, S.R.; Zaman, N.Q.; Dahlan, I. Effect of organic loading rate on anaerobic digestion: Case study on recycled paper mill effluent using Modified Anaerobic Hybrid Baffled (MAHB) reactor. KSCE J. Civ. Eng. 2015, 19, 1271–1276. [Google Scholar] [CrossRef]
  34. Chen, Y.; Rossler, B.; Zielonka, S.; Wonneberger, A.M.; Lemmer, A. Effects of Organic Loading Rate on the Performance of a Pressurized Anaerobic Filter in Two-Phase Anaerobic Digestion. Energies 2014, 7, 736–750. [Google Scholar] [CrossRef]
  35. Parra-Orobio, B.A.; Torres-Lozada, P.; Marmolejo-Rebellón, L.F. Influence of the mixing ratio on the anaerobic co-digestion of municipal biowaste with domestic wastewater sludge on methane production. DYNA 2016, 83, 86–93. [Google Scholar] [CrossRef]
  36. Yong, Z.; Dong, Y.; Zhang, X.; Tan, T. Anaerobic co-digestion of food waste and straw for biogas production. Renew. Energy 2015, 78, 527–530. [Google Scholar] [CrossRef]
  37. Ministry of Environment. National Program for Integrated Management of Solid Waste—PNGIDS, Ecuador. Available online: (accessed on 16 February 2018).
  38. Dublein, D.; Steinhauser, A. Biogas from Waste and Revewable Resourses: An Introduction, Second, Revised and Expanded Ed.; Wiley-VCH: Weinheim, Germany, 2011. [Google Scholar]
  39. Zhongfang, L.; Zhenya, Z.; Weiwei, H.; Wei, C. Recent Progress on Dry Anaerobic Digestion of Organic Solid Wastes: Achievements and Challenges. Curr. Org. Chem. 2015, 19, 400–412. [Google Scholar] [CrossRef]
  40. Chen, G.; Liu, G.; Yan, B.; Shan, R.; Wang, J.; Li, T.; Xu, W. Experimental study of co-digestion of food waste and tall fescue for biogas production. Renew. Energy 2016, 88, 273–279. [Google Scholar] [CrossRef]
  41. Fiore, S.; Ruffino, B.; Campo, G.; Roati, C.M.C. Zanetti Scale-up evaluation of the anaerobic digestion of food-processing industrial wastes. Renew. Energy 2016, 96, 949–959. [Google Scholar] [CrossRef]
  42. Adebayo, A.O.; Jekayinfa, S.O.; Linke, B. Effect of co-digestion on anaerobic digestion of pig slurry with maiz cob at mesophilic temperature. J. Natl. Sci. Res. 2014, 4, 66–73. [Google Scholar]
  43. Zhu, G.; Jha, A.K. Psycrophilic dry anaerobic digestion of cow dung for methane production: Effect of inoculum. Sci. Asia 2013, 39, 500–510. [Google Scholar] [CrossRef]
  44. Sovacool, B.K.; Kryman, M.; Smith, T. Scaling and commercializing mobile biogas systems in Kenya: A qualitative pilot study. Renew. Energy 2015, 76, 115–125. [Google Scholar] [CrossRef]
  45. Aidan, W.; Niamh, P. Biogas from cattle slaughterhouse waste: Energy recovery towards an energy self-sufficient industry in Ireland. Renew. Energy 2016, 96, 479–489. [Google Scholar] [CrossRef]
  46. Li, C.; Champagne, P.; Anderson, B.C. Enhanced biogas production from anaerobic co-digestion of municipal wastewater treatment sludge and fat, oil and grease (FOG) by a modified two-stage thermophilic digester system with selected thermo-chemical pre-treatment. Renew. Energy 2015, 83, 474–482. [Google Scholar] [CrossRef]
  47. Yoon, Y.M.; Kim, S.H.; Shin, K.S.; Kim, C.H. Effects of Substrate to Inoculum Ratio on the Biochemical Methane Potential of Piggery Slaughterhouse Wastes. Asian-Australas J. Anim. Sci. 2014, 27, 600–607. [Google Scholar] [CrossRef] [PubMed]
  48. Directive 2008/98/CE of the European Parliament and of the Council of 19 November 2008 on Waste and Repealing Certain Directives. Off. J. Eur. Union 2008, L 312, 3–30. Available online: (accessed on 2 June 2018).
  49. Kheiredine, B.; Derbal, K.; Mossaab, B.-L. Effect of inoculum to substrate ratio on termophilic anaerobic digestion of the dairy wastewater. Chem. Eng. Trans. 2014, 37, 865–870. [Google Scholar] [CrossRef]
  50. Hou, L.; Ji, D.; Zang, L. Inhibition of anaerobic biological treatment: A review. IOP Conf. Ser. Earth Environ. Sci. 2018, 112. [Google Scholar] [CrossRef]
  51. Achinas, S.; Achinas, V.; Euverink, G. A technological overview of biogas production from biowaste. Engineering 2017, 3, 299–307. [Google Scholar] [CrossRef]
  52. Huang, X.; Yun, S.; Zhu, J.; Du, T.; Zhang, C.; Li, X. Mesophilic anaerobic codigestion of aloe peel waste with dairy manure in the batch digester: Focusing on mixing ratios and digestate stability. Bioresour. Technol. 2016, 218, 62–68. [Google Scholar] [CrossRef] [PubMed]
  53. Ventorino, V.; Romano, I.; Pagliano, G.; Robertiello, A.; Pepe, O. Pre-treatment and inoculum affect the microbial community structure and enhance the biogas reactor performance in a pilot-scale biodigestion of municipal solid waste. Waste Manag. 2018, 73, 69–77. [Google Scholar] [CrossRef] [PubMed]
  54. Li, D.; Yuan, Z.; Sun, Y. Semi-dry mesophilic anaerobic digestion of water sorted organic fraction of municipal solid waste (WS-OFMSW). Bioresour. Technol. 2010, 101, 2722–2728. [Google Scholar] [CrossRef]
  55. Mao, C.; Feng, Y.; Wang, X.; Ren, G. Review on research achievements of biogas from anaerobic digestion. Renew. Sustain. Energy Rev. 2015, 45, 540–550. [Google Scholar] [CrossRef]
  56. Velmurugan, B.R.; Ramanujan, A. Anaerobic digestion of vegetable wastes for biogas production in a fed-batch reactor. Int. J. Emerg. Sci. 2011, 1, 478–486. [Google Scholar]
  57. Forster-Carneiro, T.; Pérez, M.; Romero, L.I. Anaerobic digestion of municipal solid waste: Dry thermophilic performance. Bioresour. Technol. 2008, 99, 8180–8184. [Google Scholar] [CrossRef] [PubMed]
Figure 1. Municipal solid waste (MSW) generation in Romania and Ecuador.
Figure 1. Municipal solid waste (MSW) generation in Romania and Ecuador.
Sustainability 10 01939 g001
Figure 2. MSW organic composition.
Figure 2. MSW organic composition.
Sustainability 10 01939 g002
Figure 3. Isometric view of the pilot plant used for the neutralization/stabilization of OFMSW.
Figure 3. Isometric view of the pilot plant used for the neutralization/stabilization of OFMSW.
Sustainability 10 01939 g003
Figure 4. Float cover assembly.
Figure 4. Float cover assembly.
Sustainability 10 01939 g004
Figure 5. Experiment No. 2: temperature variation with time.
Figure 5. Experiment No. 2: temperature variation with time.
Sustainability 10 01939 g005
Figure 6. Experiment No. 2: pH variation with time.
Figure 6. Experiment No. 2: pH variation with time.
Sustainability 10 01939 g006
Figure 7. Experiment No. 2: daily and cumulative biogas and CH4 production.
Figure 7. Experiment No. 2: daily and cumulative biogas and CH4 production.
Sustainability 10 01939 g007
Figure 8. Experiment No. 3: temperature variation with time.
Figure 8. Experiment No. 3: temperature variation with time.
Sustainability 10 01939 g008
Figure 9. Experiment No. 3: pH variation with time.
Figure 9. Experiment No. 3: pH variation with time.
Sustainability 10 01939 g009
Figure 10. Experiment No. 3: daily and cumulative biogas and CH4 production.
Figure 10. Experiment No. 3: daily and cumulative biogas and CH4 production.
Sustainability 10 01939 g010
Figure 11. Average cumulative biogas yields of mono and co-digestion of OFMSW.
Figure 11. Average cumulative biogas yields of mono and co-digestion of OFMSW.
Sustainability 10 01939 g011
Figure 12. Experiment No. 2: Biogas composition variations over time.
Figure 12. Experiment No. 2: Biogas composition variations over time.
Sustainability 10 01939 g012
Figure 13. Experiment No. 3: Biogas composition variation over time.
Figure 13. Experiment No. 3: Biogas composition variation over time.
Sustainability 10 01939 g013
Table 1. Characteristics and feeding conditions of substrate and inoculum used in the framework of the study.
Table 1. Characteristics and feeding conditions of substrate and inoculum used in the framework of the study.
WasteWeight W [kg]Percentage [%]Moisture H [%]TS [%]VS [% (VS/TS)]pHC/N
Dry grass62.024.676.481.67.212
Green corn stalks3511.977.312.775.36.126
Beef manure4013.772.627.473.86.318
Pig manure4013.773.226.874.37.312
Total29310079.51 *20.49 *79.17 *6.49 *22.78 *
* weighted average values. OFMSW = organic fraction of municipal solid waste; TS = total solids; VS = volatile solids; C/N = carbon and nitrogen ration.

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (
Sustainability EISSN 2071-1050 Published by MDPI AG, Basel, Switzerland RSS E-Mail Table of Contents Alert
Back to Top