Two-Phase Experimentation to Determine the Optimal Composition for the Production of Biogas and Biol Substrate Mixing Waste from the Camal de Guayaquil

Abstract

In this two-phase study, it was shown that a mixture with equal parts of manure and resulting animal blood was the optimal combination for obtaining biogas and biol. A quadratic growth trend in variable gas pressure over time—as well as its behavior—was confirmed for pHs around the neutral value for the substrate used in both the pilot phase and in the microplant, which had a mechanical implementation and mechatronic system for the control of variables that intervene in the anaerobic digestion process; this allowed for the confirmation of the results found in the first phase of research—without concerns that a lack of control over the process variables would cause—in such a way that it constituted a path for the industrialization of the waste treatment process in slaughterhouses that could be optimized by the use of the optimal combination that produces the greatest cm³ amount of gas. Anaerobic digestion in biodigesters is carried out at different constant temperature values within the mesophilic range, with a hydraulic retention time of 25 days. A direct relationship was found between temperature, biogas production and pH behavior in the buffer(s). The pH remained close to neutral and the gas pressure increased from 15 to 20. The findings indicated that the value for the C/N ratio of the blood of four was compensated for by its buffer system, composed of bicarbonate, hemoglobin, proteins and phosphates.

1. Introduction

There are several countries in every continent that have biogas production among their national strategies for energy production. Many research works have aimed at the improvement and creation of new technologies that enable a greater production of biofuels as well as recycling and reusing waste in order to generate energy; these have become viable solutions for facing the problem of the elimination of waste [1].

Anaerobic digestion is known as a biochemical process in which complex organic compounds are decomposed by anaerobic microorganisms in the absence of oxygen [1]. It has been used in Western Europe since the 1980s and one hundred and ninety-five facilities using this process have been built as of 2010, with a total annual capacity of 5.9 million tons [2].

The transformation of biodegradable matter from waste into methane gas has its origin in anaerobic digestion, which is the main process used for biosolid stabilization, the destruction of volatile solids and the production of stabilized biosolids [3]. It is known that the performance and synergy of this process is strongly related to the characteristics of the substrates used [4].

Biogas production can be a form of clean energy production in slaughterhouses [5]—taking advantage of organic residues and waste from the slaughter of large and small livestock and poultry—carried out through the implementation of plants to achieve anaerobic digestion, which can be used as a renewable energy technology for the mitigation of environmental pollution [6]. The wastewater and blood that are disposed of in the Guayaquil slaughterhouse are discharged into the Estero Salado river, causing a negative impact on this water body [7,8].

Some research has concluded that the utilization of slaughterhouse waste could result in unstable process performance and stress conditions for microbial communities due to the formation of potential inhibitory compounds. Others, to improve this performance, have paid special attention to the influence of operating parameters such as the organic load rate, the hydraulic retention time and the effect of temperature on the performance of the semi-continuous digestion process [4].

The objective of this study was to find the appropriate substrate composition when one of the substances used included a buffer solution. Two phases of experimentation were implemented: The second one was the “Modeling, implementation and automation of a micro plant of production of biogas from residual biomass” [9], which in this paper was referred to as a microplant for biogas production. The first phase aimed to characterize the waste from the Guayaquil slaughterhouse, which allowed the identification of a characteristic curve of biogas production over time for each combination of waste studied. The objective of the second phase was to study the combination of waste that produced the most gas during the first phase in a microplant with a mechanical implementation and mechatronic system for the control [10] of variables that intervene in the anaerobic digestion process, which allowed for the confirmation of the results found in the first phase of research—without the concerns that the non-control of process variables would cause—in such a way that they constituted a path for the industrialization of the waste treatment process in slaughterhouses, optimized by the use of the best combination of these residues.

The first phase, carried out at pilot-scale, was the “Characterization of organic waste from the Guayaquil slaughterhouse” [11]. The piloting of the first phase is detailed below because the tests carried out were the starting point for larger scale microplant research.

By using the optimal combination of organic waste to ensure the development of methanogenic bacteria in the process of anaerobic digestion, it is possible to find out the best values of moisture (H), total solids (TS), volatile solids (VS), average time of biogas production and pH for optimal biogas production. The process temperature was maintained within the mesophilic range [12].

The substrate used in the six test-runs during this first phase varied according to the combination of wastes. The wastes were placed in a closed digester with discontinuous flow [13], measuring the amount of biogas produced. As shown in Figure 1, the system with the storage tank consisted of a 180 cm³ plastic bottle that was left under vacuum. To transfer the biogas produced from the biodigester (DEP_01) to the storage tank (DEP_02), the valves were operated. The equipment called DEP_03 consisted of a 10 cm³ syringe to help transfer the biogas to the tank when required. Figure 2 and Figure 3 show the type of valve and the suitability of the biodigester for installation in the system, respectively [11].

Residues and Conditions Imposed for Tests One to Six

Organic waste of animal origin, such as that from slaughterhouses (blood and others), can be used in methanogenic fermentation [14].

Table 1. Amounts imposed on the substrate for tests one to six.

Test	TS Manure (g)	Fat (gr)	TS Blood (gr)	TS Liver (g)	Total Water (g)
(P1)	100	-	-	100	1400
(P2)	50	-	50	-	1500
(P3)	50	-	-	100	1450
(P4)	50	50	-	-	1500
(P5)	30	-	50	70	1450
(P6)	600	-	-	-	1000

lists the amounts imposed on the substrate for each of the tests and

Table 2. Total values and percentage values of the solids and the humidity of the six tests.

Test	TS (g)	%TS	%Humidity	VS (g)	Total (g)
(P1)	200	12.50	87.50	117.8	1600
(P2)	100	6.25	93.75	37.95	1600
(P3)	150	9.38	90.63	97.30	1600
(P4)	100	6.25	93.75	52.90	1600
(P5)	150	9.38	90.63	83.57	1600
(P6)	600	37.5	62.50	245.56	1600

relates the total values and percentage values of the solids and the humidity of the mixtures used. A temperature of 32 °C [15] was maintained due to the use of a HANYOUNG process controller—model NX4 with an RTD as a transducer, an ON/OFF setting and a hysteresis of ±1 °C [11].

Table 3. Characteristics of the residues used in the six tests.

Values	Unit	Blood	Liver	Manure
pH		7.19	6.15	6.9
C/N		4	24	23
VS	%	30.97	60.4	40.93
FS	%	15.00	25.50	19.00
Humidity	%	84.23	71.06	70

shows the characteristics of the residues used in the six tests. In Test 2 (P2)—blood, manure and water—it is important to note the differences between the C/N ratio of the cattle blood from both the Andean Plateau and the Plateau in comparison to the manure. These two regions are extensive high plains located between 2900 and 3900 m above sea level in the Central Andes.

Table 4. Biogas production parameters and pH of test (P1–P6) mixtures.

	Initial Mixture pH	Final Mixture pH	Gas (cm3)	Production (Days)	Efficiency (cm3·gVS−1)
(P1)	6.7	7.5	120.7	13	1.02
(P2)	7.2	7.0	245.2	22	6.46
(P3)	7.8	7.3	53.7	11	0.55
(P4)	6.8	5.7	146.0	15	2.76
(P5)	7.0	7.0	166.0	13	1.99
(P6)	7.4	7.5	219.3	13	0.89

shows the parameters for the biogas production and the pH values of the mixture for all tests.

In all tests, the pH remained unrchanged at the beginning and end—except in Test 4, in which there was a variation of 1.1. The gas production in Test 2 (P2), which was the only one that used blood, exceeded the others by a percentage interval between 10.7 and 78.4. The duration in days was also higher in this test by a percentage interval of 0.32–0.5; gas production showed similar behavior, surpassing that of the remaining five tests by a percentage interval of 57.3–91.5. The pH of the Test 2 (P2) mixture was measured with a “Extech PH100 meter” instrument, manufactured by Teledyne FLIR, headquartered in Wilsonville, Oregon, USA.

In Figure 4, the production, measured in days of biogas (cm³), can be seen for the six tests (P1–P6). The equation of the trend line that relates to the gas production in the six tests carried out shows exponential gas growth over time.

The studies on anaerobic digestion that were consulted maintain that the convenient C/N ratio is between 20 and 30 and that the ideal value for the pH is 7. As such, it can be inferred that the C/N ratio of blood should not be recommended for this process [16]. However, results such as those for the amount and days of biogas production are in contradiction with such an inference; as the relative humidity of the sample exceeded 80% [17], there was good heat transfer from the incubator and the total-solids concentration level was adequate.

These results suggest that a deepening of this investigation is required, which is why a larger scale and better instrumented system was built to control the temperature of the substrates throughout the anaerobic process, which allowed us to obtain reliable data on the variables under study.

The experimentation provided data customized to Ecuadorian conditions. This way, Ecuadorian bio-energy laboratories were able to replace their more general data with data pertaining to the physical–chemical characteristics of waste from Ecuador.

2. Materials and Methods

2.1. Microplant for Biogas Production

A microplant was implemented with two different systems: mechanical and electronic. The mechanical system, shown in Figure A1 (Appendix A), included accessories to hold the digital and analog instrumentation elements, to join the hoses and to measure the pressure at a determined temperature, among the other necessary functions of the experimental set.

The digestion system included the main elements such as the fermentation chamber, the fermentation chamber lid, the water chamber and the mechanical stirrer. The fermentation chamber was made of a 2 mm stainless steel sheet and had a capacity of 28 L, with the mixture occupying a quarter of this volume and the biogas the other three fourths in a cylindrical shape. The lid was made of the same material but was 6 mm thick. Additionally, a foam gasket for the chamber’s hermetic adjustment [18,19] and holes for its instrumentation were implemented, as shown in Figure 5.

The water chamber was responsible for maintaining the temperature at 20 °C and 35 °C, which are the values at which we wanted to know the behavior of the variables that affect the growth of bacteria in the methanogenic stage, within the mesophilic range [20].

The viscous mixture must be moved slowly to ensure that the fresh biomass from the top moves to the bottom and that the fermented biomass moves to the top; this also keeps the density of the microbial population constant, prevents dead spaces that form with the absence of activity, releases biogas bubbles that are present in the mixture and improves the degradation of the mixture. A turbine-type agitator [21,22] was added to maintain this movement, consisting of a crank, a central tube and two sets of blades mounted at a distance of 10 cm, as shown in Figure 6.

The gasometer used had a capacity of 20 L, an effective volume for the biodigester. The polyethylene body supported up to 5 bars and did not react with the gases involved in the process.

The desulfurizing filter was built with a container filled with steel shavings that was previously washed to remove grease and then subjected to two baths—the first with 5% HCL for a time between 5 and 10 min before being left to air dry. The second was carried out with 5% NAOH for the same time and with the same drying.

The electronic system, shown in Figure A2 (Appendix A), was composed of a sensor, control, actuator and connection system. The sensor system included 3 submersible temperature sensors for each biodigester—2 placed in the water chamber and one in the fermentation chamber—a pH electrode and a pressure sensor. To execute the recorded orders, an Arduino Mega 2060 microcontroller was placed. The store and data visualization were obtained through 12C communication, a real-time clock with an autonomous battery and internal memory that stored the set time. The data was visualized on a 3.2-inch TFT screen and stored in a datalog file, which was recorded on an SD memory card. Noise was filtered into the conditioning circuit by placing a capacitor between the signal and ground pins and the power and ground pins—thus achieving a uniform power supply.

The capacitors in the signal conditioning circuit for the pressure sensor were set up as shown in Figure A3 (Appendix A).

The analog pressure sensor required an analog/digital converter to be incorporated into the controller with a transfer function for signal conversion, as Figure A4 shows (Appendix A).

The two resistors handled a total current of 27 A in each tank; they were activated independently using solid state relays, as shown in Figure A5 (Appendix A).

A control board interconnected all of the elements. The micro production plant had two software programs, as shown in Figure A6 (Appendix A). One software program managed the data recording in the human–machine interface (HMI), with an interactive method for entering the temperature of the tank controllers. This allowed for process restarting in case of electric supply interruptions or for the continuation of the previous process.

The second software was a subroutine—implemented in a numerical computer system—for the processing of the data taken from the sensors placed in the tanks, taken every minute. The subroutine was executed by the controller and stored and displayed on a single screen, as indicated in

Table 5. Data that was recorded for each tank.

Fact	Description
T1	Water temperature
T2	Water temperature
T3	Biogas temperature
T4	Substrate temperature
P	Biogas Pressure
PH	Substrate PH

. The data processing software allowed data plotting as a function of time during the experiment.

The tanks were isolated in the final installation as shown in Figure 7.

Temperature control was carried out over a ±2 °C range to avoid negative influences on the growth of methanogenic bacteria, as well as the presence of fatty acids in the biodigester [15].

2.2. Composition of the Substrate

The composition of the substrate for both digesters—as shown in

Table 6. Description of the substrates contained in the T0 and T1 digesters.

Substrate Component	Characteristic	T0	T1
Manure	Source	Porcine	Porcine
	Quantity	5 kg	5 kg
	Humidity Physical Treatment before the test	0.43%	0.43%
		Sun dried	Sun-dried
Blood	Source	Freshly slaughtered chickens	Freshly chickens Sacrificed
	Quantity	5 Kg	5 Kg
	Humidity	90%	90%
	Physical treatment before the test	None	None
Water	Source	Rain	Rain
	Quantity	18 L	18 L
	Physical treatment before the test	None	None
Variables	Temperature of operation in the water chamber	20 °C	35 °C
	C/N ratio	7.53	7.53
	Initial pH	7.54	6.67
	PH control	None	None
	Internal pressure in the digester	−0.101 (psi)	−0.101 (psi)

—was the same, and it was decided that rainwater should be used to avoid the inconveniences that chlorine levels in the composition of drinking water entail and so that the water could be replaced in rural areas by water from wells, taking the specific weight of 1 g/1 cm³.

3. Results and Discussion

The experimentation lasted 25 days and then the results of the data collection in the different systems of the microplant during this period were listed.

3.1. Temperature in the Water Chamber

The sensors located in the water jacket of the heating system were those that controlled the switching of the heating elements. In the T0 biodigester tank, the temperature was maintained within an interval of 24.5–20.38 °C, following the set point of 20 °C, as shown in Figure 8.

The working temperature range of the T1 biodigester was 21–38 °C, as shown in Figure 9.

3.2. Temperature inside the Substrate or Fermentation Chamber

The data collection was carried out in the central part of the substrate to contrast with that taken at the edges, which was in contact with the water jacket. The initial temperature in the center of the substrate for both tanks was the same, between 21 and 22 °C. In the T0 tank there was no major change, since the temperature of the water surrounding the substrate had a set point of 20 °C. In the T1 tank, a rise in temperature was observed, reaching an interval of 34–38 °C; the set point of the water temperature in the heating system was 38 °C—see Figure 8, Figure 9 and Figure 10.

3.3. Gas Temperature inside the Fermentation Chamber

Sensors were placed in the upper part of the substrate in order to measure the temperature of the gas, registering intervals of 17.5–26 °C and 26.75–37 °C in the T0 and T1 tank, respectively—as shown in Figure 11. The variation observed was due to the transfer of heat by radiation and convection, produced by the incidence of the sun’s rays on the tank covers.

3.4. Behavior of the pH of the Substrate

The experimentation did not control the pH evolution in any way. In Figure 12, the pH evolution in the T0 tank can be observed. As can be seen, the initial value was 7.54; after 5 h it took the value of 5.27, during the following 2 days it remained at a value of 7 and then it dropped to a value of 6.71 and oscillated between that value and the neutral state for the next 2 days. After this time, the pH rose, taking a value of 7.77; later it oscillated between this value and around six. Finally, the value fell to 5.5 in the eleventh day; this indicated that the test went from a neutral state to an acidic state, staying in that area until the end of the process—showing that not all of the stages of anaerobic digestion occurred in this tank. Figure 13 shows how varied the pH was in the T1 biodigester—the first value taken was 6.72, after five hours it dropped to 5.27 and stayed at that value for two days, then it dropped to 4.29 for five consecutive days, rose to 6.5 and stayed in the neutral zone for the next two days and then it dropped again at 5.37 for a day and a half, finally decaying to 3.36 at the end of the process.

3.5. Behavior of the Pressure Values in Tanks T0 and T1

Figure 14 and Figure 15 show the pressure evolution in the T0 and T1 tanks. In the T0 tank, at the beginning of the process, the pressure was −0.41 psi for approximately 5 days. After a short time, it lowered to −0.21 kpa; then, the pressure reached the 4 kpa value, which was maintained until the end of the process.

In the T1 tank, the pressure started with a value of −0.322 kpa; after the first two days it rose to 0.229 kpa and remained approximately at that value for two more days. On the fourth day, the pressure reached the value of 1.22 kpa, on the seventh day reached the value of 3.87 kpa and then rose to the value of 25 kpa and remained at that value—approximately—until the completion of the process. The pressure growth in both tanks was exponential.

By comparing the graphs of pH, gas pressure and experimentation time in the fermentation chamber of the tanks shown in Figure 16 and Figure 17, it can be noted that in both tanks the pH values were kept close to the neutral value, and that the gas pressure increased between 15 and 20 days, approximately.

The substrates of both tanks are shown in Figure 18 and Figure 19—presenting different aspects. In tank T0 the substrate was not completely digested, but in T1 it was.

4. Conclusions

The waste combinations used in the first phase of experimentation were different and the combination that produced the highest gas production was the one used in Test 2 (P2), with 50 g of total solids for manure, 50 g of total solids for blood and 1500 mL of water.

The graphs for biogas production during the first phase—the P1–P6 tests—and the gas pressure obtained in the microplant maintained the same trend, adjusting to a polynomial curve of degree two, which indicates the exponential growth of both over time.

The gas pressure value in biodigester T1 exceeded the pressure value in biodigester T0 by 83%, because the temperature in the water chamber of biodigester T1 exceeded that of biodigester T0 by 36.84%, which shows the influence of the substrate temperature for biogas production.

The trend lines of the pH vs. time graphs for both tanks showed the same behavior due to the buffer effect of the blood and showed an approximate non-harmonic oscillatory movement function.

The production of acidogenic bacteria in the T0 biodigester was much lower than that of the T1biodigester, since in this, the temperature of the experiment was kept constant at a value approximately 16 °C lower than the temperature in the T1 tank, which makes interrupted the process before the methanogenic phase.

There was a direct relationship between the average degradation time of organic matter of approximately 25 days and the high quality of the residue in the T1 biodigester and inverse in the T0 biodigester; the amount of gas-producing bacteria in the T1 was greater than in the T0, which means that the demand for food also was, and this resulted in a better quality of residue.

The low C/N ratio of the blood was compensated for by its buffer system—composed of bicarbonate, hemoglobin, proteins and phosphates—with small variations in pH, a longer life of methane gas-producing bacteria and a longer retention time.

5. Recommendations

Several experiments should be carried out with the same substrate and different constant working temperatures to find the optimal production temperature for methanogenic bacteria in the mesophilic range.

The concentrations of the components of the blood buffer system should be broken down through clinical biochemistry practices, which will reduce uncertainty in the results of biochemical studies of buffer solutions.

A system that separates the gas product of anaerobic digestion into portions should be implemented, in which each portion could be analyzed by a chromatograph at each stage.

An experiment that allows the identification of the behavior curve of the substrate digestion and the value of the constant temperature at which the anaerobic digestion is carried out should be designed.