Biogas Production in a Variety of Sawdust and Cow Dung Mixtures Using the Intermittent Mixing Method

Cow dung is livestock waste that is very useful for renewable energy sources. The acceleration of the gas rate in cow dung biogas fermentation can be accelerated by the addition of secondary material, sawdust is one of the organic materials that can accelerate the rate of gas production in the formation of cow dung biogas. This study used a mixture composition: (B1) 675 g cow dung and 0 g sawdust, (B2) 472.5 g cow dung and 202.5 sawdust, (B3) 450 g cow dung and 225 g sawdust, and (B4) 427.5 g cow dung and 247.5 g of sawdust by fermentation for 20 days. In the research process, measurements of pH at the beginning and at the end of the study, gas pressure, gas volume, biodigester temperature, production rate, methane gas content, and flame test were carried out. In this study, the pH values ranged from 7-7.5 and the biodigester temperature was 27.5-31°C. The results of this study indicate that an organic mixture with a mixture of sawdust can accelerate production faster than without organic sawdust. Optimal results are found in the variation of the organic mixture of sawdust with a mixture of 450 g cow dung and 250 g sawdust having the highest production rate of 281.72 ml/day.


Introduction
The consumption of electrical energy in Indonesia is increasing along with the increasing number of people who use electrical energy [1].The Electrical energy that has been used daily is obtained from power plants, most of which come from fossil fuels.Fossil fuels are energy sources whose formation process takes millions of years, so fossil energy cannot be renewed.Human dependence on electrical energy makes electrical energy very important for human life.However, the increasingly expensive price of fossil energy affects the price of electrical energy.In Indonesia, there are still many areas that have not been able to gain access to the PLN electricity distribution network.Some of the influencing factors are the lack of regional development that is far from the centre of the economy and the lack of availability of electrical energy.The solution to reducing the electrical energy crisis is the existence of alternative energy sources.Alternative energy is energy with abundant and renewable availability, which is expected to be environmentally friendly, effective, efficient, and accessible fuel for the wider community.
Biogas is a good alternative energy to be developed.Biogas technology is a technology that is easy to apply, and the raw materials used to produce gas is widely available in various areas such as livestock waste, agricultural waste, industrial waste, and the like that have organic content Error!Reference source not found.. Biogas is a mixture of several gases with the main components of methane (CH4) and carbon dioxide (CO2), with a small amount of water vapour.The composition of biogas depends on the raw materials used.When using human waste, animal waste, or liquid waste from a slaughterhouse, the methane gas produced can reach 70%.Raw materials from plants such as rice stalks, straw, or water hyacinth produce about 55% methane gas Error!Reference source not found..
Cow dung is a suitable substrate for biogas utilization because cow dung contains bacteria that produce methane gas.Biogas is produced through an anaerobic process, where organic materials are converted into biogas which have the main content of methane (CH4) and carbon dioxide (CO2) [5].
Abundant sawdust waste from sawn wood needs to be handled because it will cause environmental pollution.One way is to convert sawdust into bioethanol.In addition, this study also uses the intermittent mixing method where the biodigester will be stirred every day using an agitator connected to a motor.The motor rotation speed of 50 rpm for 40 minutes every day is seen in existing research journals.

Substrates and Inoculum
Cow manure was obtained from the livestock of residents around the Campus of the Faculty of Engineering.University Hasanuddin.The air-dried sawdust is obtained from wood manufacturing around campus in Gowa Regency.

Experimental Setup
The experiment used a 1.8 L bottle jar with 75% working volume, equipped with a 2-blade stirrer, and an outlet gas port.The digester cap is sealed with a thread seal and silicon rubber glue to ensure an anaerobic environment in the digester.The digester is connected to a gas gathering with water displacement-based to measure the volume of gas produced.To keep the digester temperature in the mesophilic range, the digester was placed in a water bath at a temperature of 37±1 °C.The experiments are carried out for 21 days with intermittent stirring at 50 rpm for 40 minutes every day till the experiment is completed.

Experimental Design
The experiment was conducted with 4 ratios of sawdust and cow manure.All digesters are filled with an initial substrate of 675 g.All reactors are run in duplicate.The ratio of sawdust and cow manure of each digester is shown in Table 1.

Biogas Production
During the AD process, sawdust and rice straw will be decomposition by anaerobic microorganisms that will produce a certain amount of gas.Data on the volume of biogas produced during the AD process are presented in Figure 1.In the early days of the AD process, B3 and B4 digesters showed relatively higher biogas production than other digesters.Meanwhile, the B1 digester showed lower biogas production and reached peak production longer.This is because the ratio of sawdust in biodigesters B3, B4, and B2 is higher than in biodigester 1 which does not use sawdust.So, it affects the acceleration of biogas production in each biodigester.Cumulative biogas production is the total amount of biogas produced from each digester.Data on cumulative biogas production in this study are presented in Figure 2. On the 20th day of the anaerobic digestion process, biodigester 3 produced the highest biogas of 5635 ml, followed by biodigester 4 (4994 ml), biodigester 2 (2311 ml), and biodigester 1 (2209 ml).

Methane Yield
Methane yield is defined as the amount of methane produced per unit VS decomposed.Measurement of methane concentration (CH4) is carried out through chemical scrubbing.One that is often used is the absorption of CO2 to estimate the concentration of methane in biogas Error!Reference source not found..In its use, CO2 is absorbed using an alkaline solution, i.e.NaOH or KOH.A previous study reported a 40% potassium hydroxide (KOH) solution was used Error!Reference source not found..

Figure 3. Concentration gas of CH4
The methane content of the four biodigesters can be seen in the diagram that the biodigester with code B3 with a mixture of 450 g cow dung and 225 g sawdust has the highest methane gas content value of 75%.From these results, it can be seen that the mixture in the biodigester with code B3 has a good mixture among the other four biodigesters, in addition to the highest value of methane gas content, it can also be seen that the biodigester code B3 also has the highest gas volume and high-pressure value.
The content of methane gas from digester B1 without using secondary materials has a concentration value of 73%.Compared to the B3 digester using sawdust as a secondary material, it has a methane gas concentration of 75%.It can be seen that even though B1 does not use sawdust as a secondary material, the concentration of methane gas obtained is quite high and even higher than digesters B2 and B4 which use sawdust.This means that sawdust here only helps to accelerate biogas production, but the increase in methane gas concentration is still less significant.

pH
The pH value of the mixture largely determines the performance of AD when the value is too low (acidic) or high (alkaline).Figure 6 contains the pH values of each digester at the beginning and the end of the AD process.From this study, the pH value ranged from the range of 6.7-7.8.Therefore, the pH value has met the optimal range in supporting the environment of methanogenic bacteria in the AD process as reported in another research Error!Reference source not found..The best pH range for the AD process is 6.8 -7.2.In general, the stages of methanogenesis are very sensitive to conditions of pH, oxygen, and other factors such as free ammonia (FAN), H2S, and VFA.Methanogenesis cannot take place at pH conditions below 6.2 and pH conditions > 7.8, free ammonia (FAN) will be formed which inhibits methanogenic reactions Error!Reference source not found..The accumulation of VFA when the acid produced from the hydrolysis and acidification stages cannot be decomposed by methanogenic bacteria will cause a decrease in the pH value and instability of the AD process Error!Reference source not found..In Figure 4, the pH values of the four biodigesters are in the range of 7-7.7,where the highest value is in Biodigester 1, the pH value is at 7.7 and the lowest is in Biodigester 4, at 7.5.In general, the more neutral the pH value, the higher the CH4 value and the lower the CO2 levels, a neutral pH can stimulate the development of methanogenic bacteria so that they can reproduce optimally, the optimal pH level is in the range of 7.2-8.2.

Production Rate
The calculation of the production rate can provide an overview of the development and productivity of an organic mixture of cow dung and sawdust which was fermented for 20 days.Production rate as shown in Table 2.The most optimal production rate of the mixture with sawdust secondary material is in the sample B3 cow dung 450 g and sawdust 225 g, this corresponds to the best composition of the biogas mixture for the catalyst of about 30% of the total weight of the organic mixture, however, this also influenced by the process when mixing organic matter such as the viscosity of the organic mixture, the stirring process must be evenly distributed until the organic mixture is homogeneous, and also the selection of dry cow dung.

Flame and Digester Pressure
Flame testing is carried out by injecting biogas into a syringe with capacity, Vsyringe = 60 ml.The port of a syringe is connected with a 1/8-inch tube.Biogas in the syringe is injected with flow rate, Q = 6.38 ml/s or 0.00638 l/s.The flame time of each gas sample is calculated and an average flame time of, t = 8.8 seconds is obtained.Then the duration of this flame is calculated for 1 m3 of biogas using the equation ( 1).From equation (1), an average flame time ± 38.88 hours for 1 m3 of biogas is obtained if the gas flow discharge is maintained at 6.38 ml/s.
A flame is a chemical reaction of combustion.The flame is influenced by the quality of the fuel, in this case, biogas.The carbon dioxide content in biogas acts as an inhibitor, which will determine the calorific value of biogas, where high carbon dioxide causes a low calorific value, and vice versa.The quality of biogas can also be seen from the flame test, where the flame that has a high calorific value / low carbon dioxide, the color of the fire will be dominated by blue Error!Reference source not found.. Combustion in biogas can still occur up to the limit of carbon dioxide content of 62% of the volume of biogas.This is in line with the indication of carbon dioxide content in biogas which tends to reduce the flame speed because it absorbs some of the heat from the combustion reaction Error!Reference source not found..

Figure 5. Flame test
Biogas produced from a digester with anaerobic environment results in the presence of gas pressure in the digester.Since the dimensions of space in the digester are fixed, the pressure will be directly proportional to the volume of gas inside the digester as shown in Figure 5.

Figure 6. Digester pressure
Biogas produced from the anaerobic digestion process results in gas pressure in the digester.From Figure 6 the four digesters show an increase in pressure from day to day.Digesters 2, 3, and 4 showed an increase in biogas pressure since the beginning of the anaerobic digestion process, compared to digester 1 which had a lower pressure value which on the 7th day only showed a high-pressure value.The increase in pressure tends to be faster in digesters 2, 3, and 4 compared to digester 1 because it is influenced by secondary materials.Where digesters 2, 3, and 4 have sawdust secondary material that affects the microorganisms in the digester to produce biogas in which has a higher pressure than digesters without secondary materials.
Digesters B3 and B4 have almost the same pressure values from the first day to the 11th day, but after the B4 digester chart decreased until the 20th day, the B3 digester graph tends to be stable until the 20th day.Even though the sawdust ratio in B4 is more than B3 it does not necessarily mean that the pressure value is higher.In B3 it has a more suitable sawdust content, which is 30% of the mixture, which results in the compressive value of B3 tending to be stable and high; This is different from the B4 digester, where the sawdust content has exceeded the limit, so even though the ratio of sawdust is higher, it does not mean that the pressure value obtained is higher.
The pressure from all digesters shows a pattern that tends to be the same except for digester 1. Starting from the beginning of the anaerobic digestion process, the pressure of the digester continues to increase until it reaches the peak of biogas production which is in the range of days 4-6 depending on the mixture ratio of each digester.However, digester 1 has the highest pressure on day 11, this is because digester 1 does not use secondary materials which are secondary materials that accelerate biogas production.The highest pressure values in this study were in digester 3 of 7.72 psi, followed by digester 4, 2, and 1 of 7.5 psi, 4.44 psi, and 3.93 psi.

Conclusion
Biogas research using the intermittent mixing method from the results that have been obtained is better than research without using the intermittent mixing method.This is because the four tested biodigesters are stirred every day using a motor connected to the agitator in each biodigester, causing microorganisms to be evenly mixed in the slurry which results in more biogas production when compared to the study without stirring where microorganisms are not mixed evenly which makes biogas production.only occurs in some parts of the slurry.The best-mixed ratio in the study using the intermittent mixing method was the biodigester code B3 with a ratio (2:1) followed by the biodigester B4, B2, and B1 with the respective ratios (1.9:2.1),(2.1:0.9), and (3 :0).The results of the research from the B3 biodigester with a ratio (2:1) obtained the highest pressure value of 7.72 psi, the cumulative gas volume of 5635 ml, and the production rate of 282 ml/day.

Figure 4 .
Figure 4. Comparison of pH value in biodigester

Table 1 .
Composition ratio of each digester

Table 2 .
Production Rate