Investigation of Biogas Potential of Plantain Peels Mixed with Poultry Droppings as an Alternative Energy Source and Its Statistical Analysis

Biogas offers a clean, cheap, non-toxic with characteristics of renewability, sustainability, and environmentally friendly source of energy when compared to fossil fuel...


Introduction
in earlier reported anaerobic digestion studies for biogas production [7][8][9][10], and poultry droppings, cow dung, and kitchen wastes [9], the average biogas production according to [9] show that poultry droppings gave the highest biogas yield. The objective of this study is to employ a small scale anaerobic digestion system to assess the methane yield and treatment efficiency of poultry droppings and plantain peels. Statistical analysis and optimization using a fourfactor response surface methodology were carried out to estimate the biogas production. The thermal treatment of substrates was also carried out as an effort to remove adherent dirt and to minimize water use in the feed. The effects of process parameters including substrate composition, hydraulic retention time, and operational changes on the system performance are reported.

Material and Method Materials
The anaerobic digestion system employed in this study comprises of gas holder, anaerobic digester, a water displacement tank, and a vulgarizer tube for biogas storage. The digester has a 250L capacity and a cylindrical shape with a conical bottom for the slurry outlet. Poultry droppings were obtained from Hadebs farm Kilanko Offa garage Ilorin, while plantain peels were collected from a restaurant (item 7) within the University of Ilorin, Kwara State Nigeria. pH meter was used in monitoring the pH of the slurry. The reagents used were of analytical grades with no further purification and includes distilled water, sodium hydroxide, ferric chloride, and activated carbon pellets. Temperature sensors, amplifiers, and data loggers were used to monitor the temperatures (slurry, gas layer, ambient, and the interface between slurry and gas layer). The temperature sensors were connected to the amplifier to amplify the signal from the sensors and for easy detection on the data logger.

Evaluation Procedure
The evaluation period lasted for 50 days. The evaluation was divided into three intensive evaluation periods. The first stage was the preparation, design, and loading of substrates, the second evaluation was the steady stage of relatively consistent biogas production and intensive measurement, desulphurization, and data collection while the final stage is the data analysis whereby the data collected were optimized.
During the first stage of preparation, design, and loading; the 250L capacity digester was fabricated from galvanized steel by the method used by [11]. The digester was airtight, painted black with heights above ground level for exposure to sunlight penetration.
Poultry droppings mixed with plantain peels were thermally treated at 50 o C and mixed with distilled water in a reactor mix bucket to reduce the viscosity of the digestate and to remove adherent dirt. Plantain peels were milled to reduce the particle size, washed with distilled water before mixing with poultry droppings.
The pH of the slurry after mixing was 5.4, therefore few droplets of NaOH(aq) was added to the slurry to reduce the acidity to 7.21 for organism adaptation during anaerobic digestion. The digester occupied 2/3rd of the digester space leaving a height of 29.1cm for gas production. In order for solid accumulation at the bottom and for easy removal after digestion, the slurry inflow was directed downward, the content of the digester was manually stirred for uniformity of microbial activity in the digester daily at 9 am, 1 pm and 5 pm respectively. The ratio of feed was 1kg of poultry droppings, 2kg of distilled water, and 1kg of plantain peels.
The second evaluation which is the steady stage of relatively consistent biogas production and intensive measurement, desulphurization, and data collection, the gas was collected by water displacement method, temperatures were monitored and the pH of the slurry and height of biogas holder was measured daily. Raw biogas was allowed to pass through the desulphurization chamber which contains activated carbon pellets and ferric chloride to remove CO 2 and H 2 S from the biogas. Data on the height of digester displaced and pH were measured daily and temperatures were recorded on a data logger. The final stage of data analysis was carried out using Microsoft Excel with Linear Regression and Correlation to evaluate the regression parameters βo (intercept) and β1 (slope) too respectively with the equation below factors for biogas production were ambient temperature (T amb ), slurry temperature (T slurry ), interface temperature (T int ), and gas layer temperature (T Glt ) ( Table 1). The quality of the fit of the model was evaluated using the test of significance and analysis of variance (ANOVA). The fitted quadratic response model is described by Eq. (3).
Volume 27-Issue 3 Where: Y is response factor (biogas volume), b o is the intercept value, b i (i= 1, 2, k) is the first order model coefficient, b ij is the interaction effect, and b ii represents the quadratic coefficients of Xi, and e is the random error.

Chemical and Proximate Analyses of the Digestates
Chemical and proximate compositional analysis of the digestate such as ash content, carbon content, nitrogen content, calcium, pH, phosphorus, potassium, and C/N ratio were carried out using the method already adopted by [11].

Biogas Yield
The biogas yield contains methane (CH 4 ), with a major gas of carbon dioxide (CO 2 ) and trace gas such as hydrogen sulfide (H 2 S).
From the beginning of the digestion process, biogas yield was slow till it reached a maximum volume of 3.06 x 10 -2 m 3 /day on the 18 th day of the production process and then continued to vary till the 50 th day of the fermentation process when the biogas yield maintained a steady volume. The volume of biogas was calculated by multiplying the cross-sectional area with the height of digester displaced throughout the fermentation process. According to [10] who produced biogas from poultry droppings found out that it produced more biogas because it contains more nutrients and nitrogen in comparison with other animal waste except for pig waste, but it also produces a small proportion of H 2 S which corrodes metal fitting. Table 1 presents the daily biogas readings, cross-sectional area, and height of digester for 50 days of the fermentation period.

Conclusion
Biogas was produced from poultry droppings mixed with plantain peels. The highest biogas volume was 3.11 x 10 -2 m 3 /day on the 48 th day of the fermentation process. The temperatures (interface between slurry and gas, gas layer, ambient, and slurry) measured were within the mesophilic temperature range.