Design and fabrication of 3 . 60 m 3 household plastic bio digester loaded with kitchen waste and cow dung for biogas generation

A 3.6 m 3 pilot plastic digester for family generation of biogas was designed, constructed and evaluated through physico-chemical studies using 50% cow dung and 50% kitchen wastes. The ash content of waste increased after digestion while the fibre and fat contents of the waste was 5.10 and 1.05% but significantly (p<0.05) decreased to 2.49 and 0.70% after digestion. The carbohydrate content of the waste was 11.02% which significantly (p<0.05) decreased to 7.91%. The volatile solid content of 50% cow dung + 50% cassava peel + yam peels + vegetable was 11.10%. The biochemical oxygen demand was 44.58% while the chemical oxygen demand was 139.20% before digestion but decreased significantly (p<0.05) after digestion. The pH of 50% cow dung + 50% cassava + yam peels + vegetable waste during digestion increased from 6.71 at day of charging to 6.81 at the 8th day after which it began to fluctuate between 6.68 and 6.85 throughout the retention period. Afternoon temperatures of both ambient and slurry were within the mesophilic (30 and 40°C) temperature which was higher than the morning and evening temperatures. The production of biogas started at the 2nd day by producing 406 L and increased each day till day 8, by producing 738 L and after which its production began to fluctuate between 572 and 718 L/day. Early biogas flammability was observed on the 4th day for 50% cow dung + 50% cassava + yam peels + vegetable. At the point of flaming, the methane content of the biogas increased significantly (p<0.05) to 65.65%, while the carbon dioxide decreased significantly (p<0.05) to 25.15%, for 50% cow dung + 50% cassava + yam peels + vegetable. The average biogas (0.601 to 0.505 m 3 /day) produced from the waste using 3.6 m 3 capacity plastic bio-digester could be sufficient to cook three times a day for household of 3 to 4 persons.

2008).Achieving solutions to possible shortage in fossil fuels and environmental problems that the world is facing today requires long-term potential actions for sustainable development.In this regard, renewable energy resources appear to be one of the most efficient and effective solutions (Ofoefule et al., 2008).Many substrates are generally used as feedstock in biogas plants and the potential for biogas production varies with feedstock.Generally animal waste, human waste, kitchen waste and some crop residues are used in small scale biogas plants (Balasubramaniyam et al., 2008).The utility of biogas as fuel is derived from the high calorific value of methane.In most cities and other places, kitchen waste is disposed in landfill or discarded which causes environmental and public health hazards like environmental pollution, global warming and diseases like malaria, cholera and typhoid.These problems could be tackled successfully to a reasonable extent by biodegradation of kitchen waste with biogas plant (Kubaska et al., 2010;Suyog, 2011).Studies have already been carried out on biogas generation from cow dung, chicken droppings, food waste, dairy waste, fruits and vegetable waste using biogas digesters (Ken et al., 2005;Sagai et al., 2009;Zheng et al., 2009;Eze and Agbo, 2010;Suyog, 2011;Ljukpa, 2013).A critical analysis of literature reveals that most of the studies on biogas production are mostly laboratory studies.
The design and fabrication of cheap and movable household plastic digester loaded with kitchen wastes that could provide energy need of 4 to 5 person to cook three daily in a family have not been adequately studied.This makes this present study on the design and fabrication of 3.60 m 3 household plastic bio digester loaded with cow dung and kitchen waste for biogas generation, not only necessary but imperative.The introduction, full adoption and practice of this technology in homes will help domestically to generate biogas for cooking family meals.However, this research focused on the generation of biogas from kitchen waste and cow dung for cooking family meal.The specific objectives of the study were: (i) design and fabricate movable household plastic bio-digester (ii) evaluate the efficiency of the biogas digester through physico-chemical studies.

Wastes sources
The wastes used were cassava peels, yam peels, vegetable wastes and cow dung.The cassava and yam peels were collected in dried form from local garri processors and local yam fryers in Nsukka town respectively while vegetable waste was collected from the restaurant located on campus of the University of Nigeria, Nsukka.The cow dung was collected from the slaughter house at Nsukka Central Abattoir.

Design and construction of the plastic biogas digester
FAO (1996) assumed a daily biogas energy need of 1.5 to 2.0 Nwankwo et al. 131 m 3 for a household of six people while Vu et al. (2012) reported that 0.8 to 1.0 m 3 of biogas per day would be sufficient to supply the energy needs of a family of 5 -6 people to cook 3 times daily.The differences in these assessments can be due to variations in the quality of the biogas (that is the CH4 concentration), which is affected by the composition of the feed entering the digester.It has been reported that daily energy requirement for cooking is estimated to be 0.34 to 0.42 m 3 of biogas per person; 1.7 to 2.1 m 3 for five people (Singh and Sooch, 2004).If the digester to be designed and constructed is to serve 100% of the energy need for a household of 6 people per day, the capacity of the digester becomes By using FAO (1996) estimation Or By using Singh and Sooch (2004) estimation: 2.04 to 2.52 m 3 / day Therefore the capacity of digester that would be constructed should be able to produce biogas of up to 2.00 to 2.52 m 3 / day.For mixture of animal and kitchen waste, the rate of biogas production is about 0.5 m 3 /kg of volatile solid added (Eze, 2000;Vögeli et al., 2014).
Then with 5.40 kg volatile solid/day, the volume of digester containing waste could be calculated.But the normal range of volatile solid loading is in the range of 1 -4 kg vs /m 3 /day and the optimal loading rate for animal and kitchen waste is about 2.0 kg vs /m 3 /day (Eze, 2000;Dlabaja and Malaťák, 2013).Then with the volatile solid loading of 2.0 kg vs /m 3 /day and volatile solid added of 5.4 kg/day; Volatile solid loading rate is a measure of the biological conversion of the anaerobic digestion system.Feeding the system above its sustainable volatile solid loading rate, results in low biogas yield due to accumulation of inhibitory substances in the digester slurry (fatty acid) (Gray et al., 2008).Under such circumstances, the feeding rate of the system must be reduced.
The amount of biogas generated each day (m 3 /day) will be calculated on the basis of the daily substrate input (volatile solids content) and specific gas yield of the substrate (Sasse, 1988;Lam and Heegde 2010), such that; Daily gas production = volatile solids content × the specific gas yield (solids) Where: volatile solids content = 5.4 kg / day Specific gas yield = 0.50 m 3 / kg The maximum loading capacity of an ideal digester should not exceed 90% of the digester volume (Eze, 2000).If 2.70 m 3 is 75% of the volume of the digester containing waste required for 2.40 m 3 daily biogas production, then the entire volume (that is, 100%) is given as 75% = 2.70 Therefore, the entire digester volume is of 3.60 m 3 with a diameter of 1.28 m and height of 1.67 m.

Materials and design of the components of the digester
The digester component include: the fermentation chamber (Vf), the gas collecting chamber (Vc), the influent chamber and the effluent chamber (Figure 1).The gas storage chamber is the upper frustum section of the digester while the gas collecting chamber is the chamber through which the stored gas exits from the digester.The influent chamber is the channel through which the digester is charged, while the digested slurry is discharged from the effluent chamber.In order to calculate the total volume of the digester (V) V =Vc + Vf = 3.6 m 3 Where: Vf = Volume of fermentation chamber ≤ 75% V Vc = Volume of gas collecting chamber ≤ 25% V Then working volume of digester = Vc + Vf

Description of digester assembly and biogas production
The digester that was used was a plastic prototype and was constructed from 3.6 m 3 capacity plastic tank at the National Center for Energy Research and Development, University of Nigeria, Nsukka (Figure 2).The plastic biogas digester was made up of fermentation chamber for biogas generation and a point for biogas utilization for cooking.The digester was constructed by cylindrical PVC plastic tank (Figure 3).The plastic biogas digester had the agitator secured through the digester inlet pipe.The digester had an inlet pipe for pouring mixture of kitchen wastes, cow dung for bio-digestion, outlet pipe for removal of digested waste, could be close to the water and air tight during the digestion process.The digester cover was made of hard foam material that could withstand harsh environmental conditions and maintain anaerobic condition when used as a digester cover.The temperature of the slurry and the ambient was observed three times daily (morning, afternoon and night).The thermocouple wire that was inserted into the digester through the inlet pipe was connected to the thermocouple digital thermometer which measured the slurry and ambient temperatures respectively.The physiochemical characteristics such as moisture, fibre, fat, ash, protein and carbohydrate, total solid, volatile solid, carbon, free fatty acid, chemical and biochemical oxygen demand of the wastes were determined before and after   digestion.The wastes were studied during the period of the testing for biogas production which involved the determination of total viable count on four stages of biogas production (Eze and Agbo, 2010).The first microbial analysis was done immediately after charging to know the number of the starting microbe.The second microbial analysis was carried out immediately the produced gas started flaming.The third microbial analysis was done at the peak of gas production, when the gas production was at its maximum while fourth microbial analysis was done at the point of discharging (on each waste undergoing bio-digestion).The gas produced was characterized by portable combustion analyzer (Keeports, 2009).The first gas analysis was carried out immediately the digester started gas production.The second and third analyses were carried out at the onset of flaming and at the peak of gas production respectively (Figure 4).The pressure of the gas produced was recorded daily using the pressure gauge that was fixed on top of the digester.From the ball valve on the top of the digester, the digester was linked to the point where the gas produced was used for cooking.Nido pressure nozzle was used to control the movement of the gas and can be closed without gas leakage when the separation section is to be attached or detached from the fermentation section of the digester.A tire tube was installed to the digester to control the pressure in the digester to avoid explosion.Moisture content, ash content, crude fibre, crude fat, crude protein, free fatty acid and energy content was determined using the method of AOAC (2010).Carbohydrate content of the samples was determined by the difference as reported by Nweke et al. (2011).
Total solid was determined using the method of Meynell (1982).
The carbon content was determined as reported by Gelman et al. (2011).Free fatty acid was determined using the method of AOAC (2010).This was carried out using the methods described by Ademoroti (1996).The pH of the waste was determined by Geotechnical test method (2015).The slurry and ambient temperature was determined using the method of Ezekoye and Okeke (2006).This was carried out using the methods described by Prescott et al. (2005).The biogas generated was measured by downward displacement of water as described by Ezekoye and Okeke (2006).The flammable gas from the system was determined by portable combustion analyzer (Keeports, 2009).The pressure of the gas produced was determined using the method of Ezekoye and Okeke (2006).

Statistical analysis
Means and standard deviation of data obtained were calculated and used to plot the relevant graphs.Data generated were analysed one-way analysis of variance (ANOVA) while mean separation was

RESULTS AND DISCUSSION
Physico-chemical compositions of undigested and digested wastes were presented in Table 1.From Table 1, it is evident that the moisture, ash and protein contents of the waste increased significantly (p<0.05) while fibre, fat, carbohydrate, total solid, volatile solid, carbon, free fatty acid, chemical and biochemical oxygen demand decreased significantly (p<0.05) after digestion.The same results were reported by Sadaka and Engler (2003), Umar et al. (2005) and Yadav et al. (2014).The higher moisture content of the wastes (77.04%) before digestion would encourage the movement and growth of bacteria and reduce the limitation of mass transfer of non-homogenous or particulate substrate (Sadaka and Engler, 2003;Yadav et al., 2014).The ash content of waste increased after digestion while the fibre and fat contents of the waste was 5.10 and 1.05% but significantly (p<0.05)decreased to 2.49 and 0.70% after digestion.The waste would be low in cellulose, hemicellulose, pectin, lignin and plant wax due to low fibre content of waste which would also encourage easy biodegradation of the wastes during digestion.Moreover, Uzodinma et al. (2011) reported that the most suitable plant wastes for biogas production are those rich in biodegradable carbohydrates (sugars, starches), lipids and proteins.The carbohydrate content of the waste was 11.02% which significantly decreased to 7.91%.Although the range of total solid contents of the wastes used for biogas production were above the optimum range of total solid (7 to 9%) content recommended by Balsam and Ryan (2006) and Yadav et al. (2014) to avoid overloading of the digester and increase in acidity of the slurry, the percent reduction in total solid of the wastes after digestion was 28.48% (Table 1).This indicated that the digestion process had very small efficiency in removing the total solids.The volatile solid contents of 50% Cow dung + 50% Cassava peel + Yam peels + Vegetable was 11.10%.The reduction of total solid and volatile solid after digestion denote the process stabilization (Wan and Azni, 2009;Yadav et al., 2014).The percent reduction of carbon (59.90%) content of the wastes after digestion by anaerobic processes was therefore probably limited to the production of organic acids, methane and carbon dioxide by facultative bacteria (Yadav et al., 2014).The percent removal of carbon content was high due to enough content of carbohydrates and less cellulose and lignin content of food wastes.The free fatty acids of waste decreased significantly (p<0.05).The biochemical oxygen demand was 44.58% while the chemical oxygen demand was 139.20% before digestion but decreased significantly (p<0.05) after digestion.This could be due to reduction in organic content of the wastes caused by anaerobic digestion (Bhumesh and Sai, 2011;Sangeetha and Sivakumar, 2016).Similar result was reported by Sangeetha and Sivakumar (2016).
The pH of 50% Cow dung + 50% Cassava + Yam peels + Vegetable waste during digestion increased from 6.71 at day of charging to 6.81 at the 8th day after which it began to fluctuate between 6.68 to 6.85 throughout the retention period (Figure 5).Otun et al. (2005) reported that it is important to maintain the pH of an anaerobic digester between 6 and 8; otherwise, methanogen growth would be seriously inhibited.Furthermore, it was observed that pH of the fermentation slurry was changing in the course of biogas production which means that the pH increased as the biogas production increased.This may be attributed to higher consistent range of pH which was favourable for methanogenic bacteria.Low pH has been reported to inhibit methanogenic bacteria that are responsible for biogas production.The pH value less than 5 or greater than 8 has been reported to rapidly inhibit methanogenesis (Latinwo and Agarry, 2015).The fermenting slurry in morning (9 am), afternoon (2 pm) and evening (6 pm) temperature range of 27 to 30°C, 30 to 35°C and 27 to 31°C were observed within the monitoring period of 28 days while ambient temperature range of 27 to 30°C, 29 to 35°C and 27 to 29°C were observed within the monitoring period of 28 days for 50% Cow dung + 50% Cassava + Yam peels + Vegetable (Table 2).However, critical observation on the temperature effects reveals that lower amount of biogas was produced in the morning until the temperature is increased above 30°C.Higher temperature of the slurry than ambient temperature could be attributed to the bacterial activities within the slurry that generated heat.Slurry has high heat capacity than the ambient because of high density and can withhold heat (Mukumba et al., 2015).Afternoon temperatures were within the mesophilic (30 to 40°C) temperature which was higher than the morning and evening temperatures.It could be the reason for higher biogas production in the afternoon than in the morning and evening because there was proper digestion of the waste by the activities of microorganisms within the digester (Dahunsi and Oranusi, 2013;Abdulkarim et al., 2015).Similar results were reported by Abdulkarim et al. (2015) and Mukumba et al. (2015).
The result in Figure 6 shows the microbial total viable count at each stage of digestion.The microbial load proliferation increased significantly during gas production and continued to increase significantly (p<0.05) at the peak of gas production and then reduced significantly (p<0.05) at the point of discharge.The total viable count of 50% Cow dung + 50% Cassava + Yam peels + Vegetable was 5.40 × 10 8 cfu/ml immediately after charging and gas production started.
At peak of gas production, the total viable count of 50% Cow dung + 50% Cassava + Yam peels + Vegetable was 5.78 × 10 8 cfu/ml; however, at the point of discharge total viable count decreased significantly (p<0.05) to 4.72 × 10 8 cfu/ml.The significant (p<0.05)increase in microbial proliferation at peak gas production and significantly (p<0.05)reduction in total viable count at the end retention period could be due to temperature and acidification of the slurry during digestion (Uzodinma and Ofoefule, 2009;Eze and Agbo, 2010).
Table 3 shows the volume of biogas and pressure of biogas produced from digestion of 50% Cow dung + 50% Cassava + Yam peels + Vegetable within the retention period of 28 days.With the digestion of 50% Cow dung + 50% Cassava + Yam peels + Vegetable, the production of biogas started on the 2nd day by producing 406 L and increased each day till Day 8, by producing 738 L and after which its production began to fluctuate between 572 and 718 L/day.The rapid initial biogas production in the digestion of 50% Cow dung + 50% Cassava + Yam peels + Vegetable might also be due to shorter lag phase growth, availability of readily biodegradable organic matter in the substrate and the presence of high content of the methanogens (Aragaw et al., 2003).
The result also shows that the average biogas (0.601 m 3 /day) produced from the experimental wastes using 3.6 m 3 capacity plastic bio-digester could be sufficient to cook three meals a day for 3 to 4 persons, since Rajendran et al. (2012) and Vu et al. (2012) reported that 1.0 m 3 of biogas would be sufficient to cook three meals a day for 5-6 persons.However, if household bio digester capacity is increased up to 6 m 3 capacity, the biogas production would cook three meals a day for 5 to 6 persons depending on the substrate (Rajendran et al., 2012).The result indicates that biogas from 50% Cow dung + 50% Cassava + Yam peels + Vegetable started on the 2nd day to read on pressure gauge (11 mmHg) and had the highest pressure of about 36 mmHg on the 13th day of the experiment.Afterward, the pressure dropped a little and started fluctuating (29 to 34 mmHg) during the remaining days of the experiment.Similar results were reported by Ebunilo et al. (2016) and Olorunmaiye et al. (2016).
Early biogas flammability was observed on the 4 th day for 50% Cow dung + 50% Cassava + Yam peels + Vegetable (Figure 7).At the point of flaming, the methane content of the biogas increased significantly (p<0.05) to 65.65%, while the carbon dioxide decreased significantly (p<0.05) to 25.15%, for 50% Cow dung + 50% Cassava + Yam peels + Vegetable.Significant (p<0.05) increase in    methane yield could be attributed to specific growth rate of methanogenic bacteria in the bio-digester which allowed the use of carbon to form methane that resulted in lower percentages of carbon monoxide produced (Chukwuma and Orakwe, 2014).At the peak of biogas production, methane again increased significantly (p<0.05) to 69.74%, for 50% Cow dung + 50% Cassava + Yam peels + Vegetable, while carbon dioxide decreased drastically to 22.25%.During the measurement of the biogas at different stages of biogas production, methane was found to increase in concentration while carbon dioxide and carbon monoxide were found to decrease significantly (p<0.05) in concentration.The main and most important component of biogas is methane because biogas heating power depends on the methane concentration in biogas (Herout et al., 2011).

Conclusion
The result shows that the average biogas (0.601 to 0.505 m 3 /day) produced from the wastes using 3.6 m 3 capacity plastic bio-digester could be sufficient to cook three times daily for the household of 3 -4 persons since 1.0 m 3 of biogas would be sufficient to cook three meals a day for 5 -6 persons.As expected from a highly efficient bio digester, the proximate and physico-chemical properties such as moisture, ash and protein contents of the wastes increased significantly (p<0.05) while fibre, fat, carbohydrate, total solid, volatile solid, carbon, free fatty acid, chemical and biochemical oxygen demand decreased significantly (p<0.05) after digestion.However, if household bio digester capacity is increased up to 6 m 3 capacity, the biogas production would cook three meals a day for 5 to 6 persons depending on the substrate.Furthermore, critical observation on the temperature effects reveals that lower amount of biogas was produced in the evening when compared to biogas production in the afternoon but biogas production in the evening was higher than that of morning.Most days, during the experiment, the slurry temperature was higher during the evening because of its high heat capacity as compared to ambient.

Figure 1 .
Figure 1.Schematic diagram of the fermentation tank.

FabricationFigure 4 .
Figure 4. Flow chart for co-digestion of kitchen waste and cow dung for Biogas generation.

Figure 5 .
Figure 5. pH of the slurry for the period of 28 days.

Figure 6 .
Figure 6.Total viable count (TVC) from the waste at different stages of waste digestion.

Figure 7 .
Figure 7. Biogas composition at different stages of gas production during waste digestion.
Values are means ± standard deviation of three determination.Values on the same row with alphabets with different superscripts are significantly (p<0.05).Negative sign (-) = decrease.done by least significant different at p<0.05 using SPSS 20.0 for Windows (Armonk, NY: IBM Corp.)

Table 2 .
The slurry and ambient temperature for the period of 28.

Table 3 .
The volume and pressure of biogas from the digester.