Enhancement of Biogas Yield from Cow Dung and Rice Husk Using Guano as Nitrogen Source
P. A. Nwofe*, P. E. Agbo
(P. A. Nwofe), (P. E. Agbo)
To cite this article:
P. A. Nwofe, P. E. Agbo. Enhancement of Biogas Yield from Cow Dung and Rice Husk Using Guano as Nitrogen Source. International Journal of Sustainable and Green Energy. Vol. 4, No. 3, 2015, pp. 66-72. doi: 10.11648/j.ijrse.20150403.11
Abstract: The study reports on the influence of nitrogen source on the biogas yield from cow (N'Dama) dung and rice husk. The digester performance for both feedstocks were evaluated using standard parameters such as; initial PH, water dilution, nitrogen source (guano and poultry droppings) and heavy metals. The source of innoculum used was cow rumen fluid. The result show that for feedstock to water dilution ratio of 1:6 w/v and initial pH of 7.0, the maximum biogas yield for rice husk was 430 mL/day and 350 mL/day for cow dung. The heavy metals (Ni2+ and Zn2+) increased the biogas yield while Fe2+ (100 ppm) shows no effect. Addition of guano results in maximum production rate of 85 mL/day and 60 mL/day in rice husk and cow dung respectively. The use of guano indicates more biogas production rate in both feedstocks compared to poultry droppings.
Keywords: Cow Dung, Biogas, Guano, Poultry Droppings, Rice Husk
Renewable energy technologies are currently competing favourably in the energy sector, all geared toward achieving a more sustainable and efficient energy use. Amongst other renewable sources: solar, hydro, geothermal, wind and biomass, biomass is amongst the most vastly explored in terms of research and applications both in the developed and in under-developed countries. It is a common knowledge that fossil fuels-based conventional grid extension constitutes the major centralised power systems from urban areas to rural areas in most under-developed countries. A scenario that is not only capital intensive but also economically unrealistic in most cases. It has been established that more than a quarter of the human population experiences an energy crisis, especially those living in the rural areas of developing countries such as Nigeria [1, 2]. Biomass is generally considered as a suitable alternative to energy from fossil fuels since it can be easily converted to other forms of energy such as biogas and biofuels [3, 4]. It has been reported that Nigeria has an installed capacity of 8,425 MW of electricity but the available capacity is only about 50% of installed capacity . In Nigeria, energy conservation and energy efficiency is strongly needed, thus renewable energy technologies will be essential to the solution and are likely to play an increasingly important role for providing enhanced energy access, reduced over-dependence on fossil fuels, and to help Nigeria meet her vision 20-20-20 clean energy program. Privatisation of PHCN (Power Holding Company of Nigeria) has made many States in Nigeria to be pursuing other source of energy vigorously and Ebonyi State is playing an active role in that direction. Energy from biomass will play a significant role in that Nigeria environment is largely polluted with huge amount of wastes due to rapid urbanisation and poor waste management practices [6-8].
Ebonyi State is popularly known for rice production (Abakaliki rice) in Nigeria and thus has large reserves of rice husks from rice mills which are randomly and strategically located in various towns. These large reserves of rice husks are only utilised as cooking fuels (on a low scale) and source of income for some poor rural dwellers who scavenge those rice husk hills to earn a living. Waste from agricultural, municipal, industrial and household are also common in the study area but these wastes are not presently utilised in a sustainable manner . It has been shown that about 227,500 tons of fresh animal wastes is produced on daily basis in Nigeria , implying that Nigeria can produce 6.8 million m3 gas/day since 1kg of fresh animal wastes yields up to 0.03 m3 gas. Ogwueleka  noted that the waste density of municipal solid waste in Nigeria ranged from 280 to 370 kg/m3 with the waste generation rates in the range 0.44 to 0.66 kg/capita/day. The generation rate of solid waste in the capital cities of some South-east States of Nigeria (Ebonyi and Imo) is in the range 9.580 to 9.74 x 10-3 m3 [6, 11]. Biogas production from wastes is commonly achieved through anaerobic digestion. Biogas can be utilised as fuels both in low (household, village, community) and large scale (industry) or as fertilisers [12-15]. The use of suitable nitrogen supplement or other parameters to enhance biogas yield has been a subject of research for years. Some authors [16, 17] have used different supplement/parameters in this regard.
This paper investigates the use of locally available and abundant wastes to produce biogas using anaerobic digestion. It also reports on the effect of different parameters (initial PH, water dilution, guano and poultry droppings, heavy metal effect) on the biogas yield and it was observed that the use of guano (bat droppings) enhanced biogas production substantially.
2. Materials and Methods
2.1. Feed Materials
The feed materials were a batch of cow dung and rice husk. The cow dung was sourced locally from cow farms, and the rice husk from Abakaliki Rice Mill Industry, Ebonyi State, Nigeria. The cow dung from N'dama species was used because of its availability compared to other species in the study area. Fig. 1 gives a picture of a typical rice husk hill from Abakaliki rice mill industry. The digester feed included a batch of cow dung and rice husk. Each of these was dried using open sun-baking and then carefully stored in a stoppered polyethylene container. In order to ensure efficient stabilisation of the wastes during anaerobic digestion, innoculum obtained from the rumen of cows slaughtered at Abakaliki main market abattoir was used. The micro-organisms were maintained in an anaerobic environment by straining the innoculum in cheesecloth and then stored in an airtight container. The tip of the digesters was sealed off with clip/cello-tape in order to maintain complete anaerobic condition during the anaerobic digestion for the cow dung and rice husk respectively.
2.2. Experimental Set-up for the Anaerobic Digestions
Fig. 2 gives the experimental set-up for anaerobic digestion of cow dung (CD) and rice husk (RH). As shown in Fig. 2, each set-up included 1.5 L bottle that served as the anaerobic digester, an inverted 50 mL graduated burette containing acidified water as the biogas collector, and a rubber container which was used to collect the water discharged from the biogas collector. A rubber pipe was used to convey the gas produced in the digester to the collector. The anaerobic digestion was maintained in the mesophilic range in that the room temperature was 31±1oC [18-21]. It has been reported that stratification can lower the production rate of biogas [22, 23], thus this was avoided by mixing each digester once daily for 30 days. The water displacement method used in  was utilised in recording the biogas production rate for each anaerobic digester.
2.3. Initial pH and Water Dilution
The buffer solutions: citrate buffer of pH 4, phosphate buffer of pH 7, and borax buffer of pH 10 were used to buffer the biogas digesters containing the feedstock. This was done to investigate the impact of the initial pH on the biogas production rate. A 55 mL cow rumen innoculum was added in each case to induce the digestion. A 40 g of feeds each of cow dung and rice husks were mounted in 1.5 L conical flasks, a total of ten stands (five stand for CD and RH respectively). The feedstock to water ratio of 1:2, 1:4, 1:6, 1:8, 1:10 (w/v), were created by moistening each case with water whose volume is in the range 250-450 mL. The fermentation temperature was maintained as before (31±1oC) with a hydraulic retention time (HRT) of 30 days. The same volume of cow rumen innoculum was introduced in each digester and anaerobic condition was ensured by sealing the tip of the flask appropriately. The control experiment had no water included in the digester for the cow dung and rice husk respectively.
2.4. Nitrogen Sources and Trace Metals
A 100 g of each feedstock (CD and RH) was included in conical asks and then mixed with varying quantities of nitrogen sources (poultry droppings and guano). The poultry droppings were sourced from poultry farms while the bat droppings were obtained from aged-buildings roofed with corrugated iron sheets in the rural areas of Ebonyi State. The controls had the same quantity of feedstock without nitrogen source. A feed to poultry droppings/guano ratio of 2.5:1 (w/w) was formed by mixing a 100 g of CD and RH respectively with 40g of poultry droppings/guano in different digesters. The fermentation process in each case was induced by adding 350 mL of water to the same volume of cow rumen as before. The hydraulic retention time was 30 days while the incubation temperature was maintained at 31±1oC.
Concentrations of Fe3SO4, ZnSO4 and NiCl2 in the range 40-250 ppm were added to each digester containing 55 g of CD and RH respectively, to investigate the effect of the heavy metal on the biogas production rate. In particular, 250 mL of each solution was added to the digesters. The control in each case had 250 mL of water included only. Fermentation was induced by innoculating each digester with a 55 mL of freshly strained cow rumen liquor.
2.5. Data Acquisition and Analysis
In the experiment, each set of data was taken in quadruplicates and the average was used for the analysis. The analysis was done using the Origin Pro 8 software (trial version).
3. Results and Discussion
3.1. Effect of Initial pH and Water Dilution on Biogas Yield
It is generally known that pH plays a substantial role in the biogas yield under different conditions. This is because the activities of various microbes depend strongly on the pH of the medium amongst other factors. The best pH values in biogas production is in the range 6.5-7.5 [25, 26], though some authors have reported pH values in the range 7.6-8 [27-29]. In the literature, there are varying reports on the effect of pH on the biogas yield involving same or different feedstocks. Result obtained in this study indicate that a pH of 7 gave the best yield of biogas. This value is in agreement with the reports of other authors [16, 27, 28, 30-37] as shown on Table 1.
|Feedstock||Initial pH||Biogas yield||Ref|
|Bioethanol waste||6 - 8||increase|||
|Rice straw||6.9 - 7.28||increase|||
|Rice husks and cow dung||7.0||increase||This study|
|Pig manure and maize silage||8.0||increase|||
|Apple waste with swine manure||7.81 – 7.85||increase|||
|Cow manure||7.3 – 7.6||decrease|||
|Saccharina japonica ethanol fermentation||8.0||increase|||
|Diary manure with three crop residue||5.25 – 6.80||decrease|||
|Raw and detoxified mahua seed cake||5.9 – 7.2||decrease|||
Figs. 3 and 4 give the effect of water dilution on the biogas production rates for rice husk and cow dung respectively. The feed to water dilution ratios of 1:2, 1:4, 1:6, 1:8 and 1:10 w/v all yielded more biogas than the control for the cow dung and rice husk. The maximum biogas production rates for rice husk were observed to be 42, 148, 193, 148 and 55 mL/day respectively at day two. However for cow dung, a maximum biogas production rate of 35 mL/day and 60 mL/day were observed at day two for dilution ratios of 1:2 and 1:10 w/v. On day four, 95 mL/day and 130 mL/day for dilution ratios of 1:4 and 1:6 w/v were observed. The decrease of biogas yield observed in the case of rice husk at lower (1:2 w/v) and higher (1:10 w/v) water dilution ratios could be due to a reduction in the cluster formation of the necessary bacteria needed for biogas production. The variation in days at which maximum biogas yield was observed for the cow dung and rice husk was attributed to the difference in the C/N (carbon/nitrogen) ratio of the feedstocks. Similar behaviour has been observed by other authors . The maximum cumulative biogas yield occurred at a feed to water ratio of 1:6 w/v as shown on Fig. 5.
3.2. Effect of Nitrogen Source and Trace Metal on Biogas Yield
Fig. 6 gives the effect of nitrogen source on biogas production rate while Fig. 7 gives the cumulative biogas yield. As shown on Fig. 6, the biogas production rate peaked up in both cases compared to the control. A maximum biogas production rate of 85 mL/day and 60 mL/day were observed for the guano supplement in RH and CD respectively. The biogas production rate dropped to 50 mL/day and 40 mL/day for RH and CD respectively when poultry droppings was used as the nitrogen supplement. This could be attributed to a better balance of C:N ratio from the bat droppings. Ahmadu et al.  and Ojolo et al.  noted that the organic matter content of poultry wastes is a factor that affects the digestion environment and the microbial habitat, hence this could also be responsible for the lower yield observed for the poultry droppings supplement in this study. Table 2 gives the literature of the effect of nitrogen supplement on the biogas yield for same or different feedstocks.
Trace metals are known to affect the yield of biogas independent of the feedstock. This is because of their effect on the microbial contents of the digester during the anaerobic digestion [47-49]. Demirel and Scherer  argued that there is no direct formula for optimum composition of trace metals needed for maximum biogas production. Our result shows that for rice husk, Ni2+ (100 ppm) gave 300 mL/day while Zn2+ (100 ppm) gave 110 mL/day. For cow dung, Ni2+ gave 30 mL/day while Zn2+ gave 74 mL/day. No significant effect was observed with Fe2+ in both RH and CD respectively. Table 3 gives the effect of trace metals on the digester performance/biogas yield for different/same or different feedstock according to the literature.
|Feedstock||Nitrogen source||Biogas yield||Ref|
|Chemical source||not given||not given|||
|Rice husks||poultry droppings||increase|||
|Dairy cattle manure||urea||decrease/increase|||
|Boiled rice||human urine||increase|||
|Rice husks and cow dung||guano and poultry droppings||increase||This study|
|Cattle dung||brassica compestries||increase|||
|Rice wastes||poultry droppings||increase|||
|Feedstock||Trace metal||Biogas yield||Ref|
|Molasses silage||trace metal||decrease|||
|Palm oil mill effluent||Ni and Co||increase|||
|Food waste||K, Ni, Co, Mo, Se and W||decrease/increase|||
|Palm oil mill effluent||Fe||no effect|||
|Food waste||Co, Mo, Ni, Se and W|||
|Azolla piñata R.Br and Lemna minor L||Cd and Ni||increase|||
|Municipal waste||Fe, Co and Ni||increase|||
|Rice husks||Ni2+||increase||This study|
|Cow dung||Zn2+||increase||This study|
|Rice husk and cow dung||Fe2+||no effect||This study|
|Holy Basil (ocmium sanctum)||Se||increase|||
|Municipal waste||Cd, Cr, Cu, Pb, Ni, and Zn||no effect|||
|Municipal waste||Ni, Co and Fe||increase|||
|Chicken manure||Trace metals||increase|||
Locally abundant wastes have been investigated to establish the possibility of utilising them as alternative source of energy, thus the effect of different nitrogen sources (poultry droppings and guano) on the biogas yield from cow dungs from N'dama species and rice husk has been reported. The results show that the use of guano as nitrogen supplement is more promising than poultry droppings. A maximum biogas production rate of 85 mL/day and 60 mL/day were obtained for guano supplement in RH and CD respectively while poultry droppings in both feedstocks gave lower biogas yield. The other parameters: initial PH, water dilution, nitrogen source (guano and poultry droppings) and trace metals also affected the digester performance. Our findings show that a feed to water dilution ratio of 1:6 w/v yielded maximum cumulative biogas of 450 mL/day for RH and 350 mL/day for CD. At initial pH of 7.0 for both feedstocks, Ni2+ (100 ppm) for rice husk, and Zn2+ (100 ppm) for cow dung gave the best biogas yield while Fe2+ show no effect in RH and CD respectively. All the parameters tested increased the biogas yield without affecting the methane content. The findings reported herein could serve as a useful guide for further research to optimise the conditions needed to improve biogas yield from locally available and abundant wastes.
Dr P.A. Nwofe and Dr P.E. Agbo would wish to thank the staff of Ishieke Annex, Ebonyi State University, Abakaliki, for permission to use some equipments/chemicals from their laboratory. The authors also thank the staff of Ebonyi State Hatchery, Nkaliki and Rice Mill Industry, Abakaliki for permission to procure the feedstocks (poultry droppings and rice husks).