10.1 Global availability of grasses and other wild plants

The grass family (gramineae or poaceae) is perhaps the most successful taxonomic group in the plant kingdom. Members of this group number about 9000 species distributed in about 635 genera and they grow in all ecosystems and agroclimatic zones. From economic and ecological standpoints, they are the most important species in the plant kingdom. The pea family (leguminosae or fabaceae) is the largest family of flowering plants and also contains a large number of species found flourishing in many ecosystems and agroclimatic zones. Both families of plants contain domesticated crops and wild plants which are being researched for their potentials as reliable sources of biofuels. These plants certainly have a significant role to play in an anticipated global scenario which is 100% dependent on bioenergy in the near future.

Sparse evidence suggests that biogas was known to the Assyrians and Persians centuries before Jesus Christ was born. Further evidence is traceable to count Alessandro Volta who in 1776 concluded that there was a direct link between the amount of decaying organic matter and the amount of flammable gas produced. Sir Humphrey Davy determined in 1808 that methane was present in the gasses produced during the anaerobic decomposition of cattle manure. Helmont recorded the emanation of an inflammable gas from decaying organic matter in the 17th century (Brakel, 1980). It was not until towards the end of the 19th century that methanogenesis was found to be connected to microbial activity. In 1868, Bechamp named the organism responsible for methane production from ethanol. This organism could more accurately be described as a mixed population. Bechamp was able to show that, depending on the substrate different fermentation products were formed. Zehnder et al (1982) stated that it was in 1876 when Herter reported that acetate in sewage was converted to equal amounts of methane and carbon dioxide. Meynell (1976) noted that the first anaerobic digestion plant was built in Bombay, India in 1859. The first notable use of biogas in England occurred in 1859 when gas derived from a sewage treatment facility was used to fuel street lamps in Exeter (McCabe and Eckenfelder, 1957). Then in 1904, Travis put into operation a new two-stage process in which the suspended material was separated from the wastewater and allowed to pass into a separate ‘hydrolyzing’ chamber (Carcelon and Clark, 2002). Buswell and Hatfield (1936) and some other researchers in the 1930s identified anaerobic bacteria and the conditions that promote the production of methane. Their works also explained such issues as the fate of nitrogen in the anaerobic digestion process, stoichiometry of the reactions, as well as the production of energy from farm and industrial wastes through the anaerobic digestion process. Regarding anaerobic technology, farm — based facilities are the most common. In contemporary times low-technology biogas digesters have been most extensively used in China and India. Bui-Xuan (2004) pointed out that low cost biogas technology has been well received by small holder farms in many developing countries for producing a clean fuel to replace firewood, within the recent ten years. Stating that more than twenty thousand digesters have been installed in Vietnam, mainly paid for by the farmers; however biodigesters are still not fully integrated into the farming system as there is only limited use of by-products (effluent) as fertilizer for vegetables, fruit trees, fish pond and water plants. The paper further stated that the use of effluent from digester can be studied as a resource for small scale farmers. Interest in the technology is increasing in several other parts of the world.

The carbon:nitrogen (C/N) ratio expresses the relationship between the quantity of carbon and nitrogen present in organic materials. Materials with different C/N ratios differ widely in their yield of biogas. The ideal C/N ratio for anaerobic biodigesiton is between 20:1 and 30:1 (Marchaim, 1992). If C/N ratio is higher than that range, biogas production will be low. This is because the nitrogen will be consumed rapidly by methanogenic bacteria for meeting their protein requirements and will no longer react on the left over carbon remaining in the material. In such case of high C/N ratio, the gas production can be improved by adding nitrogen in farm cattle urine or by fitting latrine to the plant (Fulford, 1988). Materials with high C/N ratio typically are residues of agricultural plants. Conversely if C/N ratio is very low, that is outside the ideal range stated above, nitrogen will be liberated and it will accumulate in the form of ammonia. Ammonia will raise the pH value of the slurry in the digester. A pH value which is higher than 8.5, will be toxic to the methanogenic bacteria in the slurry. The cumulative effect of this is also reduced biogas production. Materials having low C/N ratio could be mixed with those having high C/N ratios so as to bring the average C/N ratio of the mixture to a desirable level. Human excreta, duck dung, chicken dung, and goat dung are some of the materials which typically have low C/N ratios.

According to Karki and Dixit (1984), typical C/N ratios of common organic materials are as shown in Table 3.

A study by Sidibe and Hashimoto (1990) documented the fact that grass straw can be fermented to methane and the yield can be relatively high. This laboratory experiment showed that the ultimate methane yield of rye grass straw (341+ 5ml/g VS) and fescue grass straw (356+ml/g VS) are not significantly different but both grass straws had significantly higher yield (p < 0.01) than dairy cattle manure (288 + 3 ml/g VS). The paper noted that nitrogen does not appear to be a limiting nutrient in the fermentation of grass straw to methane; the length of time between inocula feeding does not affect the ultimate methane yield of the straw, and longer acclimation may increase the ultimate methane yield of grass straw. Among plants themselves, differences exist regarding their potentials as feedstock for biogas production. For instance, De-Renzo (1997) reviewed anaerobic digestion of plant materials and concluded that aquatic plants such as algae and moss can be much better digested than terrestrial plants because of their toughness. Ordinarily, more digestion results in more biogas production. Akinbami et al (2001) noted that in the tropics, the identified feedstock substrates for an economically feasible biogas programme include water lettuce, water hyacinth, dung, cassava peelings, cassava leaves, urban refuse, solids (including industrial waste), agricultural residue and sewage.

Uzodinma and Ofoefule (2009) investigated the production of biogas from equal blending of field grass (F-G) with some animal wastes which include cow dung (G-C), poultry dung (G­P), swine dung (G-S) and rabbit dung (G-R). The wastes were fed into prototype metallic biodigesters of 50 L working volume on a batch basis for 30 days. They were operated at ambient temperature range of 26 to 32.8oC and prevailing atmospheric pressure conditions. Digester performance indicated that mean flammable biogas yield from the grass alone system was 2.46+2.28 L/total mass of slurry while the grass blended with rabbit dung, cow dung, swine dung and poultry dung gave average yield of 7.73+2.86, 7.53+3.84, 5.66+3.77 and 5.07+3.45 L/total mass of slurry of gas, respectively. The flash point of each of the systems took place at different times. The field grass alone became flammable after 21 days. The grass-swine (G-S) blend started producing flammable biogas on the 10th day, grass-cow (GC) and grass-poultry (G-P) blends after seven (7) days whereas grass-rabbit (G-R) blend sparked on the 6th day of the digestion period. The gross results showed fastest onset of gas flammability from the G-R followed by the G-C blends, while the highest average volume of gas production from G-R blend was 3 times higher than that of F-G alone. Overall, the results indicated that the biogas yield and onset of gas flammability of field grass can be significantly enhanced when combined with rabbit and cow dung.

Ofuefule et al., (2009) reported a comparative study of the effect of different pre-treatment methods on the biogas yield from Water Hyacinth (WH). The WH charged into metallic prototype digesters of 121 L capacity were pre-treated as: dried and chopped alone (WH-A), dried and treated with KOH (WH-T), dried and combined with cow dung (WH-C), while the fresh water Hyacinth (WH-F) served as control. They were all subjected to anaerobic digestion to produce biogas for a 32 day retention period within a mesophilic temperature range of 25 to 36°C. The results of the study showed highest cumulative biogas yield from the WH-C with yield of 356.3 L/Total mass of slurry (TMS) while the WH-T had the shortest onset of gas flammability of 6 days. The mean biogas yield of the fresh Water Hyacinth (WH-F) was 8.48 + 3.77 L/TMS. When the water Hyacinth was dried and chopped alone (WH-A), dried and treated with KOH (50% w/v) (WH-T) and dried and combined with cow dung (WH-C), the mean biogas yield increased to 9.75 + 3.40 L/TMS, 9.51 + 5.01 L/TMS and 11.88 + L/TMS respectively. Flammable biogas was produced by the WH-F from the 10th day of the digestion period whereas the WH-A, WH-T and WH-C commenced flammable gas production from the 9th, 6th and 11th day respectively. Gas analysis from WH-F shows

Methane (65.0%), CO2 (34.94%). WH-A contained Methane (60.0%), CO2 (39.94%). WH-T contained Methane (71.0%), CO2 (28.94%), while WH-C had Methane (64.0%) CO2 (35.94%). The other gases were found in the same levels and in trace amounts in all the systems. The overall results showed that treating water Hyacinth with KOH did not have a significant improvement on the biogas yield. It also indicated that water Hyacinth is a very good biogas producer and the yield can be improved by drying and combining it with cow dung.

Biomass is basically used as fuel, fertilizer, and feed. One fact which is evident in the literature is that the use of biomass, particularly livestock manure as fertilizer and feed has not grown with the continuously increasing rate of production of the manure itself. For instance, Wadman et al., (1987) pointed out that in the Netherlands, the total production of manure from housed cattle (during the winter period only) pigs, poultry, and fattening calves increased from 10 tonnes/ha in 1950 to 26 tonnes/ha in 1982. Neeteson and Wadman (1990) observed that within that same period however in the same country, the need to use animal manures as fertilizers decreased due to the widespread adoption of cheap inorganic fertilizers. These inorganic fertilizers have a number of advantages over manure namely; their composition is known, they are easier to store, transport, and apply and have a more predictive effect on crop growth than manures. Therefore, livestock manure was increasingly regarded as a waste product rather than a fertilizer.

The situation reported for the United Kingdom is another example. Using agricultural census data, Smith and Chambers (1993) estimated that around 190 million tonnes of livestock excreta per year are produced on U. K farms. Some 80 million tonnes of this is collected in buildings and yards where they are stored and hopefully applied to land later. However, land application of all the collected manure has not always been possible over the years. Chalmers (2001) in a review of fertilizer, lime and organic manure use on farms in Great Britain noted that the proportion of UK land receiving organic manures remained at 16% for tillage cropping but increased slightly for grassland, from a mean of 40% in 1983­1987 to 44% in 1993-1997. Just as in the case of the Netherlands referred to previously, livestock manure produced on UK farms constituted a burden since land application of all of it was increasingly impossible. Against the background that Netherlands used as an example has 5, 95, and 14 millions of cattle, poultry, birds, and pigs respectively, implications for other countries which have higher livestock populations are quite significant.

The option of using livestock manure as fuel merits closer investigations for its evident biogas-generation potential. Heltberg et al., (1985) pointed out that biomass could potentially contribute about 3.2 billion GJ to the United states energy resources, which is roughly the amount of energy expected to be supplied from nuclear and hydroelectric power plants in the USA as at that time. Within the USA itself, some projects are already operational. Thomas (1990) reported the case of a commercial project in California which was generating about 17.5 MW of electricity from cattle manure. Biogas typically refers to methane produced by fermentation of manure or other biomass under anaerobic conditions. Mata-Alvarezi (2002) focused on the state of research on the subject in Europe and noted that the process is popular in the rural areas, particularly in the Netherlands and Denmark because it provides a convenient way of turning waste into electricity. The use of biogas is encouraged because methane burns with a clean flame and produces little pollution or no pollution.

The use of manure to produce biogas for energy supply also has attractive prospects in developing countries. According to Akinbami et al., (2001), Nigeria produced about 227,500 tons of fresh animal waste daily. The paper noted that since 1kg of fresh animal waste produces about 0.03 m3 of biogas, then Nigeria can produce about 6.9 million m3 of biogas everyday. In addition to all this, 20kg per capita of municipal solid waste (MSW) has been estimated to be generated in the country annually. Going by the census figures 140 million inhabitants, the total generated MSW would be at least 2.8 million tonnes every year. With increasing urbanization and industrialization, the annual MSW generated will continue to increase. Biogas production can therefore be a profitable means of reducing or even eliminating the menace and nuisance of urban waste in many cities by recycling them; while at the same time contributing towards providing adequate solution to the seemingly intractable problem of energy security. In the case of Nigeria, a few small scale biogas plants have been constructed by the Sokoto Energy Research center (SERC) and the Federal Institute of Industrial Research (FIIRO) Oshodi, Lagos. As of now contributions of these small — scale biogas plants to aggregate energy supply are yet to become significant (Energy Commission of Nigeria, 1998). Similar potential as this exists in many countries across the developing world.

Processes for the conversion of biomass to biogas may be classified into two categories namely thermal processes (as in biomass gasification), and biological processes (as in anaerobic digestion). As observed by Chynoweth and Isaacson (1987), the major advantage of thermal processes is their ability to effect total conversion of organic matter at rapid rates.

The major disadvantage however is that they produce a mixture of gaseous products that must be upgraded to methane and are only economic at larger scales. Biological processes on the other hand, have the major advantages of producing biogas composed primarily of methane and carbon dioxide with traces of hydrogen sulfide, and are also low — temperature processes which are economical at a variety of scales. Biomass gasification is a process in which solid fuels are broken down by the use of heat to produce a combustible gas (Foley and Barnard, 1985). Fuels that can be gasified include wood, charcoal, coal, and a variety of other organic materials. In the sense used in this chapter, gasification should be distinguished from biogas production which uses wet organic feed stock and works by means of microbial action. Biological processes of biogas production may be aerobic (Evans and Svoboda, 1985) or anaerobic (Voermans, 1985). However, because of the high cost of aerobic processes particularly as regards the provision of energy to sustain the processes, anaerobic processes are preferred. As noted by Voermans (1985) biogas is the main purpose of anaerobic digestion and it comprises + 55-70% CH4; 30-45% CO2, water vapor, and 0.0­0.5% H2S Anaerobic digestion is brought about in anaerobic digesters.

In contemporary times, cassava is being recognized as an important source of biofuel. Research efforts aimed at investigating the potential of this sturdy crop for the production of biogas and bioethanol are currently in progress.

1.1 Global production of cassava

As observed by Adelekan (2012), Cassava (Manihot esculenta Cranz) is a very important crop grown for food and industrial purposes in several parts of the tropics. Nigeria, with a 2006 production of 49 million tonnes of cassava is the largest producer of the crop in the world (National Planning Commission, 2009). Other countries which grow significant quantities of the crop include Brazil, Congo Democratic Republic, Thailand, Indonesia, Ghana and China. A handful of other countries also grow the crop but at much lower production quantities. According to IFAD/FAO (2000) report, cassava is the fourth most important staple crop in the world after rice, wheat and maize. The present annual global production of cassava is estimated at 160 million tonnes. This huge production also results into the discharge of significant cassava-derived solid wastes and liquid wastes into the environment especially during processing. Cassava peels constitute 10-20% by mass of each tuber. Cassava tuber contains 25-30% dry matter by mass, the major portion of which is made up of carbohydrates in the form of starch and sugars. The tuber also contains 70-75% moisture. The ongoing encouragement of cassava cultivation by Governments in Nigeria, Thailand, China and other countries is gradually raising the profile of the crop as a significant cash crop. With increased crop production is also an associated increased production of peels and other cassava-derived wastes. This constitutes an enhanced risk of pollution of the environment. There is therefore a pungent need to find an alternative productive use of the peels. One area of possibility is to investigate the potential of cassava peels for the production of biogas. Finding such an important use for the peel would make it less burdensome on the environment as a pollutant and contribute towards enhancing energy security in the cassava-producing regions.

As pointed out by Barber et al., (2010) perennial grasses benefit the environment in numerous ways. They help to reduce climate change, increase energy efficiency and will constitute a sustainable energy resource for the world. Switchgrass, the most widely used perennial grass for biofuels, is also in such a manner, beneficial to both farmers as well as energy consumers in general. Perennial grasses are crucial to the ecosystem to create a sustainable energy resource for the world and also to limit the use of fossil fuels. These grasses are important because they can produce ethanol, an energy source that emits much less carbon dioxide than other fossil fuels. Reducing carbon dioxide emissions is important because carbon dioxide emissions in the atmosphere constitute one of the leading causes of climate change. Barry (2008) pointed out that 1 bale of switchgrass can yield up to about 50 gallons of ethanol. As reported by Rinehart (2006), researchers are using switchgrass as a biofuel so that they can successfully reduce carbon dioxide emissions. Switchgrass has a high energy in and out ratio because of lignin, the byproduct of the cellulose conversion that stores internal energy for its energy transformation process. Ethanol reduces carbon dioxide emissions by approximately ninety percent when compared to gasoline and consequently, carbon dioxide in the ozone layer of our atmosphere will slowly begin to deplete itself as biofuels created from switchgrass, other grasses and other ethanol sources are utilized. As a rule, all species of the grass family (poaceae) contain starch and should be able to yield ethanol.

Biomass represents a continuously renewable potential source of biogas and other biofuels and thus is certainly an option to inevitable fossil fuel depletion. Biogas can be economically converted to methane at facilities ranging from smallholder utility equipments to large scale plants and therefore can be tailored to supply rural and urban gas needs as well as meet regional and nationwide energy demands. According to Shoemaker and Visser (2000), the composition of biogas produced by anaerobic digestion as compared to natural gas is given in Table 2. It is readily seen from the table that overall, biogas is of a better quality than natural gas and possesses much less potential for polluting the environment. Biogas therefore constitutes a good alternative to natural gas.

However, the present potential of biofuels to enhance energy security is limited. Globally, the huge volume of biofuels required to substitute for fossil fuels is beyond the present overall capacity of global agriculture. For example in the year 2006/2007, the United States used 20 percent of its maize harvest for ethanol production, which replaced only three percent of its petrol consumption (World Bank, 2008). The possibility of more significant displacement of fossil fuels should be possible with second and third generation biofuels.

Theoretically, biomass includes every material of plant or animal origin. However, the focus of research and use of biomass in practical terms is on those materials from which biogas, ethanol and biodiesel may be derived at economic scales. Earlier researchers reported successes which have been advanced by more recent works. Hill (1984) conducted experiments to investigate methane productivity of some animal waste types at low temperatures and very low volatile solids concentrations. Results indicated that there are large differences between the waste types and that poultry waste produced the highest biogas yield for animal live weight (LW) while dairy waste was the least productive on a LW and total solids (TS) basis. This result corroborates those of Huang and Shin (1981), Huang et al., (1982), and Shih (1984). These studies evaluated the potential of methane generation from chicken manure and also assessed the performance of poultry waste digesters. Of further interest is the finding of the last paper, which showed that a high rate of gas produced at 4.5 v/v/day (methane 3.0 v/v/day) can be reached at 50C, 4-day retention time (RT) and 6% volatile solids (VS) concentration. Shih (1984) further pointed out that if this potential can be obtained on a poultry farm, the process of anaerobic digestion for waste treatment and energy production would be economically attractive. The potentials of other kinds of livestock waste for biogas production have also been investigated for example dairy manure (Lindley and Haughen, 1985), beef cattle manure (Hamiton et al., 1985) and pig manure (Fedler and Day, 1985). A common result however, is that these particular livestock waste types did not produce biogas as much as poultry manure in the experiments conducted. In experiments conducted on a digester (Ghederim et al., 1985) gas yields related to the organic matter fed to the digester were 0.5 to 0.6m3/kg for pig farm sludge and 0.2 to 0.3m3/kg in the case of beef cattle waste. Methane content varied between 60 and 70%.

The possibility of manure-straw mixtures producing more gas than manure alone continues to engage the interests of researchers. Jantrania and White (1985) found that high-solids anaerobic fermentation of poultry manure mixed with corn stover at 30% to 35% initial total solids produced biogas quantitatively comparable to slurry type anaerobic fermentation. However, the retention time of the process was much longer than required in the conventional process. Hills and Roberts (1979) had earlier reported a substantial increase of methane produced from rice-straw manure and barley-straw manure mixtures compared to manure alone. In a comparative study of pig manure and pig manure-corn stover, Fujita et al (1980) concluded that the mixtures produced more methane than manure alone. In a pit — scale study of wheat straw-manure mixture, Hashimoto and Robinson (1985) found a methane production of 0.25m3 CHi/kg of volatile solids (VS).

In more contemporary papers, several researchers have recently reported improvements in biofuel production from various agricultural materials including biogas from mixtures of cassava peels and livestock wastes (Adelekan and Bamgboye, 2009a), biogas from pretreated water hyacinth (Ofuefule et al., 2009), methanol from cow dung (Ajayi, 2009) fuel from indigenous biomass wastes (Saptoadi et al., 2009), ethanol from non-edible plant parts (Inderlwildi and King, 2009), as well as biogas from various livestock wastes (Adelekan and Bamgboye, 2009b). Adelekan (2012) showed that cassava, an often neglected but sturdy crop is a potent energy crop for the production of methane and ethanol, and presented production estimates for these biofuels based on cassava yield from the tropical countries. It has been discovered that, under aerobic conditions, living plants also produce methane which is significantly larger in volume than that produced by dead plants. Although this does not increase global warming because of the carbon cycle (Keppler et alv 2006), it is not readily recoverable for economic purposes. However, the methane which is recoverable for the direct production of energy is from dead plants and other dead biomass under anaerobic conditions.

Prasad et al., (2007) observed that with world reserves of petroleum fast depleting, ethanol has in recent years emerged as the most important alternative resource for liquid fuel and has generated a great deal of research interest in ethanol fermentation. The paper noted that research on improving ethanol production has been accelerating for both ecological and economic reasons, primarily for its use as an alternative to petroleum-based fuels. Based on their genetic diversity, climatic adaptation, biomass and sugar production, field crops have the best potential as large scale fuel sources. Lignocellulosic biomass is the most abundant organic raw material in the world. As observed further, the production of ethanol from renewable lignocellulosic resources will improve energy availability, reduce dependence on petroleum based fuels, decrease air pollution, and diminish atmospheric CO2 accumulation. Using the by-products of crop processing for ethanol production will also reduce waste disposal problems and lower the risks of polluting the environment.

Adelekan (2011) in laboratory experiments compared the ethanol productivity of selected varieties of cassava, sorghum and maize crops widely grown in West Africa by correlating volumes and masses of ethanol produced to the masses of samples used. The rate of ethanol production were found to be 145 l/tonne, 135 l/tonne and 346 l/tonne for cassava (variety TMS 30555), sorghum and maize respectively. In terms of ethanol productivity, the order observed in the study was maize > cassava > sorghum. The dried mash produced from the process was analysed for its nutritive quality and that from cassava was found to contain 61.8 calories of food energy per 100g; that from maize and sorghum; 59.5 and 58.1 calories respectively, making them good materials for livestock feed composition. Overall, the ethanol produced from these tropical crop varieties is of a good quality. The key advantage is that the ethanol is being produced from renewable sources which are also sustainable. The production and use of ethanol from cassava, sorghum and maize crop is recommended particularly in West African countries which often suffer crucial problems in respect of sourcing and delivery of fossil fuels and also in other tropical countries where these crop varieties are grown. In such places, ethanol can be blended with gasoline. The key production process used is fermentation and this being a natural process is very efficient, safe and not destructive to the environment.

Adelekan and Bamgboye (2009a) investigated biogas productivity of cassava peels, mixed with poultry, piggery and cattle waste types in ratios 1:1, 2:1, 3:1 and 4:1 by mass, using 12 Nos. 220l batch type anaerobic digesters in a 3 x 4 factorial experiment using a retention period of 30 days and within the mesophilic temperature range. Biogas yield was significantly (P < 0.05) influenced by the different mixing ratios of livestock waste with cassava peels. The cumulative average biogas yield from digested cassava peels was 0.6 l/kg-TS. The average cumulative biogas yield increased to 13.7, 12.3, 10.4 and 9.0 l/kg-TS respectively for 1:1, 2:1, 3:1 and 4:1 mixing ratios when cassava peel was mixed with poultry waste. On mixing with piggery waste, the average cumulative biogas yield increased to 35.0, 26.5, 17.1 and 9.3 l/kg-TS respectively for 1:1, 2:1, 3:1 and 4:1 mixing ratios. In the case of mixing with cattle waste, the average cumulative biogas yield increased to 21.3, 19.5, 15.8 and 11.2 l/kg-TS respectively for 1:1, 2:1, 3:1 and 4:1 mixing ratios. Results show that for all livestock waste types, mixing with peels in the ratio 1:1 by mass produced the highest biogas volumes, and highest in piggery waste. Cassava peels have high value of organic carbon and low value of total nitrogen, and this result in a particularly high C/N ratio. According to Karki et al. (1994) high C/N ratio is indicative of the fact that the material is not good for biogas production and will not appreciably yield biogas. However, the work points out that such a material could be mixed with another with a much lower C/N ratio to stabilize the ratio to an optimal value between 22 and 30. Biogas yield was significantly (P < 0.05) influenced by cassava peels used. The cumulative average biogas yield from digested cassava peels was 0.6 l/kg — TS. This value is low compared with values obtained by Bamgboye (1994) from other lignocellulosic materials such as chopped substrate (1.85 — 3.95 l/kg-TS) and ground water hyacinth substrate (4.01 — 5.55 l/kg-TS). Since cassava peel is a material with a high C/N ratio, it will not yield much biogas. As the paper showed however biogas production from cassava peels was enhanced by mixing with manure.

Bolarinwa and Ugoji (2010) studied biogas production by anaerobic microbial digestion of starchy wastes of Dioscorea rotundata (yam) and Manihot esculenta (cassava) aided by abattoir liquid effluent using a laboratory digester. The volume of the gas produced at 12hr intervals by feedstock varied for the 72hr of study. The cassava substrate mixture produced the highest daily average volume of gas (397ml), mixture of cassava and effluent 310.4ml; mixture of cassava, yam and effluent 259ml; mixture of cassava and yam produced 243.6ml; yam 238ml; mixture of yam and effluent 169.4ml while abattoir effluent produced the lowest volume of gas (144.4ml). The average pH of digester varied between 5.6 and 6.7 while the temperature varied between 32.30C and 33.30C. The microbial load of digester samples was determined at 12hr-intervals. Two groups of bacteria were isolated. Acid-formers isolated included Staphylococcus aureus, Pseudomonas aeruginosa, Bacillus subtilis, Escherichia coli, Serratia liquefaciens, Micrococcus pyogenes and Streptococcus pyogenes while the methane — formers were Methanobacterium sp. and Methanococcus sp. This study concluded that spoilt yam and cassava, which are otherwise of no apparent use, could provide a cheap source of renewable energy for domestic use.

Manure has for a long time been recognised as a renewable source of energy. Earlier practices involved direct combustion of manure to produce heat energy; while latter practices involved gasification of manure, followed by combustion. The more recent practices involved anaerobic biodigestion of manure to produce biogas which is scrubbed to purer methane and then combusted.

10.2 On-farm availability of manure

Huge quantities of manure are produced on farms. In fact a cattle farming operation which has a herd size of 10,000 animals can on a daily basis generate wastes equal to that produced by a city of half a million residents. Considering cattle for instance reported values of daily manure production range from 10kg (VITA, 1980) to 60kg (Safley Jr, et al., 1985) per animal. Legg (1990) reports that the 8.5, 28, 6.9 and 104 million cattle, sheep, pigs and poultry reared in England and Wales produced 80, 11, 11, and 30 million tonnes of manure respectively for the year immediately preceding. Smith and Chambers (1998) noted that manure arising from dairy beef farming comprise the majority (73 million tonnes) of the 90 million tonnes

annual animal production of livestock manure in the UK. Yet another estimate, Smith et al., (2001) reported that 4.4 million tonnes of poultry manure are produced annually in the UK; comprising about 2.2 million broiler litter, 0.3 million tonnes of turkey litter, 1.5 million tonnes of layers manure (i. e. from egg producing hens) and 0.4 million tonnes from other sources (mainly breeding hens, cocks, and ducks). Total manure production from pigs in England and Wales is estimated to be about 10.03 million tonnes per year with about 45% as slurry and 55% as farmyard manure (Smith et. al., 2001).

The yearly production of livestock wastes in the Netherlands is estimated to be about 10 million tonnes dry matter (De Boer, 1984). The paper noted that 75 to 80% of it is ruminant waste while the rest is attributed to manure from pig and poultry. Specifically in the case of Nigeria, reported values of animal waste production range from 144 million tonnes/year (Energy Commission of Nigeria, 1998) to 285.1 million tonnes/year (Adelekan, 2002). This huge production of manure from farms can constitute a threat to the environment since it may not be readily returned to or in fact absorbed by land for fertilization. The challenge is how to find effective uses for the livestock wastes out of which the production of biofuels is an attractive option.