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.

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.

Chynoweth and Isaacson (1987) observed that in any anaerobic digestion process that is not inhibited or kinetically limited, two major factors affecting methane yields are feedstock composition and inoculum characteristics. The composition of the biodegradable organic compounds can influence methane yield in that reduced compounds such as fats and proteins produce a higher percentage of methane than oxidized compounds such as sugars. Ultimate methane yields are however, influenced principally by the biodegradability of the organic components. The same paper noted further that each anaerobic environment may differ in the types of bacteria involved in the methanogenesis, depending on differing factors such as substrate, retention time, temperature, pH, and fluctuations in environmental parameter. Although certain general properties are common from one environment to another, each environment may have its own unique population of bacteria, and associated microbial activities. Key operating factors which have a direct influence on the level and efficiency of biogas include volatile solids loading rate, digester temperature hydraulic retention time, pH and carbon: nitrogen ratio (Vetter et al., 1990).

Cassava is the best energy crop used to produce ethanol. This is because the ethanol yield of cassava per unit land area is the highest among all known energy crops. The comparison of ethanol yield produced from different energy crops shows that cassava has the highest ethanol yield of 6,000 kg/ha/ yr and highest conversion rate of 150 L/tonne of all the energy crops. Though sugar cane and carrot have higher crop yield of 70 and 45 tonnes/ha/ yr respectively compared to 20 tonnes/ha/yr for cassava, the huge quantities of water which they require during their growth periods is a strong limitation when compared to cassava which can actually grow under much drier conditions. Kuiper et al., (2007) noted that a tonne of fresh cassava tubers yields about 150 litres of ethanol.

Adelekan (2010) investigated ethanol productivity of cassava crop in a laboratory experiment by correlating volumes and masses of ethanol produced to the masses of samples used. Cassava tubers (variety TMS 30555) were peeled, cut and washed. 5, 15, 25 and 35 kg samples of the tubers were weighed in three replicates, soaked in water for a period of a day, after which each sample was dried, crushed and the mash mixed with 500, 650, 800, and 950 ml of N-hexane (C6H14) respectively. This crushed mash was then allowed to ferment for a period of 8 days and afterwards pressed on a 0.6 mm aperture size and sieved to yield the alcohol contained in it. The alcohol was heated at 79°C for 10 h at intervals of 2 h followed by an h cooling. Ethanol yield was at average volumes of 0.31, 0.96, 1.61 and 2.21 litres, respectively, for the selected masses of cassava samples. This study found that a total of 6.77 million tonnes or 1338.77 million gallons of ethanol are available from total cassava production from tropical countries. The production and use of ethanol from cassava crop in the cassava-growing tropical countries of the world certain holds much promise for energy security and is therefore recommended.

Some benefits of using ethanol are that it is not poisonous and neither causes pollution nor any environmental hazard. It does not contribute to the greenhouse effect. It has a higher octane value than gasoline and is therefore an octane booster and anti-knock agent. It reduces a country’s dependence on petroleum and it is an excellent raw material for synthetic chemicals. The main crops presently being used for ethanol production are maize, sugarcane and cassava, and among these, cassava has a competitive advantage because of its lower cost of raw material and a simpler ethanol processing technology. Nguyen and Gheewala (2008) conducted a well-to-wheel analysis for cassava-based ethanol in Thailand. The aim of the analysis was to assess the potentials of cassava-based ethanol in the form of gasohol E10 for promoting energy security and reducing environmental impacts in comparison with conventional gasoline. The results showed that cassava-based ethanol in the form of E10, along its whole life cycle, reduced certain environmental loads compared to conventional gasoline. The percentage reductions relative to conventional gasoline are 6.1% for fossil energy use, 6.0% for global warming potential, 6.8% for acidification, and 12.2% for nutrient enrichment. The paper concluded that using biomass in place of fossil fuels for process energy in the manufacture of ethanol leads to improved overall life cycle energy and environmental performance of ethanol blends relative to conventional gasoline.

Marchaim (1992) noted that there is a close relationship between the biogas fermentation process and the temperature of the reactor. The higher the temperature, the more biogas is produced but when the temperature is too high, this can cause metabolic process to decline. Hobson et al., (1981) found biogas production to be greatest when the digester temperature was in the range of 32 to 400C. Hill (1982) also stated that digestion temperatures for optimum design all occur in the mesophilic range of 320C to 400 C. This work suggested that temperature beyond 400C has little effect on digester performance since the higher volumetric methane productivity is offset by the smaller digestion volume. As observed by the paper these lower temperatures also represent major savings in energy requirements when compared to thermophilic digestion (i. e. 600C). During the process of anaerobic biodigesiton in order to reach optimum operating temperatures (30-370C or 85-1000F), some measures must be taken to insulate the digester, especially in high altitudes or cold climates (VITA, 1980). Straw or shredded tree bark can be used around the outside of the digester to provide insulation. According to Carcelon and Clark (2002), anaerobic bacteria communities can endure temperatures ranging from below freezing to above 57.20C (1350 F), but they thrive best at temperatures of about 36.7 0C (980 F) (mesophilic) and 54.40C (1300 F) thermophilic. Bacteria activity, and thus biogas production falls off significantly between about 39.40C and 51.70C (103 0F and 125 0F) and gradually from 35 0C to 00 (95 0F to 32 0F). To optimize the digestion process, the digester must be kept at a consistent temperature as rapid changes will upset bacterial activity.

The potential of thermophilic digester operating temperatures (> 550C) for anaerobic biogestion of livestock waste has been investigated by several researchers (Converse et. al., 1977; Hashimoto, et. al., 1979; Hashimoto, 1983; Hashimoto, 1984; Hill, 1985; Hill and Bolte, 1985; Hill et. al., 1986) with the technical feasibility being decided in favour of the process. Hill (1990) identified the advantages of thermophilic digestion over conventional mesophilic digestion as reduced hydraulic retention time (HTR), increased loading rate, and smaller physical reactors for identical waste amounts. The major disadvantage identified is the increased use of energy required to heat the feedstock and maintain digester operating temperature. Chen and Hashimoto (1981) however suggested that the development of heat exchangers to recover energy in the effluent somewhat alleviated this advantage.

In cold climates, or during cold weather, optimal temperatures become very expensive to maintain, thus reducing the economic feasibility of the process of anaerobic biodigestion (Cullimore et al., 1985). In view of this, investigations have been conducted into the feasibility of anaerobic biodigesiton at lower temperatures. Stevens and Schulte (1979) thoroughly reviewed the literature regarding low-temperature digestion and found that methanogenesis occurs at temperatures as low as 40C, and that an increase in temperature from 40C to 250C dramatically increased the rate of methanogenesis. Cullimore (1982) reported results which indicated that as digester temperature was reduced from optimal levels, biogas production decreased linearly to extinction at between 0 and 80C. Ke-Xin and Nian-Guo (1980) successfully ran several rural digesters at ambient winter temperatures of 12 to 130C, and obtained gas yields which were 23 to 40 percent that of the optimal temperature production. Pos et al., (1985) suggested that if the anaerobic digestion process was found to function efficiently at lower temperatures, the use of large digestion units at longer retention times and without heating might be considered. It might then be possible to run full scale digesters at less than optimal temperature in order to increase their economic feasibility.

Safley Jr and Westerman (1990) reported satisfactory digester performance for both winter and summer conditions. However, biogas production was found to fluctuate seasonally with reduced biogas production being noted during the winter. Mean methane yield was found to be 0.34 m3 CH4 kg of volatile solids (VS) added. Mean biogas concentration was 69.5% CH4 and 26.8% CO2. The loading rate during the 17-month period of study was 0.12 kg VS/m3-day. Typically, anaerobic digesters are designed to operate in either in the mesophilic (200C — 450C) or thermophilic (450C — 600C) temperature ranges. However, as pointed out by Safely Jr and Westerman (1990) the production of methane (called methanogenesis) has been observed at temperatures approaching 0OC. The anaerobic decomposition of organic matter at low temperature (< 200C) is referred to as psychrophilic anaerobic digestion.

Over the recent past, cocoyam has received inadequate attraction from researchers. Relatively few works reported on considered principally as a food crop. However, as will be seen in this subsection, some papers are beginning to point out the potentials of this crop as a source of biofuel.

8.1 Global production of cocoyam

The world has focused entirely on a comparatively small number of crops to meet the various needs for food and industrial fiber; the total number of economic crops of significance to global trade hovering just above one hundred. The consequence is that thousands of plant species with a considerably larger number of varieties fall into the category of underutilised or neglected crops. These crops are marginalized by agricultural, nutritional and industrial research (Global Forum for Underutilized Species, 2009). One of such neglected crops is cocoyam which over the years has received minimal attention from researchers and other stakeholders of interest. Cocoyam (Colocasia and Xanthosoma species), a member of the Aracea family of plants, is one of the oldest crops known. It is grown largely in the tropics, for its edible corms and leaves and as an ornamental plant. On a global scale, it ranks 14th as a vegetable crop going by annual production figures of 10 million tonnes (FAO, 2005). Its production estimates vary. However, one study points out that Africa accounts for at least 60% of world production and most of the remaining 40% is from Asia and Pacific regions (Mitra et al., 2007). Another study opines that coastal West Africa accounts for 90% of the global output of the crop with Nigeria accounting for 50% of this (Opata and Nweze, 2009). Cocoyam thrives in infertile or difficult terrains that are not well suited for large scale commercial agriculture for growing most conventional staple crops. As observed by Williams and Haq (2002), since the poor are frequently the main occupants of such areas, cultivation of neglected crops such as cocoyam constitute practical alternatives for them to augment their meagre incomes. The crop’s supposed association with the poor may be a reason while conventional agricultural research has not bothered much to take a closer look at it.

According to San Thy et al., (2003) biogas fermentation requires an environment with neutral pH and when the value is below 6 or above 8 the process will be inhibited or even cease to produce gas because of toxic effect on the methanogen population. The optimum for biogas production is when the pH value of the input in the digester is between 6 and 7. Increasing the amount of feedstock or a change in the fermentation material is likely to acidify the fermentation system because of the accumulation of volatile fatty acids (VFA). In this way pH can be used to indicate if the system is being overloaded. In the initial period of fermentation, as large amounts of organic acids are produced by the acid-forming bacteria, the pH in the digester may fall below 5 causing inhibition of the growth of the methanogenic bacteria and hence reduced gas generation (Da Silva, 1979). Acetate and fatty acids produced during digestion tend to lower the pH of the digester liquid (Marchaim, 1991). Hansen et al., (1998) stated that acetate-utilizing methanogens are responsible for 70% of the methane produced in biogas reactors.

Buren (1983) pointed out that the micro-organisms involved in anaerobic biodigestion require a neutral or mildly alkaline environment, as a too acidic or too alkaline environment will be detrimental. The work stated that a pH between 7 and 8.5 is best for biodigestion and normal gas production. The pH value for a digester depends on the ratio of acidity and alkalinity and the carbon dioxide content in the digester, the determining factor being the density of the acids. Buren (1983) noted further that for the normal process of digestion, the concentration of volatile acid measured by acetic acid should be below 2000 ppm, as too high a concentration will greatly inhibit the action of the methanogenic micro-organisms. Results of a study by Jantrania and White (1985) further confirm the foregoing. The study compared the performance of a number of digesters processing poultry wastes and found that the pH of the residue from digesters that failed were between 6.1 and 6.7, while the pH from the successful digester was 7.5. The digesters which stopped producing any appreciable amount of gas after 54 days had higher hydrogen sulfide content (over 200 ppm) than the successful digester.

Adelekan (2011) produced methane from cocoyam corms and related the volumes and masses obtained to the masses of corms used; derived guiding numerical relationships for the processes and extrapolated these values using production quantities of the crop reported globally and finally submitted workable estimates as regards biogas which is derivable from aggregate global production of the crop. The scientific innovation and relevance of the work reported lies in the fact that the fermentation and anaerobic digestion methods used are applicable across countries and regions irrespective of available degree of industrialization and climate. A new vista is opened in the use of this neglected crop as a cheap renewable source of energy in view of the rapid depletion, environmental pollution and high costs of fossil fuels. Results show that the 10 million tonnes annual global production of cocoyam is potentially able to produce 39.5 million cubic metres of methane which on burning would produce 179.3 x 107 MJ of enerrgy. The mash obtained as byproduct of the processes is capable of supplying 59 calories of food energy per 100g which is an excellent feedstock for livestock. The use of cocoyam (Colocasia and Xanthosoma species) as a renewable source of energy for the production of biogas poses no threat to the environment or food supply and is therefore recommended. Furthermore, doing so helps to enhance energy security.

Adeyosoye et al., (2010) studied biogas yield of peels of sweet potato (SPP) and wild cocoyam (WCP). Buffered and sieved goat’s rumen liquor was added to 200 mg of dried and milled SPP and WCP in 100 ml syringes supplied with CO2 under anaerobic condition and incubated for 24 hr. Total biogas produced was measured at 3 hr intervals till the 24th hr when the fermentation was terminated. The inoculum was also incubated separately. The proximate composition of SPP and WCP were similar except for the higher EE content (12%) of SPP. The SPP and WCP used contained 26.81 and 26.97% DM, 3.06 and 3.83% CP, and 78.94 and 79.17% carbohydrate respectively. Both samples had the same crude fibre (7.00%) content. Total biogas produced from SPP, WCP and the inoculum varied from 13.0, 11.0 and

5.0 ml respectively at the 3rd hr through 66.5, 61.5 and 18.0 ml at the 18th hr to 77.5, 72.0 and

30.0 ml at the 24th hr respectively. The differences in biogas production across the treatments were significant (p < 0.05). There were no significant differences (p > 0.05) in the volumes of methane produced from SPP (42.5 ml) and WCP (39.5 ml) which were significantly (p < 0.05) higher than 20.0 ml produced by the inoculum. The study pointed out that peels of sweet potato and cocoyam wastes can produce significant quantities of biogas for domestic applications. The foregoing studies confirm that ultimate methane yields from biomass are influenced principally by the biodegradability of the organic components. The more putrescible the biomass, the higher is the gas yield from the system (Wis, 2009).