The solids concentration of the influent into the biodigester affects the rate of fermentation (Marchaim, 1992). In a reported experiment conducted in China, which is mostly located in the temperate latitudes, the optimum concentration of solids was considered to be 6% in summer but between 10 and 20% in winter and spring. When temperatures are low and materials take longer to decompose; it is better to have a higher total solids concentration, although this might cause a problem with impeded flows through the digesters (San Thy et al., 2003). The loading rate is defined as the amount of volatile solids (fermentable solids) per unit of active biodigester volume per day. Typical values of loading rates are between 0.2 and 2 kg VS/m3/day. This assumes that total solids (TS) are 17% of the fresh weight of the manure and that the volatile solids content (VS) is 77% (Fulford, 1988). The methane content of the gas can indicate overloading but it is more difficult to measure unless the right equipment is available. If the digester is being overloaded, the gas production will rise up initially and then fall after a while when inhibition occurs. The CH4 content of the gas will fall while the CO2 content will rise, because CO2 is not used by the hydrogen consuming bacteria or because the methanogenic bacteria are inhibited.

The feedstock concentration of volatile solids (VS), the detention time, and the operating temperature are the major design factors, which determine the maximum total daily methane production (Hill, 1982). In a study by Vetter et al., (1990), daily biogas production was determined to be directly proportional to the volatile solids loading rate, given that other factors such as digester temperature and pH stayed relatively the same. Hill (1982) found that maximum VS reduction based on developed kinetic data was 75, 56, 30 and 62 percent for pig, beef, dairy, and poultry waste respectively. No significant increase in VS destruction will occur at temperature greater than approximately 450C.

Climate change, crop failures, unpredictable commodity prices, wars, political unrest and other forms of dislocations in the established pattern of global affairs, variously show that overreliance on just a few crops is risky to the world. However, bringing those crop species with underexploited potentials out of the shadows into the mainstream would help to spread this risk and enhance the utility of marginal lands on which many of them are cultivated. Most of the comparatively few number of studies reported in respect of cocoyam have focused largely on enhancing its value as a food crop, principally to supply carbohydrates and starch; a role which it already shares with so many competing crops. However, the paper by Adelekan (2011) looked at cocoyam as an energy crop for the supply of ethanol and biogas; a role which if fully developed can raise the profile of this crop in global energy economics. Points in favour of this research are the fact that it is in line with ongoing global research efforts at discovering more energy crops and developing other sources of renewable energy. Some progress has been reported in the use of cassava (another neglected tropical crop) for the production of ethanol as a sustainable source of biofuel in tropical countries Adelekan (2010). Cocoyam also has similar potential for this, most particularly in the tropical and subtropical countries. According to Adelekan (2011) which investigated the global potential of cocoyam as an energy crop, the yield of bioethanol from cocoyam is 139 L/ tonne. This compares very favourably with 145 L/ tone obtained for cassava (Adelekan, 2010), 100L/tonne for carrot and 70L/tone for sugar cane. Given a global annual production quantity of cocoyam to be 10million tonnes, 331 million gallons of ethanol is potentially available from this.

The question always arises, with a growing demand for ethanol produced from cocoyam, is there a threat to food security in respect of the crop? The answer to this question is twofold. Firstly, the yield of cocoyam, presently about 30 tonnes per hectare (Ekwe et al., 2009) can be tremendously improved through scientific research directed at producing higher yielding varieties. With success in this area, there may not be a need to cultivate more land to increase production of the crop. The present global cultivated total hectares of the crop can still sustain higher improvements in yield. The second part of the answer hzas to do with the need to husband the crop more efficiently to plug avenues for waste. In many parts of the developing world, between the farm and the consumers, 25 to 50% losses still occur to harvested crops because of poor preservation techniques, inadequate storage facilities, deficient transportation infrastructure, weak market structures and other factors. Therefore there is a pungent need to continue to research options which will enhance preservation and lengthen the storage life of cocoyam. Improvements in the area of preservation of the crop will also increase its supply, making its use as an energy crop less potentially deleterious on its use as a food crop and thereby enhancing food security.

Lee (1997) stated that the biological process of bioethanol production utilizing lignocellulosic biomass as substrate requires: 1) delignification to liberate cellulose and hemicelluloses from their complex with lignin, 2) depolymerization of the carbohydrate polymers (cellulose and hemicelluloses) to produce free sugars, and 3) fermentation of mixed hexose and pentose sugars to produce ethanol. In Europe the consumption of bioethanol is largest in Germany, Sweden, France and Spain. Europe produced 90% of its consumption in 2006. Germany produced about 70% of its consumption, Spain 60% and Sweden 50% in the same year. In 2006, in Sweden, there were 792, 85% ethanol (i. e E85) filling stations and in France 131 E85 service stations with 550 more under construction (European Biomass Association 2007).

Biogas production is not possible without a sufficient quantity of anaerobic bacteria. In fresh manure, the concentration of these is low. Taking some effluent (10 to 30% of daily input) and putting it back into the digester is a way of inoculating the fresh manure with active microbial flora. This inoculation of fresh manure can increase gas production up to 30% and it is very important in a plug flow digester as there is almost no mixing between old and fresh slurry. The main nutrients required by microorganisms involved in anaerobic biodigestion are carbon, nitrogen, and inorganic salts. According to Buren (1983), a specific ratio of carbon to nitrogen must be maintained between 20:1 and 25:1, but this ratio will vary for different raw materials and sometime even for the same ones. The main source of nitrogen is human and animal excrement, while the polymers in crop stalks are the main source of carbon. Buren (1983) noted further that in order to maintain a proper ratio of carbon to nitrogen, there must be proper mixing of excrements with polymer sources. Since there are few common materials with a suitable ratio of carbon to nitrogen, production will generally not go well with only one source of material.