There is a great deal of environmental pressure in many parts of the country to ascertain how organic waste can best be handled in the absence of appropriate disposal methods can cause adverse environmental and health problems. Anaerobic digestion has been considered as waste-to-energy technology, and is widely used in the treatment of different organic wastes. The study was carried out for biogas production from co-digestion of cow dung, horse dung and chicken feather in producing biogas through anaerobic co-digestion with chicken feathers. This is achieved by constructing twelve (12) local digesters and gas collection systems mounted at the premises of Department of Water Resources and Environmental Engineering, Ahmadu Bello University Zaria. The digesters were used to digest the mixture of cow dung (CD), horse dung (HD) and chicken feathers (CF) at different percentage ratios for a period of thirty seven (37) days retention time until the biogas reduced significantly. Inflammability test was conducted to determine the quality of biogas produced. Proximate analysis such as nitrates, sulphates, carbon to nitrogen ratio and phosphates were determined before and after anaerobic digestion to rank the substrates in order of their biogas production capacity. The total volumes of the gas produced were 2.51E-01m3, 1.71E-01m3,1.38E-01m3, 1.33E-01m3,1.04E-01m3, 9.43E-02m3, 9.43E-02m3, 5.30E-02m3, 3.59E-02m3, 5.93E-02m3, 3.59E-02m3 and 3.04E-04m3 for 25%CD-75%HD, 100%CD, 50%CD-50%HD, 100%HD, 75%CD-25%HD, 75%CD-25%CF, 75%HD-25%CF, 50%HD-50%CF, 25%HD-75%CF, 50%CD-50%CF, 25%CD-75%CF and 100%CF respectively. These implied that the mix ratio of 25%CD-75%HD produced highest biogas production. The results of carbon to nitrogen ratio for anaerobic digestion were determined at optimum range of 20:1 to 30:1. The nitrates, sulphates and phosphates determined shown an increase after digestion for the cow dung, horse dung and chicken feather with percentage values of 15.1%, 9.7% and 3.2% respectively, which could be a good source of biofertilizer. The average temperatures of the digesters recorded in the morning, afternoon and evening range from 26°C-43°C under mesophilic condition, and the average ambient temperature observed during the study was 34°C, the pH values of the media in all the substrates digested were found almost in the optimal limits of methanogenic bacteria of 6.0-7.4. The modified Gompertz equation was used to adequately describe the cumulative biogas production from these digesters and also to assess the kinetics of the biodegradation process. It was observed that the rates of substrate biodegradability were obtained. The constants were determined using the nonlinear regression approach with the aid of the solver function of the Microsoft Excel tool pack. Biogas production was found to be feasible from the other wastes, but CF was regarded as failed digester as it does not produce significant amount of biogas because of inhibiting factor such as high keratin content. The inflammability test conducted during anaerobic digestion was found to be efficient. Biogas productions from organic wastes are having prospects in contributing towards solving the national energy crisis of most countries.

1.1       Background of Study
Biogas is a clean, environmentally friendly and renewable form of energy generated when micro-organisms degrade organic materials in an oxygen free environment. The formation of biogas can occur either in natural environment or controlled conditions in constructed biogas plants, so called anaerobic digestion (AD). Swamps, marshes, river beds, rumen of herbivore animal are some of the areas where biogas is formed naturally. The same microbial activities are achieved in both natural and controlled conditions. The feedstock for biogas production in constructed plants is more or less any organic fractions from household organic waste to dedicated energy crops like maize (Lantz et al., 2007). The potential feedstock for the production of biogas include; municipal solid waste, industrial organic waste, garden waste, agricultural waste (manure and crop residue), energy crops, cellulose rich biomass, algae and seaweed (water based), by-products of ethanol and bio diesel production (Lantz et al., 2007).

Inadequate energy supply and environmental pollutions are serious problems confronting Nigeria with high population growth rate, access to adequate energy and healthy environmental demands for a diversification of sources of energy supply, if Nigeria is to achieve any meaningful growth and development, biogas generation from anaerobic digestion of readily available wastes could contribute to solving these problems. From the global perspective, the over- dependence on fossil fuels as primary source of energy has resulted in climate change, many environmental destruction and related human health problems (Budiyono et al., 2010). The joint challenge of global pollution and depletion of fossil fuels is driving intense research into alternative renewable energy sources, among which is the biogas. Biogas is produced by the
anaerobic digestion (AD) of organic waste through the synergistic metabolic activities of consortia of hydrolytic, acidinogenic, acetogenic and methanogenic bacteria on organic materials (Yebo et al., 2011).

Currently, AD is used to treat more than 10% of organic wastes for the generation of energy in several European countries (Baere, 2000). Nigeria can do likewise. The industrial viability of this process requires a suitable combination of physical and chemical process parameters and a low- cost substrate, hence the need for process optimization. Attempts have been made to improve biogas production using mixed co-substrates (Dalhat et al., 2015). Anaerobic co-digestion of a simulated organic fraction of municipal solid wastes and fats of animal and vegetable origin has been reported (Fernandez et al., 2005). A substrate of kitchen waste with cow manure has been used to achieve a yield increase of 44% (Rongpin et al., 2009). Kaparaju and Rintala (2005) have examined the co-digestion of pig manure, potato tuber and its industrial by-products. The co-digestion of fruit and vegetable wastes, cattle slurry and chicken manure or sewage sludge for biogas production has also been studied (Ritz et al., 2007; Gomez et al., 2006). The best combination of various substrates for optimal yield remains a big problem despite the enormous number of potential substrates. It is worthy of note that the technical and economical feasibility of an industrial anaerobic digestion plant depends on how much methane is yielded and the purity and on the composition and process variables (temperature, retention time and pH).

These performances are often not available in literature; thus this could entail an increase of the risk of investments due to excessive uncertainties in the design phase. Although the anaerobic digestion of animal manures has been extensively researched and demonstrated, however, based on investment returns from energy production, the economics of diary digesters are not favorable due to the relatively low biodegradability
and biogas yield of diary manure as compared to many other types of organic wastes such as food waste. One of the approaches for improving the economics of diary digesters is to increase their biogas production rate by co-digesting the manure with more degradable waste such as food wastes as long as such wastes are available in the vicinity (Hamed and Ruihong, 2010). Co-digestion of different materials may enhance the anaerobic digestion process due to better carbon and nitrogen balance (Mshandete et al., 2004; Parawira et al., 2004). According to Mata-Alvarez et al., (2000). Co-digestion i.e digestion of more than one substrate in the same digester can establish positive synergism and the added nutrients can support microbial growth.

The process of fermentation in bio-digesters results in transformation of organically bound carbon into gaseous carbon dioxide and methane. The anaerobic environment and extended retention time also inhibit the growth of most pathogenic organisms and prevent the survival of intestinal parasites. It is therefore to be expected that both the chemical and biological parameters of livestock excreta will be improved upon by passage through bio-digesters.

The prospect of this technology is bright in developing countries like Nigeria. This is because Nigeria is an energy resource rich country in terms of both fossil fuels (such as crude oil, natural gas, coal), and renewable energy resources like solar, wind and biomass (Mshandete and Parawira, 2009). The technology can be utilized to provide energy for households, rural communities, farms and industries.

Anaerobic digestion (AD) is a highly promising technology for converting biomass waste into vast quantities of biogas (methane and carbon dioxide), which may directly be used as an energy source or converted to hydrogen.

Since biogas is a mixture of methane (also known as marsh gas or natural gas, CH4) and carbon dioxide, it is a renewable fuel produced from waste treatment. Anaerobic digestion is basically a simple process carried out in a number of steps that can use almost any organic material as a substrate. It occurs in digestive systems, marshes, rubbish dumps, septic tanks and the Arctic Tundra (Ola, 2008). The process does not require large expenditures of energy, as it is biologically driven by a mixed culture of bacteria in the absence of oxygen. Biogas is considered to be carbon neutral because all of the carbon released during combustion has been recently taken from the atmosphere through photosynthesis, unlike fossil fuels that have stored carbon for millions of years (Ryank et al, 2008). Thus biogas is a sustainable alternative to natural gas. Since anaerobic digestion only releases carbon to the gas phase, the other nutrients (nitrogen, phosphorus, and micronutrients) remain in the effluent, which makes it a high quality organic fertilizer and soil amendment wastes (Igboro, 2011). Fig 1.1 shows the biogas cycle.

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