What Problem does this research project aim to solve:
Erratic and low power supply to semi-urban residents in South Western Nigeria. Inefficient system of managing biodegradable wastes.
The research goals and objectives:
- Provision of steady and affordable electricity to semi-urban residents in South Western Nigeria.
- Scaling up the technology in Nigeria by helping to develop a national biogas masterplan.
- Contributing meaningfully to greenhouse gas emission reduction and by implication lessening the effect of global warming in Nigeria.
- Managing environmental wastes (municipal solid wastes, agricultural and food wastes etc) in a manner that will significantly reduce illnesses and deaths attributable to these wastes.
The method of anaerobic digestion that was employed is centralized co-digestion. Centralised co-digestion is a concept based on digesting animal manure and slurries, collected from farms, in a biogas plant centrally located in the manure collection area. The central location of the biogas plant aims to reduce costs, time and manpower for the transport of biomass to and from the biogas plant. Centralised anaerobic digestion plants co-digest animal manure with a variety of other suitable co-substrates (e.g. digestible residues from agriculture, food- and fish industries, separately collected organic household wastes, and sewage sludge).
Animal manure was collected from the University Farm and deposited into the biogas plant, according to an established schedule. At the biogas plant, manure was mixed with the other co- substrates, homogenised and pumped inside the digester tank. The hydraulic retention time was between 12-25 days. There was a controlled sanitation process of substrates of animal origin prior accessing the digester. This provided effective reduction of pathogens and weed seeds and ensures safe recycling of digestate.
The digester feeding system was continuous and biomass mixture was pumped in and out of the digesters in equal amounts through precise pump-sequences. Digestate, pumped out of the digester, was transferred by pipelines to temporary storage tanks. These tanks were covered with a gas proof membrane, for the collection of the additional biogas production (up to 15% of the total), taking place at lower temperature. Before leaving the biogas plant, digestate was analysed and nutritionally defined (Dry matter, volatile solids, Nitrogen, Phosphorus, Potassium, and pH).
Figure 1. Schematic representation of the closed cycle of centralised Anaerobic digestion
* Cattle manure
* Pig manure
* Poultry manure
Other biomass suppliers
* Industrial organic waste
* Sewage sludge
Storage facilities out in the fields
Centralised Biogas Plant
* Reduction of odour nuisance
* Nutritionally defined product
Fertilizer on the fields
* Improved utilisation of plant nutrients
* Reduction of the consumption
of mineral fertilizer
* Reduction of water pollution
Fig 2: Integrated concept of centralized co-digestion
Separation of digested biomass
Biogas for heat &
* Renewable energy source
* CO2– neutral
* Reduction of air pollution
* Effective energy utilisation
The digestion process began with bacterial hydrolysis of the input materials. Insoluble organic polymers, such as carbohydrates, were broken down to soluble derivatives that become available for other bacteria. Acidogenic bacteria then converted the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. These bacteria converted the resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide. Finally, methanogens converted these products to methane and carbon dioxide.
The four key stages of anaerobic digestion involve hydrolysis, acidogenesis, acetogenesis and methanogenesis. The overall process can be described by the chemical reaction, where organic material such as glucose is biochemically digested into carbon dioxide (CO2) and methane (CH4) by the anaerobic microorganisms.
In most cases, biomass is made up of large organic polymers. For the bacteria in anaerobic digesters to access the energy potential of the material, these chains must first be broken down into their smaller constituent parts. These constituent parts, or monomers, such as sugars, are readily available to other bacteria. The process of breaking these chains and dissolving the smaller molecules into solution is called hydrolysis. Therefore, hydrolysis of these high-molecular-weight polymeric components is the necessary first step in anaerobic digestion. Through hydrolysis, the complex organic molecules are broken down into simple sugars, amino acids, and fatty acids. Acetate and hydrogen produced in the first stages can be used directly by methanogens. Other molecules, such as volatile fatty acids (VFAs) with a chain length greater than that of acetate must first be converted into compounds that can be directly used by methanogens
The biological process of acidogenesis results in further breakdown of the remaining components by acidogenic (fermentative) bacteria. Here, VFAs are created, along with ammonia, carbon dioxide, and hydrogen sulphide as well as other byproducts. The process of acidogenesis is similar to the way milk sours.
The third stage of anaerobic digestion is acetogenesis. Here, simple molecules created through the acidogenesis phase are further digested by acetogens to produce largely acetic acid, as well as carbon dioxide and hydrogen.
The terminal stage of anaerobic digestion is the biological process of methanogenesis. Here, methanogens use the intermediate products of the preceding stages and convert them into methane, carbon dioxide, and water. These components make up the majority of the biogas emitted from the system. Methanogenesis is sensitive to both high and low pHs and occurs between pH 6.5 and pH 8. The remaining, indigestible material the microbes cannot use and any dead bacterial remains constitute the digestate. Fig 3 below gives a highlight of the processes leading to generation of methane.
Fig. 3: Flow chart of processes leading to methane generation
Other project partners:
Chikodi Ehumadu (Ph.D. Candidate, Agricultural and Environmental Engineering Department, University of Ibadan).
Vwamdem Kwoopna (M.Sc. in Electrical and Electronic Engineering, University of Ibadan).
Biola Oyebamiji (M.Sc. in Computer Science, University of Ibadan).
Publication linked to the project:
F.G. Oyeniyi and C.N. Ehumadu (2017) Making Renewable energy Initiatives work in Nigeria:
Lessons from success stories around the world. Abstract submission for the 8th Annual Ibadan Sustainable Development Summit, Centre For Sustainable Development, 22-25 August, University of Ibadan, Nigeria, pg 56.