A. Introduction

Biogasification can biologically recover energy in the form of methane, a combustible efficient gas. The main functions of biogasification are treating waste and producing energy. But it can not stand in the same position with compositing when it comes to technical practicality and economic feasibility. It needs more money to enhance the technology and needs more rigorous equipment than compositing. On the other hand, biogasification is better at handling degradable wastes and also when used in conjunction with sanitary landfilling. There are very few biogasification schemes that have survived and those are adhered to realism and to principles of biology and good engineering. Designers and system suppliers are finding ways to combine waste pre-processing, biogasification, and composting technologies so that  the amount of organic content and waste requiring land disposal will decrease simultaneously.

B. Principles

Biogasification is defined as being the biological decomposition of organic matter of biological origin under anaerobic conditions with an accompanying production primarily of methane (CH4) and secondarily of other gases, main of which is carbon dioxide (CO2).

C. Process description

There are two opinions of the number of stages in biogasification which are first, the two stages, comprises acid stage followed by a methane stage, and the three stage, which one is a ?polymer breakdown? stage and precedes the other two which are also acid and methane stage. The end products of a complete final stage are methane, carbon dioxide, trace gases, and a satisfactorily stable residue. Up until today, scientists have thoroughly investigated the bacteriology of methane production especially on the isolation, identification, and population size of the methane producers. In the polymer stage, the bacteria in the population must have enzymatic systems that can hydrolyse complex molecules, mainly carbohydrates and others such as lipids and proteins. Next, the acid stage converts polymer stage breakdown products into organic acids (straight-chain fatty acids) that can be utilised by methane-formers. In the methane stage, decomposition products from the acid stage are converted into CO2, CH4, and an assortment of trace gases.

D. Process rate limitation factors

The process rate limitation factors are non-required factors because it slows down the process and limits potential energy recovery. The main factors are environmental factors, performance factors, and factors in the form of elements or compounds. One of the environmental factors is temperature because some temperatures are not suitable for the microbes to work well. There are two types of temperature ranges of microbe cultures, mesophilic and thermophilic. Thermophilic which ranges between 45?C to 75?C is better than mesophilic in 10?C to 45?C because the development of a replacement culture can be accomplished in a much shorter time and more cost beneficial, which makes it the best answer. Next, the sustainability of  substrates or feedstocks depends on these three, biodegradability, chemical composition, and physical properties. Biodegradability depends on mostly on the physical properties and chemical composition of a waste. On the other hand, the chemical composition is based on the possession of nutrient elements and molecular structure of the compounds. Last, reducing physical properties including particle size, and in some cases moisture content is to improve the wastes utility as a feedstock. Performance factors such as The rate of transfer of dissolved metabolic and other products from the liquid to the gaseous phase can be overcome by agitating the culture.

E. Parameters

There are several parameters in terms of those pertinent to the cultural environment that affects digester performance and those used for judging digester performance.  The most commonly used to judge cultural performance and guide digester operation is the gas production which is a direct measure of overall microbial activity. Next, the destruction of volatile matter is a parameter because it measures of rate and extent of microbial conversion of organic solids into gas and stable or inert matter. Only when it is in a state of flux, which indicates an imbalance between proliferation and activities of acid formers and those of methanogens that become a limiting inhibitory, volatile acid concentration becomes a key parameter to the problem. Next, hydrogen ion concentration is a manifestation of volatile acid formation and could be regarded as an operational parameter, but, the pH level also depends upon the buffering capacity of the culture. The buffering capacity of the culture medium within the neutral pH range is measured by alkalinity, which is the capacity of the medium to accept protons. When parameter values indicate the approach or actual existence of an inhibitory situation and, thereby, a likely deterioration in digester performance, appropriate remedial measures must be taken. The causes should determine the remedial measures.

F. Operational procedures

Operational procedures include mixing, loading, the detention time, and starting the digester. Mixing enhances digestion efficiency regardless of the type of digestion system and is a critical feature in the digestion of some types of substrates, e.g., fibrous materials. The scum layer is a froth consisting of bubbles formed by the rising of gases released in the supernatant layer and is the uppermost layer, therefore tends to increase in thickness and interfere with the operation of a digester. An appropriate and proper mixing program can control the scum formation. Moving on, the rate and amount of loading determine the extent of energy recovery from wastes and the efficiency at which digester capacity is utilised. Next, in practice, the solid and liquid phases of the digester contents may have a common detention time. The detention time is equal to the culture volume per the throughput per unit of time. The establishment of culture and environmental conditions conducive to the proliferation of both indigenous and introduced methanogens is the loose definition of ?starting a digester?. It is the establishment of an enrichment culture for the organisms, especially methanogens because usually the necessary populations of hydrolyzers and acid-formers are developed without difficulty.

G. Digester construction design principles

Conventional digestion system is based on the type of system and digester culture involved. The volume of the digester is determined by the number of wastes to be processed each day, the moisture content of the waste, the volatile solids concentration, the loading rate, solids content of the slurry, and detention time. A high-rate system is best used in large-scale operations in urbanised situations. The contact approach digestion system or ?fixed-bed? system which provides a surface on which the microorganisms can become attached and form a film that consists mostly of active microorganisms which will give a detention period for the microorganisms. Heating the digester is important as the temperature of the digester culture should be maintained at a level sufficiently high to ensure maximum microbiological activity, especially in cold and temperate climates. There are also small-scale digester designs that cannot be applied in large-scale systems such as one in Gobar, India and in China.

H. End products of the biogasification process

The end product of the biogasification process is raw (untreated) biogas comprises two principal components, methane (CH4) and carbon dioxide (CO2), and other lesser components, H2S, N2, and H2O.The raw gas can be burned and the resulting heat can be used in any one of several uses. Biogas purification increases the potential use of it as it increases the quality of biogas.

I. Residues

There are also residues which are solid and liquid materials. Supernatant, the liquid residue, is an aqueous suspension in which the suspending medium contains an assortment of dissolved solids and a variety of suspended colloidal solids and bacterial cells that must be properly treated before being discharged into the environment. Sludge, the solid residue, is the layer of settled solids that can be used interchangeably with composted sludge in agriculture unless the sludge feedstock contains human excrement and toxic metals and toxic synthetic organic chemical compounds.

J. Feasibility considerations

Availability of the required technology and the extent of the country's economic resources are among the factors that determine the practical and economic feasibility of biogas production as a waste management option and as an energy resource. Currently, the trend of large-scale operations is towards high-solids digestion because low-solids digestion has not been successful, largely because of operational problems and deficiencies in digester design and construction. Maintenance and functioning of the digester are usually the problems in small-scale operations and can be overcome through integration into a

community installation that would be accompanied by the establishment of an organisational program designed to provide follow-up service, ensure frequent contacts with relevant agencies for technical advice, and establish a mechanism for access to reliable and regular supply of raw materials and plant components, and provide for personal contacts by biogas technicians.