Wherever there is human habituation,organic waste is produced, which consist of mainly household food waste,agricultural waste, human and animal waste. For each year the amount ofproduced organic waste is increasing dramatically, source-separation,composting and anaerobic digestion with related biogas production isincreasingly being considered as a substitute for waste management strategiesas land-filling and incineration of municipal solid waste (MSW). Breakdown oforganic materials in the absence of oxygen produces methane. The process isknown as anaerobic digestion and performed through the biological activity ofmicroorganisms. This phenomenon naturally occurs at the bottom of ponds andmarshes, which results in production of methane.
The production of naturalbiogas is the main part of biochemical carbon cycle. Temperature and thecomposition of the feedstock are two important factors that should be takeninto consideration due to the sensitivity of the process. The involved bacteriaare active within limited range of temperature, especially methanogens that arethe methane-producing bacteria.In hydrolysis, which is the first step ofbiogas production, the extracellular enzymes, such as celluloses, amylases,proteases and lipases released by the bacteria, hydrolyse the organic material.
The hydrolysis stage is also known as the polymer breakdown stage. The complexchain of carbohydrates, proteins and lipids are decomposed into shorter parts.Polysaccharides are at this stage transformed into monosaccharides and proteinsare converted into peptides 3 and amino acids. Large amount oforganic acids are also produced by acid forming bacteria in this initial periodof fermentation.Acidification is the second step in theprocess where acid-producing bacteria transforms the monomers and fermentationproducts produced in stage I, into acetic acid (CH3COOH), hydrogen (H2) andcarbon dioxide (CO2). Since these bacteria are facultative anaerobic they areable to grow in acidic environment.
For production of acetic acid, thesebacteria need oxygen and carbon. The solved oxygen in the solution or thebounded oxygen is used for this propose. An anaerobic environment is herebyobtained by acid producing bacteria, which is vital for the methane-producingmicroorganisms. Furthermore, the acid producing bacteria also reduce thecompounds with a low molecular weight into organic acids, alcohols, aminoacids, carbon dioxide, traces of methane and hydrogen sulphide. The main acidsproduced in this stage are acetic acid, propionic acid, and butyric acid;furthermore ethanol is also produced.The methane producing bacteria decomposefurther the compounds with a low molecular weight. For example, in order toform methane and carbon dioxide, the methane procucing bacteria utilisehydrogen, carbon dioxide and acetic acid.
The methane producing bacteria existunder natural conditions under water, in ruminant stomaches and in marshes,where anaerobic conditions are present. Theese microorganisms are verysensitive to environmental variations since they are obligatory anaerobic. Themethanogenic bacteria are included in the archeabacter genus in contrast toacidogenic and acetogenic bacteria. There are three types ofmethanogenic bacteria involved in the metane producing process; a.
Methanosarcina genus (spherically shaped) b. Methanothrix bacteria (long and tubular)c. Bacteria thatcatabolize furfural and sulfates (short and curved rods)Somefraction of this waste is dried out and pelletized to be used as low valuecattle feed. However, dehydration process is not economical since the peelscontain a high moister 14 content. Consequently a big portion of this unwantedwaste is discarded into the environment, which causes both economical andenvironmental problems.
Additionally, the accumulation of this industrialorange waste cause also problems for fruit processing industries, such as odourand soil pollutions. The peel holds a variety of carbohydrate polymers, whichmakes it an attractive choice for utilization in biological processes.Therefore, the orange waste can represent a probable energy source if it can beaccurately treated in order to transform it biologically into methane. Thismakes the waste a source of renewable energy, and the output of carbon dioxideto the atmosphere is zero. In recent times, anaerobic digestion ofligno-cellulosic wastes as a treatment of this renewable energy has emerged.Additionally, anaerobic digestion seems to be one of the best sustainabletechnologies existing for treatment of fruit and vegetable wastes and has beenacknowledged to be the most suitable option to incineration and composting.
Thehigh organic matter content in the orange waste makes it idyllic for anaerobicdigestion. However, previous investigations reported that orange waste mightaffect the thermophilic digestion process negatively, due to the presence ofpeel oil, limonene, which has antimicrobial effects.