Efficiency and efficacy of pre-treatment and bioreaction for bio-H2 energy production from organic waste
Highlights
► Two energy parameters as a suitable tool to score bio-processes are defined and applied. ► The efficiency (ξ) considers the quantity of hydrogen energy extracted by bioreaction. ► The efficacy (η) compares the efficiency with that of glucose under same conditions. ► Original experimental tests on Oranic Refuses have been analized with the two parameter. ► The efficacy of energy production is 45%, equivalent to 7.3 L H2/kg of undiluted Organic refuses.
Introduction
Fermentable Organic Waste is a major component of the solid waste that is produced daily by households, offices, industry, and the agricultural sectors as well as by food production. It can reach more than 50% of the total Municipal Solid Waste (MSW), as reported in [1]. Grass and tree cuttings, vegetable peels, used paper materials, straw and residues produced during the harvesting of agricultural products and the residues of food production, manufacturing and consumption are typical Fermentable Organic Waste examples. In most cases, these waste materials are burnt or dumped, with consequent environmental pollution effects, due to their high probability of forming organ-chlorine toxic compounds [2]. In the case of differential household collection, constituted by cooking food waste, the composting process, especially in the case of densely populated cities, produces a product, which is used in agriculture, with some difficulty, by farmers [3]. It would therefore be of great environmental interest to find different ways of disposing of such kinds of refuse. One possibility could be to use it as feedstock for the production of useful chemicals, thus relieving the strain on natural resources. Bearing in mind that such refuse is constituted by organic materials, fermentation processes seem to be adequate for such purposes [4]. Of among organic materials, sugars are able to ferment easily into fuels, such as ethanol, methane or hydrogen, but most organic materials are polymers of five or six-carbon sugar units, such as cellulose, hemicelluloses and lignin, which have different degrees of oxidation (Table 1) and are refractory to biological attack. Carbohydrates can be converted into fuels: ethanol, hydrogen or methane, via biotechnology processes [8]. In order to convert large quantities of organic waste materials into fuels, it is necessary to break down the carbohydrate polymers into low molecular weight, basically monomer sugars, before the microorganisms metabolize them into biofuels through a so-called bio-recycling process.
Forestry and agriculture residues and the organic fraction of MSW are by nature heterogeneous in size, composition, structure, and properties. The sugars, starches, lipids and proteins that are present in them are easily degradable by microorganisms, while some other fractions, such as lignocelluloses and keratin are more difficult to degrade [9]. Recently, several studies throughout the world have been investigated towards the possibility of producing hydrogen through dark fermentation utilizing the organic fraction of MSW [10]. Hydrogen production offers an additional opportunity of converting refuse into usable fuel with high conversion efficiency in fuel cell devices; in the case of biohydrogen production followed by a methanation process, very high energy efficiency of the whole process can be reached [11]. Physical, chemical or biological pre-treatment processes are used in ethanol production from lignocellulosic materials [12] that have been well-investigated. Furthermore, some efforts have been made to treat waste materials in order to increase biogas production [13]. Pre-treatment can enhance the bio-digestibility of waste constituents and increase the accessibility of the enzymes to the materials, thus improving the ethanol or biogas yield from the wastes [14]. The aim of the pre-treatment is to change the properties of the material in order to prepare it for enzymatic degradation, which converts the carbohydrate polymers into fermentable sugars. The goal is to break the lignin seal and disrupt the crystalline structure of cellulose [15]. The best pre-treatment method and conditions depend to a great extent on the type of lignocellulosic material. The crystallinity of cellulose, its accessible surface area and the protection against biological attack by lignin and hemicelluloses are due to the degree of cellulose polymerization and to the degree of acetylation of the hemicelluloses; these parameters are recognized to be the main factors that are able to affect the biological degradation rate of lignocelluloses [16].
Dark anaerobic fermentation has recently received a great deal of interest by researchers around the world as an environmental friendly and feasible process [17]. The number of scientific publications on this topic follows a quasi-exponential curve over the year and the technology of biohydrogen production in anaerobic conditions using organic refuse [18] represents the most promising hydrogen production biotechnology process, which will reach full-scale application thanks to its low cost compared to others (photofermentation and bioelectrogenesis) [19] and its high production rate [20]. On the basis of the previous considerations, the present paper has the aim of candidating two energy conversion parameters to quantify and compare the pre-treatment and bioreaction against glucose as the most biodegradable substrate. The parameters have been applied to our experimental results in order to establish the efficacy of a pre-treatment strategy consisting of a mechanical homogenisation step followed by either a base or an acid pre-treatment to enhance hydrogen production from Organic Waste Market (OWM) refuse collected at local vegetable and fruit markets. The most promising pre-treatment was selected through flask tests and it was tested with a 2 L bioreactor to evaluate the most adequate pre-treatment strategy compared to glucose. Glucose was selected as the reference substrate because of its easy biodegradability to compare other ligno-cellulose materials.
Section snippets
Preparation and pretreatment of feedstock
The organic waste market OWM refuse tested as a substrate for H2 production was taken from local fruit and vegetable markets in a large city in a developed country. First, it was weighed and cut into small pieces; a kitchen blade mixer was then used to liquidize the material in order to simulate an industrial milling treatment. Fig. 1 shows the merceological composition of the OWM utilized for the tests, while the composition of the refuse at its original strength is reported Table 2. Table 3
Energy conversion parameter
In order to evaluate the efficacy of the pretreatment on the OWM refuse, the produced hydrogen was compared with that produced with glucose. Glucose was selected as reference because it is the most suitable substrate for hydrogen production, due to the total absence of any ligno-cellulose structural components. Two energy parameters were used to compare the hydrogen production: energy conversion efficiency ξ, in order to take into account the quantity of energy that the pre-treatment, plus the
Pre-treatment selection
The time course of the biogas produced in the fermentation of the three parts of OWM refuse tested using the Erlenmeyer flasks are reported in Fig. 2, while the quantitative production of biogas and the mean hydrogen content of each test are reported Table 6. Fig. 2 shows that biogas was produced in all of the tests: the non-chemical treatment, acid and alkaline pre-treatments. It should be pointed out that a sufficient quantity of biogas was also produced in the non-chemical pre-treatment test
Comments and discussion
Comparing the test with the mechanically treated OWM refuse, exposed to an alkaline environment, with that conducted with the same Hydrogen Forming Bacteria (HFB) consortium at the same conditions, with glucose as the carbon source, the following comments can be made. Despite the presence of the more complex substrates, mainly composed of carbohydrates in a complex structure, fats and fibres, compared to a more simple carbon source, i.e. glucose, the mixed HFB microorganisms show a good ability
Conclusion
In the present paper two parameters that are able to evaluate the energy production as H2 using residual organic waste have been introduced and applied to biohydrogen from fruit and vegetables market residues considering as reference base glucose as a lignin cellulose free substrate. The results show that OWM is a suitable substrate for H2 production and that there is no need to add any micronutrients, proteins or other additives to support Hydrogen Forming Bacteria (HFB) activity. An alkaline
Acknowledgements
The Authors wish to acknowledge the Regione Piemonte for its financial support under the projects C16, 2004.
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