Influence of Inoculum in the Organic Fraction of Municipal Solid Waste Biodegradation

Nielfa, A.1, Fdz-Polanco, M.1, Cano, R.1, Vinot, M.2, Fernandez, E.2, 1University of Valladolid, Spain, 2URBASER


Abstract The handling and disposal of Municipal Solid Waste (MSW) is a growing concern as the volume of waste continues to increase. Anaerobic digestion (AD) process offers some advantages as it produces biogas and a digestate that could be reuse. Biochemical methane potential (BMP) tests are a high-quality method to evaluate the biogas production of wastes as it allows a variation of their conditions obtaining reliable results. Several parameters as temperature, inoculum or substrate-inoculum (S/X) ratio could have influence in the behaviour of the AD process, as result, these and other parameters have been tested in this study to evaluate the biogas generation for the organic fraction of municipal solid waste (OFMSW). Establishing a standard volume (2L), and rotational stirring, it was studied different configurations of inoculum and substrate/inoculum ratio. Different inoculums coming from 5 diverse reactors and different ratio substrate/inoculum (1/1 and 2/1) have been considered for MSW biodegradation. Results of biological methane potential test present significant differences for inoculum coming from grease and OFMSW reactors showing faster kinetics curves. The study of the acclimatization of inoculums has shown better adaptabilities for certain ones which could improve the start-up of continuous processes.

Keywords: Anaerobic digestion, Biochemical methane potential tests, Inoculum, Organic fraction of municipal solid waste, Ratio substrate-inoculum

Introduction The disposal and treatment of municipal solid waste (MSW) constitutes one of the most serious problems of contemporary societies. The volume of waste has increased very quickly (approximately 24 millions of tons of MSW has been generated in Spain last years with 50-55 % of organic fraction (OFMSW)(INE, 2010).

Anaerobic digestion (AD) is a process characterized by a series of biochemical transformation carried out by different consortia of microorganisms (inoculum) that convert complex organic macromolecules into low molecular compounds such as methane, carbon dioxide, water and ammonia (Fantozzi et al. 2011). The anaerobic process appears to be a promising alternative to composting, landfilling and incineration due to energy recovery through methane collection and reduced CO2 emission (Zhou et al. 2011). AD of MSW especially of the organic fraction is of great importance in management of solid waste because in most of the developing countries the fermentable organic waste represents the major fraction (50%). Considering the application of AD for treatment of this fraction would decrease considerably the volume destined for landfilling (Lopes et al. 2004). High solids AD also known as dry methane fermentation requires less water than wet AD, resulting in low treatment costs, less initial investment, compact digesters with high loading rate and energetically effective performance (Bolzonella et al. 2003).

In recent years, interest in biochemical methane potential (BMP) tests has increased as reflected by the wide range of research papers dealing with the BMP assays (Raposo et al. 2006). The BMP tests can be used to estimate optimum ratios substrate/inoculum (S/X) and to determine the perfect conditions to operate with a specific substrate.

Generally the inoculum used for the BMP tests is sludge originating from sewage treatments stations or some materials from animal origin (Lopes et al. 2004). Inoculum source is an important operational parameter and with the total solid percentages are responsible to accomplish rapid onset of a balanced microbial population and determines the initial activity of the microorganisms used for the test (Forster-Carneiro et al. 2008). AD is a complex process regulated by a bacteria consortium (inoculum) which is sensitive to the type of substrate and its composition. This way, specific substrate is utilized by specific types of bacteria (Zhou et al. 2011).

Temperature is another parameter that has influence in the behavior of the AD bacteria (Wan et al. 2011). Some studies have shown the efficiency of thermophilic processes over mesophilic, obtaining better results at 55 ºC taking into account the energy consumption (Cecchi et al. 1991; Ahring 1994; Griffin et al. 1998)

The substrate/inoculum (S/X) ratio is also an important parameter in BMP assays and in the assessment of anaerobic biodegradability of solid wastes (Neves et al. 2004). Although theoretically the S/X ratio has an effect only in the kinetics and not on the ultimate methane yield which depends on the organic matter content (Raposo et al. 2006), it is reported that too high S/X ratio may be toxic while too low S/X ratio may prevent induction of the enzyme necessary for biodegradation (Prashanth et al. 2006). Each substrate has its optimum S/X ratio, considering the potential amount of volatile fatty acids produced and its capacity to buffer the medium due to the ammonium produced by the hydrolysis of proteins (Lesteur et al. 2010). For these reasons S/X ratio should be recognized as one of the major parameters affecting the results of anaerobic assays.

Previous work on the effect of S/X ratio in the BMP indicates different standard ratios. The ratio proposed by Owen (1979) was approximately 1, while Chynoweth (1993) suggested a ratio of 2. Neves (2004) , used a range of ratios (2, 1, 0.74, and 0.43) when assaying the BMP of kitchen wastes. The growing interest of BMP tests for AD processes created a need for an established procedure to determine which the optimum operational conditions for a specific substrate are. The aim of this work is to evaluate the production of the Organic fraction of Municipal Solid Waste (OFMSW) and determine the best BMP conditions for this substrate.


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