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Valorization of solid brewery residues for methane and biochar production

Pitsios Michail

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Summary

A major problem of modern societies is the management of solid waste arising from agro-industrial activities. The present study aims to examine and compare the exploitation of brewer’s spent grains as a substrate for the anaerobic digestion process for methane production, as well as for the production of a biochar to be used as an adsorbent material.The brewer’s spent grains (BSG) were initially characterized regarding their elemental composition and their insoluble fibers (NDF, ADF and ADL). In addition, their moisture content, total and volatile solids were determined. Biochemical Methane Potential (BMP) tests were carried out to assess the biodegradability of the organic substrate (brewer’s spent grains) during anaerobic digestion and hence its ability for methane production. BMP assays were carried out with a substrate to inoculum ratio (SIR) of 0.25, based on volatile solids, while the inoculum used was biological sludge from a mesophilic anaerobic digester of a municipal waste water treatment plant.On the other hand, biochar production from brewers spent grains was carried out through pyrolysis in an oxygen-limited environment at a temperature of 600 °C. The produced biochar was then characterized as to its elemental composition, calorific value and point of zero charge. Thereafter, adsorption experiments were conducted in batch reactors, through which the effect of four parameters on the adsorption efficiency of two different dyes, i.e. methyl orange and methylene blue, was studied. In particular, the parameters studied were the effect of adsorbent dose, contact time, pH of the solution and initial concentration of the dye. In addition, the kinetics of dye adsorption on biochar was modelled using the pseudo-first and pseudo-second order, Intraparticle diffusion, Fractional power, Avrami and Elovich models. Finally, the adsorption equilibrium was described using Freundlich, Langmuir, Redlich-Peterson and Koble-Corrigan isotherms.Results from anaerobic digestion experiments, indicated that the use of brewer’s spent grains as a substrate in the anaerobic digestion process is feasible although the produced methane amounts were lower than expected. On the other hand, results from the adsorption experiments showed that the adsorption of dyes on biochar is affected by all the tested parameters, either positively or negatively. The kinetic model that best suited the experimental data is the Avrami model, while the adsorption equilibrium of dyes to biochar is best described by Koble-Corrigan and Redlich-Peterson isotherms, for methyl orange and methylene blue, respectively.

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