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Biogas production from microsieved biosolids in immobilized biomass anaerobic digesters – The effect of immobilization carriers

Bosiolis Konstantinos

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Summary

Biogas production with anaerobic digestion process has attracted the interest of the scientific community, as population growth, has increased the need for renewable energy production, while protecting the environment by stabilizing wastes and reducing gaseous emissions. The purpose of the present thesis, which was prepared in the Environmental Processes Design Laboratory, part of Environmental Engineering School of the Technical University of Crete, is to examine the effect of immobilization carriers on biogas production from microsieved biosolids in mesophilic batch anaerobic digesters. The carriers enhance biogas production, facilitating the growth of microorganisms on the porous surface. Immobilized microorganisms (biomass) create a biofilm on the carriers, avoiding toxic concentration of substances that would inhibit their reproduction.Microsieved biosolids, which was produced from a microscreen machine at the municipal wastewater treatment plant of Rethymno city, was suitable as a substrate with C/N ratio of 20,56 and their content of total and volatile solids (on total solids) is 37,5±1,6% and 80,1±2% respectively and were mixed with an inoculum, which was a biological sludge from a mesophilic digester of sewage treatment plant. Three anaerobic digestions of 16 days were done and substrate-inoculum ratio was 1, content of total solids for the 1st, 2nd and 3rd anaerobic digestion was 4,40%, 4,88%, 4,30% respectively, content of volatile solids was 72,95%, 64,41%, 72,75% respectively, as a result were added 3,47 gVS, 3,78 gVS and 3,47 gVS respectively.As immobilization carriers used plastic, polyethylene foam, cypress fruit and pumice stone. In the digesters with carriers was observed more cumulative biogas production than the digesters without carriers. Pumice stone had the maximum value of 466,89±0,7 ml/gVSadded. Furthermore, using carriers was observed that the maximum daily biogas production was earlier than the digester without carriers. The average maximum daily production was 57,67±0,7 ml/gVSadded*d, that produced from the pumice stone. The average percentage immobilization of the biomass for plastics was 6.69 ± 0.25%, for the foamed polyethylene 8.53 ± 1.17%, for the cypress fruit 8.96 ± 0.28% and for the pumice stone 9.13 ± 0.24%. In general, there were only few differences between the carriers. The cypress fruit stood out at the beginning of the experiments, because on its surface had nutritional substances that enhanced anaerobic digestion. However, once all carriers had immobilized biomass, they had positive effect on biogas production. Pumice stone and polyethylene foam stood out and the reason was that they had more roughness surface and they entrapped biomass into their porous more than the other materials, as a result a viscous biofilm was created that did not easily break.

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