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The effects of different mixing intensities during anaerobic digestion of the organic fraction of municipal solid waste
Mälardalen University, School of Business, Society and Engineering, Future Energy Center.ORCID iD: 0000-0002-8268-1967
Mälardalen University, School of Business, Society and Engineering.
Mälardalen University, School of Business, Society and Engineering, Future Energy Center.ORCID iD: 0000-0002-3485-5440
2014 (English)In: Waste Management, ISSN 0956-053X, E-ISSN 1879-2456, Vol. 34, no 8, p. 1391-1397Article in journal (Refereed) Published
Abstract [en]

Mixing inside an anaerobic digester is often continuous and is not actively controlled. The selected mixing regime can however affect both gas production and the energy efficiency of the biogas plant. This study aims to evaluate these effects and compare three different mixing regimes, 150 RPM and 25 RPM continuous mixing and minimally intermittent mixing for both digestion of fresh substrate and post-digestion of the organic fraction of municipal solid waste. The results show that a lower mixing intensity leads to a higher biogas production rate and higher total biogas production in both cases. 25 RPM continuous mixing and minimally intermittent mixing resulted in similar biogas production after process stabilization, while 150 RPM continuous mixing resulted in lower production throughout the experiment. The lower gas production at 150 RPM could not be explained by the inhibition of volatile fatty acids. Cumulative biogas production until day 31 was 295. ±. 2.9, 317. ±. 1.9 and 304. ±. 2.8. N. ml/g VS added during digestion of fresh feed and 113. ±. 1.3, 134. ±. 1.1 and 130. ±. 2.3. N. ml/g VS added during post digestion for the 150 RPM, 25 RPM and minimally mixed intensities respectively. As well as increasing gas production, optimal mixing can improve the energy efficiency of the anaerobic digestion process.

Place, publisher, year, edition, pages
2014. Vol. 34, no 8, p. 1391-1397
Keywords [en]
Anaerobic digestion, Continuous mixing, Continuously stirred tank reactor, Intermittent stirring, Mixing intensity, Municipal solid waste
National Category
Engineering and Technology Other Engineering and Technologies
Identifiers
URN: urn:nbn:se:mdh:diva-25313DOI: 10.1016/j.wasman.2014.04.006ISI: 000338601000006Scopus ID: 2-s2.0-84901950694OAI: oai:DiVA.org:mdh-25313DiVA, id: diva2:727154
Available from: 2014-06-19 Created: 2014-06-19 Last updated: 2018-01-03Bibliographically approved
In thesis
1. Developing the anaerobic digestion process through technology integration
Open this publication in new window or tab >>Developing the anaerobic digestion process through technology integration
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Process optimization is needed for the development and expansion of the biogas industry and to meet the ever growing demand for methane. This thesis explores process technologies for the development of the anaerobic digestion process and includes pre-treatments, studies on the effects of different mixing modes and evaluation of a water treatment technology.

Two pre-treatments were evaluated, mechanical and electroporation, for treatment of ley crop silage. Mechanical treatment included two milling machines designed for recycling of paper, Grubben deflaker and Krima disperser, and showed an increased biogas production of 59 % and 43 % respectively as well as a positive energy balance and economic results.. Electroporation increased the biogas production with 16 %, however, development is needed to increase its energy efficiency.

Digester mixing has an effect on the digestion result. The performed review and experiments show that the mixing demand increases with organic loading. Excessive mixing during process start up, instabilities and shock loads leads to increased volatile fatty acid concentrations and process inhibition. Reduction of mixing reduces the effects of process instabilities and periodical mixing with mixing breaks has been shown to be beneficial for biogas production.

A high temperature membrane filtration unit was evaluated at 70 °C, 90 °C and 110 °C to determine separation efficiencies, permeation speed when treating process water at a biogas plant.  Improved separation can increase the capacity of the substrate pre-processing and reduce process related problems. The results show a total solids separation of 60 %, and an increasing filtration speed with temperature with fluxes of between 113 and 464 L/ h m2. The substrate pre-processing could theoretically handle up to 29 % more substrate as a result.

Integration of these technologies in a biogas plant show that the pre-treatments studied exhibits a good performance when integrated and that mixing reduction has the potential to lower the process electricity demand by 23 % in the performed case study. However, even though the membrane filtration unit shows promising results it would demand a relatively high energy consumption and lead to limited benefits to a process already at it maximum organic loading.

Place, publisher, year, edition, pages
Västerås: Mälardalen University, 2014
Series
Mälardalen University Press Dissertations, ISSN 1651-4238 ; 165
National Category
Bioenergy
Research subject
Energy- and Environmental Engineering
Identifiers
urn:nbn:se:mdh:diva-26081 (URN)978-91-7485-166-3 (ISBN)
Public defence
2014-11-19, rum R2-025, Mälardalens högskola, Västerås, 09:00 (English)
Opponent
Supervisors
Available from: 2014-10-10 Created: 2014-10-09 Last updated: 2014-11-03Bibliographically approved

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Lindmark, JohanThorin, Eva

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