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Membrane filtration of process water at elevated temperatures: a way to increase the capacity of a biogas plant
Mälardalen University, School of Sustainable Development of Society and Technology. (MERO)ORCID iD: 0000-0002-8268-1967
Mälardalen University, School of Sustainable Development of Society and Technology. (MERO)ORCID iD: 0000-0002-3485-5440
Mercatus Engn AB.
Svensk Växtkraft AB.
2011 (English)In: Desalination, ISSN 0011-9164, E-ISSN 1873-4464, Vol. 267, no 2-3, p. 160-169Article in journal (Refereed) Published
Abstract [en]

 Waste water from a biogas process is often recirculated and mixed with the incoming organic material to produce a feed for the digester. The dry matter (DM) content of the final mixture should be as high as possible to maximise the capacity of the plant without exceeding the capability of the pumps. This means that the DM content of the recirculated process water has a large impact on the amount of substrate that can be processed. Experiments to reduce the dry matter content of the recirculated process water were carried out using a ceramic ultrafiltration (UF) membrane. The influence on the flux through the membrane and the separation efficiency at different operation temperatures, 70°C, 90°C and 110°C, were investigated. Higher temperatures resulted in increased flux/flow through the membrane. The DM content was reduced from 4% to 1.6%, corresponding to a 29 % increase of new material that could be added to the process. The energy required to heat the membrane when using heat recovery is small compared to the energy of the methane produced from the additional added substrate. The lifespan of the membranes and uncertainties in the substrate DM content are showed to be important for the economic result.

Place, publisher, year, edition, pages
2011. Vol. 267, no 2-3, p. 160-169
Keywords [en]
ultrafiltration, ceramic membrane, biogas, anaerobic digestion, digester, high temperature
National Category
Natural Sciences
Research subject
Energy- and Environmental Engineering
Identifiers
URN: urn:nbn:se:mdh:diva-10635DOI: 10.1016/j.desal.2010.09.020ISI: 000286851900005Scopus ID: 2-s2.0-78650294233OAI: oai:DiVA.org:mdh-10635DiVA, id: diva2:359468
Projects
BiogasoptAvailable from: 2010-10-28 Created: 2010-10-28 Last updated: 2017-12-12Bibliographically approved
In thesis
1. The wet fermentation biogas process: Limitations and possibilities for efficiency improvements
Open this publication in new window or tab >>The wet fermentation biogas process: Limitations and possibilities for efficiency improvements
2010 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The biogas process is known as an environmental friendly and sustainable way of producing energy and fuel but to be fully commercially competitive with other types of processes, efficiency improvements are needed. By doing a case study at the Växtkraft biogas plant in Västerås, Sweden, three specific limitations were identified and studied. Firstly, to improve the capacity of the plant, pre-treatments of the different substrates are needed to disintegrate the substrate and by doing so increasing the gas yield and the speed in which it is produced. Secondly, to improve the fermentation process itself more knowledge is needed around the mixing inside the digester. To be able to create an optimal and stable environment for the microorganisms the mixing is the key, because the mixing affects the mass transfer of all solids, nutrients, gases and other substances in the digester. Thirdly, the water treatment of the recirculated process water cannot reach the desired separation of dry matter (DM) and this is affecting the capacity of the plant negatively. The feed for the digester is produced by mixing the process water and the substrate to get a pumpable slurry with a DM content of 8-10 %. When there is too much DM in the process water to begin with, the mixing ratio between the substrate and the liquid changes, decreasing the amount of substrate that can be added to the mixture and later on fed to the digester.

 

The full biogas potential of most organic materials cannot be extracted during the relatively short retention time of most digesters because of their complex structures. The organic materials are broken down too slowly and the nutrients cannot become biologically available in that time span. This means that a lot of the bound energy in the organic material leaves the biogas plant with the liquid digestate. The efficiency of the process can be improved by pre-treating the material before digestion. Pre-treatment experiments to disintegrate ley crop silage using electroporation, a treatment using electrical fields, were conducted to study its effect on the biogas yield. The experiments resulted in up to twice the amount of biogas being produced from the pre-treated material compared to untreated material.

 

Numerical simulations of the mixing inside a digester were carried out to understand the effect that a gas lift mixing configuration has on the mass transfer in the system. The mixing dynamics were evaluated by testing five different flow rates of the injected gas and the effect that the liquid recirculation system has. The results indicate that there are large unmixed zones and that changing the gas flow rate only has a marginal effect on these areas. The simulation also suggests that the outlet of the liquid recirculation system is situated too close to the gas injectors, resulting in energy losses in form of diminished mixing of the digester.

 

Experiments to reduce the DM content of the recirculated process water were carried out using a ceramic ultrafiltration membrane. The flux through the membrane and the separation efficiency were investigated at different operation temperatures, 70°C, 90°C and 110°C. The results show that 59-63 % of the DM was separated in this temperature interval and that the flux/flow through the membrane increased with the temperature. These results correspond to a 29 % increase in the capacity to add new substrate. The energy required to heat the membrane, if heat recovery is used, is small in comparison to the increased methane yield.

 

In the best case scenario these above identified improvements could increase the methane yield by up to 40%.

Place, publisher, year, edition, pages
Västerås: Mälardalen University, 2010
Series
Mälardalen University Press Licentiate Theses, ISSN 1651-9256 ; 125
Research subject
Energy- and Environmental Engineering
Identifiers
urn:nbn:se:mdh:diva-10637 (URN)978-91-86135-96-6 (ISBN)
Presentation
2010-12-14, Kappa, Mälardalens högskola, Västerås, 14:00 (English)
Opponent
Supervisors
Projects
Biogasopt
Available from: 2010-11-09 Created: 2010-10-28 Last updated: 2013-12-19Bibliographically approved
2. 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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