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Enhanced biogas production from municipal WWTPs: Co-digestion of microalgae with sewage sludge and thermophilic secondary digestion of mesophilic digested sludge
Mälardalen University, School of Business, Society and Engineering, Future Energy Center.ORCID iD: 0000-0002-0861-6438
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Biogas is produced when organic material is broken down in oxygen-free (anaerobic) conditions. This process is called anaerobic digestion and is used in most large and medium-sized municipal wastewater treatment plants in Sweden. In the wastewater treatment sewage sludge is obtained, from the mecanical-, biological and chemical treatment step, which contains decomposable organic material. The sludge is pumped into a digester, which is an airtight container. In the digester raw biogas, consisting of methane and carbon dioxide, is produced. The material that comes out of the digester is a nutrient rich residue (digestate) which can be used as a fertilizer or soil conditioner. The purpose of this study was to explore ways to increase the biogas production that takes place at the municipal wastewater treatment plants by either co-digestion of sewage sludge with microalgae from a possible future biological purification steps or to use two digestion stages in series with different operating temperatures, mesophilic (37ºC) followed by a thermophilic digestion (55ºC). The challenges with these methods, which are also taken into consideration in the studies, were changes in the dewaterability of the digestate, system efficiency regarding electricity and heat consumption, the ability to recycle nutrients, changes in the carbon footprint from the treatment plant, change of the pollution level in the digestate and the ability to create a sanitization method for the digestate.

The results from the first part showed in both batch digestability tests and continuous anaerobic digestion experiment that microalgae cultivated on wastewater can be a feasible feedstock for anaerobic co-digestion with sewage sludge. Microalgae improved the biogas production in mesophilic conditions but not in thermophilic digestion. In the semi-continous experiment, with the addition of a natural mix of microalgae grown from wastewater to sewage sludge, the specific methane production was enhanced with 39 % for every gram organic matter reduced. The specific methane production for every gram added organic matter to the reactors were 9% lower in the digester where microalgae had been added. When microalgae were added the total digestibility was reduced compared to the reference digestion with only sewage sludge. Filterability tests indicated that the addition of microalgae enhanced the dewaterability of the digested sludge. Heavy metal levels in the microalgae substrate were much higher than in the sludge which could restrict the utilization of the digestate on arable land in a possible future full scale application.

The results in the second part showed that the process solution could be a self-sufficient sanitation method. The highest organic loading rates tested in this study were in the range causing an unstable process due to high ammonia levels The thermophilic digestion gave the sludge worse dewaterability. However, a subsequent aeration step could improve the properties again.

Place, publisher, year, edition, pages
Västerås: Mälardalen University , 2015. , p. 56
Series
Mälardalen University Press Licentiate Theses, ISSN 1651-9256 ; 202
National Category
Engineering and Technology Energy Engineering
Research subject
Energy- and Environmental Engineering
Identifiers
URN: urn:nbn:se:mdh:diva-27911ISBN: 978-91-7485-210-3 (print)OAI: oai:DiVA.org:mdh-27911DiVA, id: diva2:808135
Presentation
2015-06-11, Delta, Mälardalens högskola, Västerås, 10:00 (English)
Opponent
Supervisors
Available from: 2015-04-27 Created: 2015-04-27 Last updated: 2015-05-12Bibliographically approved
List of papers
1. Co-digestion of cultivated microalgae and sewage sludge from municipal waste water treatment
Open this publication in new window or tab >>Co-digestion of cultivated microalgae and sewage sludge from municipal waste water treatment
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2014 (English)In: Bioresource Technology, ISSN 0960-8524, E-ISSN 1873-2976, Vol. 171, p. 203-210Article in journal (Refereed) Published
Abstract [en]

In this study two wet microalgae cultures and one dried microalgae culture were co-digested in different proportions with sewage sludge in mesophilic and thermophilic conditions. The aim was to evaluate if the co-digestion could lead to an increased efficiency of methane production compared to digestion of sewage sludge alone. The results showed that co-digestion with both wet and dried microalgae, in certain proportions, increased the biochemical methane potential (BMP) compared with digestion of sewage sludge alone in mesophilic conditions. The BMP was significantly higher than the calculated BMP in many of the mixtures. This synergetic effect was statistically significant in a mixture containing 63% (w/w VS based) undigested sewage sludge and 37% (w/w VS based) wet algae slurry, which produced 23% more methane than observed with undigested sewage sludge alone. The trend was that thermophilic co-digestion of microalgae and undigested sewage sludge did not give the same synergy.

National Category
Environmental Biotechnology
Identifiers
urn:nbn:se:mdh:diva-26481 (URN)10.1016/j.biortech.2014.08.069 (DOI)000343091700030 ()2-s2.0-84908219564 (Scopus ID)
Available from: 2014-11-07 Created: 2014-11-07 Last updated: 2018-05-08Bibliographically approved
2. Energy efficient combination of sewage sludge treatment and hygenization after mesophilic digestion - Pilot study
Open this publication in new window or tab >>Energy efficient combination of sewage sludge treatment and hygenization after mesophilic digestion - Pilot study
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2014 (English)In: Energy Procedia, ISSN 1876-6102, E-ISSN 1876-6102, Vol. 61, p. 587-590Article in journal (Refereed) Published
Abstract [en]

Biogas production is probably the most feasible way of utilizing sewage sludge as energy source, simultaneously with nutrient recovery by recycling the biogas digestate (i.e. The residue) to arable land. However, the sludge commonly contains high amounts of human pathogenic bacteria excreted in faeces and urine. To use sludge as fertilizer on food producing land is therefore a controversial issue, partly because of the risk of spreading diseasecausing pathogens. The Swedish environment protection agency (SEPA) pre-approved two hygenization methods for the treatment of the sludge due to their positive effects on the sludge quality. One of them, conventional pasteurization (70 °C, 1 h), was investigated for its feasibility in Uppsala, Sweden, and it was found that the heat consumption was very high. The other method has the advantage of potentially increase the produced biogas. This hygenization method has been investigated in the present study through a pilot experiment where thickened mesophilic digested sludge is digested once more at thermophilic conditions (55 °C). The aim of the study was to investigate the possibility to develop this self-sufficient (in heat and electricity) hygenization method. The results showed an increase in the gas production from 430 dm3/kg VSin to 610 dm3/kg VSin by adding the thermophilic step. This increase gave an energy balance with an excess of both heat and electricity. Sludge hygenization was sufficient with the method and another important result is the significant decrease digestate volume.

Keyword
Hygenization, Mesophilic digestion, Thermophilic digestion
National Category
Energy Engineering
Identifiers
urn:nbn:se:mdh:diva-27570 (URN)10.1016/j.egypro.2014.11.1176 (DOI)000375936100136 ()2-s2.0-84922368189 (Scopus ID)
Conference
6th International Conference on Applied Energy, ICAE 2014, 30 May 2014 through 2 June 2014
Available from: 2015-02-19 Created: 2015-02-19 Last updated: 2018-02-26Bibliographically approved
3. CONTINUOUS CO-DIGESTION OF MICROALGAE AND REPRESENTATIVE MIX OF SEWAGE SLUDGE: -
Open this publication in new window or tab >>CONTINUOUS CO-DIGESTION OF MICROALGAE AND REPRESENTATIVE MIX OF SEWAGE SLUDGE: -
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2014 (English)Conference paper, Published paper (Refereed)
Abstract [en]

A natural mix of microalgae grown on wastewater was co-digested with a representative mix of sewage sludge in a semicontinuous pilot digester system. The share of microalgae in the mix were 37 % calculated as VS-content. The organic loading rate was 2.4 kg VS (volatile solids) m-3d-1 and the hydraulic retention time was 15 d in a reference reactor, with just a representative mix of sewage sludge, and a digester where microalgae were added. The results from the three retention times showed that the addition of the microalgae enhanced the methane yield with 39 % for every gram reduced VS in the reactors. The specific methane yield for every gram added VS to the reactors were 9 % lower in the digester where microalgae had been added. Less sludge was degraded when microalgae were added, but more methane was produced for every gram VS reduced. CST-measurements indicated that the addition of microalgae enhance the dewaterability of the digested sludge.

National Category
Engineering and Technology
Research subject
Energy- and Environmental Engineering
Identifiers
urn:nbn:se:mdh:diva-24808 (URN)
Conference
5th International Symposium on Energy from Biomass and Waste, Venice Italy, 19-22 Nov 2014
Available from: 2014-04-04 Created: 2014-04-04 Last updated: 2015-04-27

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Olsson, Jesper

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