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CONTINUOUS CO-DIGESTION OF MICROALGAE AND REPRESENTATIVE MIX OF SEWAGE SLUDGE: -
Mälardalen University, School of Business, Society and Engineering, Future Energy Center. (ACWA/FUTURE ENERGY)ORCID iD: 0000-0002-0861-6438
Uppsala Universitet, Sweden.
Mälardalen University, School of Business, Society and Engineering, Future Energy Center. (ACWA/FUTURE ENERGY)ORCID iD: 0000-0003-3311-9465
Mälardalen University, School of Business, Society and Engineering, Future Energy Center. (ACWA/FUTURE ENERGY)ORCID iD: 0000-0002-5014-3275
Show others and affiliations
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.

Place, publisher, year, edition, pages
2014.
National Category
Engineering and Technology
Research subject
Energy- and Environmental Engineering
Identifiers
URN: urn:nbn:se:mdh:diva-24808OAI: oai:DiVA.org:mdh-24808DiVA, id: diva2:710007
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
In thesis
1. Enhanced biogas production from municipal WWTPs: Co-digestion of microalgae with sewage sludge and thermophilic secondary digestion of mesophilic digested sludge
Open this publication in new window or tab >>Enhanced biogas production from municipal WWTPs: Co-digestion of microalgae with sewage sludge and thermophilic secondary digestion of mesophilic digested sludge
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:nbn:se:mdh:diva-27911 (URN)978-91-7485-210-3 (ISBN)
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

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Olsson, JesperNehrenheim, EmmaSchwede, SebastianThorin, Eva

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