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Co-digestion of sewage sludge and microalgae: Biogas production investigations
Mälardalen University, School of Business, Society and Engineering, Future Energy Center. (FE)ORCID iD: 0000-0002-3485-5440
Mälardalen University, School of Business, Society and Engineering, Future Energy Center. (FEC)ORCID iD: 0000-0002-0861-6438
Mälardalen University, School of Business, Society and Engineering, Future Energy Center. (FEC)ORCID iD: 0000-0002-5014-3275
Mälardalen University, School of Business, Society and Engineering, Future Energy Center. (FEC)ORCID iD: 0000-0003-3311-9465
(English)In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118Article in journal (Refereed) In press
Abstract [en]

In municipal wastewater treatment plants (WWTPs), algae could be utilised for cleaning the water and, at thesame time, produce a biomass that can be used for energy. Through anaerobic digestion, microalgae can contributeto biogas production when co-digested with sewage sludge. In this paper, previous published results onthe co-digestion of sewage sludge and microalgae are summarised and reviewed, and any remaining knowledgegaps are identified. The batch tests currently documented in literature mostly concern digestion under mesophilicconditions, and studies investigating thermophilic conditions are less common. The average biochemicalmethane potential (BMP) for 29 different mixtures co-digested under mesophilic conditions is 317 ± 101 N cm3CH4 gVS−1 while the result for 12 different mixtures investigated under thermophilic conditions is a BMP of318 ± 60 N cm3 CH4 gVS−1. An evaluation of the heat required for increasing the temperature from mesophilicto thermophilic conditions shows that increased methane production under thermophilic conditions can beenough to create a positive energy balance. For a full-scale WWTP, using thermophilic digestion on sludge, or acombination of sludge and microalgae could therefore be of interest. This is dependent on the demands onsanitation of the sludge and the possibilities for heat recovery.Most of the mesophilic investigations indicate a synergetic effect for co-digestion, with enhancements of up toalmost 70%. However, the results are uncertain since the standard deviations for some of the BMP tests are in thesame order of magnitude as the identified enhancement. Neither of the presented publications provide an understandingof the basic mechanisms that led to higher or lower BMP when microalgae were mixed with wastewatersludge. We, therefore, call for care to be taken when assuming any effects related to the specification ofsubstrates. Microalgae and wastewater sludge have several similarities, and the specific results of BMP in themixtures relate more to the specifics of the respective materials than the materials themselves.Investigations into semi-continuous processes of co-digestion of microalgae and sludge are scarce. The yieldsfor three co-digestion studies show high variation, with an average of 293 ± 112 N cm3 gVSin−1. The availableresults show strong potential for co-digestion of sewage sludge and microalgae. Further investigations are requiredto identify optimal conditions for biogas production, and analysis of microalgae implementation onwastewater treatment at a system level is also needed to identify the total mass balance of substrate and nutrientrecovery.

Keywords [en]
Biomass Wastewater treatment Batch Continuous BMP Anaerobic digestion
National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
URN: urn:nbn:se:mdh:diva-37578DOI: 10.1016/j.apenergy.2017.08.085Scopus ID: 2-s2.0-85028066228OAI: oai:DiVA.org:mdh-37578DiVA, id: diva2:1169421
Projects
MAASICA
Funder
Knowledge FoundationAvailable from: 2017-12-27 Created: 2017-12-27 Last updated: 2018-05-08Bibliographically approved
In thesis
1. Co-digestion of microalgae and sewage sludge - A feasibility study for municipal wastewater treatment plants
Open this publication in new window or tab >>Co-digestion of microalgae and sewage sludge - A feasibility study for municipal wastewater treatment plants
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The increased emissions of anthropogenic greenhouse gases over the last 100 years is the reason for the acceleration in the greenhouse effect, which has led to an increase of the globally averaged combined land and ocean surface temperature of 0.85 °C between 1880 and 2012. A small fraction of the increased anthropogenic greenhouse gases originates from municipal wastewater treatment plants (WWTPs).

This doctoral thesis was part of a larger investigation of using an alternative biological treatment based on the symbiosis of microalgae and bacteria (MAAS-process (microalgae and activated sludge)). This solution could be more energy efficient and potentially consume carbon dioxide from fossil combustion processes and also directly capture carbon dioxide from the atmosphere and thereby reduce the addition of anthropogenic greenhouse gases to the air.

 The objective of the thesis was to explore the effects when the microalgae-derived biomass from the biological treatment were co-digested with sewage sludge. The results from these experimental studies were then used to evaluate the effects on a system level when implementing microalgae in municipal WWTP.

 Microalgae grown from a synthetic medium improved the methane yield with up to 23% in mesophilic conditions when part of the sewage sludge was replaced by the microalgae. The microalgae grown from municipal wastewater showed no synergetic effect.

 In the semi-continuous experiments the methane yield was slightly reduced when implementing the microalgae. Furthermore the digestibility of the co-digestion between sewage sludge and microalgae were lower compared to the digestion of sewage sludge.

 The digestates containing microalgal substrate had higher heavy metals content than digestates containing only sewage sludge. This could have a negative effect on the potential to use this digestate on arable land in future, due to strict limits from the authorities.  Filterability measurements indicated that the addition of microalgae enhanced the dewaterability of the digested sludge and lowered the demand for polyelectrolyte significantly.

 When a hypothetical MAAS-process replaced a conventional ASP-process the amount of feedstock of biomass increased significantly due to the increased production from the autotrophic microalgae. This increased the biogas production by 66-210% and reduced the heavy metal concentration in the digestate due to a dilution effect from the increased biomass production.

 The thesis demonstrates that microalgae in combination with bacteria from a MAAS-process can be a realistic alternative feedstock to WAS in the anaerobic digestion at a municipal WWTP. A few drawbacks need to be considered when choosing a MAAS-process as biological treatment.

Place, publisher, year, edition, pages
Västerås: Mälardalen University, 2018. p. 96
Series
Mälardalen University Press Dissertations, ISSN 1651-4238 ; 262
Keywords
Microalgae, Anaerobic digestion, dewaterability, BMP-experiments
National Category
Water Treatment
Research subject
Energy- and Environmental Engineering
Identifiers
urn:nbn:se:mdh:diva-39154 (URN)978-91-7485-386-5 (ISBN)
Public defence
2018-06-18, Paros, Mälardalens högskola, Västerås, 13:00 (English)
Opponent
Supervisors
Funder
Knowledge Foundation
Available from: 2018-05-07 Created: 2018-05-07 Last updated: 2018-06-04Bibliographically approved

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Olsson, JesperSchwede, SebastianNehrenheim, Emma

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