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Energy efficient combination of sewage sludge treatment and hygenization after mesophilic digestion - Pilot study
Mälardalen University, School of Business, Society and Engineering, Future Energy Center.ORCID iD: 0000-0002-0861-6438
Uppsala University, Sweden.
Uppsala Vatten Och Avfall AB, Sweden.
Uppsala Vatten och Avfall AB, Sweden.
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2014 (English)In: Energy Procedia, ISSN 1876-6102, E-ISSN 1876-6102, Vol. 61, 587-590 p.Article 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.

Place, publisher, year, edition, pages
2014. Vol. 61, 587-590 p.
Keyword [en]
Hygenization, Mesophilic digestion, Thermophilic digestion
National Category
Energy Engineering
Identifiers
URN: urn:nbn:se:mdh:diva-27570DOI: 10.1016/j.egypro.2014.11.1176ISI: 000375936100136Scopus ID: 2-s2.0-84922368189OAI: oai:DiVA.org:mdh-27570DiVA: diva2:789624
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: 2016-10-31Bibliographically approved
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. 56 p.
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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Citation style
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