The public defense of Jesper Olsson´s doctoral thesis

Doctoral thesis and Licentiate seminars

Datum: 2018-06-18
Tid: 13.00 - 16.00
Plats: MDH, room Paros, Västerås

The public defense of Jesper Olsson´s doctoral thesis in energy and environmental engineering will take place at Mälardalen University on June 18.

The title of the thesis is “Co-digestion of microalgae and sewage sludge – A feasibility study for municipal wastewater treatment plants”.

The faculty examiner is Associate Professor Raúl Muñoz Torre, University of Valladolid, and the examining committee consists of Docent Åke Nordberg, Swedish University of Agricultural Sciences; Professor Mohammed Taherzadeh University of Borås; Associate Professor Jin Mi Triolo, The University of Southern Denmark.

Reserve: Docent Lena Johansson Westerholm, Mälardalen University

The doctoral thesis has serial number 262

Summary

The increased emissions of greenhouse gases over the last 100 years have 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 greenhouse gases originates from municipal wastewater treatment plants (WWTPs).

The most common configuration of municipal wastewater treatment comprises mechanical, biological and chemical treatment. Biological treatment, which usually consist of an activated sludge process (ASP) is based on the growth of bacteria. The bacteria require oxygen which is provided through energy-intensive (mechanical) aeration to reduce dissolved nutrients and organic matter.     

This doctoral thesis was part of a larger investigation of using an alternative biological treatment based on the symbiosis of microalgae and bacteria.  This solution could be more energy efficient since the microalgae produce oxygen through photosynthesis and the aeration is therefore not needed. The microalgae can also potentially consume carbon dioxide from fossil combustion processes or directly capture carbon dioxide from the air and thereby reduce the addition of greenhouse gas to the atmosphere.

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

The results from the experiments showed that the amount of biogas produced was slightly reduced when implementing the microalgae. Furthermore, the amount of organic matter reduced in the co-digestion of sewage sludge and microalgae was lower compared to the digestion of sewage sludge alone.

The rest material after the anaerobic digestion containing microalgae had higher heavy metals content than the rest material containing only sewage sludge. Possibly due to uptake of metals from flue gas from power plants used as a CO2 source. This could have a negative effect on the potential to use this the material as fertilizer, due to strict limits from the authorities.  

When a hypothetical biological treatment based on microalgae replaced a conventional active sludge process the amount of biomass increased significantly due to the increased production from the autotrophic microalgae. This increased the biogas production with 66-210% and increased the recovery of nitrogen to a larger extent compared to the conventional treatment.

The thesis demonstrates that microalgae can be a realistic alternative feedstock in the anaerobic digestion at municipal WWTPs, though with a few drawbacks that need to be considered.