Microbiological analysis of municipal wastewater treating photobioreactors

University dissertation from Västerås : Mälardalen University

Abstract: With the growing human population, the increasing amount of wastewater produced daily presents a challenge to the environment. When designing systems and infrastructure for wastewater treatment, we are limited by the ever-growing demands to reduce energy use. Currently used systems have many shortcomings when faced with modern treatment criteria and energy use restrictions. Microalgae reactors, commonly known as photobioreactors, have been suggested as an alternative. These systems use microalgae and bacteria to reduce pollutants and remove nutrients such as nitrogen and phosphorous compounds.Water treatment systems using photobioreactors are a relatively new technology and several aspects of their biology have yet to be studied in detail. This thesis presents a broad overview of the algal and bacterial communities present in these systems. In addition to looking at the most important species, metabolic pathways and growth dynamics of both algae and bacteria, this thesis also analyses water purification dynamics.Municipal wastewater from the Västerås wastewater treatment plant was inoculated with algae from Lake Mälaren and several different experiments were conducted with reactor volumes from 250 ml to 20 L. The inoculated reactors demonstrated better algal growth than those that were not inoculated. All tested systems were also successful in removing ammonium and phosphorous from the wastewater.The dominant algae growing in the studied photobioreactors belonged to the genera Scenedesmus, Desmodesmus and Chlorella. In addition to algae, the systems contained a large number of bacteria, mostly belonging to the Proteobacteria and Bacteroidetes phyla. These were shown to be involved in synthesis of vitamins essential for algae growth such as vitamin B12, which is not commonly synthesized by algae, suggesting cooperation between the bacteria and algae.In addition to vitamin synthesis, algal photobioreactors contained a lower abundance of genes related to nitrogen metabolism, virulence and antibiotic resistance compared to the initial wastewater, showing that a shift in the bacterial community had occurred.Overall, the information regarding algal and bacterial populations and metabolic genes presented in this thesis is important for the development of tools for the control and monitoring of full-scale wastewater treating photobioreactor plants.

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