Wastestream valorization : upcycling of municipal and industrial effluents with microalgae and cyanobacteria

Abstract

The human population is increasing steadily and with it the anthropogenic CO2, wastestreams of industrial and municipal origin and the need for basic necessities, such as human and animal nutrition or plastics. The production of these basic necessities is often based on the use of precious resources, such as fossil fuels, potable water, or nutrients contained in our soil. One way to reduce the use of finite resources is to re-use and upcycle already discarded materials, e.g. municipal wastewaters or industrial effluents. This upcycling could be done by incorporating microalgae and cyanobacteria in wastewater valorization strategies. That approach is rising in popularity because of their ability to utilize nutrients contained in wastewaters for the production of value-added products, such as pigments, nutraceuticals, bioplastics or biofertilizers. Additionally, being photosynthetic organisms, they fixate CO2 during photoautotrophic and mixotrophic nutrient upcycling processes. In this thesis, the feasibility of microalgal and cyanobacterial valorization of wastestreams from different origins has been studied.One tested wastewater was originated from wheat straw, which had been hydrolyzed to separate the hemicellulosic, cellulosic and lignin fraction, generating the fermentation inhibitors furfural (FF) and 5-hydroxymethylfurfural (HMF). Furan containing wastestreams stemming from the pulp and paper industry, such as this wheat straw hydrolysate (WSH), have not been widely studied in microalgal context. Supplementing Chlorella vulgaris medium in mixotrophic cultivation with WSH, led to an increased volumetric biomass productivity rX (mg L-1 day-1) of 10 – 20 % compared to a photoautotrophic control, depending on the amount of WSH utilized. Furthermore, a chlorophyll content of 4.40 % per dry cell weight (DCW) was obtained when cultivating Chlorella vulgaris on a medium supplemented with WSH compared to 2.66 % chlorophyll DCW-1 obtained on a defined medium. This was the highest chlorophyll content achieved in Chlorella vulgaris grown on any defined medium or wastestream, reported in literature. During this first study, the conversion of furanic compounds contained in the hydrolysates was witnessed. The products of these conversions were identified as 2-furoic acid stemming from FF and 5-hydroxymethyl-2-furoic acid from HMF oxidation. This oxidation has never been described before in any other microalgal species. The oxidation products could be used for the generation of biodegradable polymers with similar properties to fossil-basedpolyethylene terephthalate, indicating the potential valorization from the pulp and paper industry.In addition to furan containing wastestreams (i) processing waters from a potato-processing company, and (ii) municipal wastewater were tested for microalgal and cyanobacterial valorization.Four nutrient-rich wastestreams obtained from a potato processing company were investigated for their potential to be used as substrate for Chlorella vulgaris cultivation to produce a biofertilizer. All of the present P and N were taken up by C. vulgaris in the process. This led to the generation of microalgal biomass meeting the European Union’s criteria for biofertilizer, as well as meeting the EU’s regulation threshold concentration for N and P in treated wastewater. Calculations based on data obtained from the stirred-tank photobioreactor cultivations were done to assess the feasibility of a potential scale up, to close the loop from potato processing back to the potato plant.Finally, municipal wastewater was used as cultivation medium for Synechocystis sp. in an open pond photobioreactor system (100 L) to generate biomass rich in polyhydroxybutyrate (PHB). The generated biomass (≈ 3.5 g/L) contained approximately 24 % DCW-1 PHB, which is the highest content achieved in cyanobacteria grown on wastewater reported in literature. Additionally, 67 % P and 72 % N was removed from the urban wastewater during cultivation, reducing the environmental burden of the water.In conclusion, upcycling of wastestreams has been tested several times before, however my thesis fills these knowledge gaps:(i) The influence of the fermentation inhibitors FF and HMF on Chlorella vulgaris growth parameters and their respective conversion has never been described before.(ii) The utilization of potato processing water as substrate for the generation of Chlorella vulgaris based biofertilizer as a novel circular economy concept and has not been described before.(iii) PHB-producing Synechocystis sp. has not been cultivated in an outdoor raceway pond (100 L pilot scale) on municipal wastewater prior to this study.