Microalgae : Platform for conversion of waste to high value products
Year: 2018
Type: dissertation
Abstract: Microalgae have gained attention as a potential raw material for metabolic compounds suitable for human and animal nutrition, cosmetics, biofuels, pharmaceuticals and fertilizers. Furthermore, microalgae efficiently convert nutrients in the growth medium into biomass and may simultaneously be useful in wastewater treatment. Some of microalgae are able to synthesize long chain polyunsaturated fatty acids (LC-PUFAs), such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are essential parts of human and animal nutrition. Microalgal chlorophylls and carotenoids can be used as colorants, but several carotenoids are also strong antioxidants and precursors of vitamin A. Additionally, microalgae are a rich source of vitamins such as C, E, and B12 and provide a vegan protein source with all the essential amino acids. However, cost competitiveness of microalgae-based, high-value products is still a weakness. One possibility for cost reduction is the connection of microalgal cultivation to existing industrial processes and to convert spilled nutrients and side streams such as CO2 into valuable microalgal biomass. The overall aim of this thesis was (i) to study the suitability of composting leachate liquids and aquaculture wastewaters (AWWs) for microalgal cultivation, (ii) to reveal the microalgal nutrient up-take potential from wastewaters, and (iii) to evaluate the suitability of the produced biomass for human and animal nutrition. First, co-cultures of Selenastrum with associated bacteria were cultured in several composting leachate liquids and at the end of the experiments the fatty acid profiles of well grown cultures were analyzed. The ´biowaste leachate´ was selected as the substrate for experiments carried out in pilot scale photobioreactor (PBR) to study the up-scale potential of co-culture. In further experiments, the hypotheses of higher biomass and lipid production and more efficient resource utilization (i.e. nutrient reduction) and better tolerance against contaminations in mixed than in monocultures of microalgae were tested. Microalgae Euglena gracilis, Selenastrum and Chlorella sorokiniana were cultured in mono- and mixed cultures in biowaste leachate at a small scale. The experiments were up-scaled using a monoculture of E. gracilis and mixed cultures of E. gracilis with Selenastrum or with C. sorokiniana. To study the potential of microalgae in AWW treatment and to evaluate the suitability of the produced biomass as an ingredient in fish feed, mixed cultures of E. gracilis with Selenastrum were cultured in AWWs and sludge-amended AWWs from recirculating aquaculture system (RAS) used for growing pike perch or catfish. To clarify the influence of long term nitrogen (N) deprivation on fatty acid, protein, and pigment composition of E. gracilis, additional experiments were carried out in synthetic medium. In most of the composting leachate liquids and in AWWs, biomass production and nutrient reduction were efficient. As the nutrient concentrations in the composting leachate liquids were very high, the leachates had to be diluted before use, whereas the AWW sludge was used for enrichment of the nutrient poor AWWs. An interesting finding was that Selenastrum and bacteria grown together appeared to boost each other’s growth in composting leachate liquids. Pilot scale experiments showed that the growth and lipid production in co-culture can be boosted by feeding the culture with CO2, which was presumably a result of more efficient photosynthetic carbon uptake by Selenastrum. Strain survival and performance in biowaste leachate grown mixed cultures were totally different at different scales. In small scale experiments, C. sorokiniana dominated the cultures, whereas in the pilot scale, only E. gracilis thrived. Evidence of higher biomass or lipid productivity in mixed cultures as compared to monocultures was not found. However, in pilot scale cultivations a better resistance against contaminations and an enhanced lipid production and N reduction was found in mixed culture of E. gracilis with Selenastrum. In AWW grown cultures, the dominance of E. gracilis over the Selenastrum was repeated. The biomass FA and tocopherol contents were higher in AWWs without sludge-amendment than in sludge-amended cultures, but the more efficient biomass production in sludge-amended cultures resulted in higher FA and tocopherol yields. Short term nutrient stress was found to enhance lipid production in pilot scale monocultures of E. gracilis and in mixed cultures of E. gracilis with Selenastrum dominated by E. gracilis grown in biowaste leachate. However, long-term N deprivation in synthetic medium grown E. gracilis cultures resulted in poor nutritional quality of the FAs with a high content of saturated fatty acids (SAFAs) and low contents of LC-PUFAs, as well as lowered protein, chlorophyll a, and carotenoid concentrations. Thus, the recommendation for production of high quality E. gracilis biomass is that it should be cultivated under nutrient replete conditions to maximize biomass yield followed by short term exposure to nutrient limitation in the end part of the cultivation. As a conclusion, converting nutrients from waste streams into high-value microalgal biomass provides a sustainable solution that combines waste treatment with value added production. In AWW sludge treatment, connecting microalgal culturing units to RAS AWW treatment systems is an option for combined AWW treatment and microalgal biomass production. In mixed microalgal cultures, strain performance is sensitive to changes in growth conditions, and thus strains with different growth requirements can offer the solution for robust and continuous biomass production in unstable growth conditions, and also have better tolerance against bacterial contaminations. However, the biomass composition of strains varies in different growth environments and cultivation phases, and so the process parameters as well as strain combinations should be selected based on the desired products.
Source: Työväentutkimus Vuosikirja
Author: Marika Tossavainen
Institution: University of Helsinki
Language: English
Field: Energy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W2899980548