Search

The interest in microalgae products has been increasing, and therefore the cultivation industry is growing steadily. To reduce the environmental impact and production costs arising from nutrients, research needs to find alternatives to the currently used artificial nutrients. Microalgae cultivation in anaerobic effluents (more specifically, digestate) represents a promising strategy for increasing sustainability and obtaining valuable products. However, digestate must be processed prior to its use as nutrient source. Depending on its composition, different methods are suitable for removing solids (e.g., centrifugation) and adjusting nutrient concentrations and ratios (e.g., dilution, ammonia stripping). Moreover, the resulting cultivation medium must be light-permeable. Various studies show that growth rates comparable to those in artificial media can be achieved when proper digestate treatment is used. The necessary steps for obtaining a suitable cultivation medium also depend on the microalgae species to be cultivated. Concerning the application of the biomass, legal aspects and impurities originating from digestate must be considered. Furthermore, microalgae species and their application fields are essential criteria when selecting downstream processing methods (harvest, disintegration, dehydration, product purification). Microalgae grown on digestate can be used to produce various products (e.g., bioenergy, animal feed, bioplastics, and biofertilizers). This review gives insight into the origin and composition of digestate, processing options to meet requirements for microalgae cultivation and challenges regarding downstream processing and products.

This study evaluates a biorefinery concept for producing poly(3-hydroxybutyrate) (PHB) with the cyanobacterial strain Synechocystis salina. Due to this reason, pigment extraction and cell disruption were investigated as pre-treatment steps for the harvested cyanobacterial biomass. The results demonstrated that at least pigment removal was necessary to obtain PHB with processable quality (weight average molecular weight: 569-988kgmol-1, melting temperature: 177-182°C), which was comparable to heterotrophically produced PHB. The removed pigments could be utilised as additional by-products (chlorophylls 0.27-1.98mgg-1 TS, carotenoids 0.21-1.51mgg-1 TS, phycocyanin 0-127mgg-1 TS), whose concentration depended on the used nutrient source. Since the residual biomass still contained proteins (242mgg-1 TS), carbohydrates (6.1mgg-1 TS) and lipids (14mgg-1 TS), it could be used as animal feed or converted to biomethane (348 mn3 t-1VS) and fertiliser. The obtained results indicate that the combination of photoautotrophic PHB production with pigment extraction and utilisation of residual biomass offer the highest potential, since it contributes to decrease the environmental footprint of the process and because biomass could be used in a cascading way and the nutrient cycle could be closed.

Polyhydroxyalkanoates (PHAs) are inevitably a key biopolymer that has the potential to replace the conventional petrochemical based plastics that pose jeopardy to the environment globally. Even then the reach of PHA in the common market is so restricted. The economy of PHA is such that, even after several attempts the overall production cost seems to be high and this very factor surpasses PHAs usage when compared to the conventional polymers. The major focus of the review relies on the synthesis of PHA from Mixed Microbial Cultures (MMCs), through a 3-stage process most probably utilizing feedstocks from waste streams or models that mimic them. Emphasis was given to the works carried out in the past decade and their coherence with each and every individual criteria (Aeration, Substrate and bioprocess parameters) such that to understand their effect in enhancing the overall production of PHA.

Within the last decades, environmental pollution with persistent plastics steadily increased; therefore the production of biodegradable materials like poly-s-hydroxybutyrate (PHB) is essential. Currently, PHB is produced with heterotrophic bacteria from crops. This leads to competition with food and feed production, which can be avoided by using photoautotrophic cyanobacteria, as Synechocystis salina, synthesizing PHB from CO2 at nutrient limitation. This study aims to increase the economic efficiency of PHB production with cyanobacteria by using nutrients from anaerobic digestate. First, growth and PHB production of S. salina in digestate fractions (supernatant and permeate, with/without precipitating agents) and dilutions thereof and then the scale-up (photobioreactor, 200 L working volume) were evaluated. With precipitated and centrifuged digestate diluted 1/3 the highest biomass (1.55gL-1) and PHB concentrations (95.4mgL-1), being 78% of those in mineral media, were achieved. In the photobioreactor-experiments biomass (1.63gL-1) and PHB concentrations (88.7mgL-1), being 79% and 72% of those in mineral medium, were reached, but in a cultivation time 10days longer than in mineral medium. The possibility to use digestate as sustainable and low cost nutrient solution for microalgae cultivation and photoautotrophic PHB production, instead of applying it on fields or processing it to achieve discharge limits, makes this application a highly valid option.

The biopolymer poly-β-hydroxybutyrate (PHB) can be used as a promising bioplastic. It has a broad range of applications and is degraded relatively rapidly by soil organisms. Like many prokaryotes, the cyanobacterium Synechocystis sp. CCALA192 produces this biopolymer as a storage compound, especially under nutrient limitation. In a 200-L tubular photobioreactor, we cultivated Synechocystis sp. CCALA192 semi-continuously over a period of 75 days with CO2 as sole carbon source. A two-stage cultivation strategy was performed, where after 5–7 days nitrogen was depleted and the culture started to produce PHB and gradually turned from blue-green to yellow. After 16–20 days, 90% of the culture were harvested and the residual 10% were used as inoculum for the following cycle. The harvested culture had an average biomass concentration of 1.0 g/L with an average PHB content of 12.5% of cell dry weight. After restarting with fresh nutrients, the yellow culture turned blue-green again and degraded the PHB within 24–48 h. When nitrogen of the medium was consumed, PHB was produced again and the cycle continued. In the late stage of each production cycle, a ripening process was observed, where no CO2 was consumed but the PHB concentration was still rising at the expense of the existing glycogen rich biomass. Establishing a stable Synechocystis sp. CCALA192 culture under non-sterile conditions turned out to be difficult, as this small unicellular organism is very sensitive and easily grazed by protozoa. Therefore, a special cultivation strategy with partially anoxic conditions was necessary.

Political, economic and ecological reasons have recently been leading to efforts to replace fossil hydrocarbons and their products in a sustainable way. In order to replace fossil-based polymers, photoautotrophically produced polyhydroxybutryrates (PHBs), which are intracellular carbon storage products of nutrient-deprived microorganisms, seem to be a promising, biobased and biodegradable alternative. Although laboratory and pilot scale experiments have already been performed, no economic evaluation has been carried out so far. Consequently, valid claims on PHB production costs and the influence of different parameters, such as intracellular PHB-content, choice of cultivation system or location, cannot be made. In this study potential demonstration plants, equipped with different photoautotrophic cultivation systems and located at two sites, were designed to identify key parameters for a successful economic realization and implementation. Material and energy balances were determined to reveal specific PHB production costs for four different scenarios. Raw material and operating supply costs, expenditures for plant construction and operation as well as product amounts were determined using literature data for specified results from laboratory and pilot scale experiments. The lowest calculated PHB production price (24 € kg−1) accomplished in a thin-layer-system plant located in Southern Europe with 60% PHB-content of the produced biomass is significantly higher than the current market price of heterotrophically produced PHB. The most important cost factors in all scenarios are cultivation and harvesting costs accounting for 62 to 72% of the total specific production costs, followed by maintenance costs with a cost share of 11 to 14%. Therefore, the choice of a suitable cultivation system is the key driving factor for an economic PHB-production due to the currently high investment costs for photosynthetic biomass production systems. Specific production costs for a Southern compared to a Central European location amount to almost half of the costs.

A polyhydroxybutyrate (PHB) producing cyanobacteria was converted through hydrothermal liquefaction (HTL) into propylene and a bio-oil suitable for advanced biofuel production. HTL of model compounds demonstrated that in contrast to proteins and carbohydrates, no synergistic effects were detected when converting PHB in the presence of algae. Subsequently, Synechocystis cf. salina, which had accumulated 7.5 wt% PHB was converted via HTL (15% dry weight loading, 340 °C). The reaction gave an overall propylene yield of 2.6%, higher than that obtained from the model compounds, in addition to a bio-oil with a low nitrogen content of 4.6%. No propylene was recovered from the alternative non-PHB producing cyanobacterial strains screened, suggesting that PHB is the source of propylene. PHB producing microorganisms could therefore be used as a feedstock for a biorefinery to produce polypropylene and advanced biofuels, with the level of propylene being proportional to the accumulated amount of PHB.