Partner und Internationale Organisationen
(Englisch)
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AT, BE, BG, CH, DE, DK, EE, EL, ES, FI, FR, HU, IT, LT, LV, NL, NO, PL, RO, SE, TR, UK
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Abstract
(Englisch)
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We are currently working towards demonstrating the technical and economic feasibility of methane production via hydrothermal (HT) processing of microalgae, the 'SunCHem technology'. One major challenge of the SunCHem process is the recycling of nutrient-rich effluents, which upon continuous operation might become enriched in potential toxicants that in turn affect the growth of algae. Our research aimed to evaluate 1) if the nutrients obtained in the HT process are accessible by the algae and 2) if organic traces formed during liquefaction and partial gasification as well as trace metals from corrosion under the harsh HT conditions may show toxification effects on algae. In a key preliminary experiment we have demonstrated the feasibility of growing microalgae in a 5 L, flat panel airlift (FPA)-photobioreactor system using the nutrient-rich effluent from the hydrothermal step. The salt fraction contained in the HT effluent is a rather alkaline solution (pH of 9) with high concentrations of total nitrogen (17500 mg L-1) and total phosphorus (~2500 mg L-1). Important anions, cations and micronutrients were also noticed in the aqueous fraction making possible their use as an algal growth media. Biomass productivity with the diluted aqueous solution (25 fold) was comparable to that obtained with the standard growth medium. 1 g(DW) L-1 d-1 of the microalgae 'P. tricornutum' was the productivity average achieved under these conditions. Ammonium and phosphate were used completely by the algal cells. As the growth after addition of the diluted HT solution was kept constant with no inhibition evidence, it can be concluded for this concentration applied, that no toxic effects of possible residual organics were caused using the HT effluent. Concerning the study on the influence of potential toxic trace metals, a test using Aluminum demonstrated that soluble Al (mainly hydroxo-complexes) had a negative effect on algae growth but the extent was not only proportional to the metal concentration but also algae specific. The algal growth curves showed that cell densities were diminished with the addition of Al in the nutrient medium. The chlorophyll fluorescence intensity also increased, indicating that soluble Al caused some dysfunction to the photosystem in microalgae. Concentrations of Al as low as 0.025 mg L-1 were already causing 50% reduction in the growth rate of C. vulgaris while for the other two species studied, the IC-50 was above the highest measured Al concentrations tested. S. vacuolatus growth appeared to be less sensitive to Al exposure. However, Al toxicity could be diminished by adjusting the pH in the nutrient flow to the circumneutral pH region. This strategy allows the formation of insoluble complexes leaving the Al less bioavailable. Although there are further challenges to be overcome, the results obtained here are promising and an important step towards the integrated recycling of water and nutrients in sustainable biofuel production.
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