Added-value processing of ‘algae waste': 3rd quarterly report

Added-value processing of ‘algae waste': 3rd quarterly report

Background - This third report provides an update of information and interim results for a two year study being conducted at the Satake Centre for Grain Process Engineering in the University of Manchester, investigating the feasibility of producing value-added products from algae.

Project Aim - To examine in detail and optimise the production of generic fermentation feedstocks from algae wastes and evaluate the feasibility of producing a range of potential end-products (in particular ethanol and algae nutrients).

Achievements to Date - The materials under investigation were two unextracted dry powders of microalgae sent by Cellana on 17th February 2010 (samples D09 and D10). Various experiments were carried out to understand, control and optimize the glucose production that was observed in previous samples. After several tests it was concluded that none of the indigenous microorganisms were able to hydrolyze chrysolaminarin in diatoms and produce glucose. The glucose production observed in all the microalgae samples received was due to the action of in situ enzymes coming from the algae itself. Several experiments with the algae broth (filtered sterilised) after French press pre-treatment showed that the broth contained enzymes able to convert cellobiose into glucose. Moreover, the filtered broth also contained carbohydrates that could be converted into glucose. More glucose was produced when a pre-treatment such as French press was used.

The glucose produced was consumed by the microorganisms that were isolated and these should therefore seen as a nuisance in the process. The extent of the production depends on the degree of contamination. The highest glucose yield observed was 109 mg glucose/g dry sample at 30°C (56% of total carbohydrates).

The glucose production was optimized and it was found that a temperature of 50 to 60°C with no mixing gave the best yields. Presumably, mixing encouraged the diffusion of oxygen and the proliferation of thermophilic bacteria. Each individual cell contains chrysolaminarin as well as enzymes, therefore no mixing is required to bring them in contact with each other and shear can be detrimental for enzymes.

Having investigated various samples taken at various step at Cellana’s pilot plant in Hawaii (after harvest, after centrifuge and after spray-drying step) it was observed that samples obtained after the spray-drying step were systematically contaminated with bacteria and fungi because these samples were left exposed to dry. This resulted in the growth of unwanted microorganisms and the consumption of glucose which made us realise that glucose must be converted into value-added products before the spray-drying step.

A maximum glucose concentration of 73 g/L was obtained from a sample taken after the centrifuge step at Cellana’s pilot plant. This concentration was obtained by suspending algae in water, leave it for some time to extract the glucose, centrifuge the suspension and adding more algae to the supernatant. The highest conversion of carbohydrates to glucose was found to be 74%, while another test showed that glucose production could take place at high salinity (35 g/L NaCl) and high solid content (a suspension at 30% total solids) which would be the case if the algae suspension was heated to 50°C to release the glucose after the centrifuge step. Lipids staining technique showed that nothing happened to the lipids during this glucose production step. However, after 24 hours at 55°C some proteins were broken down into simple amino acids. A kinetic analysis of the glucose production was performed and the volume of the glucose production unit (a CSTR and plug flow type reactors) was evaluated based on numbers for a full-scale plant. In addition to the glucose production, tests were carried out on the filtration of the algae suspension. The characteristics of the filtration (specific cake resistance and medium resistance) were found and two tests were done on a bench-scale bottom fed rotary vacuum filter with knife discharge to investigate the filterability during continuous operation. Finally, the size of a full-scale rotary vacuum filter was also found.

It was found that an ethanol yeast can grow in the liquid medium extracted from the algae samples. A maximum ethanol concentration of 5 g/l was obtained with sample D10. The ethanol yields were found to be equal to 15 and 12.5 mg ethanol/g algae D10 and D09, respectively. However, ethanol production from these samples is unlikely to be interesting because glucose concentrations in the range 200-250 g/L will not be obtained in the algae medium given the carbohydrates content of the diatoms, except if an evaporation step is included in the process. It was also found that an oleaginous yeast (Rhodosporidium toruloides) could grow in the algae suspension as well as on the filtered broth.

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