Added-value processing of ‘algae waste': 4th quarterly report

Added-value processing of ‘algae waste': 4th quarterly report

Background - This fourth and last report provides the design data for the construction of a full-scale algae plant. These results were obtained during one and a half year study being conducted at the Satake Centre for Grain Process Engineering in the University of Manchester. The study investigated the feasibility of producing value-added products from algae.

Executive summary - The gathering of data from previous reports has been used for the detailed design of a full process pilot plant to utilise the algae wastes. This report is divided in two main sections. The first one details further experiments carried out to obtain the necessary data or information for the design of the full scale plant and the second section consists in a manual for the use of the Excel file where the process has been modelled.

In the experimental section a set of experiments was carried in aerobic and anoxic conditions to see the effect on glucose production. It was shown that it is better to keep the system closed so that we avoid contamination by airborne bacteria. If the system is kept close and there is no contamination then there is no need to flush the headspace with nitrogen. It was also previously shown that mixing was not required and could even be detrimental because it would enhance the transfer of oxygen to potential microorganism. For these reasons it is more sensible to use a plug flow type of reactor instead of a stirred tank. Both systems have however been sized in this report, and it is known that a plug flow would always give a higher conversion than a CSTR for the same volume.
It was also shown that glucose production could not be obtained with solvent-extracted samples which definitely confirm that the process should be carried out before lipids are extracted. The 55°C step was shown to facilitate the subsequent lipid extraction. A maximum lipid yield from algae was 39.85 % on a dry basis. The 55°C step resulted in the breakdown of some proteins which means that about 10 % of total nitrogen ended up in the liquor together with the glucose. This nitrogen will be useful for the growth the oleaginous yeast that will fix the nitrogen into yeast proteins. The residual free amino nitrogen and glucose after the fermentation could be recycled to the pond, so there would be no loss. A test with a rotary disk vacuum filter was carried out to show the feasibility of continuous filtration system. This type of filter gave slightly better results than the vacuum drum filter. The scale up of the process was demonstrated but it was found that contamination will be an issue in the rotary vacuum filter even though we placed it in a box flushed with nitrogen. Therefore, we would recommend keeping the rotary vacuum at 55°C as well.

As far as the microbial oil fermentation step is concerned it was shown that R. turoloides could grow in the glucose rich liquor obtained after the 55°C step and that the yeast could grow without any significant difference in dry cell weight at various concentrations of NaCl up to 55 g/L (seawater: 35 g/L). In fact the microscopic pictures revealed that the cells and the lipid vacuoles were bigger at higher salinity. The growth of the yeast was confirmed in non-sterile environment where bacteria were present.
In the second section of this report the structure of the Excel file is explained. It provides a mass balance and the dimensions of the main units of the flowsheet.

Confidential.