Large scale flat panel airlift photobioreactor for high density cultivation of vegetative cell, Haematococcus pluvialis

Abstract

The cultivation of green vegetative cells of Haematococcus pluvialis NIES-144 was carried out in flat panel airlift photobioreactors (FP-ALPBRs) of different sizes, i.e. 17, 50, 90 and 200L, operated in the evaporative room where the temperature was controlled at 27±3℃. Normal cultivation was carried out with a superficial gas velocity (U[subscript sg]) of 0.4 cm s-1 mixed with 1%vol CO₂, and a ratio of downcomer and riser cross sectional area (A[subscript d] / A[subscript r]) of 0.4. The hydrodynamic characteristics for FP-ALPBR system, i.e. liquid velocity and overall volumetric gas-liquid mass transfer, were also investigated. The hydrodynamic study reveals that the system with (A[subscript d] / A[subscript r]) of 0.4 provided the lowest liquid velocity for 17L ALPBR system (U[subscript l] ranged from 1.2-6.3 cms⁻¹), and the greatest amount of overall volumetric mass transfer coefficient (K[subscript L]a ranged from 0.000358 to 0.0066 s⁻¹). This condition provides the lowest shear stress which makes it suitable for the green vegetative stage of H. pluvialis. Several alternative growth options were proposed in order to enhance mass production of green vegetative cell of H. pluvialis and also to cut down the total operating cost for such algal cultivation. The use of natural lighting was inevitable to avoid the high electricity cost, and replacing artificial lighting with natural light was found to decrease the total production cost by as much as 307 US$ per 0.5 kg dry cell in the 50L FP-ALPBR. Nevertheless, the lack of control of diurnal light intensity resulted in a drop in the growth performance with cell density decreasing from 387 x 104 to 140 x 104 cell mL⁻¹, and specific growth rate from 0.63 to 0.53 day⁻¹. Reactor sizes (under natural light operation) appeared to be significant for the profitability of the system, and enlarging the FP-ALPBR from 17 to 200L required significantly lower total costs of production per year (121 US$ per 0.5 kg dry cell for 200L culture when compared to 197 US$ per 0.5 kg dry cell for the 17L system). Unfortunately this had to be compensated by a drop in the growth performance with cell density decreasing from 290 x 104 to 147 x 104 cell mL⁻¹ and specific growth rate from 0.49 to 0.47 day⁻¹. Finally, the reuse of spent medium (under natural light operation) with proper replenishment of nutrients caused an unexpected around 30 % drop in the growth rate and did not seem to provide an attractive response as the total cost per 0.5 kg dry cell was only saved by 8 US$ a year.