Page 181 - Refining Biomass Residues for Sustainable Energy and Bioproducts

154 Refining Biomass Residues for Sustainable Energy and Bioproducts

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in units of micromoles per square meter per second (mmol/m /s), where 1 mol
of photon equals 6.022 3 10 23 photons. The light source and intensity, and the
different vessel geometries used during the cultivation, result in a quite different
proton delivery. Light can be considered as a substrate-limiting algae growth.
Thus the typical Monod model can be applied to describe the phases of growth
resulting from changes in the specific growth rate as a function of limiting sub-
strate concentration as shown by Clark et al. (2018). These researchers studied
the cyanobacteria growth where the light was kept as the sole limiting substrate,
eliminating limitations of CO 2 and soluble nutrient availability. In their model
the growth phases of light-limited growth were determined by changes in the
spatial irradiance distribution (IzðÞ) inside the culture vessel, as described in the
following equation:

IzðÞ 5 I IN e 2αXz (7.5)

where I IN is the irradiance of photons on the surface of the vessel, z is the path
length (m), X is the biomass density (g/L), and α is the biomass specific absorbance
2
(μmol/m /s/g/L) of photons.
For low values of biomass density and irradiance below the photoinhibition
level, the Monod growth model is used to fit the data [Eq. (7.6)] and gives the
21
values for maximum specific growth rate (μ max Þ (h ) and the saturation irradiance
2
(K v Þ (μmol/m /s).
μ I
μ IðÞ 5 max (7.6)
K v 1 I

The consideration of light as a substrate in the Monod model is different with
respect to that of a molecular substrate for two reasons. Light is provided continu-
ously, as opposed to a molecular substrate which can be depleted as the growth pro-
ceeds; and, irradiance reaching the cells varies spatially due to absorbance by the
cells (cell-shading phenomenon), in contrast to a molecular substrate where concen-
tration is considered constant in spatial dimensions (Clark et al., 2018).
The use of turbid waste streams for the formulation of culture media can hinder
light penetration. Under such conditions, algae metabolism could shift toward het-
erotrophy provided that the wastewaters are rich in organic carbon. The spatial irra-
diance distribution (IzðÞ)[Eq. (7.5)] inside the culture vessel can be incorporated
into the Monod model together with other independent variables limiting the
growth, that is, molecular substrates, such as nitrogen, phosphorus, and organic car-
bon. The model can then be used to predict the algae growth under conditions of
light limitation and nutrients replenishment.
Light-limiting conditions can be extremely challenging for vital high energy to
cell growth, where cells attain a critical size and content of essential constituents,
including energy reserves.