Microalgal culture with digestate from methane fermentation - light environment in the culture solution with different digestate concentrations and microalgal cell densities

Abstract

The light intensity decreases logarithmically in the solution according to Lambert- Beer’s law that is given by equation P2 = P1 exp(-b(Z2-Z1)). Absorption coefficient is expressed as (b) = (ln (P1)-ln (P2))/(Z2-Z1), where P1 and P2 are photosynthetic photon flux densities (PPFDs) at depth Z1 and Z2, respectively, in the culture solution. The light absorption coefficients of culture solutions with different digestate concentrations and different microalgal densities were determined by spectrophotometer in the wavelength range of photosynthetic active radiation (400-700 nm). A linear regression was obtained between the absorption coefficient (cm-1) and digestate concentration (%) expressed as bdigestate = 0.0546 × "digestate concentration" + 0.005. A linear regression was also obtained between the absorption coefficient and the microalgal density (cells ml-1) expressed as bmicroalgae = 0.0655 × "microalgal density" + 0.0402. In simulation experiment conducted with microalgal density of 30×105 cells ml-1, more than 10% of light was transmitted at the depths shallower than 15 mm, using 20% diluted digestate.