Evaluating the benefits of permeate recycling in a photobioreactor using multi-component, community-level modeling

While biodiesel production from photosynthetic algae is a promising form of non-fossil energy, the process often has high water and nutrient demands. We developed a mathematical model for a photobioreactor system in which the photobioreactor's effluent is membrane filtered, with most of the permeate returned to the photobioreactor (PBR). The model considers light attenuation and inhibition, nutrient limitation, preference for ammonia consumption over nitrate, production of soluble microbial products (SMP) and extracellular polymeric substance (EPS), and the growth and impacts of heterotrophic bacteria that consume SMP. Increasing the rate of permeate recycling improves water- and nutrient-use efficiencies, but it also leads to an accumulation of more SMP in the photobioreactor and higher concentrations of heterotrophs. Increasing the dilution rate leads to increased production of photoautotrophs due to a lowering of the biomass concentration, thus increasing LI (light intensity) and μ (maximum specific growth rate). A high-enough dilution rate washes out the heterotrophs, leaving an almost mono-culture of photoautotrophs, because the heterotrophs do not benefit from the higher LI. Using high D (dilution rate) and re (ratio of permeate flow to total throughput) together gains the phototroph-productivity benefits of high D together with the substantially smaller water demand of high re. The best water- and nutrient-utilization efficiencies were associated with high re alone, but high D (alone or with high re) enriched the biomass in photoautotrophs.