Controls of Microalgal Biomass and Lipid Production In Municipal Wastewater-Fed Bioreactors

The use of wastewater as a nutrient source for algal growth as a biodiesel feedstock could increase the viability of this process and reduce wastewater discharge of N and P compounds.  In this study, four algal bioreactors were fed continuously with municipal wastewater effluent over a six month period; these were monitored weekly to determine algal biomass and lipid production, nutrient removal, and carbon and nutrient availability.  Algal biomass production ranged from 0.78 to 15.9 g dry weight m^-2 d^-1 over the experimental period.  Algal reactors removed 19% of dissolved nitrogen and 43% of dissolved phosphorus from wastewater effluent.  A stoichiometric analysis of particulates revealed that algal biomass had far lower C:P ratios (67:1) and N:P ratios (6:1), but higher C:N ratios (17:1), than Redfield values.  These data suggest that the algae produced were rich in phosphorus, and that N was likely to be growth-limiting in these systems.  Numerous studies also suggest that algal biomass can be enhanced by providing CO₂ in excess of normal atmospheric concentrations, and that algal biofuel production can contribute to CO₂ sequestration efforts.  In order to optimize nutrient removal in wastewater-fed algal reactors, and to determine the potential for CO₂ sequestration, future reactor engineering research should continue to incorporate the expertise of ecological stoichiometry.