Lipid production from algae grown on wastewater under nitrogen limitation : Opportunities for energy recovery

Daily consumption of energy reduces the life span of non-renewable energy sources, so finding an alternative has become imperative. Algae have proven their potential to be a renewable energy source, especially for biofuels. This is due to the lipid production that is faster and more efficient than by other plants. Previous studies investigated the incorporation of microalgae into wastewater treatment plants (WWTPs) as a cost-effective biological approach. These studies proved the feasibility of this synthesis to save up to 70 % of WWTP’s energy supply and generate a renewable source of biodiesel. As the global interest nowadays is to develop different approaches for biodiesel generation, different strategies are being applied and investigated to enhance the lipid yield by microalgae. For this reason, the main objective of this thesis is to take advantage of microalgae-bacteria based systems and exploit them to trigger lipid production under nitrogen limitation. A 24 h cycle of light and dark phases was implemented in two photo sequencing batch reactors (PSBRs) at room temperature and illumination of 100-200 μmole/m2.s. It was fed by artificial wastewater of 50 mg NH4+-N /L and a complex microalgae culture from IHE laboratory strains and from a canal in Delft with nitrifiers from activated sludge. Furthermore, the concentrations of NH4+-N, NO3--N, NO2--N, TN, TIC, TOC, sCOD and DO were evaluated and manipulated to achieve the highest production of lipids in microalgae cells. Both PSBRs achieved 100 % ammonium removal with an average rate of 4.4 mg/L.h after 3 weeks. Denitrification was triggered by adding sodium acetate to PSBR 1, as the initial concentration was insufficient, resulting in total nitrate removal. At the same time, nitrate concentration remained higher than 30 mg NO3--N/L in PSBR 2 because it was not supplied with an external organic carbon source. After 7 weeks, PSBR 1 was modified by adding an extra phase of nitrogen limitation and lipid was extracted by the Soxhlet method to measure lipid accumulation before and during nitrogen stress conditions. However, the available biomass was insufficient to get conclusive results on lipid accumulation