Microalgae spontaneously convert CO2 and supplementary nutrients into biomass in the presence of light via photosynthesis, and at much higher rates than convention oil-producing crops. Algal biomass can then be transformed into methane via anaerobic bacteria-mediated fermentation, or to biodiesel via lipid extraction, as well as other byproducts of secondary metabolism. Production of biofuel by microalgae can be made more sustainable through coupling microalgal biomass production with existing power generation and wastewater treatment infrastructure. On the other hand, integration of algal biofuel production into wastewater treatment plant anaerobic digestion infrastructure has the potential to increase biogas production, decrease high and variable internal nitrogen loads, and improve sludge digestibility and dewaterability.
The overall goals of this research were to investigate growth and nutrient uptake of two algal species, Spirulina platensis and Chlorella sp. in various growth conditions, while utilizing synthetic or real sludge centrate, or a mixture of centrate and nitrified effluent as a growth medium. Harvested algae were co-digested with wastewater activated sludge (WAS) at varying algae/WAS ratios and anaerobic digester performance was evaluated.
Under batch conditions, two species of microalgae, Spirulina platensis and Chlorella sp., were grown on sludge centrate and a centrate/nitrified wastewater effluent mixture. Harvested algae were co-digested with waste activated sludge (WAS) at varying ratios. High growth (6.8 g m-2 d-1), nitrogen (36.5 g m-3 d-1 ) and phosphorous (6.5 g m-3 d-1 ) uptake rates were achieved with Chlorella on centrate. No growth was observed with S. platensis under the same conditions; however, both organisms grew well on the centrate/effluent mixture. Co-digestion of algae with WAS improved volatile solids reduction. Co-digestion with S. platensis improved biosolids dewaterability; however, Chlorella had a slight negative impact on dewaterability compared to WAS alone.
Under continuous flow conditions, Spirulina platensis was cultivated in a benchscale airlift photobioreactor using synthetic wastewater (total nitrogen 412 mg/L, total phosphorous 90 mg/L, pH 9-10) with varying ammonia/total nitrogen ratios (50-100% ammonia with balance nitrate) and hydraulic residence times (15-25 d). High average biomass density (4,200 mg/L) and productivity (5.1 g m-2 d-1 ) were achieved when ammonia was maintained at 50% of the total nitrogen provided. Both high ammonia concentrations and mutual self-shading, which resulted from the high biomass density in the airlift reactor, were found to partially inhibit the growth of S. platensis. The performance of the airlift bioreactor used in this study compared favorably with other published studies.