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The use of microalgae culture technology (MCT) for mitigating CO 2 emissions from flue gases and nutrient discharges from wastewater whilst generating a biofuel product is considered with reference to the cost benefit offered. The review examines the most recent MCT literature (post 2010) focused on the algal biomass or biofuel production cost. The analysis reveals that, according to published studies, biofuel cost follows an approximate inverse relationship with algal or lipid productivity with a minimum production cost of $1 L-1 attained under representative conditions. A 35-86% cost reduction is reported across all studies from the combined harnessing of CO 2 and nutrients from waste sources. This compares with 12-27% for obviating fertiliser procurement through using a wastewater nutrient source (or else recycling the liquor from the extracted algal biomass waste), and 19-39% for CO 2 fixation from a flue gas feed. Notwithstanding the above, economic competitiveness with mineral fuels appears to be attainable only under circumstances which also feature: a) the inclusion of cost and environmental benefits from wastewater treatment (such as the energy and/or greenhouse gas emissions benefit from nutrient and CO 2 discharge abatement), and/or b) multiple installations over an extended geographic region where flue gas and wastewater sources are co-located.

BACKGROUND: The application of Chlorella vulgaris for simultaneous CO 2 biofixation and nutrient removal has been optimised using response surface methodology (RSM) based on Box Behnken design (BBD). Experimental conditions employed comprised CO 2 concentrations (C c,g ) of 0.03-22% CO 2 , irradiation intensities (I) of 100-400 E, temperatures of 20-30 ∘ C and nutrient concentrations of 0-56 and 0-19 mg L −1 nitrogen and phosphorus, respectively, the response parameters being specific growth rate , CO 2 uptake rate R c and %nutrient removal. RESULTS: Over 10 days the biomass concentration reached almost 3 g L −1 for C c,g of 5% CO 2 , with corresponding values of 0.74 g L −1 day −1 and 1.17 day −1 for R c and , respectively, and 100% nutrient (N and P) removals. At 22% CO 2 the R c and decreased by around an order of magnitude, and nutrient removal also decreased to 79% and 50% for N and P, respectively. CONCLUSION: Optimum values 5% CO 2 , 100 E and 22 ∘ C were identified for C c,g , I and T, respectively, with and R c reaching 1.53 day −1 and 1 g L −1 day −1 , respectively, along with associated nutrient removal of 100%. Regression analysis indicated a good fit between experimental and model data.

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A comprehensive mathematical modeling method for chlorella vulgaris (Cv) has been developed to assess the influence of nutrient concentration (N = 28-207 and P = 6-8 mg L-1) and UV irradiation intensity (I = 100-250 μE) at feed gas CO 2 concentrations (C c,g) of 0.04-5 %. The model encompasses gas-to-liquid mass transfer, algal uptake of carbon dioxide (C d), nutrient removal efficiency (RE for N and P with reference to total nitrogen TN and total phosphorus TP), and the growth biokinetics of Cv with reference to the specific growth rate µ in d-1). The model was validated using experimental data on the Cv species growth in an externally illuminated photobioreactor (PBR). The fitted parameters of the model were found to be in good agreement with experimental data obtained over the range of cultivation conditions explored. The mathematical model accurately reproduced the dynamic profiles of the algal biomass and nutrient (TN and TP) concentrations, and light attenuation at different input C c,g values. The proposed model may therefore be used for predicting algal growth and nutrient RE for this algal species, permitting both process optimization and scale-up.