(513b) Remediating Corn Ethanol Waste Stillage with Chlorella vulgaris

Corn Ethanol industries require energy and cost intensive operations for the treatment of corn ethanol generated stillage before safe disposal to the environment. The purpose of this study is to observe the viability of introducing the microalgae Chlorella Vulgaris to a pond simulated environment and characterize its growth on nutrients derived from the stillage and Bold’s Basal Medium (BBM) which contains required nutrients such as NPK. The Corn Ethanol stillage contains significant amounts of Nitrogen in various forms along with considerable organic matter presence indicated by COD tests, both of which are good sources of nutrition for Chlorella Vulgaris.

Growth conditions for the cultivations of C. Vulgaris were first screened in mini-photo bioreactors with working volumes of 35 mL and average light intensity of 100 μmol m^-2 s ^-1 incident to the light source facing lateral surface area of the reactors. Air flow to reactors was averaged over seven days to 1.49 min^-1 Vessel volumes per min (VVM). Change in Nitrate (mmol L^-1) and COD concentrations (mg L^-1) were observed and growth was characterized by spectrophotometric absorbance and biomass density (mg L^-1). Based on the results, we scaled up experiments to working volumes of 650 mL in a Phenometrics PBR. The culture to media volume ratios were kept the same (1:9), the vvm was kept at 1.27 min^-1 to match the 35 ml PBRs. The incident light intensity on the surface of the reactor volume was set to 1500 μmol m^-2 s ^-1 and this was kept constant by maintaining the initial reactor volume via addition of 25 ml of fresh media daily to account for sampling volumes (25ml) extracted. Experimental variables were the growth in illuminated environment with light intensity of 1500 μmol m^-2 s ^-1 and dark environment, culture media composition was tweaked to either include or exclude the presence of nitrogen to observe the nitrogen consumption, solely on the stillage, and Additional control experiments were performed without algae to determine the effects of aeration and light. The Change in nitrate concentration (mmol L^-1), the change in absorbance (at 680nm), The growth of biomass density (g L^-1), the decrease in COD (mg L^-1)and the decrease in total Nitrogen (mg L^-1) were sampled daily over a seven day period. Using these measures growth was characterized on the basis of biomass productivity (P, g L^−1 hr^−1), specific growth rate (μ, hr^-1), Removal Efficiency, R.E. (%) and elimination capacity (mg L^−1 hr^−1).

We observed the highest Removal Efficiency of 70.6 % for N₂, 87.2 % for COD for C. Vulgaris in the culture environment with light and increased nitrogen concentration. The removal efficiencies of COD was similar for both light and dark environments which suggests COD assimilation is isn’t dependent on light intensity. The highest overall elimination capacities for N₂ (0.65, 0.74 mg L^−1 hr^−1) and COD (7.38, 7.57 mg L^−1 hr^−1) were achieved by the C. Vulgaris in the culture environments with increased nitrogen concentration (dark, light). The specific growth rate over seven days was observed to be highest for C. Vulgaris in the culture environment with light and increased nitrogen concentration (0.0136 hr^-1) and was almost 70 % higher than the next highest values. Plotting the biomass productivity of C. Vulgaris in different cultivation environments indicated a more sustained productivity for the C. Vulgaris in the culture environment with light and increased nitrogen concentration.