Effects of harvesting cell density, medium depth and environmental factors on biomass and lipid productivities of Chlorella vulgaris grown in swine wastewater

A regression model was developed to determine the growth rate of Chlorella vulgaris that is affected by the environmental factors of temperature, light intensity and pH value. The optimum environmental condition for growing C. vulgaris was experimentally determined at light intensity of and pH of 7.4. At the optimum environmental condition, the growth rate of C. vulgaris in swine wastewater with 102 mg N/l and 76 mg P/l was 0.160 g/l/day, compared to 0.191 g/l/day for its growth on a modified Bold's medium with 100 mg N/l and 53 mg P/l. The regression model was further integrated with a light and heat transfer model to estimate the biomass productivity of C. vulgaris grown on the swine wastewater in an open raceway pond (ORP) with different medium depths and harvesting cell densities under the weather condition in North Carolina yearly around. At 20 cm medium depth, the highest growth rate was 0.162 g/l/day, which was obtained at 0.1 g/l harvesting cell density, in June. If the harvesting cell density increased to 0.4 g/l, the highest growth rate decreased significantly to 0.033 and 0.02 g/l/day for 20 cm and 30 cm medium depths, respectively. At 0.1 g/l harvesting cell density, the yearly algal productivity was 80 and 59 t/ha at 30 cm and 20 cm medium depths, respectively. At the average 25% lipid content of C. vulgaris grown in swine wastewater, the highest lipid yield was 20 and 14.75 t/ha/year at 30 cm and 20 cm medium depths, respectively.