The influence of light intensity and organic content on cultivation of Chlorella vulgaris in sludge extracts diluted with BG11

The cultivation of Chlorella vulgaris in sludge extract (SE) from municipal excess sludge diluted with BG11 medium was investigated to achieve sludge reduction and microalgae cultivation. A maximum biomass of 1.404 ± 0.101 g/L was achieved at 15 days in a mixture of 25% BG11 and 75% sludge extract under 10,000-lx light. In early culture periods, heterotrophy dominated the process in the group with a high proportion of sludge extract of excess sludge extract containing relatively high amounts of organic material, which led to enhanced citrate synthase enzyme activities and reduced rubisco enzyme activities. Moreover, increasing the light intensity did not significantly improve biomass production due to its inhibitory effects on respiration in the early culture stages. The response surface model showed that organic matter was the main factor influencing biomass production from 0 to 2 days. The influence of light intensity on biomass production became more noticeable only after depleting the organic matter in sludge extract.