Effect of nutrient starvation on nutrient uptake and extracellular polymeric substance for microalgae cultivation and separation

The purpose of the present study is to investigate the effects of nutrient starvation of microalgae on its nitrogen (N) and phosphorus (P) uptake, characteristics of extracellular polymeric substances (EPS), and algal sedimentation. An experiment was carried out by starving the wastewater-tolerant microalgae Chlorella sp. ADE4. The algal cultivation was put under various conditions of nutrient starvation in order to enhance nutrient removal and algal cell separation. The experimental results showed that 40 h of nutrient starvation prior to the cultivation did affect nutrient removal of Chlorella sp. ADE4. When using synthetic wastewater, the N-starved algae was the most effective in removing 82% of N in 48 h and 92% of P in 24 h. However, the starvation conditions did not cause noticeable removal improvement when microalgae were tested with real wastewater effluent. N and P removal efficiencies of 57 and 100%, respectively, were achieved in 48 h in real wastewater effluent. The lower N removal efficiency was caused by P limitation in the real sewage effluent. EPS were analyzed to evaluate if they play a role in algal cell agglomeration and subsequent microalgal separation. Carbohydrates and protein were indicated as major components in soluble and bound EPS. It was found that starvation of microalgae for 40 h could induce higher EPS production. Interestingly, the N-starved microalgae contained a large protein fraction in their EPS and low N content in their biomass. However, a significant correlation between EPS content and sedimentation efficiency was not observed in this study.