Mechanisms of microalgae-induced flocculation for urban water carbon-negative clarification

Urban rivers or water bodies play an important role in improving the ecological environment. However, many of them are turbid and uncomfortable. In this study, five species of microalgae were isolated from different natural water and used to find their growth potential, flocculation ability, turbidity and nutrient removal ability and the flocculation mechanisms were investigated. The results showed that, three microalgae species, namely, Ankistrodesmus sp. WZ3A4, Coelastrella tenuitheca W6D5, Chlorella sorokiniana W6A3 can efficiently reduce the turbidity in natural water. Optical microscope and SEM observations revealed that they can attach the suspended particles to form flocs and display high turbidity-removal efficiencies (TRE) of 74.7 %, 82.6 %, 93.8 % respectively in natural turbid water. Chlorella sorokiniana W6A3 can even reduce the turbidity from 60.3 NTU to 3.73 NTU. Through EPS component analysis, TRE changes and surface characteristics before and after EPS extraction, Fourier transform infrared (FTIR) spectrum analysis, we found that the high turbidity-removal efficiency of these microalgae species was not closely related to charge neutralization, but greatly associated with the tightly bound EPS and the high relative hydrophobicity. Hydrophobic interaction and the adhesion of extracellular macromolecular polysaccharides have important effects on the flocculation of microalgae in natural turbid water. FTIR showed that microalgae with higher turbidity-removal efficiencies all had stronger stretching vibration near 1135 cm −1 . This study demonstrates that light-driven self-flocculating microalgae can clarify natural turbid water while converting atmospheric carbon dioxide into living biomass through light energy and proposes a novel and carbon-negative technology for restoring urban turbid water environments.