Effect of extracellular organic matter (EOM) accumulation on algal proliferation and disinfection by-product precursors during cyclic cultivation†

Algal blooms, driven by nutrient enrichment from nitrogen and phosphorus, pose significant challenges to water treatment processes, particularly due to the accumulation of extracellular organic matter (EOM). This study investigates the impact of EOM accumulation on the growth of Chlorella sp. and Microcystis aeruginosa—during a 36 day cyclic cultivation period, focusing on the effects of bound EOM (bEOM) and dissolved EOM (dEOM) on nutrient uptake and disinfection by-product (DBP) formation. The cultivation period was divided into three phases (R1, R2, and R3), with algal cell counts measured every 4 days using a flow cytometer, while changes in bEOM and dEOM were quantified. Nutrient uptake rates for nitrogen (N) and phosphate (P) were also evaluated per cycle, alongside analysis of critical organic precursors for disinfection by-products (DBPs). Results showed that the N and P uptake rates remained relatively stable for both alga types across all cycles. However, Chlorella sp. cell growth decreased to 20% after the third cycle, whereas M. aeruginosa maintained approximately 80% growth. This significant difference in growth inhibition between Chlorella sp. and M. aeruginosa was closely linked to the rate of bEOM accumulation. M. aeruginosa exhibited a three times faster accumulation rate of bEOM per cell compared to Chlorella sp. after the third cycle, which resulted from fewer remaining nutrients and the significant increase in pH during cyclic culturing. Further analysis revealed that DBPs derived from intracellular organic matter (IOM) were consistently higher than those from dEOM regardless of the cultivation phase. However, the formation potential of trihalomethanes (THMs) and haloacetic acids (HAAs) decreased by approximately 62% and 37%, respectively, for M. aeruginosa, while the formation potential of THMs and HAAs showed a minimal variation for Chlorella sp. In conclusion, bEOM accumulation on the algal cell surface following cultivation significantly impacts phosphate uptake and cell proliferation, particularly in Chlorella sp.