Study On The Flocculation Of HAB Organisms By Particles And Its Fractal Numerical Simulation

Harmful algal blooms (HABs) have occurred frequently along many coasts of the world as a result of eutrophication due to the development of economy over the past several decades. HABs destroy the ecological balance, cause the enormous economic losses and the death of fish and shellfish, and even do harm to human beings by transport toxin through food chain. Studies on the effective methods of controlling HABs have important scientific meaning and application prospects. Therefore, it is of great significance to study the flocculation of HAB organisms by particles and its fractal numerical simulation.Experiment and numerical simulation were carried out to study the flocculation kinetics of HAB organisms by clay and other particles. The relation between Removal efficiency (RE) and physical and chemical features of different clays types were studied, and numerical simulation were carried out to evaluate factors influencing coagulation rate and RE of clay flocculating HAB organisms. The relation between fractal dimensions and RE were analyzed, and kinetics curves between experimental results and simulation results of clay flocculating HAB organisms using the calculated fractal dimensions were compared. Flocculation of HAB organisms by modified wheat clay was studied. The main results are given as follows:The relationships between RE and physical and chemical features of different clays types were studied. Resulted showed that surface potential differed from -36.1 to 15.9mV, BET (Brunauer-Emmett-Teller) surface area differed from 0.31 to 60.76m2.g-1, and the Si/Al ratio differed from1.35 to 3.62. RE differed from 13% to 82% with different clay types at the same loading of 0.50g.l-1.Strong linear relationships between RE and surface potential, BET surface area, Si/Al ratio were showed. High surface potential, BET surface area and low Si/Al ratio resulted the increase of RE. Analysis of grey correlation showed that the BET surface area had significant effect on RE and the correlative degree was 0.907, while the Si/Al ratio had minor effect on RE and the correlative degree was 0.618.Numerical simulations of clay flocculating HAB organisms were calculated. Based on the traditional coagulation theory and fractal theories, Factors influencing RE and clay collision rate were simulated using FORTRAN program. Result showed that RE increased from 16% to 78% as collision efficiency between clay particles and algae cells increased from 0.01 to 0.20, RE increased from 38% to 82% as clay concentration increased from 2.0×10-4g.cm-3 to 1.0×10-3g.cm-3, and RE increased from 56% to 88% as fractal dimensions of clay decreased from 2.80 to 2.00. Higher collision efficiency between clay particles and algae cells, high clay concentrations, lower collision efficiency between clay particles, lower fractal dimensions of clays and lower algal cell density higher would increase RE. Moderate shear rate and fractal dimensions of algal organisms would increase RE. Among the factors, collision efficiency between clay particles and algae cells, fractal dimensions of clay and clay concentrations had the most important effect on RE. Clay modified with PACl would increase the collision efficiency between clay particles and algae cells, and decrease the fractal dimensions of clays, which resulted in high RE. The increase of TEP(Transparent exopolymer particles)concentrations would increase the collision efficiency between clay particles and algae cells. The concentrations of TEP related with the species of algal cells, growth stage, and nutrient conditions. During the decline phase and nutrient deficit, the concentrations of TEP were higher, which resulted in a higher RE.The relationships between RE and fractal dimensions of flocs between clay particles and HAB organisms (Skeletonema costatum, Heterosigma akashiwo and Alexandrium tamarense) were studied. Skeletonema costatum had high RE, while Alexandrium tamarense had low RE. For all three species, modified clay had high RE than original clay. The fractal dimensions were calculated using the coulter counter and the image analyzer. Result showed that the clay modified with PACl had lower fractal dimensions with two-dimensional fractal dimensions(D2) of 1.84 and three-dimensional fractal dimensions(D3) of 2.50, while the original clay had higher fractal dimensions with D2 of 1.92 and D3 of 2.81. Modified clay had lower fractal dimensions and higher collision efficiency between clay particles and algae cells, which resulted high RE. The fractal dimensions of flocs and RE differed between HAB species for the difference of morphological characteristics between algal cells. The cell of Alexandrium tamarense has a shape of cylinder, and the fractal dimensions of flocs between clay particles and HAB organisms were highest with D3 of 2.59 for original clay and 2.32 for modified clay, while for Skeletonema costatum, the shape of the cells is filament and the fractal dimensions were lowest with D3 of 2.27 for original clay and 1.97 for modified clay. The kinetics curves were simulated using calculated fractal dimensions at the clay loading of 0.50g.l-1. Comparisons of kinetics curves between experiment and simulation showed basically in accordance.Studies on the flocculation of HAB organism by other particles were carried out. Modified wheat straw (MWS) were prepared and used to investigate the flocculation of Heterosigma akashiwo. The results showed that under the same loading of 0.10 g.l-1, the algal removal efficiency could reach 80% by MWS in 120 minutes, while 10% by the unmodified wheat straw. The nucleotides were released, which meant disruption of the cytoplasmic membrane. More than 15% of the nucleotides were released from the cytoplasm under the effect of 0.15 g.l-1 of MWS, indicating the irreversible damage on the cellular membrane, which resulted in the disintegration of the harmful algal cells. It is suggested that the low concentration of MWS might adsorb onto the algal cell surface and bind to the cytoplasmic membrane, which resulted in the flocculation, while the high concentration might cause the penetration into the cellular membrane, which resulted in the leakage of cytoplasm material and hence the death of cell.The work revealed that the physical and chemical features and fractal dimensions of particles, and morphological characteristics of algal cells affected RE of HAB organisms. In the control of flocculation HAB organisms using particles the fractal dimensions and other influencing factors should be taken into consideration.