The hydrodynamic behavior of diatom aggregates has a significant influence on the interactions and flocculation kinetics of algae. However, characterization of the hydrodynamics of diatoms and diatom aggregates in water is rather difficult. In this laboratory study, an advanced visualization technique in particle image velocimetry （PIV） was employed to investigate the hydrodynamic properties of settling diatom aggregates. The experiments were conducted in a settling column filled with a suspension of fluorescent polymeric beads as seed tracers. A laser light sheet was generated by the PIV setup to illuminate a thin vertical planar region in the settling column, while the motions of particles were recorded by a high speed charge-coupled device （CCD） camera. This technique was able to capture the trajectories of the tracers when a diatom aggregate settled through the tracer suspension. The PIV results indicated directly the curvilinear feature of the streamlines around diatom aggregates. The rectilinear collision model largely overestimated the collision areas of the settling particles. Algae aggregates appeared to be highly porous and fractal, which allowed streamlines to penetrate into the aggregate interior. The diatom aggregates have a fluid collection efficiency of 10%-40%. The permeable feature of aggregates can significantly enhance the collisions and flocculation between the aggregates and other small particles including algal cells in water.
Raw water from the Songhua River was treated by four types of coagulants, ferric chloride （FeC13）, aluminum sulfate （A12（504）3）, polyaluminum chloride （PACl） and composite polyaluminum （HPAC）, in order to remove dissolved organic matter （DOM）. Considering the disinfection byproduct （DBP） precursor treatability, DOM was divided into five chemical fractions based on resin adsorption. Trihalomethane formation potential （THMFP） and haloacetic acid formation potential （HAAFP） were measured for each fraction. The results showed that hydrophobic acids （HoA）, hydrophilic matter （HIM） and hydrophobic neutral （HoN） were the dominant fractions. Although both HoN and HoA were the main THM precursors, the contribution for THMFP changed after coagulation. Additionally, HoA and HiM were the main HAA precursors, while the contribution of HoN to HAAFP significantly increased after coagulation. HoM was more easily removed than HiM, no matter which coagulant was used, especially under enhanced coagulation conditions. DOC removal was highest for enhanced coagulation using FeCl3 while DBPFP was lowest using PAC1. This could indicate that not all DOC fractions contained the precursors of DBPs. Reduction of THMFP and HAAFP by PAC1 under enhanced coagulation could reach 51% and 59% respectively.
Yanmei ZhaoFeng XiaoDongsheng WangMingquan YanZhe Bi
