Effect And Mechanism Of Removing Algae From Tap Water By Calcium Carbonate Crystal

Closed water sources such as lakes and reservoirs are prone to seasonal cyanobacteria blooms.Eutrophication of lakes and reservoirs in China is still serious,and blue-green algal blooms occur frequently,posing serious threats to water supply safety,including exceeding the standard of smell,reducing the coagulation effect,blocking the filter,increasing the risk of algal toxins and disinfection by-products.Existing tap water algae removal processes can achieve satisfactory results,but there are also their own bottlenecks.CaCO3 crystallization has been proved to be an efficient and environmentally friendly algae removal technology,but the research on the kinetic process,crystallization mode and algae removal mechanism of CaCO3 crystallization in algae-containing water system is not in-depth enough,resulting in the lack of theoretical guidance for the optimization of operation conditions.In this study,the crystallization process of CaCO3 in algal water and the characteristics of the crystallization products were investigated by changing the reaction conditions,the type of algal source organic matter(AOM)and the density of algae,and the mechanism of algal removal by calcium carbonate crystallization was preliminarily explored in combination with the algal removal efficiency,providing ideas for optimizing the algal removal conditions by calcium carbonate crystallization.The main conclusions are as follows:(1)The effect of algal cells on the crystallization kinetics of CaCO3 showed a law of"low promoting and high inhibiting",that is,low SI promoted the heterogeneous crystallization of CaCO3 on the surface of algal cells,while high SI inhibited the homogeneous crystallization of CaCO3 on the main body of the liquid phase.When SI<0.71,the crystallization of CaCO3 in algal water is mainly heterogeneous crystallization on the surface of algal cells.When 0.71<SI<1.82,the homogeneous and heterogeneous crystallization of CaCO3 occurs synchronously,showing a composite crystallization mode.When SI>1.82,the crystallization process of CaCO3 is almost not affected by algal cells and is dominated by homogeneous crystallization.(2)CaCO3 crystal has good algal removal effect,and the algal removal rate increases with the increase of SI.When SI exceeds 1.52,the removal rate of algae is higher than 83.3%.When 1.52<SI<1.86,the removal efficiency of algae was the best,which was embodied in the high removal rate of algae cells ranging from 83.3%to 90%,and the effluent turbidity below 10 NTU.(3)The algal removal mechanism of CaCO3 crystal mainly includes adsorption electric neutralization and net capture and sweep.From the reaction time scale,the algal removal mechanism changes with the reaction time.In the early stage(～10 min),the adsorption electric neutralization of crystal products was the main process.With the extension of reaction time,the number of crystal products increased,and the algae removal mechanism changed to the combined effect of adsorption electric neutralization of crystal products and net capture and sweep.(4)The increase of ionic strength and C/Ca in water can inhibit the heterogeneous crystallization of calcium carbonate crystals.Heterogeneous crystallization is more beneficial to algae removal than homogeneous crystallization.With the inhibition of heterogeneous crystallization,the removal rate of algae cells decreased significantly from 30%to 8.67%.Homogeneous crystallization was the main reason for the rise of turbidity of supernatant,and the turbidity of supernatant dominated by homogeneous cry stall ization increased by 53.90%compared with that dominated by heterogeneous crystallization.(5)Both AOM and algal cell wall have Ca2+adsorption sites,and algal cell wall has a better role in promoting calcium carbonate nucleation than AOM.The Ca2+ adsorption capacity provided by the cell wall of algae was 11.84 mg/L,which was significantly higher than that of AOM(5.56 mg/L).These adsorption sites were beneficial to induce the formation of calcium carbonate on the surface of algae cells,and promoted the aggregation of algae cells and led to the destabilization and precipitation of algae cells through the effects of adsorption electric neutralization and net trapping.