Performance Of Algae Removal And Micropolluted Raw Water Treatment By Diatomite-enhanced Coagulation Process

In recent years, water pollution is serious in China. Eutrophication and algae blooms have become the threat to drinking water quality, which potentially harm the human health. It is of great significance to develop efficient and economic water treatment process for enhancement of coagulation and algae removal. Based on the coagulation process widely applied in drinking water treatment, diatomite is chosen to enhance the performance of conventional aluminum salt coagulants such as polyaluminum chloride (PAC) and aluminum sulfate (Al2(SO)3), in order to improve the pollutant removal of polluted raw water. The dosage and ratio of diatomite and aluminum salts are optimized, and the effect of environmental factors on enhanced coagulation is studied. At the same time, the mechanism of diatomite enhanced coagulation is focused on and a coagulation-sedimentation equipment in pilot-scale is set up to study the performance of enhanced coagulation in engineering application on polluted raw water. The main contents are presented as follows:1) Because PAC and Al2(SO4)3 are the most common aluminum salt coagulants in drinking water treatment, the study of diatomite-enhanced coagulation of aluminum salts was carried out using Microcystis aeruginosa suspension as research object. The experiment results showed that the diatomite favored the formation of flocs. In comparison, the combination of Al2(SO4)3 and diatomite achieved the better coagulation performance of polluted raw water treatment. At the ratio of 1:1, and 40mg/L total amount of A12(SO4)3 and diatomite, the average algae removal reached over 95%. The algal removal efficiency and coagulation ability improved with the increase of algae cell density. At a ratio of 20mg/L:20mg/L, the diatomite improved algae removal under adverse environment, such as low pH, low temperature and logarithmic phase of algae. Furthermore, the EEM results showed that algal organic matters (AOM) such as protein-like and humic-like compounds were effectively removed by diatomite, and the removal of TOC above 89%.In coagulation experiments of polluted river water treatment, adding diatomite improved the removal performance of algae, turbidity, TOC and other pollutants. In comparison, the combined PAC and diatomite had good pollutant removal, and the removal efficiencies of algae, turbidity, TOC, TN and TP reached 89.3%,90.4%, 69.4%,10.0% and 97.6% respectively under the dosage of 20mg/L PAC and 20mg/L diatomite. The analysis of EEMs showed that the composition and concentration of DOM changed significantly from season to season, and the humic-like and tyrosine-like compounds had favorable removal by the composite coagulants of diatomite and aluminum salt coagulant.2) The variation of algal cell and floc characteristics during the application of enhanced coagulation was studied. After dosing aluminum salts and diatomite, the cell surface was covered with small irregular flocs, and then bigger flocs were formed at an average size of 43μm. The enhancement of diatomite on the coagulation of PAC was better during the polluted raw water treatment. It’s speculated that the PAC hydrolysis products of Alb, and Alc and natural organic matter (NOM) favors the floc formation and pollutant removal synergistically. According to the analysis of Zeta potential, the diatomite has a slight effect on the Zeta potential in coagulation systems of Microcystis aeruginosa suspension and polluted river water, indicating that the enhancement of diatomite is mainly by the adsorption instead of compressing diffused double layer. In summary, the diatomite facilitates the aggregation of aluminum salts hydrolysis products on algae cell surface, and enhances the coagulation by improving the adsorption, bridging and sweep flocculation.3) Based on previous experiment results of enhanced coagulation, a flocculation and sedimentation equipment in pilot-scale with the treating capacity of 2.2m3/h was established in Haining 2nd Waterwork Corporation, and the process has been run for more than 8 months. Results showed that combined PAC and diatomite coagulation is the good choice for polluted raw water treatment according to the quality characteristics of high turbidity, organic matter and ammonia nitrogen in Haining area. When PAC and diatomite were added simultaneous at the ratio of 20mg/L:5mg/L, the removal efficiency of turbidity, TOC, ammonia nitrogen, TN and chlorophyll a was 96.3%,81.2%,78.3%,19.7%, and 81.2% respectively, and the performance of pollutant removal was stable. Furthermore, flocs formed by diatomite coagulation had a better settling ability and higher density, and made a contribution to little increase of sludge production. The enhanced coagulation of PAC by diatomite solves the problem of filter clogging caused by PAM and reduces the cost of coagulant usage.