| Poly-dimethyldiallylammonium chloride (PDMDAAC) is a typical cationic polymer. It has attracted widely attention in water treatment areas because of its characters of high positive charge density, easy controlled molecular weight, no toxicity. In this dissertation, the preparation processes for stable inorganic salt/PDMDAAC composite coagulants using PDMDAAC with high molecule weight and series molecule weight as coagulation-aid agents were developed, then the enhanced coagulation mechanisms using composite coagulants were studied preliminary. Based on the regional and seasonal feature of several kinds of typical tiny-polluted raw water, the enhanced coagulation processes using composite coagulants were investigated. The obtained main research progresses are showed as following.The preparation of steady composite coagulantsTo overcome the difficulty in preparing homogenous solution of inorganic coagulants with organic coagulant-aid agent PDMDAAC with high molecule weight, the composite coagulant preparation processes which was suited to industry production were developed. Three series of composite coagulants were prepared through using inorganic coagulants including Poly-Aluminum Chloride(PAC), Aluminum Sulfate(AS) and Poly-Ferric Sulfate(PFS), and organic coagulant-aid agent PDMDAAC with high molecule weight and series of molecule weight (represented as intrinsic viscosity) as raw materials. The composite coagulants have features as following:the content of inorganic component and contained impurities were corresponding the requirements of Chinese national standards of inorganic coagulants, the mass ratio of PDMDAAC to the inorganic coagulants in composite coagulants was5%-20%. and the intrinsic viscosity of PDMDAAC was0.5～3.5dL/g. The storage term of composite coagulants was more than1year under room temperature. Based on above, the foundations for the application of composite coagulants and the studies of their enhanced coagulation mechanisms were established.Enhanced coagulation mechanism of composite coagulantsAbout the enhanced coagulation mechanisms of composite coagulants, the Jar test for kaolin suspension and the settlement test for diatomite suspension under specifically condition were designed to study the charge neutralization and bridge ability of both composite coagulants and their components. The test using PAC/PDMDAAC composite coagulants was as a representative case. The results showed that in the Jar test, the charge neutralization ability of composite coagulants was higher than that of PAC, and increased with the higher content of PDMDAAC, but not with the change of intrinsic viscosity of PDMDAAC obviously. In the settlement test, the settling velocity and size of flocs of using composite coagulants increased remarkably with the increase of intrinsic viscosity or mass ratio of PDMDAAC, compared with that of using PAC. Therefore, it was found that the charge neutralization ability of composite coagulants was depended on the content of PDMDAAC, and the bridge ability of composite coagulants was depended on the intrinsic viscosity of PDMDAAC.About studies on the influences of raw water quality on enhanced coagulation effect of composite coagulants, the tiny-polluted stimulated raw water samples with different turbidity and CODMn values were formulated by adding kaolin and humus in tap water for jar tests. The results showed that under the selected coagulation conditions and ranges of simulated raw water quality, the enhanced coagulation effect of composite coagulants increased with the increase of the pollution level characterized by the value of turbidity and CODMn.Especially, the increasing of mass ratio or intrinsic viscosity of PDMDAAC in composite coagulants, which could increase ability of charge neutralization or bridge of composite coagulants, could largely enhance the coagulation effect of composite coagulants for polluted stimulated raw water.The above studies have offered a reliable theory support and an effective technology for enhanced treatment of tiny-polluted raw water.Enhanced coagulation processes using composite coagulants for several typical tiny-polluted watersFor several typical tiny-polluted raw water like net river water in Ningbo, Yangtze River water near Nanjing city in4seasons, and Taihu water near Wuxi city in winter and summer seasons, the jar tests in situ were conducted to study the performance of the enhanced coagulation process using composite coagulants under the similar coagulation condition with conventional coagulation processes used in local potable water plants. The results are showed as following.For low-turbidity and temperature Baixi reservoir raw water with turbidity of1.5NTU and temperature below10℃, the inorganic coagulants/PDMDAAC composite coagulants did not improve the turbidity removal compared with inorganic coagulants, so it could not be used for the enhanced coagulation for Baixi reservoir raw water. PFS single could get satisfying turbidity removal effect, so it could be used as a coagulant for the Baixi reservoir raw water.For net river water in Ningbo including Beidu River raw water and Yaojiang River raw water with characteristics of tiny-pollution, the suspended particles in the raw water adsorbed a certain content of organic compounds. For the Beidu River raw water in winter with temperature of8～13℃and turbidity of6～13NTU, the dose of6.0～9.0mg/L of AS, PAC and PFS were required respectively, when residual turbidity of2NTU should be achieved. But, the corresponding composite coagulants of them could decrease the inorganic coagulants dose by8%～40%respectively. For low-turbidity Yaojiang raw water with temperature8～13℃and turbidity of7～10NTU, the dose of5.0～10.0mg/L for AS, PAC or PFS were necessary for treatment to reach residual turbidity of1～1.2NTU. But, the corresponding composite coagulants of them could decrease the inorganic coagulants dose by7.76%～53.33%respectivelyFor Yangtze River water in four seasons, the suspended particles in water were mainly composed of fine sediments. The tiny-polluted level of raw water in autumn was usually higher than that in spring, the water temperature in winter was low and the water turbidity in summer was high, which all caused difficulties in treatment. Using the similar doses for inorganic coagulants to reach the residual turbidity of6NTU, which was required by the potable water plants located in Yangtze River basin, the decrease of residual turbidity using composite coagulants compared with using inorganic coagulants were as following:14.41%～61.72%in autumn,6.11%～58.50%in winter,5.31%～37.56%in summer and4.19%～39.26%in spring. When the same residual turbidity of6NTU was achieved, the decrease of dose using composite coagulants compared with using inorganic coagulants were as following:7.41%～37.04%in autumn,2.50%～37.50%in winter,2.03%～27.20%in summer and0.88%-27.16%in spring. The turbidity removal ability of composite coagulants were in the order:PAC/PDMDAAC>AS/PDMDAAC>PFS/PDMDAAC The improvement effect of composite coagulants on inorganic coagulants for Yangtze River raw water in four seasons was as following:autumn>winter>summer>spring.For Taihu Lake raw water in winter and summer which was polluted mainly by organic compounds and algae, the season characteristic of raw water quality was obvious. For the Taihu Lake raw water in winter with characteristics of low temperature, containing algae and tiny organic pollution, the turbidity removal effects of inorganic coagulants were in the order:PAC>AS≈PFS. Using the similar doses for inorganic coagulants to reach the residual turbidity of2NTU, which was required by the potable water plants located in Taihu lake basin, using AS/PDMDAAC、PAC/PDMDAAC composite coagulants could decrease residual turbidity by33.33%～62.50%,20.33%～56.22%, compared to using AS, PAC, respectively. When residual turbidity of2NTU was achieved, using AS/PDMDAAC、PAC/PDMDAAC could decrease doses by18.65%～56.15%.9.42%～23.48%, compared to using AS, PAC, respectively. For the summer Taihu Lake raw water with the characteristics of high algae and organic pollutant content, the turbidity removal effects of inorganic coagulants were in the order:AS>PAC>PFS. Using the similar doses for inorganic coagulants to reach the residual turbidity of2NTU, using AS/PDMDAAC. PAC/PDMDAAC composite coagulants could decrease turbidity of supernatant by39.67%～82.45%.45.33%～84.90%, compared to using AS、PAC, respectively. When same residual turbidity of2NTU should be achieved, using AS/PDMDAAC. PAC/PDMDAAC could decrease dose by20.18%～56.52%,28.15%～56.59%, compared to using AS, PAC, respectively. Therefore, the enhanced coagulation effect of composite coagulants on Taihu lake water in summer was higher than that in winter. Especially, the enhanced coagulation processes using composite coagulants were able to replace the function of pre-chlorination combined with inorganic coagulants in turbidity and algae removal, resulting in the improvement of the potable water security by decreasing the formation of disinfection by-products.The above Jar tests results in situ have demonstrated that for tiny-polluted raw water, the composite coagulants could improve the turbidity removal effect and decrease inorganic coagulants dose greatly under the conventional coagulation process condition. The effect of enhanced coagulation of composite coagulants was increased with the increase of intrinsic viscosity or mass ratio of PDMDAAC, and the higher pollution level of raw water, the higher effect was. Meanwhile, composite coagulants could not only obviously decrease the coagulants dose, improve the water quality of supernatant and increase the volume of water production, but also could offer an effective technology for advanced treatment of raw water in future. Therefore, based on above Jar tests results in situ, the foundations of industrializing application of composite coagulants and their enhanced coagulation processes were established.Scaled-up tests of enhanced coagulation process using composite coagulantsThe scaled-up tests in potable water plant were conducted to evaluate the effect of enhanced coagulation process using composite coagulants in treatment for Yangtze River raw water. The results showed that for the Yangtze River raw water with temperature of15～20℃and turbidity of25～40NTU in spring, the composite coagulants functioned well in the ability of decreasing residual turbidity and inorganic coagulants doses when used in the potable water plant with the production capability of120000t/d. Using the similar doses for conventional coagulation process using inorganic coagulants to reach the required residual turbidity, the enhanced coagulation process using composite coagulants could reach much lower residual turbidity. When required residual turbidity was reached, the enhanced coagulation process could decrease dose by5%-35%compared with conventional coagulation process. At the same time, the enhanced coagulation process could be able to endure strange impact of50%water amount increase than conventional coagulation process did. The enhanced coagulation processes using composite coagulant obtained from jar tests were scaled up successfully in the potable water plant.The scaled-up tests in pilot-plant with production capability of4t/h were conducted to evaluate the magnification effect and running stability of enhanced coagulation processes for treatment of tiny-polluted Taihu raw water. For tiny-polluted and algae-contained Taihu Lake raw water, the enhanced coagulation effects of composite coagulants, including the enhancement of turbidity and algae removal, the decreasing of inorganic coagulants dose and the advanced treatment process in the future, were scaled up successfully in the pilot plant. Meanwhile, these enhanced coagulation processes could be steady operated.The above scaled-up test results have demonstrated that the composite coagulants and their enhanced coagulation processes could be directly applied in industry. |
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