| Reclaimed water is a kind of water which can reach to a certain degree of water quality objectives, meet certain application requirements and can be used beneficially. Compared to seawater desalination and inter-basin water transfer, reclaimed water has obvious advantages. From the economy point of view, the treatment cost of reclaimed water is lower. From the environmental protection point of view, recycling of sewage contribute to the improvement of the ecological environment and to achieve a virtuous circle of aquatic ecosystems. Reclaimed water is the second water source of a city. Reclaimed water recharge to groundwater after advanced treatment not only can prevent seawater intrusion and land subsidence, but can also supply the groundwater resources, alleviate the shortage of water resources conflicts. It is an important measure carrying out the sustainable development.Reclaimed water need to go through some processes, reach to the standard "The reuse of urban recycling water, Water quality standard for groundwater recharge" (GB/T 19772-2005) before recharged to groundwater. The pre-treatment and deep treatment processes for reclaimed water mainly include coagulation, filtration, adsorption, reverse osmosis and disinfection. Although the processes have been more mature, but on the aspects of coagulant selection, especially the research on the new type of coagulant, filtration media selection and the type of adsorbent etc, are needed. In addition, the research on the integration of different processes need to be improved.Taking secondary effluent of a sewage treatment plant in WuHan as study object,groundwater recharge as the aim, the article research on modified natural mineral as coagulant used on pre-processing and deep processing. Using the new type of natural mineral coagulant could avoid secondary pollution and toxicity compaired with the traditional coagulants. The content of the research include:the appropriate program of modified bentonite to pre-processing and deep processing of reclaimed water, the best conditions of modified program and wastewater treatment. Finally, using secondary effluent as object, the research combine pre-treatment and deep treatment processes to study the effections by using modified bentonite as coagulant, comparied with PAC. On this basis, we could determine the feasibility of underground recharge, meanwhile, provide theoretical foundation and parameter reference for field test.Through the experiment, the following results and conclusions are gained:First:Select the most appropriate program of modified bentonite to pre-processing and deep processing of reclaimed water among roasting activation (muffle furnace calcination and microwave heating), acid activation, modification of Al-pillared and modification of micro-Ferrous sulfate.The products of roasting activation can hardly remove pollutants. The product of Al-pillared modification can partly remove the pollutants. It is worth noting that the micro-Al-pillared modified bentonite has the strong removal capacity to total phosphorus, this could explain the important role of microwave towards the modification of bentonite. The treatment effect of secondary effluent by using micro-Ferrous sulfate modified bentonite is the best, the removal rates of organic and total phosphorus are about 50% and 20%. The analysis of microstructure improve that the program of micro-Ferrous sulfate can expand the interlayer spacing of bentonite effectively and increase its adsorption capacity. Therefore, the program of micro-Ferrous sulfate is the most appropriate program to pre-processing and deep processing of reclaimed water.Second:The article conducts in-depth study of best conditions of modified program, including the influences of the concentration of modifier, the ratio of solid to liquid, the power and time of microwave irradiation to the adsorption capacity of modified bentonie. The results show that the concentration of modifier, the ratio of solid to liquid and the time of microwave irradiation effect its adsorption capacity. The concentration of modifier can directly effect the size and variety of modified group, and effect the ratio suface of bentonite as a result, and decide the degree of strength of modified product's adsorption capacity. The ratio of solid and liquid could effect the concentration of modifier as well, if happen a inadequate response, the adsorption capacity of modified product may weaken. Microwave can heat the intra-molecule of bentonite in a very short time, dredge its channel and remove its interlayer water and bound water. But, when the time of microwave irradiation is too long, curling and collapse of molecular structure will happen in the inner part of bentonite, and affect the effect of sewage treatment by using the modified bentonite. In the other hand, the power of microwave irradiation nearly has the effect to the adsorption capacity of modifed bentonite. Hence, the best conditions of modified program are:the concentration of modifier is 0.75mol/L, the ratio of solid to liquid is 1:9, the power and time of microwave irradiation are 420W and 5min. Third:The research use the best modified poduct on sewage treatment, and optimize its operation parameters.The result shows:when the dosage of modified bentonite is 1.4g/L, the adsorption time is 5min, the range of pH is 7 to 8, and the temperture is about room temperature, the effection of sewage treatment is the best.Fourth:The research integrate the obtained results (included modified bentonite,fiber ball filtration and MWB activated carbon) in the indorr pilot experiment, and product a set of pre-processing and deep processing integrated device compaired to the actual device in our field experiment base.The result shows:the integrated device can effectively remove the SS, organic and total phosphorus of sewage effluent water, and the removal rates are all higher than 80%. The effluent quality almost reach the limit of recharge standard except nitrogen pollutants. The water quality and the conditions of the integrated device are not conductive to the growing of nitrifying bacteria, this may be the reason of the low removal rate of nitrogen pollutants. At the same time, the research simulate the anti-washing step of filtration and activated carbon part.lt shows that after several times of anti-washing, the integrated device can operate stablely, and effluent quality are good without large fluctuations.Fifth:Through calculated, the project's operating cost is 0.71 yuan er cubic meter wastewater. Although the cost is a little bit higher than the cost by using PAC as the coagulation, the sludge of modified bentonite could be used in many different ways,like producing building materials or used in backfill lining. As the new type of natural mineral coagulant, it can realize self-settlement in a very short time, and truly have the basic ability of coagulation.This apply a theoretical and practical basis for future experimental research.Sixth:The research also study on the mechanism of micro-Ferrous sulfate modification.lt maybe work in this way:the iron ion from modifier could balance the negative charges on silico-oxygen tetrahedron, and make the cations from interlayer have the exchangeable ability.Thus, the adsorption capacity of bentonite is enhanced.On the other hand, some free iron ion could occur coagulation with pollutants, this will remove much more pollutants and increase the quality of water. Besides that, microwave irradiation can re-arrange the crystal structure of bentonite and make it more closely. This can increase the chemical stability of modified bentonite, and expand its applications. |
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