|Other Abstract||Vertical-flow constructed wetland is a promising wastewater treatment technology which is suitable for the conditions of China. Although its main mechanisms have been gradually recognized, the mechanism of the filtering media, which is one of the principal parts for wetland purification, is still in its infancy stage. Further understanding of treatment performance with different media and their influences on clogging are needed and necessary.
In this paper, aiming at performance of different filtering media and the problem of clogging, a year-round purification experiment was performed in the home-made experimental base, treating the simulative wastewaters which are mixed by the polluted lake water and septic tank effluent. Eight different filtering media (zeolite, anthracite, shale, vermiculite, ceramic filter media, gravel, blast furnace steel slag and biomaterial ceramsite) were chose deliberately; removal rates and mechanisms of various types of pollutants in water samples were studied also. Along with hydraulic and biological experiments, the filtering medium that was of best comprehensive advantages or outstanding removal efficiency for single pollutant was sort out. Also, a novel pathway for design, management and maintain of the vertical-flow constructed wetland was suggested, by choosing appropriate filtering media, the clogging phenomena could be effectively prevented, thus the life-span of constructed wetland would be extend.
The main results were as following:
Removal of pollutants:
1. For the removal rates of organic matters, eight media were sorted by comprehensive removal capacity as: anthracite, biomaterial ceramsite, vermiculite, zeolites, blast furnace steel slag, ceramic filter media, gravel and shale.
2. For the removal rates of various forms of nitrogen, eight media were sorted as: zeolites, ceramic filter media, anthracite, blast furnace steel slag, vermiculite, biomaterial ceramsite, gravel and shale.
3. For the removal rates of various forms of phosphorus, eight media were sorted as: blast furnace steel slag, anthracite, biomaterial ceramsite, gravel, vermiculite, shale, zeolites and ceramic filter media.
4. Simply integrating the removal rates of organic matters, various forms of nitrogen and phosphorus, eight media were sorted as: anthracite, blast furnace steel slag, zeolites, biomaterial ceramsite, vermiculite, ceramic filter media, gravel and shale.
5. For the physical and chemical indicators, the effluent of anthracite system (pilot plot) was weak acid, with TDS, ion concentration, conductivity and salinity somewhat high. In parallel, the effluent of blast furnace steel slag system was strong alkaline, with TDS, ion concentration, conductivity and salinity all extremely high. The effluents of other systems were normal.
6. For the removal rates of the total number of bacteria, eight filtering media all had good removal performance. The effluent of blast furnace steel slag system (pilot plot) was best and the effluent of anthracite system was worst among the eight systems.
7. For the operation indexes of the entire experimental cycle, the removal rates of organic matters, various forms of nitrogen and phosphorus fluctuated greatly in the effluents of vermiculite and gravel systems. And the removal rates maintained stable in the effluents of zeolites, anthracite, blast furnace steel slag and biomaterial ceramsite systems.
8. The organic matters and various forms of nitrogenin eight simulated systems were supposed to be removed mainly through physical and biological processes. And the various forms of phosphorus were supposed to be removed mainly through physical and chemical processes. But different filtering media had slightly different removal mechanisms. As for the blast furnace steel slag and anthracite systems, the main removal mechanisms were supposed to be chemical and physical processes, but for other filter media, removal mechanisms were the common results of the physical, chemical and biological processes.
9. Based on the experimental data and operational indicators for 1.5 year experimental cycle, along with various factors while wetlands operation, we could come to a preliminary conclusion that anthracite, zeolites and biomaterial ceramsite could be the high-efficient media in vertical-flow constructed wetland; vermiculite, blast furnace steel slag and ceramic filter media could be the assistant efficient media in vertical-flow constructed wetland.
Qualitative and quantitative analysis on composition:
1. Eight filtering media were compareed with their constituent elements by qualitative analysis and quantitative analysis, gravel, zeolites, shale, vermiculite and ceramic filter media had more stable components in the entire experimental cycle, and the components of anthracite, blast furnace steel slag and biomaterial ceramsite had certain changes.
2. In several media with better integrated purification results, the constituent elements of anthracite were mainly carbon and sulfur; the constituent elements of blast furnace steel slag were mainly calcium and silicon; the constituent elements of zeolites were mainly silicon and aluminum. As the media with worst integrated purification results, the constituent elements of shale were also mainly silicon and aluminum, but the contents of iron and magnesium were higher than others.
3. With the consideration of changes of composition, the main removal mechanisms in gravel, zeolites, shale, biomaterial ceramsite, vermiculite and ceramic filter media systems were physical process, and the main removal mechanisms in anthracite and blast furnace steel slag systems were chemical process.
4. Zeolites, shale, blast furnace steel slag and biomaterial ceramsite were characteristic of their adsorption ability of toxic metals. In order to find more extensive treatment capacity, their purification capacity and characteristics needed further study.
Studies of influence on clogging with different filter media:
1. Head loss and hydraulic retention time could reflect the extent of the wetlands clogging. During the entire experimental cycle, little clogging was found in biomaterial ceramsite, zeolites, vermiculite and blast furnace steel slag systems, but in the gravel system the problem was serious relatively. Different media had different mechanisms of clogging.
2. The structural characteristics of each media could influence clogging. Through the experiment of porosity, permeability, water contents and organic matter, we could comprehensively analyze the structural characteristics of the shale and gravel media and found that these two media influenced clogging greatly. And it was noteworthy that the anthracite media had a relatively higher performance for removal of pollutants, but its porosity and water content were smaller than others.
3. Through the water flow monitoring and analysis, we could find that clogging changed with the average velocity of flow, the average flow and total flow in the shale and gravel media systems. On the other hand, the vermiculite media had better performance for removal of pollutants because of its good water-holding capacity and the degree of water capacity.|