| From the latter half of the last century, there has been increasing concern regarding the elevated nutrient status and the eutrophication of rivers and lakes. One of the most widespread examples of pollution is eutrophication due to inputs of large quantities of inorganic nutrients, particularly nitrogen and phosphorus, to freshwater rivers, lakes, streams and reservoirs. Nitrogen (N) and phosphorus (P) are the two major influencing factors on water eutrophication. Therefore, removing N and P in water is an effective approach to mitigating and preventing eutrophication. Due to inability of reduction of N and P by conventional wastewater treatment systems, other appropriate measures should be taken to lower the impacts of nutrient pollution. Ecological engineering for the removal of pollutants at low cost is an emerging field dedicated to the design and construction of sustainable ecosystems that provide a balance of natural and human values. Constructed wetlands occupies a large area of lands, which is a limiting resource in countries such as China where human population density is high. Floating bed technique can be an alternative tool. Restoration using floating, floating-leaf, emergent and submersed hydrophytes is considered crucial to regulating lake biological structure, as aquatic plants limit algal growth by competing for nutrients and sunlight and can also increase herbivorous fish biomass by providing food and refuge. However, plant biomass must be removed periodically from the water bodies to maintain purification efficiency. If not harvested, the nutrients that have been incorporated into the plant tissue may be returned to the water during the decomposition processes. On the other side, rapid population growth coupled with limited cultivable land also causes serious problems maintaining a steady supply of food. Ecological treatment systems have been demonstrated to have significant potentials for both wastewater treatment and resource recovery. There are many potential economic opportunities for the use of plant biomass associated with hygrophytes. They could be dried and used as a food source for domestic animals and the food value could partially offset the cost of harvesting, if the plants were also grown and removed for nutrient abatement purposes. The plants produced on floating islands can be harvested and subsequently used as animal feeds, or even human food, or be processed into biogas, bio-fertilizer and bio-materials. People begin to realize the importance of ecological technology for water remediation due to excellent performance, minimal capital cost and ecological benefit. However, the majority study concentrated on remediation of aquatic plants in warmer condiations. There is no report about the remediation of bioenergy plants and their resource utilization. As a result, the present study was mainly concentrated on screening tall plant type of bioenergy plants in warmer seasons and perennial bioenergy grasses species in cold seasons that could not only adapt to grow well on floating beds but also with high nutrients removal efficiency, studying their biomass production and nitrogen, phosphorus reduction during the growth period, investigating the relationship between plant biomass production and nutrients removal efficiency, aiming at determine plant characters that associated with its purification capacity, and assessing the biomass as animao feed and bioenergy feedstock. The results showed:1. Field study on the response of6tall bioenergy plants in eutrophic water revealed that they could all adapt the aquatic environment. Among six tested plant species grown on floating beds in the field experiment, Chinese silvergrass (Miscanthus (sp.)) and vetiver(Vetiveria zizanioides) had greater dry biomass and were3.21kg m-1and2.34kg m-2, respectively. Chinese silvergrass and vetiver had greater phosphorus phyto-uptake carbon sequestration. The sulfur phyto-uptake of green reed (Phragmites (sp.)) was near to Chinese silvergrass. Chinese silvergrass, fewflower wildrice(Zizania caduciflord) and alligator flag(Thalia dealbata) had greater nitrogen phyto-uptake. The average removal efficiency for total nitrogen, ammonium nitrogen, nitrate and nitrite nitrogen and total phosphorus by six standing plant species were respectively50.3%,59.4%,82.4%and86.5%, during a16-day experiment on floating beds. Chinese silvergrass was most promising plant species for biomass production and nutrient removal grown on eutrophic water with floating beds. 2. It was found that root length and tillers of four tall bioenergy grasses in Hujiachi pond were greater than those in effluent of Linan municipal wastewater treatment plants. The differences of total biomass were not significant during the four tested months. However, the biomass of tall fescue (Festuca arundinacea) was significantly greater than perennial ryegrass(Lolium perenne Plateau) and had greater nitrogen phyto-uptake and phosphorus phyto-uptake. After determining the feasibility of using floating beds of tall fescue inoculated with denitrifying polyphosphate accumulating organisms (DPAOs), it was showed that tall fescue floating beds inoculated with DPAOs accomplished greater removal of NH4+-N, NO3--N, TN and ortho-phosphorus (ortho-P) after20-day treatment. The average removal rates were86.32%,93.60%,90.12%,72.09%and84.29%, respectively for NH3-N, NO3-N, TN, TP and ortho-P. The removal rates of nitrogen and phosphorus were up to137mg m-2d-1and16.0mg m-2d-1, respectively.3. After cultured in1/4Hogland solution for90days, the biomass of switchgrass (Panicum virgatum), vetiver and Chines silvergrass were significantly greater than green reed and giantreed, and were comparable to four willow species. This was related with their greater net photosynthetic rate (Pn), the maximum net photosynthetic rate (Amax) and light saturation point (LSP). Moreover, the maximum photochemical efficiency of PSII (ΦPSII) of switchgrass, vetiver and Chines silvergrass decreased less than other six plant species. After analyzing the nitrogen and phosphorus removal form synthetic wastewater, it was showed the nitrogen removal was signifcantly related with the number of bacteria and denitrifying bacteria, Pn, transpiration rate (Trmmol), water utilization efficiency (WUE), LSP and Amax, and the phosphorus removal was signifcantly related with the number of bacteria in water, Pn, LSP and Amax.4. The tested tall fescue and nine aquatic plants contained abundant crude protein ranging from128g kg-1to255g kg-1. Although Zn level of harvested biomass from remediation plants cannot meet the requirement, crude protein, Ca, Mg, Fe and Mn can meet the daily requirements of livestock and poultry at different growth stage according to National Research Council (NCR) and China Feed Database (2009). The accumulation of trace mineral elements in the tested plants was in the order of Cu> Pb> Cr> As. They were below the standard of NRC (2000) and Hygienical Standard for Feeds in China (GB13078-2001). Therefore, it should be safe when used as animal feed.5. Among six tested tall bioenergy species grown on floating beds in the field experiment, Miscanthus (sp.) and Vetiveria zizanioides were dominant in growth, annual biomass production, nitrogen phyto-uptake, phosphorus phyto-uptake, sulfur phyto-uptake and carbon sequestration. Neutral-detergent fiber, acid-detergent fiber, acid-detergent lignin, cellulose and hemicellulose contents of these species were similar to swithgrass. Miscanthus (sp.) and Vetiveria zizanioides were most promising plant species for biomass production and nutrient removal grown on eutrophic water with floating beds. |
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