| With the rapid development of the rural economy and urban modern industrialization, the loss of chemical fertilizers in agricultural production, rural and urban domestic sewage and wastewater from livestock culture have caused serious water body pollution. In some areas, waste water rich in nitrogen and phosphorus come from livestock culture and rural domestic sewage become the most important source of water pollution, which is even worse than local industrial pollution. Given the distribution of the rural sewage, an inexpensive, simple and efficient purification technology is needed which should suit the rural area and could be easily promoted. Using aquatic plants to purify wastewater is economical and effective and has received great attention. Duckweed, known as small aquatic plant, has a strong ability to absorb nitrogen and phosphorus in wastewater. This plant is rich in nutrients and can by recycled. A number of scholars have been using duckweed to purify waste water and varieties of duckweed are selected as strong candidates for removing nitrogen and phosphors from waste water.Two varieties of duckweed from Yangtze River area, Lemna aequinoctialis and Spirodela polyrrhiza, were used in this thesis. Under the condition of simulated natural illumination and temperature, the effect of growth density and temperature on nitrogen and phosphorus removal effect of duckweed was studied. The maximum phosphorus absorption capacity of duckweed in wastewater system was surveyed. Two varieties of duckweed were mixed at different ratios, in order to study the change of nutrients in plant. Finally, duckweed was collected at different growth stages to investigate the potential negative effects of duckweed growth on watsewate and to evaluate its economic benefit. This paper provides a theoretical basis for using duckweed to purify wastewater. The main results are as follows:1. The removal rate of total nitrogen (TN) and total phosphorus (TP) from the water increased with the duckweed density. The removal of TP by both duckweed varieties was significantly higher than that of TN. At low temperature, Lemna aequinoctialis had higher removal rate of TN and TN than that of Spirodela polyrrhiza.2. It was shown the optimum temperature for purifying water with duckweed was25℃. Spirodela polyrrhiza reached the maximum growth at25℃, and showed a better purification ability than Lemna aequinoctialis atboth25℃and35℃. At low temperature, Lemna aequinoctialis had a better purification effect. Therefore, Spirodela polyrrhiza should be chosen to purify wastewater at high temperatures.3. Differences were shown in the removal effects of TN and TP when Lemna aequinoctialis and Spirodela polyrrhiza were mixed at different ratios. The best removal efficiency of TN and COD was obtained when cultivating Lemna aequinoctialis and Spirodela polyrrhiza at a ratio of1:2, while the monoculture of Lemna aequinoctialis was optimum for TP removal. During the initial growth stage, Lemna aequinoctialis showed a significant purification effect of TP and TN, which became stable thereafter, while opposite trend was observed for Spirodela polyrrhiza.4. The salvage of duckweed increased the purifying capacity. The purification of TN mainly occured in the initial3-6days of the experiment, and it stopped increasing after the ninth day. The purification of TP occurred from the sixth day to the ninth day. The purifying capacity of TP is superior to that of TN, with the same tendency over the growth period of duckweed. Considering the purification effect as well as the economic efficiency, it is recommended to collect duckweed every6days.5. There was no difference in the concentrations of TP and TN nine days after the salvage. The removal efficiency of TN and TP were not proportional to the biomass ofduckweed. The salvage of duckweed didn’t affect the purification efficiency. However, to determine the reasonable salvage quantity, the concentrations of nutrients in the water, salvage period and other factors must be taken into account. |
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