Effects Of Duckweed On The Removal Of Nitrogen And Phosphorus In Two Types Of Water Bodies

With rapid development of economic and urbanization, more fresh water is needed. However, water waste and pollution exacerbate the conflict. In’fact, wastewater can be reused after treatment, which leads to a benign cycle of water in nature. As the second water source, reclaimed wastewater not only saves on clean water, but also reduces wastewater emission. As a result, rivers and lakes are less polluted and water resource is protected from being deteriorated. Cities shortage of water, including Beijing and Shijiazhuang, have initiated studies on renewing wastewater and set up a group of demonstrative projects on reclaimed water landscape. The reclaimed water has a high concentration of nitrogen and phosphorus but poor self-purification. Aquatic plants can be used for purifying water because they can absorb, transform and accumulate nutrients, which inhibits the growth of algae and accelerates sedimentation of pollutants. Duckweed has a high water purification efficiency with the characteristics of rapid growth, high absorption rate of N and P and easy to harvest. Duckweed can effectively remove N and P in landscape water and prevent the water from deteriorating.This study compared the growth of two duckweed species, Spirodela oligorrhiza and Lemna perpusilla, total nitrogen (TN) and total phosphorus (TP) absorbed by the plants, as well as their removal efficiency of nitrogen and phosphorus from water. Secondary effluent from a waste water treatment plant and landscape water from the campus were used for the experiment. The results are as follows:In the secondary effluent, biomass of the two duckweed species increased with time. Lemna perpusilla growed faster as compared with Spirodela oligorrhiza. Contents of N and P in the two species were stable during the growth period. Total N and Total P retention in Lemna perpusilla were higher than those in Spirodela oligorrhiza. Total N and Total P removed by Lemna perpusilla from the water were also higher than those removed by Spirodela oligorrhiza,In landscape water treatment, biomass of the two duckweed species also increased with time. There was hardly any change in the contents of N and P in two duckweed species after they had adapted to the solution. Absorption of P by Spirodela oligorrhiza was more than that by Lemna perpusilla, while there was no difference in N absorption by the two species. Removal of P by Spirodela oligorrhiza was more than that by Lemna perpusilla and they had similar N removal.Because content of N was more than that of P in the two duckweed species, so absorption of N were much more than that of P. Besides, total N was removed from the water not only by absorption of duckweed, but in many other ways, while total P was removed mainly by the absorption of duckweed.