Duckweed Resources In Jiangsu Province And Experimental Study On Its Nitrogen Removal Efficiency

Agricultural non-point source pollution is one of the most important factors in causing eutrophication. Aquatic plant for purification has been used as an economical, effective and easy promotion remediation technology on eutrophication. Duckweed is the optimization programme of water pollution control, which has more advantages than other aquatic plant such as fast proliferation, nitrogen absorption capacity, easy to harvest, less prone to secondary pollution, high in protein and starch, and has economic value. Duckweed has been applied to the remedia-tion of polluted water in many countries. In this field a late start in China, most of studies are limited to laboratory-scale, practical application of researchs are also scarcely.Taking the Jiangsu area as a case, a combination of investigation, laboratory simulation and field test method had been used. In this paper, the survey results of duckweed’s germplasm resources distribution and its growth environment were provided. Experimental study of lemna minor on nitrogen removal efficiency under different water depth conditions and duckweed nitrogen removal technology demonstration were also carried out. The main results were as follows:(1) Duckweed tend to flourish in calm water rather than flowing water, mainly grows in freshwater such as Mere, paddy fields and river. In summers of Jiangsu Spirodela polyrrhiza, Lemma minor, Wolffa arrhiza and Spirodela oligorrhiza are the most common duckweeds found in water bodies, and no trace of Lemma perpusilla and Lemma trisulca was found during the survey. Duckweed grows well in natural water bodies, high or low in nitrogen and phosphorus concentration, ranging from0to42,0.2to2.0,0.2to45and0.02to13mg/L for ammonium-N, nitrate-N, total nitrogen and total phosphorus, respectively.(2) The increase of water depths led to anaerobic circumstances in lower water layers, which would directly decrease the pH value in water; with lower depths, the average reduction rates of NO3--N concentration enlarged, while NO3--N concentration’s removal rates were higher. Under10cm depth, the average reduction rates of NO3-N concentration reaches1.937(mg/L·day), the corresponding reduction rate reaches62.36%. In addition, the variation of TN concentration was roughly similar as that of NO3--N concentration. Also, according to-N and TN concentration’s dynamics in different layers under a certain water depth, the experimental results illustrated that, although the depth increased, NO--N and TN concentrations stay constant except for some tiny fluctuations.(3) Seting the duckweed pond had a noticeable effect on the purification of eutrophicated water bodies. The removal rate of total nitrogen in water reached up to57.26%in40days. Duckweed grew in a high rate, environmental adaptability and easy to harvest. It was appropriate for the remediation of eutrophicated water bodies dominated by nitrogen and phosphorus in small towns or rural areas.