Research On The Removal And Use Of Different Forms Of Nitrogen In The Eutrophic Water

With the development of society and the improvement of human living standards, water eutrophication is becoming a serious problem. Among which, the devastating-algae bloom has caused a huge economic loss globally. Duckweed are widely used in waste water treatment owing to many advantages, such as fast breeding speed, high protein content, easy to harvest, causing no secondary pollution, high removal rate of nitrogen and phosphorus compared to other aquatic plants, suitable for a wide range of pH and temperature. However, research on the removal and use of different forms of nitrogen in waters by duckweeds is scarce.This paper aimed at determining the nitrogen removal efficiency of different duckweeds species as affected by nitrogen forms as well as temperature. These include (1)treatments with high (lmmol/L,14mg/L), medium (0.5mmol/L,7mg/L) and low (0.05mmol/L,0.7mg/L), concentrations of total nitrogen with five different NH4+-N/NO3--N ratios (5/0,4/1,3/2,2/3,1/4and0/5) were conducted to study nitrogen removal efficiency of Lemna minor.(2)With high nitrogen supply, the effects of temperature (10℃,20℃,30℃) and nitrogen form on the growth and nitrogen removal of three different duckweed species were studied (3) Optimal temperature for the growth of duckweeds was estimated.The results are as following:(1)Lemna minor had the highest removal rate of NH/-N and NO3--N in the treatment with low nitrogen, which reached100%in10days. After10days of cultivation, the concentration of total nitrogen was maintained at0.2mg/L. The growth rate of Lemna minor was about0.3d-1with low nitrogen supply resulted growth rate. The leaf chlorophyll content increased with high ration of NH4+-N.(2)In the treatments with medium level NH4+-N, the nitrogen removal efficiency of Lemna minor were affected by NH4+-N/NO3--N ratio. High NH4+-N ratio was accompanied by a delay in nitrogen removal. It took25days to completely remove nitrogen in water by Lemna minor when treated with only NH4+-N, while15days needed when only NO3-N was supplied. The removal rate of total nitrogen reached its highest rate at15day (76.5-94.6%). After15day, total nitrogen concentration started to increase in the treatments with high NH4+-N/NCV-N ratios (5/0and4/1), which significantly inhibited the growth of Lemna minor.(3)In the treatments with high level of nitrogen, when supplied with low ratio of NH4+-N,5～10days and15～20days were needed to completely remove NH4+-N and NO3--N respectively, andtotal nitrogen removal reached100%after15d. The growth of Lemna minor was inhibited by relative high level of ammonium nitrogen, with a relative growth rate0.1d-1and disintegrated and whitened leaves, which decreased the nitrogen removal rates The chlorophyll content in Lemna minor was the highest at the NH4+-N/NCV-N ratio2/3. The relative growth rate was the highest (1.5d-1) at aNH4+-N/NO3--N ratio of0/5, when the leaves were healthy and big.(4)There were no algae growing when treated with100%NH4+-N. With the increase of NCV-N, the algae growth increased significantly. However, when the NCV-N concentration exceeded40mg/L and the NH4+-N/NCV-N ratio was less than1/4, the algae propagated, especially in the control without duckweed. Therefore, duckweeds could inhibit the growth of the algae to a certain extent.(5) Lemna minor presented the highest growth rate in lmmol/L nitrate nutrient solution at30°C, when its relative growth rate of fresh weight reached25.1d-1. Lemna minor also showed high accumulation and removal efficiency of nutrient elements in water with containing high nitrate content and therefore could be an optimal choice for treating water polluted with high concentration of nitrate.(6)When treated with high NH4+-N, temperature played a major role in controlling the growth and nitrogen absorption of Spirodela polyrrhiza. Spirodela polyrrhiza grew the best at30°C and accumulated the most nitrogen, therefore it is suitable for eutrophic water restoration at higher temperature. However, it is not optimal for relatively low temperature since the growth decreased with time at10°C.(7)At10°C and20°C, Spirodela oligorrhiza accumulated the most macro-elements in the treatments of both high concentration of ammonium and nitrate. It is recommended to use Spirodela oligorrhiza to treat water containing high concentration of ammonium or nitrate. (8)The growth rate of Soirodela polyrhiza under10℃and20℃conditions are lower than30℃. Among the three duckweeds, relative growth rates of Soirodela polyrhiza is the lowest. At low temperature, biomass growth of Spirodela oligorrhiza is the largest, meanwhile at high temperature, relative growth rates of Lemna minor is the highest.The results showed that nutrient removal efficiencies of different duckweeds were affected by NH4+-N/NO3--N as well as by temperatures. It is recommended to choose the optimal duckweed species for improving water eutrophication based on the environmental temperature, water nitrogen content, and the ratio of ammonium and nitrate in water body. This would help to achieve high efficiency in water restoration using duckweeds.