Research On Mechanism And Technological Conditions Of Nitrogen-phosphorus Wastewater Treatment By Duckweed

With the fast development of rural economy and urbanization construction, the problem of environmental pollution caused by nitrogen-phosphorus wastewater from agriculture production and rural life is becoming more and more serious in China. How to purify nitrogen-phosphorus wastewater in an economical, effective and convenient way, in which the characteristics of rural area and agriculture production iare also taken into account, is a key technical problem to resolve urgently in the task of rural environmental protection nowadays.Aiming at the above situation, with local duckweed species in the Yangtze River delta region as experimental objects, this research was deployed in three aspects: 1) selection of optimal duckweed species for efficient treatment of nitrogen-phosphorus wastewater 2) dynamics and mechanism of nitrogen uptake by duckweed from wastewater 3) and the appropriate technological conditions for nitrogen-phosphorus wastewater treatment by growing duckweed, in hopes of provide feasibility basis for bio-treatment and reclamation of nitrogen-phosphorus wastewater in rural areas of China.Through comparative experiments among five local duckweed species (Spirodeia oligorrhiza, Spirodeia polyrhiza, Lemna perpusilla, Lemna minor, Wolffia arhiza), in aspects of sewage concentration tolerance ability at different environmental temperatures with the method of orthogonal test, protein production capacity under suitable growth condition and cold resistance capability under low temperature condition of winter, the results showed that the local duckweed species Spirodeia oligorrhiza was superior to the other local duckweed species not only in sewage concentration tolerance ability in the experimental temperature range (15~35℃), but also in protein production capacity which was almost equal to that of the foreign optimal duckweed species in terms of protein productivity per unit area. The protein content of Spirodeia oligorrhiza amounts to more than 30% (dry weight), which indicated it could be used as a valuable source of high protein animal feed. Furthermore, although Spirodeia oligorrhiza was a bit inferior to Lemna perpusilla in the capability of cold resistance, it could outperform other species and survive in the low temperature condition (5°C). Thus, according to the screening basis of strong ability of sewage concentration tolerance, high protein content and productivity, as well as good adaptability to local climate (strong cold resistance capability), Spirodeia oligorrhiza was selected as a optimal local duckweed species for nitrogen-phosphorus wastewater treatment in the Yangtze River delta region. Moreover, the great ability of sewage concentration tolerance and high protein productivity are the most distinct biological traits expressed by Spirodeia oligorrhiza among the five local duckweed species.The experiments for comparing the protein production ability and cold resistance, respectively done with 5 local duckweed species and 3 foreign optimal duckweedspecies (Lemna gibba 8678, Spirodella punctata 7776 and Lemna minor 8627), showed that the spirodela oligorrhiza was a bit inferior to Spirodella punctata 7776 and Lemna minor 8627 in protein production, while little superior to Lemna gibba 8678, For the cold resistance, exeept the Lemna gibba 8678, which couldn't survive in a.temperature under 5°C, the other 2 kinds of foreign species was obviously superior toSpmtdela oligorrhiza. The results, not only indicated the difference between local optima} duckweed species and its foreign countparts, but also proved the reliability of the screening experiments for local optimal duckweed species selection.The research on uptake dynamics of different forms of nitrogen showed that the kinetic characteristics of ammonium and nitrate uptake by duckweed could be illustrated with Michaelis-Menten equation. The results indicated that duckweed had a stronger affinity for NHU-N than for NO3-N, which indicated that duckweed had an uptake preference for NH4-N over NO3-N in water with low concentration of nitrogen and thus confirmed that "duckweed .prefer NH4-N to NO3-N" theoretically. Meanwhile, the highest uptake rate of duckweed for NO3-N was greater than that for NH4-N, which indicated that the purification rate of NO3-N would be higher than that of NH4-N if the nitrogen concentration in water was considerable high. This was probably related to the non-ion ammonia toxicity to duckweed under high NH4-N concentration condition.The research on ultimate destination of nitrogen in duckweed based system and subcellular distribution of nitrogen inside duckweed frond by 15N tracer technology indicated mat the external NH4-N uptaken by duckweed was mainly distributed in dissoluble groups such as cytoplasm and vacuole, which exist in the forms of dissoluble protein (including enzyme), polypeptide, dissociated amino acid and etc. The results directly proved mat duckweed can be used as high protein animal feeds for its high content of protein in frond.With diluted anaerobic treated swine wastewater as nitrogen-phosphorus wastewater for experiment, the influence of different technological conditions, that is sewage concentration, initial planting density, harvesting cycle, water depth and winter climate, on growth of the local optimal duckweed species and nutrient removal effect had been tested. The results showed that:1) In summer with high temperature when duckweed is in the most disadvantageous periods of its sewage concentration tolerance, Spirodela oligorrhiza could grow well and nutrient (TN and TP) could be removed efficiently with the concentrationv of TN below 80mg/L and the ratio of N: P matching 4 :1 in wastewater2) The removal efficiency for NH4-N was basically immobile with different initial planting densities of duckweed system, while the removal efficiency for TN and TP was increasing with accretion of initial planting densities of duckweed system. It was founded that the algae grew rapidly with lower initial planting density of duckweed system, which lead to increasing of pH, and then could probably inhibit the growth of duckweed to some extent. In order to raise the removal efficiency of...