| Population growth and society development have generated increasing amounts of wastewater containing high content of nitrogen (N) and phosphorus (P) from agriculture, industry and urban life. On the contrary, the treatment and quality control of sewerage disposal lagged far behind. Large amount of wastewater is discharged directly into rivers, lakes and other water bodies. This is frequently causing eutrophication of surface water and groundwater contamination. Our country is facing more and more serious situation of N and P pollution and eutrophication of reliable freshwater resource. A low-cost and easy-operate measure characterized with high reduction rate of N and P for wastewater treatment and bioremediation of surface water is urgently needed to establish.The application of aquatic macrophytes in wastewater decontamination not only can remove the pollutant out of the system. In addition, they assimilate nutrients into a high-quality biomass that may be processable to new resource, feedstuff, fertilizer or industrial material. This technique combines wastewater treatments with source reuses and is an economical-efficient and easy-spreading option for the purification of wastewater.In this study, the growth competence and tolerance ability to the forms and strength of N and P of several duckweed species were studied through population growth experiment. Lemna perpusilla is chosen for its highest relative growth rate (RGR) and good endurance to N and P to study the removal rate of N and P in high organic load wastewater by static experiments indoors, in order to get some useful information for further application of duckweed in decontamination of nutrient elements from wastewater and eutrophicated water body. The following were the results:1.Lemna perpusilla has high efficiency in absorbing and concentrating N and P from wastewater. The reduction speed is increasing along with the concentration of N and P in water, whereas the removal rate of N and P is descending. The ammonia concentration descends as a negative exponent, while TP and phosphate decrease linearly.2.When the ratio of nitrate/ammonia in medium is 7.5: 2.5, Lemna perpusilla grows best and has the most biomass. In the meantime, the reduction of ammonia and TN is optimal with removal rate of 99.9% and 65.3%, and that of nitrate is 68.6%. The content of N in Lemna perpusilla is increasing when the ammonia strength is higher.3.The absorption capability to ammonia of duckweed is stronger than to nitrate. As in the same strength level, the removal rate of ammonia is 10%~20% higher than nitrate. Even as the growth of duckweed is inhibited in a full ammonia circumstance, the removal rate of ammonia is 4% higher than nitrate.4.If phosphorus is enough, when the strength of P is higher, the content of P in Lemna perpusilla is higher too, but the biomass of Lemna perpusilla isn't increasing significantly. This suggests that Lemna perpusilla can absorb P more than demand and store it inside its body. Lemna perpusilla grows well in any phosphorus strength and absorbs TP and PO43-efficiently. The removal rate of TP and phosphate is above 90% and 88%.The great tolerance and removal capability of duckweed to strong strength N and P is very considerable, so the application of duckweed in bioremediation of eutrophicated water body and post treatment of domestic sewage and septic tank effluent etc is reliable. Duckweed population grows rapidly and removes the nutrients mainly by biomass increasing. Harvest termly can keep duckweed population in high growth rates, therefore the duckweed-based system can work steadily. As duckweed has rich nutrients, it can be used as feedstuff for fishes, pigs, ruminants and chicken. And it can be used as ecological fertilizer and material of biogas. Under reasonable and efficient utilization, duckweed-based technology can improve the development of fish culture, stockbreeding and ecological agriculture greatly. |
