Environmental Impacts Of Decaying Water Hyacinth In Eutrophic Water

Water hyacinth, Eichhonia Cassipes Solms, has been widely used in wastewater treatment. It plays an important role in remediation of phytoplankton-dominated eutrophic water. However, its application has been limited because it causes many ecological and environmental problems. Most research on the purification of water hyacinth was done during the vigorous growth period of water hyacinth, and it is very few that studying the effect on water environment and contaminants during the whole growth of water hyacinth. Accordingly, when water hyacinth began to decline and decay, they were considered as "secondary pollution". But when and what conditions do they come into being "secondary pollution"? How deep of the degree are they? How long can the purification of water quality persist? All these questions need to thorough research. The experiment began in early winter and simulated the effect of water hyacinth's natural decline and decay of different density in a eutrophic water, the following conclusions were gotten:(1) After the water hyacinth was planted in eutrophic water in early winter, it can reduce the concentration of CODMn.TP, TN, NH4+-N and Chla although the temperature of water was low and its growth rate was very slow. But the capability of purification in early winter was weaker than that of in its vigorous growth, such as the capability of purification to the concentration of TP, TN, NH4+-N, CODMn of the experimental enclosure D3 in early winter, which planted 100 kg water hyacinth, was only 83.1 %, 25.7%, 55.2% and 21.3% of in its vigorous growths(2) During the period from the beginning of water hyacinth death to disappeared completely in water surface, the remains of water hyacinth can influence the dissolved oxygen (DO). The DO in the experimental enclosure D8 was higher than DO in other enclosures with remains of water hyacinth all the time. The time when the nutrients began to increase was related to the original biomass of water hyacinth in each enclosure. The NH4+-N was the earlist pollution and it happened in the sixty-first day in the experimental enclosure D3. The TP was the latest and it appeared in the ninety-second day. There wasn't clear pollution to the CODMn and Chla because of natural purification.(3) After all the remains disappeared from water surface and deposited to the water bottom, the water quality become worse and worse day by day. During this time, the nutrients of water of experimental encolsure D3, D5, D7 kept increasing. The periods of TP and TN pollution in enclosure D3 were 60 days and 87 days longer than that of D7 respectively. The original biomasses were 100, 50, 25 kg in enclosure D3, D5, D7.(4) After all the remains deposited to bottom, the concentration of nutrients in sediments increased obviously. There was positive correlation between the degree of sediment-pollution and the original quantity of water hyacinth in each encosure. The TP, TN, TOC of the sediment in D3 increased 31.78%, 31.17% and 52.17% respectively, while these indexes increased 18.03%, 17.25% and 24.32% respectively in D7.