| Eutrophication is already one of the most major water pollution problems in the world today. It's very hard to solve the serious eutrophication problem in theYangtze river delta. Due to the cement embankment, the cost is extremely high to restore natural banks and wetland. However, floating treatment wetlands (FTW) can offer the advantages of providing effective treatment, easy management and economic benefits, and occupying water surface only, without being constrained by the requirement of water depth. Thus, this technology has great practical significance for remidiation of eutrophicated river and sustainable water resource management. At present, most of the existing studies are static or closed water box tests, with few river field experiments. The only field tests were also just confined to purification function. There has been very little information published to date about FTW plant management in the river. The aim of this paper is to seek effective and reasonable methods for FTW management, which can provide scientific foundation for the application of this technology.Ipomoea aquatica was used as FTW plant in both field experimentation and controled experimentation to evaluate the effects of different cutting frequency and stubble heights. With field experimentation, we assessed the effects of different cutting regimes on the output of floating plants by analyzing sprout, biomass, stem-leaf ratio; while with controlled experimentation, we evaluated the effects of cutting regimes on water purification function, by measuring TN,NF4+,-N,TP,CODcr in the water. The main research results of this study are as follows:The effects of cutting on plant productivity(1) After cutting the plants could grow healthily during its whole lifecycle. The number of tillers was 5 times that of the initial value, with the longest roots of 27 cm, average roots length of 15 cm, and average root system diameter of 11 cm. The fresh root biomass of single plant was 146 g, which could adsorb 3.36 kg suspended particulates per square meter of floating mat.(2) Sprout growth rate went gradually up and then down with the increase of cutting times, at the range of 0.54-3.7 cm·d-1 in different cutting treatments, compared with 1.63 cm·d-1 of no-cutting. Only appropriate cutting could promote sprout growth rate.The change of biomass harvested resembles that of sprouts, and the average biomass growth rate was between 3.83-37.9 g·m-2·d-1.(3) Concerning total biomass, the treatment of cutting every 4 weeks, and stubble heights at 25 cm or 35 cm could obtain maximum biomass, at 2112 g·m-2; Concerning stem-leaf ratio, the treatments of cutting every 2 weeks and every 3 weeks with stubble heights at 15 cm or 25 cm were better than other treatments; Concerning both biomass and quality, as well as management convenience, cutting every 3 weeks with stubble heights at 15 cm was the best. Under this cutting regime, sprout growth rate was 1.88 cm·d-1, stem-leaf ratio was less than 1, and the total biomass was 1966 g·m-2.The effect of floating treatment wetland system on water purification(1) The concentration of TN reduced with the time passing. Concerning cutting frequencies and stubble heights, the effects of FTW on TN removal were found to be significantly different (P<0.05) among different treatments. The removal rate for TN from the treatment of non-harvest (78%) is much higher than that from no-floating bed system (48%). Concerning TN removal, the treatment of cutting every 3 weeks and stubble height at 16 cm or 24 cm, or cutting every 4 weeks with stubble height at 16 cm will result in good performance. The average TN removal rate is about 80% for these treatments.(2) The concentration of TP also reduced with the time passing. No significant different (P>0.05) for TP reduction was found among all the treatments. Therefore, there was no effect of cutting on TP purification. The removal rate for TN (80%) is better than that for TP (64%), taking non-harvest treatment as an example.(3) The change for the concentration of CODcr was fluctuating with the time passing. The effect of floating bed on CODcr removal is not apparent.(4) The concentration of NH4+-N also reduced with the time passing, for which the purification rate was above 95%, even close to 100% in most of the treatments, among with, no significant different effects was found. |
