3　Vegetation Management

The experience,accumulated by researchers locally and abroad in using constructed wetlands to treat sewage shows that to attain and maintain optimal treatment effect,vegetation management is required in order to achieve suitable vegetation balance.There are mainly two kinds of balance:the balance between the biomass for reaching treatment effect and excessive accumulation of plant waste;and the balance between removal capacity caused by abundant biomass and nutrient load resulting from phytodegradation.

In the newly-built treatment system,plants which grow rapidly can absorb lots of nutrients,mineral substances and so on.Plants living in nutrient-rich water and comfortable climate can produce large amounts of biomass within a short period of time.During this growth stage,nutrients get reduced because of the rapidly increasing biomass.Indeed,compared with the system without plants,treatment efficiency of the systems with plants is improved significantly(1995;1996).As plants grow,the work mechanism in the treatment process keeps changing.The mature plants growing stably can drive the denitrification and nitrogen cycle of the system(2005),which plays a critical role in water treatment,but the removal ability mainly activated by plants which are growing rapidly decreases.

There are a series of positive roles played by plants growing stably in a treatment system,such as,plants absorb nutrients and other pollutants(2000),provide attachment points(1998)for microbial and invertebrates;oxygen from developed roots is beneficial for mineralization and nitration(2001);and plant degradation provide some carbon source for heterotrophic microorganisms like denitrifying bacteria,which can help to remove nitrogen.However,intensively grown-plants will lead to a list of problems when the biomass gradually approaches maximum.Some of them are as follows(1999;2000):nitration is confined owing to the lower DO concentration(usually less than 1.0mg/L)(2000;2000)resulting from dense vegetation;organic load increases evidently because of plant degradation largely;and plant shadow negatively affects the growth of algae and autotrophic microorganisms,which results in their decreasing retentivity for nutrients.

Naturally,plants'accumulation of nutrients by absorption is temporary;actually the impact of plants on treatment efficiency divides into two stages:purification and degradation(2010).In the purification stage,nutrient concentration decreases quickly because of the indirect effect of plants on water(microorganism,physical parameter,etc)and assimilation of nutrients.However,in the degradation stage,nutrients accumulated in plant tissues return to water,which leads to increased nutrition load and water degradation.It is reported that 31%of the initial quantity will remain in the system after plants die one year(2006).Therefore harvest is one of methods effectively carried out in vegetation management.At present,the harvest in engineering is only done for the plants above water,not for the roots under water which have not received much study.However,plant roots play a vital role in systems where plants are used for sewage treatment.According to research,the resident time of sewage near plant roots has a crucial influence on the assimilation and contaminant degradation(2003).Clearly,plant roots are important to the purification effect of the system.

Nitration-nitrification by microorganisms is the primary mechanism of the system for removing nitrogen(1995).Nitrobacteria can convert ammonia nitrogen to nitrate nitrogen under aerobic conditions,and nitrate nitrogen can be converted to N2 anaerobically.Organic nitrogen can be nitrated or absorbed eventually under conditions in which organic nitrogen is decomposed into inorganic nitrogen.Oxygen is transmitted to roots by plant tissues,and released to water by roots.Then aerobicanoxic-anaerobic environments(2008;2003)are able to exist in seriously polluted water and provide habitats for different microorganisms,which are beneficial for removal of nutrients including nitrogen,phosphorus,etc.

Developed roots can provide a huge surface area for microorganism attachment(2010),meanwhile root exudates and insoluble contaminants intercepted by roots can supply abundant carbon source and nutrients for microorganisms,all of which contribute to make full use of biological degradation.It has been reported(2008)that the microorganisms in the rhizosphere play major roles during the degradation processes of organic pollutants,ammonia nitrogen,etc.Some functions occurring in plant roots,such as,the interception,the degradation resulting from microorganisms attached to the roots,oxygen secretion and so on,lead to the strong ability of plant roots to remove contaminations.

Plant roots also play a significant role when plants are used for removing heavy metals.There are two parts in the removal process aroused by plant roots:rapid adhesion,absorption and accumulation(2008).Actually,the heavy metal removal by plant absorption only accounts for a part(2007).Plants play more important roles in the settlement or sedimentary processes for heavy metal removal(2001),for instance,various biological geochemical reactions happening around plant roots can motivate the settlement of heavy metal.And research indicates that the capacity of sediment to store heavy metal can get enhanced due to the change in water environment caused by the biochemical reactions around roots(2008).

The decline in gas permeability of the cell wall,gradually appears with the growth of plant roots,and results in the minimization of radial leakage oxygen quantity,which makes more oxygen reach the apical meristem(1997).The aerobic effect of oxygen which is released by the apical meristem can detoxify potentially-toxic substances.Therefore,the periodic harvest of the above water vegetation in ecological floating bed systems can conserve mature plant roots,which contribute to the degradation of contaminants.Aged roots,however,which are bound to fall away naturally,will increase nutrient load.It is suggested that plant rotation is adopted,and plants including their roots should be reaped when one round of plants begin to die.Thus,we can take full advantage of the seasonal differences of various plants.

Harvest by reaping the plants above the water surface when plant biomass is close to its maximum,aims to keep plants growing rapidly;while plants,including their roots,are removed at the end of the growing season to start a new round of cultivation and management.All of these can reduce pollution as much as possible.Better purification effect is obtained when plant rotation and harvest are used to manage vegetation in the floating bed.