2　Systematic Structure Technology

2.1　Hydraulic Load

Hydraulic load is one of the important factors which have influence on the purifying effect of systems such as constructed wetland and ecological floating bed.A lot of researches(1999,2005)demonstrate that the removal of nitrate nitrogen is strengthened by increasing hydraulic loading of the constructed wetland.Generally,a higher hydraulic load means a relative increase in organic load and thus more carbon source for denitrifying bacteria,which contribute to remove nitrate nitrogen.But other reports(2009)show that the removal efficience of nitrate nitrogen by constructed wetlands with different hydraulic retention times have no significant difference.That is likely related to the scale and working time of wetlands.

The study of Wu and Luo(2008),which was about the effect of hydraulic load on nitrogen removal of floating island,showed clearly that TN removal load increased with increased hydraulic load of up to 0.3m3/(m2·d),and decreased gradually with an increase of hydraulic load beyond this value.In research,in which Eichhornia Crassipes was used to remove nitrogen and phosphorus under various hydraulic load conditions(0.14;0.20;0.33;1.00m3/(m2·d)),decreasing removal rates and increasing removal loads were observed with the ascension of the hydraulic load Under high hydraulic load conditions,research elsewhere has established that nitrate nitrogen can be removed significantly only when the hydraulic load does not exceed a certain value(2010).

Hydraulic retention time reduces accordingly with the augment of hydraulic load in a certain range,but contaminants can be degraded normally and properly increased pollution load can impel the degradation to be more thorough,which enhances the purifying effect.However,excessively high hydraulic loads result in some change in the precipitation and reaction times and cause some problems such as weakened plant uptake,incomplete nitrification or denitrification,inadequate pollutant settlement,and increased difficulty in absorbing suspended solids.And the restricted degradation possibly resulting from excessively high hydraulic loads will reduce the purifying effect.

In the ecological bed system,treatment efficiency can be enhanced with the augment of hydraulic load to less than a critical value,whereas treatment efficiency worsens when the hydraulic load exceeds the critical value.However,the critical load changes owing to different conditions,such as structure or scale,in various systems(2008;2004).

2.2　Inflow Configuration

Dissolved oxygen is one of the important indicators used to inspect the purifying effects of sewage treatment systems.The shortage of dissolved oxygen prevents further improvement in the further improvement in the treatment ability of the treatment systems.To solve the oxygen shortage problem,micro-pumps are utilized to make the sewage circulate in the floating bed system,and achieve higher removal rate under experimental conditions(2004).Batch-flow,widely used in the constructed wetlands,can enhance re-aeration ability and contribute to enhance purifying capacity.Researchers introduce sewage into the vertical flow reed-bed filter by intermittent inflow,and the removal rate is greatly increased.

In the comparison research about treatment effects under batch-flow and continuous-flow conditions,Nie and Nian et al.(2007)concluded that batch-flow makes a contribution to higher DO concentration and has great impact on TN andremoval and less influence on NO3-N,TP and CODMn removal.The researchers found that the intermittent MBR(membrane bioreactor)had a significantly better effect on the removal of TN and NH3-N than the aerobic MBR(2010).Batch-flow can strengthen reaeration ability,and help to aerobically decompose pollutants.Meanwhile,influent discharge of batchflow is much greater than that of continuous-flow for the same daily hydraulic load,which can make local pores(ground substance of constructed wetland,artificial media of floating bed)filled with water.Then the conditions beneficial to nitration reaction will be developed.The stronger water impact reduces dead space inside the system,and this can explain the increasing treatment efficiency of TN and(2009).

Nowadays,the study about hydraulic load and batchflow in the ecological floating bed system is mostly confined to experimental investigation of the comparison between different hydraulic loads or between batch-flow and other inflow configuration.Since there are close relations between hydraulic load and discharge as well as between batch-flow and flow characteristics of rivers controlled by gates,ecological floating bed systems in practice are adjusted and optimally combined with the scheduling schema.

2.3　Planting Pattern

When using the macroohytes to treat sewage,people get used to applying various plant cultivations instead of single plant cultivation,mainly because when macrophytes are used to treat sewage,the most obvious functions are landscaping and the influence on wildlife habitat.Combination cultivation can increase the diversity of wildlife habitat,and the seasonal differences between plants result in different landscaping for different seasons.Additionally,it was proved that plant configuration can enhance nitrogen and phosphorus removal(1997).And different plants in the same environment as well as plants in different environments have different purifying capacities.Therefore,appropriate combination cultivation helps to establish a perfect ecological system,improve purifying capacity,and ensure long-term stability of the system.

2.4　Application of Plants

Nitrogen and phosphorus are restrictive elements of primary producers in most freshwater ecosystems,consequently,the control target of freshwater ecological restoration is to reduce nitrogen and phosphorus(2004).Nutrients-nitrogen,phosphorus-are accumulated in the grown plants and taken away through harvest.This method is feasible(2001;2007).However,there are two key steps required to enhance the role of plants:application of plants before running the system and vegetation management during running.

Generally,terrestrial plants are difficult to grow in aquatic environments because they have adapted to dry environments;but most hygrophytes which grow in wetland,marsh and river banks can be cultivated in floating beds because they can adapt to aquatic environments.But how well these plants adapt to the aquatic environments differ from plant to plant.The research of Li and Nie(2003)shows that four plants,rice,Chrysopogon zizanioides,Canna lily,and water taro,are able to grow in floating beds very well;the biomass and root activity of rice and Canna lily change slightly while the biomass of Chrysopogon zizanioides decline significantly.

Even if in the extreme environment,such as a strong reducing environment,meta-acid/partial alkaline environment,and toxic water conditions,hygrophytes,including reed,Juncus roemerianus,cattail and so forth,can grow well because of their special growth mechanisms(2003).In addition,hygrophytes have great tolerance for heavy metal(2005;2008)so that they can live in the water of high heavy metal concentration,whereas different plants have different accumulation abilities for various heavy metals(1997;2010).As has been stated in the Zayed's(1998)report,Typhalatifolia has higher accumulation ability for Cd,Cu,Ni,and Pb while Needle Lin can accumulate high levels of Cr.It has been found that plant biomass has strong affinity for Cu,and Zn by the adsorption/resolution of watermifoil for Co,Cu,Ni and Zn as studied by Lesage and Mundia(2007).

According to the research on the purification ability of plants(1997;2010;2011),the removal ability of emergent plants for nutrients and heavy metal is stronger than of submerged plants and floating plants,which is related to their sturdy roots and developed adventitious roots.Researchers report that the accumulation of nitrogen,phosphorus(2012)and heavy metal(1983;2010)in the plant tissues follows an ascending order of root,stem,and lamina.Plant roots mainly absorb contaminant.Developed roots conduce for the adhesion of rhizospheric microorganism as well as the absorption and deposition of pollutants.Hence,hygrophyte with developed roots can help effectively to remove water contaminant.Briefly,during the process of building an ecological floating bed system,it is necessary to investigate first the pollution characteristic of the water to be treated,then according to growth characteristics,pollution resistibility;etc researchers can select appropriate plants.Choosing plants must be done carefully because some plants,such as Eichhornia crassipes,which have very high reproductive capacity,can cause an ecological invasion.

All in all,a critical factor that determines the purifying effect is structure technology of the ecological floating bed system.Combined with pollutant characteristics and surroundings,the best ecological floating bed system can be designed only when the designer carefully takes into consideration the structure technology which has crucial effects on biological degradation and removal mechanisms of the pollutants.