1.2　The mechanism of constructed wetland for contaminant removal

With the development and application of CW in wastewater treatment,the mechanism of CW for various pollutants removal becomes clearer.The combination of degradability of vegetation,substrate,and microorganisms was the obvious features compared with other treatment processes.In the treatment process,the filler was acted as the support material to provide a good environment for the growth of vegetation and microorganisms.Meanwhile,the precipitation,filtration and adsorption function of substrate was benefit for the removal of suspended solids(SS),chemical oxygen demand(COD)and total phosphorus(TP)from the effluents.However,the different types of substrate showed the significant difference in the removal performance of contaminant.For example,the use of shale,bauxite,and limestone as the substrate of CW could improve the removal efficiency of TP.

The pollutant,including ammonia nitrogen(NH3-N),nitrate nitrogen(NO3-N),nitrite nitrogen(NO2-N),organic nitrogen,TP and organic matter,could be used as the metabolic substrate for the growth of vegetation and microorganisms.For refractory organic compounds,the phytoextraction,phytotransformation and rhizodegradation function also played an important role in the contaminant removal(Li et al.,2016).However,the biodegradation of these compounds by microorganisms was significantly higher than that of plants.Therefore,the mechanism of microbial degradation ability in removal of organic matter,nitrogen,phosphorus,and heavy metal was reflected in the sections below.

1.2.1　Biodegradation of organic matter

Organic matter in sewage is divided into soluble and insoluble forms according with the formation of dissolution.The insoluble organic matter exists in SS or colloid formation.The adsorption,co precipitation and chemical adsorption functions of CW medium would intercept the dissolved organic matter,and then these compounds were degraded by the extracellular enzymes of microorganisms into small molecular organic compounds.So,the soluble organic matter can directly penetrate and be degraded by the cells of microorganisms(Wu et al.,2016).In the process of metabolic degradation,microorganisms decompose pollutants into CO2,H2O,NH3 and so on,and release a large amount of energy for the microbial synthesis and metabolism(Tremouli et al.,2016).The degradation process of organic matter contains the anaerobic,anoxic and aerobic processes,and the process of energy metabolism is often accompanied by electron and proton transfer.In this period,the by-products of electron donors can be used as electron acceptors by microorganisms in the reduction of O2,NO3-N and NO2-N(Feng et al.,2013).

1.2.2　Removal of nitrogen

The forms of nitrogen in wastewater mainly included two forms: inorganic nitrogen and organic nitrogen.The ammonia nitrogen,nitrite nitrogen and nitrate nitrogen are the main formation of inorganic nitrogen in wastewater.In these compounds,the ammonia nitrogen has the strongest toxicity.Meanwhile,the high concentration ammonia nitrogen in wastewater is more toxic to plants and microorganisms.In particularly,the nitrogen content could be used as an executive indicator for the occurrence of red tides.The removal of nitrogen in the CW mainly included the following steps: assimilation,ammonification,nitrification,denitrification,shortcut nitrification and denitrification(Coban et al.,2015).

Assimilation refers to the part of ammonia nitrogen or organic nitrogen assimilated into the composition of microorganisms in sewage treatment.The content of nitrogen in weight cell dry was about 12.5%.Although after assimilation,nitrogen exists in the form of organic nitrogen in microbial cells,but part of the organic nitrogen could be re-released in the formation of ammonia by the microbial endogenous.

Ammonification is the process of decomposition or conversion of organic nitrogen from sewage into ammonia nitrogen under the respiration of ammonification bacteria(Huang et al.,2010).Most ammonification bacteria are heterotrophic bacteria,so ammonia nitrogen is often combined with organic decomposed into small molecular compounds.The amination of amino acids was listed as follows(Formula 1.1).

(1.1)

Nitrification was consisted of two processes:①ammonia was oxidized into nitrite by the action of Nitrosomonas;②nitrite was oxidized into nitrate by the action of Nitrobacter. These bacteria are collectively called nitrifying bacteria,and most of these microorganisms were heterotrophic bacillus or coccus.They widely existed in the wetland soil and could be used carbon dioxide as carbon source to obtain their growth energy in ammonia oxidation process(Jadhav et al.,2014).The previous studies showed that the energy production of ammonia and nitrite nitrogen in the oxidation process was under a low level.This also resulted in the growth rate of nitrifying bacteria was relatively slower compared to other bacteria.Therefore,the nitrification reaction rate of microorganisms during water treatment process was significantly limited to the production of metabolic energy.The common nitrifying bacteria included Nitrosomonas,Nitrosococcus,Nitrosolobus,and Nitrococcus(Park et al.,2006).The reaction process of nitration function is listed as follows:

Oxidation process of ammonia nitrogen:

(1.2)

Nitric acid oxidation process:

(1.3)

Total reaction process:

(1.4)

From the above reaction,it could be concluded that the oxidation of NH4-N molecular into nitrite nitrogen required 1.5 molecular oxygen,and oxidized to nitrate required 2 molecules of oxygen.In addition,proton is also generated during the nitrification process.Therefore,the CW with a good airflow diffusion level showed a good ability in nitrification function.

Denitrification usually occurs under anoxic conditions.The nitrite and nitrate produced in the process of nitrification could be reduced to gaseous nitrogen(N2,NO,N2O)under the effect of denitrifying bacteria.The most of common denitrifying bacteria(Pseudomonas,Bacillus proteus,Micrococcus etc.)are facultative bacteria.These bacteria could not only use oxygen as an electron acceptor to oxide organic matter but also use nitrite-nitrate nitrogen and electron acceptor for denitrification to removal nitrogen(Chen et al.,2014).The process of denitrification is listed as follows:

(1.5)

(1.6)

In denitrification process,microorganisms provide electrons(e-)and protons(H+)in the oxidation of organic matter.While the denitrification process occurs mainly in the processes of microbial anabolism and catabolism,and the removal of nitrogen by catabolism effects accounted for 70%-75% of total nitrogen removal(Zhu et al.,2015).However,the nitrogen removal during water treatment process by denitrifying effect is mainly limited by the lack of denitrification carbon source,which is closely related to the C/N composition of sewage in China.Therefore,for advanced wastewater treatment,the denitrification carbon source was employed to improve the total nitrogen removal rate.

The shortcut nitrification-denitrification refers to the ammonia nitrogen was oxidized to produce nitrite by the microbial nitrification function and then directly reduced to gaseous nitrogen forms(such as N2,NO,N2O).The advantage of this method could save a large amount of denitrifying carbon source(about 40% CH3OH)and reduce oxygen consumption(25%).The effect of shortcut nitrification-denitrification often requires more stringent reaction conditions(temperature,sludge etc.).However,the study of CW mainly concentrates on the operation of nitrogen removal under natural environment.The mechanism of nitrogen removal includes assimilation,ammonification,nitrification and denitrification.Even though there is a small amount of shortcut nitrification-denitrification in CW,the nitrogen removal rate by this function is under a relatively low level.At the same time,there are also some autotrophic bacteria in CW,which can directly convert ammonia to N2 and NO3-N under aerobic conditions.This process is called completely autotrophic nitrogen removal over nitrite.The reaction is listed as follows:

(1.7)

1.2.3　Absorption and biodegradation of phosphorus

The main mechanisms of phosphorus removal in CW are media adsorption,matrix filtration,co-precipitation,microbial metabolism,and plant uptake.Phosphate salts are inorganic compounds containing the phosphate ion,the hydrogen phosphate ion,or the dihydrogen phosphate ion,along with any cation.These soluble phosphate salts can be quickly formed compounds under the function of media contains calcium,magnesium,iron and aluminum ion(Calheiros et al.,2015),so that the phosphorus content was enriched in the substrate of CW in a short time.Notably,after several years,the phosphorus content of the constructed wetland can reach 10-1000 times of the influent concentration.Therefore,it is necessary to replace the substrate of CW periodically to ensure the efficient removal of phosphate.In addition,the phosphorus accumulating bacteria generally absorb phosphorus in aerobic environment,the release of phosphorus under the anaerobic condition.So,the reasonable optimization of CW treatment process can effectively promote the effect of biological phosphorus removal after the anaerobic and aerobic process of vertical flow CW.At the same time,inorganic phosphorus is also the nutrient element of wetland plants.Some wetland plants have strong absorption of phosphorus(such as Windmill grass),and the removal efficiency of phosphorus is generally maintained at 25%-70% in CW.It is found that the removal of inorganic phosphorus in sewage is closely related to the absorption of emergent plants,but organic phosphorus is mainly through biodegradation and matrix adsorption(Ciria et al.,2005).

1.2.4　Heavy metal removal

The removal of heavy metals from sewage by CW is mainly based on the function of co-precipitation,plant absorption and enrichment,chemical adsorption(Vymazal and á,2016).The heavy metal ions in wastewater diffused by plastid flow to c plant roots,and plant roots absorb heavy metal ions by root zone cell,which can be distributed in plant roots,stems and leaves through the phytotransformation(Jing et al.,2015).In addition,the microorganisms distributed in the media gap of wetland also showed a strong adsorption capacity to remove heavy metals.The process of microbial metabolism is accompanied by the transfer of electrons,which can be used to change the valence of heavy metals.For example,some bacteria have a reduction effect on Fe2+ and Cr6+,and some bacteria have an oxidation effect on Fe0 and Fe2+.The change of valence of these metal ions will reduce the toxicity of heavy metals and enhance their stability.At the same time,microorganisms can c2+bine so6+ heavy metal ions with proteins by assimilation,a0d final2+ form polymers through biological chelation to remove heavy metal from sewage(Matamoros et al.,2012).

1.2.5　Bacteria removal performance

The removal of bacteria in CW is mainly based on the function of matrix filtration,plant metabolism,plankton predation,chemical decomposition.The environmental adaptation ability of bacteria is extensive,but the drastic changes in environmental conditions might inhibit bacterial growth.Among these environmental conditions,the ultraviolet radiation contained in sunlight is an effective natural sterilization method,which has a harmful effect on most bacteria and other microorganisms.Therefore,the operation of tanks after CW with a lower water level is benefit for the bacteria removal during the wastewater treatment.