Design of Constructed Wetland for Treatment of Tailwater from Wastewater Treatment Plant

Kun LI,Ling WANG,Zhaohua LI,Jifa MIAO,Xiangrong WANG

1.Department of Environmental Sciences and Engineering,Fudan University,Shanghai 200433,China;2.Institute of Subtropical Agriculture,Chinese Academy of Sciences,Changsha 410125,China;3.Faculty of Resources and Environmental Science,Hubei University,Wuhan 430062,China

With the rapid development of economic and improvement of people’s living standards,the water pollution,caused by discharge of industrial wastewater and urban sewage has become more and moreserious.A largeamountof wastewater that is rich in nitrogen,phosphorus,organic matter and other nutrients is discharged directly into rivers,lakes and even oceans,resulting in eutrophication of water bodies.Therefore,water pollution control has been a hot area in environmental and ecological studies[1].In recent years,the construction of wastewater treatment facilities has been accelerated in China,so the number of wastewater treatment plants is increased every year.Although the nitrogen,phosphorus and organic matter contents in outlet water from most of the wastewater treatment plants reach the national discharge standards,the quality of outlet water still cannot meet the practical requirements.In this situation,the upgrading and transformation of wastewater treatment plants are necessary.Moreover,they have been focuses in new wastewater treatment works[2].Constructed wetland,as a treatment process for tailwater from wastewatertreatmentplants,has been applied more and more widely[3-4].

Constructed wetland,as an enhanced ecological treatment facility,is an artificial ecosystem that simulates naturalwetland.Itis constructed,monitored and controlled all by hand.Constructed wetland is similarto swamp,and can purify wastewater more efficiently through filtration,adsorption,precipitation,ion exchange,absorption by plants and microbial degradation[5].The purification of wastewater is achieved by the triple synergy of physical,chemical and biological effects in the ecosystem.Constructed wetland can be constructed based on local conditions.In addition to purifying wastewater,constructed wetland also has excellent ornamental effect.It helps to achieve the decentralized processing ofwastewater.Therefore,constructed wetland has been widely applied in decentralized middle and low-concentration wastewater treatment at home and abroad,and it can effectively remove organic matter in polluted water bodies[6].Constructed wetland can be treated as a technical supplement for conventional wastewater treatment processes.The treated water,by constructed wetland,can meet different requirements,and can also be used as a supplementary water source for restoration ofcontaminated water bodies.Therefore,constructed wetland can produce good environmental and economic benefits[7-8].In this study,the design of constructed wetland for tailwater treatment from the wastewater treatment plant in Wudang Mountain was taken as an example so as to provide technical support and reference for upgrading and transformation of urban sewage treatment plants in China.

Engineering Background

The Jianhe River is located in the Wudang Mountain Tourism Economic Zones of Shiyan City,Hubei Province.It is a tributary in the right bank of the Zhihe River.The length of the main channel is 26.5 km.Jianhe River enters the Taiji Lake through the Qiaojiayuan Bridge,then flows through the cities,factories,roads,wastewater treatmentplants in the Wudang Mountain,and finally enters the Danjiangkou Reservoir.The wastewater treatment plant is located in the right bank of the Taiji Lake,which is located in the downstream of the Wudang Mountain Tourism Economic Zones.It has ample room for expansion,convenient transportation and superior engineering geologicalconditions.The wastewatertreatmentplantin the Wudang Mountain is mainly used to treat industrial wastewater and domestic sewage around the Jianghe River basin.Currently,the wastewater treatment capacity of the plant is 7 000 m3a day.The treated water is directly discharged back into the Jianhe River.At present,the treated water from the wastewater treatment plant has reaches the Standard B of the first class in Discharge Standard ofPollutants for MunicipalWastewaterTreatment Plant(GB 18918-2002).In 2006,the State CouncilapprovedtheDanjiangkou Reservoir Area and Upstream Water Pollution Control and Soil Conservation Planning(GH[2006]No.10).Ifthe tailwaterfrom the wastewatertreatmentplantis not treated but discharged directly into the Jianhe River,it will undoubtedly pollute the water in the Danjiangkou Reservoir,affecting the core area of protection of water source for the South-North Water Diversion Project,threatening water source security of the middle route of the South-North Water Diversion Project.Therefore,to improve the water ecological environment in the Wudang Mountain Tourism Economic Zones and to eliminate the pollution of water source of the South-North Water Diversion Project,the tailwater from the wastewater treatment plant in Wudang Mountain is treated and purified further.Thus the outlet water from the wastewater treatment plant will reach the Standard A of the first class,and the water quality in the Danjiangkou Reservoir will meet the requirements by function zoning.

Overall Engineering Design

Determination of process

The wastewater treatment plant is located in the Wudang Mountain Tourism Economic Zones,so the environmental and ornamental requirements are particularly high.The analysis on stabilization pond,activated sludge,oxidation ditch and other conventional wastewater treatment processes shows that conventional wastewater treatment processes require a large area of buildings.Even worse,most of the buildings are directly exposed to the ground[9].According to the overall planning of Wudang Mountain Special Economic Zone,a wetland park is planned near the wastewater treatment plant.The conventional wastewater treatment processes will produce great environmental perturbations,and they are uncoordinated with the overall planning of the Wetland Park beside Taiji Lake from the aspects of view and landscape.So constructed wetland,which has both landscaping and ornamental effects is more appropriate.Currently,based on the flow type of wastewater,the structural design of constructed wetland wastewater treatment system can be divided into three types,surface flow constructed wetland,subsurface flow constructed wetland and vertical flow constructed wetland.

The terrain,elevation and other parameters around the plant can be measured and analyzed by total-station electronic speedometer and GPSRTK method.Combining the native characteristics of surface flow constructed wetland,subsurface flow constructed wetland and vertical flow constructed wetland,it can be found that the qualities of both inlet and outlet water of subsurface flow constructed wetland just meet the practical requirements (Table 1).It also means that the tailwater from the wastewater treatment plant,which reaches the Standard B of the first class in Discharge Standard ofPollutantsfor MunicipalWastewaterTreatment Plant(GB 18918-2002)can be purified by subsurface flow constructed wetland,and the treated water will reach the Standard A of the first class.It is also indicated that the treated tailwater can be directly discharged into the Taiji Lake Reservoir.At the same time,subsurface flow constructed wetland is characterized by low capital investment,low operating costs,relatively simple maintenance and management and good treatment effect,etc[10-12].

Location selection

Based on analysis on terrain parameters around TaijiLake and wastewater treatment plant,there are two sites suitable for the construction of constructed wetland.The first site A is near the wastewater treatment plant.It is a valley located between the levee of Taiji Lake and a mountain.The usable area is 7 528 m2.The site A does not require the construction of pipelines.The outlet water can flowdownstream itself and can be applied directly by green and other water-consuming projects.The second site B is located 807 m upstream the wastewater treatment plant.It is a beach inside the protection embankment of Taiji Lake.The site B covers a relatively large area,and has opener ground.However,due to the site B being located upstream the wastewater treatment plant,pipeline and upgrading project are needed to be constructed.In addition,the filling also requires huge amount of work (Fig.2).Based on field investigation and native characteristics of wetland itself,the first site A is selected as the construction location of wetland.

Table 1 Requirements of outlet water quality from constructed wetland mg/L

Engineering Design of Subsurface Flow Constructed Wetland

Referring to technology research results of constructed wetland,practical experience of demonstration projects and related design manuals at home and abroad[13-19],the current latest design parameters in the field of research on constructed wetland are applied in the design of constructed wetland for treatment of tailwater from the Jianhe wastewater treatment plant.

Systematic design parameters Basic parameters of porous tubes

The water collecting pipes and water distributing pipesall adoptU-PVC plastic pipes.The U-PVC plastic pipes have three specifications,including 250,160 and 60 mm of diameters.The water collecting and distributing pipes all require to be opened.The diameter per opening is all 6 mm,and the spacing between two adjacent openings is all 20 cm.Thus clogging will be effectively prevented.

Basic parameters of collecting well

According to current treatment capacity of the wastewater treatment plant in Wudang Mountain,the scale of collecting well is designed as 7 000 m3/d,equivalent to 291.7 m3/h.The hydraulic retention time (HRT)is calculated as 1.5 h.So the volume of collecting well is designed as 437.5 m3,with length of 10 m,width of 8 m and height of 6 m.

Table 2 Calculation of surface area of constructed wetland

Parameters of water pumpAccording to the volume of collecting well and scale of constructed wetland,two water pumps with model of 125-230C and flow of 291.7 m3/h are selected.One is utilized,and one is prepared as a standby.The head,speed,power,efficiency and working hours of the two water pumps are 7.3 m,1 480 r/min,11 KW,74%and 24 h/d.

Basic design parameters of subsurface flow constructed wetland

Surface area of wetlandThe pollution loads of constructed wetland include chemical oxygen demand(COD)surface load,total nitrogen (TN)surface load,ammoniacal nitrogen(NH3-N)surface load and total phosphorus(TP)surface load.The pollution loads can be used to calculate the surface area of constructed wetland.

Wherein,Qrefers to wastewater flow,m3/d (Qis assigned as 7 000 m3/d);C0refers to pollutant concentration in inlet water,mg/L or g/m3;Cerefers to pollutant concentration in outlet water,mg/L or g/m3;Nrefers to surface load of pollutant,g/(m2·d).

TP and TN are core pollutants for eutrophication of water bodies.Combining the actual area of wetland site,the surface area of the subsurface low constructed wetland is designed as 7 227 m2.

Hydraulic retention timeThe calculation formula of hydraulic retention time is as follow:

Wherein,trefers to hydraulic retention time,d;vrefers to volume of the pond,m3[v=7 227 m2×1.2 m(depth of filler in the wetland)=8 672 m3];d refers to porosity of the wetland,also the percentage between spacing among filler and volume of the pond(it is assigned as 30%in this study);Q refers to average flow rate,m3/d (it is assigned as 7 000 m3/d in this study).Therefore,t=8 672 m3×30%/7 000 m3/d=0.432 d=10.368 h.

Calculation of hydraulic loading

The calculation formula of hydraulic loading is as follow:

Wherein,Q=7000m3/d;As=7227 m2.Therefore,HLR=7 000 m3/d/7 227 m2=1.0 m3/(m2·d).

Graphic design

The ratio between length and width of constructed wetland should be better controlled between 1∶1 and 3∶1.The area ofan independentunit should not be larger than 800 m2.The designed subsurface flow constructed wetland covers an area of 7 227 m2.The slope gradient of the bottom slope is 1%.A total of 20 units are arranged.Among the 20 units,the lengths and widths of the 18 units are all 27 and 14 m;the lengths and widths of the remaining 2 units are 26.5,9.5 m and 18,9.5 m,respectively.

Structural design

Arrangement of inlet and outlet water systemsTo ensure the distribution evenness of inlet water system of the wetland bed,several ponds are connected in parallel in the horizontal flow zone of the constructed wetland.In the inlet zone,V-shaped groove and overflow groove are arranged.The water distributor will divide wastewater evenly into all units of the wetland.In the subsurface flow zone,the wastewater is imported through perforated wall,and then distributed by perforated pipes and finally flows through the filler layer. Moreover, the wastewater should be ensured to flow vertically and evenly through the filler layer on the surface of the constructed wetland.The outlet water system should be designed based on the regulation requirements for water level in wetland bed.The perforated water collecting pipes are arranged above the antiseepage at the end of the outlet water area.The staggered openings are all 45°downward.In addition,rotation elbows and control valves are designed to regulate the water level in the wetland bed.

Selection of bed fillerThe filler of the wetland adopted rubble and other 5 kinds of materials.The wetland bed is constructed with different-size materials according to different proportions.At the bottom,30 cm of rubble layer is laid.The diameter of rubble ranges from 8 to 10 mm.On top of the gravel layer,20 cm of anti-seepage layer is laid.Then 20 cm of zeolite layer(diameter of 0.3 to 1.0 mm),30 cm of ceramicite layer(diameter of 10 to 20 mm),20 cm of gravel layer(containing 2-2.5 kg/100 g of calcium,diameter of 5 to 8 mm)and 30 cm of quartz layer(diameter of 2 to 6 mm)are laid in turn.

Water level control of wetland bed

The subsurface flow constructed wetland needs to maintain a certain depth to ensure the growth of plants and necessary aerobic conditions in the units of constructed wetland.The roots of plants should be submerged in water in the wetland bed with an even depth.The ultra-high waterlevel,caused by stormwater runoff in the rainy season,should be prevented.To ensure the normal growth and functions of most of the plants that are submerged in the water,the depth of water in the wetland bed should be controlled below 200 mm.

Freeze prevention of wetland bed

The constructed wetland is located at an altitude of 171 m.So in the winter,two heat-conservation and anti-freezing measures must be taken.First,the water level in the constructed wetland must be increased(≥50 mm)when cold air comes.Thus the surface ice will conserve heat for filler and roots of aquatic plants.Second,leaves,twigs or agricultural plastic film can be used to cover the surface of constructed wetland so as to reduce heat loss from the wetland.

Anti-clogging of constructed wetlandConsidering thesuspended solids concentration,organic load,dosing methods,filler size,microorganisms,operating cycle and other factors,the following ways are adopted to reduce clogging.First,the intermittent operation mode is adopted.The constructed wetland is divided into many units and the units are connected in parallel.Thus the water supply for parts of the units can be cut off for 1-2 d every 5-7 d.Second,pre-aeration is carried out for wastewater so as to improve dissolved oxygen in filler of constructed wetland,better play the role of microbial decomposition and prevent soil accumulation of extracellular polymeric substances.Third,the filler size and mixed proportion should be chosen appropriately so that the filler not only can purify wastewater but also can prevent clogging.Finally,on the premise that constructed wetland accepts the maximum designed flow,the inlet water system must not be clogged so as to prevent the occurrence of surface flow.

Selection of plants

According to the actual situation of pollutant content in the tailwater of the wastewater treatment plant,the selection of plants must consider the following principles:a.the selected plants should have long growth period;b.the selected plants should have ecological safety and will not produce threats to local ecological environment;c.the selected plants should have certain economic benefit,cultural value,landscape value and comprehensive utilization value;d.the selected plants should have good ecological adaptability,ecologicalconstruction capacity,strong screening and purifying ability, uniform resistance but smaller growth(i.e.local or local natural wetland-native plants);e.the selected plants should have strong vitality and growth potential,high antifreeze,anti-heat and anti-pest capacity and strong adaptability to surrounding environment;f.since aquatic plants mainly depend on the microorganisms that are attached to or near the surface of plant roots to eliminate BOD5,COD,TN,TP and other pollutants from wastewater,the selected plants should have developed rootsystem and strong pollution tolerance[20].

Based on the six principles above,cattail,calamus and canna are finally selected for the constructed wetland.The row and plant spacings of cattail,calamus and canna are 30×30,25×20 and 30×20 cm,respectively.When the selected plants are planted,the ponds should have certain-depth water,and the planting density is a litter higher in the inlet water zone so as to provide flocculation conditions for inlet water.When the planting is completed,the hydraulic loading will be increased gradually so as to make the plants adapt to wastewater.The same-batch plants should have uniform size,and the too young plants should be forbidden.The growth of roots of the selected three kinds of plants can guarantee the necessary aerobic conditions for units of the constructed wetland.Thus the oxygen can be fully delivered to the bottom of the wetland,preventing the formation of stagnant water.The three kinds of plants not only can foil each other during their growth from the perspective of view,but also can enhance their own diseases and pests resistance.In addition,the three kinds of plants also foil each other from the perspective of treating wastewater.

Results and Discussion

After the constructed wetland is completed and put into operation,a self-healing wetland ecosystem composed by different-scale and irregular subsurface flow wetland units will be formed.It can not only effectively optimize wastewater flow pattern,improve the utilization of pond capacity,reduce floor space and reduce clogging,but also can simplify the water distribution system,reducing the cost.The process can treat 7 000 t of tailwater from the wastewater treatment plant a day,and can reduce the emission of COD,BOD5,TN,TP and NH3-N by 25.55,25.55,12.78,1.28 and 17.89 t respectively a day.Moreover,the outlet water is proved to reach the Standard A of the first class inDischarge Standard of Pollutants for Municipal Wastewater Treatment Plant(GB 18918-2002).The system requires no other mechanicalequipmentexcept water pump.It is characterized by low energy consumption,simple operation and easy maintenance and management.Therefore,subsurface flow constructed wetland can be used to deeply treat the tailwater from wastewater treatment plant in Wudang Mountain.The pollution load to river will be mitigated,and the self-purification capacity of water bodies will also be enhanced.At the same time,the multiple roles of Jianhe River,such as conserving water,regulating floods,storing floodwater,maintaining biodiversity and constructing landscape diversity,will be fully played.

Different from the construction of conventional landscape water bodies,the construction of wetland has more strict requirements for anti-seepage treatment and distribution evenness of pipes.However,due to insufficient running data,the constructed wetland still needs to be studied further.The constructed wetland has applied recently in treatment of tailwater from wastewater treatment plant in China,so people may have no thorough understanding.Coupled with lagged technology,the application potential of constructed wetland has not been dug deeply.During the construction and operation of constructed wetland,three issues still need to be paid attention to.First,subsurface flow constructedwetland,asanadvanced treatment process,is characterized by low operation and maintenance costs,less pollution,etc.But it requires a larger area.The construction of constructed wetland should better foil with surrounding open place and valley,thereby combining the wastewater treatment and landscape design.Second,the larger filler can effectively prevent the occurrence of clogging,but they may also shorten the hydraulic retention time,affecting the purifying effect.So the purification and anti-clogging should be balanced.Third,in addition to purifying effect and seasonality,the propagation speed and visual effect of selected plants should also be taken into account.

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