OPTIMIZED DESIGN OF NATURAL ECOLOGICAL WASTEWATER TREATMENT SYSTEM BASED ON WATER ENVIRONMENT MODEL OF DYNAMIC MESH TECHNIQUE*

XU Zu-xin, WEI Zhong, YIN Hai-long

State Key Laboratory of Pollution Control and Resource Reuse, Tongji University, Shanghai 200092, China, E-mail: xzx@stcsm.gov.cn

HUANG Li-hui

College of Environmental Science and Engineering, Shandong University, Jinan 250100, China

(Received June 7, 2009, Revised October 9, 2009)

OPTIMIZED DESIGN OF NATURAL ECOLOGICAL WASTEWATER TREATMENT SYSTEM BASED ON WATER ENVIRONMENT MODEL OF DYNAMIC MESH TECHNIQUE*

XU Zu-xin, WEI Zhong, YIN Hai-long

State Key Laboratory of Pollution Control and Resource Reuse, Tongji University, Shanghai 200092, China, E-mail: xzx@stcsm.gov.cn

HUANG Li-hui

College of Environmental Science and Engineering, Shandong University, Jinan 250100, China

(Received June 7, 2009, Revised October 9, 2009)

A Natural Ecological Wastewater Treatment System (NEWTS) is usually built on natural terrain with necessary topography modification to improve water flowing route and pattern, and then the topography modified NEWTS should also have a reasonable water storage volume and hydraulic retention time so as to achieve the anticipated water purification effect. In this study, the dynamic mesh technique based on the finite element method and element storativity coefficients was presented to develop a two-dimensional hydrodynamic and water quality model, which was used to optimize the design of NEWTS under the dynamic land-water boundary due to various water storage volume. The models were employed in the optimized design of NEWTS from a large abandoned coal mine, which purifies the polluted water flowing into a large water storage lake, as part of the East Route South-to-North Water Transfer Project in China. Specifically, the natural topography modification scheme was presented, and further, a reasonable water storage volume and hydraulic residence time was obtained, based on the reasonable estimation of roughness coefficient and pollutant removal rate of the NEWTS with phragmites communis.

ecological wastewater treatment, dynamic mesh, hydrodynamic model, water quality model, abandoned coal mine

1. Introduction

A Natural Ecological Wastewater Treatment System (NEWTS) is usually formed in terrain modified beaches, lakes, or ponds where aquatic plants are cultivated to remove water pollutants through biological, physical and chemical processes such as sedimentation, plants adsorption and microorganisms degradation. The typical NEWTS include the wetland treatment system, stabilization pond treatment system, water detention pond system, etc.. Because of its simple management, low running cost and strong adaptability to the variation of wastewater pollutants load, the NEWTS has increasingly accepted in recovery and purification of polluted water throughout the world.

The key technical aspects in the design of NEWTS can be summarized as follows: (1) to optimize the design of water flow pattern, (2) to improve water purification effect. Most of NEWTS are surface flow systems. It is not surprise that theNEWTS contains poor water flow patterns such as dead flow and short-circuiting flow due to its irregular topography. Therefore, the modification of original topography of NEWTS is often necessary to obtain an optimized water flow pattern to achieve ideal form of uniform flow and plug flow. Based on the reasonable flow pattern, the water storage volume and Hydraulic Residence Time (HRT) should be further optimized so as to assure the anticipated outflow water quality. To achieve all these simultaneously, the design cannot be purely rely on empirical values, and the hydraulic analysis method must be introduced.

Tracer experiment is the typical method to identify the flow characteristics of NEWTS. David et al.[1]studied water flow pattern over surface wetland system by injecting dye into the wetland inlet and observing the tracer concentration at different stations of the wetland. In other studies, Rhoda-mine WT was used to study residence time distribution characteristics of storm water pond under different water depths and flow velocities[2], and the flow characteristics of surface water wetland that received agricultural drainage water and the relationship between hydraulic efficiency and phosphorus removal rate[3]. Joan et al.[4]evaluated the hydraulic efficiency of horizontal subsurface artificial wetland for different length-width ratios and substrate granular size. Although the tracer experiment could reveal the general operation status of NEWTS, it is still technically a “black-box”, which cannot illuminate the specific flow field inside the domain of the system.

To solve this problem, water environment mathematical model has been used to analyze the characteristics of flow pattern and water quality of NEWTS. Jenkins[5], Ranjit[6]et al. used a horizontal two-dimensional hydrodynamic model to study the wetland hydraulic efficiency under different length-width ratios and vegetation densities. Water environment model was employed to optimize the design of NEWTS including vegetation cultivation, out-flowing section configuration, flowing induced device application, etc.[7-9]. Another two-dimensional horizontal numerical model was used to evaluate treatment efficiency of stabilization pond with 13 types of layout so as to determine the optimized pond shape[10]. Synoptically, the above systems were characterized by fixed shape and constant flooded area, which were similar to the sewage treatment facilities with fixed shape. However, many NEWTS are formed on natural open terrains, and as the water storage volume varies, there will be dynamic changes in land-water boundary and flooded area. Thus, NEWTS may not be in a fixed shape. The terrain modification such as channel excavation also causes the change in land-water interface in the system. With considering these characteristics, the water environment model with dynamic mesh technique is worked out to achieve the optimized design of NEWTS.

2. Materials and methods

2.1 Site description

The South-to-North Water Transfer Project in China is a major infrastructure project aiming at supplying water resource to the Northern China where water shortage is serious. The Water Transfer Project consists of eastern route, middle route, and western route. For the eastern route, the project draws water from the downstream of the Yangtze River near Yangzhou City, and supplies water to Shandong Peninsula and Tianjin through the old Great Canal and the parallel watercourses.

The Nansi Lake, located in Shandong Province, serves as a very important water storage lake in the eastern route, and plays an important role in assuring water quality and safety of the project. Based on the advice on implementing water pollution control plan of South-to-North Water Transfer Project issued by the State Council of China, the Nansi Lake watershed is one of the most important water pollution control regions, where water quality of rivers flowing into the lake should meet the goal of Grade Ⅲ of national standard for surface water quality.

Fig.1 Planning NEWTS in the Nansi Lake watershed

Tengzhou City is located in the Nansi Lake watershed, and all rivers in the city flow into the Nansi Lake, among which the Cheng River and Guo River are two major rivers with serious pollution, as shown in Fig.1. During the past years, the water quality of the two rivers ameliorated to some extent due to regional industrial structure adjustment and discharging wastewater treatment, and as a result the blackness and odor in middle-stream and down-stream of the rivers was eliminated. Nowadays, the new wastewater treatment plants are still in construction so as to make sure that river water quality meets the goal of Grade Ⅴor even better. However, to meet the goal of Grade Ⅲ, river water is still left to be purified further.

Tengzhou belongs to Zhaozhuang City, famous for its coal mine industry. Many abandoned coal mine collapsed in this region. It is intended to re-utilize the abandoned coal mine to construct NEWTS for further purifying the Cheng River and Guo River to meet Grade Ⅲ (as shown in Fig.1). As Fig.1 shows, water in the Cheng River will diverse into a planned NEWTS based on an abandoned coal mine called Quanshang, and then discharge into the Nansi Lake after achieving the goal of Grade Ⅲ water.

As Fig.2 shows, with the total area of 1784 acres, the Quanshang abandoned coal mine is made up of 5 large collapsed pits, which are disconnected among each other, featuring dead-zone waters with the maximum water depth of 6.35 m. Even if some isolated pits link to some extent during rainfall, the water flowing is still very poor, and therefore the Quanshang collapsed mine coal must be re-modified for serving as a NEWTS.

Fig.2 Study domain

2.2 Governing equations of hydrodynamic and water quality model

Since the NEWTS is usually presented as surface water flow system with relatively shallow water depth, the averaged-depth two-dimensional hydrodynamic and water quality model is applied to simulate its hydrodynamic patterns and water purification effect.

The governing equations for simulating the system flow patterns can be expressed as

where t is the time,x and y are the horizontal Cartesian coordinates, h is water depth, U, V are the depth-averaged flowing velocity components in thex and y directions,Zsis the water surface elevation, g is the gravitational acceleration, ρ is the density of flow, Txx, Txy, Tyxand Tyyare the depth-averaged turbulent stresses, τsxand τsyare the shear stresses on water surface due to wind driving, determined by

in which ρa(bǔ)is the density of air, cfais the friction coefficient, Uwand Vware the components of wind velocity inx and y directions,τbxand τbyare the bed shear stresses determined by

in which c=gn2/h1/3and n is Manning’s

f roughness coefficient andfcis the Coriolis coefficient.

The wind force and Coriolis force exerting on the Quanshang NEWTS with such spatial scale are insignificant, so the two kinds of forces can be excluded from above equations. The bed shear stress is compounded by the aquatic plants, which is related to the roughness coefficient n in the governing equations.

The governing equation for simulating water quality of the NEWTS is

where C is concentration of water quality constituents,Exand Eyare dispersion coefficients in x and y directions, and S is pollutant removal rate of aquatic plants.

2.3 The water environment modeling technique based on dynamic mesh

Recently numerical modeling based on dynamic mesh technique has been proposed and used in the simulation of tidal flowing in estuaries[11-14], dam break flows[15]and flood propagation[16]. However, there is no study concerning the flowing and pollutants removal in NEWTS with dynamic land-water boundary. In this study, we present a modeling technique based on finite element method and element storativity coefficients.

First, to describe the NEWTS with irregular topography, the finite element method is presented to discretize the above governing equations: at each element, integrating governing equation by the Galerkin weighted residual method and descending second-order derivative by the Green equation and integral identity on the basis of boundary conditions, give the finite element equation of each element, further, assembling the equations in each element of the study domain, yield the global matrix system, finally, the global matrix system is solved to give the variables in the governing equations in each computational node.

Second, we introduced element storativity coefficientbλ, which is the ratio of changes in stored water per unit element area with respect to changes in water elevation. The coefficients are calculated as follows:

where zbis the mean land elevation in the vicinity of node n,ζ is storativity depth, a is minimum element storativity,ηbis storativity depth factor, and

is the depth below zbat which λb=a. Element storativity λb=a forzb-b≥zs≥zb-ηbζ.

Numerical experiments show that assigning a=0.01and ηb=3 provided a good means of controlling element transition from wet to dry states. The dynamic mesh technique in the NEWTS is depicted in Fig.3.

Fig.3 Dynamic mesh technique of NEWTS

3. Results and discussion

3.1 Optimized design of NEWTS

3.2.1 Critical flooding level

Figure 2 shows the study domain, including the largest impounding area and the peripheral land. Based on the water environment model of dynamic mesh technique, the flooded area under different impounding water levels was simulated (Fig.4, Table 1). It was known that the water passageway linking Region 1 and Region 2 existed in the western part of the abandoned coal mine when the impounding water level was between 43.5 m and 43.8 m, whenimpounding water level rose to 43.9 m, the water passageways linking Region 1 and Region 2 existed on both western and eastern sides. Considering that a road (marketed in Fig.4) runs across the abandoned coal mine, the maximum impounding water level was assumed to be between 43.7 m and 43.8 m, provided the road was not flooded frequently.

Fig.4 Simulated flooding area of abandoned coal mine under different impounding water levels

Table 1 Simulated flooded area and water storage volume of the Quanshang abandoned coal mine under various impounding water levels

3.2.2 Modification of original topography

Through the analysis of flooded area, the optimized water passageway and the corresponding water flow pattern was further designed by reconstructing the topography of the Quanshang abandoned coal mine. To allow water to outflow, an outflow channel was constructed to the southwest of the abandoned coal mine. Using the impounding water level of 43.7 m, we simulated the water flow through the modified topography under general HRT of 30 d (Fig.5). The simulation result indicated that Region 2, Region 3, and part of Region 1 was dead water zones. The results also indicate that the water flow is not uniform, resulting in insufficient HRT in some areas.

Fig.5 Simulated flowing pattern of the NEWTS under first topography modification

Figure 6 shows the topography of modified scheme, in which the land linking Region 1 and Region 2 were excavated deeper, and in addition, Region 2 was also connected to Region 3 by a culvert, and Region 3 served as a water storage pond. The corresponding water flow pattern of was simulated (Fig.7). The results show that the dead water zone is eliminated in the modified system, while the water flow pattern becomes more uniform than before. This can promote water purification effect.

Fig.6 Final topography modification scheme of the NEWTS

Fig.7 Simulated flowing pattern of NEWTS after final topography modification

With the topography modification scheme of original abandoned coal mine, the NEWTS was designed. Nowadays, in the area of the Xinxue River joining the Nansi Lake, the wetland of phragmites communis with the area of more than 80 hectares have been constructed[17]. It played an important role in intercepting water pollutants into the Nansi Lake while bringing economic profits for local residents. Based on the experience, phragmites communis was selected for the NEWTS, and the phragmites communis was planted less than 2 m in depth in the water. The planning NEWTS is schematized in Fig.8.

Fig.8 Designing aquatic plants in the NEWTS

3.2.3 Roughness coefficient of NEWTS

Planting paragmites communis obviously influences water flowing pattern of the NEWTS, which is reflected in the bottom roughness coefficient nin the hydrodynamic model. Some studies have concerned the effect of aquatic plants on bottom roughness coefficient, which is usually a function of either water depth or dragging coefficient[18-24]. Especially, the study[18,19]about the effect of phragmites communis on water flow resistance indicates that the roughness coefficient can be calculated as

where nbis the bottom roughness coefficient, which is set to be 0.05, c is a constant set to be 2.4.

For the NEWTS shown in Fig.8, the average water depth is 1.686 m under impounding water level of 43.7 m. Based on Eq.(7), the average roughness coefficient is 1.18, which is nearly identical to the values of Somes et al.[8]. It further proved the reliability of estimated roughness coefficient n and the modeling results.

3.2.4 Optimized HRT of the NEWTS

The HRT was further designed to assure the anticipated water purification effect. The water quality monitoring data show that COD concentration of the inlet water is 40 mg/L (Grade V of Chinese national surface water quality standard) and the concentration of NH4+-N is 3.5 mg/L (inferior Grade V). However, the outflow concentrations of COD and NH4+-N of the NEWTS are required to be 20 mg/L and 1.0 mg/L respectively, reaching the goal of Grade Ⅲ.

As is shown in Eq.(4), the pollutant removal rate of the aquatic plants is the key parameter for water quality modeling. A pilot study of artificial wetland with phragmites communis in the Nansi Lake watershed revealed the degradation rates of COD and NH4+-N in the NEWTS[25]. The wetland covered an area of 1000 m2and its substrates were taken from the sediments of the old Great Canal of the Nansi Lake watershed, similar to the design of NEWTS in this study. The receiving water of the pilot-scale artificial wetland was 24 m3/d, and the average water quality of COD and NH4+-N were 84 mg/L and 28 mg/L respectively. From those results, the pollutant removal rate of phragmites communis wetland can be calculated using the following equation:

where C0is the inflow water quality, Q is the inflow discharge, k is the degradation rate of water quality constituents, t is the hydraulic residence time, and A is the area of wetland.

With the measured degradation rates of COD and NH4+-N in the artificial wetland[25], COD removal rate of the system was estimated to be 1.34 g m-2/d, and NH4+-N removal rate of the system was estimated to be 0.24 g m-2/d.

Fig.9 Outlet water quality of NEWTS under different HRTs

Further, we simulated the anticipated water purification effect of the NEWTS under different HRTs. Figure 9 showed the anticipated outlet water quality under different HRTs. With the simulated data, the relationships between the pollutants removal ratio and HRT were determined, which are listed as follows:

For COD,

where Coutis the outlet water quality, Cinis the inlet water quality, T is the hydraulic residence time, R2is correlation coefficient of the above fitting equations, which proves the accuracy of the expression.

Based on Eqs.(9)-(10), it is concluded that the HRT for anticipated outlet COD concentration is 13 d, and the HRT for anticipated outlet NH4+-N is 30 d. Generally, the designing HRT of the NEWTS is set at 30 d. Under this parameter, the outlet COD concentration is 17.15 mg/L, and the outlet NH4+-N concentration is 1.03 mg/L, meeting the goal of GradeⅢ. Correspondingly, the designed treatment capacity is 3×104m3d-1, anticipated water purification effect along the NEWTS is shown in Fig.10.

Fig.10 Anticipated water quality in the NEWTS under 30 d

4. Conclusions

A water environment model with dynamic mesh technique based on finite element method and element storativity coefficients has been developed to optimize the design of NEWTS with open natural topography and dynamic water storage level. A case study of NEWTS based on the Quanshang abandoned coal mine demonstrates that the presented model can be effectively used to simulate the hydrodynamic pattern and water quality of NEWTS with the characteristics of dynamic land-water boundary under open topography and varying water impounding level. First, the critical flooded area is determined, and the natural topography modification scheme is implemented to assure the optimized water flow pattern. Further, the roughness coefficient and pollutant removal rate ofNEWTS with phragmites communis is estimated, and the anticipated water purification effect of the system under different HRT has been simulated to obtain the ideal level of HRT, assuring that outlet water quality met the proposed requirement by the East Route of South-North water Transfer Project in China.

As a result, the designed NEWTS based on Quangshan abandoned coal mine has been constructed in Tengzhou, Shandong Province, assuring the purification effect of wastewater discharged into the east route of South-North Water Transfer Project.

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10.1016/S1001-6058(09)60021-4

* Project supported by the Key Program on the S and T for the Pollution Control and Treatment of Water Bodies (Grant Nos. 2009ZX07210-008, 2009ZX07316-005), the Science and Technology Commission of Shanghai Municipal People’s Government (Grant No. 072312050).

Biography: XU Zu-xin (1956-), Female, Ph. D., Professor

YIN Hai-long,

E-mail: yinhailong@#edu.cn