Science of the Total Environment 628–629 (2018) 509–516
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Study of the spatial and temporal distribution of accumulated solids in an experimental vertical-flow constructed wetland system Mengyao Yang a,c, Mingzhi Lu b, Lianxi Sheng c, Haitao Wu a,⁎ a b c
Key Laboratory of Wetland Ecology and Environment, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun 13012, PR China Ministry of Education Key Laboratory of Vegetation Ecology, Institute of Grassland Science, Northeast Normal University, Changchun, Jilin 130024, PR China State Environmental Protection Key Laboratory of Wetland Ecology and Vegetation Restoration, School of Environment, Northeast Normal University, Changchun 130117, PR China
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• Solids accumulation process was monitored in experimental VF CW systems. • Adsorption was the major accumulation mode for particles (d50 = 8.26 μm). • Particles were intercepted or adsorbed into the matrix at a constant rate. • The proportion of particles (N20 μm) kept increasing in the top layer of medium. • The accumulation of particles could be simulated by a first-order kinetics model.
a r t i c l e
i n f o
Article history: Received 18 December 2017 Received in revised form 7 February 2018 Accepted 8 February 2018 Available online xxxx Editor: Jay Gan Keywords: Vertical flow constructed wetland Clogging Accumulated solids Spatial and temporal change
a b s t r a c t Clogging is the most serious problem in the operation of subsurface flow constructed wetlands (SSF CWs) and is caused by the accumulation of solids in substrates. Study of the solids accumulation process can provide a more accurate reference for the management and maintenance of SSF CWs. In this study, an experimental vertical-flow constructed wetland system was recreated in the lab, and substrates with different depth were sampled through different operation time to reveal the spatial and temporal distribution of accumulated solids. During the study, particulates mainly accumulated through adsorption along the gravel surface. Therefore, the matrix could still provide sufficient space for the particles to pass through and be intercepted or adsorbed into the system at a constant rate. At the end of the study, an increasing number of large particles had been intercepted and were accumulated in the 0–2 cm layer of the matrix, indicating a significant decrease in the pore diameter at the top substrate layer. The spatial and temporal accumulation of substrate particulates during the study period was accurately simulated by first-order kinetics models, and the simulated results were in good agreement with measured values. © 2018 Published by Elsevier B.V.
1. Introduction
⁎ Corresponding author. E-mail addresses:
[email protected] (M. Yang),
[email protected] (L. Sheng) ,
[email protected] (H. Wu).
https://doi.org/10.1016/j.scitotenv.2018.02.086 0048-9697/© 2018 Published by Elsevier B.V.
Subsurface flow constructed wetlands (SSF CWs) have been used worldwide for sanitation, especially in small communities, because of their natural design and low operational and maintenance costs (Cooper et al., 1990; Vymazal, 2010). However, clogging is the most
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serious but inevitable problem that is faced when using this technology (Blazejewski and Murat-Blazejewska, 1997; Caselles-Osorio et al., 2007; Pedescoll et al., 2013; AI-Isawi et al., 2015). This phenomenon is due to the build up of solids in substrate, which mainly come from biofilms, roots, and macrophytes, macrophyte litter, as well as any undissolved matter present in raw wastewater (Knowles et al., 2011; Nivala et al., 2012). Accumulated solids can be divided into organic and inorganic parts. Due to high moisture content and low density, organic matter occupies more pore volume comparing with inorganic solids (Matos et al., 2017; Miranda et al., 2017). However, researches conducted by Matos et al. (2017) and Miranda et al. (2017) also suggested that most of the accumulated solids were inorganic, which was confirmed by previous studies (Caselles-Osorio et al., 2007; De Paoli and von Sperling, 2013). Although Nguyen (2000) concluded that organic matter was the main factor in clogging, his study revealed that up to 90% of the organic solids were recalcitrant parts. In anaerobic conditions, only little organic matter is biodegradable, and aeration can promote the decomposition of accumulated organic matter in CWs (Carballeira et al., 2017). Therefore, in the SSF CWs that provide appropriate conditions for organic matter to decompose completely, mineral composition plays a dominant role in the accumulation of solids (García et al., 2007; Ruiz et al., 2010). SSF CW performance indicators, such as contaminant removal efficiency, porosity, and hydraulic conductivity, change during the process of solids accumulation (Langergraber et al., 2003; Hua et al., 2010; Ranieri et al., 2013; Aiello et al., 2016). Thus, research on solids matter accumulation is critical for understanding and forecasting the performance of SSF CWs. There are two modes of accumulation for solids in substrates: on the substrate surface or within the pores, depending on the size of solid particles involved (Caselles-Osorio et al., 2007; De Paoli and von Sperling, 2013). Particles with a larger diameter clog substrate pores, whereas solids absorbed on substrate surfaces mostly have a diameter b100 μm (Hua et al., 2010). These two accumulation modes result in varying effects on their hydraulic parameters and behaviors. Solids embedded into pores have a stronger influence on hydraulic conductivity than adsorbed solids (Pan and Yu, 2015). Therefore, it is important to consider the dynamic change of distribution modes influenced by accumulated particle size when studying the accumulation of solids in substrates. Some studies already exist that focus on the quantification of these accumulated solids (Caselles-Osorio et al., 2007; Pedescoll et al., 2013). Temporal changes occur in the dynamics of accumulated solids with a significant increase in the amounts of accumulated solids over operation time (Pedescoll et al., 2011; Fu et al., 2013; Zhong et al., 2013). And the distribution of solids in SSF CWs declines along the length of the flow path (Ye et al., 2008; Lancheros et al., 2017). In vertical-flow CW systems, where hydraulic behavior is less complicated, the accumulation of solids is negatively correlated with depth (Yan et al., 2008; Hua et al., 2010), with an obstructing layer of substrate existing no deeper than 20 cm (Todt et al., 2014; Xie et al., 2010). In horizontal-flow CWs, which have more complicated hydraulic behavior, the variation in accumulated solids distribution is less significant (Lancheros et al., 2017). However, there is little study on the process of solids accumulation in CWs, which could not be elucidated clearly by just measuring the content change of accumulated solids. The temporal and spatial variance of accumulation mode together with particle diameter of accumulated solids should be taken into account. Due to the lack of real-time monitoring technology in SSF CWs, accumulated solids can be measured only through substrate sampling, which has significant limitations as the sampling process itself disrupts the internal environment in the substrate around the sampling sites. And data based on regular substrate sampling in a SSF CW system over time to reveal the temporal inner changes is still sparse (Corbella et al., 2016). This study aimed to investigate the accumulation process of solids in an experimental vertical-flow constructed wetland system. To achieve this goal, the spatial and temporal changes in quantity and particle
size distribution of accumulated solids, both in pores and on the substrate surface, were monitored and analyzed. Further, based on the observed data, a model was established to describe the spatial and temporal dynamics of accumulated solids distribution, which could help better understand the clogging process in SSF CWs. 2. Material and methods This study was conducted in an experimental laboratory system over a period of 180 days. Because inorganic solids and recalcitrant organic solids account for the vast majority of accumulated solids in the current literature (García et al., 2007; Ruiz et al., 2010), insoluble inorganic particles in the inflow entering the SSF CWs were focused on to study the accumulation process. To exclude any contribution from biofilms or macrophytes, organic matter and nutrients were not added into the inflow of experimental systems. In addition, macrophytes were not planted. 2.1. Experimental system Due to the disruptive effect of the sampling process on substrates, 12 identical systems were set up to allow for the sampling of 12 stages within the operational period without interference. At the end of each operational stage, substrates were collected from a system for the measurement of accumulated solids. The same conditions were used for all systems. Fig. S1 shows the configuration of each experimental vertical flow constructed wetland system. The suspensions were prepared using zeolite powder (consisting of aluminosilicate) with the particle size distribution shown in Fig. S2, and then uniformly mixed with tap water using a magnetic stirrer in the upstream tank. This mixture was pumped up through the influent pipe to an experimental column filled with uniform gravel, which was sieved to 3 mm–4 mm in diameter and carefully cleaned in water using ultrasonic waves. The experimental column was 5 cm in diameter and 15 cm in height and made of Perspex with scales marked on the lateral wall. As mentioned before, in vertical-flow CWs systems the accumulation of solids is negatively correlated with system depth (Yan et al., 2008). And Hua et al. (2010) established that 80%–90% of total accumulated solids in vertical-flow CWs were distributed in the top 6 cm of substrates. Considering this, the height of packed gravel was set to be 10 cm. The column was equipped with a manometer connected near the bottom to monitor the water head. The outflow was sent to a downstream tank through an effluent pipe at the same rate as the inflow and controlled by pump-2. 2.2. Operation All 12 systems, as shown in Fig. S1, had the same gravel packing and mode of operation. Steady inflow conditions were maintained throughout the whole experimental period. The concentration of total suspended solids (TSS) was about 500 mg/L, and the inflow rate to the experimental column was 37 mm/day in height, resulting in a value of 36.5 mg/day for the TSS load. Initial substrate effective porosity and infiltration rates were 0.372 and 0.458 cm/s, respectively. The air temperature in the laboratory where the apparatuses were installed was kept at 20 °C throughout the experiment. 2.3. Measurements 2.3.1. Quantity of accumulated solids According to Caselles-Osorio et al. (2007), there are two types of accumulated solids in substrates: interstitial solids that are trapped in the empty spaces between the gravel and easily released when the spatial structure is lost, and adhered solids that are tightly adsorbed onto the surface of the gravel particles and are not easily released. Both types of accumulated solids were considered in this study. Every 15 days,
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the column from one system was dismantled and the inner matrix was divided into five layers of 2 cm in height according to the scales marked on the lateral wall, and the substrate from each layer was removed in turn from top to bottom. This gravel was collected in a beaker and mixed with distilled water. The mixture of gravel and water was gently stirred with a glass rod to release the solids entrapped in the empty spaces between the gravel. The soaking solution was retained and vacuum-filtered with a pre-dried microporous membrane (0.45 μm), which was then dried and weighed. The difference between the membrane weight pre-and post-filtration represented the amount of interstitial solids. After releasing the interstitial solids, the gravel was again mixed with distilled water and subjected for ultrasounds (JiuPin JP10-200). In the study conducted by Caselles-Osorio et al. (2007), the time for ultrasounds was set to be 7 min. Considering the different type of ultrasonic wave cleaner, a small experiment was carried to determine the ultrasonic time in this study, and it was found that after 10 min of ultrasounds no more solids were released into the water from the surface of the gravel. The collected soaking solution was vacuum-filtered with a pre-dried microporous membrane (0.45 μm), which was then dried and weighed. The difference between the membrane weight pre-and post-filtration represented the level of adhered solids. 2.3.2. Particle size of accumulated solids Particle sizes of accumulated solids (both in the pores and on the substrate surface) were measured using a Malvern MS-2000 laser particle size analyzer. 3. Results 3.1. Changes in quantity of accumulated solids over operation time At the beginning of the experiment (0-30th day), the quantity of accumulated matter in the pores of each layer decreased with depth. After the 45th day, the interstitial matter content at the 8–10 cm layer exceeded that of the 0–2 cm layer until the end of the experiment. In other layers, the trend of interstitial particles decreasing as matrix depth increased continued, with the lowest content in the 4–6 cm and 6–8 cm layers (Fig. 1). During the experimental period, there was a significant positive correlation (at the 0.01 level) between run time and the accumulation of particulate matter in the pores of each layer of the matrix. Pearson correlation coefficients ranged from 0.843 to 0.941 (Table S1) with a good linear relationship fit (R2 between 0.681 and 0.727), indicating that particulate matter accumulated at a constant rate in the pores of the matrix. The accumulation rate of interstitial particles in the 8–10 cm layer was
Fig. 1. Temporal and spatial distribution of solids that accumulated in pores.
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the highest compared with other substrate layers, at 1.11E-4 g/g gravel·day. The other four layers had decreasing accumulation rates with increasing matrix depth. The accumulation rates in the 4–6 cm and 6–8 cm layer were very similar, at 1.21E-5 g/g gravel·day and 1.3E-5 g/g gravel·day, respectively. With respect to particulate matter adsorbed on the substrate surface, it was generally observed that quantity decreased as matrix depth increased throughout the full experimental period. Quantities of adsorbed particulate matter on the gravel surface were similar among the 4–6 cm, 6–8 cm, and 8–10 cm substrate layers, and all were lower than the values for the top layers (0–2 cm and 2–4 cm) (Fig. 2). The quantity of particles adsorbed on the substrate surface also increased with experimental run time. However, the correlations between these two parameters were significant only in the 0–2 cm layer (0.01 level), the 2–4 cm layer (0.05 level), and the 8–10 cm layer (0.01 level), with respective Pearson correlation coefficients of 0.912, 0.603, and 0.49. Accumulation of particles adsorbed on the gravel surface in the 4–6 cm and 6–8 cm layers was not directly related to run time (Table S1). Good linear relationships were observed between run time and the quantity of adsorbed particles in the 0–2 cm, 2–4 cm, and 8–10 cm layers, indicating that particle adsorption in those three substrate layers was constant. The rates of accumulation decreased as matrix depth increased, with values of 2.51E-5 g/g gravel·day and 1.45E-5 g/g gravel·day in the top two layers, and 7.96E-6 g/g gravel·day in the bottom layer. The ratios of accumulated interstitial particulate matter to total accumulated solids at different operation times were analyzed (Fig. 3). The proportion of interstitial matter that accumulated in the 8–10 cm layer increased from 0.267 at the beginning of the experiment to 0.807 at the end. The same proportion also increased in other layers but remained below 0.5, indicating that more solids accumulated through adsorption on the gravel surface than through pore interception. It should be noted that although the particulate matter content that was adsorbed on the gravel surface was higher than that of the interstitial accumulated particles (except for the 8–10 cm layer), the accumulation rates were lower for the adhered solids (Table S1). It can be inferred that if the experiment were to continue, the solids would shift substrate accumulation modes from being adsorbed on the gravel surface to being intercepted in the pores. The variance in the distribution of total accumulated solids was determined by both the distribution of interstitial and adhered solids. Accumulation of total solids decreased as depth increased, except in the 8–10 cm layer (Fig. 4). The accumulation of total solids in the 8–10 cm layer was similar to that of the interstitial solids, because interstitial solids predominated over adhered solids in that layer. As for the other
Fig. 2. Temporal and spatial distribution of solids adsorbed on the gravel surface.
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Fig. 4. Temporal and spatial distribution of total accumulated solids.
four layers (0–2 cm, 2–4 cm, 4–6 cm, and 6–8 cm), the total solid accumulation was dominated by the adsorption of the matrix surface. Total accumulated solids in the 0–2 cm, 2–4 cm, and 8–10 cm layers were significantly correlated with operation time (0.01 level), and had respective Pearson coefficients of 0.599, 0.438, and 0.624. Linear regression results showed that among these three layers, the accumulation rate in the 8–10 cm layer was the highest, at 1.19E-4 g/g gravel·day, followed by the 0–2 cm layer (6.55E-5 g/g gravel·day), and the lowest accumulation rate was observed in the 2–4 cm layer (3.03E-5 g/g gravel·day). The accumulation of particulate matter in the 4–6 cm and 6–8 cm layers was not directly related to operation time (Table S1). 3.2. Change in particle size distribution of accumulated solids over operation time The particle size of solids in the inflow ranged from 0 to 2000 μm with a d50 of 8.26 μm. Particles with a diameter of b2 μm made up the highest proportion in the total solids (24.4%). Most of the particles had a diameter of b100 μm, and the particles with diameter N100 μm accounted for only 5.3% of the total (Fig. S2). As the particle size of solids in the inflow was generally small, accumulated solids in the pores during the experiment were mostly made up of fine grains (Fig. 5). However, the composition of interstitial particles was different than that of the inflow water. In the case of
Fig. 3. Temporal and spatial distribution of the ratio between the interstitial solids and the total accumulated solids.
particulates with a diameter of b20 μm, 75.4% was found in the inflow and after 15 days of operation, 100% was found in the 2–4 cm and 8–10 cm layers. In the 0–2 cm, 4–6 cm, and 6–8 cm layers, the same percentages after 15 days were 86.9%, 83.4% and 75.2%, respectively, all higher than that of the inflow water. As the experiment continued, the proportion of interstitial particles smaller than 20 μm in the 0–2 cm layer continued to decrease. This ratio dropped to 66% at the end of the experiment, and proportions in the remaining four layers were still around 80%, with no significant change over time. This indicates that the pore diameter of the matrix in the top layer gradually decreased, resulting in an increasing number of large particles being intercepted and accumulating in the 0–2 cm layer and reducing the proportion of small particles. As for the other four layers, the proportion of large particles did not increase significantly, indicating that the deep pores of the matrix could still allow large particles to pass through. For the adhered particles, the particulate size distribution was similar to that of the particles accumulated in the pores above. The proportion of the adhered particles smaller than 20 μm was higher than that of the inflow water and the ratio always fluctuated around 80% during the experiment, showing no significant change trend over time (Fig. 6). It is important to note that there was no distinction in particle size distribution among adsorbed particulates in the top, middle, and bottom layers. These results indicated that the adsorption of particles on the substrates was quite stable and not affected by a decrease in the pore diameter during the experimental period. 4. Discussion 4.1. Solids accumulation process in the experimental system In general, the amount of particulate matter carried by water flow and deposited in a wetland will decrease in concentration along the flow path through the substrate. However, in this study the accumulation of particulate matter in the outlet zone exceeded that of the inlet zone. This phenomenon was related to an inappropriate exit setting in the experimental system. Because the exit diameter of the experimental system was quite small (1 cm in diameter), water flowing through the underlying matrix was intercepted by the plexiglass that formed the bottom boundary of the experimental system, resulting in more particulate matter accumulating in the bottom layer than in the remaining layers of the matrix. The fact that the particles were easily intercepted by the boundary at the exit of the constructed wetlands has also been
Fig. 5. Temporal and spatial distributions of particle sizes accumulated in pores.
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Fig. 6. Temporal and spatial distributions of particle sizes adsorbed on the gravel surface.
confirmed by the results of other scholars (Ye et al., 2008; Pedescoll et al., 2009). Suspended particles are filtered and trapped by SSF CWs through transport and collision mechanisms (Yao et al., 1971). These, like the flocculation mechanism, create an opportunity for collisions between particles, which cause the particulates to become integrated (Swift and Friedlander, 1964). This phenomenon of particulate matter accumulating over time at a constant rate in the upper and lower layers of the system, rather than in the intermediate layers, could be due to the fact that water flow was stable in the inlet and outlet zones under peristaltic pump operation. This ensured a steady transport of particles and led to stable contact time between particles and substrate. Disorderly water flow in the middle layers of the matrix, on the other hand, resulted in variable particle adsorption. According to Knowles et al. (2011), the reduction in particulate matter in wetlands is based on the size under a series of different physicalchemical processes. Large particles can be removed through a variety of mechanisms, including sedimentation and uplift, fluid mechanics, inertial separation, interception, filtration, and capture. The removal of small particles includes mechanisms such as Brownian motion, electrostatic interaction, bridging, and coagulation. Therefore, particle interception in pores mainly relies on inertial forces and is less influenced by the water flow, compared with particle adsorption on the substrate surface, which mainly relies on particles and matrix irregular collision. Thus, even in the middle layer of the matrix where the water flow was unstable, the amount of accumulated interstitial solids had a strong linear relationship with experimental operating time. In this study, the content of particulate matter that was adsorbed on the substrate surface was larger than that of the interstitial particulate matter, which was not in agreement to the results of previous studies. Caselles-Osorio et al. (2007) investigated this in a horizontal-flow CWs system in operation for four years and found that 99% of accumulated solids were distributed within the pores, with only 1% adsorbed on the surface of substrates. This was further supported by De Paoli and von Sperling (2013), who studied a horizontal-flow CWs system that had been in operation for two years, finding 60–80% of accumulated solids in the substrate pores. The reason for the different results may be the difference between the accumulated solid particle sizes in these three studies. According to Hua et al. (2010), solids adsorbed on substrate surfaces mostly have a diameter of b100 μm, and the particles in that range in this experiment made up the majority of particulates in the inflow water. Due to the small particle size, adsorption onto the substrate surface was a major mode for solids to accumulate in the SSF CWs in this study. Generally, during the experimental period, solids accumulated in the matrix at a constant rate and no clear changes in particle size
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distribution were observed. The results indicated that particle adsorption on the substrate surface only reduced the diameter of the pores, rather than completely blocked the water flow channel. The substrate was still able to provide sufficient space for particles to pass through and be intercepted or adsorbed into the system at a constant rate. The system was also able to maintain a stable state of operation. Nevertheless, it is important to note that while adsorption onto the substrate surface was the major mode of solid accumulation in substrate over the entire experimental period, interstitial particulate matter accumulated more rapidly than adhered particulates. In addition, an increasing number of large particles were intercepted and accumulated in the 0–2 cm layer, indicating a significant decrease in the pore diameter in the top substrate layer. It can therefore be inferred that with a longer experimental period, more particles would block the pores of the matrix, particularly in the top layer, until the wetland system operation would eventually deteriorate and fail. It is noteworthy that in this study the accumulation process of undissolved matter present in wastewater was focused on, and other contributors to accumulated solids in CWs, such as plants and biofilms, were not considered. Contradictory results exist about the effect of plant presence on solids accumulation. Some SSF CWs planted with macrophytes generate significantly more solids compared with non-planted treatment systems (Tanner et al., 1998; De Paoli and von Sperling, 2013; Fu et al., 2013). But Chazarenc et al. (2009) reported that the presence of plants reduced accumulated solids by 26%. Moreover, in the study conducted by Carballeira et al. (2017), it was found that the presence or absence of plants did not significantly affect solids accumulation. However, the decomposed debris of macrophytes can form a significant sludge layer on substrate surfaces (Gagnon et al., 2013). Hence, harvesting has been treated as a key to limit solid accumulation above-ground in SSF CWs. An important contribution to the process of solids accumulation by biofilm populations is the production of extra-cellular polymeric substances, which are apt to associate with other materials (Thullner, 2010; Hua et al., 2013). To avoid this, measures such as aeration and application of a resting period to facilitate oxygen diffusion have been proposed to mitigate bio-clogging (Pozo-Morales et al., 2013; Hua et al., 2017). 4.2. Simulation of temporal and spatial distribution of accumulated solids During the experiment, the removal rate of TSS in the inflow was steady, which has been discussed in detail before (Yang et al., 2017). The retention of particulates in substrates was regarded as a firstorder kinetic process. The amount of solids that were intercepted or adsorbed was related to the TSS concentration and contact time between water flow and matrix. The concentration of TSS in the matrix at time t can be expressed by Eq. (1): CðtÞ ¼ C0 e−kðt−t0 Þ
ð1Þ
where C0 is the initial concentration of TSS, t0 is the initial time, and k is the reaction rate constant. According to the TSS removal rate experimental results, the value of k in this experiment was 0.00009576. The experimental system in this study was simplified into a twodimensional model to investigate the accumulation of material in the vertical profile. The abscissa range of the simplified model was between 0 and 0.05 m (the diameter of the gravel column), and the ordinate range was within 0–0.1 m (the height of the gravel column). Considering the spatial distribution of TSS concentration at time t, Eq. (1) can be adjusted to the following form: Cðx; y; tÞ ¼ C0 e−kðtx;y −t0 Þ
ð2Þ
It was assumed that the fluid in the experimental system only flowed along the vertical direction and did not move in the horizontal
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Based on Eq. (4), the quantity of accumulated solids in the experimental system under different operation times can be simulated. The simulated data was compared with the measured data (Fig. 7), and simulated values were in good agreement with the measured values. The Nash-Sutcliffe Efficiency factor (NSE), which is recommended for evaluating the efficiency of a hydrological model, was selected to evaluate the model simulation results (Moriasi et al., 2007). NSE uses normalized statistical data obtained by comparing the residual variance of the simulated data with the variance of the measured data. NSE characterizes the degree of fit between the measured data sequence and the simulated data sequence to 1: 1, which is calculated as follows: 2
2 3 Pn obs sim Y −Y i i i¼1 6 7 NSE ¼ 1−4 2 5 Pn obs mean Y −Y i i¼1
Fig. 7. Contrast between the measured and simulated values of accumulated solids in the experimental system.
direction. For a representative unit of the matrix, after Δt time, the accumulated solids content can be described by Eq. (3): MðΔx; Δy; ΔtÞ ¼ ½Cðx; y; tÞ−Cðx þ Δx; y þ Δy; t þ ΔtÞ vx;y;t
ð3Þ
where vx,y,t was the volume of fluid flowing through the representative unit at time t. According to Eq. (3), the total amount of particulate matter accumulated in the system matrix at time t can be expressed by the following equation:
M¼
Z t (Z 0
0:05 0
"Z 0
0:1
# ) ðMðΔx; Δy; ΔtÞÞdy dx dt
ð4Þ
is the i-th measured data for the evaluated data sequence; where Yobs i Ysim is the i-th simulated data; Ymean is the mean value of the measured i data sequence; and n is the number of the data sequence. The value of NSE is between −∞ and 1 (inclusive). When NSE is 1, the simulation results of the model are the best. When the value of NSE is between 0 and 1, the simulation result is considered acceptable. When the value is less than or equal to 0, the mean value of the measured data is better than the simulation results, indicating that the simulation result is considered unacceptable. In this study, the NSE value was 0.35, showing that the simulation of experimental results for temporal change in accumulated solids were acceptable. It was found that all the simulated values were smaller than the corresponding measured values (Fig. 7), which might be caused by the extra solids generated in the packing process due to the fracture of gravel. Furtherly, a parameter representing the amount of endogenous solids in the experimental systems was introduced to the model. And after calibration, the NSE value was 0.9, showing that the simulation of experimental results for temporal change in
Fig. 8. Temporal and spatial distributions of accumulated solids in the experimental system.
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accumulated solids were in excellent accordance with the measured values. Taking the simulation results at days 30, 75, 120, and 165 as examples, the distribution of particles accumulated in the matrix for these four experimental stages are shown in Fig. 8. The accumulation of particles increased with the experimental run time but decreased along with the depth of the matrix, which was consistent with the distribution of the measured data. It should be noted that the simulation did not consider the interception of TSS by the underlying boundary of the system. As a result, the simulated values of accumulated particulate matter at the bottom of matrix were much lower than the measured results. 5. Conclusions The clogging process of SSF CWs can be divided into the stable phase and the deteriorating phase. In this study, due to the small size of particulates in the inflow, adsorption on the gravel surface was the main pathway for particulate accumulation, which only reduced the diameter of the pores instead of completely blocking the water flow channel. The substrate still provided sufficient space for the particles to pass through and be intercepted or adsorbed into the system at a constant rate. The system was also able to maintain a stable state of operation. However, at the end of the experiment, an increasing number of large particles had been intercepted and accumulated in the 0–2 cm layer of the matrix, indicating a significant decrease in the pore diameter of the top substrate layer. The performance of the experimental system began to enter the deteriorating phase following the stable phase. During the stable phase of the SSF CWs, the accumulation of particulates in the substrate was successfully simulated using a first-order kinetics model. The results were evaluated using NSE with a value of 0.35, demonstrating that the simulation results for temporal change in accumulated solids of the experimental system were acceptable. Because TSS interception by the bottom of the experimental column was not considered in the models, there was a disparity between the simulated and measured data for accumulated solids in the bottom substrate layer. Solids accumulation process is related to both concentration and diameters of TSS in the inflow of SSF CWs. From this aspect, pre-treatment of the influent will have a significant effect on prolonging the lifespan of SSF CWs, which not only reduces the concentration of pollutants, but removes particles with larger sizes to avoid immediate clogging around the inlet of CWs. Supplementary data to this article can be found online at https://doi. org/10.1016/j.scitotenv.2018.02.086. Acknowledgments This study was supported by the National Key R&D Program of China (2017YFC0505901, 2016YFC0500408), and the National Natural Science Foundation of China (41371260, 41671261). References Aiello, R., Bagarello, V., Barbagallo, S., Iovino, M., Marzo, A., Toscano, A., 2016. Evaluation of clogging in full-scale subsurface flow constructed wetlands. Ecol. Eng. 95, 505–513. AI-Isawi, R., Scholz, M., Wang, Y., Sani, A., 2015. Clogging of vertical-flow constructed wetlands treating urban wastewater contaminated with a diesel spill. Environ. Sci. Pollut. Res. 22, 12779–12803. Blazejewski, R., Murat-Blazejewska, S., 1997. Soil clogging phenomena in constructed wetlands with subsurface flow. Water Sci. Technol. 35, 183–188. Carballeira, T., Ruiz, I., Soto, M., 2017. Aerobic and anaerobic biodegradability of accumulated solids in horizontal subsurface flow constructed wetlands. Int. Biodeterior. Biodegrad. 119, 396–404. Caselles-Osorio, A., Puigagut, J., Segú, E., Vaello, N., Granés, F., García, D., García, J., 2007. Solids accumulation in six full-scale subsurface flow constructed wetlands. Water Res. 41, 1388–1398. Chazarenc, F., Gagnon, V., Comeau, Y., Brisson, J., 2009. Effect of plant and artificial aeration on solids accumulation and biological activities in constructed wetlands. Ecol. Eng. 35 (6), 1005–1010. Cooper, P.F., Hobson, J.A., Findlater, C., 1990. The use of reed bed treatment systems in the UK. Water Sci. Technol. 22, 57–64.
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