The use of constructed wetlands (CWs) in the treatment of raw wastewater in China has proved to be very successful in recent decades. However, it is not known whether surface-flow constructed wetlands can effectively purify irrigation return water. To investigate the performance of a constructed wetland in terms of meeting the goals of pollutant purification, the 8th drainage of Ulansuhai Lake was used for this study. Pollutant removal performances, as well as hydrological characteristic variations in relation to specific characteristics of plants, were investigated utilizing two years of monthly average data. The results indicated that surface-flow constructed wetlands can effectively change the physical characteristics of return water and lead to a sharp decrease in pollutant concentrations. The 1200 m long, narrowly constructed wetland resulted in the average reduction rates of total nitrogen (TN) and total phosphorus (TP) of up to 22.1% and 21.5%, respectively. The overall purification efficient of the constructed wetland presented seasonal variations in four different monitoring periods (May, July, September, and November). Constructed wetlands with multiple types of plants exhibited higher efficiencies in pollutants removal than those with a single type of plant. The current study can provide meaningful information for the treatment of agricultural wastewater.
Lake eutrophication has become a significant ecological environmental problem facing freshwater lakes in China [
Constructed wetlands serve as a valid treatment measure because investment and running costs are low, and maintenance and management are easy [
CWs, which are designed to return irrigation water, have certain boundary conditions to meet [
Ulansuhai Lake (N40°36′–41°03′, E108°43′–108°57′) is located in Ulate County, Inner Mongolia, China. It covers an area of 292 km2 (Figure
Location of the Ulansuhai Lake.
The CW analyzed for this study is situated in the 8th drainage. The primary source of the drainage is from spring irrigation (April) and autumn irrigation (October) return water from the upstream region. The largest flowing velocity of these return flows reaches 0.3 m/s. The dimensions of the constructed wetland are 1200 m (
Detailed information of the three segments of the CW.
Segment | Segment I (400 m) | Segment II (400 m) | Segment III (400 m) |
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Types of plants |
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Dominant plant |
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Plant density | 30–35/m2 | 30–35/m2 | 30–35/m2 |
A total number of 12 sampling sites were set in the CW at 100 m intervals. Three parallel samples were collected weekly from each segment and stored at −18°C until analysis.
Water flow velocity (FV) was measured with a velocimeter, and the suspended solids (SS) were measured using the gravimetric method. TN concentrations were measured using the alkaline potassium persulfate digestion-UV spectrophotometric method, and TP concentrations were measured by the ammonium molybdate spectrophotometric method.
The removal ratios (RRs) of the total N and P were calculated by (
To determine the removal efficiency (RE) of nutrients at different locations, (
As depicted in Figure
Flow velocity and suspended solids distribution in the CW.
The average SS levels decreased from 41.0 mg/L to 21.4 mg/L, a total of 48%, indicating that the CW was working well in controlling water quality. The largest concentration (55 mg/L) of SS was observed in May due to the large amounts of fine solids that were brought by the return water of spring irrigation. The largest drop (59%) appeared in July. This was attributed to the vigorous growth of the aquatic plants, as well as the lower initial inflow velocity in this region. The average drops in the SS level in the four periods were 48.5%, 59.6%, 42.0%, and 46.8%. The lowest removal ratio of SS appeared in November, when many plants withered up. During the months of May and July, the first part (before 500 m) of the CW had lower removal ratios (an average of 27.5%) than those of the back part (an average of 34%). Conversely, September and November showed higher removal ratios in the front part of the CW (an average of 24%) than those of the back part (an average of 20%). Therefore, the growth conditions of the aquatic plants are thought to be a decisive factor in the removal ratios of suspended solids.
The average removal ratios of N and P in the four monitored periods are listed in Table
Average removal ratio of N and P in four monitoring periods.
May | July | Sep. | Nov. | Average | |
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RRN | 0.235 | 0.282 | 0.203 | 0.164 | 22.1% |
RRP | 0.224 | 0.313 | 0.211 | 0.113 | 21.5% |
The average RRTN and RRTP were 22.1% and 21.5%, respectively, indicating the CW performed well in handling fast-flowing return wastewater from agricultural irrigation. The RRTN of the four monitored periods was 23.5%, 28.2%, 20.3%, and 16.4%. The largest RR of TN appeared in July and the least RR appeared in November. The RRTP of the four periods was 22.4%, 31.3%, 21.1%, and 11.3%. In consideration of the temporal distribution, July had the largest RR of total N and total P, which was closely related to the exuberant growth of the aquatic plants in that season. Contrarily, a peak removal ratio appeared in November, when most of the aquatic plants were in a stage of growth retardation. Compared to RV of 60%–80%, or higher, in other constructed wetlands [
The temporal and spatial distributions of RRs in the 8th drainage are depicted in Figure
Temporal and spatial distribution of RRs in the 8th drainage.
Due to variations in water depth, vegetation, sediments, and so forth, each same-length CW presented a different level of removal efficiency. The results of the
Temporal and spatial distribution of REs in the 8th drainage.
The average RETN was 0.08 × 10−2 mg (L·m)−1 with a slight decreasing tendency. For example, the RE in the first segment was 1.2 × 10−2 mg (L·m)−1, which was almost triple the amount in the last segment 0.44 × 10−2 mg (L·m)−1. However, the largest RE appeared in the second segment. The high FV of the return water was thought to have contributed to this result. In consideration of temporal distribution of REs, July exhibited the highest average RETN among the four monitored periods. That was in accordance with the variation law of RVs. A similar variation tendency was observed in the REs of TP. The average RETP was 0.083 × 10−3 mg (L·m)−1 with a slight decreasing tendency. The peak and valley values of RETP appeared in July (0.13 × 10−3 mg (L·m)−1) and November (0.03 × 10−3 mg (L·m)−1), respectively. Dissimilarly, the seasonal variation of the RETP was smaller than the RETN. Generally, both regularities and complexities existed in the temporal and spatial distributions of the REs. Many factors, including flow flux, velocity, and landform, produced different influences on the experimental results.
Absorption by aquatic plants is the primary way of reducing N and P in a surface-flow CW [
Nutrient concentration in the leaves of tree types of aquatic plants.
The average percentage concentration (APC) of TN in the
The Pearson correlation index was used to analyze the simple correlations among five environmental indicators (Table
Correlation analysis among five environmental indicators.
TN | TP | FV | SS | CW length | |
---|---|---|---|---|---|
TN | 1 | .970∗∗ | .953∗∗ | .954∗∗ | −.956∗∗ |
TP | .970∗∗ | 1 | .983∗∗ | .988∗∗ | −.972∗∗ |
FV | .953∗∗ | .983∗∗ | 1 | .987∗∗ | −.975∗∗ |
SS | .954∗∗ | .988∗∗ | .987∗∗ | 1 | −.989∗∗ |
CW length | −.956∗∗ | −.972∗∗ | −.975∗∗ | −.989∗∗ | 1 |
∗∗With a significance level of 0.01.
The FV presented a strong positive correlation with TN (
In the current study, a 1200 m long CW was constructed to detect the effectiveness of surface-flow CWs in the treatment of fast-flowing water in Ulansuhai Lake in Northern China. With monitoring and experimental data, the performances of surface-flow CWs were investigated. A preliminary conclusion was that the current CW effectively reduces the nutrient concentrations of the irrigation return water, even with a relatively high water flow velocity. Plant types and the CW length obviously affected the removal ratio of nutrients, and different types of plants had different capabilities of absorbing various nutrients. Moreover, the performance of the CW fluctuated with the seasonal fluctuations of aquatic plants. Overall, there were strong positive correlations among the TN, TP, FV, and SS, suggesting these indicators present similar variation trends in CWs. The length of the CW was an important factor in the RR of TN, TP, and SS. Moreover, plant density had a significant effect on the RE of various pollutants.
A limitation of the current study was the absence of daily, or even hourly, monitoring data. More accurate conclusions can be drawn only after a longer test and operation period. Future work should focus on plant configurations, plant density, and substrate construction in this field. The present study exhibits the great potential of CWs in dealing with agricultural wastewater.
The authors declare no conflict of interests.
This work is supported by National Natural Science Foundation of China (no. 51409144, 51209003, and 51478026), the National Water Pollution Control and Management Technology Major Project (no. 2010ZX07320-002 and 2011ZX07301-004), and key projects in the National Science & Technology Pillar Program (no. 2012BAJ21B08).