To improve landfill management in Southeast Asia, reduction of landfill leachate volume with low cost and easy operation is required. The present study attempted to utilize constructed wetland (CW) for reduction of landfill leachate by evapotranspiration. The objective of this study is to evaluate the influence of operation and season on evapotranspiration of CW applied to landfill leachate treatment in tropical regions. The pilot-scale CW planted with cattail was constructed in Sainoi landfill site, Thailand. CW could siginificantly reduce the leachate volume by the evapotranspiration in daytime than nighttime. Frequent inflow of horizontal subsurface flow (HSSF) during daytime resulted in active evapotranspiration. Evapotranspiration of CW in HSSF with high frequency of inflow showed the similar that in free water surface (FWS). Throughout the year including both rainy and dry seasons, evapotranspiration of CW were significantly higher than the evaporation volume which is regarded as water reduction by existing landfill leachate treatment, i.e. stabilization pond. CW should be served as the techonology for appropriate management of landfill leachate in Southeast Asia.

INTRODUCTION

In most of the region of Southeast Asia, a treatment of landfill leachate has largely depended on the natural evaporation and purification at stabilization ponds. Increase of landfill leachate volume in rainy season has been concerned to cause pollution of groundwater and the surrounding environment. Although advanced physicochemical and biological treatments were introduced to tropical developing countries by international development aid, these treatments are expensive, consume large amounts of energy and require technical capabilities in operation. First step on management of landfill leachate in Southeast Asia, reduction of leachate volume is important to prevent environmental pollution with taking into consideration of their situation of economic, technical and climate.

Constructed wetland (CW) is artificial water treatment system using ecological function including media, plant and microorganisms designed to remove contaminant. In the past several decades, CW has been mainly applied for sewage and municipal wastewater treatment to remove nutrient in Europe and United States (Vymazal 2010). Recently, lab- to full-scale CW has been investigated to remove the pollutants from landfill leachate (Bulc 2006; Polprasert & Sawaittayothin 2006; Nivala et al. 2007; Chiemchaisri et al. 2009; Kadlec & Zmarthie 2010). In addition to ability of purification by CW, there has been increasing intesst in the function of water reduction of CW by evapotranspiration in Europe (Borin et al. 2011; Milani & Toscano 2013; Pedescoll et al. 2013), Africa (Bojcevska & Tonderski 2007) and Australia (Headley et al. 2012). CW should be one of the most attractive method of landfill leachate treatment which can effectively reduce leachate volume in Southeast Asia. However, to our knowledge, there are no reports in application of CW for landfill leachate treatment in terms of leachate volume reduction in Southeast Asia. It is importance to understand performance of CW throughout the year including dry and rainy seasons.

The objective of the present study was to evaluate the influence of operation of CW on water reduction by the field appllication for landfill leachate management in tropical regions. Seasonal effect on its performance was also investigated through the operation.

METHODS

Site description

The pilot-scale of CW (1 m [width] × 2 m [length] × 1 m [depth]) was installed in Sai Noi landfill site, Nonthaburi province, Thailand (Latitude: 14°, 00′ 24.4″ N; Longitude: 100°, 19′ 02.2″ E). Landfill leachate has been treated by only stabilization pond in Sai Noi landfill site. Climate conditions during the experimental period (Jan. 2014 to Jul. 2014) are shown in Figure 1. Supply of landfill leachate to CW was obtained from stabilization pond. Characteristics of the landfill leachate were as follows: pH 9.8, EC 17.1 (mS cm−1), TS 13,900 (mg L−1), SS 190 (mg L−1), CODcr 1,500 (mg L−1), DOC 550 (mg L−1), TP 7 (mg L−1) and TN 66 (mg L−1) [sampling days: Jan. and Jun. 2014].
Figure 1

Monthly climate data (Jan. 2014 to Jul. 2014).

Figure 1

Monthly climate data (Jan. 2014 to Jul. 2014).

Experimental designs and operating conditions

Several experimental conditions with different frequency of inflow, flow pattern, load were conducted during Jan. 30 2014 to Jul. 26 2014 (Table 1). The bed was filled with sand at 1,600 mm (length) × 650 mm (depth), and 50 mm of gravel (inlet) and 10 mm gravel (outlet) in side of bed at 200 mm (length) × 650 mm (depth), respectively. The effluent pipe was placed at the bottom and the outlet level was set under 100 mm from media surface as horizontal sub-surface flow (HSSF) in operations A to C. The outlet level was designed above 100 mm from media surface like as free water surface (FWS) with effluent leaving through the bottom in operations D and E. Cattails were planted 26 shoots as a density of 13 shoots/m2. Daily inflow, hydraulic retention time (HRT) in HSSF and FWS were 80 L d−1, 4.8 d and 8.1 d, respectively. Frequency of inflow was changed as once, twice and five times on a day: A; 80 L of landfill leachate was applied to CW at 8:50, B; 40 L of landfill leachate was applied to CW at 8:30 and 16:00, C to E; 16 L of landfill leachate was applied to CW at 8:30, 10:30, 12:30, 14:30 and 16:30.

Table 1

Experimental conditions

Operation Duration Each inflow volume × Frequency Feeding time Flow pattern Range of COD load (g m−2 d−1
Jan. 30 – Apr. 30 2014 80 L × 1 8:50 HSSF 52–116 
May 1 – May 17 2014 40 L × 2 8:30, 16:00 HSSF 129–158 
May 20 – Jun. 3 2014 16 L × 5 8:30, 10:30, 12:30, 14:30, 16:30 HSSF 158–166 
Jun. 5 – Jun. 13 2014 16 L × 5 8:30, 10:30, 12:30, 14:30, 16:30 FWS 159* 
Jun. 16 – Jul. 26 2014 16 L × 5 8:30, 10:30, 12:30, 14:30, 16:30 FWS 84–194 
Operation Duration Each inflow volume × Frequency Feeding time Flow pattern Range of COD load (g m−2 d−1
Jan. 30 – Apr. 30 2014 80 L × 1 8:50 HSSF 52–116 
May 1 – May 17 2014 40 L × 2 8:30, 16:00 HSSF 129–158 
May 20 – Jun. 3 2014 16 L × 5 8:30, 10:30, 12:30, 14:30, 16:30 HSSF 158–166 
Jun. 5 – Jun. 13 2014 16 L × 5 8:30, 10:30, 12:30, 14:30, 16:30 FWS 159* 
Jun. 16 – Jul. 26 2014 16 L × 5 8:30, 10:30, 12:30, 14:30, 16:30 FWS 84–194 

HSSF; horizontal sub-surface flow, FWS; free water surface.

*n = 1.

Water balance

For evaluation of water reduction of CW, volumes of inflow and outflow water were periodically measured. Water reduction by evapotranspiration at CW (ETcw) was calculated by a daily water balance as follow: 
formula
where ETcw = volume of evapotranspiration at CW (mm d−1), Inf = volume of influent (mm d−1), Eff = volume of effluent (mm d−1), P = precipitation (mm d−1).
The ratio of ETcw at daytime to ETcw at nighttime (Rdn) was calculated as follow: 
formula

Mean hourly ETcw in daytime was calculated using ETcw at each inflow event (8:30–10:30, 10:30–12:30, 12:30–14:30, 14:30–16:30). Hourly ETcw in nighttime was calculated (16:30–8:30).

To measure water surface evaporation (Epan), pan (radius 53 mm) was set on close to the pilot scale of CW. Landfill leachate that was applied for CW treatment was added to the pan. The coefficients of water reduction of CW related to evaporation (Kpan) was determined as follow: 
formula

RESULTS AND DISCUSSION

Influence of COD load on daily ETcw was evaluated by the results of operations A and B (Figure 2). COD load of landfill leachate did not affect the condition of cattail, and resulted in no correlation between COD load and daily ETcw (r = 0.27, P > 0.01). This indicated that CW could provide stable evapotranspiration regardless of load of inflow under the experimental condition.
Figure 2

Influence of COD load (g m−2 d−1) on daily evapotranspiration (ETcw) in operations A and B.

Figure 2

Influence of COD load (g m−2 d−1) on daily evapotranspiration (ETcw) in operations A and B.

Rates of evapotranspiration at each inflow event were stable in daytime (data not shown). Hourly ETcw in daytime was higher than that in nighttime (Figure 3), and resulted in 4.2 to 5.3 of Rdn in operation C. The results exhibited that water reduction of CW is mainly occurred by evapotranspiration in daytime.
Figure 3

Mean hourly evapotranspiration (ETcw) at daytime and nighttime and the ratio of ETcw at daytime to ETcw at nighttime (Rdn) in operation C.

Figure 3

Mean hourly evapotranspiration (ETcw) at daytime and nighttime and the ratio of ETcw at daytime to ETcw at nighttime (Rdn) in operation C.

Frequent inflow of HSSF during daytime (C) resulted in active evapotranspiration (Figure 4). Frequent water addition would increase the plant available moisture at CW and the rate of transpiration of plant might be increased. CW in HSSF with high frequency of inflow during daytime (C) could reduce the water volume as well as CW in FWS with high frequency of inflow (D). On the other hand, ETcw in HSSF with low frequency of inflow (A and B) was obviously less than that with high frequency of inflow (C). ETcw in FWS was supposed be unaffected by frequency of inflow because water level had been kept above the surface of wetland in FWS. ETcw in HSSF with once inflow migh be less than that in FWS with once inflow. ETcw in operation B did not siginificantly differ from that in operation A (P > 0.01), suggesting that additional feeding at evening could not affect the ETcw.
Figure 4

Comparison of daily evapotranspiration (ETcw) by different feed patterns (operations A to D).

Figure 4

Comparison of daily evapotranspiration (ETcw) by different feed patterns (operations A to D).

ETcw was higher than Epan in all operations (Figure 5). Kpan, which indicates efficiency of evapotranspiration to surface evaporation, revealed that the CW could effectively reduce the leachate volume compared to the stabilization pond from dry to the begining of rainy season. Furthermore, CW could reduce leachate volume even on rainy day. Stable leachate reduction by CW throughout year should be a beneficial feature of tropical climate, and CW must be served as the technology for appropriate management of landfill leachate.
Figure 5

Temporal evaluation of daily evapotranspiration (ETcw), daily evaporation (Epan), the coefficients of ETcw related to Epan (Kpan) and daily precipitation during operations A to E.

Figure 5

Temporal evaluation of daily evapotranspiration (ETcw), daily evaporation (Epan), the coefficients of ETcw related to Epan (Kpan) and daily precipitation during operations A to E.

CONCLUSIONS

The present study evaluates the influence of season and operation on ETcw applied to landfill leachate treatment in tropical regions. CW could significantly reduce the leachate volume by the evapotranspiration in daytime than nighttime. Frequent inflow of HSSF during daytime resulted in active evapotranspiration. CW in HSSF with high frequency of inflow during daytime could reduce the water volume as well as CW in FWS. Leachate volume was effectively reduced by CW even in rainy season by this operation. Stable leachate reduction regardless of seasons performed by this study indicated that the CW can be served as the technology for appropriate management of landfill leachate under the tropical climate.

ACKNOWLEDGEMENTS

This research was supported by the Environment Research and Technology Development Fund (3K113027) from the Ministry of the Environment, Japan.

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