Franz Kevin F.Geronimo
(Franz Kevin F. Geronimo)
홍정선
(Jungsun Hong)
김이형
(Lee-Hyung Kim)
†
-
국립공주대학교 건설환경공학과
(Department of Civil and Environmental Engineering, Kongju National University)
© Korean Society on Water Environment. All rights reserved.
Key words(Korean)
Bioretention, Hydrologic and hydraulic factors, Low Impact Development, Tree box filter
1. Introduction
Low impact development (LID) is a stormwater management approach that mimics the predevelopment
hydrology of an area by using decentralized technologies that infiltrate, filter,
store, evaporate and detain runoff close to its source (Flores et al., 2015; Hinman, 2005). One example of LID technology was bioretention systems which were widely implemented
in different countries due to its minimal space requirements, versatility and appropriateness
as small landscape areas, and good pollutant removal and peak hydraulic flow reduction
efficiency (Geronimo et al., 2017). Bioretention systems treat stormwater via a range of physical, chemical and biological
processes (Geronimo et al., 2014; Lucke and Nichols, 2015). Bioretention media including soil and sand supports the growth of plants during
intermittent wet and dry periods (Winston et al., 2016).
Tree box filters were an example of bioretention system that have large planting pit
with additional storage, a storm flow inlet from the street or sidewalk and an underdrain
system (Hinman, 2005). These tree box filters were compacted urban stormwater low impact development technique
which allowed volume and water quality treatment performance to be adjusted based
on the hydrologic, runoff quality and catchment characteristics. Tree box filters
were usually applied to treat stormwater runoff from transportation land uses including
roads, bridges, parking lots, highways and sidewalks. In USA, tree box filters attained
pollutant removal greater than 85 % and 45 % for total suspended solids (TSS) and
nutrients, respectively while heavy metal removal ranged from 54 % to 88 % (Penmetcha, 2015).
While there were several researches evaluating long term hydrologic and water quality
performance of bioretention systems both for field scale and laboratory scale, there
was no study conducted to identify hydrologic and hydraulic factors affecting long
term performances of tree box filters (Lucke and Nichols, 2015; O’Neill and Davis, 2011; Thomas et al., 2015). Evaluation of long term performance was used as a common performance indicator
for LID practices including bioretention systems (Li and Lam, 2015). Limited research works were conducted at present completely assessing the long
term performance of tree box filters for stormwater management. In addition, most
of the recent studies have considered limited storm events and short term monitoring
which is not sufficient and may not accurately represent the system’s long term performance.
As such, this study evaluated the long term treatment and hydraulic performance of
tree box filter. Specifically, hydrologic and hydraulic factors affecting the long-term
treatment performance of the tree box filter were identified and investigated.
2. Materials and Methods
2.1. Description of tree box filter and its catchment area
The tree box filter with infiltration function was constructed in 2010 inside Kongju
National University, Cheonan city, South Korea to manage urban stormwater runoff from
a 100 % impervious parking lot. The aspect ratio (L:W:H) of the tree box filter was
1:0.67:0.87. The schematic representation of the media arrangement in the tree box
filter and its catchment area was illustrated in Fig. 1. The tree box filter was equipped with a small sedimentation basin (2.8 % of the
total facility storage volume) with geotextile fabric for the retardation clogging
in the system and for ease of maintenance. Filter media including woodchip, sand and
gravel were employed in the tree box filter resulting to a total facility storage
volume of 0.71 m3. Metasequoia tree (Dawn redwood) was planted in the tree box filter
to enhance phytoremediation potential of the system and to increase capability of
soils to remove pollutants. The 379 m2 parking lot was constructed using rough asphalt
with slope and runoff coefficient (mean ± standard deviation) of less than 1 % and
0.27 ± 0.42. The surface area of the tree box filter occupied less than 1 % of the
parking lot area.
Fig. 1. Schematic representation of tree box filter and its catchment area.
2.2. Storm event monitoring, data collection and analyses
In order to evaluate the long-term performance of the tree box filter in managing
parking lot storm water runoff, storm events were monitored for years from July 2010
to April 2016. Following the typical sampling scheme in South Korea, six grab samples
were collected during the first hour when the stormwater runoff entered and was discharged
by the tree box filter for the inflow and outflow water samples, respectively (Jung et al., 2008). The first grab sample was collected as soon as the runoff entered the tree box
filter and after 5, 10, 15, 30 and 60 mins respectively. After the first hour, a sample
was collected at an interval of 1 hour until the storm water runoff stopped flowing
into the tree box filter. Similar sampling scheme was conducted for the storm water
discharged by the tree box filter. Runoff and discharge flow rates were measured at
a 5-min interval. Climatological characteristics of the monitored rainfall events
including the antecedent dry days (ADD), rainfall depth and rainfall intensity were
obtained from the Korea Meteorological Administration. Water samples were analytically
analyzed for particulates (TSS); organics including biochemical oxygen demand (BOD)
and chemical oxygen demand (COD); nutrients such as total nitrogen (TN) and total
phosphorus); soluble heavy metals such as chromium (Cr), copper (Cu), zinc (Zn) and
lead (Pb), and their total heavy metal counterpart namely, total chromium (TCr), total
copper (TCu), total zinc (TZn) and total lead (TPb) parameters in the laboratory in
accordance with the standard methods for the examination of water and wastewater (APHA, AWWA, and WEF, 1992).
The efficiency of tree-box filter was evaluated using load and volume ratio which
were calculated by dividing the discharged load and volume with the runoff load and
volume, respectively. Statistical analyses including Pearson correlation (r) and paired
sample sign test were conducted using SYSTAT 12 and Origin Pro 8. Significant correlations
and differences between parameters were accepted at 95 % confidence level implying
that probability (p) values were less than 0.05.
3. Results and Discussion
3.1. Characteristics of monitored storm events and water quality load
A total of 21 storm events were monitored in the tree box filter. 70 to 80 % of the
monitored storm events were less than 10 mm which was in the same range of daily rainfall
depth occurring in Cheonan city (Maniquiz et al, 2010). Cheonan city received an annual rainfall (mean ± standard deviation) from 2010
to 2016 amounting to 1176 ± 340 mm. Listed in Table 1 was the statistical summary of hydrologic and hydraulic characteristics of monitored
storm events in the tree box filter. The minimum and maximum rainfall depth monitored
corresponded to 34 % and 80 % to 90 %, respectively of the rainfall occurring in Cheonan
city. These findings implied that the storm events monitored in the study objectively
represented the rainfall depth occurring in Cheonan city. Among the 21 storm events
monitored, only ten storm events were able to produce outflow wherein 40 % of these
occurred from June to August. The mean rainfall intensity of storm events that produced
outflow was 34 % greater than those which have no outflow. Excluding the storm events
where the tree box filter has no discharge, the significant volume, average flow and
peak flow reduction were 14 % to 86 %, 14 % to 71 % and 20 % to 88 %, respectively
(p < 0.001). Rainfall depth was found to be negatively correlated with volume, average
flow and peak flow reduction of the system implying that the increase in rainfall
depth causes decreased hydraulic performance of the system (r = -0.53 to -0.59; p
< 0.01). The hydraulic retention time (HRT) in the tree box filter was found to be
significantly correlated with antecedent dry days (ADD) and runoff duration (r = 0.55
to 0.63; p < 0.05). This finding implied that the intermittent dry and wet condition
affected by evapotranspiration mechanism of the tree box filter affected the system’s
hydraulic performance. Lastly, there was no correlation found between ADD and the
system’s hydraulic and pollutant reduction performance. The mean runoff and discharged
unit pollutant loads in the tree box filter were demonstrated in Fig. 2. Inflow TSS unit load amounting to 0.26 ± 0.49 g/m2 was significantly reduced to outflow TSS unit load of 0.02 ± 0.04 g/m2 (p < 0.001). Despite the negative organic loads removal efficiency in some of the
monitored storm events, the inflow unit BOD and COD loads were significantly reduced
by -50 % to 100 % and –13 % to 100 %, respectively (p < 0.001). Negative BOD and COD
removal efficiencies were observed for the rainfall depth of 17.5 mm which was beyond
the design rainfall capacity of the system of about 5 mm. Similarly, significant nutrients
and heavy metals unit load reduction with mean value ranges of 79 % to 83 % and 77
% to 86 %, respectively (p < 0.001). The heavy metals in stormwater runoff including
Cr, Cu, Zn and Pb were mostly in dissolved or soluble form ranging from 46 % to 67
% of TCr, TCu, TZn and TPb, respectively. On the other hand, the percentage of discharged
soluble heavy metals further increased to 64 % to 83 % implying that the system developed
was more effective in reducing particulate heavy metals compared to soluble heavy
metals. This finding was attributed to the filtration capability of the system where
in larger particles tend to be reduced through filter media in the tree box filter.
In addition, larger soluble heavy metal fraction in the outlets could also be attributed
to the hydraulic retention time in the tree box filter similar to the study conducted
by Egemose et al. (2015) which evaluated heavy metal retention in stormwater treatment ponds. High mean unit
pollutant load reduction were attributed to the storm events with rainfall depth less
than 6.5 mm in which the runoff were either retained or infiltrated fully in the system
resulting to 100 % pollutant load reduction. These findings suggested that the tree
box filter showed efficiency in volume, flow, and pollutant load reduction. Retention,
infiltration and evapotranspiration were the volume, flow and pollutant removal mechanisms
that occurred in the tree box filter.
Table 1. Statistical summary of hydrologic and hydraulic characteristics of monitored storm events
|
Parameter
|
Unit
|
n
|
Minimum
|
Maximum
|
Mean
|
Median
|
Standard deviation
|
|
|
Antecedent dry days (ADD)
|
day
|
21
|
0.20
|
19.30
|
5.21
|
3.90
|
4.76
|
|
Rainfall depth
|
mm
|
21
|
1.00
|
22.50
|
6.29
|
3.50
|
5.99
|
|
Rainfall duration
|
hr
|
21
|
0.53
|
8.22
|
3.66
|
3.07
|
2.62
|
|
Average rainfall intensity
|
mm/hr
|
21
|
0.36
|
25.40
|
3.23
|
1.22
|
5.65
|
|
Runoff duration
|
hr
|
21
|
0.08
|
7.00
|
1.70
|
1.17
|
1.64
|
|
Hydraulic retention time (HRT)
|
hr
|
10
|
0.12
|
2.63
|
1.12
|
0.96
|
0.97
|
|
Runoff volume
|
m3 |
21
|
0.01
|
5.07
|
0.84
|
0.10
|
1.37
|
|
Discharged volume
|
m3 |
10
|
0.01
|
3.97
|
0.97
|
0.42
|
1.30
|
|
Average inflow rate
|
m3/hr
|
21
|
0.01
|
3.20
|
0.58
|
0.08
|
0.94
|
|
Average outflow rate
|
m3/hr
|
10
|
0.02
|
2.25
|
0.64
|
0.34
|
0.69
|
|
Peak inflow rate
|
m3/hr
|
21
|
0.02
|
11.61
|
2.26
|
0.30
|
3.41
|
|
Peak outflow rate
|
m3/hr
|
10
|
0.03
|
5.12
|
1.64
|
1.52
|
1.50
|
Fig. 2. Runoff and discharged unit pollutant loads (mean ± standard deviation).
3.2. Relationship between pollutant load, volume and flow rate reduction
Apparent in Fig. 3, TSS has the lowest pollutant load ratio of 0.15 which is equivalent to 85 % TSS
load reduction for volume ratio 0.5 among all the stormwater constituents. Similarly,
reduction of runoff volume by 50 % will result to organics and nutrients constituents’
reduction ranging from 30 % to 55 % and 40 % to 60 %, respectively. Lastly, the corresponding
total and soluble heavy metal reduction ranged from 55 % to 70 % (except for TCr and
TPb) and 50 % to 55 % for volume reduction of 50 %. Lower reduction for TCr, TPb and
soluble heavy metals supported the findings of a previous study wherein soluble heavy
metals tend to by-pass the system only with the volume reduction through infiltration
and retention as the main factor for heavy metal reduction (Geronimo et al., 2014). While the reduction of heavy metals may be attributed to ion exchange and adsorption,
the retention of Pb decreased significantly as a result of competition with other
heavy metals being adsorbed in the system (Oh et al., 2009). Among the pollutants analyzed, BOD, TPb and Cr and Cu were found to have probability
of leaching when the discharged volume in the tree box filter were greater than 75
%, 65 %, 95 % and 95 % of the runoff volume. The leaching of these pollutants was
observed during the two storm events with the highest rainfall depth amounting to
17.5 mm and 22.5 mm. In a study conducted by Scholes et al. (2008), medium potential reduction of BOD may be expected through mechanisms including adsorption,
settling, microbial degradation, filtration and plant uptake. Similar studies about
bioretention systems have found that volume reduction through infiltration, retention
and intermittent conditions in the stormwater treatment systems, specifically bioretention,
have affected pollutant load reduction (Geronimo et al, 2014; Mangangka et al. 2015; Subramaniam et al., 2015).
Fig. 3. Polynomial regression of the changes in pollutant load ratio with respect to volume ratio.
By attaining average flow reduction of 50 %, greater pollutant load reduction by the
system may be expected ranging from 55 % to 90 % exhibited in Fig. 4(a) except for BOD. Lower BOD removal amounting to approximately 43 % with respect to
50 % average flow reduction was observed. This finding was attributed with the BOD
leaching during the monitoring of storm events with highest rainfall depth amounting
to 17.5 mm and 22.5 mm and the increase in rainfall depth which caused the pollutants
to by-pass the system. Similarly, 25 % to greater than 80 % pollutant reduction may
be expected from the tree box filter for a 50 % peak flow reduction as can be seen
in Fig. 4(b). Among the pollutant constituents, only TSS, TP, COD and TZn had greater pollutant
removal amounting to 55 % to 90 % at 50 % peak flow reduction. Greater pollutant reduction
was observed with corresponding average flow reduction compared to peak flow reduction.
These findings suggested that hydrologic and hydraulic factors including volume, average
flow and peak flow were found have affected the pollutant load reduction performance
of tree box filter for all the stormwater constituents. The hydraulic performance
of the system was also found to be important not only for pollutant removal but also
to support plant life and growth in bioretention systems such as tree box filters
(Fassman-Beck et al., 2015; Guo et al., 2015).
Fig. 4. Polynomial relationship of pollutant reduction with a. average flow and b. peak flow reduction.
3.3. Runoff duration and hydraulic retention time
The varying pollutant load ratios with respect to the ratio of HRT to runoff duration
(RD) were demonstrated in Fig. 5. Among the stormwater constituents, TSS was found to have the greatest load ratio
of about 0.1 corresponding to 90 % TSS reduction at an HRT to runoff duration ratio
of 0.5. Increased HRT to runoff duration ratio from 0.2 to 0.5 resulted to increased
organics and nutrients removal by 20 % to 30 % and 25 % to 35 %, respectively. Consequently,
0.5 HRT to runoff duration ratio caused 67 % to 71 % and 55 % to 82 % soluble and
total heavy metal reduction, respectively. Low HRT in constructed wetlands, another
type of low impact development technology used for stormwater and wastewater treatment,
resulted to incomplete denitrification and nitrogen removal required longer HRT compared
with that required for organics removal (Lee et al., 2009). Heavy metal retention of a stormwater treatment facility depended on several facility
related factors such as type, age, volume/CA ratio and HRT and runoff characteristics
including pH, alkalinity, organic matter and other substances (Egemose et al, 2015). Previous studies in USA and South Korea confirmed that longer HRT in the media
or in the stormwater treatment system resulted to greater pollutant reduction efficiency
(Geronimo et al., 2017; Peterson et al, 2015). Longer HRT enabled longer contact time between stormwater runoff and filter media
in the tree box filter increasing the pollutant reduction efficiency of the system.
Fig. 5. Logarithmic regression of pollutant load ratio at varying HRT to runoff duration ratio.
4. Conclusion
In this study, a tree box filter developed to treat parking lot stormwater runoff
was evaluated to identify the long term treatment performance and the corresponding
hydrologic and hydraulic factors affecting its performance. Based on the results of
this study, the increase in rainfall depth caused the decreased hydraulic performance
of the tree box filter including volume, average flow and peak flow reduction (r =
-0.53 to -0.59; p <0.01). The intermittent dry and wet condition of the tree box filter
affected the hydraulic performance since ADD and runoff duration were found to affect
the HRT of the system (r = 0.55 to 0.63; p <0.05). Significant reduction in volume,
flow and pollutant loads through media filtration, infiltration and evapotranspiration
were observed after the runoff entered the tree box filter (p < 0.001). Significant
volume, average flow and peak flow reduction amounting to 14 % to 86 %, 14 % to 71
% and 20 % to 88 %, respectively for storm events where the tree box filter has no
discharge. Among the pollutants, TSS showed the greatest removal amounting to 70 %
to 98 % for rainfall events in which tree box filter produced outflow. This was followed
by satisfactory total and soluble heavy metal removal that ranged from 18 % to 93
% and 14 % to 88 %, respectively. Apart from volume reduction, total heavy metal removal
was attributed to the filtration mechanism in the system wherein greater particulate
metals tend to be reduced compared to the soluble heavy metal evident in the higher
percentage of soluble heavy metal in the discharged volume of the tree box filter.
Both organics and nutrients, which were observed to have leached at a rainfall depth
of 17.5 mm, have the lowest removal efficiencies with ranges of -50 % to 90 % and
-20 % to 94 %, respectively. Significant hydrologic and hydraulic factors including
volume, average flow, peak flow, HRT and runoff duration affected the pollutant reduction
efficiency of the tree box filter. An increase in the volume ratio corresponded to
an increase in pollutant load ratio. In addition to volume reduction, an increase
in both average and peak flow reduction corresponded to increased pollutant reduction
efficiency of the tree box filter. However, greater pollutant reduction was observed
with corresponding average flow reduction compared to peak flow reduction. The hydraulic
performance of the tree box filter was not only important for pollutant removal but
also for sustaining plant life. At an HRT to runoff duration ratio of 0.5 the removal
of TSS, organics, nutrients, and total and soluble heavy metals were 90 %, 60 % to
70 %, 70 % to 80 %, 55 % to 82 % and 67 % to 71 %, respectively. Longer HRT resulted
to longer contact time between stormwater runoff and filter media in the tree box
filter and increased pollutant removal efficiency of the system. For similar tree
box filter design and application, it is suggested that a small sedimentation basin
be included to slow down clogging and for the ease of maintenance through coarse particle
and debris removal. These findings were especially useful in applying similar tree
box filter which may be designed by considering tree box filter surface area to catchment
area of less than 1 %.
5. 국문요약
식생체류지 기법 중 하나인 나무여과상자는 유역면적 및 강우유출수의 특성에 따라 기법의 용적 및 수질 저감 능력 조정이 가능한 도시 저영향개발 기술이다.
본 연구는 주차장 강우유출수 처리를 위해 6년동안 운영된 나무여과상자의 성 능을 평가하기 위하여 수행되었다. 또한 나무여과상자의 저 감 능력에 영향을
미치는 수리·수문학적 요인들을 조사하였 다. 분석 결과, 강수량의 증가는 나무여과상자의 유출량, 평 균유량 및 첨두유량 감소 등의 수리·수문학적 성능이
감소되 는 것으로 평가되었다(r = -0.53 to -0.59; p < 0.01). TSS, 유 기물, 영양물질 및 중금속 등의 오염물질은 나무여과상자
내 충진된 여재의 여과 및 흡착, 침투, 증산발 기작 등을 통하여 저감되는 것으로 나타났다(p < 0.001). 또한 유출량, 평균유 량, 첨두유량,
체류시간 및 강우지속시간 등과 같은 수리·수 문학적 요인의 영향을 받는 것으로 평가되었다. 이는 나무여 과상자 시설을 유역면적 대비 시설의 표면적을
1 % 미만으 로 설계 시 특히 유용한 것으로 나타났다.
Acknowledgement
This research was supported by a grant (E416-00020-0602-0) from Public Welfare Technology
Development Program funded by Ministry of Environment of Korean government. The authors
are grateful for their support.
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