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Clogging Potential in Constructed Vertical Flow Wetlands Employing

Different Filter Materials for First-flush Urban Stormwater Runoff Treatment

Article · August 2018

DOI: 10.17663/JWR.2018.20.3.235

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Journal of Wetlands Research ISSN 1229-6031 (Print) / ISSN 2384-0056 (Online)
Vol. 20, No. 3, August 2018, pp. 235-242 DOI https://doi.org/10.17663/JWR.2018.20.3.235

Clogging Potential in Constructed Vertical Flow Wetlands Employing Different

Filter Materials for First-flush Urban Stormwater Runoff Treatment
Yaoping Chen･Heidi B. Guerra*･Youngchul Kim*†
School of Earth and Environment, Anhui University of Science and Technology, Huainan, China
*
Department of Environmental Engineering, Hanseo University, Seosan, Republic of Korea

도시 초기 강우유출수 처리를 위한
수직흐름습지에서 여재별 폐색 잠재성 분석
진요평･게라 하이디*･김영철*†
중국 안휘이공대학 지구환경과학부
*
한서대학교 환경공학과
(Received : 15 March 2018, Revised: 24 May 2018, Accepted: 23 July 2018)

Abstract
The function of vertical subsurface flow wetlands can potentially be reduced with time due to clogging and are often
assumed to be occurring when ponding and overflow is observed during rainfall. To investigate their clogging
potential, three pilot-scale vertical subsurface flow (VSF) wetland systems were constructed employing woodchip,
pumice, and volcanic gravel as main media. The systems received stormwater runoff from a highway bridge for seven
months, after which the media were taken out and divided into layers to determine the amount and characteristics of
the accumulated clogging matters. Findings revealed that the main clogging mechanism was the deposition of
suspended solids. This is followed by the growth of biofilm in the media which is more evident in the wetland
employing woodchip. Up to more than 30% of the clogging matter were found in the upper 20 cm of the media
suggesting that this layer will need replacement once clogging occurs. Moreover, no signs of clogging were observed in
all the wetlands during the operation period even though an estimation of at least 2 months without clogging was
calculated. This was attributed to the intermittent loading mode of operation that gave way for the decomposition of
organic matters during the resting period and potentially restored the pore volume.

Key words : media clogging, stormwater, vertical flow wetland

요약
운전시간이 경과함에 따라 공극폐색 문제로 인하여 수직 흐름형 습지의 기능은 저하되는데 이와 같은 문제는 폰딩(ponding)
이나 월류 현상에 의하여 쉽게 관측할 수 있다. 공극폐색 잠재성을 조사하기 위하여 도로주변에 설치된 파일럿 규모의 습지
운전자료를 분석하였다. 습지에는 각각 우드칩과 마사(부석), 그리고 화산석을 충진하였다. 약 7개월 동안 도로 강우유출수
처리시험을 수행한 후 충진된 여재를 비운 후 여재 층별로 분류하여 여재에 의해 포획된 고형물 입자의 양과 특성을 분석하였다.
분석결과 대부분의 포획물질은 외부기인 부유물질 이었으며 다음으로 여재표면에 증식한 생물막인 것으로 나타났다. 특히
다른 여재와 비교하여 유기성 여재인 우드칩에서 생물막의 증식이 왕성하였다. 또한 전체 포획량 중 30% 이상이 상부 20cm
이내에 집중되어 있어 폐색으로 인한 폰딩 발생시 이 부분의 여재를 우선적으로 교체하여야 할 것으로 판단된다. 또한 모델계산
결과 우드칩 충진 습지에서 폐색에 도달하는데 약 2달 정도가 소요될 것으로 산출되었으나 실제로는 전혀 폐색 기미는 발생하지
않았는데 이는 강우시에만 운영되는 특성상 강우활동이 없는 무강우 기간 동안 포획된 유기물질이나 생물막이 자연적으로
분해되어 일정기간이 경과되면 공극이 회복되었다.

핵심용어 : 도시 강우유출수, 수직흐름형 습지, 여재폐색

1)

†
To whom correspondence should be addressed.
Department of Environmental Engineering, Hanseo University
E-mail: ykim@hanseo.ac.kr

Journal of Wetlands Research, Vol. 20, No. 3, 2018

236 도시 초기 강우유출수 처리를 위한 수직흐름습지에서 여재별 폐색 잠재성 분석

1. INTRODUCTION as compared to the growth of biomass.

Undesirable effects of wetland bed clogging include the
Structural best management practices (BMPs) are creation of a highly polluted top media layer and reduction
typically designed to reduce the negative impacts of in the treatment volume of water due to an increase in
stormwater pollutants and control the amount of urban overflow frequency (Larmet et al., 2007; Le Coustumer
sediment (Yong et al., 2013; Li and Davis, 2008). BMP et al., 2012). Hence, of interest for wetland applications
structures such as vertical subsurface flow (VSF) is the time until clogging occurs, which can be identified
constructed wetlands usually employ settling, filtration, and when overflow, ponding, and decrease in maximum water
biological mechanisms in reducing diffuse pollutants from content is observed. Langergraber et al. (2003) conducted
stormwater runoff. However, the performance of VSF experiments on a pilot-scale vertical flow constructed
wetlands can ipotentially be reduced with time due to wetlands dosed every 6 hours and observed clogging after
clogging as can be observed through the occurrence of 1 month, 2 months, and up to 18 months at hydraulic
ponding and overflow. Clogging is a common phenomenon loading rates of 250, 150, and less than 180 mm/d. Bavor
that occurs in any filtration system. It is defined as the and Schulz (1993) reported up to 100 days without clogging
formation of a semi-pervious layer throughout a range of on large-scale constructed wetland systems. However,
depths due to the combined effect of physical, biological, clogging can also occur as early as 6-43 days under
and chemical processes (Langergraber et al. 2003; Bouwer, high-concentration loadings such as in the study of Zhao
2002). According to Blazejewski and Murat-Blazejewska et al. (2004). Thus, the typical time period when clogging
(1997), the main factors affecting clogging are the can possibly occur depends on a number of factors
accumulation of solids and vegetation debris, the growth including the type and characteristics of the inflow,
of biofilm within the medium, roots and rhizomes, and hydraulic loading rate, mode of operation (continuous or
the deposition of chemical precipitates. Langergraber et al. intermittent loading), media size, etc. Therefore in this
(2003) and Winter and Goetz (2003) reported that study, three pilot-scale structural VSF wetland was
suspended solid loading plays a more vital role in clogging operated and its treatment performance in terms of

Fig. 1. (a) Photos (top and diagonal view), (b) schematic diagram,
and (c) longitudinal section of the wetland portion of the VSF system (not to scale)

한국습지학회 제20권 제3호, 2018

 진요평･게라 하이디･김영철 237

pollutant reduction as well as water retention with or higher density of 840 kg/m3. All the media were washed
without overflow was observed. To determine its clogging with clean lake water before packed carefully avoiding
potential as well as to estimate its longevity without unnecessary compaction.
clogging, the accumulation of solid matters within the To improve the treatment efficiency while providing
media were collected and analyzed after the operation aesthetics, Acorus Calamus (sweet flag) was planted in
period. the wetlands. The roots were embedded between the
small pot gravel and big stone layers and no soil was
2. MATERIALS AND METHODS added to avoid clogging. Considering the porosity of the
main media, the storage volumes were 0.245 m3 for the
2.1 Constructed VSF wetlands woodchip and volcanic gravel wetlands, and 0.211 m3
for the pumice wetland.
To conduct the experiments, three vegetated pilot-scale
wetland systems made of opaque acryl plates were 2.2 Operation, monitoring, and sampling
constructed as shown in Fig. 1(a). Each wetland consists
of a settling tank (0.5Lx0.6Wx1.1H m), a VSF wetland Stormwater runoff from a bridge was collected and
(0.8Lx0.6Wx1.1H m), and a pipe system Fig. 1(b). The stored in the settling tank for 24 hours before feeding to
pipe system is connected to a pump in the settling tank the VSF wetlands at an approach velocity of 55 m/d.
wherein the stormwater was stored for a period of time This corresponds to a rainfall event that has a return
before being fed to the VSF wetland. On one side of the period of 5 years. The treatment cycle corresponding to
wetland, a recycling system was built to recirculate the the hydraulic retention time (HRT) in the VSF wetland
effluent back to the surface of the bed thus providing was designed as three days. Within this period, recycling
multiple treatment. Sprinklers were provided to distribute is conducted every 6 hours and entails pumping the
the inflow or recycled stormwater evenly on the surface water at the bottom of the wetland back to the surface.
of the wetland. After each treatment cycle, another batch of stormwater
The arrangement of the media in each wetland is was fed into the systems.
shown in Fig. 1(c) and the physical properties of the To achieve the same water level in all the wetlands,
main media are summarized in Table 1. Small pot gravel the required inflow volume was 126, 102, and 120L for
and quartz stones were laid at the surface to facilitate the woodchip, pumice, and volcanic gravel, respectively.
the distribution of incoming water and to provide space Every stormwater influent as well as effluents per day
for the growth of plant roots. In the same manner, were sampled for water quality analysis. Water levels in
larger stones were employed at the bottom to provide the tank were also recorded. The wetlands were
proper drainage of the effluent. Woodchip, pumice and operated throughout a total of 28 rainfall events from
volcanic gravel were selected as the main media and are May to November for a total of seven months. After
all locally available. Woodchip is a kind of renewable the end of operation, all the media were taken out and
organic material, which have the lowest density of 260 divided into 5 layers: 0-20, 20-30, 30-40, 40-50, and
3
kg/m among the three media. The pumice used in this 50-60 cm from the surface. Each layer of the dirty
study is also a kind of lightweight material with a media were washed and the wash water were sampled
3
density of 400 kg/m and the highest specific surface to measure the total suspended solids (TSS), chemical
2
area of 29.55 m /g. Volcanic gravel is a porous material oxygen demand (COD), total nitrogen (TN), and total
2
with a specific surface area of 4.56 m /g and a relatively phosphorus (TP).

Table 1. Physical characteristics of the main media used in the study

Size d10a d50 d60

Media Ub Porosity (%)
Range (mm) (mm) (mm) (mm)

Woodchip 15.0~65.0 20.0 31.0 34.0 1.70 64.0

Pumice 6.0~13.0 7.0 9.0 9.0 1.29 55.0

Volcanic gravel 11.0~20.0 13.5 16.0 16.5 1.22 65.0

a
d10 = effective diamater (dN = particle size wherein N% of the total amount by mass is smaller);
b
U = uniformity coefficient

Journal of Wetlands Research, Vol. 20, No. 3, 2018

238 도시 초기 강우유출수 처리를 위한 수직흐름습지에서 여재별 폐색 잠재성 분석

3. RESULTS AND DISCUSSION resting period of a cycle, and can be removed at a rate
depending on the decomposition process. Blazejewski and
3.1 Characteristics of the matters accumulated in Murat-Blazejewska (1997) assumed that biofilm growth
and decomposition can be balanced and do not contribute
the wetlands
to clogging. However, Platzer and Mauch (1997) reported
Table 2 summarizes the accumulation of clogging matter a linear decrease in bed conductivity with increasing COD
in the wetlands throughout the operation period. The loading, although it was likely that TSS loading also
amount of these matter were determined to be 18.8 L, 5.1 increased. Langergraber et al. (2003) concluded that
L, and 2.3 L corresponding to void space reductions of biomass growth plays only a minor role compared to
11.9%, 3.8%, and 1.4% in the wetlands containing suspended solids over short terms.
woodchip, pumice and volcanic gravel, respectively. While Thus, for non-biodegradable inorganic media, the grain
the woodchip displayed the highest content of clogging size as well as its solids trapping capacity mainly contributes
matter, its TSS removal was the lowest among the three to the speed of clogging matter accumulation. On the other
wetlands as shown in Table 3. Moreover, an increasing hand, roots and biofilms were anticipated to block only
COD concentration along with retention time was a small portion of the pore.
observed. This shows that the accumulation of clogging
matter in the woodchip wetland was mainly caused by the 3.2 Distribution of clogging matter
combination of incoming particulates deposition and
The distribution of clogging matter within the depth of
possibly, biomass production. It has been reported that
the media also plays an important role in the development
higher concentrations of COD should promote biofilm
of the clogging process. Generally, distributed filtration is
growth within the media bed. Woodchip is a biodegradable
more beneficial than surface filtration when it comes to
organic material and the debris generated from the process
delaying clogging. At the end of the experiment, the
of biodegradation contributed to the amount of clogging
distribution of clogging matters were measured and the
matter.
profiles are shown in Fig. 2.
On the other hand, very high TSS removals were observed
A sharp S-shaped distribution of mass as well as volume
in the wetlands containing pumice and volcanic gravel
was observed in the woodchip wetland, showing more
suggesting that the blockage of pores by inorganic solids
clogging matters accumulated in the top layer and the
has potential greater contribution to the removal of solids
bottom layer (Fig. 2(a)). This is in contrast to previous
in these wetlands. Meanwhile, the idea of organic matter
reports wherein clogging matters mainly accumulated in
contributing to pore blockage has been arguable. Organic
the top layer (Zhao et al., 2009; Hua et al., 2010). However,
matters are subject to oxidation, especially during the
this is most probably due to the fact that sand was used
as the substrate in those studies whereas woodchip, which
Table 2. Accumulation of clogging matter in the wetlands is bigger and provides more void spaces for sediment
Accumulated Void space transport down the bed, was used in this study. Woodchip,
Void volume
Media clogging matter reduction
(L) as an organic material, can gradually decay under wet
(L) (%)
conditions. In this case, the clogging matters in the top
Woodchip 158.4 18.8 11.9
layer were mainly from the entrapped solids and the growth
Pumice 136.1 5.1 3.8
of biofilm, while the accumulation in the bottom layer can
Volcanic gravel 160.8 2.3 1.4
be mainly due to the settling of the woody materials that

Table 3. Mean pollutant inflow concentrations, outflow concentrations, and pollutant removal efficiencies
Woodchip Pumice Volcanic gravel
Pollutant In Out In Out In Out
Rem. % Rem. % Rem. %
(mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L)
TSS 22.5 7.5 48.7 22.5 1.30 90.4 22.5 1.30 92.1
TCOD 65.0 119 -127.1 65.0 31 39.3 65.0 38.0 29.4
NH4-N 1.10 0.12 82.8 1.10 0.14 88.0 1.10 0.04 88.7
TN 4.73 2.45 40.4 4.73 2.70 39.0 4.73 3.03 33.7
TP 0.16 0.11 29.0 0.16 0.03 79.3 0.16 0.05 59.6

한국습지학회 제20권 제3호, 2018

 진요평･게라 하이디･김영철 239

Fig. 3. Distribution of TSS in the VSF wetland beds

The detachment and settling of solid particles during the

stormwater retention period can be confirmed by the
distribution of TSS accumulated at the different layers of
the media as shown in Fig. 3. It is apparent that the amount
of accumulated solids increased with depth except at the
upper layers of woodchip where biofilm growth was
observed.

3.3 Distribution of COD, TN, and TP

To understand how basic pollutants collected within the
wetland bed, the contents of COD, TN, and TP in the
clogging matters were also measured. Fig. 4 illustrates the
profiles of the pollutant distributions in these VSF wetlands.
COD accumulation was found to be higher at the upper
layers than the lower layers in all the wetlands. This
corroborates the lower density of the clogging matter in
the upper layers as observed in the previous section (Section
3.2). The accumulation of organic matter at the top layer
forms a mat on the top of the bed which acts as a trap
and spares the underlying layers from clogging. In the
woodchip wetland, however, an increase in COD was
Fig. 2. Distribution of clogging matters in the VSF wetland beds observed at a depth 40-50 cm from the surface. This can
be attributed to the leached COD from the woodchip
were detached from the submerged woodchip. material itself as well as to the biofilms created in that
For pumice and volcanic gravel wetlands, similar trends layer.
were observed (Fig. 2(b) and 2(c)). Higher volume than Higher amounts of TN were also observed in the top
mass percentages of the clogging matter at the top layer layer in all the wetlands suggesting that a large portion
implies that they have lower densities as compared to that of this pollutant is particulate-bound and that filtration
found in the lower layers. This means that at the tope laye is more significant in this layer as compared to the lower
of both wetlands, the accumulated matters were mostly layers. In the woodchip wetland, a sudden rise in
organic in nature. The nutrients from inflow were always accumulated nitrogen was observed in the 4th layer where
firstly trapped by the top layer and, if combined with good the increase in COD was also observed.
aeration, can enhanced the growth of the biofilm. In As with nitrogen, most of the phosphorus were removed
addition, the increase of clogging matters in the bottom at the upper layers of the wetlands which is expected due
layer were most probably due to the accumulation of to the high affinity of phosphorus to sediments. However,
broken debris from the media during the discharge. in the wetland containing volcanic gravel, TP was observed

Journal of Wetlands Research, Vol. 20, No. 3, 2018

240 도시 초기 강우유출수 처리를 위한 수직흐름습지에서 여재별 폐색 잠재성 분석

to have been released by the media resulting to negative

removal efficiencies and higher TP content as compared
to woodchip and pumice.

3.4 Changes in water head

Clogging in granular medium is a process that develops
with operational time and significant clogging can be
ultimately reflected by the change of the water level or the
occurrence of ponding on the surface of the wetlands. In
this study, the factors influencing the change of water level
in the VSF wetlands include transpiration, evaporation, daily
collection of water quality samples, and the occurrence of
rainfall. Fig.5 shows the fluctuation of the water depth in
these wetlands over time. The significant declines of water
levels in some periods, especially in the initial operation
period of 10 to 30 days, were mainly caused by transpiration
and evaporation. During these periods, no stormwater was
fed into the wetlands to replace the treated stormwater
because of the longer dry days. In contrast, sudden rises
in water level occurred during rainfall and were caused
by the raindrops which directly fell to the wetlands. Except
for these instances, the water levels were stable throughout
the operational time and no ponding was observed on the
surface of these wetlands. Therefore, clogging did not
happen within the operation period which lasted for 180
days or approximately 6 months.
Among the three media used, woodchip was found to
be more susceptible to clogging due to the organic leaching,
woody decay, and higher potential for biofilm growth.
Therefore, the use of this media should be controlled and
the right amount for a target organic requirements for
biological removal processes should be determined.

3.5 Estimation of time to clogging

An early attempt to model clogging in VSF wetlands
assumed that porosity was diminished cumulatively by the
Fig. 4. Distribution of pollutants in the media layer
of the VSF wetlands volume of influent suspended solids loaded into the system

Fig. 5. Changes of the water heads in the VSF wetlands over time

한국습지학회 제20권 제3호, 2018

 진요평･게라 하이디･김영철 241

Table 4. Estimation of time to clogging

ρsolid q Ci α tclog adjusted tclog
Media
(kg/m3) (m/d) (g/m3) (m) (days) (days)
woodchip 20.0 55 22.5 2.98 0.048 69
pumice 518.1 55 22.5 0.74 0.311 448
volcanic gravel 489.4 55 22.5 1.56 0.617 888

over time, such that system longevity corresponded to zero biofilms. In addition, variation in the inflow TSS concentration
porosity (Blazejewski and Murat-Blazejewska, 1997). can also affect the time to clogging.
Subsequently, this approach was validated and extended Nonetheless, the estimated values show that among the
to make it applicable to solids fractions with a biodegradable three media, woodchip is the most susceptible to clogging,
component (Hua et al., 2010). Kadlec and Wallace (2009) followed by pumice, and volcanic gravel. This information
summarized the time to clogging using the relationship is useful when deciding which filter media to select in terms
below: of maintenance frequency and longevity of the constructed
wetland.

log    (Eq. 1)
 
4. CONCLUSIONS
where tclog = time to clogging in days, ρsolid = bulk density
The results of the experiments in the study revealed that
of accumulating solids in kg/m3, q = hydraulic loading rate
the potential major contributors of clogging in vertical flow
in m/d, Ci = inlet TSS concentration, g/m3, and α is an
constructed wetlands are suspended solids deposition
empirical coefficient determined using Eq. 2 as proposed
followed by the formation of biofilms within the filter
by Blazejewski and Murat-Blazejeska (1997).
interstices. Up to more than 30% of the clogging matter
α = 150ε*d (Eq. 2) were found in the upper 20 cm of the media suggesting
that this depth is required to be replaced once clogging
In the equation above, ε is the porosity of the clean occurs. For organic materials such as woodchip, the
media in m3/m3 while d is the particle diameter in m. Using formation of biofilms were more evident through less dense
the properties of the employed media and the accumulated accumulated matter and its occurrence on the deeper part
clogging matters as well, the time to clogging was estimated of the bed. This contributed to a more rapid occurrence
and is presented in Table 4. of clogging. Therefore, for this type of wetland, intermittent
It is important to note that the calculated time of clogging loading with periods of rest is a recommended mode of
was adjusted to consider intermittent loading of stormwater inflow to allow these organic materials to decompose
as well as the resting period between cycles. As a result, thereby restoring pore volume. In addition, due to the
the estimated clogging time was 2, 15, and 30 months for affinity of most pollutants to TSS, majority of them were
woodchip, pumice, and volcanic gravel wetlands respectively. found in the upper layers of the bed, although a leaching
However, the experimental data in this study shows that of COD from woodchip and phosphorus from volcanic
the wetlands were operated for 6 months without clogging gravel was observed in the lower layers of the wetland.
which is 3 times longer than that estimated for woodchip Meanwhile, no signs of clogging were observed during the
wetland. The most probable factors to consider in this case operation period even though an estimated 2 months
is the settling of solid particles at the bottom of the bed without clogging was calculated for the woodchip wetland.
and decomposition of organic matters during the resting Thus, the equation for time of clogging used in the study
period which opens up previously blocked voids and clearly created underestimations. This can possibly be due
restores the hydraulic conductivity of the media bed (Platzer to the intermittent feeding cycle which allowed the opening
and Mauch, 1997). Previous studies have verified that Eq. of previously blocked pores through detachment of solid
1 provides a reasonable approximation to clogging due to particles from the media and decomposition of organic
filtration. Therefore, an underestimation can be made if materials in the media interstices. Based on the findings,
other factors that can restore pore volume is not considered. it can be concluded that woodchip was the media more
These factors include the detachment of previously attached susceptible to clogging due to biofilm growth and decay
particles, decay of the filter media, and decomposition of of the woody material under saturated conditions.

Journal of Wetlands Research, Vol. 20, No. 3, 2018

 242 도시 초기 강우유출수 처리를 위한 수직흐름습지에서 여재별 폐색 잠재성 분석

Acknowledgement metals concomitant transfer in an infiltration basin:

Columns study under realistic hydrodynamical conditions.
This research was partially supported by the Public Proceedings of the NOVATECH 2007: 6th International
Welfare Technology Development Program of the Korean Conference on Sustainable Techniques and Strategies in
Urban Water Management, Lyon, France, pp. 615–622.
Ministry of Environment (Grant No. 2016000200002) and
Le Coustumer, S, Fletcher, TD, Deletic, A, Barraud, S, Poelsma,
the Education Department of Anhui Province of China
P (2012). The influence of design parameters on clogging
(Grant No. KJ2017A074).
of stormwater biofilters: A large-scale column study.
Water Research, 46(20), pp. 6743–6752. [DOI:
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한국습지학회 제20권 제3호, 2018

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