Journal of Engineering Research xxx (xxxx) xxx

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Journal of Engineering Research

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Performance evaluation of a pilot wetland system for wastewater treatment

Mishari Khajah *, Mohd. Elmuntasir Ahmed
Water Research Center, Kuwait Institute for Scientific Research, Kuwait

A R T I C L E I N F O A B S T R A C T

Keywords: Wetland systems are inexpensive and easy-to-operate wastewater systems and are widely used globally at
Heavy metals different scales for the safe treatment of wastewater. This study aims to evaluate a pilot-scale vertical flow
Nutrients wetland system treating wastewater under different loadings. The wetland system was constructed using
Organic matter
indigenous plants (Imperata cylindrical) and operated using actual wastewater for twelve-months. Samples from
Wastewater treatment
Wetland
influent and effluent of the wetland system were collected weekly. They were analyzed for parameters listed in
the Kuwait Environment Public Authority standards for reuse in irrigation, such as organic matter, nutrients,
heavy metals, and microorganisms. The results showed that the high hydraulic loading rate (1.67 m3/m2.d)
phase was more efficient than the low hydraulic loading rate (1.04 m3/m2.d) phase in terms of organics and
nutrient removal efficiency. The range of the removal efficiency of organic matter in low and high loading rates
was around 48.5–49.0% and 59.1–59.1%, respectively. In addition, the range of the removal efficiency of nu­
trients in low and high loading rates was around 43.7–67.7% and 24.7–76.6%, respectively. Moreover, the range
of the removal efficiency of heavy metals under the low and high loading rates was − 134.3–93.8% and −
1545.7–92.6%, respectively. It was evident that he metals were leaching from the soil. Moreover, it was found
that the system performance was closely linked to the ambient temperature effects during the start-up (r = 0.59)
and high hydraulic loading (r = 0.43) phases for example. While ambient temperature has wider effects on
wetland biota, its effects on the plants are more pronounced. In conclusion, wetland systems can be constructed
using indigenous plants to treat office wastewater efficiently under the harsh climate conditions of Kuwait with
adjustments in capacity, loading rates, and operational conditions.

Introduction of the limited freshwater [4,5].

Many parts of the world are not connected to a centralized sewer
Wastewater treatment could offer a valuable water resource for system and mainly depend on limited wastewater treatment and
Kuwait and other arid and semi-arid countries with shortages in fresh­ disposal practices such as septic or storage tanks [6,7]. On the other
water supply [1] and could significantly bridge the gap between water hand, centralized wastewater management systems are capital-intensive
supply and demand. However, the main concern with wastewater and often need to be more economical. In-situ wastewater management
treatment reuse or recirculation is its composition (that is, nutrients systems could offer an alternative inexpensive source of water for reuse
[nitrogen; N, phosphorus; P], chemicals, and pathogens), which can be at the generation point, and, therefore, it is advantageous to have a
detrimental to human health and aquatic life and might present envi­ suitable and cost-effective in-situ wastewater treatment system to treat
ronmental hazards if not appropriately treated [2,3]. Additionally, wastewater and reuse it for irrigation [8,9].
inappropriate wastewater discharge would release nutrients and Wetland systems are considered one of the in situ wastewater
waterborne pathogenic microbes into the receiving environment and treatment technology that is easy to operate, cost-effective, consumes
may raise environmental issues. Small-scale (in situ) wastewater treat­ less energy, and is environmentally friendly (green systems) [10–12].
ment technology is receiving significant attention worldwide and could Wetland systems can be used for small communities, as well as for areas
help address the issue of rising water demand. For example, recently, the that are not connected to the sewer system [13–15]. Therefore, this
government of Kuwait has turned its attention to wastewater treatment research study aimed at designing, constructing, and evaluating the
and reuse in the irrigation of landscapes to reduce the cost and demand performance of a small-scale wetland system to treat the wastewater

* Correspondence to: Kuwait Institute for Scientific Research, P.O. Box 24885, Safat 13109, Kuwait.
E-mail address: mkhajah@kisr.edu.kw (M. Khajah).

https://doi.org/10.1016/j.jer.2023.11.027
Received 25 July 2023; Received in revised form 26 November 2023; Accepted 29 November 2023
Available online 2 December 2023
2307-1877/© 2023 The Author(s). Published by Elsevier B.V. on behalf of Kuwait University. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article as: Mishari Khajah, Mohd. Elmuntasir Ahmed, Journal of Engineering Research, https://doi.org/10.1016/j.jer.2023.11.027
M. Khajah and Mohd.E. Ahmed Journal of Engineering Research xxx (xxxx) xxx

generated at Sulaibiya Research Plant (SRP). A wetland system is a VFCW. Specifically, this research aims to design and construct a small-
system that can biodegrade/remove organic matter (OM), nutrients, scale wetland system using indigenous plants suitable for treating of­
heavy metals, and so on by the bacteria attached to the media and in the fice wastewater generated at a selected office complex site. Specifically,
plants. The treatment process is also enhanced by passing the waste­ under the harsh climate conditions of Kuwait, the project objectives are
water media (filtration) and plant roots horizontally or vertically and to infer about the feasibility of VCWS suitability, evaluate the quality of
reusing it for irrigation [16]. treated wastewater from the wetland treatment system, and understand
Several research studies [17–19] evaluated the performance effi­ its limitations. This work is vital since many areas in Kuwait and globally
ciency of vertical flow constructed wetland (VFCW) systems. These are not connected to centralized wastewater treatment systems and need
studies revealed that the system is a suitable treatment technology with adequate environmentally friendly treatment.
respect to water quality parameters. In addition, these studies revealed
that VFCW systems’ performance was satisfactory in suspended solids Methods and materials
(SS) and OM, as represented by chemical oxygen demand (COD) and
biochemical oxygen demand (BOD), and was limited in P removal due to This study utilized the wetland system to treat wastewater and reuse
the insufficient interaction between the wastewater and the substrate it for irrigating the landscape at Kuwait’s SRP. A VFCW system was
media. Other studies mentioned that VFCWs can also achieve a satis­ fabricated in the mechanical workshop at the KISR and transferred to
factory level of nitrification [20–22]. However, some research studies SRP. The CW has an impermeable lining and is slightly inclined to
pointed out that the performance of VFCWs was poor regarding deni­ facilitate the movement and collection of the water flow from the
trification [23,24]. Furthermore, many recent research studies have influent to the effluent. The VFCW system has a height of 1 m, a width of
demonstrated that VFCWs with intermittent loading regimes can deni­ 60 cm, and a length of 60 cm. It was fabricated using acrylic sheets
trify effectively with some amendments and modifications to the system installed on a steel base, and its outlet is situated at a height of 50 cm
[25–27]. VFCWs are primarily used in Europe, predominantly in from the ground surface, as shown in Fig. 1. The single CW system
Austria, Denmark, Germany, the United Kingdom (UK), France and the consists of the following three layers:
United States of America (USA) [28]. The application of VFCWs is
gradually developing in Asia and Africa [29,30]. • At the bottom, a drainage layer, filled with gravel media, having a
In addition, in Kuwait, the wetland system was applied at Jleeb Al- height of 20 cm, to facilitate the drainage of the wastewater,
Shuyoukh Camp from 2008 to 2013 for 500 people to treat their • On the top of the drainage layer is the primary filter consisting of a
wastewater onsite [31]. Results showed that the system was successfully random packing media with a height of 50 cm, and
applied under Kuwait’s conditions with removal efficiencies of 93.3% • At the top is the distribution layer, primarily soil media of 10 cm
for COD, 92.5% for BOD, 74% for NH+ 4 -N, 43% for PO4-P, and 99.9% for depth planted with macrophytes.
total coliform. Also, the results showed that the treated wastewater met
the specifications for its reuse for irrigation purposes (Kuwait Environ­ The top layer is designed to distribute the wastewater uniformly over
ment Public Authority (KEPA) standards). the surface area of the VFCW system. Random packing was used as a
However, the disadvantages of the constructed wetlands (CWs) biological support media in the VFCW system since it provides a large
include high land area requirements (depending on the design), the need surface area for microbial growth, enhancing the biodegradation
for a preliminary treatment before the treatment of the wastewater by component of the treatment process. The type of macrophytes used is
the system, and the need for higher retention time than a conventional Imperata cylindrical.
system [32,33]. For that reason, VFCWs have the advantage over hori­ The wastewater was pumped from the existing septic tank located at
zontal flow type since they are more efficient and require less land area the parking area of the SRP to a storage tank (volume 500 L) to feed the
[34,35]. wetland system, as shown in Fig. 1. Initially, during the start-up period
This research paper examines the VFCW pilot for wastewater treat­ (3 months, from 12 January to 9 April 2022), the system was fed with
ment under Kuwait conditions and its reuse potential according to KEPA wastewater from the storage tank every 1–2 d to allow biofilm devel­
standards using tidal flow strategy to enhance the oxygen into the opment. During the start-up period, the CW system was filled 2–3 times

Fig. 1. Schematic diagram of the wetland system.

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M. Khajah and Mohd.E. Ahmed Journal of Engineering Research xxx (xxxx) xxx

per week (the system can treat up to 130 L per cycle). One sample was to the operation range (0.25–1.5 m3/m2.d) as mentioned by Metcalf and
collected weekly, 1 L (influent and effluent, before and after the wetland Eddy [38].
system). In situ measurement was conducted for pH, temperature, dis­ Statistical methods were used separately to assess raw wastewater
solved oxygen (DO), electrical conductivity (EC), and oxidation- concentrations and removal rates for the start-up, low, and high HLRs.
reduction potential (ORP). The collected samples were analysed for Analysis was first carried out using the typical statistical measures to
other wastewater parameters, including BOD5, COD, TPO4, NH3, TN, Al, determine the data’s location (mean and median) and variability
As, Ba, B, Cd, Cr, Ni, Hg, Co, Fe, Sb, Cu, Mn, Zn and Pb at KISR labo­ (standard deviation and coefficient of variation). The statistical analysis,
ratories and compared with KEPA standards for irrigation water pur­ including parametric and non-parametric tests, was conducted using
poses [36], as shown in Table 1 along with the characteristics of the Excel (2016).
wastewater. Field and laboratory analysis was conducted according to
Carranzo [37]. After the start-up, full operation with a low hydraulic Results and discussion
loading rate (HLR) was started on 10 April 2022 with 3 cycles per day
(100 L per cycle). Each cycle comprised 6 h (3 h wet and 3 h dry). One The wetland system was operated in three phases; 1) Start-up phase
sample (influent and effluent, before and after the wetland system) per (12 January to 9 April 2022), 2) Low HLR phase (10 April to 30 July
week was collected during this stage with parameters similar to that of 2022), and 3) High HLR phase (31 July until 31 December 2022). The
the start-up stage. During the low HLR period, a volume of 0.3 m3/d results of the three phases are presented as follows:
(2.1 m3/week) of wastewater was used to operate the system, resulting Wastewater samples were collected once every week from influent
in a hydraulic retention time (HRT) of 10.4 h. The low HLR stage ended and effluent and analyzed for the parameters listed in Table 1. In total,
on 30 July 2022, and the high HLR stage started on 31 July until 31 22 samples were collected (11 influent and 11 effluent). The start-up
December 2022 with 3 cycles per day (160 L per cycle). Each cycle phase was 3 months [39]. It is important to know that during the
comprised 6 h (3 h wet and 3 h dry). During this stage, one sample was start-up period, plant growth, microbiological growth, and the inter­
taken every two weeks, with the same parameters as in the previous two ception properties of the wetland are expected to change until the
phases (influent and effluent, before and after the wetland system). It is wetland system is fully developed and its efficiency reaches a stable
important to mention that the HLRs (1.67 and 1.04 m3/m2.d) are similar value. VFCWs utilize microbial-mediated removal pathways to treat
biodegradable contaminants such as organics and nutrients [40]. This is
clearly reflected in Fig. 2.
Table 1 As shown in Fig. 2, the removal efficiency after start-up reached
Characteristics of the wastewater and guidelines for water used for irrigation. (74.7% and 74.0% for BOD and COD, respectively) which is typical of
Parameters Symbol Unit Wastewater Max. Limit VFCW systems > 70.0% [41–43] with an average of 46.9% and 48.1%
characteristics KEPA for BOD and COD, respectively.
pH pH - 5.94–7.85 6.5–8.5 Table 2 shows the average concentrations of the measured parame­
Biochemical oxygen BOD mg/ 37–318 20 ters and their average removal efficiencies during the low HLR phase
demand l (1.04 m3/m2.d), which was operated for a period of (4.5 months). An
Chemical oxygen COD mg/ 61–534 100
average efficiency of 48.5% for BOD and 49.0% for COD was similar to
demand l
Dissolved oxygen DO mg/ 0.2–4.58 >2
previous results [44–46]. One investigation Li, M. et al. [44] examining
l a pilot-scale CW planted with Salicornia bigelovii and treating artificial
Phosphate PO4-P mg/ 13.3–45.6 30 mariculture wastewater under different hydraulic operating regimes
l using haydites as a main filter media. This study achieved removal rates
Ammonia NH3-N mg/ 0.32–80.69 15
between 33.9% and 44.6%. Another study Wang, Q. et al. [45] focused
l
Total nitrogen TN mg/ 12.55–117 65 on using intertidal wetland sediment (IWS) as an innovative source for
l treating saline wastewater in CWs, both with and without Phragmites
Aluminum Al mg/ 0.0979–3.79 5 australis plants, using gravel and sand as the primary filter media,
l
achieving removal rates from 51% to 80%. Additionally, another
Arsenic As mg/ 0.00036–0.0143 0.1
l
Barium Ba mg/ 0.01116–0.40656 2
l
Boron B mg/ 0.2028–2.15 2
l
Cadmium Cd mg/ 0.0003–0.1496 0.01
l
Chromium Cr mg/ 0.01171–0.86614 0.15
l
Nickel Ni mg/ 0.00137–0.04472 0.2
l
Mercury Hg mg/ 0.00175–0.55367 0.001
l
Cobalt Co mg/ 0.00015–0.00972 0.2
l
Iron Fe mg/ 0.1441–10.55 5
l
Antimony Sb mg/ 0.00012–0.01068 1
l
Copper Cu mg/ 0.0141–0.81884 0.2
l
Manganese Mn mg/ 0.0108–0.67086 0.2
l
Zinc Zn mg/ 0.03882–1.12 2
l
Lead Pb mg/ 0.00111–0.01904 0.5
l
Fig. 2. a) COD and b) BOD vs time during the start-up phase.

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M. Khajah and Mohd.E. Ahmed Journal of Engineering Research xxx (xxxx) xxx

Table 2 Table 3
Parameters and removal efficiencies during the low HLR phase. Parameters and removal efficiencies during the high HLR phase.
No. Parameters In Out Removal % No. Parameters In Out Removal %

1 Temp. (C) 24.58 24.47 - 1 Temp. (C) 22.95 23.01 -

2 pH 7.50 7.0 - 2 pH 6.89 6.90 -
3 Ec (µS/cm) 1061.98 1083.73 - 3 Ec (µS/cm) 909.69 966.31 -
4 DO (mg/l) 0.682 3.74 - 4 DO (mg/l) 1.94 3.72 -
5 Eh (mV) -217.8 52.4 - 5 Eh (mV) -151.54 71.08 -
6 BOD (mg/l) 101.67 52.4 48.5 6 BOD (mg/l) 80.92 33.08 59.1
7 COD (mg/l) 168.2 85.8 49.0 7 COD (mg/l) 131.62 53.85 59.1
8 TPO4 (mg/l) 30.61 17.24 43.7 8 TPO4 (mg/l) 18.93 14.25 24.7
9 NH3 (mg/l) 49.15 15.90 67.7 9 NH3 (mg/l) 27.99 6.56 76.6
10 TN (mg/l) 62.83 33.90 46.1 10 TN (mg/l) 48.87 21.98 55.0
11 Al (mg/l) 0.9686 0.2652 72.6 11 Al (mg/l) 0.7831 0.2508 68.0
12 AS (mg/l) 0.0071 0.0058 18.3 12 As (mg/l) 0.0051 0.0080 -57.3
13 Ba (mg/l) 0.1464 0.0965 34.1 13 Ba (mg/l) 0.1499 0.0864 42.3
14 B (mg/l) 1.562 1.652 -5.8 14 B (mg/l) 1.208 1.614 -33.6
15 Cd (mg/l) 0.0050 0.0060 -19.5 15 Cd (mg/l) 0.0265 0.0047 82.4
16 Cr (mg/l) 0.1642 0.0102 93.8 16 Cr (mg/l) 0.1957 0.0144 92.6
17 Ni (mg/l) 0.0126 0.0296 -134.3 17 Ni (mg/l) 0.0104 0.0237 -128.8
18 Hg (mg/l) 0.0823 0.0141 82.9 18 Hg (mg/l) 0.0455 0.0090 80.3
19 Co (mg/l) 0.0025 0.0024 5.8 19 Co (mg/l) 0.0024 0.0010 57.5
20 Fe (mg/l) 2.955 0.4971 83.2 20 Fe (mg/l) 2.310 0.5947 74.3
21 Sb (mg/l) 0.0078 0.0056 28.4 21 Sb (mg/l) 0.0004 0.0058 -1545.7
22 Cu (mg/l) 0.2546 0.0227 91.1 22 Cu (mg/l) 0.1433 0.0543 62.1
23 Mn (mg/l) 0.1585 0.1126 29.0 23 Mn (mg/l) 0.1762 0.1026 41.8
24 Zn (mg/l) 0.2884 0.0831 71.2 24 Zn (mg/l) 0.1863 0.1130 39.3
25 Pb (mg/l) 0.0069 0.0078 -12.9 25 Pb (mg/l) 0.0070 0.0087 -25.2

investigation Saeed, T. et al. [46] aimed to enhance the removal per­ The removal rates achieved in this study are superior to the low HLR
formance of organics and nutrients in four hybrid wetland systems, efficiencies and could be attributed to a lower F/M ratio resulting in
which employed landfill leachate as the influent. These wetlands used inferior biological degradation [52]. Additionally, the roles of nutrient
either organic materials like coal and coco peat or construction materials concentration and organics to nutrient ratio (COD/TN is 2.7 and COD/
like concrete blocks, bricks, and sand as fillers, and they were planted TPO4 is 5.5) may have played a role in the process performance [38].
with Phragmites australis or Chrysopogon zizanioides (Vetiver) species.
This study achieved removal rates ranging from 55% to 76% in line with Overall performance of the wetland system
these studies. The low removal rates were attributed to hydraulic
operating regimes (Intermittent or Batch operating regimes), the mi­ It is usually believed that a single-stage CW system cannot effectively
crobial communities abundant, and oxygen transportation and con­ remove organics, nutrients, and heavy metals because of its inability to
sumption rate. In our study, the main reasons for lower values could be provide alternate aerobic and anoxic conditions within the single stage
attributed to the characteristics of the offices wastewater, HRT and ox­ [53]. Consequently, to enhance removal effectiveness, either a recircu­
ygen transfer into the system. However, for some metals, a negative lation strategy or multistage CWs should be used [54].
removal was observed (for example, Cd and Ni in Table 2). This typical As described in the methodology, an independent samples t-test, or
fluctuation in metal concentrations could be attributed to resuspension its non-parametric equivalent Mann-Whitney test, was performed for
or leaching from the soil layer [47,48]. each variable, comparing inflow and outflow concentration levels dur­
The high HLR was run for a period of (4.5 months). Table 3 shows the ing the start-up, low HLR, and high HLR operational modes.
average concentrations of the measured parameters and their average The results of these tests indicate, with p exact (<0.05), that some
removal efficiencies during the high HLR phase. During this phase, a variables (for example, As, B, Cd, and Co) are not effectively removed in
removal efficiency of 59.1% and 59.1% for BOD and COD was reached, the wetland system. However, during both low and high HLR, the
similar to previous studies [49–51]. For example, Tan, X. et al. [49] wetland system effectively treated some organics, nutrients, and metals.
investigated the long-term operation of a Tidal Flow Constructed
Wetland (TFCW) for treating domestic wastewater and achieved Low hydraulic loading rate phase
pollutant removal rates in the range of 37.5–68.8%. This TFCW used
activated alumina as a substrate in parallel with shale ceramsite layer. During this period, the organics (BOD and COD) were the main in­
Another study Ung, T. et al. [50] demonstrated a straightforward dicators for the system’s performance. BOD and COD concentrations and
method to enhance the treatment efficiency and contaminant removal in efficiency variation of the wetland system with time is shown in Figs. 3
the CW process for domestic wastewater. They combined a CW (planted
with Cyperus involucratus) with a flowform cascade system and used
crushed stone as the primary filter media, achieving removal rates be­
tween 60% and 65%. Furthermore, another investigation Mittal, Y. et al.
[51] explored various types of CWs for the treatment of domestic
wastewater. These CWs employed gravel as the primary filter media and
were planted with Canna indica. They were followed by a slow sand filter
(SSF) for wastewater treatment and disinfection, achieving an average
removal efficiency of 67.0 ± 2.4%. The low removal rates in this study
were attributed to high organic content, oxygen transfer into the system,
and HRT. In our study, the main reason for lower values could be
attributed to the characteristics of the offices wastewater, HRT and ox­
ygen transfer into the system limitations. Fig. 3. Removal efficiency for BOD in low HLR phase.

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M. Khajah and Mohd.E. Ahmed Journal of Engineering Research xxx (xxxx) xxx

Fig. 4. Removal efficiency for COD in low HLR phase. Fig. 6. Removal efficiency of NH3 in low HLR phase.

and 4, respectively. The increase in efficiency was steady and almost

identical for BOD and COD, indicating that the office wastewater does
not contain hard-to-degrade organics and is similar to domestic waste­
water in terms of organic load [55]. The average efficiencies achieved
were 48.5% and 49.0% for BOD and COD, respectively, similar to pre­
vious results [44–46]. While these efficiencies seem slightly more than
those achieved during the start-up period, the effluents’ quality was
much better than those for both BOD and COD. For example, the average
effluent for the start-up period was 76.9 and 120 mg/l for BOD and COD,
respectively. During the low loading rate, the effluent concentrations
were 52.4 and 85.8 mg/l for BOD and COD, respectively. During the low
Fig. 7. Removal efficiency of TN in low HLR phase.
loading rate stage, the COD was below thresholds (Table 1), indicating
that possible improvements in effluent quality could be observed during
the next high loading rate stage. Kuwait reuse standards, which ranged from 2.2 to 29.3 mg/l, with an
In addition, nutrients (N and P) are the leading indicator for the average removal efficiency of 43.7%, within the range of previous
system’s performance and the plants. Excessive nutrients would findings [62,63]. In a previous study, Ruan, W. et al. [62] investigated
contaminate the water, causing severe problems for human health and and evaluated the purification capacity of 15 CWs that treated rural/­
the environment, which affect the economy: for example, the eutro­ urban domestic sewage and industrial park wastewater in China using
phication phenomenon, deterioration of water quality, contamination of several types of plants. It also assessed the growth, adaptation, and
soils and plants, and transmission of diseases [56]. Eutrophication can contribution of plants to the performance of these CWs, reporting a
be defined as an ecosystem rich in nutrients in the water body, which removal efficiency of 45.2%. Another study Shang, Z. et al. [63] focused
causes a dense growth of plant life (e.g., algae) on its surface. Eutro­ on the use of a novel substrate media called
phication reduces the light reaching the bottom of the water body Lanthanum-ammonia-modified hydrothermal biochar (La-A-HC) to
resulting in a decreased amount of sunlight for infiltration [57]. As a enhance P removal and planted with Phragmites australis. As reported in
result, the death and decomposition of blooming algae affect the DO Fig. 4, in this study, a removal efficiency of 47.8% was achieved. As for
levels by depleting and minimizing the DO in the water. It reduces the the effluents of NH3, five out of fifteen samples were above the Kuwait
DO that fish and other aquatic life need to survive, leading to consid­ reuse standards, which ranged from 26.2 to 41.3 mg/l, whereas the
erable fish sickness and subsequent death [58]. remaining samples were below the Kuwait reuse standards, which
Moreover, anoxic and anaerobic reactions take place in the water ranged from 0.2 to 15.9 mg/l, with an average removal efficiency of
system. In addition, human health is affected by blooming algae due to 67.7% as previously reported [64,65]. Another study by Zhang, Q. et al.
high toxins produced from the algae and bacterial growth, which can [64] investigated the performance of TFCWs in removing ammonium
make people sick if they come into contact with or drink contaminated through substrate optimization, specifically using gravel and a mixed
water and consume tainted fish or shellfish [59]. The concentrations of filler of gravel and zeolite. This study achieved removal rates ranging
TPO4, NH3, and TN and the efficiency variation of the wetland system from 61% to 89.7% without the use of plants. Another study Xu, J. et al.
with time are shown in Figs. 4–6. The increase in efficiency was steady, [65] explored the impact of root exudates from M. aquaticum and the
and a single CW system was expected to have a low-efficiency removal, effects on ammonium removal. This investigation used washed sand as
especially for TPO4, as the CW system was not designed to treat P. The the primary filter media and achieved removal rates between 69.3% and
increased nutrient removal indicates healthy wetland plants, microor­ 90.4%. In addition, all of the TN effluents were below the Kuwait reuse
ganisms, and flora and fauna growth [60,61]. The concentrations of standards, ranging from 16 to 62.6 mg/l, with an average removal ef­
TPO4, NH3, and TN and the efficiency variation of the wetland system ficiency of 46% as reported earlier [66,67]. Xiong, C. et al. [66] exam­
with time are shown in Figs. 5–7. The effluents of TPO4 were below the ined the impact of a combination sequence of traditional hybrid
constructed wetlands (HCWs) on N removal in raw sewage wastewater
and achieved removal rates of approximately 50%. Another investiga­
tion Liu, Y. et al. [67] assessed the effectiveness of CWs in treating
carbon-limiting wastewater and utilized pyrite and steel slag as sub­
strates for N removal. This study reported N removal efficiencies in the
range of 40–54%.
The removal rates of nutrients were attributed to influent charac­
teristics, CW design, operational conditions, substrate media, microbial
activities, the availability of oxygen and carbon source, climates, and
plant uptake. In our study, the main reasons for the nutrients removal
rates could be attributed to the HRT, oxygen transfer into the system for
nitrification process, the availability of carbon source for denitrification
Fig. 5. Removal efficiency of TPO4 in low HLR phase. process, climate, microbial activities, and plants uptake.

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M. Khajah and Mohd.E. Ahmed Journal of Engineering Research xxx (xxxx) xxx

Heavy metals in wastewater, even at low concentrations, could harm

receiving water bodies, the surrounding environment, and public health
[68,69]. CWs can also remove heavy metals through different kinds of
media and macrophytes [70,71]. The results demonstrate an increase in
the heavy metals removal efficiency during the low HLR phase
(13.6–93.4, 80.2–97.8, − 1 to 99.5, 51.2–92.6, 62–97.2, and − 118.1 to
90.9% for Al, Cr, Hg, Fe, Cu, and Zn, respectively). In addition, the
average removal efficiencies for Al, Cr, Hg, Fe, Cu, and Zn were above
70%, conforming to previous findings [72–74]. Kumar, S. et al. [72]
investigated the effectiveness of different macrophytes in adsorbing
heavy metals from domestic wastewater. They used various substrate Fig. 9. Removal efficiency for COD in high HLR phase.
media such as crushed stone, sand, and soil and achieved removal effi­
ciencies ranging from 75.9% to 92.1%. Another research Nguyen, T.
the high loading rate stage, the COD was below thresholds, and the BOD
et al. [73] evaluated the feasibility of using clamshells as a substrate
was slightly above the thresholds (Table 1).
media in CWs planted with cattails for removing heavy metals from acid
The concentrations of TPO4, NH3, and TN and the efficiency variation
mine drainage. In this study, removal efficiencies ranged from 83.8% to
of the wetland system with time are shown in Figs. 10–12. The effluents
99.7%. Furthermore, Faisal, A. et al. [74] focused on the effectiveness of
of TPO4 were below the Kuwait reuse standards, which ranged from 1.9
VFCWs planted with Typha domingensis and Canna indica. They used
to 22.5 mg/l, with an average removal efficiency of 24.7%. This low
sewage sludge as the primary substrate media to remove heavy metals
removal was previously reported [77,78]. On average, the removal ef­
from wastewater containing cadmium ions, achieving removal rates
ficiency was higher at a high hydraulic loading rate compared to a low
exceeding 82%. Except for B and Hg, some of the effluent samples were
hydraulic loading rate. Nevertheless, in certain cases, better perfor­
above the Kuwait reuse standards, and their average removal effi­
mance was observed at the low hydraulic loading rate, attributed to the
ciencies were − 5.8% and 82.9%, respectively. However, all heavy
use of actual wastewater with varying characteristics depending on the
metals were either influents or effluents within Kuwait reuse standards.
various staff activities at the SRP and the treatment efficiency of the
As a result, there was an improvement in the removal efficiencies of
wetland system i.e. type of contaminants, microbial interactions, cli­
heavy metals in this phase compared to the start-up period.
matic factors, etc. [79,80]. Wu, H. et al. [77] evaluated the feasibility
The high removal rates were attributed to sedimentation, filtration,
and effectiveness of achieving simultaneous efficient removal of N and P
precipitation, adsorption, plant rhizofiltration uptake, microbial re­
in CWs planted with Iris pseudacorus and using gravel as the primary
actions, and types of substrate media. In our study, it can be concluded
substrate media. This research reported removal rates ranging from 10%
that the main reasons for high values could be attributed to the plants
to 20% for P. In addition, Krzeminska, D. et al. [78] examined the
uptake and substrate media, whereas the negative removal due to
sediment and P removal efficiency in CW over the long term and through
leaching and resuspension yielded.
seasonal variations, particularly in treating agricultural runoff. In this
As evident from the results of the system under low HLR of 1.04 m3/
case, the study achieved a removal efficiency of 22% for P. As for the
m2, the system’s effluents met KEPA standards for all of the listed pa­
effluents of NH3, two out of thirteen samples were above the Kuwait
rameters (Table 1), except BOD and Hg, which failed to meet the stan­
reuse standards (19.8 and 30.9 mg/l), whereas the remaining samples
dards. The average removal efficiency of the majority of the organic
were below the Kuwait reuse standards, which ranged from 0.3 to
elements was 48.5%. However, the effluents’ Hg concentrations were
10.6 mg/l, with an average removal efficiency of 76.6% similar to pre­
slightly high, although the average removal efficiency was 82.9%. The
vious results [81,82]. In a previous study Liu, C. et al. [81] evaluated the
SRP laboratory’s disposal of industrial wastewater into its septic tank is
application of shale ceramsite overlaid onto active alumina to function
the only reason or source of the Hg [75,76].
as a double-layer substrate in unplanted TFCW for decentralized do­
mestic sewage treatment, specifically focusing on N removal. This
High hydraulic loading rate phase
investigation achieved ammonium removal rates in the range of
72–80%. Another research Luo, P. et al. [82] investigated ammonium
The concentration of organics (BOD and COD) and the efficiency
removal in swine wastewater with varying strengths in CWs planted
variation of the wetland system with time are shown in Figs. 8 and 9. The
with Myriophyllum aquaticum. The substrate used in this study was local
average efficiencies achieved were 59.1% and 59.1% for BOD and COD,
paddy soil containing sand, silt, and clay. Ammonium removal effi­
respectively. These efficiencies were found within the range of previous
ciencies in this research ranged from 40% to 90%. In addition, all of the
results [49–51]. Although these efficiencies appear higher than those
TN effluents were below the Kuwait reuse standards, which ranged from
attained during the low HLR phase, the effluents’ quality was signifi­
3 to 39.7 mg/l, with an average removal efficiency of 55% within the
cantly better for both BOD and COD. For example, the average effluent
range of values reported earlier [83,84]. Additionally, Wang, H. et al.
concentrations for the low HLR were 52.4 and 85.8 mg/l for BOD and
[83] investigated the removal of N and Pb using a novel material, peanut
COD, respectively. During the high loading rate, the effluent concen­
shell biochar, as a substrate in CWs. This research focused on the com­
trations were 33.1 and 53.8 mg/l for BOD and COD, respectively. During
bination of organic and inorganic electron donors to drive

Fig. 8. Removal efficiency for BOD in high HLR phase. Fig. 10. Removal efficiency for TPO4 in high HLR phase.

6
M. Khajah and Mohd.E. Ahmed Journal of Engineering Research xxx (xxxx) xxx

in total), the load and characteristics fluctuated in addition to the

climate. All of these factors have a crucial role in the VFCW system.
Although it is impossible to infer these effects during the whole year due
to changes in loading, the variation can be seen in Figs. 3–12 and Table 1
for every phase. However, a Pearson correlation [87] was conducted to
detect the effects of the wastewater characteristics and ambient tem­
perature on the removal efficiency during start-up, low HLR, and high
HLR phases, separately. The average temperatures for the three phases
were 18.0, 35.5, and 27.2 ◦ C, respectively.
The results of Pearson’s correlation for the start-up phase indicated
Fig. 11. Removal efficiency for NH3 in high HLR phase. that during start-up, the removal efficiency was dependent on almost all
parameters (r = 0.59, 0.80, 0.74, 0.63, 0.53, and >0.50 for ambient
temperature, BOD, COD, TP, TN, and metals, respectively). For the low
HLR phase, dependency was limited to BOD and COD (r = 0.64 and
0.63, respectively). The high HLR was similar to the low HLR but with
some dependency on metals and a weaker dependency on ambient
temperature (r = 0.43). Since the climate and temperatures affect plant
functioning, these findings suggest that ambient temperature is not the
main factor to explain removal efficiency variation and that there is no
explicit dependency on ambient temperature.
The findings suggest that temperature changes explain some varia­
tions during the start-up and high HLR. In contrast, temperature is not
responsible for the variation in removal efficiency during low HLR.
Fig. 12. Removal efficiency for TN in high HLR phase. In summary, organic and metal loads have the most significant effect
on the operation of the VFCW system [88]. The complicated nature of
the wetlands removal processes (such as filtration, biodegradation, plant
denitrification for treating wastewater containing both N and Pb. The
and uptake, among others) may have dampened the effect of seasonal
study reported N removal efficiency of 52.5%. Another investigation
variation in this research [89].
Zheng, X. et al. [84] explored the enhanced removal mechanism of
iron-carbon CWs planted with Phragmites communis and Iris tectorum for
Conclusion
N removal. They used coarse sand, gravel, and iron-carbon as substrate
media and achieved N removal efficiency of 58.3%.
This study has successfully constructed a pilot scale VFCW systems
The results demonstrate an increase in the heavy metals removal
using indigenous plants to treat office wastewater efficiently under the
efficiency during the high HLR phase (− 167.2 to 97.2, 53.7–97.1,
harsh climate conditions of Kuwait. The VFCW system was operated
45.6–99.9, − 49.7 to 97.7, − 386.5 to 95.8, and − 43.4 to 88.6% for Al,
under various operating conditions, and the results showed that removal
Cr, Hg, Fe, Cu, and Zn, respectively). In addition, the average removal
efficiency was better operating under high HLR (1.67 m3/m2.d) than the
efficiencies for Cd, Cr, Hg, and Fe were above 70%, and similar values
low HLR (1.04 m3/m2.d). Evidently, at low HLR, the amount of nutrients
have been reported [68,85,86]. Different types of substrates in recir­
and organics is low, thereby reducing the activity of the biota in
culating standing hybrid constructed wetlands (RSHCWs) planted with
removing wastewater contaminants, which leads to the conclusion that
Canna indica were investigated by Zhang, X. et al. [68]. They used sub­
wetland systems can be operated at higher loading rates. This "design
strates such as gravel, bio-ceramic, or lava rock and exposed them to
conclusion" is essential for the future implementation of the wetland
varying heavy metal inflow loads to assess their potential and the un­
system on a full scale.
derlying mechanisms for removing heavy metals from simulated mixed
The specific conclusions of this study are as follows:
wastewater. The removal efficiencies reported were greater than 85%.
Another research Chang, J. et al. [85] examined the efficiency of Hg
• The average removal efficiencies in the high HLR phase were higher
removal in continuously running biochar-packed CWs planted with
than those in the lower phase.
Lythrum salicaria compared to traditional gravel substrates. In this study,
• For OM (expressed as BOD and COD), the average removal effi­
the removal efficiencies reported were greater than 97%. Furthermore,
ciencies for BOD and COD were 48.5% and 49%, respectively, in the
another investigation Yang, Z. et al. [86] aimed to create a suitable
low hydraulic loading rate phase. In contrast, the average removal
environment for multiple-phase interactions with soluble metals. In this
efficiencies for BOD and COD were 59.1% and 59.1%, respectively,
study, removal efficiencies for heavy metals ranged from 83% to 100%.
in the high hydraulic loading rate phase.
Except for B and Hg, some of the effluent samples were above the Kuwait
• The average removal efficiencies for nutrients (expressed as TPO4,
reuse standards, and their average removal efficiencies were − 33.6%
NH3, and TN) were 43.7%, 67.7%, and 46.1%, respectively, in the
and 80.3%, respectively. However, all heavy metals were either in­
low hydraulic loading rate phase, and were 24.7%, 76.6%, and 55%,
fluents or effluents within Kuwait reuse standards.
respectively, in the high hydraulic loading rate phase. Interestingly,
To sum up, the results of the system under high HLR (1.67 m3/m2.d)
the removal efficiency for N was good with respect to a single unit
indicated that the system effluents met KEPA standards for all of the
treatment (CW); however, there was marginal P removal as this CW
listed parameters (Table 1), except BOD and Hg, which failed to meet the
system was not designed to treat P.
standards due to the same reasons in low HLR phase, with average
• For heavy metals, the average removal efficiencies for Al, Cr, Hg, Fe,
removal efficiencies of 59.1% and 80.3%, respectively. The BOD
Cu, and Zn were above 70% in the low hydraulic loading rate phase,
removal efficiency increased in this phase compared to the previous
whereas the average removal efficiencies for Cd, Cr, Hg, and Fe were
phase, whereas Hg removal efficiency remained above 80%.
above 70% in the high hydraulic loading rate phase. However, some
of the analyzed parameters’ concentrations in the effluents were
Effect of wastewater characteristics on the VFCW performance
greater than those in the influents (indicating leaching), yet they all
remained within KEPA standards.
During the start-up, low HLR, and high HLR phases (one-year period

7
M. Khajah and Mohd.E. Ahmed Journal of Engineering Research xxx (xxxx) xxx

• Ambient temperature affected the removal during the start-up and [17] D. Xu, B. Li, X. Dou, L. Feng, L. Zhang, Y. Liu, Enhanced performance and
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Declaration of Competing Interest vertical flow subsurface constructed wetlands, Bioresour. Technol. Rep. 15 (2021),
100801.
[19] Y. Chu, W. Liu, Q. Tan, L. Yang, J. Chen, L. Ma, Y. Zhang, Z. Wu, F. He, Vertical-
The authors declare the following financial interests/personal re­ flow constructed wetland based on pyrite intensification: Mixotrophic
lationships which may be considered as potential competing interests: denitrification performance and mechanism, Bioresour. Technol. 347 (2022),
126710.
Mishari Khajah reports financial support was provided by Kuwait [20] X. Tan, Y.L. Yang, X. Li, Z.W. Zhou, C.J. Liu, Y.W. Liu, W.C. Yin, X.Y. Fan,
Institute for Scientific Research. Intensified nitrogen removal by heterotrophic nitrification aerobic denitrification
bacteria in two pilot-scale tidal flow constructed wetlands: influence of influent C/
N ratios and tidal strategies, Bioresour. Technol. 302 (2020), 122803.
Acknowledgments [21] X. Tan, Y.L. Yang, Y.W. Liu, X. Li, W.B. Zhu, Quantitative ecology associations
between heterotrophic nitrification-aerobic denitrification, nitrogen-metabolism
The authors would like to thank the Kuwait Institute for Scientific genes, and key bacteria in a tidal flow constructed wetland, Bioresour. Technol.
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Research (KISR) for their financial support (Grant number WT083K). [22] C. Li, L. Feng, J. Lian, X. Yu, C. Fan, Z. Hu, H. Wu, Enhancement of organics and
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online version at doi:10.1016/j.jer.2023.11.027. microbial community, J. Environ. Manag. 278 (2021), 111564.
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