Combined biochar vertical flow and free-water surface constructed wetland system for dormitory sewage treatment and reuse

Journal Pre-proof Combined biochar vertical flow and free-water surface constructed wetland system for dormitory sewage treatment and reuse

X. Cuong Nguyen, T.C. Phuong Tran, V. Hoan Hoang, T. Phuong Nguyen, S. Woong Chang, D. Duc Nguyen, Wenshan Guo, Ashok Kumar, Duong Duc La, Quang-Vu Bach PII:

S0048-9697(19)36400-9

DOI:

https://doi.org/10.1016/j.scitotenv.2019.136404

Reference:

STOTEN 136404

To appear in:

Science of the Total Environment

Received date:

10 October 2019

Revised date:

26 December 2019

Accepted date:

27 December 2019

Please cite this article as: X.C. Nguyen, T.C.P. Tran, V.H. Hoang, et al., Combined biochar vertical flow and free-water surface constructed wetland system for dormitory sewage treatment and reuse, Science of the Total Environment (2020), https://doi.org/10.1016/ j.scitotenv.2019.136404

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© 2020 Published by Elsevier.

Journal Pre-proof Combined biochar vertical flow and free-water surface constructed wetland system for dormitory sewage treatment and reuse

X. Cuong Nguyena, T.C. Phuong Tranb, V. Hoan Hoangb, T. Phuong Nguyenb, S. Woong Changd, D. Duc Nguyenc,d,*, Wenshan Guoe, Ashok Kumard, Duong Duc Lae, Quang-Vu Bachf,*

Center for Advanced Chemistry, Institute of Research and Development, Duy Tan University, 03 Quang Trung, Da Nang, Vietnam

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Faculty of Environmental Engineering Technology, Hue University – Quang Tri Campus, Vietnam d Department of Environmental Energy Engineering, Kyonggi University, Suwon, Republic of Korea b Centre for Technology in Water and Wastewater, School of Civil and Environmental Engineering, University of Technology Sydney, Sydney, NWS 2007, Australia d Department of Biotechnology and Bioinformatics, Jaypee University of Information Technology, Waknaghat, Solan - 173 234 e Institute of Chemistry and Materials, Hanoi, Vietnam f Sustainable Management of Natural Resources and Environment Research Group, Faculty of

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Environment and Labour Safety, Ton Duc Thang University, Ho Chi Minh City, Vietnam

* Corresponding authors: [email protected] (B.Q. Vu) [email protected] (D.D. Nguyen)

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Journal Pre-proof Abstract A two-stage treatment system that included vertical flow (VF) and free-water surface (FWS) constructed wetlands was investigated for the dual purposes of sewage treatment and reuse. The VF included four layers (biochar, sand, gravel, and sandy soil), and the FWS was installed after the VF and used as a polishing tank. Two types of local plants, namely Colocasia esculenta and Canna indica, were planted in the VF and FWS, respectively. The system operated for approximately six months, and the experimental period was categorized into four stages that corresponded to changes

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in the hydraulic loading rate (HLR) (0.02–0.12 m/d). The removal efficiencies for total suspended

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solids (TSS), chemical oxygen demand (COD), biological oxygen demand (BOD5), ammonia

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(NH4-N), and total coliform (Tcol) were 71 ± 11%, 73 ± 13%, 79 ± 11%, 91 ± 3%, and 70 ± 20%,

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respectively. At HLRs of 0.04–0.06 m/d, the COD and BOD5 levels satisfied Vietnam’s irrigation standards, with removable rates of 64% and 88%, respectively, and the TSS and Tcol levels

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satisfied Vietnam’s standards for potable water. Furthermore, the NO3-N levels satisfied the reuse

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limits, whereas the NH4-N levels exceeded the reuse standards. At high HLRs (e.g., 0.12 m/d), all

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the effluent parameters, except Tcol and NO3-N, exceeded the standards.

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Key words: biochar; constructed wetland; dormitory sewage; reuse; sewage treatment system

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Journal Pre-proof 1. Introduction Until recently, the legislation of Vietnam did not officially regulate the illegal discharge of untreated domestic wastewater. Under pressure from citizens due to the polluted water, several large cities in Vietnam constructed municipal wastewater treatment plants. However, the proportion of the total domestic wastewater problem that these facilities have solved is small. In fact, only about 10% of Vietnam’s municipal wastewater is being treated in these centralized plants (WB, 2013).

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In general, the strength of domestic wastewater is relatively moderate, which makes it appropriate

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for combining treatment and reuse, and the opportunity for domestic wastewater reclamation and

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reuse have been realized with regard to water scarcity, unbalanced distribution, and the mitigation

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of pollution (Lyu et al., 2016; Maryam and Büyükgüngör, 2017). The combination of wastewater treatment and reuse in low-cost natural technologies, such as with constructed wetlands (CW)

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(Ávila et al., 2013; Li et al., 2018; Shao et al., 2014; Wu et al., 2016a), rapid infiltration (Wang et

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al., 2010), multi-soil-layering systems (Chen et al., 2009), subsurface infiltration (Sun et al., 2018), and soil aquifer treatments (Sharma and Kennedy, 2017), is ideal for addressing the current water

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issues in Vietnam. Both vertical flow (VF) and free-water surface (FWS) systems, in particular,

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have demonstrated their suitability for wastewater treatment and reclamation (Valipour and Ahn, 2015; Wu et al., 2015). However, the VF and FWS systems are limited by their low removal efficiencies when constructed using normal filter layers or simple designs. In addition, the lack of normal rapid infiltration plants and low filter media height in FWS systems are some additional disadvantages (Kizito et al., 2015; Wang et al., 2010). To overcome the shortcomings of the VF and FWS systems with regard to wastewater treatment, the materials, designs, and operations of these systems have been improved. These improvements include the use of a novel FWS (Chyan et al., 2013; Fang et al., 2018; Vo et al., 2017) with ecology filter-integrated rapid infiltration (Wang et al., 2010), multi-layer artificial wetlands (Kizito et al., 2017; Nguyen et al., 2019), zeolite-containing filter sands (Bruch et al., 2011), integrated biochar and woodchip filters (Baltrėnaitė et al., 2017; Kaetzl et al., 2018), and a combination of multi-soilPage | 3

Journal Pre-proof layering systems and sand filters (Latrach et al., 2016). The combination of CW and biochar has been reported to partly remediate the chemical oxygen demand (COD), contents of nutrients and organic compounds, and turbidity (Gupta et al., 2016; Kaetzl et al., 2018; Kasak et al., 2018; Yan et al., 2017; Zhou et al., 2017). These amendments have increased the level of wastewater purification via CW. However, the integration of these improvements has still not received much attention, and there are many additional aspects that can be exploited. As vital components of CW and soil-based wastewater treatment systems, numerous plant species

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have been utilized such as Cyperus papyrus, Phragmites australis, Scirpus, genera Typha, and

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Canna sp. (Sandoval et al., 2019; Shelef et al., 2013; Vymazal, 2011). Canna sp. is an ornamental

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plant that has demonstrated a high potential for simultaneous organic matter and nutrient removal

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in FWS and hybrid infiltration systems (Nguyen et al., 2017; Sandoval et al., 2019; Zhang et al., 2011). In addition, Rana and Maiti (2018) and Nguyen et al. (2019) first reported using Colocasia

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esculenta in FWS for the treatment of rice noodle and municipal wastewater, and the results

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indicated a sufficient adsorption capacity for organics, total suspended solids (TSS), COD, total

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Kjeldahl nitrogen, Cu, Cd, Cr, Zn, and Pb.

This study is the first to combine VF, using biochar, and FWS for dormitory sewage treatment and

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reuse. The combination of biochar, which is a highly adsorbent material, with other natural and locally available materials to improve the constructed filter system represents a new and promising application for biochar. The main purpose of this study is to assess the potential of a combined VFFWS system, with biochar, for wastewater purification and reuse. The implementation of the second step (i.e., free-flowing FWS) was based on the assumption that the effluent of the first step could be further improved to meet the reuse standards. This study also assesses the removal efficiency of the treatment system under different operating conditions in terms of hydraulic loading rate (HLR). 2. Materials and methods

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Journal Pre-proof 2.1. System set up and description The treatment system included four types of filter media: biochar, sand, gravel, and sandy soil. The sandy soil, which was collected from the bank of a local river, consisted mostly of sand, with only a small portion being soil. The gravel had a mean diameter of approximately 2–3 cm, and the sand had a mean diameter of 2 mm; both were purchased from a local material store. The biochar was produced from the bark of the Acacia auriculiformis plant, which was collected from a local processing site as a type of agriculture waste, by heating the material to 500 °C (10 °C/min) in a

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furnace under anoxic conditions for 2 h. The mean diameter of the biochar was approximately 1–3 cm.

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The VF tank (0.25 m3, with internal dimensions of 1.0 m height  0.5 m width  0.5 m length) and

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FWS tank (0.3 m3, with internal dimensions of 0.6 m height  0.8 m diameter) were filled with 0.1

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m3 and 0.2 m3 of filter media, respectively. The sandy soil was added to the FWS until it reached a height of 20 cm, and the other media were placed in the VF in the following order: gravel to a

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height of 10 cm, biochar to a height of 40 cm (50 cm in total), sand to a height of 20 cm (70 cm in

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total), and sandy soil to a height of 10 cm (80 cm in total).

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Elephant ear (Colocasia esculenta), a local plant, was planted in the sandy soil layer of the VF. Seedlings were taken from a home garden, cut into 25 cm pieces, and planted with 10 cm between cuttings. A total of 16 cuttings were planted in the VF at a density of 64 seedlings per m2. Meanwhile, the ornamental plant Canna indica was planted in the FWS using the same methods. To help the plants acclimate and grow, clean water was supplied to the tanks for 15 d before operating the treatment system.

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Fig. 1. Experimental wastewater treatment system used in the present study: a) diagram and b)

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small pilot scale.

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The diagram and basic dimensions of the experimental system are illustrated in Fig. 1. Wastewater was pumped directly from the internal sewer of the dormitory (located in Hue University, Quang

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Tri province, Vietnam) into the VF using a water pump that was mounted to a timer device, and the

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wastewater flowed through the VF and then the FWS. The wastewater in the VF flowed vertically

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with the effluent point at the bottom, and the effluent point was at the surface level for the FWS. The level of water in the VF fluctuated over time according to the water pump schedule. During pumping, the water level increased to 12 cm above the surface of the filter material; this level steadily decreased until the next pumping cycle. Wastewater was pumped from the sewer into the VF twice a day, from 7:00 to 8:00 am and from 4:00 to 6:00 pm. Meanwhile, the water level of the FWS was stable at 8 cm above the soil level. The operation of the treatment system was divided into four stages. During the first stage (i.e., the start-up state), the wastewater was mixed with water (1:1) to obtain an HLR of 0.02 m/d. The HLR of each successive stage was increased, until it reached 0.12 m/d in the fourth stage, which corresponded to flow rates of 0.015–0.09 m3/d (Table S1). During the operational period, the study

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Journal Pre-proof area was characterized by moderate temperature conditions: 26.0–26.9 °C, 26.9–28.3 °C, 28.3– 26.1 °C, and 26.1–24.1 °C during stages I, II, III, and IV, respectively. Several pollutants such as TSS, organic matter, nitrogen, phosphorus, and pathogens can be removed in VF (Stefanakis et al., 2014) and FWS (Kadlec and Wallace, 2009). In VF, organic matter decompose by both aerobic and anaerobic processes; the aerobic decomposition is dominant due to the abundance of oxygen in the water column. The typical feeding regime of intermittent loadings of wastewater drainage makes high rates of oxygen available for aerobic microbial

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processes in VF, enhancing the removal of BOD5 and COD (Stefanakis et al., 2014; Vymazal,

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2007a) and nitrifying the ammonia (Vymazal, 2005). The aerobic condition in VF also biologically

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converts and removes nitrogen by ammonification and nitrification processes (Guo et al., 2008;

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Saeed and Sun, 2012b; Vymazal, 2007b). FWS is used as the tertiary CW to improve water quality, especially with regard to the denitrification process converting nitrate into nitrogen gas (Ghermandi

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because of the limited oxygen content.

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et al., 2007; Saeed and Sun, 2012b). The removal of organic matters in FWS is not dominant

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2.2. Sampling and analytical methods

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Influent (S1), and VF and FWS effluent (S2 and S3, respectively) samples were collected every 3 d or 4 d (Fig. 1), and all three samples were analyzed for all the parameters, except for total coliform (Tcol), for which only two samples were analyzed (influent and effluent). Totals of 4, 12, 12, and 10 sample sets were collected during stages I, II, III, and IV, respectively. TSS (2540D), COD (5220D), BOD5 (5210B), NO3-N (4500 NO3-B), NH4-N (4500-NH3 F), and Tcol (9221 B) were analyzed using standard methods (APHA/WEF/AWWA, 2012), using an Agilent Cary 60 ultraviolet–visible light (UV–Vis) Spectrophotometer (Agilent Technologies, Inc., USA). Meanwhile, dissolved oxygen (DO), pH, and conductivity (EC) were measured using a multiparameter water quality meter (HQ40D; Hach Company, USA).

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Journal Pre-proof 2.3. Statistical analysis The experiment data were organized and analyzed throughout the study period to generate simple parameters, such as mean and standard deviation, using Microsoft Office Excel 2013 (Microsoft, USA). Meanwhile, statistical analyses were performed using The R Project for Statistical Computing (R Version 3.5.2). The effect of operation stage on the effluent variables was investigated using Tukey’s Honest Significant Difference (HSD) test for normally distributed data and an analysis of variance (Kruskal-Wallis test) for non-normally distributed data at the 95%

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confidence level. The Anderson-Darling test was used to check whether the data were normally

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distributed.

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3. Results and discussion

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3.1. Influent characteristics and reuse standards

The influent samples were collected from the internal sewer of the dormitory. The mean and

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standard deviation values of the influent samples and influent standards are shown in Table 1. All

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the influent parameters, except pH and NO3-N, exceeded the Vietnamese effluent standards

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(QCVN 14-MT:2015/BTNMT) (14:2015/BTNMT, 2015) and those of several other countries (China, Turkey, USA, and Italy) by approximately two times. Discharge limits vary by country and the specific demand. For instance, the effluents used for potable water must meet higher quality standards than those for domestic use or agricultural irrigation. Moreover, Vietnam’s standards tend to be higher than those of some other countries, in terms of TSS, BOD5, NH4-N, and COD. The mean influent parameters were in the range of those reported by previous studies (Awang and Shaaban, 2016; Guo et al., 2014; Koottatep et al., 2018; Öövel et al., 2007). Several studies have also reported high values of these parameters in influent domestic wastewater. For example, a study by Çakir et al. (2015) carried out in Büyükdöllük village about the application of constructed wetlands for treatment of domestic wastewaters determined the influent pollutant values were 243– 456 m/L (mean 325 mg/L) for BOD5, 372–556 mg/L (mean 490 mg/L) for COD, and 113–187

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Journal Pre-proof mg/L (mean 147 mg/L) for TSS. Higher values of these parameters were obtained in municipal wastewater: 102 ± 67 mg/L, 241 ± 120 mg/L, 32.4 ± 7.8 mg/L, and 4.0 ± 1.8 mg/L for BOD5, COD, TN, and TP, respectively, as reported by Vymazal and Kröpfelová (2015b). To meet the discharge standards for reuse (agricultural irrigation), the effluent concentrations of TSS, BOD5, COD, and NH4-N obtained in the present study must be reduced by 61%, 80%, 80%, and 98%,

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respectively.

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Journal Pre-proof Table 1. Influent and reuse standards of Vietnam and other nations Standards for discharge and reuse Influents Parameter

Vietnam (08-MT:2015/BTNMT, 2015; 14:2015/BTNMT, 2015) A B C D

China (Lyu et al., 2016) A1 B

TSS (mg/L)

128.2 ± 19.6

30

50

100

50

-

BOD5 (mg/L)

74.1 ± 11.5

6

15

25

30

10

COD (mg/L)

146.7 ± 31.8

15

30

50

-

100

Coliform (MPN/100 mL)

9515 ± 5105

2500

7500

10,000

3000

NO3-N

1.36 ± 0.33

5

10

15

30

NH4-N

20.2 ± 5.03

0.3

0.9

0.9

pH

6.7 ± 0.2

6–6.8

5.5–9

5.5–9

5–9

DO (mg/L)

0.2 ± 0.5

≥5

≥4

≥2

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1.0

0.5

-

EC (µS/cm)

750 ± 185

-

-

-

-

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Notes:

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Italy (Licciardello et al., 2018) B

30

5–30

10

30

5–30

20

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30

o r p

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r P 5

60

USA* (Bastian and Murray, 2012) A, B1

Turkey (Ayaz et al., 2015) A1 B

40

20

-

-

-

-

100

-

-

-

240

-

-

-

-

5–30

-

1-4

2

6.5–8.4

6-9.5

-

-

-

-

700–3000

-

10

6–9

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A: Water supply (with appropriate treatment process) A1: Domestic use (non-potable water use) B: Agricultural irrigation B1: Agricultural irrigation, environmental and urban use C: Water transportation and other low-quality water uses D: Permitted to discharge into water bodies *: Range or mean

values

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of

US

state

standards

Journal Pre-proof 3.2. DO, EC, and pH variation The parameters evaluated in the present study included the DO, pH, and EC of the system influent and effluent (Table S2). The difference between the pH of the influent and effluent was not significant and varied between 6.04 mg/L and 7.25 mg/L, whereas the mean DO of the influent (0.22 mg/L) rapidly increased to 1.78 mg/L and 6.32 mg/L for the effluents of the VF and FWS, respectively. The DO of the influent was only 0.22 mg/L, which may be related to the

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enclosed dormitory sewer system. Interestingly, Oovel et al. (2007), who studied schoolhouse wastewater, reported mean DO levels of 6.2 ± 3.5 mg/L. However, in agreement with the results

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of the present study, Ávila et al. (2013) reported that the DO level of sewer wastewater was only

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0.8 ± 0.8 mg/L. The mean EC values of the influent wastewater were higher than those in the VF

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and FWS effluents, as previously reported by both Ávila et al. (2013) and Oovel et al. (2007). The reduction in EC and increase in DO levels indicated that the quality of the wastewater was

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3.3.1. Pollutant removal

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3.3. Overall performance

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improved by the treatment tanks.

Differences in the mean wastewater parameters of the system influent and effluents are presented in Figs. 2–4 and Table 2. The mean TSS of the influent was 128.2 ± 19.6 mg/L, which decreased significantly to mean concentrations of 42.3 ± 26 mg/L and 37.4 ± 24 mg/L in the VF and FWS effluents, respectively (Fig. 2b). Therefore, the system removed 71 ± 11% of TSS, which was lower than that reported by Zhai et al. (2011) (>95%) and Abou-Elela et al. (2014) (~93%), who both used a two-stage FWS system. Furthermore, the reduction in TSS by the VF (68 ± 18%) was noticeably greater than that by the FWS (10 ± 17%), likely owing to the absence of filter layers and development of algae in the FWS (Mbow et al., 2014). The water in the VF flowed from top to bottom through the layers in an intentional order, making it easier for the

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Journal Pre-proof TSS to settle. By contrast, the flow in the FWS was scalar and on the surface, which hinders the process of gravity deposition and is easily disturbed, potentially leading to high TSS

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concentrations in the effluent.

Fig. 2. Variation in (a, b, and c) TSS, (d, e, and f) BOD5, and (g, h, and i) COD during the experiment. The mean BOD5 of the influent was 74.1 ± 11.5 mg/L, which was slightly more stable compared to the effluents of the VF and FWS, which yielded mean BOD5 values of 18 ± 8.9 mg/L and 15.1 ± 8.1 mg/L, respectively (Table 2). The BOD5 removal efficiency of the VF (75 ± 12%)

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Journal Pre-proof was significantly greater than that of the FWS (16 ± 13%; Fig. 2f). Similar to the removal of organic matter, the FWS, without filter layers, exhibited a relatively low removal efficiency for BOD5 compared to the VF, which included filter layers that functioned as microbial substrates, supporting growth, biodegradation, flocculation, settlement, and absorption that reduced BOD5 in terms of both particles and soluble organic matter. Together, the entire treatment system removed 79 ± 11% of BOD5. However, this was lower than the results reported by Monte and Albuquerque (2010) (94.8%), who used a horizontal flow FWS, and Saeed et al. (2014) (97%),

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who used a vertical-horizontal flow FWS. Other studies of hybrid FWS systems have also reported high removal efficiencies for BOD5 (Vymazal and Kröpfelová, 2015a).

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The mean COD of the influent was reduced by the treatment tanks to 50.1 ± 18.9 mg/L and 38.2

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± 18.7 mg/L in the VF and FWS effluents, respectively. The removal efficiency of the VF (65 ±

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13%) was greater than that of the FWS (25 ± 14%; Fig. 2i). The lower COD removal performance of the FWS can be attributed to the absence of filter layers, unlike the VF, which

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contains filter layers, because the FWS only functions as a polishing basin in the treatment

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system. The principal COD removal mechanisms in these systems are similar to those of the

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BOD5 removal; this was mainly attributed to uptake by plants, bacterial metabolism and growth, and adsorption in the media. The total removal efficiency (73 ± 13%) was lower than that reported by Zhai et al. (2011) (83%) but higher than that reported by Monte and Albuquerque (2010) and Uggetti et al. (2016) (66–68%). The removal efficiencies were also in line with several previous investigations into FWS systems in which COD removal efficiencies ranged between 50% and 95% (Ayaz et al., 2015). Table 2. The concentration and removal efficiency of water indicators

Parameter Influent

TSS

BOD5

COD

NH4-N

NO3-N

Tcol.

(mg/L)

(mg/L)

(mg/L)

(mg/L)

(mg/L)

(MPN/100 mL)

128.2 ± 19.6 74.1 ± 11.5 146.7 ± 31.8 13.5 ± 3.35 1.36 ± 0.33

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9516 ± 5,106

Journal Pre-proof Effluent Removal (%)

37.4 ± 24.1 15.1 ± 8.1 38.2 ± 18.7 1.64 ± 0.52 3.58 ± 0.76 71 ± 11

79 ± 11

73 ± 13

87 ± 5

2206 ± 883 70 ± 20

The trends of NH4-N and NO3-N concentrations were nearly opposite to each other (Fig. 3). The NH4-N and NO3-N levels in the influent were 13.5 ± 3.35 mg/L and 1.36 ± 0.33 mg/L, respectively, at the beginning of the operation. The NH4-N levels of all the influent samples exceeded the limits for discharge and reuse (Fig. 3a). However, the mean NH4-N levels were

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reduced to 2.9 ± 0.66 mg/L and 1.64 ± 0.52 mg/L in the VF and FWS effluents, respectively. The removal efficiency of the VF (85 ± 6%) was greater than that of the FWS (42 ± 18%) and

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was also greater than the removal efficiencies reported by Gupta et al. (2016) (58.3%) and de

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Rozari et al. (2018) (79%). Furthermore, the amounts of NH4-N removed by the VF and FWS

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were 3.54 ± 2.44 g/m2∙d and 0.13 ± 0.09 g/m2∙d, respectively. The greater removal of NH4-N by the VF indicates that the system includes both nitrification, in

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which ammonia is oxidized to nitrate, and ammonia adsorption, in which cation exchanges take

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place in the detritus, inorganic soils, and biochar (Lehmann et al., 2011; Vymazal, 2007b).

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Biomass assimilation also proceeds through the incorporation of NH4-N in the heterotrophic biomass by plant roots in the soil matrix (Saeed and Sun, 2012a; Tao, 2018) or is adsorbed into clays and humus in the substrates (Lee et al., 2009). Even though this experiment did not include control systems that would allow for the comparison of this VF system with another that did not contain biochar, previous investigations have thoroughly verified the high cation exchange capacity and large surface area of biochar, which promotes the absorption of nitrogen into the biochar (Gupta et al., 2016; Kasak et al., 2018; Kizito et al., 2017).

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Fig. 3. Variation in (a and b) NH4-N and (c and d) NO3-N during the experimental period. The NO3-N content of the influent was increased to 8.62 ± 2.14 mg/L in the VF effluent and was then reduced to 3.58 ± 0.76 mg/L in the FWS effluent. This result could involve nitrification in the VF and both nitrification and denitrification in the FWS. NO3-N is an intermediate step in the nitrogen cycle, and it converts organic nitrogen to N2, which is not individually defined by a removal mechanism. The enhanced nitrification observed in VF was in agreement with the conclusion of Kasak et al. (2018) but contrary to that of de Rozari et al. (2018). Tcol was substantially reduced by the treatment system, i.e., by 70 ± 20% in terms of MPN/100 mL and by 0.63 ± 0.25 in terms of Log10MPN/100 mL (Fig. 4, Table 2). These results are in line with the results from horizontal subsurface flow FWS systems with 1 d of the hydraulic

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Journal Pre-proof retention time (HRT, 72.5%) and lower than those with 3 d of HRT (90.1%) as reported by Mbow et al. (2014). Several studies have reported achieving greater Tcol removal efficiencies, in terms of Log10MPN/100 mL, than those in the present study, with reductions ranging from 0.8 Log10MPN/100 mL to 4.46 Log10MPN/100 mL (Ayaz, 2008; Caselles-Osorio et al., 2011; Richter and Weaver, 2003). The mechanisms underlying Tcol removal are complicated and involve operational parameters, vegetation, filter materials, seasonal fluctuations, pH, HRT,

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oxygen, and water composition (Wu et al., 2016b), although HRT may be the most influential.

Fig. 4. Variation in Tcol influent and effluent during the experimental period, in terms of (a)

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MPN/100 mL and (b) Log unit

3.3.2. Effect of stage on effluent parameters Another aim of the present study was to assess the reduction efficiencies of the treatment system under various HLRs. The effects of HLR on the variation of effluent parameters and removal capacities are summarized in Fig. 5 and Table 3. It is apparent that increases in HLR elevate the TSS, BOD5, COD, and NH4-N levels of both the VF and FWS effluents, except during stage I, which is in agreement with previous research (Chung et al., 2008; Cui et al., 2006; Trang et al., 2010). All effluent parameters, except NO3-N and Tcol, significantly increased in Stage IV (Fig. 5). In addition, the relatively high COD and BOD5 levels observed during Stage I (Fig. 5) may have been caused by the insufficient development of microorganisms in the filter layers; this

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Journal Pre-proof insufficient development significantly contributes to the degradation of organic matter. Indeed, Cui et al. (2006) reported that there may be some organic matter left in the treatment wetland which may have partly contributed to the high BOD5 and COD levels in the effluents during Stage I. With regard to hydrology, higher HLRs imply shorter HRTs, thereby reducing the contact between bacteria and ammonia, and organic matter. Furthermore, increasing the wastewater flow rate disturbed the inside of the treatment tanks; this made it difficult for the TSS to settle, thereby increasing the TSS levels of the effluents.

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As shown in Figs. 5e-f, the effluent levels of NO3-N and Tcol may not be proportional to the increasing HLRs. This is because NO3-N mediates the process of nitrogen removal. Moreover,

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the change in Tcol is considered to be abnormal. The reduction in NO3-N removal by increasing

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the HLR has been reported previously (Dong et al., 2011) in a study that was conducted during

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the flood season.

Fig. 5. Variation in effluent parameters during different stages of operation. Table 3. Reduction (%) in pollutants at different experimental stages of the treatment system Experimental stage Parameter I

II

III

IV

TSS

(mg/L)

78 ± 7

85 ± 2

77 ± 6

46 ± 7

BOD5

(mg/L)

78 ± 2

87 ± 4

84 ± 3

64 ± 9

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Journal Pre-proof COD

(mg/L)

66 ± 7

82 ± 6

81 ± 4

56 ± 7

NH4-N

(mg/L)

78 ± 2

87 ± 4

84 ± 3

64 ± 9

Tcol

(MPN/100 mL)

47 ± 24

73 ± 21

74 ± 17

70 ± 11

0.3 ± 0.26

0.71 ± 0.26

0.64 ± 0.25

0.56 ± 0.23

(Log unit)

The ANOVA and post-hoc tests revealed significant differences between the water parameters of the treatment system effluents at different stages, with H-values of 22.7–30.5 and a P < 0.005 (Table 4). However, the differences were only significant for certain pair comparisons. The HLR

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of Stage IV (0.12) contributed notably to differences in the effluent parameters of the four phases. For example, the Kruskal-Wallis test indicated that there were only significant

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differences in the BOD5 of effluents from Stages II and III and from Stages III and IV. These

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differences demonstrated the lowest removal efficiency at Stage IV of 64 ± 9% and a removal efficiency range of 78–87% in the other stages. The BOD5 removal results are shown in Table 3.

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Similar to other water indicators, statistically significant differences in the effluent parameters of

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the different stages indicate that HLR is an important factor in determining the efficiency of

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pollutant removal.

Parameters

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Table 4. Effect of stage on the water quality parameters of the treatment system effluent Difference among stages (F-test)

Difference between stages (post-hoc test)

H

df

P

True (P < 0.005)

TSS

30.5

3

1.092 × 10-6

I-IV, III-IV

BOD5

22.7

3

4.564 × 10-5

II-IV, III-IV

COD

25.7

3

1.102 × 10-5

II-IV, III-IV

NH4-N

26.7

3

6.684 × 10-6

II-IV, III-IV

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Journal Pre-proof 3.3.3. Correlations among water indicators To identify the relationships among the variables of the system, the correlation coefficient (r) was used (Fig. 6). The values of r range from -1 to 1, thereby denoting the strongest negative and positive relationships, respectively, with 0 representing the absence of a correlation. In this study, the effluent values of BOD5 (BODeff) and ammonia (NH4-Neff) were considered dependent variables (predicted response), whereas the independent variables included the influent parameters (i.e., CODinf, BODinf, HLRs, NH4-Ninf, NO3-Ninf, and TSSinf). Both BODeff

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and NH4-Neff were significantly correlated with HLR (r = 0.82 and r = 0.71, respectively). A correlation was found between BODeff and NH4-Ninf (r = 0.5). Thus, HLR plays an important

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role in determining the organic and ammonia content of treatment system effluents. Previous

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reports have also recognized the importance of HLR in constructed wetlands (Kadlec and

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Knight, 1996; Nguyen et al., 2018). This correlation clearly demonstrates the importance of conducting small-scale pilot studies to verify the efficiency of treatment systems under different

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HLRs.

Fig. 6. Correlation matrix of pollution variables. The output variables, BODeff and NH4-Neff were influenced by input variables, and the r values reflect the magnitude of the correlations between them. The value of the column and row intersection of the two variables is the r.

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Journal Pre-proof 3.4. Reuse potential and future perspectives The long-term experimental results of the system demonstrated excellent performance in pollutant removal, regardless of varying influent concentrations for each stage (Table 2, Figs. 2a, d, g and 3a-c). This suggests that the effluents can fulfill the various discharge standards for reuse; therefore, the reuse potential of the system effluents was evaluated by dividing the operational period into several stages, which corresponded to different ranges of effluent values.

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As shown in Fig. 2a and Table 2, the mean TSS level of the system effluent was 37.4 mg/L,

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which met the Vietnamese standard for agricultural irrigation and the Chinese standard for

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domestic use (non-potable) (Lyu et al., 2016), and the system had a removal rate of 76.3%. Considering that Stage I was a start-up period and that the HLR of Stage IV was unusually high,

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the mean TSS of the system effluent during Stages II and III was 25.5 mg/L, which met both the

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Vietnamese and American standards for water supply quality (Bastian and Murray, 2012) with a removal rate of 75%. This suggests that, with a mean HLR of 0.06 m/d, the treatment system is

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suitable for water reuse in terms of TSS.

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The mean COD of the treatment system effluent (38.2 ± 18.7 mg/L) did not fulfill the

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Vietnamese standard for agricultural irrigation (<30 mg/L; Table 2, Fig. 2g) but did satisfy the Turkish (Ayaz et al., 2015), Chinese (Lyu et al., 2016), and Italian (Licciardello et al., 2018) standards for domestic use (non-portable) with a removal rate of 100%. To achieve a more comprehensive assessment for reuse criteria, the effluents of Stages I–III were considered, and the mean COD levels of the effluents of the treatment system during Stages I and III and during Stages II and III were 28 mg/L and 27 mg/L, respectively. These levels met the Vietnamese standard for irrigation with a removal rate exceeding 64%. Consequently, with a mean HLR of 0.04–0.06 m/d, the treatment system evaluated in this study is appropriate for producing reused water for irrigation, at least in terms of COD levels.

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Journal Pre-proof The mean BOD5 of the treatment system effluent (15.1 ± 8.1 mg/L) exceeded the Vietnamese standards for reuse (<6 mg/L for domestic use and <15 mg/L for agricultural irrigation). The levels satisfied the Chinese (Lyu et al., 2016) and Italian (Licciardello et al., 2018) standards for agricultural irrigation and the Turkish (Ayaz et al., 2015) and American (Bastian and Murray, 2012) standards for domestic use. The mean BOD5 of effluents during Stages I–III and Stages II–III were 11 mg/L and 10.9 mg/L, respectively, which satisfied the Vietnamese standards for irrigation with a removal rate of 88%. This suggests that the VF–FWS system conforms to the

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water reuse standards for agriculture irrigation regarding the BOD5 criteria when operated under a mean HLR of 0.04–0.06 m/d.

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The mean NH4-N of the treatment system effluent (1.36 mg/L) exceeded the high Vietnamese

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standards for reuse (<0.9 mg/L) in place regardless of stage (Fig. 3a–c), but it satisfied the

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Chinese discharge limits for domestic use (non-potable, <10 mg/L) (Lyu et al., 2016), Italian standards for agriculture reuse (<2 mg/L) (Licciardello et al., 2018), and American requirements

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for water supply (1–4 mg/L). For NO3-N, all effluents contained lower levels than what is

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treatment process).

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required by Vietnam and the USA for the reuse of water for water supply (with an appropriate

The mean Tcol of the treatment system effluent (2206 ± 883 MPN/100 mL and 3.34 ± 0.25 Log10MPN/100 mL) satisfied the Vietnamese standard for domestic use (<2500 MPN/100 mL; Table 2) but failed to meet the American standard (240 MPN/100 mL) (Bastian and Murray, 2012). In a nutshell, the levels of TSS, BOD5, COD, and Tcol in the effluent of the treatment system investigated in this study, along with the various standards of Vietnam and other countries, suggest that the VF–FWS system should operate under an HLR of 0.04–0.06 m/d to efficiently process water to be reused for agriculture irrigation. With regard to nutrients, this study demonstrated that a second treatment step (i.e., polishing tank or free-flowing FWS) was

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Journal Pre-proof necessary to reduce NO3-N to acceptable levels, since NO3-N levels normally increase during the first treatment step (i.e., in the VF). Due to the extremely low reuse standards for NH4-N, the treatment system investigated by the present study might not be satisfactory. The results also indicate that the use of natural treatment systems, such as VFs and FWSs, is more practical for agricultural irrigation than it is for water supply. To meet the standards for water supply or domestic use, natural treatment systems may need to be further improved, such as by the addition of a preliminary treatment, an advanced tertiary treatment, recirculation, or a larger

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surface area.

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4. Conclusions

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The present study revealed that a VF–FWS system is effective in removing pollutants from wastewater and that the removal efficiencies of such systems are inversely related to HLR. The

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removal efficiencies for TSS, COD, BOD5, NH4-N, and Tcol were 71%, 73%, 79%, 91%, and

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70%, respectively, and the FWS was less efficient in removing pollutants than the VF. Considering Stage I as the start-up period, the COD and BOD5 levels of the system effluent met

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Vietnamese standards for irrigation, with removable rates of 64% and 88%, while the TSS, NO3-

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N, and Tcol levels of the system effluent met Vietnamese standards for water supply. The NO3N levels of the effluent satisfied Vietnamese reuse standards, whereas the NH4-N levels exceeded these standards. To reuse wastewater for irrigation, the VF–FWS system should operate under an HLR of 0.04–0.06 m/d. Under these conditions, the system will meet the water quality standards of Vietnam and some other countries. Acknowledgments This research is funded by National Foundation for Science and Technology Development (NAFOSTED) under grant number 105.99-2019.25. The research collaboration among the groups, institutions, and universities of the authors are also grateful.

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Journal Pre-proof Declaration of competing interests The authors declare that they have no known competing financial interests or personal

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relationships that could have appeared to influence the work reported in this paper.

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Journal Pre-proof

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Graphical abstract

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Journal Pre-proof Highlights  A sewage treatment system with biochar and a constructed wetland was studied.  The COD and BOD5 levels of the system effluents met the irrigation standards.  All indicators except NH4-N satisfied the Vietnamese water supply standards.  The system most optimally processes water for irrigation at an HLR of 0.04–0.06 m/d.

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 Biochar is a promising medium for soil-based wastewater treatment systems.

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