Accepted Manuscript

Title: Constructed floating wetland for the treatment of

domestic sewage: A real-scale study

Authors: Tatiane Benvenuti, Fernando Hamerski, Alexandre

Giacobbo, Andréa M. Bernardes, Jane Zoppas-Ferreira,
Marco A.S. Rodrigues

PII: S2213-3437(18)30521-9
DOI: https://doi.org/10.1016/j.jece.2018.08.067
Reference: JECE 2615

To appear in:

Received date: 19-7-2018

Revised date: 24-8-2018
Accepted date: 27-8-2018

Please cite this article as: Benvenuti T, Hamerski F, Giacobbo A, Bernardes AM, Zoppas-
Ferreira J, Rodrigues MAS, Constructed floating wetland for the treatment of domestic
sewage: A real-scale study, Journal of Environmental Chemical Engineering (2018),
https://doi.org/10.1016/j.jece.2018.08.067

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Constructed floating wetland for the treatment of domestic sewage: a real-scale

study

TATIANE BENVENUTIa,*, FERNANDO HAMERSKIb, ALEXANDRE GIACOBBOb, ANDRÉA

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M. BERNARDESb, JANE ZOPPAS-FERREIRAb, MARCO A. S. RODRIGUESa

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a
Laboratório Aquário, Universidade Feevale, Rodovia 239, n. 2755 – Vila Nova, Novo

Hamburgo/RS, Brasil. Tel: +555135868800

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b
Programa de Pós-Graduação em Engenharia de Minas, Metalúrgica e de Materiais – PPGE3M,
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Universidade Federal do Rio Grande do Sul – UFRGS. Av. Bento Gonçalves, n. 9500, Porto
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Alegre/RS, Brasil. Tel: +555133089428 - Fax: +555133089427
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*Corresponding author e-mail:benvenuti.tatiane@gmail.com

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Abstract
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A constructed floating wetland using macrophytes Typha domingensis Pers. was applied to the

treatment of raw sewage in a municipal sewage treatment plant in south Brazil. During 12
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months, the average removal of organic matter was evaluated by chemical oxygen demand

(COD), 5-day biochemical oxygen demand (BOD5) and by total suspended solids (TSS) analysis
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and a removal efficiency of 55, 56 and 78 % was, respectively, obtained. For nutrients, total

Kjeldahl nitrogen (TKN) was reduced in 41 % and total phosphorus, in 37 %. The floating mats

supported satisfactorily the macrophytes. This floating arrangement was applied as a single step

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on the sewage treatment. It is an effective alternative in Brazilian sewage treatment systems,

where wetlands are normally used as a polishing step. The evaluation of parameters in the

treatment system may give useful information in order to improve the removal efficiency and

increase the quality of the water bodies.

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Keywords: Constructed wetland; Constructed floating wetland; Floating system; Domestic

sewage treatment; Typha domingensis Pers.; Macrophytes

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1. Introduction

Municipal wastewater treatment is a widespread concern throughout the world. Wastewater

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and sewage threaten human safety and development, impairing the sustainable development of

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the economy and society. Due to the acceleration of industrialization and urbanization, sewage
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discharge is increasing, which bring many sewage treatment plants to operate on the limit of their
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capacity. This problem is intensified in developing countries because the volume of sewage

treated is very low. In Brazil, according to the 2015 annual report “Diagnosis of water and
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sewage services” [1], only 43 % of the sewage was treated in average, with some regions where
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this index strongly drops, as the North region, which treats only 16 % of the generated sewage.
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These low rates are associated to different reasons, as the lack of public agents awareness and the

high costs of sewage treatment.

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The design and operation of the wastewater treatment plant should consider the targeted

pollutants to be eliminated, the volume of sewage and hence the size of the served population,
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the local financial budget and the geographical characteristics [2]. In this scenario, the

development of more economical and environmentally sustainable alternatives of sewage

treatment technologies must be a constant search.

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 In the last decades, there has been a great interest in natural biological systems that can act in

water purification. The ability of natural and constructed wetlands to filter and remove sediments

and pollutants from water is well known [3], bring eco-remediation technologies developed for

engineered wetlands increasingly researched and applied [4].

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Wetland systems reduce or remove contaminants including organic and inorganic matter, trace

organics and pathogens from the water [5]. The treatment processes are performed through

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artificially designed and constructed complexes composed of substrate, plants, microbes, and

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water bodies, as well as the threefold synergies established by the interaction of substrate,

hydrophytes and microbes in the ecosystem [3,4,6].

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Constructed wetlands (CW) are becoming an increasingly popular option among water

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agencies because of their low operation cost, energy consumption and environmental impact [7].
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According to Chernicharo et al. [8], CW is the fifth major technology for municipal wastewater
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treatment in Latin America. In addition, CW provide ancillary benefits, such as the creation of

aesthetically appealing green spaces and wildlife habitats [9].

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In the last few years, Constructed Floating Wetland (CFW), a variant of CW, has gained
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prominence. This technology, also called ‘planted floating system beds’, ‘artificial or vegetated
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floating islands’ or ‘ecological floating beds’ [10] consists of emergent vegetation established

upon a buoyant structure, floating on surface water. The upper parts of the vegetation grow and
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remain primarily above the water level, while the roots extend down in the water column,

developing an extensive under water-level root system. Thus, the vegetation grows
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hydroponically, performing direct nutrient uptake from the water column. Biofilm is attached on

the roots and rhizomes and, as physical and biochemical processes take place, the system

functions as a natural filter [11]. The main biological component of CFW is the macrophytes.

3
They not only assimilate pollutants directly into their tissues, but they also act as catalysts for

purification reactions by increasing the environmental diversity in the roots zone and promoting

a variety of chemical and biochemical reactions which enhance purification. Additionally, many

of them are bioaccumulators [12–14].

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The researches with CFW are mainly directed towards the treatment of stormwater, river

water, and either primary or secondary effluents, with only a few studies targeting raw urban

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sewage [10]. Van de Moortel et al. [15] evaluated a small-scale CFW system, having 1.44 m2

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surface area, treating (in batch) the raw sewage from a municipal treatment plant in Drongen

(Belgium). Weragoda et al [16] also studied a small-scale CFW system treating raw sewage. The

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CFW had 3 m2 surface area and was applied to treat the raw sewage from an University of Sri

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Lanka. However, in both studies [15,16], the raw sewage is characterized as a weak sewage [17],
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presenting 5-day biochemical oxygen demand (BOD5) below 80 mg L-1 and total nitrogen
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around 25 mg L-1. Additionally, the studies regarding CFW are predominantly performed in

small scales (microcosm, mesocosm and pilot scale), being real scale studies valuable
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contributions about this matter.

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In this sense, the present work aims to assess, over one year of monitoring, the efficiency of a
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CFW in real scale treating a strong raw urban sewage (BOD5 around 300 mg L-1), generated by a

600 inhabitants community.

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2. Materials and Methods

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2.1. Establishment of the CFW System

A preexisting tank in a municipal sewage treatment plant (MSTP) in Novo Hamburgo, located

at the metropolitan region of Porto Alegre, in South Brazil, was changed in a constructed floating

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wetland (CFW). The macrophytes T. domingensis Pers. were placed in an artificial flotation

system (Fig. 1), covering the whole surface of the treatment tank, which has a volume of

722.5 m3 (17 m x 17 m x 2.5 m). The CFW is in operation in full scale since 2012. The

monitored period reported in this study started two years after the system implementation. The

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influence of rainwater flow rate over the performance of the treatment system was also evaluated

based on the rainfall index, obtained from the website of the National Institute of Meteorology

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[18], and on the CFW surface area.

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A
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Fig. 1 Floating structure. A – complete floating mat; B – intermediate hitch; C – Plant support; D
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– Installed floating system with T. domingensis Pers.

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The CFW treats the raw sewage generated by 600 inhabitants from a residential condominium.
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The sewage feed flow rate was 67.4 ± 8.0 m3 d-1, and the hydraulic retention time (HRT) was

11.5 ± 1.3 d. The system uses a continuous recirculation pump with a flow rate of
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53.5 ± 8.1 m3 d-1. The raw sewage is distributed along the entire length of the tank with points

every 2 meters and was inserted into the plants roots zone.

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 Fig. 2 shows the schematic operation diagram of the MSTP with the CFW system. It should be

highlighted that the CFW treats the raw sewage that goes only through a grid system as a

preliminary treatment in the receiving tank.

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A
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D
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Fig 2 Schematic operation diagram of the CFW.

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2.2. Sampling and Analysis

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The monitoring data presented in this paper corresponds to 12 months of operation. Samples

were collected every 15 days, maintained, transported and analyzed according to the Standard
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Methods for the Examination of Water and Wastewater [19]. Parameters as chemical oxygen

demand (COD), BOD5, total Kjeldahl nitrogen (TKN), total phosphorus (TP), total suspended

solids (TSS), dissolved oxygen (DO) and pH were monitored. The analyses were conducted, at

least, in duplicate.

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3. Results and Discussion

The CFW was evaluated during the third year of operation, showing a good plants growth and

development (Fig. 3). In this application of CFW as a single step of sewage treatment, the

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behavior of many important monitoring parameters related to organic matter (OM) and nutrients

contamination was evaluated.

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A
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Fig. 3 Constructed floating wetland monitored in this study

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In the sewage treatment by CFW using T. domingensis, physical, chemical and biological

mechanisms may act to remove and degrade contaminants. The water flow carries substances to

the microbial population in the plant roots region, giving conditions to the biochemical reactions.

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In raw sewage, the pH varied between 6.7 and 8.8. After the CFW treatment, pH remained

between 6.6 and 7.4. In terms of system maintenance, it is evident the good buffering effect of

the CFW system, since the pH remained in the neutral range as reported in other studies [20].

Similar to the pH behavior, Fig. 4 indicates a wide variation of the influent characteristics,

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although the sampling was always carried out early in the morning. On the other hand, the values

of effluent samples indicate a constant quality of the treated sewage, regardless of the wide

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influent variation.

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a) 700 In

N Out
BOD5 Concentration (mg L-1)

600
A
500
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400

300
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200
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100
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0
0 50 100 150 200 250 300 350 400

Sample (d)
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A

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 b) 700 In
Out
600
COD Concentration (mg L-1)

500

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400

300

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200

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100

0
0 50 100 150 200 250 300 350 400

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Sample (d)

N
A
In
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700
c) Out
600
TSS Concentration (mg L-1)

500
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400

300
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200

100
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0
0 50 100 150 200 250 300 350 400
Sample (d)
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Fig. 4 Temporal variation of the contaminants concentration (a) BOD5 (b) COD and (c) TSS in

the CFW influent (In) and effluent (●Out) (Dashed line indicates the constant behavior for the

treated effluent).

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 For the parameters related to the organic load in the sewage, the CFW allowed an average

reduction of 55 % for BOD5 and COD, while TSS were reduced around 78 %. This good OM

removal rate could be related to the COD/ BOD5 ratio of sewage that was always higher than 1.5,

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characterizing this wastewater as highly biodegradable [21], being these results in line with the

ones reported in the literature [22,23].

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For the treatment systems with artificially floating plants, the load removed per plant area is an

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interesting way of evaluating yields. For BOD5 and COD, the removed load was determined

considering the average feed flow rate (67.4 m³ d-1) and was linked to the rainfall in the

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metropolitan region during the monitored period, according to Fig. 5.

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The load removal could be evaluated in two distinct periods: the driest (the first 200 days) and
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the rainiest (day 200 to 361). In the driest period, the average organic load removed was
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79.9 ± 11 and 42.2 ± 13 g m-2 d-1, respectively, for COD and BOD5. On the other hand, the

rainiest period showed lower load removal: 41.4 ± 25.1 and 22.5 ± 16.8 g m-2 d-1, respectively,
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for COD and BOD5. The rainfall caused a dilution effect in the influent load, reducing the HRT
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from 11.5 d to 10 d and, consequently, decreasing the organic load removal. However, due to the
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CFW stability, the characteristics of the treated samples showed a linear behavior, as displayed

in Fig. 4.
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10
 100 100
BOD5
BOD5 and COD load removal (g m_² d_¹) COD
Rainfall
75 75

Rainfall (mm)

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50 50

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25 25

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0 0

0 50 100 150 200 250 300 350 400

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Sample (d)

Fig. 5 BOD5 () and COD () load removal and rainfall () during the CFW monitoring
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period (Dashed line separates two periods and behaviors).
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Removal of suspended particulate matter in CFW occurs through a combination of settling - as

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flow velocities are reduced in the pond-like basin - and physical filtering/entrapment of

particulates within the hanging root-biofilm network [24]. The removal mechanism is very
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dependent on the size and nature of solids present in the sewage.

The removal of suspended solids and OM can be related to each other. As reported for the
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treatment in floating wetland systems of palm oil effluent [25], of domestic and municipal

wastewater [26] and of stormwater [24], oxygen demand removal depends on the combination of
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physical and microbial mechanisms. Because of the physical root filtration mechanism,

suspended solids could be filtered and trapped in the plant rhizomes and roots, thereby allowing

better biodegradation of particulate OM. It has been demonstrated that biodegradation takes

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place when dissolved OM contacts the biofilm via diffusion processes. Plants provide the

medium for microbial degradation and carry oxygen to their rhizosphere for aerobic degradation

[27].

For BOD5 and COD the removal rate ranged from negative – indicative of the release of solids

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and OM, in some episodes – to high values as 73 and 80 %, respectively. This release can be

related to some characteristics of the floating wetland treatment [24]: the floating coverage

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excludes light from the water column, minimizing algal growth, which can provides an

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endogenous source of suspended solids in pond systems. Additionally, the wetland treatment

system operates in recirculation, which can resuspend sediments.

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Resuspended sediments directly increase turbidity levels; they also facilitate phosphorus (P)

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release from themselves. The performance of constructed wetlands is not always satisfactory,
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with considerable variations in P removal efficiency, ranging from 70 % net retention to 48 %
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net release of soluble P [28]. In the present study, the monitored nutrients were TKN and total

phosphorus (TP), as indicated in Fig. 6.

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A

12
 30
a) In
Out
25
TP Concentration (mg L-1)

20

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15

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10

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0
0 50 100 150 200 250 300 350 400

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Sample (d)

N
A
200
In
b)
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Out
160
TKN Concentration (mg L-1)

120
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80
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40
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0
0 50 100 150 200 250 300 350 400
Sample (d)
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Fig. 6 Temporal variation of the nutrients concentrations (a) Total Phosphorus as TP and

(b) Nitrogen as TKN (In) and (Out) (Dashed line indicates the constant behavior for the

treated effluent).

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 For nutrients, the treated effluent presented the same constant behavior observed for OM. The

influent concentration of phosphorus ranged from 3 to 14.7 mg L-1. After the CFW system, TP

varied from 3.7 to 7.2 mg L-1, and the efficiency ranged from negative (release) to 63 % removal.

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The average phosphorus removal for the overall monitored period was 37 %.

Nutrients requirement for growth of wetland macrophytes, mainly nitrogen and phosphorus,

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are taken up primarily through their root systems. Growing plants take up nutrients like

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phosphorus, thereby reducing levels in the wetland. In general, OM accumulation is also an

important biogeochemical process for long-term P storage. The most important process

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responsible for phosphorus removal in wetlands is precipitation with soil, calcium, iron and

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aluminum [29,30]. Verhoeven and Meuleman [31] cited also magnesium among the metals that
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react with phosphorus by the adsorption phenomenon. According to the authors, adsorption of
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phosphorus by iron takes place under aerobic and neutral to acidic conditions, while in anaerobic

conditions, the adsorption by calcium and magnesium takes place and is favored by the basic to
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neutral pH conditions. During the monitored period, pH of the influent varied between 6.7 and
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8.8, while the one from the effluent ranged from 6.6 to 7.4, turning this neutral to basic
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characteristic under anaerobic conditions was the likely route for phosphorus absorption.

Sediment accumulation is the major long-term phosphorus sink and microorganisms and
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vegetation are a short-term sink. Phosphate from water is fixed in the matrix of phosphates and

metals. Decomposition of litter (dead plants) and OM in the wetland also takes up phosphorus.
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This process results in storage of phosphorus in the OM which will be released eventually [32].

Thus, the lower efficiency in phosphorus removal, when compared to COD and BOD5, can be

explained, in part, by the low influent concentration, and also, by the low oxygenation of the

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inlet sewage, once Dunne et al. [30] reported that under anaerobic conditions, phosphorus

mobility increases.

DO in the inlet sewage was low, ranging from 0 to 2.8 mg L-1. For effluent samples, DO was

not detected. Once COD and BOD5 represent the amount of oxidisable material in the sewage,

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these parameters also indicate the potential for reducing DO in the receiving sewage. Thus, the

DO of the influent samples was probably consumed for the partial reduction in COD and BOD5.

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Tanner and Headley [33] also detected the DO reduction in floating wetland systems. In the same

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way, Yang et al. [34] monitored a CFW with complete coverage of the water surface receiving a

synthetic agricultural runoff (initial DO < 1.0 mg L−1), and found that the DO concentration

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rapidly declined to zero and did not increase thereafter.

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In this sense, Headley and Tanner [24] stated the floating mat of CFW provides a barrier
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against aeration via diffusion from the atmosphere, wind-driven entrainment and algal
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photosynthesis, while also providing an endogenous source of OM (oxygen demand). Tanner and

Headley [33] additionally suggested that benthic sediments may also affect processes beneath
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and within CFW by exertion of benthic oxygen demand (promoting deoxygenation), and release
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of reactants (e.g. sulphides) and/or organic ligands (e.g. humic acids).

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The DO does not act only in the phosphate adsorption but also in nitrification reactions. Ding

et al. [35] reported that DO limitation is quite common in horizontal subsurface flow constructed
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wetland (SSF-CW) treating high organic or N load wastewaters. It is generally accepted that DO

concentration above 1.5 mg L-1 is essential to the occurrence of nitrification. Considering that
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part of the available DO in the inlet (0 - 2.8 mg L-1) could have been consumed in the OM

oxidation, the deficit of DO for nitrification would be higher.

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 Nitrogen-compounds removal was evaluated as TKN. Similar to total phosphorus, TKN

removal was low, around 41 %, varying from release to 62 % removal. As previously discussed

for phosphorus, in CFW, the suspended roots in the water column can physically remove

nutrients either by their incorporating into the plant tissue through biosynthesis, or by settling

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caused by rhizofiltration. Biosynthesis occurs both for N and P, while settling is the main process

for P removal. Moreover, the removal efficiency depends on the metabolism of the biofilm of

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fungi, bacteria, and beneficial algae that builds up along the suspended roots and the mat. There

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is an ongoing argument whether phyto-uptake or biofilm metabolism is the main contributor for

nutrient removal in CFW system [36].

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The plants uptake nitrogen in the form of ammonium or nitrate, which is then stored in their

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tissues in the organic form. The uptake capacity of emergent plant species, in constructed
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wetlands, can vary from 200 to 2500 kg ha-1 year-1. Factors affecting nutrients uptake by plants is
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their growth rate, concentration of nutrients in the plant tissues and climatic conditions. The

major portion of the nitrogen removal is through bacterial conversion as compared to its uptake
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by plants [37]. While aiding biological degradation or organic pollutants, oxygen in combination
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with carbon drives the nitrification process. The physical process of volatilization also
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contributes to nitrogen removal in wetlands. The matrix of aerobic and anaerobic environments

that develop in the wetlands helps to achieve nitrification and at the same time denitrification
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[27]. In CW, nitrogen removal ranges from 25 to 85 % [38]. In the present study, the absence of

DO in the treated sewage and the OM decomposition (dead plants) in the wetland could also be
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related to the low efficiency in TKN removal.

As can be seen in Table 1, the values of critical parameters like COD, BOD5 and TSS after

CFW treatment comply with the current Brazilian regulation for effluent discard [39]. On the

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other hand, ammonia nitrogen (NH3-N) and total phosphorus exceeded the limit established in

the Brazilian regulation, but compliance with these parameters is not always required by the

environmental agency, being it responsible to decide whether these parameters should be

monitored or not.

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Table 1. Average physicochemical characteristics of the raw sewage and after CFW treatment,

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and maximal allowed values for sewage discharge according to Brazilian regulation (n = 29).

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Removal Brazilian
Parameter Raw sewage After CFW
efficiency (%) Regulation
COD (mg O2 L-1) 487 ± 138 220 ± 68.4 55 330

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BOD5 (mg O2 L-1) 269 ± 76.8 118 ± 25.5 56 120
TSS (mg L-1)
TKN (mg L-1)
168 ± 38.9
105 ± 44.7
N
37.6 ± 16.4
61.9 ± 9.91
78
41
140
n.r.
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NH3-N (mg L-1) 76.2 ± 40.0 47.2 ± 14.2 38 20*
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TP (mg L-1) 8.88 ± 3.83 5.62 ± 0.84 37 4*

n.r.: does not require monitoring
* The requirement to comply with this parameter is at the discretion of the agency responsible for environmental
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licensing.
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Although the treated sewage complies with the regulation, the treatment efficiency must be as

high as possible to aquatic life preservation and human health safety. The treated (or not) sewage
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is commonly discharged in surface water, which is also used as raw water supply to produce
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drinking water, characterizing indirect potable reuse – a common practice worldwide [40,41].
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4. Conclusion

The constructed floating wetland presented in this paper operated as a single step for the

sewage treatment in a municipal treatment plant for three years. The reduction in organic load

was satisfactory. For nutrients, the lowest reduction could be related to the anaerobic condition

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verified in the effluent samples. In order to improve the treatment efficiency, the system could be

adjusted. One of the challenges is to increase the DO and hence the nutrients removal. Future

evaluations may include aeration with air diffusers. The monitored samples indicated a robust

CFW system that represents a sustainable alternative for sewage treatment plants, especially in

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regions where advanced wastewater treatment cannot be applied either from the economic or

infrastructure point of views.

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Acknowledgments

Authors would like to thank the Municipality of Novo Hamburgo-RS, the COMUSA (Water &

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Sewerage Services to Novo Hamburgo) in partnership (Nº. 0004/2013) with the Feevale

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University. Additionally, the financial support from CAPES, CNPq, BNDES and SCIT.
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