Authored by Érika Alves Tavares Marques
Itacuruba has ideal climate to install fish farms with a production
capacity of about 100,000 tonnes per year. Located in the semiarid
region
where rainfall regime is extremely irregular, it is necessary to adopt
strategies to improve the water use efficiency. The objective of this
paper is
to discuss about the efficiency of a constructed wetland (CW) on outlet
water quality of a fish farm located in the Brazilian semiarid region.
The
system consists of an excavated tank using two aquatic macrophytes:
Eichhornia crassipes and Egeria densa, which are endemic in the region.
Water
samples were taken from April 2015 to March 2016 for physical and
chemical analysis of parameters during dry and rainy seasons. The
results were
compared with current legislation. During the rainy season, the maximum
removal efficiency for total phosphorus was 78%, while for total
nitrogen
it was 35%. During the dry season, the maximum removal efficiency for
phosphorus was 43% while for total nitrogen it was 53%. These results
reveal that a constructed wetland is a viable alternative to improve the
outlet quality of a fish farm, but it requires a complementary
treatment
system to achieve a satisfactory result.
The Sub-Middle region of the São Francisco River features
exceptional conditions for pisciculture in terms of water resources,
regarding both volume and quality. Attracted by the warm climate
and the 820km² Itaparica Reservoir, the region has become a major
aquaculture production area in Brazil, reaching 36 thousand tonnes
per year [1]. The development of the aquaculture sector has proved
to be an alternative source of income for families, improving their
quality of life. In addition, it contributes an alternative supply to
the regional market, reducing the negative effects of fishing on local
rivers [2]. However, as aquaculture assumes greater significance as
a global food production system, concerns about its environmental
and social impacts have arisen. As in other animal production
sectors, several aquaculture inputs-land, freshwater, feed and
energy-are associated with significant environmental impacts.
At the same time, the availability of these inputs is limited, and
will likely become even more so in the future. In addition, water
pollution, fish diseases, and escapes continue to compromise the
sustainability of the sector [3].
The inappropriate discharge of industrial outlets and
wastewater into the environment are causes of public concern as
well as grounds for legal sanctions according to current Brazilian
legislation [4]. Brazil has experienced recent changes in its National
Aquaculture Legislation, which incorporate principles such as
public participation, shared responsibilities and recognition of
traditional knowledge of fishermen [5].
At Brazilian scale, freshwater fish farming is essentially
regulated through Conama 430 [6] which complements and alters
Resolution 357 [7] from the National Council of Environment
and aims at achieving good chemical and ecological status of the
rivers. Conama Resolution 357/2005 determines the classification
of water bodies and environmental guidelines for its framing,
as well as the conditions and standards of effluents, and makes
other provisions. Conama Resolution 430/2011 provides for
the conditions and standards for the discharge of effluents,
complements and alters Resolution 357, dated March 17, 2005,
from the National Environment Council-CONAMA.
In traditional flow-through aquaculture systems water passes
through the land-based tanks only once before being discharged
back into the aquatic environment. The flow of water through the
culture system supplies oxygen to the fish and carries dissolved
and suspended wastes out of the system [8]. Water is taken from a
river, circulated through the farm and treated before being released
downstream. In this sense, land-based systems can be a mitigation
strategy over in-lake net-cages for aquaculture farms.
According to the report Opportunities and challenges for
aquaculture in developing countries [9], aquaculture is not simply
a matter of producing fish -the practice is part of a complex value
chain that is itself influenced by a range of environmental, societal
and governmental factors that together determine a successful or
failed initiative. Waite et al. [3] emphasized that aquaculture not
only consumes freshwater, but also can cause freshwater pollution.
Discharges from aquaculture can contain excess nutrients from fish
feed and waste, antibiotic drugs, inorganic fertilizers, and a variety
of chemicals (e.g. pesticides, hormones, antifoulants), contributing
significantly to water quality degradation and eutrophication
around the projects. Despite these effects, no treatment-or even
adequate wastewater management practices - occurs in many
regions throughout Brazil, leading to negative impacts on the
environment [10,11].
Brazilian Northeast presents problems of water scarcity and
there are conflicts between multiple uses of water. According to
International Panel on Climate Change [12], the region will suffer
a decrease in water resources due to climate change: it will tend
to become more arid, presenting an increase in the frequency and
the intensity of droughts and consequent reductions in resource
availability of water resources. Access to water of adequate quality
is seen as essential in the production process in semiarid regions
[13]. Changes in climate values do have an impact on hydrological
processes and the water regime [14]. Semiarid regions are more
vulnerable to eutrophication and its negative effects on water
bodies, especially during periods of severe drought, as occurred
during the present study. There are few studies about the use of
constructed wetlands in semiarid regions, especially in Brazilian
semiarid.
In this rapidly growing and maturing industry, adaptive
management will be important to ensure economic, social and
environmental sustainability [3]. Considering the typical water
limitation in the region, it is extremely necessary to adopt ecoefficient
measures in order to minimize the impacts of pisciculture
on receiving water bodies. While the amount of water needed
is great, fish farming also allows for its reuse. This is crucial for
water-scarce areas such as the semiarid region of Brazil, where
aquaculture can be integrated with agriculture by potentially using
the outlet from fish farms to irrigate crops [15]. It is therefore
necessary to develop more efficient wastewater treatment systems
in intensive aquaculture to achieve sustainability. Among these
systems, constructed wetlands are considered a well-established
and low-cost method of wastewater treatment. The use of wetlands
for remediation of polluted soils and waters has been increasing
steadily over the past decades [16]. This technology can be used in
developed and developing countries [17], improving water-quality,
providing flood control, among other benefits [18].
Sezerino et al. [19] reported that wastewater treatment
technology using CW was first implemented in Germany by Kathe
Seidel from the Max Planck Institute in the 1950s for phenol
removal and organic load reduction from dairy wastewaters [20].
In Brazil, the first experiments that used wetlands to improve water
quality and control pollution were carried out in the early 1980s by
Salati-Jr, et al. [21]. The concept of Green Liver Systems (GLS) was
developed by [22], who pointed out the similarities between the
techniques of animal and plant biotransformation [23]. GLS can be
regarded as CWs, which can be used for aquaculture wastewater
treatment. Depending on the type of waste to be removed, the
plant(s) used may vary [24].
The first pilot project of a GLS was built in a water treatment plant
on Chao Hu Lake (Hefei, China) and consists of 6 compartments:
1+2 covered with Lemna minor; 3+4 planted with Ceratophyllum
demersum; and 5+6 planted with Phragmites Australis [23]. The
second GLS pilot project is this current experiment within the
INNOVATE Project (binational research project between German
and Brazilian researchers that had one focus on the entire watershed
of the São Francisco River and another one on a portion of the
watershed -the Itaparica Reservoir aimed at suggesting practices
and pathways towards ecologically and socioeconomically sound
management of land, water and biodiversity. It ran from January
2012 through December 2016). The GLS was used for treating
aquaculture wastewater, according to legislation on water quality.
Wastewater treatment systems using macrophytes are a
feasible alternative for fish farmers to minimize negative impacts on
aquatic ecosystems. Although many studies have demonstrated the
efficiency of aquatic plants for treating aquaculture wastewaters in
Brazil [10,11,25-28] there are relatively few studies investigating
the use of this technique in Brazil’s semiarid Northeast [29,30].
Therefore, more studies are required to evaluate the potential
and feasibility of phytoremediation processes using Eichhornia
crassipes and Egeria densa by GLS in the region. The objective of
this paper is to discuss about the efficiency of a CW on outlet water
quality of a fish farm located in the Brazilian semiarid region.
Location description
This study was carried out at a fish farm located on the
shore of the Itaparica Reservoir in the municipality of Itacuruba
(08° 50’22.50” S and 38° 41’47.38” W), State of Pernambuco,
northeastern Brazil (Figure 1). Itacuruba has 4,369 inhabitants
according to the last census of the Brazilian Geographical and
Statistical Institute [31] (Figure 1).
As stated by Melo (2007) [32], the Itaparica Reservoir is located
between the Brazilian states of Pernambuco and Bahia, in the
physiographic region designated as the Sub-Middle São Francisco
and was built in 1987 for the primary purpose of electricity
generation. The reservoir, however, provides for multiple uses
including public and industrial water supply, irrigation, pisciculture,
livestock, tourism and leisure.
The municipality of Itacuruba covers an area of 430,000km2 and
is in the Pajeú River Basin within the Sub-Middle Sao Francisco and
features a hot semiarid climate with a mean annual temperature
of 26.1oC [31]. According to the classification of Köppen [33], its
climate is classified as BSwh (B -arid climate; S - weather of the
steppes; w-with a wet season in the summer that may not occur; h
-dry and hot). The vegetation is characterized as hyper xerophilic
Caatinga [34].
As a small municipality, Itacuruba’s economy relies on
pisciculture in excavated tanks and net-cages, as well as the use of
drylands for livestock and subsistence agriculture [35]. Belfort [36]
reported that ten years ago, a group of 12 people were producing a
few kilograms of fish in the Itaparica Reservoir. Today, there are 96
fish farmers who together produce 100 tonnes of tilapia per month.
This multiplication occurred because of a simple fact: the producers
grouped together in associations, which now number eight. They
then formed a cooperative and later a Local Productive Arrangement
- a structure that attempts to bring together a production cluster in
a coordinated form: the production of fingerlings and the fattening
of tilapia (primary sector); manufacture of aquaculture structures
and factories for both fish feed and ice for processing (secondary
sector); marketing and services (tertiary sector) [37].
Kubitza [38] noted that the great vocation and market
potential created by the aquaculture sector attracts domestic and
international investors interested in fish farming and several other
segments of the production supply chain, especially fish feed,
medicines, vaccines, genetics and equipment. The fish farm where
the experiment took place operates under an intensive production
system, with six excavated 1,200m3 tanks for juvenile tilapia
(Oreochromis niloticus) production, as well as two excavated
tanks of 1,600m3 and nine excavated tanks of 1,800m3. The tanks
are supplied with 75,000L water under a continuous flow rate
and water renewal is every 10 to 15 days. Excavated tanks of
6,000m3 are dedicated to the production of tambaqui (Colossoma
macropomum Cuvier 1816).
Wastewater treatment was carried out in a horizontal subsuperficial
flow CW (named as GLS) 100m x 25m x 2.0m in size, with
six baffles of 0.4m x 20m (Figure 2). During the rainy season, mean
outflow was 3.03Lmin-1; during the dry season it was 2.05Lmin-
1. The hydraulic retention time (HRT) of wastewater was at least
72hrs, with an operating volume of 1,200m3. The GLS was divided
into six sections, with the first treatment stage (first 30 m after the
inlet) using Eichhornia crassipes (popularly known as aguapé), a
floating plant. The remainder of the tank was planted with Egeria
densa, a submerged rooted species. E. crassipes removes high
concentrations of nutrients and its roots form a dense biomass that
retains solids, thus reducing turbidity; for this reason, it is placed
near the system input (Figure 2).
Macrophyte establishment
The GLS was introduced in August 2013. Due to the initial
turbidity of the water column, the aquatic plants were first
installed after a period of 60 days. For a more efficient treatment
process, young samples of E. crassipes and E. densa were taken
from an oxbow lake due to their higher productivity at this stage of
development. In the first stage was installed E. crassipes, covering
about 60% of the water surface. In the remaining 70m of the
wetland was installed 25 seedlings of E. densa per m². Management
of the macrophytes was done monthly.
Field sampling and analysis
From April 2015 to March 2016, twelve water samples were
collected from five sampling points in the morning (totalizing
60 samples), to monitor physical and chemical water quality
parameters and the efficiency of the GLS system. The dry season
at the time of sampling (n=9) was from April to December 2015
and the rainy season (n=3) was from January to March 2016.
The samples were collected monthly from raw water and outlet
subsurface in triplicate. The raw water sampling point (RW) was
in the Itaparica Reservoir between coordinates S 08°48’21.27” and
W 038°44’26.18”. The wastewater inlet (WWin) of the GLS was
located between the coordinates S 08°48.302’S and W 038°44’29.2”.
The wastewater outlet (WWout) of GLS was located between
coordinates S 08°48.357’and W 038°44’35”.
The air temperature (AT, oC) was measured using a
thermometer; water temperature (WT, oC), pH, electrical
conductivity (EC, μScm-1), salinity (‰) and total dissolved solids
(TDS, mgL-¹) were analyzed in situ using a multiparameter Oakton
probe. The samples were kept in 250mL plastic bottles, maintained
at approximately 4oC and analysed in triplicate. Total phosphorus
(TP, mg L-¹), soluble reactive phosphorus (SRP, mgL-¹), total nitrogen
(TN, mgL-¹), ammoniacal nitrogen (AN, mgL-¹), nitrite (mgL-¹) and
nitrate (mgL-¹), dissolved oxygen (DO, mgL-¹) and turbidity (NTU)
were analysed in the laboratory of the Chemistry Department at the
Federal University of Pernambuco. TP, SRP, nitrite and nitrate were
analysed according to Golterman et al. (1978) [39]; AN was analysed
according to Koroleff [40]; DO was analysed according to Winkler
(1988) [41] and turbidity was analysed using Nephelometric
methodology.
Water samples were collected from below the surface in
clean plastic or glass 500mL bottles previously washed with the
same water to be collected. For the evaluation of some specific
parameters, the samples were maintained at a pH<2. The National
Council for the Environment (CONAMA) sanctioned Resolution
430 [6], which classifies water bodies, establishes limit levels of
wastewater discharge, complements and alters Resolution 357 [7].
This was used to compare the Maximum Permissible Values (MPV)
with the monitored results from the outlet to verify if the treated
outlet water complied with the legislation.
Climatic parameters
The climatic data were obtained from the Brazilian National
Institute of Meteorology (INMET 2016) [42] from its Data
Collection Platform (PCD) A-351 located in the municipality of
Floresta, 40 km from Itacuruba. The mean monthly rainfall (mm)
and air temperature (oC) during wet and dry months were used to
analyze the effects of seasonality (means of the periods and their
standard variations).
Statistical analysis
To calculate the Mean Removal Efficiency (MRE) of nutrients
and solids by the wetland, the following formula was used based on
Kadlec and Wallace (2009) [43]:
MRE Wetland %=[(WWin-WWout)/WWin] x 100
where WWin is the concentration in wastewater before
treatment (inlet) and WWout is the concentration in wastewater
after treatment (outlet).
Calculating the mean of the parameters was done for the dry
season considering the 9-month period (April to December 2015)
and the rainy season considering the 3-month period (January
to March 2016). Statistical analysis was carried out using the
IBM Statistica 23 software. All data were tested for normality of
distribution with the Shapiro-Wilk test. The parametric data were
statistically subjected to ANOVA. When significant differences were
observed among means, the Tukey test HSD (p > 0.05) was applied
using the IBM Statistica 23 software.
Result
Physicochemical parameters
Air temperature varied between 23.5 °C during the dry period
(April to December 2015) and 45.4 °C during the rainy period
(January to March 2016). The rainy season in Itacuruba begins
in January and finishes in April, being this period rainier. Rainfall
occurs in summer, and winters, in most of the area, are dry. The
strong irradiation during rainy season, coupled with the low relative
humidity, conditions a high potential evapotranspiration, whose
annual average is 2,042 mm. Water temperature varied between
22.7 ºC and 31.4 °C in the dry and rainy seasons, respectively,
tolerable for both macrophytes. According to INMET (2016) [42],
the accumulated rainfall during the dry season was 281 mm and
342 mm in the rainy season.
In accordance with CONAMA Resolution 357 [7], Class 2
freshwater in Lake Itaparica can be used for aquaculture and
fishing activity. The pH of the water was in accordance with the
Resolution’s limits (between 6 and 9) in all samples. The electrical
conductivity was 77.70 μS cm-¹, salinity was 43.9 ‰ and TDS were
42.76 mg L-¹. CONAMA Resolution 430 does not establish any limit
for salinity or SRP. Waters with salinity that are equal or superior
to 30 ‰ are considered as Saltwater by Conama Resolution 357. In
the semi-arid region, the reservoirs are subject to high evaporation
rates, becoming salty, reaching, in some cases, concentrations of
salts that prevent its use for human consumption and agriculture.
Raw water TP concentrations varied between 0.02 mgL-¹ to 20
mgL-¹ Wastewater TP concentrations remained high at both points
(Fig. 3) both in the dry period and in the rainy season in relation
to the limit recommended by CONAMA Resolution 357 [7] (≥0.03
mgL-¹ for lentic environments). Semi-arid reservoirs are highly
vulnerable to eutrophication, due to their large catchment to
surface area ratio and high-water residence time, which points their
potential for high retention of nutrients and sediments exported
from the watershed [45]. Additional phosphorus from uneaten fish
feed also contributes to the high concentrations of phosphorus in
the wastewater. In semiarid regions this impact, associated with
elevated temperatures and high evapotranspiration, is even more
intense. The highest concentrations of TP occurred at WWin and
WWout in January 2016 (both presenting 0.47 mgL-¹). SRP varied
between 0.02 mgL-¹ and 0.10 mgL-¹ after the treatment (Figure 3).
Total nitrogen (TN) concentrations presented values above
CONAMA Resolution 430 [6] (≥1.27mgL-¹ for lentic environments)
in all samples, with exception of the sample collected in April/2015
(Figure 4). Raw water concentrations of TN reached 3.25mgL-¹ and
17.51mgL-¹ in the dry and rainy seasons, respectively. The highest
TN concentration occurred during March 2016 (21.18mgL-¹). The wetland promoted efficient removal for TN during both periods,
with exception of the sample collected in April 2015 (-40.0%).
About total nitrogen, the outflow suffered more influence from the
raw water than from the pisciculture. All samples presented low
values for ammoniacal nitrogen (between 3.7mgL-¹for pH≤7.5 and
1.0mgL-¹for pH 8.0 < pH ≤ 8.5), nitrite (≤1 mgL-¹), nitrate (≤10 mgL-
¹) during the studied period and were in accordance with the law)
(Figure 4).
Regarding Dissolved Oxygen (DO), 88% of the samples
presented values below the CONAMA Resolution 430 limit [6]
(≥5.0mgL-¹). For turbidity (NTU) all samples were according to
law (<100mgL-¹). Mean concentration of total phosphorus did not
indicate statistical differences between sampling points, aside
from point RW that corresponds to raw water from the Itaparica
Reservoir (p-value>0.05). Mean concentration of TN showed
statistical differences between all sampling points. The mean values
of the physicochemical parameters monitored in Itaparica Reservoir
(point RW) and the wetland (WWin, WWout), their respective ranges
and the maximum values established by CONAMA Resolution 357
[7] for Class 2 rivers are shown in (Table 1).
Evaluation of the mean removal efficiency (MRE) of the GLS
The wastewater treatment with E. crassipes and E. densa
showed better removal efficiency during the rainy period for total
dissolved solids, total phosphorus (Figure 5), nitrite, nitrate and
turbidity possibly due to the dilution of nutrients and organic
matter by rainfall. During the dry period, the treatment system
showed better removal efficiency for SRP, ammoniacal nitrogen and
total nitrogen (Figure 5&6) (Table 2).
The concentration of total phosphorus in the outlet after the
treatment system was above the maximum permissible value by
CONAMA Resolution 430 (2011) during both periods due to the
high concentration of this nutrient in the sediment and water
column. The results showed that the wetland using E. crassipes and
E. densa promoted better MRE for total phosphorus in the rainy
season (33.36%) while for total nitrogen the best MRE occurred
in the dry season (28.72%). With respect to seasonality, analysis
of variance (One-way ANOVA) showed that there was significant
difference in the results of total phosphorus (p<0.005) between dry
and rainy seasons (Table 3).
Nutrient uptake by macrophytes
CWs have been used worldwide for wastewater treatment [46].
Researchers have demonstrated that CW can remove significant
amounts of suspended solids, organic matter, nitrogen and
phosphorus from wastewater [47]. As water flows through the built
wetland, the roots of the E. crassipes, located near the entrance to
the tank, act as a barrier, slowing the water towards the receiving
body, causing the sediments and pollutants it carries to precipitate.
In this way, these sediments and pollutants can be captured by
the vegetation and, soon after, metabolized. E. crassipes is a plant
with long roots, which can reach 1 m in length. In its metabolic
process, this species can absorb some pollutants from the water
and transform them into fresh matter through photosynthesis
[48]. On the other hand, E. densa has a high specific growth rate
and relatively short doubling time. It occupies large tracts and
accumulates about 50 tons per hectare in more colonized regions
[49].
Parameters results
In this study, water temperature at the outlet of the treatment
system was 0.2 ºC to 4.6 ºC higher than at its inlet. This result
possibly arose because the part of the wetland corresponding to E.
densa (rooted macrophyte) receives radiation directly into the water
column while E. crassipes causes shading and decreases the water
temperature at the entrance to the wetland. Electrical conductivity
was above 100 μScm-¹ in 80% of the samples. In the rainy season
electrical conductivity was higher in comparison to the dry season.
Electrical conductivity is an indirect measurement of the pollutant
concentration in aquatic environments [50]. Several authors have
observed that in eutrophic environments electrical conductivity
is elevated, with values varying from 48uScm-¹ to 240uScm-¹.
Carvalho-Júnior et al. [51] reported that generally elevated values
are related to a high rate of decomposition. According to Rotta &
Queiróz (2003) [52,53], in most cases changes in the physical and
chemical parameters of water quality are caused by activities in
areas adjacent to the reservoirs.
Lower pH values were observed in the inlet samples both
in the dry season (6.65 + 0.18) and the rainy season (7.30 +
0.21) compared to the outlet samples (7.78 + 0.17 and 8.21 +
0.30, respectively). Hussar et al. [53], analysing water samples
from inlets and outlets of aquaculture tanks, reported the same
finding. Similarly, Cavalcante & Sá [54] noted that the water pH
in aquariums without and with the presence of phytoplankton
was 7.32 ± 0.39 and 8.45 ± 0.40, respectively with the difference
being explained by the phytoplankton’s capacity for removing CO2
from water during the photosynthetic process. However, Tundisi &
Tundisi (2008) [55] found that pisciculture tended to reduce pH in
accordance with the increase in organic matter to be decomposed
by heterotrophic organisms, while phytoplankton can increase pH,
as, during photosynthesis, CO2 and HCO3 are removed by primary
producers.
During the dry period all DO samples at the outlet (WWout)
presented low values (<5mgL-¹). Hussar et al. [53] attributed this
to oxygen use by the root systems of macrophytes as well as the
nitrification process within the system. Souza et al. [56] observed
that wetlands that had Spartina alternifolia macrophytes had
higher levels of dissolved oxygen and pH in relation to wetlands
that did not have macrophytes.
According to Casillas-Hernández et al. [57], only 25 to 30%
of nitrogen and phosphorus supplied in food and fertilizer for
aquaculture will be used for the formation of fish biomass, with the
rest being retained in the sediment of the nurseries or eliminated
via the outlet. Batista et al. [58] found similar results in relation to
total phosphorus (TP) at the Orós Dam (Ceará, Brazil) in an area
close to a fish-cleaning community that disposes a great amount of
wastewater.
In the rainy season mean TP varied between 0.05mgL-¹ to
0.11mgL-¹; in the dry season it varied from 0.14mgL-¹ to 0.13mgL-
¹. [24] achieved good removal efficiency for E. crassipes with
regard to nitrogen, particularly for NH3 (63.4%), but not for TP
(-204.0%), whereas biofilm was efficient in capturing TP (23.89%).
There are additional factors that may exacerbate P concentrations
in aquaculture systems as reported by Cooke et al. [59,60]: low
dissolved oxygen concentration, high temperature, shallow water
and large tank surface area relative to depth. All these contribute
to resuspend and re-dissolve P in the tanks leading to a high degree
of internal P loading in addition to the external factors mentioned
previously (agriculture runoff and urban/industrial discharges).
During the rainy season an increase of AN occurred presenting a
MRE of -323.95%. [61] also observed an increase in AN and nitratewhile
still remaining within the values established by CONAMA
Resolution 430 [6] for areas of a reservoir under tilapia farmed
in net-cages systems located in Pariquera-Açú, State of São Paulo,
in the regional pisciculture hub of [62] obtained a MRE between
5.6% and 93.9%. It is emphasized that the raw water from the
Itaparica Reservoir, which supplies the fish nurseries, already has a
total nitrogen concentration above the limit allowed by legislation
(>1.27mgL-¹). Contaminated outlets, when released directly into
river and lake waters, provoke potential risks to public health,
especially when untreated waters are used in food preparation,
personal hygiene or crop irrigation [63,64]. As shown by Biudes
& Camargo [65], the efficiency of wetlands can vary according to
the species of macrophytes used. Deficient performance can also be
related to an increase in nutrient load, as shown by Lin et al. [66].
In fact, there is a negative relationship between the nutrient load of
the inflow and the percentage of removal by the wetland.
Performance of the CW
Most of the performance data available for CW systems are
from temperate regions. While the efficiency of the treatment could
be considerably higher in tropical areas under certain conditions
it may lead to eutrophication [67,68]. A wetland’s efficiency can
also be impaired when carrying capacity of the plants is reached.
Submerged water plants are commonly covered by periphytic
algae, especially with high concentration of nutrients in the water.
These algae remove a large amount of nutrients, which can hamper
the efficiency of the treatment; additionally, if the macrophytes are
not periodically removed, the stored nutrients tend to return to the
water column during plant senescence as reported by Niegel [69].
In a study by Barros et al. [70], regarding the treatment of
domestic sewage with E. crassipes, the authors verified that the system
showed better efficiency in reducing the concentration of
TDS in the aerobic lake mud-obtaining removal levels between 64%
(with E. crassipes) and 25% (without macrophytes). According
to the authors, this reduction was due to the surface area of E.
crassipes roots, which retains and absorbs the materials. In Brazil,
the use of CWs for treating several types of wastewater is on the rise.
Scientific publications that evaluate performance of the treatments
over the long term are rare, which reinforces the need for combined
measures among research-funding institutions, education and
research institutions, public authorities, the private sector and the
communities involved. Though high, water use by fish farming is not
particularly consumptive and allows for water reuse. This is crucial
for water-scarce areas such as the semiarid region of Brazil, where
fish farming could be integrated with agriculture, with outlets from
aquaculture potentially being used to irrigate crops [15].
The constructed wetland evaluated in this article consists of an
excavated tank with two aquatic macrophytes: Eichhornia crassipes
and Egeria densa. From the operational point of view, it can be said
that the treatment system was tested under critical conditions
going through a drought period in 2015. The wetland demonstrated
efficiency in the reduction of the analyzed parameters, except for
total phosphorus and total nitrogen from the fish outlet, setting
most of the analyzed parameters to the standards required by
CONAMA Resolution 430 [7] and reducing the impact of fish
farming on the receiving water body.
The removal efficiency results reveal that during the rainy
season the system presented a better efficiency than in the dry
season. To achieve a satisfactory result, the wetland should be
associated with another complementary treatment system, such
as stabilization lagoons or overflow channels. A limitation of
the technique used is that its efficiency depends on seasonality.
Flooding may not be a high risk in the semiarid region, but if many
fish tanks release effluents at the same time, a similar effect occurs.
On the other hand, in case of severe drought the operator must
decide how much water can still be channeled to the wetland, first
considering the fish, then the wetland. If the drought persists for a
long time, the wetland may have to be discontinued temporarily.
Another limitation is that a CW needs periodic maintenance due
to plant senescence while leads to decrease in water quality. During
the experiment there was quick growth of macrophytes’ biomass
due to the semiarid climate and excess of nutrients. To improve
the removal efficiency of the CW regarding total phosphorus and
total nitrogen, it would be necessary to carry out management by
pruning and regular harvesting bi-weekly-or maybe even weekly -
since in the typical climatic conditions the plants tend to enter the
senescence stage more quickly compared to other regions. Another
related problem is that this macrophyte is a focus of mosquitoes,
among others. It is also important to make use of the biomass,
since it can be part of the nutritional composition of feed for cattle,
fertilize the soil and promote cost reduction, among other uses.
The relevance of the present study is that it was the first CW
installed in a fish farm located in the Brazilian semiarid region. It
can be useful for local fish farmers in the region to improve outlet
quality and it helps the scientific community to understand the
behavior of physico-chemical variables of the wastewater in a CW
with the aquatic macrophytes E. crassipes and E. densa in a semiarid
climate, contributing to sustainable management of aquaculture.
Due to water shortage in semiarid regions, it is necessary to
adopt strategies to improve water use. The use of eco-friendly
technologies must be prioritized to reduce the negative impacts of
aquaculture. The use of constructed wetlands for effluent treatment
is an alternative that if handled correctly provides economic,
social and environmental benefits to assure the sustainability of
aquaculture, an important economic activity, especially in semiarid
regions.
So far, there is no effluent quality monitoring in most fish farms,
and the cheapest and easiest way to dispose of untreated effluent
is simply to return it to the reservoir. Therefore, to ensure water
quality and promote the sustainable use of natural resources it is
fundamental that there is an intensification of the environmental
control of aquaculture activity in the semiarid region by
governmental institutions. For future works it is recommended that
E. crassipes must be replaced by another plant; its rapid growth -
related to the semiarid climate and excess of nutrients-requires
frequent handling which increases maintenance costs due to hand
labor.
Acknowledgments
The authors would like to thank CNPq (Brazilian Ministry of Science, Technology, Innovation and Communication), FACEPE (Foundation for the Support of Science and Technology of the State of Pernambuco, APQ 1248_3.07/15) and the INNOVATE project- Interplay among multiple uses of water reservoirs via innovative coupling of substance cycles in aquatic and terrestrial ecosystemsfinanced by BMBF (German Ministry for Education and Research, 01LL0904C) and MCTI (Brazilian Ministry for Science, Technology and Innovation, 490003/2012-5), with the following institutions: Federal University of Pernambuco, State University of Bahia and Technische Universitat Berlin.
To read more about this article....Open access Journal of Oceanography & Marine Biology
Please follow the URL to access more information about this article
No comments:
Post a Comment