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Dynamic root floating technique: An option to reduce electric power consumption in
aquaponic systems
Laura Silva, David Valdés-Lozano, Edgardo Escalante, Eucario Gasca-Leyva
PII: S0959-6526(18)30392-5
DOI: 10.1016/j.jclepro.2018.02.086
Reference: JCLP 12033
To appear in: Journal of Cleaner Production
Received Date: 23 June 2017
Revised Date: 7 February 2018
Accepted Date: 8 February 2018
Please cite this article as: Silva L, Valdés-Lozano D, Escalante E, Gasca-Leyva E, Dynamic root floating
technique: An option to reduce electric power consumption in aquaponic systems, Journal of Cleaner
Production (2018), doi: 10.1016/j.jclepro.2018.02.086.
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Dynamic root floating technique: an option to reduce electric
power consumption in aquaponic systems
Laura Silva
a
, David Valdés-Lozano
a
, Edgardo Escalante
b
, Eucario Gasca-
Leyva
a, *
a Centro de Investigación y de Estudios Avanzados del IPN-CINVESTAV,
Antigua carretera a Progreso Km
6, C.P., 97310 Mérida, Yucatán, México;
e-mails: lapasile@gmail.com (L.S.);
dvaldes@mda.cinvestav.mx (D.V.L.)
b Centro Regional Universitario de la Península de Yucatán, Universidad Autónoma Chapingo. Ex
Hacienda Temozón Norte, C.P. 97310 Mérida, Yucatán, México
; e-mail: erer512002@yahoo.com.mx
*
Corresponding
author: eucario.gasca@cinvestav.mx
; Tel.: +1-999-942-9400 (ext. 9460)
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1 Introduction
Aquaculture currently provides half of all fish used for human consumption. It has been
proposed as the only means to ensure that the demands of worldwide fish consumption are
met and constitutes an efficient way to increase protein supplies for a growing human
population (Bunting, 2013; FAO, 2016). However, because of the release of effluents
containing organic matter and elements that cause eutrophication such as nitrogen and
phosphorus into water bodies, it is believed that aquaculture could pose a threat to the
environment (Bunting, 2013; Godfray et al., 2010). One of the released compounds is total
ammoniacal nitrogen (TAN), which is composed of ionized and unionized ammonia (NH
4+
-
N, NH
3
-N respectively). In the walls of the aquaculture systems, part of the NH
4+
-N is first
transformed by bacteria to nitrite nitrogen (NO
2-
-N) and, subsequently, to nitrate nitrogen
(NO
3-
-N), through the nitrification process. The NH
3
-N , and NO
2-
-N and compounds have
been reported to be either toxic or impede the growth of tilapia, depending on their
concentration in water (El-Shafai et al., 2004; El-Sherif and El-Feky, 2008; Monsees et al.,
2016; Yildiz et al., 2006). Also, NO
3-
-N, NH
4+
-N together with phosphate (PO
43-
) are
nutrients for plants and algae and the release of these compounds into the environment can
cause water eutrophication (Bunting, 2013; Godfray et al., 2010).
Modern aquaculture needs to adopt a new strategy of using ecologically sustainable
production systems which could come from integrating aquatic and terrestrial food
production (Costa-Pierce, 2002; Godfray et al., 2010). Aquaponics is an integrated
aquaculture system in which the water containing fish waste is used as a nutrient source for
plant production (aquaponic solution) (Rakocy et al., 2004). Thus, aquaponics is an
ecologically sustainable way to produce food.
Agricultural systems, are nowadays significant energy consumers (Nabavi-Pelesaraei et
al., 2017). Energy savings and emission reductions, which can come from reductions in
electric power consumption and technical improvements among other strategies, are
important issues gaining global attention (Wallgren and Mattias, 2009; Xu and
Szmerekovsky, 2017). In agriculture, energy consumption is directly related to the
development of technology and cultivation, with electricity being one of the inputs that
contributes significantly to the energy supplies of the cultivation system in modern
agriculture (Kusek et al., 2016). Thus, studies have recently attempted to identify ways
of minimizing the electricity consumption used in food production systems (Fang et al.,
2017; Surendran et al., 2016). In aquaponics, according to Boxman et al. (2016) ,
Forchino et al. (2017) and Maucieri et al., (2017), electricity is also one of the main
contributors to the impact on the environment, and it is believed that even a small
reduction in electricity could significantly reduce the environmental impact of the
system.
In aquaponics, hydroponic culture refers to plant production (Somerville et al., 2014),
which is separated from the fish culture tanks but is connected to them through water
recirculation pipes. One of the hydroponic systems used in aquaponics is deep water culture
in which the water circulates through channels at a depth of about 0.20 m (Rakocy et al.,
2004). The most widely used method in deep water systems is the root floating technique
(RAFT), which involves a sheet of floating material punctured with several holes and is
placed on top of the water of hydroponic tanks, the plants grow through the holes while
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their roots are totally immersed in the aquaponic solution (Ghaly et al., 2005; Liang and
Chien, 2015; Rakocy et al., 2004).
In deep water systems, plant aeration may be active or passive. Active aeration is achieved
by bubbling air into the nutrient solution of hydroponic tanks through air diffusers, which is
usually the case for aquaponic RAFT systems (Al-Hafedh et al., 2008; Liang and Chien,
2015, 2013; Roosta, 2014). In contrast, passive aeration may be achieved by the dynamic
root floating technique (DRFT). In the DRFT, an air space is left between the sheet that
supports the plants and the nutrient solution. The roots occupying the air space above the
solution are called oxygen roots and their main function is plant oxygenation (Kao et al.,
1991; Kratky, 2010). The DRFT avoids or reduces the use of active aeration in
hydroponic systems, thereby lowering the electric requirements (air pump) and,
therefore, costs.
Nature-based solutions are inspired and supported by nature, which are cost-effective,
and simultaneously provide environmental, social and economic benefits and help build
resilience (European Commission, 2015). The biological principle behind the DRFT is
the formation of root hairs (Kao et al., 1991). Root hairs are tubular extensions of
epidermal cells which originate from cells in the differentiation zone of the root epidermis
(Bibikova and Gilroy, 2002; Gilroy and Jones, 2000). The DRFT in aquaponics systems,
can be considered therefore as a nature-based solution for reducing the electric power
consumption in aquaponics systems which may lead to more environmentally sustainable
food production.
Pak choi (Brassica chinensis) is a plant that belongs to the Brassicaceae family, and grows
well in intertropical regions (Remy and Singh, 2006; Tavares et al., 2015). In bulk, pak choi
has a relatively high price (1.2 dollars/piece of 0.4 kg) in comparison with plants like
lettuce (Latuca sativa, with price ranges of 0.5-1.25 dollars, with the higher price being
achieved when the product is hydroponics and packaging in pet boxes) or coriander
(Coriandrum sativum, 0.25-0.5 dollars/piece), which have similar growth periods but lower
market costs. Tilapia (Oreochromis spp.) is a tropical fish species which, due to its
biological characteristics of rapid growth, has omnivorous feeding habits, a high resistance
to extreme water quality conditions, is the most widespread species of fish in aquaculture of
the world (FAO, 2014) and has been successfully used in aquaponic systems (Castillo-
Castellanos et al., 2016; Medina et al., 2016; Rakocy et al., 2004). Two of the most
important water quality parameters for tilapia culture are dissolved oxygen (D.O.) and
water temperature (El-Sayed, 2006). Together with D.O., conductivity and pH are relevant
solution quality parameters for hydroponic plant culture (Resh, 2001; Sánchez del Castillo
and Escalante Rebolledo, 1998). Therefore, in aquaponics it is important to maintain these
aquaponic solution quality parameters within the correct range for tilapia and pak choi
growth.
Aquaponics research has been directed towards intensive systems, employing high fish
stock densities and highly controlled technical systems technology ( Kloas et al., 2015;
Rakocy et al., 2004; Suhl et al., 2016). However, less intensive or small-scale aquaponic
systems have also been suggested as an option for food production (Love et al., 2015;
Mahfujul et al., 2015; Somerville et al., 2014). Also, it is believed that smallholder
agricultural systems will benefit from the adoption of technologies that support sustainable
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principles, and nature-based solutions have been suggested as sustainable agricultural
solutions for smallholder farms (Peter et al., 2017).
The decision to evaluate the DRFT in a semi-intensive aquaponic system was made since
semi-intensive tilapia culture systems have been installed as a complementary activity for
small-scale rural farmers who receive governmental support in Yucatan, Mexico (Flores-
Nava et al., 2016). On these small-scale farms, this type of aquaponics could be an option
for farm diversification, and a first step towards farm intensification.
To our knowledge, there have not been any studies in aquaponics that compares the food
production in a RAFT system against a DRFT system. In addition, current research has
yet to address whether similar yields can be obtained with less electric power
consumption or to improve understanding of the water quality parameters and nutrient
concentrations during a plant production cycle under these techniques. To date there has
only been one study of energy saving in aquaponics systems (Fang et al., 2017). Hence,
the intention of the present study was to prove that the DRFT allows a reduction in the
electric power consumed in aquaponic systems without compromising plant production,
fish growth and water treatment achieved in RAFT systems by using a nature-based
solution.
2 Materials and Methods
The experiment was carried out in Yucatan, Mexico over a period of 32 days from May to
June. Two aquaponic treatments were evaluated in the production of tilapia and pak choi;
one using the RAFT system with active aeration and the other using the DRFT as a
substitute for active aeration. Fish culture tanks were installed under a roof and hydroponic
culture tanks were inside a shade net house.
2.1 Experimental conditions
Each system consists of three fish tanks, a settler for solid deposition, a reserve tank, a
water pump, an elevated tank for water distribution by gravity and four hydroponic tanks.
The aquaponic systems and their operation have been previously described by Silva et al.
(2015). In the aquaponic systems, water recirculation was constant during the entire trial. In
both treatment groups, a single air diffuser (8 cm height by 3 cm diameter) was placed in
each of the fish tanks, and nine air diffusers were distributed across the bottom of each
hydroponic tank. In the DRFT system, a PVC support was used to keep the polystyrene
sheet above the water level.
For the fish, three tanks (replicates) were used for each treatment. In each tank, 24 fish
were introduced, weighing 91.4 g on average. Fish were fed three times a day with
commercial pellets with 32% crude protein. The food manufacturer indications were
followed and according to the individual fish weight, daily feeding rate was restricted to a
biomass percentage which was decreasing from 4% to 3% throughout the trial. Every two
weeks, all fish were weighed individually and the average weight of the fish of each tank
was calculated and used to adjust the food supply. At the end of the trial, the fish were
weighed individually, the average weight, average feeding conversion rate (FCR), average
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specific growth rate (SGR), total biomass gain (kg) and productivity (kg/m
3
, gained) per
tank were calculated since they are important parameters to measure the culture
performance of a tilapia production system (El-Sayed, 2006).
The pak choi seeds were sown in polystyrene seed trays with vermiculite as the substrate.
In both treatments, the plants were transplanted in the hydroponic system at a density of 20
plants/m
2
15 days after being sown. When transplanted, the plants were placed in the
perforations of the polystyrene sheet with a sponge around the stalk to provide support.
During the first week, there was no difference in the operation of the system of each
treatment group; the polystyrene sheet remained floating on the water and the air diffusers
were switched on. At the beginning of the second week, in the DRFT system, air diffusers
were turned off, the PVC support was introduced in each hydroponic tank and the drain
pipe was substituted for a shorter one to establish a 5cm gap between the polystyrene sheet
and the water column (Figure 1).
At the beginning of the plant culture, in both treatments, 1 ml of micronutrient stock
solution was added to each 10 L of water (Sánchez del Castillo and Escalante Rebolledo,
1998). The micronutrient solution provides the necessary minerals that represents less than
0.1% of plant dry weight (Resh, 2001). The solution was made with agricultural fertilizers
and provided a concentration of 1 mg/l of iron, and 0.5 mg/l of manganese, boron, cooper
and zinc. Chelated iron was added during the experiment to reach a concentration of 1 mg/l,
since iron precipitated due to the high alkalinity levels and high calcium content in the
water (Hernández et al., 2014) which was obtained from an underground well. Since
preliminary studies (unpublished data) showed that this was the only nutrient for which the
plants displayed symptoms of deficiency when micronutrient solution was added, because
high calcium content in the water causes iron to be precipitated, making it unavailable to
plants, and using chelated iron improves the availability of this micronutrient. Iron was
measured every three days using HACH iron color disk kits, model IR-18A and IR-18B.
When the concentration dropped to 0.3 mg/l, iron was added again to reach the 1mg/l
concentration. The iron was added approximately every three days. At the end of the trial
the total fresh weight, total dry weight, edible individual fresh weight, foliar height and
basal diameter, which are parameters used to measure plant quality, of 5 plants in each
hydroponic tank were measured individually for both treatments. To compare plant
moisture, a typical plant quality measurement, dry weight was obtained from plants which
had been dried in an oven at 68°C for 76 hours. Yield (kg/m
2
), a unitary important
parameter used to measure food production systems (FAO, 2007), was calculated using the
average edible fresh weight obtained per square meter in each treatment.
2.2 Water quality and Nutrient compounds
To measure how the DRFT system affects the water quality of fish and plant culture, the
following parameters were measured daily in the hydroponic and fish culture tanks: water
temperature, dissolved oxygen (D.O.) and conductivity, which were measured with a digital
multi-meter YSI 85, and pH, which was measured using a multiparameter 35 Series Oakton
Eutech instrument. It is known that these parameters are important to the survival and
growth of tilapia and plant species (El-Sayed, 206; Resh, 2001). The measurements were
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taken on alternate day times: morning (M) and afternoon (A). The morning measurement
was taken before the fish were fed, between 8:30 - 9:30, and the afternoon measurement
was made between 15:00 – 15:30, before the last feeding of the day. The most relevant
parameter was the D.O. since in the DRFT system, less aeration was provided than in the
RAFT system.
Since TAN (NH
4+
-N, NH
3
-N), NO
3 -
-N, NO
2-
-N and PO
43-
, are compounds produced in fish
culture tanks that are potentially toxic or represent a threat to the environment, but are a
nutrient source to plants (in the case of NH
4+
-N, NO
3 -
-N and PO
43-
) these were measured
in the trial. To record TAN, NO
3 -
-N, NO
2-
-N and PO
43-
concentrations in hydroponic and
fish culture tanks, water samples were collected twice a week from each replicate, with
morning and afternoon samples being taken on alternate days, according to the parameter’s
measurement timing. The concentrations of TAN, NO
3-
-N, NO
2-
-N and PO
43-
were
analyzed and quantified in the Aquaculture Laboratory at CINVESTAV by colorimetry
(APHA, 1992) using a Technitron Analyzer II and processed by the New Analyzer Program
(NAP) software.
2.3 Statistical analyses
INFOSTAT and R were used for the statistical analysis. For all data, normality and
homogeneity of variance was verified. For the fish culture tanks, individual fish weight,
productivity, FCR, SGR, water quality parameters, NO
2-
-N, NO
3-
-N and PO
43-
concentrations in the water were analyzed using the Student´s t-test for comparison
between treatments. Differences between morning and afternoon water quality parameters,
NO
2-
-N, NO
3-
-N and PO
43-
concentrations, were compared using a dependent Student´s t-
test for paired samples.
For the hydroponic tanks, a random complete block design was applied, with 4 blocks (B1,
B2, B3 and B4). Total fresh weight, total dry weight, edible individual fresh weight, foliar
height, basal diameter, water parameters, and nutrient concentration in the hydroponic tanks
of both treatments, obtained at the same time of day, were analyzed with a two-way
ANOVA; however, since there was no block effect, the Student´s t-test was applied to these
variables. The dependent Student´s t-test for paired samples was applied to compare NO
2-
-
N, NO
3-
-N and PO
43-
concentrations in the hydroponic tanks and water parameters between
the morning and afternoon.
2.4 Electric power consumption
Electric power consumption was calculated for both DRFT and RAFT systems since
electricity has been reported to be a major contributor to the impact on the environment in
aquaponic systems (Boxman et al., 2016; Forchino et al., 2017; Maucieri et al., 2017) and,
in this study, the objective use of DRFT was to reduce the electric power consumption. The
electric power consumed by the operation of the experimental aquaponic system was
calculated based on the electricity used by an air pump which fed the 36 total air diffusers
that were used to aerate the hydroponics tanks, a second air pump feeding the 3 total air
diffusers used to aerate the fish tanks and the electricity used to operate the water pump
used to drive water recirculation. The percentage of electric power consumption by each
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appliance was calculated for each treatment group, and the percentage reduction in the
electric power consumption of DRFT compared to RAFT was also calculated.
3 Results and discussion
3.1 Aquaponic food production
Fish culture
A survival of 100% was recorded for the tilapia culture. In each treatment, the initial
density was 2.6 kg/m
3
and the final density was 4.29 ± 0.22 kg/m
3
for the RAFT and 4.31 ±
0.24 kg/m
3
for the DRFT. No significant differences were observed between the variables
used to measure fish culture performance (Table 1). Thus, the lower water aeration in the
DRFT system, compared to the RAFT system, did not affect fish culture performance.
Table 1
Average values of fish culture results in the aquaponic systems ± standard
deviation (n=3).
Variable Treatment
RAFT DRFT
Individual initial weight (g)
81.4 ± 1.6
81.
4
± 0.
5
Individual final weight (g)
134.1 ± 7.0
134.
9
± 7.
6
Productivity (kg/m
3
)
1.69 ± 0.2
2
1.71± 0.23
FCR
2.1 ± 0.
2
2.1 ± 0.
2
SGR
(%/day)
1.56 ± 0.16
1.56 ± 0.16
RAFT= Root floating technique, DRFT= Dynamic root floating technique, FCR= Feeding
conversion rate, SGR= Specific growth rate.
In both treatments, the obtained FCR was higher than previously reported for tilapia with
similar weight and sex characteristics (Al-Hafedh et al., 2008; Rakocy et al., 2004). There
could be several reasons for poor fish culture performance, however, in this case the main
cause was probably the origin of the tilapia. The fish were obtained from a commercial
tilapia farm and the previous culture conditions and age were unknown. At the beginning of
the trial a manual selection was performed to select only males. However, at the end of the
experiment dissections were performed and females were identified with eggs in their
abdominal cavity, but without oviduct. The energy derived from the food consumed by
these females may, therefore, have been directed towards reproductive functions rather than
growth (Mair and Little, 1991).
Plant culture
The plants achieved a survival of 100% in both treatment groups. Plants reached
commercial sizes of 20 cm - 50 cm in foliar height (Ronzio, 2003; Hu, 2005) and an
individual fresh edible weight of 225 g - 260 g (Hu, 2005) (Table 2). The yield obtained
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was 7.43 ± 1.23 kg/m
2
with the RAFT and 7.32 ± 0.76 kg/m
2
with the DRFT, both values
were greater than the 0.5 to 7 kg/m
2
range reported by FAO (2007) for soil culture.
Table 2
Pak choi average growth results ± standard deviation
(n=4)
.
Variable
Treatment
RAFT
DRFT
Total fresh weight (g/plant)
433.9 ± 120.6
406.6 ± 95.
6
Total dry weight (g/plant)
17.8 ± 5.6
18.4 ± 5.2
Moisture %
95.
9
± 0.5
95
.
5
± 0.
8
Edible fresh weight (g/plant)
371.5 ± 111.
8
351.6 ± 82.8
Foliar height (
cm
)
35.5 ± 2.8 ª
32.
3
± 3.7
b
Basal diameter (
mm
)
12.8 ± 1.
8
13.2 ± 1.7
RAFT= Floating root technique, DRFT= Dynamic root floating technique. Different superscript
letters denote statistically significant differences between treatments (p < 0.05).
Based on the results, the DRFT system successfully produced pak choi in aquaponic
arrangements and was equivalent to the production achieved by the RAFT system. The
measured growth variables only showed differences between treatments for foliar height,
with higher values (p<0.05) for the RAFT than for the DRFT (Table 2). This statistical
difference was, probably due to the relatively low variability in the data.
There are three types of plants according to the cellular origin of the root hairs. In type I
plants, all root epidermal cells are capable of producing a hair. In type II plants, root hairs
originate from smaller cells produced by an asymmetric cell division in the meristem, and
Type III plants, which include the Brassicaceae family (at which pak choi belongs), have
alternating rows of atrichoblasts and trichoblasts (Cormack 1937 in Bibikova and Gilroy,
2002). The development of root hairs increases the root’s effective surface area for nutrient
and water uptake (Bibikova and Gilroy, 2002; Peterson and Farquhar, 1996), and in this
case for oxygen uptake. Root hairs grow well in moist air (Grierson et al., 2014; Robbins
and Dinneny, 2015); therefore, when an air space was established over the water column in
the hydroponics tanks of the DRFT system a moist air environment was created, which
promoted the root hair growth from the trhicloblasts rows in the pak choi root (Figure 2).
In a DRFT system, part of the plant oxygen was taken up by the root hairs, whereas in
RAFT there was no root hair formation and the plant oxygen uptake occurred directly
through the aerated aquaponic solution.
The use of the DRFT, or similar systems, for plant oxygenation, has previously been
reported as successful in several hydroponic studies of plants such as amaranth
(Amaranthus tricolor) and cucumber (Cucumis sativus) by Kao et al (1991), lettuce (Latuca
sativa) by Kratky (2010), and pak choi by Kao et al.(1991) and Remy and Singh (2006).
Therefore, these plants could also be used in aquaponic systems where only pak choi has
been reported as a successful plant culture (Silva et al., 2015). Other plants that can be used
in DRFT systems, due to their ability to develop root hairs, have been studied by Clowes
(2000), Kim et al. (2006) and Pemberton (2001) who sought to identify root hair formation
patterns in root meristem of several angiosperm species.
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3.2 Water quality parameters
Fish culture tanks
In the fish culture tanks, all the measured water quality parameters were within the ranges
necessary for healthy tilapia growth (El-Sayed, 2006). During the morning D.O.
concentrations were similar between treatments (p>0.05), while water temperature and
conductivity were higher, and pH was lower (p<0.05) in the DRFT than in the RAFT
system (Table 3).
Although the air supply was lower in the DRFT system than in the RAFT system (since
there was no active aeration in DRFT hydroponic tanks), the D.O. concentration in the fish
culture tank was similar (p>0.05) between treatments. This proves that the implementation
of the DRFT in an aquaponics system does not affect the oxygen concentration in the fish
culture tanks. The differences in water temperature between treatments may be associated
with the location of the fish tanks, since the DRFT tanks were next to a concrete wall which
at night retained some of the heat received during the day. Although water temperature and
D.O. are inversely related, different temperatures did not produce differences in D.O.
concentrations between the RAFT and DRFT system for fish tanks.
In the afternoon, only conductivity was significantly greater (p<0.05) in the DRFT system
compared to the RAFT system, while no significant differences were observed for the
parameters: D.O., temperature and pH. Conductivity is a good indicator of ion availability
for plants (Resh, 2001) thus, in the fish culture tanks, there were a higher number of
available ions in the DRFT system than in the RAFT system. This could have been caused
by a higher ion absorption by plants or bacteria in the RAFT system than in the DRFT
system or by the effect of aeration in the RAFT hydroponic tanks, which caused ion
precipitation (Stumm and Morgan, 1995).
It was found that the parameters differed depending on whether samples were taken in the
morning compared to the afternoon irrespective of the treatment system. For instance, D.O.
was higher in the morning compared to the afternoon (p<0.05), water temperature was
higher in the afternoon compared to the morning (p<0.05), conductivity was similar
Table 3
Average values of D.O., water temperature, conductivity and pH in the fish culture tanks during the
morning (M) and afternoon (A) throughout the experiment, ± standard deviation (n=3).
Time
of day Treatment D.O.
(mg/L)
Water
temperature
(°C)
Conductivity
(dS/m) pH
M
RAFT
6.44 ± 0.11
a,1
28.95 ± 0.01
b,2
0.98 ± 5.7 E
-
04
b
,1
8.80 ± 0.02
a,1
DRFT
6.34 ± 0.04
a,1
29.09 ± 0.01
a,2
1.02 ± 1.15 E
-
03
a
,1
8.74 ± 0.01
b,1
A
RAFT
5.87 ± 0.22
a,2
30.50 ± 0.04
a,1
0.98 ± 0.50E
-
04
b
,1
8.75 ± 0.04
a,2
DRFT
5.91 ± 0.11
a,2
30.51 ± 0.01
a,1
1.01 ± 0.32E
-
03
a
,1
8.71 ± 0.01
a,2
RAFT= Floating root technique, DRFT= Dynamic root floating technique.
Different superscript
letters denote statistically significant differences between treatments (p < 0.05) and different
superscript numbers denote statistically significant differences between the morning and afternoon
for the same treatment (p < 0.05).
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between times of the day (p>0.05) and pH was higher (p<0.05) in the morning than in the
afternoon (Table 3). The lower D.O. level in the afternoon than in the morning could be due
to the influence of the water temperature, since these are inversely related, and the
temperature increased in the afternoon due to solar radiation during the day. Similar
conductivity values in the morning and afternoon indicate that an equilibrium was reached
between ion input (by fish) and uptake (by plants and bacteria) during the day.
Hydroponic culture tanks
Greater D.O. concentrations were recorded during both the morning and afternoon for the
RAFT system compared to the DRFT system (p<0.05) which is due to the active aeration
through air diffusers in the RAFT system. In DRFT systems, oxygen uptake was carried out
directly by the root hairs, which had developed in the moist air space provided, thus pak
choi production was similar in both treatments, even when the D.O. concentration was
lower in DRFT, and was not affected by the suspension of the active aeration in the
hydroponic tanks. The DRFT, as part of its design, uses a physiological and anatomical
characteristic of the pak choi (capacity to form root hairs) to reduce the consumption of
electric power.
Table 4
Average values of pH, D.O.,
water
temperature and conductivity
during t
he morning (M) and
afternoon (A) in the hydroponic culture tanks during the experiment ± standard deviation (n=4).
Time
of day
Treatment
D.O.
(mg/L)
Water
temperature
(°C)
Conductivity
(dS/m)
pH
M
R
A
F
T
6.39 ± 0.07
a,1
29.11 ± 0.07
b,2
0.98 ± 0.44 E
-
03
b,1
8.80 ± 0.02
a,1
DRFT
5.86 ± 0.07
b,1
29.23 ± 0.04
a,2
1.01± 0.67 E
-
03
a,1
8.70 ± 0.01
b,1
A
R
A
F
T
6.41 ± 0.08
a,1
30.76 ± 0.04
b,1
0.97 ± 0.14E
-
03
b,2
8.80 ± 0.02
a,1
DRFT
5.80 ± 0.06
b,1
30.86 ± 0.01
a,1
1.00 ± 0.31E
-
03
a,2
8.72 ± 0.01
b,1
RAFT= Floating root
technique
, DRFT= Dynamic root floating technique.
Different superscript
letters denote statistically significant differences between treatments (p < 0.05) and different
superscript numbers denote statistically significant differences between the morning and afternoon
for the same treatment (p < 0.05).
Water temperature was higher for the DRFT than for the RAFT (p<0.05), this may be
associated with the aeration that occurred in the RAFT which cooled the water.
Conductivity was lower for the RAFT than for the DRFT at both times of day and pH was
higher for the RAFT than for DRFT (p<0.05). Lower conductivity and higher pH values for
the RAFT than for the DRFT may be due to the active aeration in the RAFT system which
allows CO
2
liberation, increasing the pH and the concentration of CO
3
, which leads to
CaCO
3
precipitation and reduces the number of ions in solution and therefore the
conductivity (Stumm and Morgan, 1995). However, for a complete explanation, all the
elements and compounds present in the solution would need to be analyzed and this was
out of the scope of this study.
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There were no differences (p>0.05) between D.O. concentrations recorded during the
morning compared to the afternoon, even when the water temperature was higher in the
afternoon than in the morning (p<0.05) due to solar radiation during the day. The
conductivity was higher in the morning compared to recordings made in the afternoon in
both treatments (p<0.05). An explanation for lower conductivity in the afternoon is that,
despite the input of ions by fish feeding during the day, ion uptake by plants led to a
decrease in ions in the afternoon. No significant differences were observed between pH
values recorded at the different times of day (p>0.05).
In aquaponic systems, lower conductivity and higher pH values than those recommended
for hydroponic systems is a common situation (Liang and Chien, 2013; Medina et al., 2016;
Roosta and Hamidpour, 2011; Wortman, 2015). In the hydroponic tanks, for both
treatments, the aquaponic solution conductivity values (Table 4) were outside of the
recommended range of 1.5 – 4.0 dS/m for hydroponic culture (Resh, 2001; Trejo-Téllez
and Gómez-Merino, 2012). Lower conductivity is due to a constant process of supply (by
fish) and uptake by bacteria and plants, which is in accordance with previous observations
by Endut et al. (2009). The uptake prevents the accumulation of ions in the system, and in
the current study this is also due to the semi-intensive fish culture. In both treatments, the
water pH values during the experiment were close to 8.5, which is outside of the 5.5 – 7
recommended range for pak choi culture (Aminuddi et al., 1993; Tavares et al., 2015;
Wang et al., 2007). Regarding pH, Tyson et al. (2011) indicated that there is a dichotomy
between optimum pH levels needed for hydroponic plant culture (5.5-6.5) and the optimum
necessary for nitrifying bacteria activity (7.5-9.0). Furthermore, in the present study the
culture water was obtained from an underground well and due to the karstic soil in Yucatan,
the pH was high.
Despite the low conductivity and high pH levels, the pak choi culture was successful (FAO,
2007; Hu, 2005; Ronzio, 2003). The reason for the success of the pak choi culture was that
the pH levels of the water during the study limited the availability of several nutrients,
including Fe, but did not inhibit the uptake of Fe by the plant root (Resh, 2001). In the
present study, only Fe was measured and added, in a chelated form, to the aquaponic
solution. As a result of the addition of iron, pak choi did not show any visual symptoms of
nutrient deficiency (Resh, 2001). The application of foliar iron is another strategy that has
been suggested to prevent nutrient deficiency, the availability of which diminishes with pH
values higher than 7 (Roosta and Mohsenian, 2015).
3.3 Nitrogen compounds and phosphates in water
Groundwater was used for aquaponic culture, initial values from measured compounds
were < 0.047 mg/L for TAN, 0.062 mg/L for PO
43-
, 4.701 mg/L for NO
3-
-N and < 0.0014
mg/L for NO
2-
-N.
Fish culture tanks
In the tilapia culture, the NO
2-
-N concentration in the morning was higher in the DRFT than
in the RAFT system (p<0.05) and in the afternoon the concentration was similar in both
treatments (p>0.05) (Figure 3). The results suggest a greater accumulation of NO
2-
-N
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during the night in the DRFT than in the RAFT system, which is compensated during the
day when similar concentrations
of NO
2-
-N were reached. The NO
3-
-N concentration was
similar for the RAFT and DRFT during the morning and afternoon (p>0.05). Meanwhile
PO
43-
was higher in the DRFT than in the RAFT system at both times of the day (p<0.05).
From the third week, TAN values for both the fish and hydroponic culture were lower than
the detection limit (0.04 mg/L) of the autoanalyzer, thus the values have been omitted.
With regards to the comparison of parameters recorded at different times of day in the fish
culture, NO
2-
-N concentration in the RAFT system was similar during the morning and
afternoon (p>0.05). In the DRFT system, the NO
2-
-N concentration was higher in the
morning than in the afternoon (p<0.05) due to the higher temperatures during the afternoon
which increased the nitrification process (Antoniou et al., 1990; Mieczkowski et al., 2016).
However, in the RAFT system, the effect of a higher water temperature in the afternoon
than in the morning had no significant effect on NO
2-
-N concentration. The NO
3-
-N
concentration was higher in the afternoon than in the morning in both treatments (p<0.05),
this was also related to the higher water temperature in the afternoon compared to the water
temperature recorded in the morning. The PO
43-
concentration was similar at both times of
day in the DRFT system (p>0.05) and was lower in the morning than in the afternoon in the
RAFT system (p<0.05).
The chronic NO
2-
-N, NO
3-
-N and NH
3
-N concentrations that reduce tilapia growth are 0.5 –
1.38 mg/L (El-Shafai et al., 2004; El-Sherif and El-Feky, 2008), 500 mg/L (Monsees et al.,
2016) and 0.1 mg/L (Yildiz et al., 2006) respectively. In the current study, none of these
values were reached (Figure 4) because the hydroponic culture worked as a biofilter and
due to the low fish density. It is believed that the chronic concentrations of NH
3
-N were not
reached because TAN values were lower than 0.04 mg/L; therefore, even if 100% of the
TAN had been in the form of NH
3
-N, the concentration would never have been higher than
0.5 mg/L. Since the concentrations of the nitrogen compounds were not sufficient to
impede tilapia growth and the water quality parameters were within the optimum range for
tilapia growth, the hypothesis that fish origin is the cause of poor tilapia culture
performance is reinforced.
Hydroponic culture tanks
In the hydroponic tanks, the pattern of the NO
2-
-N concentration was the same as for the
fish culture tanks, with higher concentrations in the DRFT than in the RAFT system during
the morning (p<0.05) and similar concentrations recorded in the afternoon (p>0.05) (Figure
4). These observations were probably a result of a combination of two factors, firstly owing
to a larger surface area in the RAFT and, secondly, the NH
4+
-N assimilation by plants in
both treatments. Nitrification occurs spontaneously and in a constant way in the aquaculture
system surfaces that are in contact with the water and provides a fixing medium for
bacterial growth (Rakocy et al., 2004) and the RAFT system provides a larger surface area
for bacterial fixation than DRFT, because the polystyrene sheet is placed such that it floats
on the water. Through nitrification, the bacteria, established on the floating bed, initially
transform the TAN into NO
2-
-N and subsequently into NO
3-
-N, which is a continuous
process that also occurs at night since a nitrification does not require a light source. Thus, in
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the RAFT system the number of bacteria and the nitrification rate should be greater than in
the DRFT system, which leads to lower concentrations of NO
2-
-N during the morning. On
the other hand, plants can assimilate NH
4+
-N under high radiation conditions (Resh, 2001),
which especially occur in intertropical areas, such as Yucatan. Probably due to the plant
uptake of NH
4+
-N during the day in both treatments, the effect of the larger surface area of
the RAFT system compared to DRFT on NO
2-
-N concentration was less during the day
than during the night. Therefore, in the afternoon the concentrations were similar between
treatments.
NO
3-
-N concentrations recorded during the morning and afternoon were similar between
treatments (p>0.05) which means that its removal is not affected by the lack of active
aeration or minor surface area in the DRFT when compared to the RAFT. Finally, at both
times of day, the PO
43-
concentration was higher in the DRFT than in the RAFT system
(p<0.05). It is not possible to infer the cause of these differences from the data obtained in
this study; however, there are several possibilities. One reason for these differences could
be that the effect of aeration on the composition of the solution in the RAFT system,
described previously for pH and conductivity, may have had an effect on the form of
phosphorus present in the solution, causing the precipitation or the formation of other
compounds, like hydroxyapatite, besides PO
43-
. Another reason might be that the PO
43-
plant
removal is lower in DRFT than in RAFT. A complete analysis of compounds
containing P or an analysis of total P in the plant would be necessary to determine the
reason for the lower level of PO
43-
in
RAFT
than in DRFT. However, P
mass balance and
phosphorus speciation in the systems were out of the aims of the present investigation.
In the hydroponic tanks, the NO
2-
-N concentration was higher for both treatments in the
morning compared to recordings made in the afternoon (p<0.05), since nitrification
increases during the day. The NO
3-
-N concentration comparison between times of day for
the same treatment shows higher concentrations in the afternoon than in the morning for
both RAFT and DRFT (p<0.05). This was because the nitrogen released from fish feeding
during the day combined with higher water temperature caused by solar radiation, increased
nitrification (Antoniou et al., 1990; Mieczkowski et al., 2016). In contrast, for both
treatments, PO
43-
concentration was similar at both times of the day (p>0.05), which
indicates that this compound does not accumulate in the hydroponic tanks at the times of
day when the samples were taken (Figure 4).
Given the lower levels of NO
2-
-N in the RAFT than in the DRFT system, the former proved
to be better for water treatment for the fish culture, nevertheless the levels reported as toxic
were not reached in either treatment. Being a toxic compound for fish, it is important to
highlight that the experiment was carried out with a low fish density (2.6 to 4.3 kg/m3),
thus the levels of compounds released were relatively low. Therefore, the use of DRFT in
intensive culture conditions should be studied before implementation.
3.4 Electric power consumption
The required electric power consumption for water recirculation and fish tank aeration was
the same in the RAFT and DRFT systems since in the equipment was turned on all day and
throughout the trial. Meanwhile, for plant culture, electric power consumption was less for
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DRFT than for the RAFT due to the use of an air pump for seven days in DRFT and during
all the cycle (32 days) in RAFT (Table 5).
Table 5
Electric power consumption calculated by equipment in RAFT and DRFT systems for the
32 days of the experiment. The value given for centrifugal pump and the air pump for the
fish tanks represents the electric power consumption for both systems in an individual way.
Equipment
Equipment
electric power
consumption
(watts)
Aquaponic system electric power
consumption (kWh)
Day
(Average) Pak choi
production cycle
Centrifugal pump 372 8.9 285.7
Fish tanks air pump 25 0.6 19.2
Plant culture air pump
RAFT
60
1.4
46.1
DRFT 60 0.32 10.1
Total electric
power consumption
RAFT
10.9
351
DRFT 9.8 315
RAFT= Floating root technique, DRFT= Dynamic root floating technique.
The equipment with the greatest electric power consumption (Figure 5) in both systems is
the centrifugal pump, constituting 81.4 % of the electric power used in the RAFT system
and 90.7% in the DRFT system. This pump was used for water recirculation, the electric
power consumption depends on the system design and in this case the system had a 3m
elevated tank from water recirculation, therefore, a pump with a high electric power (372
watts) was needed, which resulted in the high electric power consumption.
In the RAFT system, the second greatest consumer of electric power was the plant culture
air pump (60 watts pump) which represented 13.1% of the electric power used, and,
subsequently, the fish tank air pump (25 watts pump) represented 5.5 % of the electric
power used. In contrast, in DRFT, the fish tank air pump was the second greatest consumer
of electric power representing 6.1%, and thirdly the plant culture air pump with only a 3.2
%. The DRFT reduced the electric power consumption by 78% when compared to the plant
culture of the RAFT system and reduced the total electric power consumption of the system
by 10.3%. Taking into account the complete system, the aeration electric power
consumption needed to obtain 1 kg of tilapia was 5.05 kWh for RAFT treatment and 4.99
kWh for DRFT treatment, and to obtain 1 kg of pak choi, these values were lower than the
range reported by Fang et al. (2017) of 21.95-39.11 kWh. The aeration electric power
consumption needed to obtain 1 kg of pak choi was 1.55 kWh for RAFT treatment and 0.34
kWh for DRFT treatment, these values were lower than the range reported by Fang et al.
(2017) of 4.81 to 8.8 kWh.
The electric power rate is $0.19 USD/kWh. The total cost of electric power during a pak
choi production cycle of 32 days was $66.7 USD in RAFT and $ 59.9 USD in DRFT
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system, which represented a saving of 11.4%. In this case an experimental system was
examined; however, systems for commercial purposes will vary since more efficient
equipment and designs should be used given the scale.
According to the definition provided by Maes and Jacobs (2017), the use of DRFT could
be considered a nature-based solution for the aquaponics systems since there is a decreased
input of non-renewable natural capital (not renewable electric power) and increased
investment in renewable natural processes (hair root formation). Even when nature-based
solutions have been generally proposed for urban areas or smallholder agriculture
applications (Bosch and Sang, 2017; Kabisch and Bosch, 2017; Peter et al., 2017), the
DRFT could be used in either intensive or semi-intensive aquaponic systems and in both
cases, there will be reductions in energy consumption, which will be reflected in reduced
operational costs and environmental impact (Fang et al., 2017). It has been proved that the
environmental impact caused by aquaponics production is highly sensitive to changes in
electricity inputs and it has been suggested that even a small reduction in electricity could
contribute to a correspondingly large change in the environmental impact of aquaponic
systems (Boxman et al., 2016). Thus, the DRFT could be a more environmentally friendly
system than the RAFT because of the reduction in electricity caused by turning off the air
diffusers in the hydroponic component of the system.
4. Conclusion
In semi-intensive aquaponics systems, the DRFT represents a good option for aquaponic
production for several reasons: in both RAFT and DRFT chronic exposure to
NO
3-
-N, NO
2-
-N and NH
3
-N at levels that reduce tilapia growth were not reached, water quality
parameters were within the optimal range for tilapia culture in both systems; however, plant
culture pH and conductivity were out of the optimum range. However, the use of DRFT in
intensive aquaponics systems should be studied. In the DRFT system, the electric power
consumption was reduced by 10.3% and the associated cost was reduced by 11.4% in
comparison to those in the RAFT system. This research contributes to the improvement of
an aquaponic system design, by using a nature-based solution (DRFT) that reduces electric
power consumption without compromising the production of tilapia and pak choi.
Acknowledgments
The authors thank Victor Ceja Moreno for their support with water analyses and to Tiburcio
Castro Suaste, Pedro Tec Tec and Rodrigo Mendoza Quezada for his support during all the
stages of the experiment.
Funding: The experimental work performed in this study was supported by the “Departamento
de Recursos del Mar”, CINVESTAV, Merida, Yucatan, Mexico and the “Consejo Nacional de
Ciencia y Tecnología” (CONACYT) [grant number
323715
].
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Figure captions
Figure 1. Side view of the hydroponic tank used during the experiment from the second
week to the end of the trial, a) Hydroponic tank for the RAFT b) Hydroponic tank for the
DRFT.
Figure 2. Pictures of roots from the floating technique system (RAFT) in the left side a),
and from the dynamic root floating technique system (DRFT) in the right side b). At the
top: roots in culture tanks, these were totally and partially submerged in RAFT and DRFT
respectively but for the picture they were lifted. In DRFT the root close to the polystyrene
sheet looks slightly clearer than the rest of the root because it had root hairs. At the middle:
roots view in a stereoscopic microscope at 2X. At the bottom: roots view in a stereoscopic
microscope at 3.2X. The root hairs can be seen in DRFT pictures.
Figure 3. Average concentration of nitrite nitrogen (NO
2-
-N), nitrate nitrogen (NO
3-
-N) and
phosphate (PO
43-
) in the fish tanks of the floating bed system (RAFT) and dynamic root
floating technique system (DRFT) during morning (M) and afternoon (A). In the graph,
whiskers represent minimum and maximum values of the sample, and the second and third
quartiles and the median are represented in the box plot.
Figure 4. Average concentration of nitrite nitrogen (NO
2-
-N), nitrate nitrogen (NO
3-
-N) and
phosphate (PO
43-
) in the hydroponic tanks of the floating bed technique system (RAFT) and
dynamic root floating technique system (DRFT) during morning (M) and afternoon (A). In
the box plot, whiskers represent minimum and maximum values of the sample, the second
and third quartiles and the median is displayed.
Figure 5. Percentage contribution to electric power consumption from the equipment used
in the floating bed technique system (RAFT) and dynamic root floating technique system
(DRFT)
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Air
diffusers
on
a)
Polystyrene sheet
Air space
Water
Drain
b)
Pak choi
Air
diffusers
off
PVC support
Air
bubbles
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Percentage of contribution to electric power consumption by each equipment required for RAFT and DRFT systems operation
Equipment
RAFT
DRFT
Cetrifugal pump
81.4
90.7
Fisht tanks air pump
5.5
6.1
Plant culture air pump
13.1
3.2
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
80.0
90.0
100.0
RAFT DRFT
% of electric power
consumption
Cetrifugal pump Fisht tanks air pump Plant culture air pump
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Highlights
Root hair principle was used to avoid active aeration for plants in aquaponics
Dynamic root floating (DRFT) is suitable for pak choi production in aquaponics
In semi-intensive aquaponics culture, DRFT does not affect water quality for tilapia
DRFT reduces the electric power consumption in aquaponic systems
