Samir Khanal

1. Nitrogen transformations in
aquaponic systems
Introduction
Aquaponic system refers to cultivation of fish and plants together in a
constructed, re-circulating ecosystem utilizing natural microbial cycles to
convert fish wastes to plant nutrients. Aquaponics is a sustainable food
production system that combines traditional aquaculture (raising fish in
tanks) with hydroponics (cultivating plants in water) in a symbiotic
environment1.
Nitrogen is the most important component of an aquaponic system. The
understanding of nitrogen transformations is essential to maintain better
water quality, and fish and plant growth in an aquaponics system. The
overall goal of this study is to examine nitrogen transformations within the
aquaponic system under different hydraulic loading rates, dissolved oxygen
concentrations and feeding rates. The study also examined nitrogen isotopic
composition (δ15N) to identify nitrogen loss from the aquaponic system. The
understanding of nitrogen transformations and nitrogen mass balance would
improve the nutrient utilization efficiency (NUE) of plant in aquaponic
systems.
Objectives
The main objective of this study is to investigate the nitrogen
transformations in an aquaponic system by integrating nitrogen mass
balance and natural abundance of nitrogen via isotope analysis. The
specific objectives are:
• Investigate the effects of hydraulic loading rate (HLR) and dissolved
oxygen (DO) concentration on nitrogen utilization efficiency (NUE)
of vegetables.
• Evaluate the effect of feed-to-plant ratio on NUE.
• Investigate the baseline natural abundance isotopic nitrogen
compositions in aquaponic systems.
• Identify the denitrification process in aquaponic systems by
investigating the nitrate concentration and natural abundance
isotopic composition (δ15N).
Methodology
Figure1. Set-up of an aquaponic system
Results and Discussion
A series of three replicated aquaponic systems were operated in the
Magoon Greenhouse Facility at the University of Hawai’i at Manoa. Pak
choi (Brassica rapa, sub. chinensis) (24 plants per raft) and tilapia
(Oreochromis sp.) (initial stocking density of 30 kg/m3) were selected as the
growing species. Recirculating water (RC water) in the aquaponic system
was maintained at 650 liters. Plastic media with high surface area were
used in biofilters (up-flow and down-flow with partial aeration) to promote
the nitrification process. Fish in each tank was fed with commercial fish
feed with 40% protein content at a rate of 35-50 g/day.
Water was sampled from the fish tanks for DO, total Kjeldahl nitrogen
(TKN), total ammonia nitrogen (TAN), nitrite nitrogen, nitrate nitrogen and
chemical oxygen demand (COD). The pH in the tanks was maintained at
around neutral by dosing 2:1 mixture of KOH:Ca(OH)2 daily. The vegetable
seeds were germinated for 14 days before they were transplanted into the
grow bed. The plants were harvested at the end of a 37-day experimental
period.
In experiments 1 to 3 (Figure 3), nitrogen transformations and NUE
were studied at four HLRs (1.0, 1.5, 2.0 and 2.5 m/d). In experiments 4 and
5, nitrogen transformations and NUE were studied at two DO levels in fish
tanks (low DO (~ 3.5 mg/L) and high DO (~ 7 mg/L). Aquaponic systems in
experiment 4 and 5 were operated at HLR of 1.5 and 1.0 m/d, respectively.
The experiments were conducted in triplicates and statistical analyses of
the collected data were carried out using an analysis of variance (one-way
ANOVA) at a confidence level of α = 0.05.
Nitrogen mass balance was carried out based on nitrogen contents in
fish feed (input), fish biomass, plant biomass , nitrate accumulated in RC
water, sludge in the effluent and nitrogen loss.
Fish feed, mixed-liquor suspended solids (MLSS) in the biofilter, RC
water, water in the biofilter (BF water), Pak choi’s roots, stems and leaves
were randomly collected at the end of growing batch to analyze for bulk
nitrogen isotopic composition. RC water and BF water were analyzed for
nitrate isotopic composition (Nitrate was the major nitrogen species that
was present in the aqueous phase).
Conclusions
Acknowledgements
References
Sumeth Wongkiew and Samir K. Khanal
Department of Molecular Biosciences and Bioengineering
University of Hawaii at Manoa
• This project is being supported by Agriculture and Food Research Initiative
Competitive Grant no. 2013-67019-21376 from the USDA National Institute
of Food and Agriculture.
• 1Hu, Z., Lee, J.W., Chandran, K., Kim, S., Brotto, A.C., and Khanal S.K. 2015.
Effect of plant species on nitrogen recovery in aquaponics. Bioresource
Technology 188: 92-98.
• 2Robinson, D. 2001. δ15N as an integrator of the nitrogen cycle. TRENDS in
Ecology & Evolution 16(3): 153-159.
26%
16%
19%7%
32%
Feed 35 g/d at low DO
26%
16%
18%9%
31%
Feed 35 g/d at high DO
21%
12%
10%
5%
52%
Feed 50 g/d at high DO
Plant biomass (NUE)
Fish biomass
Nitrate-N accumulated
Sludge effluent
Loss to gases
0
5
10
15
20
25
30
Experiment 1 Experiment 2 Experiment 3
NUE(%)
20
21
22
23
24
25
26
27
28
29
30
185
190
195
200
205
210
215
220
0 3 6 9 12
Watertemperature(°C)
nitrate(mgN/L)
Days
low DO-
nitrate N
high DO-
nitrate N
water
temperature
This experiment was specifically designed to identify the effect of DO
on denitrification process. Three replications of aquaponic systems with no
vegetables were operated at HLR of 1.5 m/d for 12 days. The δ15N of
nitrate in high DO condition (DO ~7 mg/L at biofilter inlet) was compared
with δ15N of nitrate in low DO condition (DO ~3.5 mg/L at biofilter inlet).
Results showed that the δ15N of nitrate in the low DO condition was
significantly higher than that at the high DO condition. This indicates that
the denitrification process occurred at a higher rate in the low DO
condition relative to the high DO condition2. To confirm the nitrogen loss
due to denitrification, nitrate concentrations were measured and analyzed
together with the isotope results. Figure 7 shows the nitrate concentration
in these two conditions. Nitrate was accumulating in the systems; however,
nitrate accumulation rate was faster at high DO condition. The difference in
nitrogen accumulation rate could be attributed to the difference in
nitrogen loss due to the denitrification process. Thus, from the results, it
can be concluded that nitrogen loss in aquaponic systems is attributed to
the denitrification process and low DO conditions, which resulted in higher
rate of denitrification.
The δ15N values of nitrate in RC water (18.3%) was consistent with
large isotope fractionation associated with denitrification or ammonia
volatilization. However, TAN concentration was below 4 mg N/L during
the experiments and pH was below 7, suggesting that the ammonia
volatilization was low. High δ15N value of nitrate in RC water supports the
occurrence of high denitrification in the aquaponic systems (Figures 5, 6
and 7). The δ15N values of MLSS was about 10%. The δ15N enrichment in
MLSS relative to fish feed could be attributed to a kinetic isotope effect
associated with assimilation of nitrogen by the tilapia. The δ15N values of
nitrate in RC water and BF water were homogeneous indicating that the
tank content was well mixed.
The δ15N values of plant increased from the roots up to the leaves
(Figure 5). This suggests that nitrate nitrogen in the water was taken up
by the roots and accumulated in the plant’s stems and leaves.
4.15
2.00
0.43
86.5
199.5
77.4
0
50
100
150
200
250
0
1
2
3
4
5
6
TKN TAN Nitrite N Nitrate N
(before)
Nitrate N
(after)
COD
Nitrateconcentration(mgN/L)&
CODconcentration(mg/L)
TKN,TAN&Nitriteconcentration
(mgN/L)
Figure 5. Nitrogen isotopic composition (δ15N) of bulk nitrogen of fish feed, mixed-
liquor suspended solids (MLSS), plant roots, stems and leaves, and δ15N of nitrate of
RC water and BF water
1.0
1.5 2.0 2.5
1.5 2.0
Figure 6. Nitrate isotopic composition (δ15N) of low DO and high DO condition
Figure 7. Nitrate nitrogen concentration of low DO and high DO condition
Figure 2. Concentrations of TKN, TAN, nitrite N, nitrate N and COD from all experimental
conditions. Nitrate N (before) is the concentration at the beginning of experiment 1 and N
(after) is the concentration at the end of experiment 5.
Figure 3. Nitrogen utilization efficiency (NUE) comparisons between HLRs of 1.0 vs 1.5
(Experiment 1), 2.0 vs 2.5 (Experiment 2) and 1.5 vs 2.0 m/d (Experiment 3)
Figure 4. Nitrogen mass balance of input (fish feed) and nitrogen outputs at feeding rate of 50
g/d (high DO) and feeding rate of 35 g/d (low DO and high DO)
Fish tank Grow bed
Aquaponic system Biofilter
20
22
24
26
28
30
32
34
17.5
18.0
18.5
19.0
19.5
20.0
20.5
0 3 6 9 12
Watertemperature(°C)
δ15N(%)
Days
low DO-δ15N
nitrate
high DO-δ15N
nitrate
water
temperature
• The accumulation of nitrate in recirculating water, “Balloon Effect”,
occurred because nitrate input exceeded the amount that the plants
could utilize.
• Denitrification was the major contribution to nitrogen loss in aquaponic
systems.
• When the Balloon Effect occurs, reducing the feeding rate will increase
NUE and decrease the denitrification in the system.
• To reduce the nitrogen loss in aquaponic systems, higher rate of sludge
draining and higher plant-to-fish ratio are recommended.
Nitrate nitrogen continuously accumulated during steady state operation while TKN,
TAN, nitrite and COD concentration remained fairly steady (Figure 2). The accumulation of
nitrate in RC water was discovered, which is termed as “Balloon Effect”, indicating that
nitrate input was more than the plant requirements. HLR of 1.5 m/d or higher significantly
improved the NUE for Pak choi after 37 days of growing (Figure 3). This could be attributed
to sufficient DO level in the grow beds; providing effective nitrification around the plant’s
root surface area. Result also showed that nitrogen loss was relatively high compared with
other outputs such as organic nitrogen in whole plant biomass, suspended solids (MLSS)
accumulated in biofilter and nitrate accumulated in the recirculated aquaponic water. The
high loss of nitrogen was attributed to COD in the water, which led to heterotrophic
denitrification and anoxic conditions (low DO) in biofilter. Nitrogen loss decreased from
52% to 31-32% and NUE increased by 5% when the feeding rate was decreased from 50
g/d to 35 g/d (Figure 4). In addition, the tilapia biomass yield was independent of HLRs at
the given stocking density.
0
5
10
15
20
25
Fish feed RC water
(nitrate)
BF water
(nitrate)
MLSS Roots Stems Leaves
δ15NofbulkNornitrate(%)
Source of nitrogen