Borgmann Aquaponics Hydroponics
Table 1. Observed sunshine hours (per month) and the respective estimated reference evaporation
(ETo in mm/day) for Köln-Bonn.
Figure 2. Reference evapotranspiration (ETo in mm/day) upon plain natural lighting (Köln‐Bonn) or
at constant radiation using (additional) artificial light.
3.3. Input Data and Parametrization for RAS
The modelled RAS comprised four fish tanks with a volume of 1 m3 each. Additional RAS
components (i.e., biofilter, drum filter, sump, etc.) add another 3 m3 of volume. The RAS water
temperature is held stable at about 28 °C. Temperature is an important factor for fish growth and its
influence on tilapia growth (cm∙month−1) can be predicted given the following formula for a
maximum temperature of 29.5 °C [32]:
The modelled RAS comprised four fish tanks with a volume of 1 m3 each. Additional RAS
components (i.e., biofilter, drum filter, sump, etc.) add another 3 m3 of volume. The RAS water
temperature is held stable at about 28 °C. Temperature is an important factor for fish growth and its
influence on tilapia growth (cm∙month−1) can be predicted given the following formula for a
maximum temperature of 29.5 °C [32]:
where T is the average temperature (in ˝C) of the RAS.
Knowing the fish length allows to calculate the fish weight [32]:
where WT is the weight of the tilapia (in g), and L the length of tilapia (in cm).
In the model, fingerlings 10 cm in size were sequentially added to the fish tanks. The total fish
growth cycle was set at 200 days. Thus, every 50 days fish were harvested and re-added to one of the
fish tanks. The fish were fed with species specific feed with a protein content of 35%, of which 9.2% of
proteins become TAN [32]. The feeding rate was calculated based on a percentage of body weight that,
in turn, was depending on the feed conversion ratio (FCR) [32]. It was assumed that the optimal FCR
is 1.1–1.3 for tilapia depending on their growth stage [32], even though in aquaponics FCRs of 1.7–1.8
have been observed [33]. It was also assumed that water quality parameters of the RAS lie within
acceptable limits (Table 2) and did not have any impact on the mortality rate. The main factor for the
fish’s survival rate was thus the density [34]. The passive denitrification rate was 10% [32]. The fish
feed uptake rate (i.e., the percentage of feed the fish actually consume) was assumed being 95% [32].
In the model, fingerlings 10 cm in size were sequentially added to the fish tanks. The total fish
growth cycle was set at 200 days. Thus, every 50 days fish were harvested and re-added to one of the
fish tanks. The fish were fed with species specific feed with a protein content of 35%, of which 9.2% of
proteins become TAN [32]. The feeding rate was calculated based on a percentage of body weight that,
in turn, was depending on the feed conversion ratio (FCR) [32]. It was assumed that the optimal FCR
is 1.1–1.3 for tilapia depending on their growth stage [32], even though in aquaponics FCRs of 1.7–1.8
have been observed [33]. It was also assumed that water quality parameters of the RAS lie within
acceptable limits (Table 2) and did not have any impact on the mortality rate. The main factor for the
fish’s survival rate was thus the density [34]. The passive denitrification rate was 10% [32]. The fish
feed uptake rate (i.e., the percentage of feed the fish actually consume) was assumed being 95% [32].
Given these parameters, the optimization experiment that has been conducted with AnyLogic
with the decision variable “amount of fish” suggested to start with 60 fingerlings to gain a fish biomass
of approximately 50 kg¨ m´3 fishes per tank after 200 days.
with the decision variable “amount of fish” suggested to start with 60 fingerlings to gain a fish biomass
of approximately 50 kg¨ m´3 fishes per tank after 200 days.
3.4. Input Data and Parametrization for Hydroponics
The growth cycle for lettuce was set at 35 days. They were grown in DWC systems with a depth
of 30 cm. The inflow from the RAS to the DWC was determined by the evapotranspiration rate, that is
ETc times area of the hydroponic system, and is given by:
where ANRC represents the anaerobic nutrient remineralization component.
Under optimal conditions, lettuce head weights of 150 g can be achieved 4 weeks after
transplanting the seedlings into the hydroponic system [33,41]. 16 lettuce plants were cultivated
per m2, each of which takes up 6 g of N and 50 mg of P within the growth cycle [9].
Under optimal conditions, lettuce head weights of 150 g can be achieved 4 weeks after
transplanting the seedlings into the hydroponic system [33,41]. 16 lettuce plants were cultivated
per m2, each of which takes up 6 g of N and 50 mg of P within the growth cycle [9].
3.5. Input Data and Parametrization for Remineralization
Nitrate and nitrite are degraded to ammonia and dinitrogen gas under anaerobic conditions [42].
Both Zhao et al. [43] and Cuervo-López et al. [44] observed nitrate and nitrite degradation efficiencies
of approx. 99% in UASB reactors operating with an hydraulic retention time (HRT) of 9–12 days
and 2 days, respectively. Mirzoyan and Gross [17] show similar nitrate and nitrite removal rates.
However, in their work, the effluent consisted of 17.7 mg¨ L´1 TAN (for every 102 g¨ kg´1 entering
the UASB reactor) at an HRT of 8 days and a total suspended solids sludge concentration of 3.8%.
The N composition of the inflowing sludge is a sum of N water content and N in dry matter sludge.
The composition of the latter has been shown by Neto and Ostrensky [10], who report an N content of
approximate 15% of dry sludge for a feed N content of 5.3%, and a P feed content of 1.5% [10,12].
The volume of the UASB reactor has been determined by the following HRT formula:
where HRT is the hydraulic retention time in hours.
The desired HRT was 10 days, because it showed best digestion performance recently [17].
Since an HRT of 10 days cannot practically fulfill this requirement, an additional circulation pump
was required for each UASB reactor. The flow rate going into the reactor has been determined by
the amount of daily sludge produced by the fish, which has been recovered from the settling basin
(see Figure 3) with a specific TSS. In our case the sludge retention time (SRT) was 80 days considering
that the sludge blanket occupied 60% of the volume of the reactor with a TSS of 3%. The amount of
sludge withdrawn on a daily basis has been assumed to be 10% of the organic loading rate (ORL);
The desired HRT was 10 days, because it showed best digestion performance recently [17].
Since an HRT of 10 days cannot practically fulfill this requirement, an additional circulation pump
was required for each UASB reactor. The flow rate going into the reactor has been determined by
the amount of daily sludge produced by the fish, which has been recovered from the settling basin
(see Figure 3) with a specific TSS. In our case the sludge retention time (SRT) was 80 days considering
that the sludge blanket occupied 60% of the volume of the reactor with a TSS of 3%. The amount of
sludge withdrawn on a daily basis has been assumed to be 10% of the organic loading rate (ORL);
i.e., of the added fresh sludge. Mirzoyan and Gross [17] showed a fresh sludge TSS removal of 90%.
The same TSS removal has been assumed in this study.
The same TSS removal has been assumed in this study.
An upflow velocity of 0.5 m¨ h´1 must be maintained to keep the sludge blanket in suspension [45].
The upflow velocity formula is as follows
The upflow velocity formula is as follows
where Vup is the upflow velocity (m x h-1), H is the reactor height (m), and HRT is the hydraulic retention time (h).
Figure 3. Process flow drawing of a basic DAPS layout. The blue tags comprise the RAS component,
the green tags the hydroponic component, and the red tags the ANRC components. The level of each
component is illustrated numerically in the small box and refers to the vertical direction the flow needs
to travel to; whereas high numbers refer to high positioning and low numbers to low positioning.
Gravity flow occurs, when water flows from high levels to low levels, and pressurized flow is required
when the flow goes from low to high numbers.
the green tags the hydroponic component, and the red tags the ANRC components. The level of each
component is illustrated numerically in the small box and refers to the vertical direction the flow needs
to travel to; whereas high numbers refer to high positioning and low numbers to low positioning.
Gravity flow occurs, when water flows from high levels to low levels, and pressurized flow is required
when the flow goes from low to high numbers.
4. Theory
4.1. Decoupled System Design
4.1.1. Justification for Decoupled Systems
Nitrate concentrations in aquaponic systems can be controlled either by water exchange, plant
uptake, and/or denitrification through anoxic bacterial reduction. Water exchange rates can be
determined via mass balance, whilst N removal by denitrification or plant uptake must be calculated
using available information on N removal rates by different plants and denitrification reactor
configurations. Unlike N, which is present in aquaponics mostly in dissolved forms that tend to
accumulate, several importantplant nutrients are found almost exclusively in biosolids originating
from uneaten food and feces.
Until recently, the principal focus of publications in the field of aquaponics was mostly on
the availability, concentration, and accumulation of nitrate, which has been considered the most
important macronutrient for vegetative growth. Even though it increases the complexity of the overall
process and is not a focus of this paper, it is inevitable to address the accumulation of all micro- and
macronutrients that are necessary for optimal growth conditions. This becomes more relevant given the
fact that in DAPS the nutrient accumulation in the hydroponic component results from several sources
(i.e., inflow from RAS, inflow from remineralization component, and nutrient supplementation) and
reduces dilution due to the fact that the ANRC outflows only dilutes in the hydroponic component
instead of system-wide. As the specific nutrient needs of fish and plants depend on a large number
of dependencies and interactions, design trade-offs towards best practicable means need to be made,
to achieve best possible conditions for both the fish and the plant and thus the optimization of the
sub-systems with regard to the use/recycling of nutrients. These heterogeneous conditions of the
respective sub-systems and their impact on nutrient accumulation have a significant effect on the
overall system performance.
4.1. Decoupled System Design
4.1.1. Justification for Decoupled Systems
Nitrate concentrations in aquaponic systems can be controlled either by water exchange, plant
uptake, and/or denitrification through anoxic bacterial reduction. Water exchange rates can be
determined via mass balance, whilst N removal by denitrification or plant uptake must be calculated
using available information on N removal rates by different plants and denitrification reactor
configurations. Unlike N, which is present in aquaponics mostly in dissolved forms that tend to
accumulate, several importantplant nutrients are found almost exclusively in biosolids originating
from uneaten food and feces.
Until recently, the principal focus of publications in the field of aquaponics was mostly on
the availability, concentration, and accumulation of nitrate, which has been considered the most
important macronutrient for vegetative growth. Even though it increases the complexity of the overall
process and is not a focus of this paper, it is inevitable to address the accumulation of all micro- and
macronutrients that are necessary for optimal growth conditions. This becomes more relevant given the
fact that in DAPS the nutrient accumulation in the hydroponic component results from several sources
(i.e., inflow from RAS, inflow from remineralization component, and nutrient supplementation) and
reduces dilution due to the fact that the ANRC outflows only dilutes in the hydroponic component
instead of system-wide. As the specific nutrient needs of fish and plants depend on a large number
of dependencies and interactions, design trade-offs towards best practicable means need to be made,
to achieve best possible conditions for both the fish and the plant and thus the optimization of the
sub-systems with regard to the use/recycling of nutrients. These heterogeneous conditions of the
respective sub-systems and their impact on nutrient accumulation have a significant effect on the
overall system performance.
The advantages of independently controllable components are underlined in the Tables 3 and 4.
The in Table 3 given examples of optimal conditions for different plant and fish species, as well as
the bio filtration and ANRC components are cases in point. The more the conditions of RAS and
hydroponics deviate from their optimal conditions, the lower the production efficiency that can be
expected. Instead of accounting for trade-offs between those component parts, the objective should be
to provide the best practicable conditions for each component and combination of species. Even though
similar growth performances between aquaponics and hydroponics have been observed, optimizing
the respective conditions could lead to an enhanced fish and plant growth [6]. This is only achievable
in independently controlled and running sub-systems. This concurs with Table 4. Here too, one can
see major differences in environmental and nutritional factors. The significant difference in terms of
nutrient concentration can be explained by the fact that trade-offs have to be made with respect to
plants’ and fish’ specific needs, which is the reason for these gaps.
Table 3. Optimal conditions for different fish in RAS (i.e., tilapia, European perch and rainbow trout), plant species in hydroponic conditions (i.e., lettuce and tomato), biofiltration efficiency being a part of RAS, and anaerobic digestion.
Table 4. Suggested hydroponic nutrient solution (ppm) vs. observed aquaponic nutrient solution (ppm) for lettuce, and the percentage difference of aquaponics with respect to optimal hydroponic conditions.
With respect to nutrient dynamics, the flow of each nutrient depends upon many factors
including species, system design, biofilter performance, remineralization method, feed composition,
etc. Around 25% of feed (dry matter) becomes sludge [32,73]. Neto and Ostrensky [10] analyzed the
nutrient compositions of sludge (Table 5). They report that 55% of the P that entered the system via the
fish feed accumulates in sludge. The sludge is composed of 37% feces and 18% non-consumed feed.
The percentage of non-consumed feed, however, must be treated with caution as the data is collected
from RAS cage breeding systems. Even though the values of Neto and Ostrensky [10] base on cage
breeding systems, our observed values (Table 5) are closer to these values, than to observations made
by Refiee and Saad [74] in RAS. We observed similar concentrations, using fish feed consisting of 9.7%
N and 1.7% P. However, more P accumulated in the fish than in the sludge. Yet, the implementation of
remineralization technology has great potential to recycling a high proportion of macronutrients such
as P, K, and N (i.e., if carried out under anaerobic conditions) etc. [15,16,75].
including species, system design, biofilter performance, remineralization method, feed composition,
etc. Around 25% of feed (dry matter) becomes sludge [32,73]. Neto and Ostrensky [10] analyzed the
nutrient compositions of sludge (Table 5). They report that 55% of the P that entered the system via the
fish feed accumulates in sludge. The sludge is composed of 37% feces and 18% non-consumed feed.
The percentage of non-consumed feed, however, must be treated with caution as the data is collected
from RAS cage breeding systems. Even though the values of Neto and Ostrensky [10] base on cage
breeding systems, our observed values (Table 5) are closer to these values, than to observations made
by Refiee and Saad [74] in RAS. We observed similar concentrations, using fish feed consisting of 9.7%
N and 1.7% P. However, more P accumulated in the fish than in the sludge. Yet, the implementation of
remineralization technology has great potential to recycling a high proportion of macronutrients such
as P, K, and N (i.e., if carried out under anaerobic conditions) etc. [15,16,75].
Table 5. Nitrogen (N) and phosphorous (P) flow from Nile tilapia feeding in RAS cage production.
As compared to one-loop aquaponics, the nutrient flow in DAPS is slightly different.
As exemplified in Section 4.2, the extent to which the sludge remineralization process can add to
the development of an integrated system approach. Apart from nutrient supplementation, nutrient
remineralization can be used to accumulate nutrients in the hydroponic component. This may have a
commercial advantage, as the root:shoot ratio (i.e., the ratio between the edible parts and the residuals)
of plants is dependent on nutrient concentration in the plant. According to Lambers et al. [76] the
root:shoot ratio of plants is decreased under sufficient presence of N, P, and very likely sulfate deficiency,
which is the case in one-loop aquaponics systems.
The most important principle in aquaponics design states that the nutrient load of the system can
be balanced between both the nutrient load, as a function of fish biomass and feeding rate, metabolic
conversion and subsequent excretion as well as uneaten feed and feces, and the nutrient requirement
of the plants [9]. Consequently, the determination of fish:plant ratios has become the most commonly
used design approach for balancing the systems. However, as every plant and fish species have
different nutritional needs that are also dependent on the growth stage/life-cycle and external factors
(including system design), the exact determination of this ratio is complex, system-dependent and
commonly carried out with empirical data. The more system designs are studied, the more accurately
fish:plant ratios can be estimated and, as a consequence, the more efficient nutrient flows and yields
can be managed. Although specific estimations might apply to a particular system setup, adopting this
practice to size or balance entirely new designs without experimental evaluation can be problematic.
Irrespective of the system’s input and the specific optimized nutrient solution, the nutrient
uptake of plants is highly dependent on nutrient availability, illumination, temperature, pH, etc.
These deviating optimal conditions are reflected by plant specific coefficients. Being able to select the
best combination of plants and fish as well as feed that achieves the best possible water quality and thus
growth performance provides a good argument for DAPS. Achieving the best possible water quality
still remains a minor criterion in system design engineering. However, when both fish and plant
species and their specific nutritional needs are known, it will be possible to predict the plant component
size based on the estimation of the RAS nutrient loading as well as the remineralization capacity of the
As exemplified in Section 4.2, the extent to which the sludge remineralization process can add to
the development of an integrated system approach. Apart from nutrient supplementation, nutrient
remineralization can be used to accumulate nutrients in the hydroponic component. This may have a
commercial advantage, as the root:shoot ratio (i.e., the ratio between the edible parts and the residuals)
of plants is dependent on nutrient concentration in the plant. According to Lambers et al. [76] the
root:shoot ratio of plants is decreased under sufficient presence of N, P, and very likely sulfate deficiency,
which is the case in one-loop aquaponics systems.
The most important principle in aquaponics design states that the nutrient load of the system can
be balanced between both the nutrient load, as a function of fish biomass and feeding rate, metabolic
conversion and subsequent excretion as well as uneaten feed and feces, and the nutrient requirement
of the plants [9]. Consequently, the determination of fish:plant ratios has become the most commonly
used design approach for balancing the systems. However, as every plant and fish species have
different nutritional needs that are also dependent on the growth stage/life-cycle and external factors
(including system design), the exact determination of this ratio is complex, system-dependent and
commonly carried out with empirical data. The more system designs are studied, the more accurately
fish:plant ratios can be estimated and, as a consequence, the more efficient nutrient flows and yields
can be managed. Although specific estimations might apply to a particular system setup, adopting this
practice to size or balance entirely new designs without experimental evaluation can be problematic.
Irrespective of the system’s input and the specific optimized nutrient solution, the nutrient
uptake of plants is highly dependent on nutrient availability, illumination, temperature, pH, etc.
These deviating optimal conditions are reflected by plant specific coefficients. Being able to select the
best combination of plants and fish as well as feed that achieves the best possible water quality and thus
growth performance provides a good argument for DAPS. Achieving the best possible water quality
still remains a minor criterion in system design engineering. However, when both fish and plant
species and their specific nutritional needs are known, it will be possible to predict the plant component
size based on the estimation of the RAS nutrient loading as well as the remineralization capacity of the
used digestion method. Several mathematical N-uptake models have been developed [77–79] and can
provide a good estimation for the plants’ N-consumption. However, the nutrient removal performance
for different plant species and life stages under aquaponic conditions remains to be studied in order to
be able to model the flows for all important nutrients within DAPS.
provide a good estimation for the plants’ N-consumption. However, the nutrient removal performance
for different plant species and life stages under aquaponic conditions remains to be studied in order to
be able to model the flows for all important nutrients within DAPS.
4.1.2. Hardware Layout
The schematic design of a DAPS layout is illustrated in Figure 3. It consists of three parts:
(1) conventional RAS; (2) hydroponic component; and (3) ANRC. Implementing such an ANRC into
an aquaponic system and following a one-way flow approach require several design considerations,
which are outlined below:
The schematic design of a DAPS layout is illustrated in Figure 3. It consists of three parts:
(1) conventional RAS; (2) hydroponic component; and (3) ANRC. Implementing such an ANRC into
an aquaponic system and following a one-way flow approach require several design considerations,
which are outlined below:
1. The RAS is intended to manage the fish sludge and to provide control over the most important
water quality parameters (dissolved oxygen, TAN, suspended solids, and carbon dioxide).
2. Since UASBs have a high denitrification potential that is dependent on both HRT and SRT, a
direct one way flow from RAS to the hydroponic component is required to control the nitrate
dosing (i.e., provide the plants with N). This also includes a return overflow option; e.g., in case a
sub-system needs to be re-coupled.
3. Sludge thickening is a necessary prerequisite for the anaerobic digestion process [15]. An offline
settling tank prior to the UASB is used for this process. In practice, activated sludge denitrification
reactors can be installed upstream of the UASB to use some of the carbon source to get rid of
nitrate and reduce sludge volume [80]. In our case, N is preferred to be kept in the system.
However, N recovery in anaerobic sludge treatment is very much HRT dependent and only
marginal, when exceeding an HRT over several hours [16].
4. Hypothetically, digester effluents to RAS, even though they contain a high amount of sulfide,
are not expected to affect water quality and therefore fish welfare due to dilution [17]. This also
applies for the hydroponics-RAS return overflow.
5. Two mechanical filtration steps are used to minimize the TSS in the UASB effluent.
6. The hydroponic system is a hybrid system that utilizes dosing systems to manage nutrients,
pH, electrical conductivity (EC), dissolved oxygen, and redox potential to maintain acceptable
nutrient levels with precision.
7. The outflow from both the ANRC and RAS, and the utilized water in the hydroponic component
are congruent. Consequently, the utilized water in the hydroponic component, and thus the
replenishment of water to the fish tanks must be high enough to avoid accumulation of nutrients
in the RAS.
8. The ANRC can be complemented with an aerobic pre- or post-treatment, as better
remineralization performance can be assumed, and chemical oxygen demand (COD) reduction
has been observed [81]. Yet, it must be noted that the drawback of an additional aerobic step
is additional production of biomass (i.e., bacteria growth) that consumes part of the available
nutrients. Whether the aerobic treatment proceeds or follows the anaerobic treatment depends on
whether one prefers to increase the carbon dioxide concentration in the greenhouse (pre) or use
the anaerobic digester as a biogas producing device (post). Even though the aerobic treatment
most likely provides additional advantages (e.g., H2S reduction), it is no part of our software
calculations. For the sake of completeness, we nonetheless added it to this scheme.
9. The produced energy in form of electricity and heat, gained through CH4 combustion,
can be returned to the system. This combustion also reduces greenhouse gas emissions.
The combustion’s products (i.e., CO2 and H2O) can be lead back to the system as clean water and
CO2 (greenhouse required) in order to enhance plant growth.
water quality parameters (dissolved oxygen, TAN, suspended solids, and carbon dioxide).
2. Since UASBs have a high denitrification potential that is dependent on both HRT and SRT, a
direct one way flow from RAS to the hydroponic component is required to control the nitrate
dosing (i.e., provide the plants with N). This also includes a return overflow option; e.g., in case a
sub-system needs to be re-coupled.
3. Sludge thickening is a necessary prerequisite for the anaerobic digestion process [15]. An offline
settling tank prior to the UASB is used for this process. In practice, activated sludge denitrification
reactors can be installed upstream of the UASB to use some of the carbon source to get rid of
nitrate and reduce sludge volume [80]. In our case, N is preferred to be kept in the system.
However, N recovery in anaerobic sludge treatment is very much HRT dependent and only
marginal, when exceeding an HRT over several hours [16].
4. Hypothetically, digester effluents to RAS, even though they contain a high amount of sulfide,
are not expected to affect water quality and therefore fish welfare due to dilution [17]. This also
applies for the hydroponics-RAS return overflow.
5. Two mechanical filtration steps are used to minimize the TSS in the UASB effluent.
6. The hydroponic system is a hybrid system that utilizes dosing systems to manage nutrients,
pH, electrical conductivity (EC), dissolved oxygen, and redox potential to maintain acceptable
nutrient levels with precision.
7. The outflow from both the ANRC and RAS, and the utilized water in the hydroponic component
are congruent. Consequently, the utilized water in the hydroponic component, and thus the
replenishment of water to the fish tanks must be high enough to avoid accumulation of nutrients
in the RAS.
8. The ANRC can be complemented with an aerobic pre- or post-treatment, as better
remineralization performance can be assumed, and chemical oxygen demand (COD) reduction
has been observed [81]. Yet, it must be noted that the drawback of an additional aerobic step
is additional production of biomass (i.e., bacteria growth) that consumes part of the available
nutrients. Whether the aerobic treatment proceeds or follows the anaerobic treatment depends on
whether one prefers to increase the carbon dioxide concentration in the greenhouse (pre) or use
the anaerobic digester as a biogas producing device (post). Even though the aerobic treatment
most likely provides additional advantages (e.g., H2S reduction), it is no part of our software
calculations. For the sake of completeness, we nonetheless added it to this scheme.
9. The produced energy in form of electricity and heat, gained through CH4 combustion,
can be returned to the system. This combustion also reduces greenhouse gas emissions.
The combustion’s products (i.e., CO2 and H2O) can be lead back to the system as clean water and
CO2 (greenhouse required) in order to enhance plant growth.
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