Aquaponic Systems: Nutrient recycling from fish wastewater by vegetable production
Desalination XXXXXXXXXX–156
Aquaponic Systems: Nutrient recycling from fish wastewate
y vegetable production
Andreas Graber, Ranka Junge*
ZHAW Zurich University of Applied Sciences, Institute for Natural Resource Sciences Gruental,
CH-8820 Waedenswil, Switzerland
Tel: XXXXXXXXXX; Fax: XXXXXXXXXX; email: XXXXXXXXXX
Received 10 December 2007; revised 06 March 2008; accepted 18 March 2008
Abstract
This chapter describes the possibility to combine wastewater treatment in recirculating aquaculture systems
(RAS) with the production of crop plants biomass. In an aquaponic RAS established in Waedenswil, Zurich,
the potential of three crop plants was assessed to recycle nutrients from fish wastewater. A special design of trick-
ling filters was used to provide nitrification of fish wastewater: Light-expanded clay aggregate (LECA) was filled
in a layer of 30 cm in vegetable boxes, providing both surface for biofilm growth and cultivation area for crop
plants. Aubergine, tomato and cucumber cultures were established in the LECA filter and nutrient removal
ates calculated during 42–105 days. The highest nutrient removal rates by fruit harvest were achieved during
tomato culture: over a period of >3 months, fruit production removed 0.52, 0.11 and 0.8 g m�2 d�1 for N, P
and K in hydroponic and 0.43, 0.07 and 0.4 g m�2 d�1 for N, P and K in aquaponic. In aquaponic, 69% of nitrogen
emoval by the overall system could thus be converted into edible fruits. Plant yield in aquaponic was similar to
conventional hydroponic production systems. The experiments showed that nutrient recycling is not a luxury
eserved for rural areas with litlle space limitation; instead, the additionally occupied surface generates income
y producing marketable goods. By converting nutrients into biomass, treating wastewater could become a profit-
able business.
Keywords: Aquaponic; Nutrient recycling; Nutrient removal; Trickling filter; Biomass production
1. Introduction
Trickling filters offer well-known and widely
applied technical solutions to treat wastewaters of
different sources and compositions. Their main
purpose is to provide nitrification and removal of
Biochemical Oxygen Demand (BOD) [1]. Treat-
ment capacity is related to the total surface of the
filter medium, providing area for bacterial growth.
As each filter medium has a specific index of
area per volume, the treatment capacity can be indi-
cated as mass removal per volume per time.*Co
esponding author.
Presented at Multi Functions of Wetland Systems, International Conference of Multiple Roles of Wetlands, June 26–29, 2007, Legnaro (Padova) Italy
XXXXXXXXXX/09/$– See front matter © 200 Elsevier B.V. All rights reserved.8
doi:10.1016/j.desal XXXXXXXXXX
Unfortunately, the conventional trickling filte
approach does not recycle nutrients in the waste-
water but merely transforms them into non-toxic
(H2O, NO3) or gaseous forms (CO2, N2) [2].
We studied the possibilities of combining
wastewater treatment in constructed wetlands
with the production of crop plants biomass. The
idea was to use the surface of trickling filters to
grow crop plants, to combine the processes of
nutrient transformation (mainly nitrification) and
nutrient recycling. The original concept was estab-
lished by aquaponic producers [3,4], which
achieved remarkable fish to plant production ratios
in market-scale production systems. For each kilo-
gram of fish produced in feedlot aquaculture, the
nutrients in the resulting wastewater allowed a
vegetable biomass production of 7 kg [5].
Merging the two disciplines, wastewate
treatment and crop production, requires moving
the focus from optimizing the degradation, nitri-
fication, denitrification and absorption rates to
maximizing the recycling rates of phosphorus
and nitrogen and to fulfilling the quality require-
ments of the resulting products such as plant bio-
mass and effluent water.
In contrast to bacterial degradation, nutrient
assimilation by plants is limited by surface, as
photosynthesis is dependent on solar radiation.
Therefore, to achieve maximum nutrient recy-
cling rates, trickling filter systems should pro-
vide a large surface area for plant growth and
photosynthesis in relation to their volume. In
theory, a planted trickling filter could double
its nutrient recycling capacity when constructed
halve as deep and with a surface twice as large.
Given this, possible applications of the concept
would not be in the classic wastewater treatment
disciplines where land use is regarded as loss of
efficiency, but rather in industry sectors already
using large surfaces for plant production. In
other words, the idea is not to replace existing
filter techniques in municipal wastewater treat-
ment but to recruit producers of hydroponic veg-
etables as recipients and users of nutrient-rich
wastewaters. This approach reflects postulations
that suggested to re-integrate agronomic produc-
tion forms nowadays separated in monocultures
to combined production systems [6].
This study describes one possible application
of this concept: planted trickling filters adapted
to provide nitrification in recirculating systems
for fish production. This combination of fish
and plant production in an integrated recirculat-
ing system is called aquaponics [7]. Many differ-
ent system concepts are feasible, depending on
the target crops or resources available [8]. Ou
esearch focused on the selection of crop plants
characterized by high productivity and thus
nutrient recycling capacity. To test the suitabil-
ity of the new combined production system fo
vegetable producers, we compared plant produc-
tivity in aquaponic and in conventional hydro-
ponic systems.
Microbiological contamination of the target
plants is not a problem discussed in aquaponic
literature, as the harvested crops have only con-
tact with the fish water by their root system.
Nevertheless this topic should be addressed, as
heterotrophic plate counts revealed similar con-
centrations in salmonid farm effluent (105 CFU
ml [9]) as in mechanically pre-treated municipal
wastewater (106 CFU/ml [10]). In case hydro-
ponics are used to recycle nutrients from mun-
icipal wastewaters, this topic is of primary
importance. In this case, pre-treatment with
planted sand filters followed by UV radiation
should be considered [11]. This approach was
already tested; lettuce and capsicum indeed are
able to utilize nutrients from municipal waste-
waters [12].
2. Materials and methods
2.1. Aquaponic system
Aquaponic is a special form of recirculating
aquaculture systems (RAS), namely a polyculture
consisting of fish tanks (aquaculture) and plants
A. Graber and R. Junge / Desalination XXXXXXXXXX–156148
that are cultivated in the same water circle (hydro-
ponic) [3–5]. The primary goal of aquaponic is to
euse the nutrients released by fishes to grow crop
plants. Most systems separate fish faeces as
quickly as possible to reduce the BOD load in
the RAS, to enhance nitrification performance
and to reduce clogging of plant roots, which
could lead to loss of crop productivity.
Our aquaponic system established in Wae-
denswil, Zurich, was a new concept using light-
expanded clay aggregate (LECA
TM
) [13] as filte
medium for the trickling filter. LECA is a type
of clay, which is super-fired to create a porous
medium. It is heavy enough to provide secure sup-
port for the plants’ root systems and was used in
indoor and outdoor hydroponic systems [13–17].
LECA was filled in standard boxes (0.4 ďż˝ 0.6 ďż˝
0.4 m, green PVC), aligned in three rows in an
adjoining greenhouse (Fig. 1). In total, 74 boxes
were used, holding 3.0 m3 of LECA.
A primary goal of the system was to test a
completely closed RAS, making use of fish fae-
ces, too. The raw effluent from the 2.5 m3 fish
tank (2 ďż˝ 2 m square, water depth 0.65 m,
green fi
e glass with central outlet at the bot-
tom) was pumped and distributed in the LECA
filter at a rate of 10–15 m3 h�1 with a specially
developed i
igation system. To achieve a homo-
genous load of Total Suspended Solids (TSS)
distribution to the plant boxes, standard sewage
piping (d = 0.11 m, orange PVC) was levelled
horizontally over the LECA rows and each box
i
igated through a drilling hole of 6 cm diame-
ter. The water exchange rate was set to zero,
and the system was operated as a closed looped
system. Only evaporation water was replaced
with tap water. Technical details were described
only in an unpublished report so far [18].
Fish species cultivated were tilapia (Oreo-
chromis niloticus) during aubergine production
in 2004 and eurasian perch (Perca fluviatilis) in
the later experiments with tomato and cucumber.
Tilapia was a natural strain imported from Lake
Turkana, Kenya, perch was obtained from Perci-
tech S.A., a Swiss company specialized in perch
eeding. The fish were fed ad libitum with a
swimming pelleted feed (Trouvit Tilapia Starter,
3.5 mm, 45% raw protein).
As a control, a row of 29 boxes in a hydroponic
system was installed, holding 1.2 m3 LECA
(Fig. 2). Tap water was pumped from a separate
0.3 m3 water reservoir at a rate of 5 m3 h�1, and
LECA filter boxes
Water pump
permanent flow
Water pump
level controlled flow
Outflow from fish basin
Backflow to fish basin
Valves to control flow
Greenhouse 1 Greenhouse 2
Side view
Fig. 1. Aquaponic research unit at Waedenswil, Zurich.
149A. Graber and R. Junge / Desalination XXXXXXXXXX–156
fertilizer [19] was applied two or three times a
week to maintain a fertilizer concentration with
an electrical conductivity of 2.5 mS cm�1. This
salt concentration was known to be the uppe
limit for vegetable growth from previous experi-
ments at the institute [20–22]. Evaporated wate
was replaced continuously, holding the wate
level constant. During tomato trials, a second con-
trol culture was planted in a natural, non-amended
soil in the same greenhouse. Soil cultures were
i
igated every few days with water from the
fish tank, according to the i
igation need of the
tomatoes (Fig. 2).
2.2. Nutrient budgeting
As in the recirculation systems the complete
water volume passed over the trickling filte
once every 20 min, concentration differences
etween in- and outflow were within detection
limits. Thus, samples were taken in the main
water reservoir only (fish tank or water reservoir).
Input and removal rates were calculated through
mass balance, calculated over a specified time
interval, by the addition of nutrient input in the
form of fish fodder, nutrient removal in the form
of fruit and plant biomass harvest, change in the
nutrient reservoir in the water, and nutrient losses
y water exchange.
Normally, nutrient input would be calculated
using the nutrient concentration in the feed, and
nutrient incorporation into fish biomass would
have to be considered as a sink. Instead, nutrient
input was calculated using fertilizer coefficients
of the fish feed, which were determined in an
experiment with two replicates. Tilapia (25
fish weighing 1330 g and 31 fish weighing
1730 g) were placed in a 220-l glass aquaria,
equipped with an unplanted LECA Box as a
nitrification filter. Fish were fed with tilapia
feed and build-up of nutrients (NH4, NO2,
NO3, PO4, K) measured after 14 days.
Aquaponic row 1
Aquaponic row 2
Hydroponic
To fish tank
From
fish
tank
Soil culture row 1
Soil culture row 2
I
igation
with fish wate
every few days
Fig. 2. Plant production in top view with expanded clay boxes