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Aquaponic Systems: Nutrient recycling from fish wastewater by vegetable production Desalination 246 (2009) 147–156 Aquaponic Systems: Nutrient recycling from fish wastewater by vegetable production Andreas Graber, Ranka Junge* ZHAW Zurich University of Applied Sciences, Institute for Natural Resource Sciences Gruental, CH-8820 Waedenswil, Switzerland Tel:+41589345928; Fax: +41589345940; email: ranka.junge@zhaw.ch 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 rates 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 removal 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 reserved for rural areas with litlle space limitation; instead, the additionally occupied surface generates income by 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.*Corresponding author. Presented at Multi Functions of Wetland Systems, International Conference of Multiple Roles of Wetlands, June 26–29, 2007, Legnaro (Padova) Italy 0011-9164/09/$– See front matter © 200 Elsevier B.V. All rights reserved.8 doi:10.1016/j.desal.2008.03.048 Unfortunately, the conventional trickling filter 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, wastewater 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]. Our research 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 for 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 246 (2009) 147–156148 that are cultivated in the same water circle (hydro- ponic) [3–5]. The primary goal of aquaponic is to reuse 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 filter 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 � 0.6 � 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 � 2 m square, water depth 0.65 m, green fibre 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 irrigation 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 irrigated 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 breeding. 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 246 (2009) 147–156 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 upper limit for vegetable growth from previous experi- ments at the institute [20–22]. Evaporated water was replaced continuously, holding the water 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 irrigated every few days with water from the fish tank, according to the irrigation need of the tomatoes (Fig. 2). 2.2. Nutrient budgeting As in the recirculation systems the complete water volume passed over the trickling filter once every 20 min, concentration differences between 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 by 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 Irrigation with fish water every few days Fig. 2. Plant production in top view with expanded clay boxes