water Article Designing Aquaponic Production Systems towards Integration into Greenhouse Farming Ragnar Ingi Danner 1,2, Utra Mankasingh 1, Kesara Anamthawat-Jonsson 1 and Ragnheidur Inga Thorarinsdottir 2,3,* 1 Faculty of Life and Environmental Sciences, University of Iceland, Sturlugata 7, 101 Reykjavik, Iceland; [email protected] (R.I.D.); [email protected] (U.M.); [email protected] (K.A.-J.) 2 Svinna-verkfraedi Ltd., Fannafold 61, 112 Reykjavik, Iceland 3 Faculty of Civil and Environmental Engineering, University of Iceland, Taeknigardur, Dunhagi 5, 107 Reykjavik, Iceland * Correspondence: [email protected]; Tel.: +354-8964830 Received: 31 July 2019; Accepted: 9 October 2019; Published: 13 October 2019 Abstract: Aquaponics is a sustainable method of food production, whereby aquaculture and hydroponics are combined in one circular system. A few aquaponics startup companies are emerging in Europe with a limited production area of a few hundred or a few thousand square meters, whereas hydroponics is a common practice in a commercially viable manner most often with production units of several hectares. In Iceland, greenhouse farmers operate on relatively small production units, often between 2000 and 5000 m2. The aim of the present study was, therefore, to develop and design aquaponic production systems towards integration into small greenhouse farming strengthening economic viability and sustainability. Since the local market in Iceland is small and import is relatively expensive due to the distance from other markets, the suitability of commercially available fish feed and the selection of plant species were assessed in relation to production efficiency and available market and resources. The effects of water flow on plant growth and on nutrient utilization in culture water were measured and evaluated. Four aquaponics test systems were designed, built and operated, and results were used to develop a pilot commercial aquaponics system implemented for greenhouse farming in Iceland. One of the test systems was a media filled flood and drain system and the other three were deep water culture systems. Tilapia (Oreochromis niloticus), one of the most popular fish in aquaculture, was reared in all systems, while different leafy greens and fruiting vegetables were grown in the hydroponics. The fish was fed with commercial aquaculture feed made for cod and charr. The feed conversion ratio (FCR) was used to assess the effectiveness of feed on fish growth. The FCR observed in this research was between 0.9 and 1.2, within the typical values for tilapia growth in aquaculture. The production of the leafy green plants (e.g., pak-choi) was approximately four times, by weight, that of the production of fish, a similar yield as shown in other researches in the field. The continuous rise of nitrate and phosphate concentrations in the aquaponic system indicated the potential to support even higher crop yield. Long daylength in the summer in Iceland is clearly beneficial for crop production in aquaponics. Based on the results, it is concluded that aquaponics can be a feasible opportunity for greenhouse farming at least to diversify the current business model. Not only can the fish provide an extra income but also the effluent from the aquaculture is easily used as fertilizer for the plants, thus the circular production system offers new innovative ideas for diversifying and value-adding the business further, for example into crayfish production and/or into educational and experience tourism. Keywords: aquaponics; nutrients; horticulture; hydroponics; RAS (recirculating aquaculture system); FCR (feed conversion ratio) Water 2019, 11, 2123; doi:10.3390/w11102123 www.mdpi.com/journal/water Water 2019, 11, 2123 2 of 21 Water 2019, 10, x FOR PEER REVIEW 2 of 21 1.1. Introduction Introduction DwindlingDwindling natural natural resources resources are are forcing forcing us us to to look look into into more more sustainable sustainable ways ways of of living. living. Aquaponics isis a developinga developing sustainable sustainable food productionfood production method couplingmethod aquaculturecoupling aquaculture and horticulture and horticulturetogether in onetogether circular in systemone circular mimicking system nutrient mimicking and water nutrient cycles and in water nature. cycles The methodologyin nature. The of methodologyaquaponics has of gained aquaponics increased has interestgained inincreased recent years interest and thein numberrecent years of aquaponic and the practitionersnumber of aquaponichas increased practitioners greatly since has 2007 increased [1]. greatly since 2007 [1]. AquaponicsAquaponics isis aa combination combination of of recirculating recirculating aquaculture aquaculture systems systems (RAS) (RAS) and hydroponics,and hydroponics, which whichis growing is growing plants withoutplants without soil in circulating soil in circulating water containing water containing nutrients [nutrients2]. A typical [2]. aquaponicA typical aquaponicsystem (Figure system1) consists(Figure of1) aconsists fish rearing of a fish tank, rearing a solids tank, removal a solids tank, removal a biofilter, tank, a a hydroponic biofilter, a hydroponicunit, and optionally unit, and aoptionally sump tank a [sump3]. In tank RAS, [3]. it isIn important RAS, it is toimportant maintain to good maintain water good quality water for qualitythe fish for health the fish by removing health by solidremoving waste solid and waste dissolved and nutrientsdissolved that nutrients would that become would toxic become to the toxic fish toat the high fish concentrations. at high concentrations. The release The of release large of amounts large amounts of nutrients of nutrients into the into environment the environment can cause can causeeutrophication, eutrophication, leading leading to imbalances to imbalances in the in ecosystem the ecosystem such assuch algal as algal bloom bloom and changes and changes in the in local the localfauna fauna [4]. Conversely, [4]. Conversely, in hydroponics, in hydroponics, it is beneficial it is beneficial to keep to highkeep nutrienthigh nutrient concentrations concentrations for good for goodgrowth growth and health and ofhealth the plants. of the Combining plants. Combining these two productionthese two systemsproduction thus systems reduces nutrientthus reduces waste nutrientthrough waste reuse, through making thereuse, food making production the food system production more sustainable. system more sustainable. FigureFigure 1. AA schematic schematic representation representation of of an an aquaponic system. The The arrows arrows indicate indicate the the water water flow flow throughthrough the the system. system. TheThe water water flowing flowing from from the the fish fish rearing rearing tank is is first first mechanically filtered filtered to remove suspended solidssolids that will adhere toto plantplant rootsroots and and may may a affectffect rhizosphere rhizosphere health. health. Accumulated Accumulated solids solids can can lead lead to todepleted depleted dissolved dissolved oxygen oxygen in thein the water, water, due due to a to high a high biological- biological- and chemicaland chemical oxygen oxygen demand demand (BOD (BODand COD) and andCOD) elevated and elevated ammonia ammonia levels [5]. levels Organic [5]. particles Organic can particles also lead can to reduced also lead eff ectivenessto reduced of effectivenessthe biofilter [6 of]. Afterthe biofilter removal, [6]. the After solids removal, can be mineralizedthe solids can to be release mineralized nutrients to and release then nutrients re-introduced and theninto there-introduced aquaponic system,into the whereaquaponic the released system, nutrientswhere the can released be of further nutrients use can for be the of plant further growth use for [7]. the plantAfter growth solids [7]. removal, the water is passed through biomedia (biofilter), whereby the biological conversionAfter solids takes placeremoval, via athe two-step water bacterialis passed nitrification through biomedia of the ammonia (biofilter), in the whereby fish waste. the biological In the first conversionstep, ammonia takes is convertedplace via a to two-step nitrite, and bacterial in the secondnitrification step, nitriteof the isammonia converted in tothe plant-usable fish waste. nitrate.In the firstThe nitrate-enrichedstep, ammonia is water converted then flowsto nitrite, to the and hydroponic in the second unit, step, where nitrite the plants is converted remove to nutrients plant-usable from nitrate.the water The by nitrate-enriched utilizing them for water growth. then The flows main to types the hydroponic of hydroponic unit, setups where are the deep plants water remove culture nutrients(DWC), nutrient from the film water technique by utilizing (NFT), them and for flood-and-drain growth. The main (F&D) types systems, of hydroponic and these setups differ mostlyare deep in waterthe method culture of (DWC), irrigation nutrient [7]. film technique (NFT), and flood-and-drain (F&D) systems, and these differ mostly in the method of irrigation [7]. Water 2019, 11, 2123 3 of 21 The success of recirculating aquaponic systems relies on a healthy environment for both fish and plants, by securing good aeration, a suitable pH in the system, efficient biological conversion of harmful ammonia to beneficial nitrate and the plant uptake of dissolved nutrients. Plants utilize nitrogen mostly + as nitrate (NO3−) or ammonium (NH4 ), but the total ammonium nitrogen (TAN) in the fish waste + consists of two forms, i.e., ammonia (NH3) and ammonium (NH4