WATER SOURCE MATTERS: ASSESSING RAINWATER USE IN AQUAPONIC PRODUCTION SYSTEMS

Introduction

Aquaponic systems significantly reduce water consumption compared to conventional agriculture due to water recirculation, making them suitable for water-scarce contexts (Barbosa et al., 2015; Love et al., 2015; Maucieri et al., 2018). They also support sustainable urban food production through low land use and nutrient recycling (Benke & Tomkins, 2017; Specht et al., 2014; Geissdoerfer et al., 2017; Klinger & Naylor, 2012). However, these systems still require external water inputs, making water source a critical factor (Tyson et al., 2011). In urban contexts, decentralized solutions such as rainwater harvesting can enhance resilience and reduce reliance on centralized supply systems (Campisano et al., 2017; Fletcher et al., 2015; Pahl-Wostl, 2015). Integrating rainwater harvesting into aquaponics raises challenges related to water chemistry and system stability (Goddek et al., 2019; Tyson et al., 2011). Despite this, existing research has mainly focused on internal system processes, overlooking alternative water sources (Goddek et al., 2019; Maucieri et al., 2018). This study addresses this gap by comparing an aquaponic system supplied with treated rainwater to one using tap water, evaluating biological and environmental performance. The aim is to assess the feasibility of rainwater use and contribute to more sustainable and resilient urban agriculture models.

Materials and Methods

The experimental activity was conducted at the Urban Farming Lab (University of Salento, Lecce, Italy), using two identical recirculating aquaponic systems (RAS) (5000 L each), one supplied with treated rainwater and the other with municipal tap water. Both systems consisted of fully coupled loops including fish tanks, sedimentation units, biofilters and deep-water culture (DWC) plant beds, ensuring continuous recirculation (≈10 turnovers per day). Rainwater was collected, filtered with preformed biopolymer hydrogels, obtained from a chitosan/alginate matrix used in clariflocculation processes and mineralized prior to use. Two hundred and forty Nile tilapia (Oreochromis niloticus) were divided and reared for four months in two 1600 L tanks (120 fish in control tank with tap water, Wi = 79.08 ± 28.64 g , Li = 16.26 ± 2.06 cm, and 120 fish in treatment tank with treated rainwater, Wi = 74.59 ± 28.52 g , Li = 15.98 ± 1.99 cm) under controlled conditions (24 ± 0.8 °C; pH 6-7; DO ≈6.5 mg L^-1), fed with a commercial diet (Veronesi C.F.W. with 35% crude protein) via automated feeders. Lettuce (Lactuca sativa var. capitata) was cultivated for four weeks in DWC systems (25 plants m^-2) under LED lighting (16.000 lux, 12 h photoperiod). Water quality parameters (D.O., pH, nitrogen compounds, ORP, turbidity, TDS) were continuously monitored using automatic sensor-based systems, while fish growth (biometrics and somatic indices) and plant performance (morphological traits, biomass and SPAD index) were periodically assessed.

Results

The comparative analysis showed no significant differences between the rainwater and tap water systems in terms of both fish and plant performance. Fish exhibited similar final weight (Wf = 198.42 ± 66.99 g for control vs 188.30 ± 64.72 g for rainwater), final length (Lf = 21.37 ± 2.51 cm for control vs 20.98 ± 2.58 cm for rainwater), growth rates (specific growth rate SGR ≈ 0.68% day^-1 for both conditions), feed conversion ratios (FCR = 1.65 control vs 1.69 rainwater), and condition factors (K = 1.96 ± 0,18 for control vs 1.96 ± 0,18 for rainwater), with no statistically significant differences in somatic indices or survival. Likewise, four weeks lettuce production showed comparable vertical and radial growths (Figure 1A and 1B), leaf number (Figure 1C), and overall yield, with statistical tests confirming the absence of significant differences between treatments. Minor variations were observed in plant morphology, with rainwater slightly enhancing stem diameter and tap water promoting leaf number and height. Early differences in chlorophyll content (SPAD) converged over time (Figure 1D). Sensory analysis with 15 volunteer participants also revealed no detectable differences in lettuce quality (Figure 2). Overall, the results demonstrate that treated rainwater can effectively replace conventional water sources in aquaponic systems without compromising productivity, stability, or product quality.

Figure 1 - graphs for A) vertical growth, B) radial growth, C) number of leaves, D) SPAD Index

Figure 2 - Spider plot of the results obtained after the sensory analysis of aquaponic salad grown with tap water (blue) and aquaponic salad grown with rainwater.

Acknowledgment

We would like to thank all the individuals who made possible the start-up and ensured the proper management of the aquaponics system in the Urban Farming Laboratory, thereby contributing significantly to the success of this study. This research was supported by the REAP project (Rainwater Exploitation for urban Aquaponic Production), funded by the PNRR, missione 4, "istruzione e ricerca" - componente 2, "dalla ricerca all'impresa" - linea di investimento 1.4, finanziato dall'unione europea – nextgenerationeu", progetto "agritech - National Research Centre for Agricultural Technologies", Codice Progetto CN00000022, CUP H93C2200044007.

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