Introduction
Aquaponics is a non-traditional approach to producing both fish and crops, integrating aquaculture with hydroponic systems. It can be implemented in areas with rocky, saline, or non-arable soils, without the need for fertilizers or chemical pesticides. This method helps to reduce land use and degradation, eases pressure on water supplies, and lowers the environmental footprint of agriculture [1].
Urban wastewater is increasingly being utilized in agriculture due to growing water scarcity and stress, alongside rising populations, greater volumes of wastewater, and pollution incidents. Urban environments should explore new opportunities for repurposing wastewater, channeling more of it inland for productive uses—not just in agriculture, but also in aquaculture and aquaponics [2,3]. For instance, reclaimed water can lessen aquaculture’s dependence on freshwater sources, enabling it to be located near urban wastewater treatment facilities. This would reduce production costs and provide communities with fresh, protein-rich food [4]. However, limited public acceptance and the absence of clear regulations remain major challenges [5].
The main objective of this work is to demonstrate (i) the efficiency of the advance tertiary wastewater treatment to produce acceptable water quality for aquaponics reuse; and (i) thatthe yields of plant and fish in tap water-fed aquaponic systems are comparable to those obtained with reclaimed water from urban wastewater treatment plants. Within the framework of the project Aquaponics from WAstewater REclamation (AWARE) project, GA No. 101084245, a small-scale experimental aquaponic system has been integrated with a pilot urban wastewater treatment facility at URJC in Madrid, Spain. Wastewater undergoes advanced tertiary treatment specifically adapted to feed the aquaponic system, which is used to grow lettuce and tilapia. The water quality of the aquaponic farm using reclaimed urban wastewater has been assessed.
Materials and Methods
An advanced tertiary water treatment process was applied to further purify the effluent from a wastewater treatment plant (WWTP). This treatment system included sequential modules of Ultrafiltration (UF), electrooxidation (EO), and UVC-LED irradiation, using a total of 60 UVC-LEDs. The UF membrane, made of Al₂O₃, had a pore size of 100 nm and dimensions of 0.23 m² by 1.2 m. In the EO module, the anode was composed of titanium with mixed metal oxides (Ti/MMO), while the cathode was titanium. Operating parameters included a surface area of 270 cm², a current of 5.2 A, voltage between 10–15 V, and energy input of 78.85 J. The UVC-LEDs emitted radiation within a 268–280 nm range, peaking at 275 nm, with a total energy output of 102.99 J. The system operated at a flow rate of 200 L/h.
The study involved a 1 m³ aquaponic system using reclaimed urban wastewater, stocked with 50 Nile tilapia (Oreochromis niloticus L.) at a biomass density of 5.7 kg per system, and 72 lettuce plants (Lactuca sativa, Batavian Red variety), cultivated across 22 five-week crop cycles. The experiment was carried out over 19 weeks in a small-scale, closed-loop, recirculating aquaculture system (RAS). A control setup using dechlorinated tap water served as the baseline for comparison.
The measurements analyzed involved physico-chemical such as nitrogen and phosphorus species, organic carbon, turbidity, total suspended solids (TSS) apart from temperature, oxygen, electrical conductivity, pH. Microbiological parameters were quantified by serial dilution and plate count method and plate count not only in water, but also in lettuce and fish.
Results
Compounds which are toxic to fish in aquaponics are nitrite (NO2-) and ammonia (NH4+). During the aquaponic cycle with reclaimed water, levels below the toxicity threshold were achieved. The difference between both cycles was significant in terms of some nutrients such as total phosphorus, reaching over 5 ppm in the reclaimed water source, compared to levels below 0.5 ppm in the tap water source. This is probably because reclaimed water contains more nutrients than dechlorinated tap water. Since nutrients are beneficial to the hydroponic system, this increase may increase the produce yields.
Regarding parameters related to organic matter, similar values have been observed in terms of Chemical Oxygen Demand (COD) (10–40 ppm mg O₂/L) and turbidity (<0.5 NTU) in both cycles.
Regarding bacterial load in water within the aquaponic system, total aerobic bacterial growth was slightly higher in the reclaimed water cycle, as this type of water initially contained a certain microbial load. However, levels gradually stabilized over time, reaching values comparable to those observed in the tap water production cycle. This is a highly positive indicator of the water quality within the system. Notably, no significant pathogens such as Escherichia coli ,Salmonella , Clostridium perfringens, MS2 coliphage, Enterococcus were detected in any of the water sources. On the other hand, during the first weeks of the cycle, reclaimed water has fewer coliform bacteria compared to the aquaponic cycle with tap water. However, over the weeks, coliform levels in both cycles become similar.
Conclusions
The physicochemical and microbiological parameters analyzed do not indicate that feeding reclaimed water into an aquaponic system affects the yields of plants and fish within the system. Key parameters related to organic matter suggest that organic content in the medium is limited, thus not depleting the available dissolved oxygen and preventing the proliferation of anaerobic bacteria that produce undesirable compounds such as hydrogen sulfide, methane, or ammonia. It has been demonstrated that the introduction of reclaimed water into an aquaponic system does not result in an increase in bacterial load, nor is this reflected in the bacterial populations found in the animal and plant species within the system. No significant pathogenic species, as previously mentioned, were detected. Meanwhile, levels of aerobic bacteria and coliform bacteria were found to be comparable across both water sources.
Literature
[1] Greenfeld, A., Becker, N., McIlwain, J., Fotedar, R., & Bornman, J. F. (2019). Economically viable aquaponics? Identifying the gap between potential and current uncertainties. Reviews in Aquaculture, 11(3), 848-862. doi:10.1111/raq.12269
[2] Urban Aquaculture. 2005. Eds. 10.1079/9780851998299.0001
[3] Stuart W. Bunting, Confronting the realities of wastewater aquaculture in peri-urban Kolkata with bioeconomic modelling, Water Research, Volume 41, Issue 2, 2007, 499-505.
[4] Cifuentes-Torres Liliana, Correa-Reyes Gabriel, Mendoza-Espinosa Leopoldo G. Can Reclaimed Water Be Used for Sustainable Food Production in Aquaponics? Frontiers in Plant Science, 12 2021, 981
[5] Official Journal of the European Union, 2020 “REGULATION (EU) 2020/741 OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 25 May 2020 on minimum requirements for water reuse”.