Similar to mineral wool, pumice is also a neutral material containing less than 1% organic matter. Thus, it showed minimal sorption affinity to most of the studied compounds during the initial contact time. Caffeine is a notable exception. We speculate that caffeine’s low molecular mass allowed high mass transfer into pumice’s large number of pores. Some limitations to this study are apparent, and limit a complete mechanistic interpretation due to lack of statistical significance in some kinetic and sorption equilibrium data. In order to obtain mass balances and insights in reversibility of ad- and desorption, we suggest to identify suitable elution agents for these novel substrata to conduct desorption experiments. Furthermore, this study did not specifically investigate the effect of phosphate buffer on sorption. Previous studies report that phosphate sorption is inhibited on adsorption sites due to surface complexation with metals and repulsion of phosphate ions by negatively charged organic matter . However, specific adsorption of phosphate could also occur by ligand exchange with surface hydroxyl groups. Also, cation bridges may also increase phosphate sorption sites and may result in competitive interaction for sorption sites between phosphate and micro-pollutants . Further study is required to get more insights into this.Organic substrata wood fibre and coconut fibre sorbed micro-pollutants more than the inorganic substrata mineral wool and pumice .
A similar order of sorption affinity for trimethoprim, carbamazepine and sulfamethoxazole was found for soils by Kodešová et al. . Ibuprofen and naproxen, containing -COOH groups, and sulfamethoxazole, containing -SO2-NH moieties, hydroponic dutch buckets have acidic protons which can dissociate and form anionic species . Subsequently, a strong repulsion occurs between these anionic species and negatively charged surface of the organic substrata. Although sulfamethoxazole has a basic NH2 group , it has acidic protons due to the presence of an acidic -NHand an electron-withdrawing -SO2- in the vicinity. The presence of two basic NH2 groups in trimethoprim explains its highest sorption affinity. Carbamazepine has one amido group and the basicity of an amido group is lower than of an amino group. This could explain the sorption order: trimethoprim is followed by carbamazepine. Caffeine has a lone pair of electrons at the non-methylated N site . Therefore, caffeine acts as a proton-acceptor, basic, so it is positively charged and attracted to the negatively charged surface.The hydrophobicity positively affects the sorption . Therefore, carbamazepine is followed by caffeine in the sorption order. The sorption of micro-pollutants on organic rich materials appears to be a trade-off between electrostatic interaction and hydrophobic interactions between the organic matrices and the micro-pollutants.
This is further depicted in a four quadrant matrix . In the high pKa and high Log Kow quadrant, both electrostatic interactions and hydrophobicity positively affect the sorption, as indicated by high KF values for carbamazepine on the organic substrata.For the low pKa and high Log Kow quadrant, the positive effect of hydrophobicity of iburprofen and naproxen appears to be largely counteracted by strong electrostatic repulsion due to deprotonation . At the quadrant of high pKa and low Log Kow, an intermediate sorption on the organic substrata was observed for caffeine and sulfamethoxazole. In this study, no compounds falling into the low pKa and low Log Kow quadrant were investigated. We speculate based on our results, that even lower sorption is to be expected for compounds in this quadrant on our selected substrata. Inorganic substrata mineral wool and pumice generally exhibited much lower sorption and slower sorption kinetics towards the studied micro-pollutants. Mineral wool is a non-reactive neutral material. Therefore, unlike wood fibre and coconut fibre, mineral wool has no extensive net negative surface charge. Furthermore, mineral wool has the lowest surface area among all the substrata . These properties together explain the low sorption of the micro-pollutants. Similar to mineral wool, pumice is also a neutral material containing less than 1% organic matter. Thus, it showed minimal sorption affinity to most of the studied compounds during the initial contact time. Caffeine is a notable exception. We speculate that caffeine’s low molecular mass allowed high mass transfer into pumice’s large number of pores.
Some limitations to this study are apparent, and limit a complete mechanistic interpretation due to lack of statistical significance in some kinetic and sorption equilibrium data. In order to obtain mass balances and insights in reversibility of ad- and desorption, we suggest to identify suitable elution agents for these novel substrata to conduct desorption experiments. Furthermore, this study did not specifically investigate the effect of phosphate buffer on sorption. Previous studies report that phosphate sorption is inhibited on adsorption sites due to surface complexation with metals and repulsion of phosphate ions by negatively charged organic matter . However, specific adsorption of phosphate could also occur by ligand exchange with surface hydroxyl groups. Also, cation bridges may also increase phosphate sorption sites and may result in competitive interaction for sorption sites between phosphate and micro-pollutants . Further study is required to get more insights into this.Selection of suitable substrata is important when designing CWs like systems. Substrata with a high sorption capacity efficiently remove a variety of pollutants, including micro-pollutants. Generally, higher Kd values for the six selected compounds were obtained on the organic substrata . These values were compared with the available literature data for natural sorbents, like soil . Kd values of wood fibre and coconut fibre for the selected compounds were mostly 5-20 times higher than that of soil. High Kd values suggest a larger partitioning of the micro-pollutants onto the organic matter. When compounds can accumulate on- and into the substrata, they would have a strongly retarded mobility in the solid-liquid matrix of a filter bed .
This characteristic of the organic substrata illustrates the suitability of organic hydroponic substrata for use in wastewater treatment technologies. For example, CWs could be amended with such a substrata filter to mitigate the emissions of micro-pollutants to the environment. Wood fibre is particularly suited for application in CWs. To illustrate this, we compared two generic CWs with either wood fibre or soil as a filter bed and determined the theoretical removal of carbamazepine and trimethoprim from wastewater . These compounds were selected considering carbamazepine’s recalcitrance in conventional wastewater treatment plants and presence of trimethoprim residues in wastewater may lead to the development of antibiotic resistance,bato bucket affecting human health and environment. Based on the calculations, hydroponic substrata like wood fibre have a great potential to remove micro-pollutants by sorption, which could reduce the size of a CW. A wetland constructed with wood fibre requires significantly less surface area to achieve the same micropollutant removal as a wetland using soil. A wood fibre wetland requires only 35% and 66% of the surface area of a traditional CW to remove trimethoprim and carbamazepine respectively from the wastewater . It is important to mention that under natural conditions, there are multiple compounds competing for available sorption sites on a substratum. Also, additional removal mechanisms, like biodegradation and phytodegradation would play a role in the removal of these compounds. Therefore, it is recommended to study the sorption behaviour of the substrata and micro-pollutants at trace concentrations taking into account the natural conditions that may restrict sorption, and other additional removal mechanisms that may support micro-pollutant removal. Additionally, to optimize the treatment performance of the envisioned CWs, further studies on the operational parameters, for example, hydraulic loading rate, hydraulic retention time and plant types, are recommended .Globally, current food consumption and trade are placing unprecedented demands on agricultural systems and increasing the pressure on natural resources. This circumstance requires trade-offs between food security and environmental impacts, especially given the tension between market-driven agriculture and agro-ecological goals .
Intensive greenhouse horticulture has spread in response to the year round demand for fresh agricultural food products. The total area of greenhouse cultivation in the EU increased at a rate of 4.5 % between 2005 and 2013, reaching 210,000 ha in 2018 . Inthe Mediterranean region, greenhouse cultivation constitutes the most productive form of primary agricultural production, where a total area of about 120,000 ha was registered in 2016 . For several decades, the greenhouses in Almería region have been the main driver for its socio-economic and demographic development. Annually, its 31,000 ha of greenhouses, which represent 14.7 % of the total covered surface in the EU, produce more than 3 million tons of fruits and vegetables , generating a value of almost €2 billion . This fruitful business is currently facing significant challenges in terms of sustainability. This is partly related to the new environmental demands of customers and markets, but the main issue is the fragility of the natural resources, especially fresh water . Additionally, uncontrolled underground water withdrawal and non-efficient fertigation plans have caused a decrease in water resources quantity and quality. The water quality deterioration is mainly caused by in-depth filtration of fertilizers and phytosanitary products . In order to address the problem of water quality deterioration and overexploitation, research has mainly put the focus on the development and implementation of management practices aimed at reducing nitrate leaching and regulating water withdrawals from the aquifer. For instance, Gallardo et al. and Granados et al. reported a reduction in nitrate discharge to the environment through the accurate management of the leaching fraction and nitrogen supplies in horticultural crops. Later, Mag´ an et al. supported that finding by showing that with correct management of nitrogen and irrigation, nitrate leaching was reduced by 63 % compared to conventional management. In addition, research has shown desalinated seawater as a viable option for irrigating fruits and vegetables in greenhouses that can help to alleviate aquifer over-exploitation.
Facing these issues requires a good insight into the behaviour of agricultural systems and how their inputs and outputs may impact the environment . Accordingly, farmers and managers of agri-food businesses must understand the impacts’ sources and how to deal with them in order to optimise the production systems . In this sense, Life Cycle Assessment is the reference method to quantitatively evaluate the environmental impacts across the entire supply chain. It enables the comparison of different production systems in terms of resource-use efficiency and environmental effects. For agri-food systems, LCA is also increasingly being used to evaluate and analyse food security issues . The ISO standards provide guidelines to perform LCA studies so as to guarantee their objectivity. Several studies using LCA in agriculture have evaluated the environmental impacts of greenhouse tomato cultivation. Some examples are: the evaluation of the use of different nitrogen fertilizers in hydroponic and soil production systems with open and closed loop systems , the comparison of greenhouses, screenhouses and plastic tunnels for cherry tomato production ,the irrigation with brackish desalination water versus conventional water ,the footprint estimation of increasing nitrogen doses for hydroponic tomato and the environmental assessment of rooftop greenhouse tomato with water-harvesting systems . Nevertheless, none have yet applied LCA to quantify the environmental impact of recycling effluents using reverse osmosis powered by photovoltaic solar energy.
The aims of this study are as follows: Firstly, to quantify the potential environmental benefits of the implementation of an on-farm solar-powered drainage treatment plant to recycle effluents from hydroponic tomato versus the traditional soil cultivation; Secondly, to explore, from an environmental perspective, the pros and cons of the use of desalinated seawater as an alternative to locally available brackish groundwater; and Finally, to carry out a sensitivity analysis to assess the potential environmental benefits of increasing the use of renewable energy for desalinated water production and curbing the common over-fertilisation malpractice reported in the study area. Two cultivation systems were used: i) conventional soil: sanded soil called ‘enarenado’, commonly used by farmers. This soil is a mix of clay, manure and sand arranged on top of the original soil base ; and ii) hydroponic system: coconut fibre substrate in bags. The most common fertigation management for both cultivation systems by farmers in Almeria is a free-draining open system . However, in our experimental design, the effluent of the HS was collected and driven to a tank. Once the tank was full, the effluent was then treated in an on-farm reverse osmosis system plant fully powered by photovoltaic solar panels .