Living walls
Living walls are often used in architecture to provide aesthetic, ecological and environmental benefits in urban areas. The modular panels, comprised of polypropylene plastic containers or geotextile mats, support plants which provide benefits not only in visual terms, but also with regards to amenity, biodiversity, thermal efficiency and amelioration of air pollutants, all for a very small ground level footprint (Manso & Castro-Gomes 2015; Perini et al. 2013).
Two universities have been investigating the potential for living walls for growing edible crops using aquaponics. A series of experiments were conducted at the University of Greenwich, UK, to identify the most suitable type of system, and the best growing medium (Khandaker & Kotzen 2018). The first experiment used a Terapia Urbana Fytotextile living wall panel. This semi-hydroponic modular panel system is made from a patented geotextile fabric composed of three layers of synthetic and organic material including PVC, Fytotextile and Polyamide. Each square metre holds up to 49 plants in individual pockets. Depending on the vegetable species grown, approximately 98 plants/m2 can therefore be grown using back-to-back elements of this living wall system, compared with 20-25 leafy greens per square metre in a horizontal system. The felt panel was attached to an east-facing external wall, and planted with seven different plants (spinach, basil, chicory, asparagus pea, lettuce, mint and tomato) in seven different growing media (horticultural-grade mineral wool, vermiculite, charcoal, coconut fibre, sphagnum moss, pond grown algae, and straw). Each plant species was arranged vertically in columns, and the growing medium was arranged horizontally in rows (Figure 18). Water was pumped up to an internal drip irrigation pipe from a surrogate aquaponics tank with added hydroponic nutrients. The water then flowed down the back of the panel where it was made available to the substrate and the plant roots. Excess water dripped from the bottom of the living wall panel into a gutter and then back to the water tank (Khandaker & Kotzen 2018).
Figure 18: The Terapia Urbana living wall (Photos: M. Khandaker)
The results of the first experiment showed that mineral wool and vermiculite were the best substrates, resulting in greater yield and better root growth. The plants located at the top and along the sides performed best, which suggests that overshadowing was an issue for the plants in the middle of the wall. However, the main problem with this type of living wall was that the plant roots grew into the geotextile, which made harvesting difficult. If one were to grow cut-and-come-again varieties, this would not be an issue (Khandaker & Kotzen 2018).
The second experiment was set up adjacent to Experiment 1 using the Green Vertical Garden Company (GVGC) pot system. The individual plant pots were attached to a stainless steel reinforcing mesh panel, with five horizontal rows and eight vertical columns of pots. Only one plant (basil) was used across the whole living wall, with different growing media used in the vertical columns (two columns each of hydroleca, vermiculite, horticultural-grade mineral wool and coconut fibre) (Figure 19). The system was irrigated using an irrigation pipe to supply nutrient-rich water to the top row of pots and the water then flowed through each pot to the one below via a small irrigation tube from a hole located at the bottom of each pot. The third experiment used the GVGC system and one plant (chicory) planted in two columns each of hydroleca, vermiculite, horticultural-grade mineral wool and coconut fibre (Khandaker & Kotzen 2018).
In the second and third experiments, the basil and chicory performed best in the coconut fibre and mineral wool. There are advantages and disadvantages to using both of these substrates. While coconut fibre and the roots within can be readily composted, blockages can occur if it is used in a system with small irrigation pipes. Horticultural-grade mineral wool performs well, but it cannot be readily recycled and thus is likely to be considered to be less sustainable. Hydroleca and vermiculite were more difficult to work with, as the material was easily displaced at planting and at harvest. Again, overshadowing caused plants in the middle of the wall to grow less well (Khandaker & Kotzen 2018).
Figure 19: The Green Vertical Garden Company living wall Photos: M. Khandaker
Researchers at the University of Seville, Spain, have compared the performance of a felt pocket living wall system with small-scale NFT and DWC systems for growing lettuces and goldfish in a greenhouse (Peréz-Urrestarazu et al. 2019). The living wall system is composed of two layers, the outer one made of a porous material to favour the aeration of the roots, and the inner one of geotextile which helps to distribute the water. The panel was angled at 20° with respect to the vertical plane. The planting pockets were filled with expanded clay in order to favour a better aeration of the root zone, given that the felt was intended to be receiving water at all times. Although the living wall has a maximum capacity of 20 plants/m2, not all the pockets were used in order to have an equivalent planting density to the other two systems. In terms of plant productivity, the living wall had the worst performance of the three systems. Part of this may have been due to a lower radiation influx due to the vertical nature of the growing space, even though it had a slight slope. While water was distributed through the felt, the evaporation rate was high, and the expanded clay inside the pockets did not receive enough water and nutrients, due to the slope; a substrate with a greater capillary action, such as perlite, might have helped to alleviate this issue. Another problem was the growth of algae on the felt, caused by the humid environment and the high levels of nutrients and light. This caused competition with the crop resulting in higher water consumption, caused obstructions in the irrigation emitters, and resulted in more hours being required for maintaining the system. In terms of fish production, on the other hand, the living wall system outperformed the NFT and DWC systems. This is most likely because the water had to be replenished more frequently due to the high rate of evaporation, resulting in better water quality (Peréz-Urrestarazu et al. 2019).
The results of the experimental studies by Khandaker & Kotzen 2018 and Peréz-Urrestarazu et al. 2019 suggest that geotextile living walls may not be the most suitable sort of system to use for vertical aquaponics, despite the potentially high number of plants that can be grown in them in ratio to occupied floor area, due to the problems encountered with algae growth, uneven biomass and yield, and difficulties with harvesting the plants. In addition, it is important to bear in mind that most geotextiles consist of a polymer from the polyolefin, polyester or polyamide family, and additives to improve their stability. Over time and under various conditions the polymer may degrade into microplastic particles, which could be ingested by the fish. Generally, a higher ambient temperature accelerates the degradation rate, and different degradation mechanisms may act in synergy. Leaching of additives is also likely when micro-sized plastic particles have been formed, and may even occur from non-degraded material, as the additives are often not covalently bound to the polymer backbone (Vé Wiewel & Lamoree 2016). The ecotoxicology of a geotextile living wall therefore should be tested before it is used with an aquaponic system. A geotextile made from biopolymers constructed from natural fibres, such as jute and coir, would be more suitable than a synthetic geotextile. Other types of living wall might also be suitable, such as the hydroponic system produced by Biotecture, which consists of rigid plastic panels filled with horticultural-grade rockwool.
Copyright © Partners of the Aqu@teach Project. Aqu@teach is an Erasmus+ Strategic Partnership in Higher Education (2017-2020) led by the University of Greenwich, in collaboration with the Zurich University of Applied Sciences (Switzerland), the Technical University of Madrid (Spain), the University of Ljubljana and the Biotechnical Centre Naklo (Slovenia).