Integrating membrane processes into hydroponics systems to promote plant growth, recover added-value root exudates and recycle nutrients

Hydroponics are controlled soilless agricultural systems that enable crops to be grown out of season on land otherwise poorly suited for crop production. In 2015, hydroponic farming was estimated to be worth $21.4 billion, with an expected annual growth of 7%. Hydroponic farms have several advantages over traditional farming, including 3 to 10 times more plant production per unit space, and up to 90% more efficient use of water in well-managed farms. Many horticultural crops are routinely grown in commercial vertical hydroponic farms because of the high quality and yields these systems provide. However, plants in hydroponic culture exude high amounts of phytochemicals into the nutrient solution. Continuous recycling of nutrient solutions in closed hydroponic systems causes these phytochemicals to accumulate, leading to autotoxicity. Replacing the nutrient solution is typical, but is costly, labour-intensive, inefficient and causes system downtime.

In contrast, phytochemicals extracted from plant wastes are increasingly finding a range of technological applications, offering additional revenue within a circular economy. Plants exude many metabolites from their roots, such as polyphenols, which have antioxidant properties that promote human health, along with molecules that have roles in regulating plant growth and development, and in plant-microbe interactions. Root exudates are therefore a potential source of novel activities for use as plant biostimulants or plant protection products.

This project seeks to use hydroponic cultivation of pea shoots as a model system to solve autotoxicity problems and allow nutrient recycling, whilst simultaneously exploiting efficient membrane separation to recover organic molecules from root exudates and evaluate their properties. To achieve this, two parallel approaches will be followed to minimise the negative effects of phytotoxic exudates. First, we will seek to optimise the growth environment (recirculation flow, temperature, etc.) to understand how hydroponic culture conditions influence the production of phytotoxins. Secondly, we will try to establish a semi-pilot scale membrane filtration process within a hydroponic system to continuously remove exudates. Since root exudates may contain valuable compounds (e.g. in human/animal nutrition) or can be screened for novel activities (e.g. as plant biostimulants or antimicrobial agents), such integrated filtration provides additional opportunities to exploit the fractionated phytochemicals.

The proposal is multidisciplinary and involves groups of various complementary backgrounds. In particular, the project involves chemical/bio-process engineering (nutrient composition and/or flow rates to facilitate the production and recovery of exudates), membrane science (use of appropriate membranes), analytical chemistry (use appropriate methodologies to characterise the composition of the exudates), and plant physiology (assessing plant growth and in-vitro and in-vivo bioassays to identify novel applications of exudates). If successful, this innovative project could revolutionise hydroponic culture systems. Our results will provide evidence for the technological feasibility of using merged systems for future soilless plant growth and chemical-producing farms. When developed further, our ideas will contribute towards establishing next generation biorefinery principles, able to isolate valuable chemicals from the plant root system while producing more crop biomass.

In summary, we propose a highly innovative, but relatively simple, chemical-free and scalable process to stimulate the production and recovery of compounds from hydroponic exudates. This will maximize plant growth and resolve an existing commercial problem of autotoxicity in such systems, whilst simultaneously introducing the potential for new revenue routes for hydroponic farming.