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

Lead Research Organisation: Lancaster University
Department Name: Engineering


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.
Description This work was able to provide systematic evidence in the peashoot exudation in respect to the plant density, the temperature and the introduction of flow during the plant growth. Increasing temperature and the plant density decreased individual plant biomass and increased biophenol concentrations. Recycling of hydroponic nutrient solutions (HNS) decreased plant biomass by ~15%. The addition of flow rate in the hydroponics did not affect plant growth, but high flow rates lead to an increase on biophenols concentration from 7.5 to 16.9 mg/L. Pea shoots were grown in 1.1 L tubs for 20 days in controlled environment chambers. Plants were grown in factorial combinations of low (~25 plants) or high (~100 plants) density and low (220C) or high (320C) temperature. The HNS was stagnant (no flow) or re-circulated (at 100 or 1.1L/min). Following pea shoot growth HNS was collected and stored at -200C for further use. Two flat sheet (microfiltration, MF 0.2 µm PVDF and ultrafiltration, UF, 500 kDa RC), two tubular (ceramic 500 nm with an internal diameter, ID, 3.5 mm; and UF 400-500kDa PVDF, 13 mm ID) and a hollow fiber (60kDa PES/PVP, 0.19-0.2 m ID) UF membranes were evaluated to clarify HNS. A hollow fibre nanofiltration (NF) membrane (400 Da, modified PES, 0.7 mm ID) was selected for the subsequent biophenols recovery. The UF/NF cross-flow cascade studies were performed using a prototype equipment composed of a jacketed feed tank (30 L), a permeate tank and a pump. The temperature (30 ±20C) and pressure (0.6 bar for the UF and 5 bar for the NF, respectively) were kept constant during the concentration experiments up to a specific volume reduction factor (VRF). In case of the water membrane permeance measurements, the permeate flux was recorded continuously every 10 s under various pressures.The PVDF UF tubular membrane performed better in removing suspended solids from HNS. Adsorption experiments with HNS and the membranes carried out before rejection measurements showed that UF PVDF tubular membrane lost only ~16% of pure water flux permeance (PWFP). An additional (~8%) PWFP loss, after HNS processing up to VRF of 5.5, was observed. The UF membrane fouling after HNS processing was largely reversible, as its cleaning with sodium hydroxide recovered 99% of the initial PWFP, the turbidity was decreased to 0.09 NTU (~ 99% reduction) and most biophenols (72%) passed in UF permeate, as expected. PWFP losses for NF after HNS processing was only ~7%. After cleaning NF recovered 85% of initial PWFP. The NF step proceeded up to a VRF of 8.7 and rejected 72% biophenols. Therefore, the enriched in biophenols fraction of NF retentate could be used for other applications and the permeate can be recycled as HNS. Environmental effects on root exudation, the membrane process stability, NF permeate autotoxicity and biophenols recovery was covered.
Exploitation Route The outcomes of this research have been already communicated in relevant conferences and two more publication are planned till September. We are in negotioations with other colleagues to submit some additional research proposals in this area, thus to magnify the impact of this work.
Sectors Agriculture

Food and Drink



Description A sharing PhD studentship 
Organisation University of Strathclyde
Country United Kingdom 
Sector Academic/University 
PI Contribution I am able to provide support to the experimental implementation of this studentship
Collaborator Contribution They are able to provide modelling of the membrane processes
Impact An abstract has been submitted in the Euromembrane Conference 2024
Start Year 2023