Ultrafiltration cleans up for hydroponic cucumbers

Cucumbers_webAll water used in a greenhouse and hydroponic system should be disinfected before use, and there is an extensive range of disinfection methods that could be used to treat water. In this article, the author gives a benefit cost assessment of ultrafiltration as a treatment option for water disinfection, a Horticulture Innovation Australia project undertaken by Primary Principles Pty Ltd.


Key cucumber diseases such as Fusarium oxysporum have long been ‘a risk too large’ in reusing nutrient run-off but recent trials by Primary Principles, with the assistance of NSW Department of Primary Industries and supported through the vegetable levy by Horticulture Innovation Australia, have confirmed that available ultrafiltration technology can cost effectively remove key fungal pathogens from run-off water without affecting the nutrient profile.

Hydroponics is a highly efficient production system and closed systems make the most of this technology. While an open system—one in which the nutrient solution is used only once—requires only a small fraction of the water and fertiliser that is needed in a field cropping system to produce an equivalent yield, a closed hydroponic system is even more efficient and sustainable.

A closed system reuses the nutrient solution several times. The excess nutrient solution (containing fertilisers and water) that drains from the crop is collected, disinfected and reused. A typical nutrient solution for cucumber production can contain approximately $4 per kilolitre of fertiliser. For growers using municipal reticulated water supplies, the water alone is worth around $2 per kiloLitre. Reusing nutrient run-off saves thousands of dollars per year.

Closed system hydroponics can offer up to 40% savings in fertiliser and water costs but a common obstacle faced by growers seeking to improve water and fertiliser efficiency through the reuse of the run-off is the risk of spreading disease.

Some 65% of the more common pathogens of hydroponic cucumber production can be spread through water, making the reuse of the nutrient run-off a high disease risk. Any source of water that has contact with soil or plant material is potentially a disease risk and needs to be disinfected before it is used in the greenhouse or hydroponic system. In a typical greenhouse vegetable enterprise, up to 10% of potential income is lost through disease—and this does not include losses from a major disease outbreak.

Industry experience has shown that some methods of water disinfection can be unreliable and, particularly for smaller farms, can be cost prohibitive.

Trials were undertaken to confirm that ultrafiltration technology can reliably remove key pathogens from the run-off water and benefit cost analyses were conducted to assess the potential economic value to cucumber growers under different scenarios.

Typical purple growth of F. oxysporum from unfiltered samples.

Typical purple growth of F. oxysporum from unfiltered samples.

Fusarium oxysporum was selected as the primary model pathogen for use in the trials as this fungal pathogen can be very damaging and difficult to control once it enters a hydroponic cucumber enterprise. Three species of Pythium were also assessed in separate trials. Concentrated suspensions containing the fungal pathogens were prepared and pumped through replicated ultrafiltration units. A series of samples of the filtrate, as well as a positive control sample (the unfiltered solution) were then screened in the laboratory. No evidence of the fungal pathogens was detected indicating that the ultrafiltration units successfully and reliably removed these organisms from the water, offering an effective disinfection option for use with cucumbers.

Money matters
For smaller enterprises, the affordability of reliable disinfection systems can be an obstacle to implementing and reaping the benefits of closed hydroponic systems. To gauge the economic feasibility of ultrafiltration, the purchase and installation costs were collated and a range of scenarios were modelled to calculate the benefit to cost of an investment in this technology.

Fine pore filter membranes prior to installation.

Fine pore filter membranes prior to installation.

Key considerations used include the size of the enterprise and the volume of water to be treated (and recovered) as well as the discount rate of interest which reflects the potential change in the value of money over time. Different scenarios considered variations in the starting cost and potential rate of inflation of the primary inputs—electricity, labour, fertilisers, cleaning chemicals and water. Day-to-day use and ‘real world’ assessment of commercially available ultrafiltration systems shows that general maintenance and repairs are negligible, though with all technology and utilities a consideration for breakdown and failure is prudent so this was also included in the modelling. Depreciation is not considered.

Overall, the benefit to cost analyses showed that there is almost no reasonable economic situation in which a hydroponic cucumber enterprise would not be better off by implementing closed hydroponics with an ultrafiltration disinfection system.

Graph - Comparison of economic returns at 3 enterprise sizes, 3 target run-off levels and treating all waterOne of the dominating reasons for ultrafiltration disinfection compared with other non-chemical disinfection technologies, is that the capital investment is typically lower, which is a critical factor for smaller farms. A comparison of three different size enterprises (0.2 ha, 0.5 ha and 1.0 ha) in which run-off water is disinfected, illustrate that investment in a closed system for larger enterprises is resoundingly beneficial. For a smaller enterprise (0.2ha), the target run-off volume is a significant factor. There is no stand-alone economic benefit of only treating and reusing run-off water at a target run-off level of 20%, however at higher volumes of run-off (and in situations where the new water supply is also a disease risk), disinfection with ultrafiltration and reuse of water has a positive economic return. In a 0.2 hectare enterprise with a 30% target run-off, the payback period is just 4.4 years. This is a 5.07% return on investment over a five-year period (Graph 1).

Graph 1

Graph 1

Closed system hydroponics is now simple and cost-effective for cucumber growers to adopt—even for a small farm.

Ultrafiltration has been shown in trials by Primary Principles, with the assistance of NSW Department of Primary Industries to cost effectively remove fungal pathogens such as Fusarium oxysporum. There is a link to the project website for more information and financial modelling examples at Primary Principles

This valuable work was supported through the vegetable levy by Horticulture Innovation Australia.

(Further tables and graphs can be accessed directly from PH&G December 2015 /Issue 162).

Have you installed a disinfection system, or are you planning to?

Please help to improve information about the use of water disinfection systems in hydroponics by participating in the short project survey. There are just a couple of questions we need you to answer (a minute or two). If you want to give more information (than the survey asks for) or make a comment, please feel free to post a comment. https://sites.google.com/site/sustainablehydroponics/asking-you
(NB No contact details are shared with any third party.)

About Primary Principles
Primary Principles is an innovative, independent agricultural and rural development consultancy. Email: info@primaryprinciples.com.au  Ω

PH&G December 2015 / Issue 162